Michael, see if Paul White may respond – I have done a design for such a device( the electronics part at least..) for a hobber that Paul has – not sure if it is the Jacobs – I used a Nucleo processor module ( STM micro processor) – the Nucleo is an Arduino work-alike, but a lot faster and easier for me to use! Drives a stepper to turn the gear blank, with an encoder on the hob shaft. 7 segment digital display to set things up – tooth count, etc – very simple stuff – Paul managed to get all the bits email-order in quick time.

I would think the mechanics should not be too difficult – basically a rigid mount to take the rotating blank, driven by a geared down stepper, and on a slide to feed the blank into the hob. If you do not need a fully stand alone device, you could make the blank axle and stepper drive a component that fits in place of the tool post on your lathe, and fit an encoder to the lathe spindle. Happy to pass the design on and assist if you wish to try my electronics

Regards

Joe

edit – syntax…

Edited By Joseph Noci 1 on 07/08/2018 16:04:18

Joe

I would also be very interested in details of your design.

Mike

The 'obvious' solution to me is something that senses cutter rotation and synchronises to a stepper motor driven spindle carrying the blank.

I can confirm that the CMOS phase lock loop, CMOS divider chain and digital encoder approach to synchronising two stepper driven spindles is practical and relatively easy to implement. Main gotcha is sorting the frequency response components on the phase lock loop right for stable operation. If possible switched ranges set-up is better than trying to do it all in one range.

Stable operation at better than a tenth of a step error is easily achieved, you will need to gear down. Its arguable whether two steppers and all digital control is better than one stepper slaving to one free running servo motor. I've used both but this was back in the late 1970's when CMOS logic was still the latest, greatest thing. No nice microprocessors then. Pretty sure I ended up with a dual CMOS PLL and a voltage control oscillator or two.

Clive.

Edited By Clive Foster on 07/08/2018 16:58:34

All was detailed by John Stevenson on one of the forums (HSM probably) and on his own website when it operated for the system he used on his milling machine for commercial gear production (for the Myford metric approximation gears). Initially using hard divider chips and later a micro when someone else wrote the software.

In essence a 20:1 dividing head plus 200 step motor requires 4000 steps per revolution. The spindle needs to generate 4000 pulses per rev. then it can be divided down to drive the stepper. As a 4k encoder is rare he used I think an 800 pulse one and a 5:1toothed belt drive speed increaser. Other combinations can be used and digital doubling or using falling/rising edges helps reduce the gearing.
If you have quadrature outputs to give a direction and the circuit can follow it you can reverse and keep in step to start a second pass, otherwise you have to have a zeroing procedure or do single pass.

TTL makes it fairly easy to do the dividing if you can get the chips. A software solution has to do integer division not just slap it into the floating point algorithm in the compiler.
I'm not sure if a digital PLL keeps accurate count as the swallow counter means it is only correct on average.

If you are using the lathe for it then once you have the encoder you can do an electronic leadscrew.
Don't forget the blank axis is not perpendicular to the hob axis.

Bazyle an ordinary PLL doesn't keep a accurate count. It just acts to minimise the error between two pulse strings. So long as the PLL frequency response is fast enough and the mechanical system stability is sufficient it won't loose steps. But if it does drop a step out it can't recover. Actually if it does drop out the system design is wrong in some way or other. Something I had major league issues with as that particular system had to three way sync over a wide speed range.

Digital PLL can be made to track lost steps and institute a recovery mode. I'm sure that microprocessors make it far easier today but I'd probably use a set of offset grey code counters to pick up the slip.

In retrospect if I'd had the sense of a midwife toad I'd have re-worked that system into an electronic leadscrew as it had everything needed short of a set of thumb wheels to set TPI and was cheap. Been kicking myself for years over leaving the design behind when I changed jobs.

Clive.

Thanks Jason!

A hall sensor and a 20MHz timer just won't cut it I fear…

The easiest and most optimal method is to use the DDA algorithm – an extension of Bresenham's line draw method.

It's simple, works for ANY combination of encoder count value ( sensible encoder values, that is – use at least a 1000PPS encoder) and any number of gear teeth on the blank. Very easy to implement in software, but a 16MHz Arduino will not cut it either! Hence my use of the Nucleo with a small STM processor @ 120MHz – The Nucleo module cost around US$15.00. I can give you all that software if you wish Michael.

A PLL implementation 'does' work, but it's a nightmare working out all the diveders, etc, if done in discrete logic. Also, PLL's are a pain to make stable – phase lead and lag can kill it. A digital PLL is better, easier to stabilise, but needs a really fast processor to up the sample rate.

The DDA method is used in all modern CNC threading and gear cutting implementations.

Joe

Edited By Joseph Noci 1 on 07/08/2018 20:33:13

Hi Michael
Back in issue MEW 193 i wrote the article Gear hobbing in the mill,the system designed by Richard Bartlett (Compucut) is not at all similar to the published hobbing unit in Mew 108 as it remains in sync when the spindle is stopped .The article shows the constuction of the spindle ,the offset design keeps the table in close to the column up to the maximum size of gear that can be cut.I don't know if the unit is still available from Richard, info@compucutters.co.uk
The photo shows the the unit as supplied you just dial in the the tooth count.
Although you are only interested in making spur gears the unit as shown is only a little more work to be able to produce helical gears.The unit can be mounted at either end of the milling table making it easy to produce small left and right hand helical gears with high angles as seen in the photo's .
Arc Euro used to sell the hobs but they are no longer available which is a shame as they were very good cutters at a reasonable price.Would be interested to know where you are getting these from.
John