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Ajohn

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Everything posted by Ajohn

  1. Looks like you wont get an answer. Some reckon you would gain using it. Some such as a comment on Oldham Optical's web site point out that people used to make 12in mirrors out of plate glass but wouldn't these days. This ties up with Texereau's comments - up to 12in as well. He mentions that the thermal effects even at that size would be scarcely visible in actual use. He also mentions that it's easier to work - it is compared with pyrex. On my first I tried to hog a pyrex blank out once with a plate glass tool and gave up. The wear on the glass was impressive, the effect on the pyrex was minimal. This is probably why video's like the sky at night one show a blank being hogged out with any old piece of steel that is available. Followed by a tile tool. Some use a tile tool straight off or one that uses steel nuts rather than tiles on very hard materials. I'm having a bit of a problem with a 220mm low expansion blank with an F3 curve that was pre milled for me and a tile tool. Not sure why yet but it doesn't seem to be correcting the fact that the usual pre milling doesn't produce a true curve. Probably just need coarser grit. Where you might run into trouble with plate is warming the mirror up for hot pressing a lap - too hot to quickly and it might crack. On the other hand many people have done it in the past and pored hot pitch onto a plate glass tool. Texereau mentions warming up at a distance from a gentle source of heat and water at 35C. I've never used plate but he is usually a reliable source of info. I've managed with glass tools via things lying about all except that very 1st plate glass tool. It's a tough call really. Plate same thickness tool and mirror are likely to work more quickly. For my 1st I should have given it a try. Instead after the pyrex problem I bought matched same material low expansion premilled blanks and it was easy until it came to figuring - that took me a while. I don't do enough of them to do that quickly. I don't know if you have texereau's book - it is well worth a read but some aspects are dated. His aim was a telescope capable of general use with it's max magnification capabilities given most peoples viewing conditions. Seeing often limits resolution. So still true. Shape in nebulosity etc can be seen with a scope of this size if light pollution isn't a problem. As he puts it - good views of nebulae. Big mirror light bucket designs aren't considered at all - it's a book for starters. Another one not so high quality working wise is Muirden Beginners Guide To Astronomical Telescope Making isn't too bad of it's type. That may be on the archive as I don't think there are any commercial interests. Might be about on Abbe Books too used. His telescope design is dob like after a fashion mounted on a simple alt az stand Actually I have a scan of Tex...... book 1 which is well out of print and not available any more. I can try and upload it somewhere over night - it'll take ages. If I do I will post a link. It was on the internet archive. Trouble is commercial interests removed it and there may be some like minded people around here. John -
  2. Afraid it is the kill switch - I can grab it as I fall over and turn the lathe off. It's fitted with an inverter now but currently that's dangling on wires as I want to reposition it. I may fit an emergency stop at the same time. I thought that the miller might interest some. There is a small Chinese one with a rather narrow bed and limited travel (75mm or so) that way that isn't too bad but they may have widened it. I switched to a Dore Westbury. Made by model engineers from castings but I am not sure how much was pre made for them. I've come across several people who are happy with them. 2 morse with a myford spindle nose so the spindle is pretty rigid. All the 2 morse means really is the need for a set of 2 morse draw bar collets. It has a 14x7in table and can machine all over it. I bought it off some one who seemed to think it was a load of rubbish - demo'd end milling with a blunt slot drill with the work in a totally inadequate vice and as usual slides set way too loose for a small machine. Looked ok to me and it came with a small rotary table and dividing head made by the same person, rather well actually so I went for it. The rotary table had a bit of a problem as some things do that are made from castings - too thin and warped slightly so had to be re machined a bit. The head can be tilted - bit of a problem setting it square to the table afterwards. I rigged up a jack screw to set it up and have left it like that. I have a pillar drill upstairs so don't use it much for that but the head can be moved up and down. Have to laugh some one on another forum saw the shot and said I am sad to tell you it's a Mk1 - as if I cared. Actually I would find the typical spring ram return pillar drills and the MkII have a pain for milling. The lathe came of some one who was giving up via ebay with a lot of tooling. It's well equiped so I was prepared to offer his reserve which was surprisingly low really all things considered. Boxfords seem to have gone up some what now. They are a well designed lathe copied from southbend. My father was a metal removal expert at a large company and even he rated them for what they are. He wanted me to have a CVA - if only I could especially fully equipped. I have used one. They are unbelievable for that size of machine but probably weigh well over a tonne as small as they are. What I did is emailed the seller and asked him to turn 6in of bar and tell me how much taper there was. Couple of thou which I could live with. Turned out that the bearings needed a bit of adjustment and his bar was probably bending. I have made my mistakes even though I should know what I am