cutting spur gears on a mill

The spiral path goes along the rotating tap regardless of the angle so there will be a pressure on the blank from the tap whether angled or not.

Martin C

Not sure I see that Andrew. My headstock dividing attachment is a worm gear (which is the same form as a tap without the flutes but also with involute instead of straight flanks) driving the bull wheel on the lathe spindle. It is cranked over at the helix angle of the worm and it certainly drives the spindle. It's a poor mans worm gear as it has only line engagement but it's only called on to do light duty. In the free hobbing of straight spur gears the hob/tap is as you say set over at the helix angle but the gear has to be gashed first in order to be driven. The hob/tap then just modifies the tooth form. Well thats how I see it anyway. I've only ever done it to produce curved knurling wheels.

regards Martin

Please all excuse my lack of knowledge in this subject.

I'll address my question to Andrew J.

If I were to use the same tap to cut a number of gears with varying diameter with the same tap would they mesh or would they not?

If I were to introduce a "worm" of same thread between gears would I be able to build a gear train to increase/decrease speed?

I've never cut a gear in my life so have no idea, would like to know where to start If I wish to!

Nathan.

I should have said "mesh on their outer edges. Sorry.

Second edit repeated myself in para 1.

Edited By Nathan Sharpe on 19/08/2021 22:44:19

Ooops, I think you're correct.

However, that raises a question; if the tap/hob can drive the work why do hobbing machines go to the trouble of actively driving the gear blank?

Andrew

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I think it’s because they want the cutting teeth to be dedicated to their main purpose … thus improving accuracy and reducing wear.

MichaelG.

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Edit: __ having thunk, I went away and found this:

https://www.geartechnology.com/issues/0993x/gt0993.pdf

p46 [p48 of the pdf file] et seq may be of interest.

Edited By Michael Gilligan on 20/08/2021 08:33:39

It, as you say Andrew, can drive the work but you get periodic errors as can only follow the path it's just cut. If it drifts out its going to stay out rather than self correct. Driving the gear blank ensures that the pitch remains constant. The other obvious advantage is with a driven blank you don't need to gash the teeth before you start. That's my appraisal.

regards Martin

See answers embedded above.

Andrew

Driving the blank with the hob only puts pressure on one side of the tooth form. As the hob is advanced from one face of the blank to the other the amount of rotation force varies, only a small amount at the start and end and maximum rotation force between these extremes. That will result in a less than perfect tooth form that is unpredictable. Going from one face to the other at different rates will introduce other variables into the process as there will be additional wear on the pushed face from a long slow cutting action compared to ploughing through aggressively. Driven blanks in a real gear hobbing machine do not have this issue of one sided pressure on the tooth form and form tools in an indexed setup will also be free of it. Using the free hobbing method is only good for one thing and that is light duty worm drives where there is no need to worry too much about the tooth form or contact face but even here it is important to not overdo the cutting because even when the tooth form is completed you can carry on removing material from one side of the teeth until all the teeth are worn away.

Trying to make spur gears with a free hobbing method using a tap will produce teeth with an odd pressure angle so will either wear faster with fat teeth under load or have thinner teeth than standard gears and so be more prone to breaking. Standard pressure angles have been chosen to give the best compromise between the strength and the wear of gear teeth.

Two worm gears cut using the free hobbing method will not mesh unless the blank is moved through its full thickness relative to the cutting tool, pointless even considering it.

Anyone who has ever cut gears will know that there is great satisfaction in it because it is not a quick and easy process however it is done and there are no simple shortcuts.

Martin C

Edited By Martin Connelly on 20/08/2021 08:40:53

Must be careful not to get over-excited.

Andrew

Any Meccano experts available? Can't find my copy but I think Brian's method is described in the Meccano Constructors' Guide (BJ Love), where a ½" BSW tap is used in a Meccano Project to cut Meccano gears.

It's not an accurate system. One fault is a tendency to cut an extra tooth, which is bad news whenever an exact ratio is needed! Just as bad for most other gear applications is the poor tooth form. Ideally gear teeth are shaped 'just so' for efficiency, low wear and low noise. Two particular mathematical curves have this property, of which Involutes are best for power-transfer and Hypocycloid are favoured by clock-makers, who need large low-friction step up ratios. A BSW tap produces neither tooth form, it's a bodge.

Although the tap method "works", it's unpredictable and doesn't make efficient gears. OK if you don't mind the occasional extra-tooth, high-rate of wear, noise, and terrible backlash. (Meccano comes to mind!) I suggest the tap technique is rarely used in practice because alternative methods make much better gears.

