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Ajohn

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Everything posted by Ajohn

  1. I was thinking in terms of using a shelf going back into the case to support the horizontal gears Gina. Backing paper running down from that an also in the "pocket" formed by the shelf leaving all of the gearing visible. I suggested making the clock face a bit smaller to get round the lack of visibility in the area you indicated with arrow A. That might mean changes to the size of the case or it might not. As it's a visual thing hard to say. The shelf gets around having a slot and leaves everything visible. I'm no artist though. Just a humble engineer. Mmmm you would have to support the prawl from the top of the case. Maybe something printed ? John -
  2. Maybe a smaller dial just showing the teeth of the larger gear and the rest of the drive sitting on a shelf hidden by the background. The case would probably look better oblong then rather than square. I nearly suggested larger gears to keep the dial the same but the case would still probably look better oblong. John -
  3. Gina every time I see the name of a 3D printer mentioned I have a look at it on the web. Is the Titan you mentioned the one that uses a closed loop servo drive ? Price very ouch for me but the construction is interesting. My Weller is still going strong on it's original supply. It dates from the 70's. Just hope it remains like this. John -
  4. As I am slowly collecting bits for AP I bought one of the ZWO one's I linked to. I've a lot to sort out first. May as well mention here as using it will be a bit diy. Nice and light. Pleased about that as finders can be pretty heavy. Things lock up nicely. Having looked through a lot of optics I reckon I can get a good idea using them terrestrially. Nice and bright and very sharp even with a 10mm eyepiece and then a 22mm Vixen. 280mm fl. If the sky ever clears I'll see what stars look like but strongly suspect it will be ok. I was bit dubious when I saw that there were no baffles in it. It will focus with an eyepiece in it but a televue plossl wouldn't unless it was pulled out about 6mm. Distance around 30m. Another old Vixen was ok. So due to looking how much the focus was sticking out I thought I might get away with a 1 1/4 diagonal in it. No. Might be able to by modifying one but doubtful. There is some scope for modifying by making new bits for it where this could work out - with a lathe. The fact that this one has a rotating focus doesn't seem to be much of a problem to me. It certainly isn't with an eyepiece in it and I can't see it being much of a problem with a guide camera either. It's a nice holder with 3 adjusting screws that can be locked on each ring but it sits on a short Vixen dovetail. Small holders for these are available but making one or adapting would be a lot cheaper. So, wont focus with one eyepiece so I could send it back legit but o/all think I will keep it. I was surprised just how light it was given it's 60mm objective. Pity about the work needed. The focus adjustment could have been longer. May well not be a problem with a cross hair eyepiece but it would benefit from an eyepiece focal length that maximises the field of view, say 32mm giving a mag of just under 9x the edge of the field might be a bit colourful using a 1 1/4" eyepiece like this. It's only intended to be used over a small sensor. Then there is the work needed to make some sort of camera par focal with an eyepiece. John -
  5. Googling uk skywatcher guider conversion came up with this link http://www.rothervalleyoptics.co.uk/rvo-finderguider-adaptor-skywatcher-to-c.html However looking at prices selling yours and buying this might work out cheaper - not sure about mounting but it does come with a decent holder. It might mean sawing of the base of a bracket and attaching it some how. http://www.365astronomy.com/ZWO-60mm-Finder-and-Guide-Scope-60280.html They also do the adapter - probably cheaper than the other link John -
  6. I've found this thread interesting and wonder if the earlier post ( which I hope was on here ) concerning noise against temperature is the important aspect. John -
  7. Not sure if it will be of interest to you Herra but full frame has been reckoned to offer 2 to 3 stops better noise performance than APS. There is a problem though. As their pixel count gets higher and higher that becomes less true. Where is was true is cameras such as the canon 6D but at the 3 stop level even that one may be dubious. In the case of levels like the D800 one of the aims seems to be to do away with the anti aliasing filter as the lenses do that anyway. It looks like this has been absent on compact cameras for some time. All very confusing though. Sensors have improved but so has noise removal software. I own an interesting example of the effect. A Nkon V1 with CX sensor. They did a cheaper Nikon 1 with less pixels and lo it has less noise too. My most used "dslr's" now are olympus m 4/3. I haven't touched aps for some time now. The 6D isn't very well rated by serious photographers - af and etc is too simple. My last ff was a 5D and no interest any more so no idea of used prices. John -
  8. Gunson have made an automatic batter charger with a float setting for permanent connection for a long long time. It works rather well. John -