doing. A peatol initially. Amazingly accurate and I really mean that until the head bends. A Unimat - the dog clutch on the lead screw has problems even after just 1/2 hr or screw cutting. The next owner manage to crack the tool post casting while tightening the bolts on a tool. A myford ML7 and I really should have spotted this - the belt to the head way to tight to keep the spindle in place as the bearings were shot. I didn't realise how bad the beds can be and it was hardened - difficult to fix. Kept this due to the problems and bought a heft Chinese machine. Finish not bad but basically very poorly aligned and impossible to fix. Found a super 7 head and fitted it to the ML7, sold to a friend for a song really mostly sorted out telling him to send it to Myford for a bed regrind and saddle fit. Remains like that unused. I should have sold the bits. I've now had the boxford for a long time. Must be about 12 years at least now. The 1st larger lathe was the Raglan. Fine after a bit of work but then came the rust problem. The best garage is an attached one really - a bit of heat. Madness - as I make odd bits for microscopes at times I would like a small high speed lathe - had the idea of modifying an ML10 for 4,000 rpm. That is kicking around and the need for that has more or less disappeared. I also have an early German clock makers lathe in pretty good order really and should have just got on with that but screw cutting would be nice. One of the problems with having ideas. When I bought that I heard one of the guys there, private sellers say I would love to see his face when he tries to part off with it. Bearing problems again. Lots of old expensive swiss and german gear about probably hardly used if at all - too tidy. I've found real machine tool sellers can be more honest than others but they seldom have things suitable for home use - like my Raglan. They did say that they didn't usually sound like that - cracked variable speed pulley. Others though including these types will sell people a load of rubbish. Some sellers will deliberately fit a useless tool in order to give people the impression that they will be able to do better. A worry with the miller but he clearly had no real idea what he was doing. Some sellers don't. One "gent" on here might feel that all of this is not relevant but a bit of not all that uncommon experience can help others avoid similar mistakes. It's a bit of a mine field really. LOL I even bought a little Schaublin at an auction at a large factory that was closing down. Another mistake really as the bearing were totally shot and nigh on impossible for me to repair. I even played around with it doing some things that I knew couldn't be done that way to put others of biding. Couldn't power it up but would still have bought it. I sold all of the parts with a staggering profit. Even the head very cheaply to some one who fancied having ago at making new bearings on a bigger Schaublin. John -
  3. I have some stuff knocking about too that was going on ebay but if any of it helps -- Finding the maker on a chuck sometimes needs side light and even a magnifying glass. Measure the spindle nose up completely. You might be lucky. One thing to note is the chuck key - it looks like an older forged one. They weld the ends on the newer ones and they snap off even for me. John -
  4. I probably didn't mention the joys of owning a set of blacksmith drills - they will open holes out to an inch used in sensible stages. If you have a four jaw the jaws will be reversable - they also grip parts a lot more rigidly than a 3 jaw but need the work centring in them. Seeing the money pit comment - I agree. It always pays to buy lathes with as much gear as possible. I've been lucky in that respect but still keep buying things from time to time. Seeing the earlier photo's of a wonderful bench - Eventually it all becomes a problem for some. This is what I have to put up with 1/2 way through a tidy up It's even worse at the moment - another tidy up, not enough space etc and I want to do some work. The WD40 dropped there by my son - clutter from others is also a problem at times. John -
  5. Perhaps Damian it would be best to just let some one go out and buy an inverter and motor and then find out for themselves. Some do and it proves expensive in the end if some one isn't aware of the problems. Good point about mdf and water but I wouldn't be using it unless I though I could get round that. Worn out myfords can cost rather a lot of money as can many others. The other problem is that they get to low speed using gears but some of the pulleys would prove useful and can be bought off ebay on their own - fairly cheaply of late. Washing machines can be a good source of high reduction ratios but best to make sure the belts are available, V pulleys can have problems with too hight a reduction in one go. Due to where it has to go mine might slip as I know I am pushing it a bit too far. The mirrormatic design is probably sensible on that score and just the turntable drive parts could be used. Some reckon anything involving gears is a no no. I'm disregarding that. An easy way if one can be found is an industrial right angle gearbox. There is one on ebay at the moment 10:1, a decent start in the right direction, new other, £125. That is really cheap for what some fetch. Chain drive at least in part could be the cheapest and most efficient method. The B'ham Astro Society has one based on that - last time I heard no one could figure how to put it back together correctly - a few min look defeated me. Tables - I did look around at marble and granite circles but came up with zilch that suited me. Concrete maybe with mesh in it - have to see if I run into trouble with what I have. I did have a bell cement mixer until recently. Finished with it, one small job, lent it to others and sold it cheaply as lots of rust but still solid - might be a feasible source of parts. John -
