The 22" mapstar mirror

[Ajohn]

I am in the process of planning a fixed post grinding/ polishing machine. First I am going to try to make a powered turntable and see how I go from there. I have one pulley so far and need another one or two, I am probably going to use the motor from my drill press. I need to calculate the sizes of the pulleys to get the right rpm. This will all take a while especially the construction!

 But in any case I first need to grind both sides of the mirror flat, as it has ridges, and this will take a lot of work and quite a long time. I will be doing that by hand.

David

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

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[mapstar]

I am in the process of planning a fixed post grinding/ polishing machine. First I am going to try to make a powered turntable and see how I go from there. I have one pulley so far and need another one or two, I am probably going to use the motor from my drill press. I need to calculate the sizes of the pulleys to get the right rpm. This will all take a while especially the construction!

But in any case I first need to grind both sides of the mirror flat, as it has ridges, and this will take a lot of work and quite a long time. I will be doing that by hand.

David

Thanks for replying David

It will be nice to see your progress on the table build and mirror so I hope you will be documenting it on here if not whilst your doing it then later when you've completed it.

On the right RPM, fitting an inverter would give you infinitely variable speed control and you wouldn't need to be accurate with pulleys.

Grinding the faces flat does take a while but I'm sure if I can do it anyone can.

Damian

[JamesF]

I am in the process of planning a fixed post grinding/ polishing machine. First I am going to try to make a powered turntable and see how I go from there. I have one pulley so far and need another one or two, I am probably going to use the motor from my drill press. I need to calculate the sizes of the pulleys to get the right rpm. This will all take a while especially the construction!

 But in any case I first need to grind both sides of the mirror flat, as it has ridges, and this will take a lot of work and quite a long time. I will be doing that by hand.

David

Please do start a thread for the grinding machine.  I'd love to see how you get on.

James

[Ajohn]

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

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Edited April 30, 2015 by Ajohn

[mapstar]

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

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Looks like someone has 25years of experience in the industry

[Ajohn]

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

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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.

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Edited May 1, 2015 by Ajohn

[Physopto]

A suggestion only!

If you are going to build a variable speed motorized table have a look at the way an old Myford lathe or a large pillar drill press works. Just use pulleys and a slow 4 pole motor ( 1500 rpm). Easy to change speeds and reliable. Just over size the motor and any bearings used if possible. MDF can always be glued and screwed together to increase thickness to anything wanted. But it is not the best product for use near water regardless of what protection used.

Derek

[mapstar]

Nice short answer there Derek thank you proving there is more than one way to crack a nut and doesn't need a huge irrelevant explanation to get there.

[RLWP]

A suggestion only!

If you are going to build a variable speed motorized table have a look at the way an old Myford lathe or a large pillar drill press works. Just use pulleys and a slow 4 pole motor ( 1500 rpm). Easy to change speeds and reliable. Just over size the motor and any bearings used if possible. MDF can always be glued and screwed together to increase thickness to anything wanted. But it is not the best product for use near water regardless of what protection used.

Derek

A further DIY tip - you can cut wooden discs either by hand or with a router to make pulleys. They won't be turning quickly for that kind of application, you can make quite a few from a bit of plank in several diameters

I can do cheapskate when required...

Richard

[Ajohn]

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

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[dark star]

My turntable is wooden, varnished and covered with plastic used for kitchen work tops, how well this will work I don't know. I am determined to make a powered turn table at some point, whether I will try to make an actual grinding machine just yet I am not sure, I will see how hard the turn table is to construct!

I am planning to use my 750 watt drill press as the power, it has pulleys for changing speeds, I haven't figured out if I should try to connect one of these pulleys to another pulley to get the correct rpm or try to connect a pulley direct to the spindle (if that is the right word) where the chuck goes. The lowest rpm of the drill press is 210 and I want to get it down to around 40.

I did buy a 4 pole electric motor from Machine Mart, but was advised on another forum not to connect it up to the mains until I have a greater understanding of electricity! Which is probably very good advice. As it will probably take me a few months to understand the wiring, DOL switches and No Volt switches (if I have remembered the terminogy correctly) for the motor it will hopefully be quicker to use the drill press which I can just plug in to the mains safely. I hope to eventually develope an understanding of electric circuits etc. so I can safely use the 4 pole motor.

I will try to plan the pulleys and construction of a way of holding/ supporting the part of the drill press I want to use under the grinding table this weekend. I need belts and some kind of rod to fit to the underside of the turn table. Also start grinding the mirror flat by hand.

If I succeed in getting the powered turntable working I will post some pictures, it will take me a few weeks at least to get to that stage.

I will start a new thread if I make any progress with the turntable.

