Variable speed Motor

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J-G

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My grandson (in his 20's - not a 'child') regularly has 'fanciful' ideas about what he wants to do or make - a past 'project' was a one man submarine !!

He makes jewellery using Silver & gemstones (ie. not plastic or glass) and having just returened from a holiday in South Africa, asked if I could make him a '5" dia gear with a groove' - hardly sufficient information but it did start a train of thought and I evetually discovered that he wants it to make a 'Facet Cutter' - totally impractical of course, but it did start me thinking and researching.

It seems that he now has contacts in SA who can supply rough gemstones at 'silly' money - ~10Rand (~£2.50) per kilo - so he's wanting to look at grinding/polishing them himself rather than buy in finished product.

Commercial Facetting Machines can be bought for about £300 but for serious work you are talking about £3-4k. There is nothing particularly onerous about these machines - though naturally there is some precision needed. I'm quite happy that I have both the skill & machinery to make a suitable machine but the one thing I do lack is knowledge about Variable Speed Electric Motors.

What I need (I think) is a motor running at 0 - 2000(ish) rpm - probably 120w and physically about 100-120mm long. I've done some research but only found low speed or geared AC motors. I could of course use a low speed motor and (say) 10:1 belted pully drive but would appreciate the opinion of much more knowledgable contributors here-abouts who may well have such information at their fingertips or even in their head!!
 
Would a sewing machine motor do the job? I don't recall which make but some are around that wattage
 
Well i would start with a small 4 pole 3 phase induction motor and a vfd and run it up to 80 hz so that will give you the 2000 rpm and at say 20hz will give you 560 rpm or so….do you need to go down to 0 rpm?
 
The first thought is an ac induction motor because they can be entirely sealed and good for dirty dusty environments. 2 pole, common motors are 2800 rpm. 120W is a little under 1/5 of a horsepower. Unfortunately it will be bigger than your dimensions.

More compact will be a brushed universal motor - mains or a low voltage dc version. This will probably need to include a belt reduction or a gearbox to drop it to 2000 rpm. Not so many of these are sealed against dust.

I'd start searching ebay ....
 
Well i would start with a small 4 pole 3 phase induction motor and a vfd and run it up to 80 hz so that will give you the 2000 rpm and at say 20hz will give you 560 rpm or so….do you need to go down to 0 rpm?
Actually no, I don't need to go down below ~1000rpm probably and I can use a belt drive to reduce the raw speed.

I have a VFD on my Myford S7 and that is quite a large unit - which I would want to avoid.
 
Look at motors under the parvalux name for an example of specifically small stuff.
Oddly, I have a Parvalux Motor but it's not variable speed. It runs at 4k and is 38W. The physical size would pass muster though :)
 
The first thought is an ac induction motor because they can be entirely sealed and good for dirty dusty environments. 2 pole, common motors are 2800 rpm. 120W is a little under 1/5 of a horsepower. Unfortunately it will be bigger than your dimensions.

More compact will be a brushed universal motor - mains or a low voltage dc version. This will probably need to include a belt reduction or a gearbox to drop it to 2000 rpm. Not so many of these are sealed against dust.

I'd start searching ebay ....
I'm not too concerned about 'dust' - although grinding gemstones might be thought of as creating dust - the reality is that water is used as a lubricant so it creates little or no air-born dust.

I'll search for "brushed universal motor" - If I find a low-volt DC version, what would I need by way of a converter from 230v AC mains? or should I expect a controller of some kind to be included?
 
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The only stone polishing I can remember is a women who had a couple of modified cement mixers that she used to tumble stones in for weeks before hand polishing but it sounds like you want flats on the stones so no good. Would there not be different speeds for say roughing to say polishing so the ability to alter the speed might be handy so therefore why not a small bench mounted drill press ?
 
The only stone polishing I can remember is a women who had a couple of modified cement mixers that she used to tumble stones in for weeks before hand polishing but it sounds like you want flats on the stones so no good. Would there not be different speeds for say roughing to say polishing so the ability to alter the speed might be handy so therefore why not a small bench mounted drill press ?
You are quite correct in that the 'variable' speed is specifically to accomodate the needs of 'Grinding' or 'Polishing'.

The initial shaping of a rough gemstone benefits from higher speed (say 2k ish) but the final polishing of the facets is better at 1k ish. This is (one of the reasons) why the cheap units are 'toys'.