  6. I spent around 10 years working on electric vehicles so as far as motors go know enough to ask the right questions. Like most things that are ahead of their time in the UK they went no where other than the designs to California but the Royal Mail ran a number of them around - 1 tonne vans. Also a bus and a taxi. One of the royals ran a rather luxurious version of the vans around on one of their estates. I've driven it. The sound proofing and seating was awesome. Inverter drives have 2 problems really. One I have outlined but I feel that the manufacturers overstate the problems a bit. Some do run motors at very low speeds but only for short periods. The other problem is motor power. If say a 1/2kw AC motor has it's speed reduced "mechanically" by a factor of 2 there will then be 1kw available at that speed ignoring efficiency. Do the same thing with an inverter and loosely speaking 1/4 kw will be available. They often mention constant torque in all information on inverters and that's the problem in terms of power - they do provide more or less constant torque. There are some ways round the low speed overheating problem. The extra fan is one. Another is modelling the motor in the inverter and guessing what it's temperature is and regulating power to suit or even just shutting down if there is risk. Another which doesn't seem to be popular with motor manufacturers is building a temperature sensor into the motor. So really it all boils down to if the maximum power available at low speeds is needed and used for long periods because if it is the extra fan is the best option. If only some fraction of it is used people may get away with it. Given how fans function with speed it will only be a small fraction of it as well. One of the worst aspects with AC inverters is many of the yoyo's that sell them. They will tell people what they want to hear but it is possible to phone and talk to the both the inverter and motor technical people and ask about the problems. What a lot of people who fit them for home use on machine tools is fit a more powerful motor but the results are still a bit imponderable. Many are aware of the problems so take care with settings and how long the machine is run like that. Some people, like one on another forum, get told conflicting information by different suppliers so ask. In this case running a milling machine from 20 to 2000 rpm. He really thought he could do that without any problems and became a bit angry when he found out it wasn't as simple as that. Inverters to him are now a misleading load of rubbish. He doesn't really want to even accept what he sees in a catalogue. Mirrors - there isn't a need to make many to get a decent feel for the problems. There is no need for a mirror making machine really and I would estimate that it's possible to spend rather a lot of money making one properly with some certainty of the outcome. Will my pavalux universal motor be powerful enough - probably going on machines others have built and according to the web which can always be a problem. Will I be able to reduce speed by 1/2 electronically. As it runs at 4,000 rpm and has a gearbox on the end I hope so but will be fitting an ammeter to get some idea what is going on. It still needs a fair reduction after the gearbox to get the power up. That's part of the reason for 33rpm. Will my mdf frame break under the load ? I hope not but I'm not about to try and determine what loads will exist and do stress calculations. I happen to have a lot of 19mm mdf about that was used for shelves. The table is more difficult. I had hoped that I would find a cheap suitably sized lathe face plate but cheap proved impossible. I have disk of aluminium that may be to thin, have to wait and see. If it is more plate, bandsaw and route the edge or maybe I can find some one who will cut one for me. Really for me making one is an addition to the fun. I've intended to do it for some time and have been keeping my eye out for bits for several years. John - mmmmmmmm Maybe a circular slab would be an option or such like caste in concrete but I don't have much interest in very large mirrors. -
  7. 2nd hand stuff like micrometers off ebay are usually ok but for a 1:2n you ideally need a setting guage for it. Just pick one that looks decent with nice clear markings - if they are faded it probably means it's been cleaned up. You could make the dummy spindle out of a few inches of free cutting mild steel, if you scrap one end turn it round and try again on the other. It's not rocket science. If you google on things like screw cutting you will probably find clearer explanations than I can write on here. If you have an 8tpi lead screw you can engage screw cutting for 8 tpi anywhere it will go in but do remember to start 1/2 in or so from the end of the work to take out play. The chasers are a good idea too really and wont cost much. There is a way of screw cutting without an indicator. I've never needed to do it but if you google screw cutting no indicator something some where is likely to come up. Don't mess with refurbing a lathe unless you are certain you are doing the right thing - it's easy to make things worse. I have a feeling my straight edge came of amazon but why not just turn and see what happens. Myford, Boxford, Viceroy and many other smaller lathes use 8 tpi lead screws. I suspect this man will make you a screw cutting indicator. To be honest though I find the bulk of my screw cutting is done with taps and dies http://www.latheparts.co.uk/ I've heard that he is hard to get hold of especially on the phone. He's fairly famous actually and his prices are remarkable really even for things like face plates. John -