David

Edited May 1, 2015 by dark star

[Beulah]

My turntable is wooden, varnished and covered with plastic used for kitchen work tops, how well this will work I don't know. I am determined to make a powered turn table at some point, whether I will try to make an actual grinding machine just yet I am not sure, I will see how hard the turn table is to construct!

I am planning to use my 750 watt drill press as the power, it has pulleys for changing speeds, I haven't figured out if I should try to connect one of these pulleys to another pulley to get the correct rpm or try to connect a pulley direct to the spindle (if that is the right word) where the chuck goes. The lowest rpm of the drill press is 210 and I want to get it down to around 40.

I did buy a 4 pole electric motor from Machine Mart, but was advised on another forum not to connect it up to the mains until I have a greater understanding of electricity! Which is probably very good advice. As it will probably take me a few months to understand the wiring, DOL switches and No Volt switches (if I have remembered the terminogy correctly) for the motor it will hopefully be quicker to use the drill press which I can just plug in to the mains safely. I hope to eventually develope an understanding of electric circuits etc. so I can safely use the 4 pole motor.

I will try to plan the pulleys and construction of a way of holding/ supporting the part of the drill press I want to use under the grinding table this weekend. I need belts and some kind of rod to fit to the underside of the turn table. Also start grinding the mirror flat by hand.

If I succeed in getting the powered turntable working I will post some pictures, it will take me a few weeks at least to get to that stage.

I will start a new thread if I make any progress with the turntable.

David

If you have time, starting a thread on this and your progress would be great. 

[Soupy]

I agree with Beulah, this is Damian's thread for his mirror grind and we are here to see how he gets on. I am sure I rotating tables are very interesting but if you want to discuss rotating tables please start a new thread and we can follow there if we are interested.

Cheers for the brew today Damain, the mirror is looking amazing!

[mapstar]

Thanks Guys

I suppose the table comments just ran away tas I did ask out of interest if it was going to be powered.

It's the 25 years in industry dealing with motors and drives from 0.25kw up to 4500kw that make's me curious I guess.

Anyway back to the mirror making

Damian

[Beulah]

I agree with Beulah, this is Damian's thread for his mirror grind and we are here to see how he gets on. I am sure I rotating tables are very interesting but if you want to discuss rotating tables please start a new thread and we can follow there if we are interested.

Oops, I didn't mean in that way, I apologise to both David (Dark Star) and soupy!

I simply want to encourage more people to share their astronomy related stuff, especially in the area of ATM as it's interesting.

It's just that I have an interest in mirror making as I came very close to grinding a 12" blank myself. I was looking into building a mechanised grinding machine...

[RLWP]

Anyway back to the mirror making

Damian

[mapstar]

He he

Excellent, best post so far Richard

As the sketch says I certainly will tomorrow

Damian

[dark star]

Sorry if I rambled a bit (well rather a lot) off topic, I tend to get carried away! I will stick to questions about mirror making and testing here (of which I will have lots!) and start another thread about turntables etc.

David

[mapstar]

Sorry if I rambled a bit (well rather a lot) off topic, I tend to get carried away! I will stick to questions about mirror making and testing here (of which I will have lots!) and start another thread about turntables etc.

David

No problem David.

It didn't bother me to be fair, I try my best to encourage people to have a go at anything (some here don't have that same attitude though)

I shall try to pick up your thread when you put it on the forum and read it with interest

Damian

[Soupy]

Think we all will David.

How's the TDE going now Damian, more progress I hope

[mapstar]

Think we all will David.

How's the TDE going now Damian, more progress I hope

Hi all

I have spent the last couple of days with a little more edge work then 4 hours through the centre which has cleaned up quite a lot.

TDE is still present so I will continue tomorrow with the centre through strokes to see how things progress.

I'll up date with some images tomorrow after some more work

Onwards and upwards

Damian

[Soupy]

Look forward to seeing them Damian

[mapstar]

Hi all,

Well a busy couple of days have passed although I've not managed much on the mirror since the weekend when I did about 6 hours on it. I think I have now managed to get through at least a quarter of my music collection up to now although it's quite therapeutic so no big problem

So onto the updated images and after a bit more edge work I went onto straight through the centre with little to no overhang. I've lined up a series of images so yet again everyone can see how the surface changes over time. The centre is a hill and get's smaller as the image's progress.

Images left are inside the centre of curvature and the right outside the centre of curvature

Moving through from approx 5mm inside centre of curvature to 20mm outside is quite interesting to see how the banding changes 

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

[mapstar]

Still a turned edge and with a week's holiday from tomorrow It will now have to wait till I return for more and will be raring to continue 

Damian 

[Beulah]

Looks like progress is being made.