Gear Hobber : Design ideas please …

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Pace on 07/08/2018 22:24:59:

Hi Michael
Back in issue MEW 193 i wrote the article Gear hobbing in the mill,the system designed by Richard Bartlett (Compucut) …

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.

.

That's another very helpful response, thank you John

I'm doing rather well on this thread !

My hob was just a one-off purchase from a German supplier on ebay … perhaps rather whimsical, given that I don't yet have a machine; but whilst I was browsing for ordinary MOD_0.8 or 32DP cutters, it cried 'Buy Me'

MichaelG.

[John HaineParticipant @johnhaine32865]

I can't agree that a PLL is tricky in this case. There will only be a limited range of spindle speeds – possibly only one, and a fixed division ratio between the VCO and the spindle pulse – following JS's practice perhaps 4000, or 4096 to be convenient in binary. The 4046 has an edge triggered frequency/phase detector which makes locking pretty reliable provided the VCO frequency is well centred. It may jitter rather, but there is also a pretty large division ratio between the VCO and the spindle carrying the blank so the phase errors will be reduced by this ratio. It's true that one will not be able to stop the milling spindle and restart it in sync but I'm not really sure that's a requirement – anyway it may be possible to add some such synch as a refinement. The programmable divider is simple, the MEW article had a circuit for one, or it comes from the application notes for the ICs – 4000 series CMOS has programmable dividers, in fact the 4059 chip will divide by any number between 3 and 15999 but I think has to be programmed in binary which is a bit tricky.

Yes the whole thing is possible in software if you are skilled at real-time programming and using a processor with high enough clock speed, but such skills are rarer than building fairly simple logic circuitry using 4000 series devices on strip board.

[Neil WyattModerator @neilwyatt]

Posted by John Haine on 08/08/2018 09:37:30:

Yes the whole thing is possible in software if you are skilled at real-time programming and using a processor with high enough clock speed, but such skills are rarer than building fairly simple logic circuitry using 4000 series devices on strip board.

I'm not sure that's true these days. Show a 'maker' a quad NAND they will (a) ask where the SPI pins are and (b) if they do know what it is, they will say that you can emulate one on a microprocessor chip at a fraction of the price…

[I.M. OUTAHEREParticipant @i-m-outahere]

Wouldn't just be a matter of dividing the spindle speed by the number of teeth to be cut on the gear blank ? Isn't a hob is basically a thread with cutting teeth gashed into it ? If so one turn advances what ever it is engaged with by one pitch or in the case of a hob one tooth , if the spindle is rotating at 250 rpm and we are cutting a 25 tooth gear the gear blank would be rotating at 10 rpm to match the hob .

What if the spindle was driven by a stepper motor & controller that is driven by a square wave generator ( crystal controlled ? ) couldn't the gear blank drive be exactly the same set up and use the same square wave signal that is controlling the spindle but fed through a microcontroller to do the frequency division then fed to the gear blank drive stepper controller and motor ? Or would the gear blank drive be better served if its square wave drive was generated by the encoder on the hob spindle ? That way it would sort of be able to sense the speed of the spindle and any variation because of the load when cutting .

Just some thoughts !

[John HaineParticipant @johnhaine32865]

You could drive the spindle with a stepper as you say, but normally the spindle is rotating rather fast for a stepper to cope with and give reasonable torque/power to drive the cutter.

You mention having an encoder on the hob spindle – that's just what a number of the schemes described above do. The problem is that you need a very fine encoder to make the numbers work, so JS for example used one with effectively 4000 lines – 4000 pulses per hob rotation. Other schemes described here try to avoid this by having for example 1 pulse per rev and electronically multiplying this up using a phase locked loop, implemented either in hardware or software, then dividing down to drive the blank.

[Michael GilliganParticipant @michaelgilligan61133]

Please feel free to continue debating the electronic options

… Its.s all good, interesting , educational stuff.

I should just mention though, that Joe's solution appears to fit my requirements very well.

MichaelG.

[Mark RandParticipant @markrand96270]

Quite a few years ago, I wrote the programs for some racing motorcycle (and car) combined rev-indicator lights and rev limiters. With a 4MHz PIC controller, I could accurately measure frequencies up to 200kHz (had calibrated correction factors to allow for the time taken to service interrupt routined etc). We even made a modified version to indicate the speed of steam turbines on barring gear down to 10 rpm, with an RS232 interface!

Anyhow:- I've got some ideas along the divider route to work as a modification to the recent lathe based gear hobbing setup described in MEW. I've got all the ingredients in various caches around the house and shed, just need to find the time to do it!

[Another JohnSParticipant @anotherjohns]

Andy Pugh is one of the LinuxCNC'ers who cuts gears with a hob – one of his videos is linked to above.