  8. I wouldn't get too carried away with inverter driving a grinding/polishing machine. They can fairly safely used for a frequency range of around 40-80 Hz giving a 2 :1 speed range but the manufacturers would prefer them to be fitted with a separately powered cooling fan below 50Hz really as prolonged use will cause them to overheat. There wasn't much info about when I put one on my lathe but TEC are pretty clear about it all now. I kept asking the suppliers some rather pointed questions and guessed the figures TEC now quote. They can be run at 100Hz but that just makes gearing down more difficult. When asked about 100Hz the motor men say I don't think we would make one that would burst at that speed and things like that. The cooling fans shove the price up a lot. A 100:1 gearbox might work out cheaper and also provide a good solid spindle. Inverters also cause the motors to make a more irritating noise but some have random switching which helps with that. MDF - yes on mine but not the table. John -
  9. If you know of a place that strips and repairs washing machines you might find some cheap pulleys. After much thought and looking around I've settled on 33 rpm so I can play my LP's when it's not being used for mirrors. No seriously it's looking around and the Waits video's. I've happened on a Pavalux motor so can also vary the speed if a really cheap speed controller works. If not I think I would go down to maybe 16 rpm via another pulley set up for hand work. If I added a stroke arm, unlikely, going off data on a commercial machine I would set that at a lower speed and try and arrive at a set up where the tool doesn't pass over the same path very often or never if possible. The primes the speeds are based on might sort that - makes my head hurt. I may get round to cutting the MDF for mine this weekend, soon anyway. John -
  10. The new lighting they are putting up in B'ham is a lot more powerful than what was there before - the usual tall low pressure sodium lights as used in most cities. They are also a lot taller now. The history of lighting is interesting. Lots used to be turned off at 11 PM - everybody should be asleep in bed so that they wouldn't be late to work in the morning maybe. Ok if the moon was up if some one was out and about and the skies at least partly clear. Some people who live in real rural areas are probably aware of that but would still like more lighting. Some one had the idea of leaving them on all night at junctions and round abouts and it seems this did reduce accidents. Due to the accident reduction they then moved to on all of the time. While this was all going on people complained about driving through maybe a mile or what ever of street lights and then being in a black hole suddenly when they had passed through them. True as it takes time for eyes to adjust to headlamp light levels. Maybe it wouldn't be so bad if they were dimmer. Also going back in time but not all that far there was a drive on dipped headlights campaign for 2 reasons. Mainly in cities as people were driving on side lights as they could see perfectly well via the street lights. Probably a lot of gas lighting about for most of their driving experiences. The 2nd reason, done by many, same sort of people who had been doing it for years, driving on country roads with side lights only if they could - the moon again. More - there was a press campaign when low pressure sodium took off big time. It illustrated that people wearing certain coloured clothes didn't stand out very well especially standing directly under one. I understand the councils were there are real observatories are requested to stick with low pressure sodium because it's so easy to filter out. The filters are very narrow band so hardly change light levels at all - but why did they disappear? I bought a SCT some time ago and bought a filter to go with it. Went back maybe a month after to get a 2in one and was told that they couldn't get them any more, only the types that block out lots of light all over the spectrum. The thing that sort of gets me about the whole thing is no one seems to have asked the question what levels of lighting are needed. It's easy to see why broader spectrum lighting is desirable but frankly I have doubts about the press campaign. But why ever brighter and brighter. I've seen a claim by a pundit that peripheral vision is improved via brighter lights - rubbish as many of us know - averted vision. I feel much better now I have had a rant but feel that official thinking has gone seriously astray in this area. I grew up in a place that used ordinary bulbs with a very long life for street lighting. No colour problems and plenty of light when it was dark. Stars easy to see too. I know that because when I was a yob I climbed up one and removed the bulb and stuck it in my bedroom and it lasted for years and years. Maybe I was just curious really and not just a yob. John -
  11. If you are going to make a back plate it's best to make a dummy spindle first. Most lathes have a thread and a register = a short parallel section that actually locates the chuck. It's important to get the register right if you can. A dummy spindle head can help a lot with that also ensure that the thread will fit ok. The easy way to do that is to bore close to size and then reduce the size of the cut. If for instance you reduce to 0.0005 and you do this till the dummy spindle head goes in you know you have a fit to 0.001. The biggest problem people have boring is not realising that the work moves so will often take another cut if it's run through again at the same setting. It's important to run them out if you can. Also to set the tool slightly above centre in case it bends. If it's on centre the tool will dig in if it bends. Slightly is say 0.010 to 0.005. Depends how much it bends. You might have problems due to worn head stock bearings. That can make it impossible to take good light cuts and the machine will work a lot better with heavier cuts as they take out the play. Best option then is a micrometer