  9. I've not tried to do this but have a feeling that the lens needs to be focused onto the mirror not infinity. The reason for changing the lens on a web cam is to get a bigger image as the standard ones are fairly wide angle. The feeling comes from eyeball testing and also messing about with taking photo's from what comes out of an eyepiece where the camera does have to be focused on infinity. I'd be interested to know if I am correct as the idea of a nice big image on a PC screen appeals. So basically place the camera with the lens a little outside the centre of curvature, focus the camera onto the mirror. Having the knife edge close to the camera is mentioned here for instance. I'm sure I have seen the same thing mentioned elsewhere. http://foucault.sourceforge.net/#docs That test might be worth a go but I would cross check with a mask the usual way. A mask on the mirror would help to see if the web cam can be focused. John -
  10. I found the attached file on the web for download so no problems attaching it. It covers just about everything I have ever seen mentioned on dental stone but avoids using epoxy for sticking tiles. I suspect that is what people will have to look for in the UK not plaster. Some have used hydra stone over here, or a name very like it. It's a much stronger type of the usual modelling plaster. Some one on the youtube uses a mixer to mix dental stone up quickly (Gordon ??). It sets pretty quickly so probably best to mix a little to see what happens rather than just diving in. I've attached the file in pdf format. I saved it in odt which is a Linux word format so used something else to convert to pdf. If Adobe wont accept I can post in some other format if needed. Tex casts squares, heats them over a low flame and plonks them down on a warm mirror. From other sources I have seen this seems to be the pro way even on very large mirrors. He mentions that Ritchey ( as in Chretien telescope) painted hot bees wax on poor quality pitch. I've used Howard's method. Let it cool somewhat, pour in a spiral on the warm mirror, cover everything with rouge and rub around to achieve full contact, then cut slots with a wet saw when it's fully set. Messy so next time I'm going to cast strips, cut squares and stick them down tex style. Bet it sticks to what ever I cast it on. Whoops One thing to add. For sub diameter Gordon ??? again seems to have decent method. He uses an annular ring of wood of some sort to act a former with what ever is used to form the sides running down onto the mirror. Looks like he uses the thin plastic film (fablon) that can be stuck to shelves etc in the kitchen to stop the plaster from sticking to the mirror. John - tiletool.pdf
  11. I wish I had one Damian but I wont spend what they usually cost plus some only have 1/2" travel. Some people who test thin mirrors test vertically to avoid mirror flex problems. That makes an easy build x-y jig difficult but it's possible to manage without the 2nd axis testing in the normal fashion. Many do. It's also ideal for that caustic test I mentioned but if your having the mirror checked some other way that's over the top. That mic spindle on mine has a 1/10000" vernier on it and that has real meaning on that test. The only woolly part is the actual mirror rad. I tried push pull and a rule as a scale for rough figuring but found I prefer the mic spindle. That could even be a chopped up mic. Also pin pricks on paper measured afterwards. Stub mentioned the bolt. Knobs can be found with the graduations on them. There are cheaper digital machine scales about. ArcEuro maybe. There are all sorts of options really. Once some sort of stage is made it's pretty easy to try a number of things out including stationary source, slits, different ways of moving and measuring etc all part of the "fun" - ? if that's the right thing to call it. My feeling is that it is worth spending some time on the stage what ever type it is. Sometimes it's just a straight piece of wood with the moving part pressed against it. I simply thought that a piece of rod and a couple of brass V's was just as easy and it would definitely slide well. The tilt bolt could also run on brass or glass even. The main problem with mine was the knife tilt bolt - the thread is too coarse so I should have added a big knob or use something finer and probably a knob as well. Texereau gets round that by making the platform a lot wider and uses a finer bolt than I did. I'm might just soak the rusty bits in a mix of 25% molasses and water for a few days to get rid of the rust and more or less use the stage as it is. It's time for an upgrade on it anyway to try a big moving source but I'll probably still make another slit. John -
  12. I think Howard's book is on the archive. I did try drilling holes in callipers but found I needed a carbide drill and went of the idea. Lots of people like the idea of using a dti. Some move by hand some move mechanically. A lot depends on what is around. A spare focuser can be used as has been shown. Used micrometer spindles can be cheap. DTI's too. The metal parts I used don't cost much either and where metal is best used really easy to make. John -