A drill press wouldn't pass muster for many reasons . . . . Here's an image of a $5500 machine . . . .
Faceting machine.png
 
That makes things clearer, it is like a very precise drill sharpener that has a coolant / lube feed . Are these machines expensive due to technology, low volume sales or just because they can ? What is that readout for, angle or depth because like drills I would imagine that you want all angles the same to get symmetry. Would it be possible to get a cheap unit and then upgrade it yourself as then you have a starting point ?
 
Hear's one reason that I get very confused when looking at Electric Motor details - this eBay item (304999069206) is described as "MY6812 12V 120W High Speed Small Brush Motor W/ Belt Pulley for Electric Scooter" which tells me 12v but a further image has the anotation "Rated voltage range of 3c: below 36V DC" ?????

Would this motor be a decent starting point ? What sort power converter could I use to provide the 'variable' element?

That makes things clearer, it is like a very precise drill sharpener that has a coolant / lube feed . Are these machines expensive due to technology, low volume sales or just because they can ? What is that readout for, angle or depth because like drills I would imagine that you want all angles the same to get symmetry. Would it be possible to get a cheap unit and then upgrade it yourself as then you have a starting point ?
Yes VERY Precise. - good analogy - the idea of getting a cheap unit and adding a quality 'column' is certainly a thought - though I doubt that it's just the lack of precise adjustment that is it's only weakness!

The digital readout is to display the angle of the facet - for a Standard Round 'Brilliant' cut, 45.7° is one of the angles that has to be set. The cheap machines have a quardrant which is stamped in degrees and a simple screw clamping system which I'm sure you will appreciate is hardly likely to be that closely adjustable. I'm not sure whether the $5500 machine has screw adjustable angular setting, but that is certainly in my design.
 
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A cheap variable speed hand drill will have everything you need. Can be battery powered too!
 
A cheap variable speed hand drill will have everything you need. Can be battery powered too!
Are you suggesting the purchase of a new drill and stripping it down for the motor and controller (switch) ?

That may well be an option but I'd be a little concerned about housing the components - and I might need a great deal more assistance/guidance from forum contributors :)
 
Yep, just use it as a source of bits. I would look at just lifting out the guts and with your skills making a holder for the bearings of the motor. If you go for a cheap brushed motor, drill the trigger can be replicated by a on off switched wired in series with a variable resistance potentiometer.
 
Yep, just use it as a source of bits. I would look at just lifting out the guts and with your skills making a holder for the bearings of the motor. If you go for a cheap brushed motor, drill the trigger can be replicated by a on off switched wired in series with a variable resistance potentiometer.
Your idea certainly as me 'thinking' :) - - - I'm quite capable of machining housings for bearings!

After some further research/reading I'm wondering about noise and what type of motor I could drive with the likes of this AC Voltage Regulator - - -

Controller.png

or similar - of which there appear to be a plethora on eBay.

I could be being totally silly of course.
 
That makes things clearer, it is like a very precise drill sharpener that has a coolant / lube feed . Are these machines expensive due to technology, low volume sales or just because they can ?
I didn't answer this question in my previous reply :(

I suspect that it's a combination of of all of those reasons - with 'because they can' being high - and I would not think the technology is that sophisticated - well I think that I could easily overcome any technical problems as a hobby engineer (though I did train as a tool-maker, so do have some advantage!)

The industry is concidered 'high value' mainly due to DeBeers controlling the price of diamonds - not that Steven is contemplating polishing Diamonds!! - even the 'cheap' machines are beyond his means :)
 
Looking at your images, it makes me think of precision lapping machines. In fact that is what it is.

So the parts most critical to quality will be a flat, rigid disk spinning with low vibration.
Rotational forces are probably not too high.

I am reminded of a hifi turntable platter. Heavy and with a good main bearing.

Having the motor decoupled from the platter via a belt drive would help to reduce vibration.
I wonder if 4000 rpm parvalux driving through a 1:4 ratio pulley & belt would work ?

Brush motors with commutators are called "universal" because they can run off both ac and dc. Regardless of their names, they don't care what you use. Only the voltage matters and below their design maximum voltage they will still run but at lower speed and making less power.

There are many cheap modules available on ebay that convert one dc voltage to another. Up (boost converters) or down (buck converters). They are specified by their max voltages and by their power rating.

If you preferred to feed the motor with AC, then a big enough transformer would do.

This expensive lapping machine happened to pop up used on ebay today. Just for your curiousity.

https://www.ebay.co.uk/itm/266695404087
 

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