and telescopic gauges - a vernier wont really measure close enough for this sort of thing especially internally. The telescopic gauge needs to be a firm fit in the bore when you rock it to take a measurement. When turning OD's it's best to set the tool slightly below centre for the same reason and again running the same cut might remove more metal. Usually down to head stock bearings or the work bending. The easiest way to cut what are really big threads for the size of lathe is to set the compound slide over at 1/2 the thread angle and set the cut with that. Then the lathe is only cutting one side of the thread so the load is reduced. The cross slide can be zero'd and used to wind out and reset to zero again for the next cut. Remember to take out play in the cross slide screw by always winding into the cut direction from far enough away to remove play in it. I usually use a 55 degree tool and set the angle slightly less so that one side is scraped a little and the other cut but it's easier really to forget that. You will need to do the sums using a single point tool as they don't cut a round topped thread so the o/d needs to be reduced and the total depth of cut worked out to get the flanks of the thread correct. You can also cut with chasers especially the backing plate itself as they can be run straight through and also the dummy spindle too really but you might find disengaging and winding out to clear the register frightening. Same as a single point tool really. That will give the correct form without any messing about and can still be fed in the same way. There are plenty of used 9 tpi whit chasers on ebay. Machine types are best but the hand ones can be used as well more especially externally. Lastly your thread form is 1 1/8 in dia whit. It's slightly undersized on the lathe to clear a perfect one in the things fitted to it. That's how accurate real lathes are made. You might find a tap but even just sizing a thread that has been cut a few thou short takes a lot of effort at that size. It's an option some use. I'd guess in a very rigidly fitted vice and bench. It might be possible to very lightly clean a thread up in the lathe with one. Before buying of RG I would see what ArcEuro have as well. Might be cheaper. They also have a tendency to sell good stuff. There is also another ebayer with a good reputation harry something or the other. Rotagrip may be able to sell you external jaws for your chuck. Just tell them the make and size and see what the say. They may offer soft jaws. I've had to spend a couple of hours with a file and stone to get them to fit properly so ask if there is any doubt about fitting. Chances are that the chuck on your lathe is better quality than the cheaper ones. I mostly use HSS tools I grind myself but also of late some carbide tooling from these people http://www.shop-apt.co.uk/ I would advise the smaller triangular tips. They will be recommended for finishing - stainless etc and work well on smaller lathes. Some on on the Schaublin yahoo group suggested that I should try them. They work well on most materials. Ebay bargains can be a mixed bunch. I suspect some have come off cnc machines just before they are worn out. That's what they do with production machines - stop using them just before the wear out. HSS tool bits can be hand ground even on a 6in grinder but only really with better wheels than they usually come with. Axminster do some white aluminium oxide grinding wheels that work rather well on HSS. The cut slow and cool without clogging quickly. They also do a hammer head dresser which makes dressing the wheel easy when it needs it. They should be polished with a slip stone afterwards and sharpened when in use from time to time too. Good slip stones are hard to find now so people use diamond sticks etc. Braised tip carbide tools can be reground using a green grit wheel but I feel that needs an 8in grinder. Some people lap worn beds if they have one and it proves to be a problem. It's important to know where it is worn. That can be done with a decent length straight edge and a 0.0005in feeler gauge reasonably well. Main problem is that the affordable straight edges are usually straight to 0.001in per foot so it's important to make sure that isn't making an area that is ok look worn. Right up at the tail stock end beds usually have no wear at all so that area can be checked. My father mentioned that in days of old when and knights were bold and plain bearing lathes and high accuracy lathe work was common people made things to fit on the bed so that new sets could be rebored from the bed or the headstock by hand to finally size them. As only thous are being removed turning by them by hand is easy. Actually I came across a lathe at work that had twin bearings so that this could be done using a normal boring tool. The outer bearings were usually locked and just had to be unclamped to allow them to rotate. These days accurate work usually finishes up on a grinder and lathes are seen as things that struggle to work to better than say 0.010in. Then comes lubrication. Small bottles of slideway lubricant can be bought of ebay- wonderful stuff, not much is needed. Stays there and doesn't evaporate etc as much as other oils. I understand that people who have plain bearing lathes lubricate them with some grade of hydraulic fluid - a web search should sort that out. I bought 5ltrs of cutting oil from Morris Lubricants a long time ago and sometimes smear it on work before a cut with a kiddies paste brush from a pound shop. It can really help with finish. Warding files are useful on a lathe - or any other type at times but make sure they have a handle especially needle files as a slip might send the end through the palm of your hand. Since Stubbs stopped making them there is a lot of junk about however Axminster sell Swiss Vallorbe - the miniature file set are warding files and