  13. Thanks for mentioning off setting the knife. I've been webbed. That tester crops up all over the place and is what I intend to make but it can't work on axis, the mirror will return the image of the source right back above it and the knife wont cut it at all so it needs offsetting to cause the return beam to hit the knife. Leaves me wondering if it would be better to leave the led unobstructed and just have the knife central too it and above it. Minimising the offset does improve the accuracy. This is what I have used. It was in a loft room when we had the roof done. Covered in dust and got damp. It a fixed source version but the idea for the sliding table can be used with any type. It's easy to make. MDF etc would do but the sliding table needs some weight added even in aluminium. The tilt screws just run on a bit of metal plate under them. I added my own thoughts. As I used a slit I tilted the source rather than having a separate piece that can be tilted mounted on the knife. I also added a zero facility for the mic spindle - set mic to zero, slide table about to pick up the centre and then slide the bar up against the mic spindle and lock it in position. I didn't add a spring to pull back the table onto the mic, just used finger pressure on the table. Most people don't mount the bar the table runs on in blocks and just screw it down onto a board of some sort. In that case the mic spindle can be arranged to press on something on top of the table. It is best to make the V's that rest on the bar out of brass. It slides very smoothly. The slide going across has the V's made of aluminium and seemed ok. The other gubins that moves the knife across is for a version of the caustic test. For that a wire is mounted on the knife and I just held a 10x eye cup type loupe in my eye focused on the wire by moving my head. This test does need a slit. I've lost that but it was made as per Texereau. His way does work. No shadow reading at all with this test as it uses diffraction. I used it as a final check. The all thread - for playing with a Dall null test. I might play with that again and get a better lens for doing it. Abuse of the Ross tests looks easier though. Just move the lens and tester around until the mirror nulls flat and look for ripple and small zonal errors etc. John -
  14. That's odd really. The camera should work just the same way as the telescope does in the photo of the test rig I pinched of the web. It doesn't care that the frame of the tester and the knife edge is blocking the view. The knife edge has to for the test to work. I'd guess that getting the camera on axis and square to it would be a bit tricky though and some zoom lenses can do odd things used on systems like this. There was a mention of the Ross test. This can tempt people but on faster and larger mirrors the measurement accuracy needed form tester to lens and lens to mirror to make it worth while is very extreme. John -
  15. Trying to help again. The problems with Foucault testing are likely to be down to the tester used. There are lots of nvg info about however there is a sensible one shown on this page especially for some one with an x-y stage but that can be made up in all sorts of ways. http://www.stathis-firstlight.de/atm/foucault_tester.htm?sa=X&ved=0CDwQ9QEwEzgoahUKEwiw0eCp_frGAhUOKtsKHa0IBFg This one The telescope is optional. Note the comment on the web page about sanding the led or using a diffuser. I would add a flat ended led, If it's round ended just sand it flat. Some sort of plastic diffuser as mentioned isn't a bad idea anyway. What he doesn't mention is alignment. He's done many mirrors so will have some sort of bench set up so that the mirror can be placed on a stand and align well with the mirror. What I would do is fix the knife edge more securely and have a piece of cardboard with 2 holes in it. One to let the light through and the other smaller one to indicate where the knife edge is. If the centre of the led was say 10mm down from the top of the mounting this small hole should be 10mm above it. Best make that distance as small as possible but the return image or the led must clear the woodwork so the distance depends on the size of the led in some respects. If the cardboard is place in front of the tester it will catch the return image of the led from the mirror. At the ROC the image of the led and knife edge will be sharp and the set up can be manoeuvred around to place the image on the knife edge hole. Fiddle with the knobs and it should be fairly easy to null a near sphere now but the adjustments will be very sensitive. That can be helped by mounting say 50mm dia disks some how on the end of micrometer spindles. When a mask is used for measurements the shadows have to move equi spaced around the centre of the mirror as the knife is moved back and forth. For initial rough measurement the shadows can be evened up side to side with the other axis but that needs to remain fixed for final figuring. Old time testing using a fixed source will already have taken care of that and people without an x-y stand can use this sort of tester in the same way to set it up - alter it's angle until the shadows move evenly. This can also be done roughly with the cardboard in front of the tester as the return image will move from side to side. To use Ronchi just remove the knife edge and clip the screen on but set up in the same way. Setting up is a pain. A simple solution for most people if they haven't got a bench that is long enough and things can't be left in place, lack packing to raise the tester up etc might be a shelf on a wall. Maybe 2 one for the mirror and another for the tester so that mirror sizes and tester height can be at least part adjusted. I have seen a professional who made lots of mirrors daily work on a shelf. It seems to work well but he was mostly making 10in F6.2 mirrors so his tester was built for that size. Me I had a bench and a bookcase which made up the lengths The bookcase was taller than the