Author Topic: Cutting Spur Gears -- A neat technique  (Read 2479 times)

Offline philf

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Re: Cutting Spur Gears -- A neat technique
« Reply #25 on: May 18, 2017, 12:33:03 PM »
Paul,

Your cutter profile looks all wrong to me and would explain why the resulting gear teeth look thin.

The cutter teeth should resemble a rack profile and the tooth/space should be equal at the pitch circle line and not at the tip of the tooth which yours looks.

Phil.
« Last Edit: May 18, 2017, 01:08:03 PM by philf »
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Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #26 on: May 19, 2017, 03:52:21 AM »
Quote
Then a circular array of this cut is done 'Tooth count' times to form the gear.
That's not how a gear hob works.

your cutter needs to be a symmetrical rack form and (in your reference frame) it needs to move along the tangent as it is copied around the blank (as it it were rolling around the circumference).

If you need a gear model (reasonable accuracy, good enough for printing) I have a couple, mod & dp,  based on a modified button method (uses elliptical buttons for a better involute match). mod version attached

Bill
« Last Edit: May 19, 2017, 04:16:27 AM by BillTodd »
Bill

Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #27 on: May 19, 2017, 05:07:18 AM »
Hi Paul

I've just been looking at your first diagram.  The figures you have used for pitch and depth of cut are exactly as described on Helicron's web page linked at the beginning of this thread but what isn't particularly clear on his page is that these figures are for a 20 degree pressure angle.  If you change the angle in your solidworks model I think you will find it works.

I have tried this method of gear cutting - my first attempt at gear cutting to test my home made dividing head - and the teeth when examined closely have three distinct facets on each side rather than the two seen in your model.

Russell

Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #28 on: May 19, 2017, 11:13:02 AM »
Using the straight hob  method described you'll have a different number of facets depending on the numbercof teeth engaging with the hob, larger gears (more teeth)  will have more facets as they'll be flatter against the hob

Have a look on you tube for a method using a straight hob on a small cnc mill that indexes both the gear blank and hob to get many more facets for a closer involute form.

bill
Bill

Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #29 on: May 19, 2017, 11:35:29 AM »
The original post describes using a 5 tooth cutter central to the gear blank.  Under those circumstances it should only give three facets using this method.  If the gear were small enough there might only be two facets but there were three on the 16 tooth gear I tried.

Russell

Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #30 on: May 20, 2017, 05:05:09 AM »
OK, so in lieu of actual machines, I broke out SolidWorks as well  :lol:

Unfortunately, I just can't get mine to work properly  :scratch:

Please look at the 3 attached pictures:

The first picture shows my "cutter" lined up with a gear blank. The dimensions are all taken from Helicron's site, namely: 4.32mm maximum depth-of-cut (assuming pointy-tipped cutter). 1/2 way point between (2.16mm) is the contact point (gap & tooth width equal), so I assume this is my PCD. Actual DOC (assuming pointy cutter) is 3.41mm (again, taken from Helicron's site). And finally I've lopped off the top to 1.4mm above the PCD (red circled dimension).

This results in the "gear" as seen in the 2nd picture. Looks to me like the teeth are too skinny, have an odd "dink" where the tall cutter is catching just inside the PCD, and a strange "pit" at the bottom where the cutter is at max depth.

So... I reduced the cutter height-above-PCD to 1mm... and got the result in pic 3. The teeth still look too skinny, but at least that "dink" has gone away. Still got the weird pit though.

Obviously, I've mangled something somewhere. Can anyone set me back on the right track? The measurements are all based on Module 1 gear (as far as I understand Module...) & 20 degree pressure angle.

I've attached a ZIP containing the Solidworks (2013) part, so if someone fancies fixing it for me! Note that I'd like to use Helicron's method (i.e. simple 40-degree included angle pointy tool) to actually cut the cutter.

Thanks!
Cheers!
Ade.
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Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #31 on: May 20, 2017, 05:23:11 AM »
Your rack still does not look quite right . The width of the V at the circumference of the blank should be at least equal to the flat at the top of the rack.  it looks like the flat is too wide given the depth of the cut.