well worth the money as are the bigger hand files. This is how some produce some amazing work on lathes with problems along with a strip of emery cloth on them. With a bit of practice it's easy to remove the taper some lathes produce when it should be parallel and also improve the finish if needed and even still get very precise sizes. Astronomer machinists might have problems boring thin walled tubing because vibrations set in and really wreck the finish. The finish will look like the noise sounds. The answer is to slow the machine right down and use the automatic feed. Maybe even using the back gear to get it right down. Ok a single cut might take 10min but if needed it's the only certain way and only needed for the last few cuts. Some people get rather tied up with cutting speed tables. These are really aimed at maximising metal removal rates with reasonable tool life. There is no harm working at slower speeds. The things to watch for are say blue swarf coming off a steel that will harden - it probably will as you cut it making work harder than it need be. Aluminium often stick to tools because friction is causing it to melt. The finest feed rates a lathe has might not give the best finish. The feed rate really needs to be fine compared with the rad on the end of the tool if there is one - this is why the small triangular tips work well - they have a small rad. Big rads use up more horsepower taking the cut, more of that means more heat and metal tearing and distortion as it comes off so they aren't really a good idea on any lathe. There is a technique of old called fine turning that's a bit different. Huge rad, tool well over centre with a lot of front clearance ground on it. Really it's more like using a broad ended scraper and goes back to pure hand metal turning days. An interesting thing to try and may work well if the lathe is up to it. I had better book mark this post as I ain't typing that lot again. So sorry if I have made some really weird typo's. I do sometimes. John -
  12. The lights round me have been changed recently. They were low pressure sodium and powerful. Some time ago I managed to obtain a filter that just killed the main sodium line they produce. The difference was amazing more or less like going to a dark site. These filters disappeared off the market long before the lights did. They tell me the latest ones are LED but the spectrum looks wrong to me so probably high pressure sodium. More powerful and higher than they were of course even though all they really need to do to achieve moon light brightness. I'm part way through knocking up a spectroscope to see what they put out. Worse still they have put lower powered LED lighting round the corner and used high CRI lighting - this has to put out an awfully high amount of blue light to achieve the colour balance they achieve. They are extremely inefficient because of this. They will be using more electricity than the lights that were there before. It seems councils will cut all sorts of services in the UK to save money yet still spend large amounts of money on "updating" street lighting which always involves more and more of it one way or the other. It even just has to brighter and lamps closer and closer together. Not that this annoys me of course John -
  13. I've been wondering that for some time too RAC and suggested periods fixing the centre which should be short earlier, also not working with the tool edge past centre when trying to fix the edge plus not much overhang when doing that. This will still leave a hump in the middle but not an extreme one which could turn out to be difficult to fix. However from comments I shouldn't post in this thread so over and out. It's just difficult to resist as it seems to me that Damian has spent more that sufficient time to finish the entire mirror even accounting for figuring for the first time. John -
  14. If your thinking the powered grinding machine way Damian for this or other projects there is a lot of information on this mans youtube video's on all sorts of things. https://www.youtube.com/user/GordonWaite/videos He uses a fixed post machine - no stroke arm. Much simpler to make. Using them is a mix of hand and pure action of the machine depending on what's being done. The bearings for mine arrived today - working in the house and not much space so a sensible way to go. Out of interest it looks like dental paster is dental stone in the UK and easy to get. I think another goes by the name of hydrastone but I can't find it. John -
  15. LOL - I mentioned the Raglan as a proper machine with plain bar slideways and then edited it out - I think. Much better design than the 7's and bigger. I had one but alas rust in the garage put me off it and I moved into the house and it couldn't go with me. What happened to Raglan - bought up by Myford and pole axed. Interesting lath. The bed slides are bolted to the bed, carefully set up and then dowelled in place. Mine did have a bit of wear on the side of them. I had them reground very lightly and the people who did it clocked up the inside the tail stock runs on and took an incredibly light cut on the outside. They wondered why I had it done as the cut was so light so I pointed out that I didn't have a cylindrical grinder. I'm a bit like that with lathes. They also ground up shims for me to set the saddle up. I've also shimmed up a Myford saddle - it's a painful long winded job to get it as it should be. The first serious job I did on my Raglan was after I found that one of the sides of the variable speed pulley was cracked and had a bit missing so I made another one in 2 parts. plate for the big dia and bar for the other. Maybe the ML10 wasn't set up well or too worn. Some have under powered motors off washing machines etc fitted. The problem with little machines like that is that they have to have everything set fairly tightly so that slides