bench so height adjustment wasn't a problem, just adjusted the height of the mirror. Once set up the mirror could then be just dropped in place, the tester checked for axial alignment and measurements taken. I haven't got that any more so am looking for a wall for shelves that wont upset the wife. One warning about bright sources. After a long session I found the pupil in my right eye was a tiny pin [removed word] and the other a lot larger. It took a while to settle down. The telescope on the tester in the link - just used to make the mirror look bigger. A converse view is to use one backwards to improve the view of the shadows. Pass never tried either. Whoops - People need to be able to get their eye behind the tester. Might not be easy as shown. Remove one side / turn it around etc but turning it around would need some slots in the cardboard to clear the frame or what ever is used for that. The knife will probably need moving forwards by 1/2 the thickness of the cardboard if the return image has been sharply focused. John -
  16. There is a difference when you are doing that Nigel - you can see from the testing you do where glass needs removing as per the Palomar mirror. There is also the brave way - forget the sphere and go for the parabola. Trouble is when surfaces start going up and down probably even with slopes in different directions it can be very difficult to see what needs to be done. One problem Damian has is that he can't actually measure how much glass needs to be removed. He could try taking measurements and sticking them into some software and setting the conic to zero. I would say exactly the same as before - if a small lap is used or just one area of the mirror is worked because of the size of the lap do spend some time blending for each session. The blending in this case will still lower the hill. Same with the earlier work further out from the centre - do it often enough and no hill. As to back stepping - just thought that it had to mentioned and would be surprised if others were not thinking the same thing. It's much nicer to have some measurements to base that decision on though - if they can be taken well enough and that can be tough. John -
  17. Blanks are never junked unless they are dropped etc and broken. Seems I am rude because I pointed out that Damian has spent more than enough time to finish this mirror. Comment made because I am annoyed that he is having such a bad experience. It could put others off having a go. Anyway as my comments are not appreciated and making another wont do any harm there comes a stage in mirror making where things have gone wrong and the only and quickest answer is to back step - even when forming the parabola. In that case it's working it back to a sphere again or at least part way. Turned edges can mean going back to fine grinding as there may be too much glass to remove with a lap. Other defects can mean the same thing even a hole in the middle as that means a huge area of glass has to be lowered just like with a seriously turned edge. Actually going back is beneficial as people then know that what they did at the next stage didn't work out and can hopefully react accordingly. Personally I think Damian has 2 problems. Pitch hardness and lap size. The lap is about the right size for forming a parabola on a mirror of this size. Good for lowering the centre region and working down towards the edge. From the description it's also pretty light if made from wood. If I remember correctly John advocates concrete - I'd seal it with diluted water proof pva or diluted exterior grade wood glue - same stuff. Pressing down on a lap to make it work like a heavier one can't be a good idea as it will be impossible to apply an even pressure. Personally I feel it's best to work up toward elbow height maybe above, easier to just push and muscles will work to their best advantage. I have this aquarium I have used standing in end with a board on it - not a good idea really. I've never measured it. It doesn't move around up to 10in with a bucket of water in it. Pitch hardness is a toughy. I've tried the Texereau way - small piece in the mouth, let it warm up and see if you can chew it and still had sleeks at times. I'd guess this has something to do with him using pine rather than coal tar pitch and also him having very high skill levels. He does show coating the lap with beeswax - I couldn't find any at the time that happened. Another more recent view is pressing hard with the thumb nail. If an impression forms in under say 1/2 min or some level approaching that forget it as it's too soft. Pitch hardness will alter the amount of drag the lap has. Some people pre polish - charge the lap with one of the abrasives rather than rouge. Might correct things more quickly but I have no idea if the lap is any good for anything else afterwards. Too small a lap and it will probably just cause problems more quickly. For the next one I want to make a machine. Main reason is that I want to put it on a shelf in my workshop when it's running - bit difficult to walk round a shelf. No stroke arm as I can do that for the brief time it's needed compared with the time to do the rest. RPM has been tricky to sort out but from looking around I'm settling on about 15rpm, no more that 20. That's based on pro machines - usually intended for smaller work. Adding a stroke arm makes things more tricky even to the point of deciding on a sensible rate that wont produce patterns in the work. The Waite's video's on