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Gear Hobber : Design ideas please ...

Compact and Simple ...

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Neil Wyatt07/08/2018 19:23:01
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I have to be honest, I wouldn't use a PLL, I'd just time the rotations accurately with a hall sensor and do the rest in software. a 20MHz timer is going to give you plenty of resolution.

Neil

Joseph Noci 107/08/2018 20:24:25
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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

Michael Gilligan07/08/2018 22:00:20
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Posted by XD 351 on 07/08/2018 19:05:33:

https://youtu.be/ZhICrb0Tbn4

Doesn't give any plans but does show his general set up and software he is using .

.

Another thank you yes

MichaelG.

John P07/08/2018 22:24:59
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268 photos

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.
Johngh7.jpggh5.jpggear1.jpg

Michael Gilligan07/08/2018 22:50:34
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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 Haine08/08/2018 09:37:30
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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 Wyatt08/08/2018 12:29:09
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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. OUTAHERE09/08/2018 03:41:23
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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 Haine09/08/2018 07:25:35
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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 Gilligan09/08/2018 07:52:51
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Please feel free to continue debating the electronic options

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

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

MichaelG.

Mark Rand09/08/2018 11:53:03
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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 JohnS09/08/2018 12:32:06
842 forum posts
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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 Connelly09/08/2018 13:22:47
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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 Haine09/08/2018 15:52:10
5563 forum posts
322 photos

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 109/08/2018 17:20:38
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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 JohnS09/08/2018 19:55:04
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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 Stevens09/08/2018 20:00:18
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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

JasonB09/08/2018 20:10:54
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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 Rand09/08/2018 20:12:35
1505 forum posts
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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. smiley

Bazyle09/08/2018 20:59:10
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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?

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