offer a bit of resistance. I have come across people who are happy with them and do accurate work but size is limited. Even a Boxford benefits from well adjusted slides. It would have no problem taking 1/16 cuts of mild steel bar. The only problem I have is making m2 thumb screws and silly things like that. The thread dia has to be cut in one go otherwise if more than a thou or so is taken off the dam things they bend even in stainless. I also tried a Chines lathe - it turned more of a taper than I am prepared to put up with and went to some one who mostly shortened bolts. Then I looked at a lot of them. All sorts of problems. One interesting one is not being able to turn up to a centre in the end of the work without using a morse taper extension in the tail stock. It makes the centre distance they quote bigger than it really is. Accuracy problems are sometimes put down to the headstock bearings they fit. I reckon it's more to do with how they are made. Poor quality cast iron as well. Older lathes tend to be a lot better in that respect but a lot of this could be down to weight. Pass really forking out for one put me off. I warned a friend at work not to buy one of the small mini lathes - he did and later told me he thought it was really intended to turn plastic. Might be technique. I don't know. Adds for them sometimes say so many microns run out on the head stock - pretty useless really as it doesn't give any idea how much taper parts will have. Real lathes have usually been specified properly. John -
  16. Being fair to Myford the ML and Speed 10 should be a decent smaller lathe. These use a dovetail bed which has it's own problems but is better than beds formed by 2 oblong bars etc providing it's not worn too much. Easily checked by tightening them up by the head stock and seeing if it can still be moved all along the bed. These Myford's use taper roller bearings so easy to change or adjust. The other type of Myford beds have to be shimmed up but shims can not be used to take up any wear as the bed will just be too tight some where or the other. Explaining a bit more as this applies to any lathe the Boxford and others use prismatic beds. The saddle* on this type run along to upward pointing triangles, sometimes just one at the front. The mating part in the saddle is usually much longer than other types so the area to resist wear is much greater and the shape itself resists movement under cutting loads. The ends often go past the head and the tailstock at the extremes of travel making them even longer on some models. Some run the tailstock along it's own prism - unlikely to ever wear out and will remain as accurate as it was made within reason. The length to width ratio of the guides is important as well. Say a piece of 25mm bar is waggled about passing though a 0.1 mm oversized hole that is 25mm deep. It will waggle by so many degrees. If it passed through hole that was 250mm deep it would waggle by around 1/10 of the amount. The basic idea on lathes is usually referred to as the narrow guides principle. The Myford 7's try to use this by running on the front rail but another factor comes into it as well - cutting forces have to push the saddle hard up against it for it to work and they haven't designed that in very well. Really the bed needs to wider. There is also a lot of mass to move if there is any clearance - the prismatic beds are always in direct contact. Dovetail beds take a lot of the force on the top of them which helps but some is also taken on the rear slopped edge so can have the same problem as the usual Myford bed. Headstock bearings are a pain. They can be a reason for a lathe only giving a decent finish when heavier cuts are taken and also usually explain why the same cut setting keeps removing metal. I notice that the Portass on here uses split phos bronze bearings - not a bad idea at all as a new set could be made on the lathe if needed and then scraped in. The worst ones just run in cast iron split on one side and tightened with a bolt on the side with the split. When fully tightened the castings often split. Lathe bearings always tend to wear oval due to tension from the belts and cutting forces so this sort of adjustment only helps rather than cures unless there is very little wear. *The part that moves back and forth on the bed carrying the cross slide and compound slide. John -
  17. It's possible to way way more ticks than that see http://www.renishaw.com/en/resolute-linear-absolute-encoder-options--10936 Some version of this has been fitted round a ring for the direct drive telescope motor project that is about on the web. I've been wondering about cheaper versions which are bound to be about and may be based on vernier caliper etc type technology. They also produce rotational version http://www.renishaw.com/en/resolute-rotary-angle-absolute-encoder-options--10939 John -
  18. This might or might not be of interest http://code.google.com/p/wheel-encoder-generator/ It will print an absolute grey encoder with a resolution of 4096 such as this, it proves a little bit too much for a 600dpi laser to print accurately at 150mm dia. I just tried it. A resolution of 4096 gets down to about 5' direct absolute encoding. I'm not sure what the resolution of the rather expensive absolute encoders are that are used on some high end mounts. It will also print ordinary encoder wheels. Those might be of interest to people who are interested in servo motor drive. John -
  19. Sounds like they are planning a 42in as well but have problems with a wind farm being built around them. Wind farms - I wonder how much subsidy they get. As I see more and more of them they are beginning to give me the same feeling as over lit streets do. John -
