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cutting spur gears on a mill

a rogue method?

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JasonB20/09/2021 12:56:06
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Andrew, cutter would be rack form in section, think ACME form but with no helix so with the work moving in small increments you would get a facet per angular movement. So one cutter per DP or MOD size but it could cut any number of teeth save for very small ones. Much like a Sunderland but with a rotating action to the cutter instead of linear.

JasonB20/09/2021 13:01:01
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Posted by brian jones 11 on 20/09/2021 08:16:56:

Ok JB, I agree with all your comments AND

the fundamental limitation is the choice of screw thread pitches available for practical use

To keep it simple, and assume that the CP is approx = to screw thread pitch then you could realistically go from 2mm say 1/2 UNC (or M14 x 2mm) to M24 x 3mm to M36 x 4mm

That was one of the points I listed, "Limited tooth size as large threads are screwcut not cut with taps" even your M24 and M36 are doubtful unless you have means of holding that diameter in spindle.

As for plastic gears find a friend with a 3D printer far better and predictable results from them.

Also worth noting that in another thread we have a relative beginner who like several others has bought a rotary table for other jobs long before the need to cut gears. So many will have the equipment and can either make a cheap single point tool of for less than the cost of a large tap buy an involute cutter from the far east.

Edited By JasonB on 20/09/2021 13:01:37

Andrew Johnston20/09/2021 14:16:36
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Posted by JasonB on 20/09/2021 12:56:06:

So one cutter per DP or MOD size but it could cut any number of teeth.......

Thanks for clearing that up, I understand one DP or Mod, but any number of teeth, per cutter. I originally interpreted the post as one cutter for all DP or Mod, which is why i got confused. embarrassed

In theory it should be able to cut small numbers of teeth as well. The undercut arises automatically. In some of the 1930s books I've got on gear cutting there are formulae for calculating whether a hob will undercut or not, even when not needed.

Andrew

Dave S20/09/2021 14:39:54
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You can always profile shift if you want to avoid undercutting on low number tooth pinions.
I suspect with appropriate CNC magic and a pointed V cutter you could actually cut a number of mod sizes and any tooth count with just one cutter - you would do offset passes without the rotation to clear the bottom of the tooth gap appropriately. Needs a bit more thought, but imagine cutting an acme thread but using a pointed tool and moving the top slide parallel with the cut to clear the root.

Dave

Andrew Johnston20/09/2021 14:48:27
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Posted by Dave S on 20/09/2021 14:39:54:

I suspect with appropriate CNC magic and a pointed V cutter you could actually cut a number of mod sizes and any tooth count with just one cutter........

For me it's easier to use standard ballnose cutters and let the CAD/CAM do the grunt work. These prototype 10 tooth, 6DP, pinions have undercut as part of the CAD model:

Bevel Gear Pinions

Andrew

DC31k20/09/2021 15:19:53
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Posted by Dave S on 20/09/2021 14:39:54:

I suspect with appropriate CNC magic and a pointed V cutter you could actually cut a number of mod sizes and any tooth count with just one cutter

A tapered cutter is not necessary. Assume your A-axis is aligned with your X-axis. If you move in Y- and rotate in A-, that presents the side of a parallel cutter to the stock in the correct manner.

What sets the upper limit of the cutter diameter is the root detail of the tooth (i.e. part geometry). What sets the lower limit is the more practical issue of length-to-diameter ratio.

Please have a look at Gearotic, written by Art, the originator of Mach 3. There is a 'gearheads' forum associated with the software where historical posts can be seen. You will see a number of contributions there by John Stevenson who was closely involved in the development of the software.

John Haine20/09/2021 15:30:42
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Using a parallel sided cutter is very attractive except for small gears one needs a small cutter! Given my mills limited top speed I can't go muck below 1mm and using Gearotic to look at some designs recently this limited the minimum module such that I couldn't make a gear sufficiently small.

I have wondered about using an engraving type cutter with the point ground off to make a "single vee" but I think this could be a bit fragile - might be worth a try though. You can buy 40 degree point TC engraving cutters for about a quid each from eBay, made originally for PCB cutting.

Dave S20/09/2021 15:30:51
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What you are describing is using the cutter “vertically” I think. With a standard end mill.

what I was envisaging was the cutter “horizontally” more like a slitting saw.

so as the blank rolls the gearotic code rolls A and moves Y. I was thinking roll A and move Z

This would mean the “stick out” issue of a long thin cutter is now a diameter issue of a thin slitting saw like cutter.

I have experience with Gearotic, and knew John quite well. Clumsy Bstard lived 5 mins up the road.

Dave

Martin Connelly20/09/2021 16:30:24
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A single cutter formed like a rack with one tooth can be raised or lowered one or two pitches above and below the centreline of the blank's rotational axis to create DP or Module gears. This will be like having a 5 tooth rack cutter. Rotate the blank one tooth position (0, 1, 2, 3 etc) and repeat until finished. Then if you want less faceted faces you can rotate the blank half a tooth (0.5) and move the cutter to the intermediate positions between the first five followed by rotating the blank to the remaining positions (1.5, 2.5, 3.5 etc.) and repeat until finished. Easy with CNC and an a axis, fiddly but doable without CNC.