Dave

I couldn't agree more with Martin and Andrew

Rod

Is this what you wanted Dave?

I suppose a lot depends on what you want out of the the hobby your hobby. many on here enjoy the building as much and if not more than the finished product, I certainly don't do much with my models once built and test run.

Others use their workshop as a means to an end to support other hobbies so may look at things differently if they just want that gear to get their RC model or full size stationary engine back up and running ready for the next outing. In this case getting the job done is more important than actually doing it.

Then there are those that enjoy making tools and tooling to make a part which can be all absorbing and they never get to finish off that model but they are happy doing what they do.

Edited By JasonB on 20/08/2021 11:23:49

Remember the original proposition

" Primitive but very quick and only suitable for light load low speed apps and probably noisy "

Some mentioned mechano gears - well Im not pompous - it worked for a million kids

I asked if anyone else had actually tried this method rather than generate a whole sheaf of plausible conjecture

Yes I tried the method when I was a brash beginner. it didn't work. I suggest you give it a try and let us all know the results.

Andrew.

Dave (SoD) commented on tooth forms above - as was mentioned earlier the thread form on a tap is in effect a rack and will hob an involute tooth (in an ideal hobbing setup anyway) provided the number of teeth is not too small..

Brian, the basic method has been used a lot for making non-critical worm gears. It will work reasonably well if you gash the blank first to get the right number of teeth, even better if you drive the blank from the cutter arbor with the right ratio like a hobbing machine. Used in a hobbing machine then yes it will work to make gears but the pressure angle will be non-standard and the DP or module, depending on your preference, will be non standard. So only good if you make both gears from the same tap pitch.

What people are trying to work out is whether, if you take the method further to cut a spur gear with parallel teeth, it would also work. If you want to make a gear for a project then usually you need a bit more certainty so would use more of a standard method. I think that's why no one has tried it. Clearly it would be a useful experiment and I urge you to have a go and report back.

I don't think the thread suffers too much from 'plausible conjecture'! Using a tap to cut gears isn't new. I've experimented with it, but got much better results with a Sunderland Rack cutter and Rotary Table:

Neil Wyatt made a lot of gears this way for his Jovilabe Orrery a few years ago.

Unlike a tap, Rack Cutters get the number of teeth right and, with a little care, they cut an accurate involute. Rack cutting and Hobs do take longer!

Boils down to how much the operator needs an accurate gear as opposed a quick approximation. If gashing gears with a tap is 'good enough' for the job in hand, go for it! No-one objects. But I suspect better specified gears are needed almost all the time. If there was an easy quick way of cutting good gears, everyone would be doing it. They don't because the method has significant shortcomings. Tried and found wanting, so the Flat Earth Society wins this one on points!

Dave

Yes indeed I fully recognise its a crude method hence "rogue method" but there maybe some mileage for simple apps

After all there is a whole science you could spend years on perfecting the perfect tooth cos users wanted efficiency and also quiet operation and long life

none of the above would be satisfied by my dirty method

As you can imagine it would vibrate under load

Something tells me this isnt the thread form I am going to get, it doesnt feel right, I am assuming a tpi tap of 16

Perhaps its why members have reported that they didnt get the number of teeth they expected? Hmmmm

Edited By brian jones 11 on 20/08/2021 15:52:22

Edited By brian jones 11 on 20/08/2021 16:56:55

You will probably get a bit more curvature or faceting of the teeth as the tooth above and below the central one on the tap will take a small amount off the gear teeth above and below ctr height, more so on larger gear tooth numbers

Couple of sketches I did some time ago to illustrate the effect, you will also get this with the cutter Dave (SOD) shows and that is why one cutter does all gear tooth counts unlike commercial involute ones where you may need several.

so its not so far off the involute as expected then, hmm I must get on and make some

you see I have a large qty of plug taps 1" to 2"W and xl metric MS washers from M30 - M80 all 4mm thick

though I may have trouble with the steel used? Perhaps might need to anneal one, might be work hardened from being punched out of strip.

I have ordered som mtl to make a simple mandrel jig to be held in my lathe QCTP. In theory I can raised the tooll holder so as to get a straight cut across the witdth

I think I can see one reason why it will fail and that is that when starting the cut your tap tpi will spread across the OD of the blank but as you go deeper the tpi have to spread over the inner ID of the thread so less space for the original cut? which is why teeth numbers turn out less than expected?

Edited By brian jones 11 on 20/08/2021 17:18:34

Rack and pinion or worm gear, both have been shown to work to an extent.