  20. Oldham optical will do you a 30in for £"Ask" maybe he would undercut the astrobuy sell price which to me sounds too expensive plus the need for a 1 tonne trailer. If it's an older mirror it might be 5in thick at least and I suspect it wouldn't be far off that if a recent mirror as well. Hope some one on here buys it and posts. I'd love to see how they got on with the mirror cell and tube. Telescope House - probably sell anything if there is money in it. No mirror blank kits any more though. Sad that. The pre grinding was brilliant, tool too which wouldn't be needed these days. John -
  21. It's an ancient method of keeping optical bits in place. Instrumentation people are fond of using socket screws - the key handles are long so minute turns are easier to do. It's also still possible to buy precision screws - I asked for unbrako equivalents from a local screw supplier but they are more expensive than the usual type. It might even be possible to get metric fine even though ISO seem to have decided they don't exist any more. I bought some 40 TPI model engineer taps and dies for this sort of thing some time ago from the London Tap and Die Co. Not used for this yet but I know I am likely to need them. I've bought a mirror blank now so if that goes ok they will get used. I did a couple of years of tool making training before going into design. Every die stock that came in was opened out by something like 0.010in so that split dies could be opened out allowing rather precise threads to be cut. Open them too much and they crack. Might help people get threads that mate well to prevent rock but I'm not sure anyone makes taps and dies that accurately any more. ISO again. The other way to help with rock is to make the female part longish - say 2 to 3 diameters. There are also some 0.5mm pitch taps about at various sizes thanks to the thread being used one things like board lenses. The male parts might have to be screw cut but metric has the advantage that a 3 square needle file can be used to finally fit the thread short lengths at a time. Do use one with a handle and the lathe running at low speed - I had one more or less go through the palm of my hand once. Fortunately it didn't do any serious damage - just hurt in an odd sort of way, more like a severe ache. On the other hand when both the push and pull screws are tight the threads will be resting on their flanks so wont be going anywhere. Maybe the best option on the parts shown would be long push nuts as they could stick out of the back of the cell. The pulls could also be nuts on all thread / a stud. This sort of thing would minimise the space needed in the scope's mirror cell. John -
  22. Has anyone tried adding "push screws" to these. Some don't use springs. One screw adjustment pulls and the other pushes so there is no play - other than bolts moving around if they can. The springs on the pulls should help setting up as the mirrors normally shift slightly when the push screws are used to lock the setting up - so the pull screws have to be backed off a mite and then the push ones tightened. If nothing can move around this set up should hold collimation. There is a need to make sure nothing is being bent when the screws are tightened as well. John -
  23. The grinding on the ebay blanks is what I would call so so. I have had much better. It might mean running down from a full set of grits to get the basic sphere correct initially. Normally the coarser ones aren't required if the pre grinding is done well. I suspect I will buy a bag of suitable shot blasting grit to do that if it needs it. The back has been ground as well, finish wise better than the front. It will also need more of a chamfer putting on it. No rush though as for other reasons I have decided to make a fixed post grinding / polishing machine - mainly so that it might encourage me to pick up my cassegrain project again. I feel that pure hand reworking of 2in dia lenses wont be on. John -
  24. The castings have self aligning bearings in them and they are a form of pillow block. They are basically a ball race with a spherically shaped outer diameter so that they can tilt in the housing to align with each other when a shaft is fitted. I did know that they were available but not what they were called but on ebay at least when the usual pillow block is searched for this type come up as well. The other type were very popular for making home made table saws etc. John -
  25. In case some one gets into a Bath interferometer I must stress a red laser pointer - the others use IR lasers to excite a crystal of some sort and are not suitable. I did toy with the idea of working on a 2nd hand or cheap mirror. Maybe just refiguring it if needed. There are usually some on Astroboot. There was a 10in recently but I came across ebay 251250299718 and bought one of those as it's pre ground to what I want. If some one wants something else it wouldn't surprise me if they could organise it. F5 is getting a bit fast though. About 2um of glass to remove if mostly removed from the edge. Moving to F6 would virtually halve that. It's not the amount of glass to remove that counts within limits. It's the steepness of the slopes and the rate they change at on the mirror surface that they indicate. A boys own 6in F8 hardly departs from a sphere at all. It goes up with size and decreasing F ratio. Texereau uses Diameter^4/128*F^3 for the figuring depth. Others use the difference in depth at the centre. Trouble is until some one has done it they don't know what it means in practice so it's mostly just another thing to worry about. I haven't tried F5 before so feel it will be harder than F6. Hopefully not too much harder. Another source of blanks is this one http://www.stathis-firstlight.de/spiegelschleifen/materialeng.htm They too may be able to organise pre milling the radius but personally I wouldn't worry to much about hogging a mirror out. The only reason I have gone for a pre milled one is that it's easily available. John -
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