You can turn HSS with a ground and polished carbide insert.

This is a test piece left over from one I did for a specific need for a 0.4 module 27 tooth gear where an off the shelf gear was not suitable, the actual part was made from brass.

Single tool gear cutting

Martin C

Edited By Martin Connelly on 20/09/2021 16:32:19

JasonB20/09/2021 18:47:30
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The advantage of a cutter with a rack profile of say 5 teeth is that it will cut several teeth at each pass so the total number of A and Z movements can be reduced therefore making the run time less

As I think Neil posted images of all his brass gears were done with a cutter like that but only indexing the work by 1 tooth at a time so facets are quite marked, if you could get the CNC to give you another couple of positions you would get much smaller facets and a smoother face to the gear tooth. At some point it would not be worth making small movements as material removed would be so small (depending on DP or MOD size)

Something like this of reasonable diameter would give a good cutting speed and with 4 or 6 flutes machined into the cutter would allow it to move a lot faster than the usual 2-flute ball nosed milling cutter for the same chip load and it would be far more robust.

straight hob 1.jpg

Several facets cut each pass

straight hob 2.jpg

Just by indexing one tooth you start to get curved gear faces, imaging the facets being 3 or more times as many once A and Z start moving.

straight hob 3.jpg

Neil Wyatt20/09/2021 21:11:51
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cutting gear.jpg

This works. One pass per tooth. Works for small gears. For really small you can shift by half a tooth and go round again.

Also cuts slightly ugly but perfectly meshing gears on slightly non-standard PCDs, over and under size.

jovilabe (4).jpg

Neil

Neil Wyatt20/09/2021 21:12:23
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When my workshop rises phoenix like, finishing that beast will be a priority!

brian jones 1121/09/2021 10:59:20
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Posted by JasonB on 20/09/2021 18:47:30:

straight hob 1.jpg

Several facets cut each pass

straight hob 2.jpg

Just by indexing one tooth you start to get curved gear faces, imaging the facets being 3 or more times as many once A and Z start moving.

straight hob 3.jpg

Funny that - looks like my OP

and the concept of using a bolt with 6 straight flutes

Hmm

Must get on with that, too many distractions around me simulating gear rotation. Cant be doing with GIFs and all that stop motion stuff, its like cutting racks

JasonB21/09/2021 12:53:12
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Look again Brian, NO helix on the cutter

brian jones 1121/09/2021 13:33:18
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Well if a cutter like that had a helix?

cnc scr copy.jpg

I think there is carpet on the floor

SillyOldDuffer21/09/2021 16:22:51
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Posted by brian jones 11 on 21/09/2021 13:33:18:

Well if a cutter like that had a helix?

...

A helical rack cutter would grind off all the metal leaving zero teeth, because the blank isn't free to rotate. However, no teeth at all still meets Brian's extremely low Statement of Work - push two plain wheels together and it's a friction drive!

Dunno what others think, but I score free hobbing at one out of ten. Thumbs down because the method's many limitations result in gears that are unacceptable for most practical purposes.

As H L Mencken said: 'For every complex problem there is an answer that is clear, simple, and wrong."

Dave

JasonB21/09/2021 16:53:17
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I might give free hobbing with a half decent hobbing cutter a bit more than 1, but doing it with a tap is scraping the barrel.

brian jones 1121/09/2021 18:44:47
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Now SOD you have not been paying attention in the back

the blank isn't free to rotate.

A rotating rack!!!!!!!!!!!

What are you on

One out of ten for imagination

You would need a mill with a freely moving cross slide

So I would put a lead-in taper on that cutter (just like a taper tap)

That would shunt the whole job in 5 mins NOT 5 hours

I didnt get where I am by sitting in an armchair sniping at "can do" MEH devil

 

JB - you never seen an ACME tap?

 

Edited By brian jones 11 on 21/09/2021 19:02:05

JasonB21/09/2021 19:20:41
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I think You need to be on the meds Brian if you are not all ready.

SOD was talking about Neil's and my cutters which are rack form and rotate and the gear blank is fixed to an arbor that is indexed.

Also never seen a mill with a cross slide maybe you don't know your mill from your lathesmile p

Yes I have seen ACME taps and wondered how long it would take you to twig that an ACME thread form with it's 29degree angles might be worth trying as it could possibly give the 14.5pa of old gears such as the ones found on a Myford and the pitch is reasonably coarse too. Biggest problem is they are rather long, not that cheap and mostly straight fluted.

brian jones 1121/09/2021 19:30:37
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Also never seen a mill with a cross slide maybe you don't know your mill from your lathe

Indeed I am, as ever, scrambling my glossaries

what is the mill X slide called in shop talk

I spose the Y slide would be a cross slide?dont know

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