He's very active on LinuxCNC (old name was EMC2) and is great at answering questions. Both on the mailing list, and on the online forum.

A link to a wiki entry:

**LINK**

Another youtube video:

**LINK**

The software's all there, and it's proven, if that is the end goal you want.

[Martin ConnellyParticipant @martinconnelly55370]

I knew I had seen something about this on YouTube. It is a series of videos titled "Home Hobby Hobber" by F Cleff.

Martin C

[John HaineParticipant @johnhaine32865]

Another reason to use LinuxCNC! It looks like he is using a gear as a spindle encoder with 3 sensors offset presumably each by 1/3 tooth and x-ored to get a x3 pulse rate as well – but impressive how LinuxCNC is effectively implementing a gearbox between the hob and the blank.

[Joseph Noci 1Participant @josephnoci1]

As some may know, I have made many add-ons for my machines. ELS on both my lathes, my NC Shaper, my 'computer' controlled Spot welder, My CNC Mill, etc…And That I use Mach-3 for the CNC mill, the CNC Router, the CNC PCB Engraver, the CNC Foam-cutter, etc…..

It would seem that I am therefore not that dense…

But I give up with LinuxCNC, in fact with Linux in all its glory, at that kind of level. I am pretty computer literate, do well in C and assembler, but you have to be born with that special (mis)gene to work Linux. Most of the time getting stuff to work and debugging processes is spent getting to grips with linux's obscure terminology and syntax and has one greps-ing all over the place. And in the end, it is but a tool, would it only realise that and behave accordingly…

Anyway, the intention is not to hijack this thread with a linux group tussle – I just don't believe Michael needs the consternation of setup for the simple implementation he is after!

Joe

[Another JohnSParticipant @anotherjohns]

Hi Joe;

Thank goodness everyone has different strengths – I enjoy (and, am envious of!) your strengths that you show here.

One of my strengths is the ability to develop software that runs internally on graphics chips, which means that I have to be able to easily write programs on Linux, Windows, Android, MacOS and IOS.

Of those, I find Windows obtuse, confusing, and hard to do anything on, compared to the others!

I think we should focus more on the ease of the end results rather on the specifics of getting there – there can be many paths to a destination.

Regards – John.

[Tim StevensParticipant @timstevens64731]

It seems to me that in essence, what you need is a drive linking the rotation of the workpiece to the rotation of the cutter. So, a different solution would be to fit a tooth-belt sprocket on the cutter shaft and a (much larger) one on the workpiece spindle, choosing the numbers of teeth to suit the gearing, of course. Then work out a system of pulleys so that the drive is tight, bearing in mind the wonderful flexibility of these belts, and the need for an adjustment of length, as well as the relative movement of cutter and workpiece.

In other words, a meccano-type drive which I can understand rather than all this smoke and mirrors electric stuff.

Ho hum

Tim

[JasonBModerator @jasonb]

Tim, isn't that essentially what a Jacobs machine does which is what Michael said he did not want, the electronic solution avoids the need for pullies or gears in the case of the Jacobs.

A couple of presses of a button and you have a setup for whatever tooth count you want rather than having to work out a gear train or pulley ratio then obtain suitable gears or pullies

[Mark RandParticipant @markrand96270]

Only problem with that approach is similar to the problem with a conventional hobber:- you need a supply of sprockets/gears with the right number of teeth or which can be arranged to have the right ratios to generate all the gears that you want to make, including any prime numbers that might crop up. The electron shuffling approach allows you to get those ratios via the magic of arithmetic.

[BazyleParticipant @bazyle]

We have discussed encoders to provide pulses but, especially in a stand alone machine, at what point doe it become a better idea to drive the hob spindle from a stepper as well? This also enables the system to ramp up to speed in a controlled way which may help.
What direction are the forces in both mechanical and electronic versions. As we know you can 'free hob' with the hob running a freewheeling blank does this mean in a Jacobs or electronic machine the blank is being pulled round and the gears/stepper is acting as a brake to keep it in step, rather than forcing it round?
Any ideas about detecting missing steps – not the electronics failing to command the step but the motor stalling and not making the move due to power/friction problems?

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Alexander Stewart on 09/08/2018 19:55:04:.

…

Of those, I find Windows obtuse, confusing, and hard to do anything on, compared to the others!

I think we should focus more on the ease of the end results rather on the specifics of getting there – there can be many paths to a destination.

Regards – John.

Agree. But Joe can't possibly believe Windows is an easy platform to program on can he? He must be comparing Linux with something else entirely, perhaps an embedded environment that has no operating system at all. Programming hardware is much easier when you don't have to share the computer with other users and processes.