Bill
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Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #32 on: May 20, 2017, 05:48:05 AM »
Your rack still does not look quite right . The width of the V at the circumference of the blank should be at least equal to the flat at the top of the rack.  it looks like the flat is too wide given the depth of the cut.

Doh! You're quite right of course... I got the cutter blank diameter wrong - I was working with an outside diameter of 48mm, not a PCD of 48mm...

Re-set the diameter to 50.50mm (PCD + 2x 1.25mm), and it's looking better. I increased the cutter depth back to 1.4mm, as the tooth looked a bit shonky at 1mm.

Still got that "pit" at the bottom, is that because 1.4mm is too deep a cut? Or because it's only an approx tooth form?

Thanks for the prod anyway, made me realise what I'd done wrong  :thumbup:
Cheers!
Ade.
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Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #33 on: May 20, 2017, 06:22:11 AM »
Well... they mesh... more or less...

Might need a spot of virtual grinding paste in there to smooth things off a bit  :lol:

Cheers!
Ade.
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Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #34 on: May 20, 2017, 11:34:42 AM »
They don't look too bad  :D

what happens if you do another cut with the rack and gear half a tooth around (if you see what i mean).?

Bill
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Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #35 on: May 22, 2017, 06:36:00 AM »
Hi Ade

Helicron suggests the DOC below the PCD should be 1.25*MOD and you seem to have 1.4.  I haven't gone any further investigating but I think that might deal with the pit at the bottom of your teeth.

Russell

Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #36 on: May 22, 2017, 08:20:57 AM »
Hi Russell,

Every time I look at this again it makes my brain ache... For the pics I did above BTW, I did reduce the cut to 1.25mod (i.e. 1.25mm), it reduced the "pit" but didn't eliminate it.

So, attached is another pic of the cutter sketch.

The four dotted lines, starting with the top one are:
 - My maximum depth of cut (note the triangular peak marked 3.41 - that's the theoretical DOC if the cutter were infinitely sharp)
 - The next dotted line is the PCD (point where tooth & gap are equal, exactly half of the absolute maximum DOC, 4.32mm approx)
 - The third dotted line is the outer edge of the gear blank, and represents the point where I stop cutting
 - The fourth and final dotted line is the bottom of the cutter (=bottom of the rack).

Now... according to Helicron's site, the total DOC should be 3.41mm based on a 2.16 + 1.25 cut. But I'm not cutting as deep as 2.16, is that a problem??

Thing is, if I go full depth (i.e. the full (theoretical) 3.41mm cut, I end up with vampire teeth (pic 2)...?

 :scratch: :scratch: :scratch:

A bit more information:
Gear blank is 50.5mm diameter (increased to 52.32mm to account for the increased DOC)
I'm "cutting" a module 1 gear, so 48 teeth.
20 degrees pressure angle.
Cheers!
Ade.
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Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #37 on: May 22, 2017, 08:22:45 AM »
Ignore that last one... my DOC's gone all wonky...

No... don't ignore it... the DOC is actually 3.41mm from the base dotted line to where I've trimmed off my cutter.

Last modification....! Do ignore it, I've got my blank WAY too big. I'd missed the bit where the OD of the blank is N(o of teeth) + 2 in mm... i.e. 50mm.

I've still got a "pit" though. I can only assume it must be because it's an approximation rather than an actual involute.
« Last Edit: May 22, 2017, 08:49:06 AM by AdeV »
Cheers!
Ade.
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Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #38 on: May 22, 2017, 02:24:16 PM »
Hi Ade

Don't forget there are two DOCs here.  The first one is for making the cutter - 3.41*MOD as you show.  The next one is the DOC when cutting the gear blank which according to Helicron is 2.25*MOD or 2.4*MOD for fine gears.

It may not help but I attach the spreadsheet I used to calculate the numbers when I tried this method - I wanted to make some 14.5deg PA gears. :smart:

Russell