youtube are a good source of info on how to use a fixed post machine. Could add a manual stroke arm anyway easily. Maybe Damian might realise that any comments I make are largely down to concern about the experience he is having and nothing else. As I see it there is a lot of junk about on the web concerning making precision mirrors and people can easily be miss lead and I feel that is where Texereau comes in. Little has changed really other than the materials used. It's a pity he didn't do another on bigger faster mirrors because that does need some changes. He does give sufficient info to work out how much glass needs to be removed to figure if I remember correctly - if that gets up to 0.001in it will be a lot of work. I don't think Damian is any where near that though and probably in real terms due to the size of a lap trying to figure a sphere. Something up to 15,16in should be a lot better but will still take a while to polish out to the edge and too much overhang will probably turn the edge. The idea of the sphere forming is all down to random motion as a sphere is the only shape that can cope with that and remain in contact. Anyway Damian probably has muscles like Popeye now. Don't give up. You've learnt a lot all ready. I just feel some one should have posted the above some time ago. Some will have been there. I have been lucky. John -
  18. I think you need a good spell with the larger lap now Damian to minimise the possibility of a hole, centre over centre and zero overhang, probably even stopping a bit short of that. This will also reduce any hill in the centre and a 10in lap on a mirror of this size is a small lap really. Pass on what might happen if the pitch is too soft. I feel I would be thinking and maybe struggling with a 15 or 16in lap on a mirror of this size. Even bigger if I could manage it. One thing I am curious about is if you are using cold tar or pine pitch. John -
  19. A pointless comment really RAC from Nick. No doubt Mr Dob doesn't both collimating the scope before he does it so then it is simpler but still not a reliable way of checking a mirrors figure. Any form of testing needs some form of collimation. I use Mr Dob's method to check any scope I buy - high magnification so that I can see diffraction rings. John -
  20. If you do use Faucault Damian not squaring up the tester to the mirror can cause some confusion. Vertical comes first. If the source was dead on axis the mirror would throw an image exactly back onto it. Pity a knife can't be used then. You seem to using a vertical knife 1/2 over source and below it set up so the source needed to be slightly above centre and the reflection down below it onto the knife. The return beam can be caught on a piece of card and this sorted out. You should try and keep the separation from the source as small as you can. I'd get them vertically aligned as well. Then comes axial alignment. You may be able to do that at least roughly with the same set up. Perhaps a more usual method is to watch to see that shadows in a mask or between pins change equally side to side across the mirror as the stage goes back and forth. Some people use an x-y stage so that at least initially until things get really close they can align it every time a separate reading is taken. That alignment is easier with a stationary source. Actually I wonder just how much this area can give odd effects with a ronchi screen. Uncle Mel reckons that radial measurements can be taken by looking for a constant line width. So say you get one size at the centre and then move the tester it's now on the radius of the area of the mirror that has the same line width. I don't think anyone really reckons that this is a good way of measuring a mirror but when I look at the shots posted I wonder about some of the changes. Pass really as I just knocked one up and played with it but only on a sphere and I found other methods were a lot better but in some ways tied up to a fixed source tester. Not the knife exactly on centre though but I feel that may be hard to call unless it's pretty close to a sphere and it takes an incredibly fine adjustment even then with a slit anyway, 40um slit means the image is that size too so the knife has to be put 1/2 way through it or there abouts. As mentioned earlier - there doesn't seem to be much about on lining up a moving source tester. I maybe initially people could locate the tester on a pin at the back and swing it slightly if needed as readings are taken. Accuracy. People have made mirrors of all shapes and sizes using Faucault and other checks have found them to be equally as good. Even some famous ones later finally checked with a Hartmann test. One of the major catches as I see it is R. That needs measuring well enough to make the error insignificant. Easy to check with say fringexp. Bigger mirrors in some respects help and faster F ratio's make things worse. Another which might be significant on some mirrors is the formulae used - it's part of a series. Another term may be needed on some. There are some ideas about on maximum separation between knife and source as well. Pass, just seen and forgotten. John -
  21. 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 -
  22. 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. -
  23. 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 -
  24. 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 -
  25. 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 -
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