Dave

[I.M. OUTAHEREParticipant @i-m-outahere]

Another thought i had ( yes it is true that you only need two brain cells to think! ) is to use a single pulse generator feeding two identical steppers so in theory they should be rotating at the same speed . One drives the hob the other drives the gear blank but the gear blank mandrel is not directly driven like the hob spindle is .

The gear blank mandrel is driven by a worm wheel that is the same diameter and pitch as the hob and this is driving a gear which is the same tooth count and diameter as the gear to be cut . This should give the correct drive ratio required to cut the gear and the dummy gear and worm wheel would be drawn up in cad then 3D printed , once the hob has cut deep enough to free hob the drive system on the gear mandrel could be disengaged if needed .

Once again just a thought !

[Mark RandParticipant @markrand96270]

Posted by Bazyle on 09/08/2018 20:59:10:

We have discussed encoders to provide pulses but, especially in a stand alone machine, at what point doe it become a better idea to drive the hob spindle from a stepper as well? This also enables the system to ramp up to speed in a controlled way which may help.
What direction are the forces in both mechanical and electronic versions. As we know you can 'free hob' with the hob running a freewheeling blank does this mean in a Jacobs or electronic machine the blank is being pulled round and the gears/stepper is acting as a brake to keep it in step, rather than forcing it round?
Any ideas about detecting missing steps – not the electronics failing to command the step but the motor stalling and not making the move due to power/friction problems?

If there were going to be missing steps, they would occur on a stepper driving the hob, that has an intermittent load. The gear drive itself should only have friction to deal with, since the gear PD is going to be moving at exactly the same speed that the hob pitch is moving at, so there should be no torque imposed on the blank by the hob. On that basis, it's probably simpler to use a conventional drive and a rotary encoder for the hob and divide that down for a stepper to drive the blank.

Another advantage of the electronic solution is that there are no universal couplings, bevel drives needed, just spur gears (or straight helical gears, once they've been made) to turn the 200 steps/rev of the stepper motor into enough to give smooth rotation of the blank . Also the gears can be whatever changewheels are to hand initially.

 

Again, the reason for the electronics is to avoid the need for any specific collection of gears in order to create the gears you want to make.

Edited By Mark Rand on 09/08/2018 22:34:32

[Clive FosterParticipant @clivefoster55965]

Missed step detection is, as I recall it, relatively easy with the electronic PLL based control system. Encoder on the output feeding a second PLL and count both output and input steps using a simple grey code divider chains. Pulse outputs should be temporally aligned so any miss is obvious. Recovery isn't quite so simple but it can be done.

System I made way back had this capability, including automatic recovery, but for different purposes. I used both halves of a CMOS Dual PLL chip and monitored the delay between the grey code counts to tweak the output of the voltage controlled oscillator driving the stepper so it could track a moderately unstable "axis". Including ramp up to speed from stationary over a somewhat undefined period of a few seconds varying by up to 50%. The major issue being persuading the PLL to operate over a too wide frequency range.

I'm really annoyed that I've lost the circuit because it would have done for a fair few things. Ideal for Michaels job. Simple too as it was built as part of technology demonstrator for a "£100" sensor and control package for terminally guided submunitions.

On the microchip user side I wonder if there is anything useful in this book :- **LINK** . As I recall it Elektor projects had the reputation of being reliable and well thought out from the "doing what it says on the tin" viewpoint.

Clive.

[John PParticipant @johnp77052]

Hi Michael

Have sent you a PM

John

[WalterParticipant @walter]

Hello Michael,

here is a solution that works.

My homemade gear hobber

("https://www.homemodelenginemachinist.com/threads/my-homemade-gear-hobber.30291/&quot

He cuts straight aluminum gears with a mini mill , a homemade hob , uses a Due and no stepper and the software is included in the posts.

I found this by typing "gear hobbing arduino" in google

br

Walter

[Michael GilliganParticipant @michaelgilligan61133]

Many thanks, Walter … I will have a look at that.

Joe Noci was very supportive on the electronics side … but I regret to say that my hardware costs were escalating out of control, and I have shelved the project for the moment.

MichaelG

[Howard LewisParticipant @howardlewis46836]

M choice would have been a Dividing Head, or Rotary Table on a mill, as long as the capital cost for a variety of gear cutters would have been within budget. Having said that a set of eight cutters (probably about £160 ) would cover every tooth count from 12 to a rack.

That way, no worries about the electronics being upset by power surges, stray electric fields, or not being temperature stable.

No wonder I am a mechanical engineer!

Howard

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Howard Lewis on 11/11/2019 14:38:59:

M choice would have been a Dividing Head, or Rotary Table on a mill, as long as the capital cost for a variety of gear cutters would have been within budget. […]

.

I could have done that, Howard … but I fancied a hobbing machine, for its ‘purity’

MichaelG.

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