Do you particularly want to hob gears, of just make prime number gears? These days, I would have thought that a stepper driven dividing head, using for example Steve Ward's controller, would be much easier.

http://www.worldofward.com/rotarycontroller/overview/

FYI.

I've looked at the Martin Models castings (on line) for their gear hobber.

But – a Brit (Andy Pugh) has done interesting things gear hobbing with LinuxCNC (old name of software: EMC). Go to Youtube, and look for "Andy Pugh Gear Hobbing" and see what comes up.

(This might be much easier/quicker to build than a mechanical hobber, but I *do* like the Martin Models version…)

John.

I would have thought the simplest and easiest way to make a 127 and others would be to use a dividing head with yet another worm and wheel driving the worm on it as per the G Thomas design of old.

In some ways it's better than differential dividing as the flank errors can be worked out and then averaged rather than getting as close as possible with summed fractions.

Must add though that some people desire things like 127T gears for conversion sets to allow metric to be cut on imperial machines. There is no need for them as approximations can be derived which exceed the accuracy that's likely to be achieved anyway even with it.

Stepper generally have a step accuracy of 5% by the way. I suppose that the usual argument can be made that this is reduced by the ratio of the worm and wheel they drive but I reckon dividing plates are better.

John

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Edited By Ajohnw on 21/02/2016 23:10:04

You are correct Bazyle. No matter people probably understood anyway. I should go to bed rather than looking to see what's going on here.

In terms of change wheels people have generated very precise approximation for metric on imperial but I can't recollect seeing any for imperial on metric.

John

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Gustav

Just be aware that if you do decide to build a gear hobbing machine, the small hobs are not that easy to find these days.

I have a good selection for my own Mikron 112 hobber, but it took a good few years to obtain them and I still look every day on the off chance something I need turns up.

Phil.

.

Have a look at information from companies like Oriental Motor, & remember that the quoted error is [of course] non-cumulative.

Micro-Stepping does not help much, because it the intermediate micro-steps are not true detents … but motors are available with 800 real steps/rev.

MichaelG.

If I have done my sums correctly a 200 step driving a 90T worm, 5% is equal to around +/- 0.001" error per inch of radius.

Cranky old calculator. VPAM drives me up the wall but I'm sure the calc acts up now and again, I even liked reverse polish, even less key strokes.

Loosely speaking 800 steps would 1/4 that if it's still 5%. That's more like it.

John

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The Sherline rotary table…

400 steps per handle revolution. Worm and gear makes 72 turns for one full rotation of the table. So 28,800 steps per 360 degrees. That equates to 80 steps per degree, which in turn equates to 0.75 minutes per step. Final controller, electronic position is accurate to +/- 0.5 step.

"This turns out to be a very small amount because the sine of this angle is only 0.0001091. This will amount to 0.109 mm per 1 meter when compared to a flat plane. This is just 0.0055 mm at the edge of the table"

http://www.sherline.com/8700inst.pdf

Good enough for me!

Proper CNC machine I would hope John. The few I have bothered to look at in detail – older used one seem to use low step count motors. I assume like the far more accurate ones I mentioned. So far I have resisted buying one.

As to sin and pdf. I used tan but the answer is virtually the same anyway. Really a different type of trig should be uses as sin and tan don't equate to arcs.

360/ (90*200) = 0.02 degrees, 5% of that is 0.001 degrees. Sin of that 0.0009999999 tan 0.001000. So both come out at one thou per inch of rad. The assumption that 800 steps would 1/4 this is fairly valid if the error is the same. 400 would halve it. A 72T worm would make it worse.

That sort of error with 200 pulses is a bit debatable on larger gears such as 127T. As Michael pointed out micro stepping is more iffy and steps will be made anyway. Where high accuracy is needed on things like astro telescopes the answer is generally a lot of gearing and closed loop feedback – the star is tracked and the steps corrected as a result. The other option used is to fit an encoder and drive according to that – not step counts. Some do the obvious then and use an ordinary motor to drive instead.

One aspect of cnc is interesting. When I visited chester m/c tools the sales man pointed out that numbers come out of their simple digital scales and he wondered why some one hadn't done a machine that used these.

It could be done with +/- 1u scales too. The real ones of course if they exist.

John

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Edited By Ajohnw on 22/02/2016 20:31:51

Gustav,

You may be pleasantly surprised [as was I] to find that lathes.co.uk has this useful page about the Jacobs hobber.

MichaelG.

For the benefit of those who may be better able to advise Gustav

The page that I linked includes two excellent photos [4 & 5] of the clever device

"To allow gears with numbers of teeth equal to prime numbers to be cut, a useful addition is a differential gear built, very cleverly, into one of the gears in the index train"

This looks very neat … Surely someone will remember, if they have seen its manufacture described.

MichaelG.