Questions per cutting 12/14 stub tooth bevel gear

OK, I have what appears to be a stub tooth 45 deg bevel gear to cut. (yes I COULD buy one, for $117, but I'm not going to)

It looks like this



It goes in the hand-cranked table-raising setup of a drill press. it turns a 3/4-8 "lead screw" that raises and lowers the table. Low speed hand cranking means noise etc isn't an problem, but locking, or trouble with loads or pressure would be an issue.

Various stats.... in case I am FOS about the gear.

Circular pitch measures out to about 0.260
Whole depth of tooth measures a bit over 0.160

Back cone radius is about 1.13", and its an 18 tooth gear, 45 deg bevel.

Now, I know there are cutters for stub tooth form. I have some, in huge sizes that I have no use for (came with other useful stuff).

But, while I can get normal cutters, I have no source for stub tooth form cutters. Nor would I likely ever have another use for this one, unless I want to make atlas gears (It's a big Clausing DP, and the gear is actually an Atlas lathe part from a 10F).

I already have adjustments to make to cut a bevel gear anyway. I have to swing the gear slightly each way around the cone apex to generate an approximate correct shape.

That being the case, is there a way to use an appropriate pitch and cutter number in the standard full form series to approximate this stub form?

I would assume there is. If I had a rack of cutters for various DP, I would go thru them until I found a suitable-looking profile, and go ahead.

But, I need to figure out which to buy, as I have no suitable ones here. Is there any sensible way to decide what cutter number and DP I would need, somewhat sight-unseen?

If I were doing another one on the shaper, I'd grind out the form and have at it, but I'm not going to try the shaper on this gear. Been there, have a mill now, and the stock I have for it is 4140, anyhow. The little 7" Atlas shaper would not like that, and I'd likely have to re-sharpen a couple times....wotta pain.

Per tables in Colvin and Stanley, if it were not a stub form, a #5 cutter would be the one.

But, while the Colvin and Stanley book is normally pretty helpful, on the standard cutter for stub form aspect of the issue they are not.


BTW, the pic shows a zamak part, but Clausing had the sense to at least make teh mating part steel, so I can use steel without a problem for the replacement. In fact that is what Clausing would sell for $117 plus shipping.

I'm not sure why you would designate this as a "stub tooth bevel gear"?

The tooth form is a simple convergant projection, and does not actually change shape, but only changes scale from the small end to the large end.

So what you would need to find would be a "standard cutter" with a thinner than normal chordal thickness, so that it can pass through the small end of the tooth, yet has enough depth to cut the large end of the tooth to depth without cutting away too much at either end.

AFAIK, no such cutter exists. For a single use, you could make a flycutter and chomp it out that way, but as for the exact shape to give it, grind the profile of each half of the tool so that it matches the sample gear at the large end of the tooth. However, each half of the cutter needs to be offset towards the tool centerline enough so that the tip can pass through the small end of the tooth.

However, such a tool cannot cut the tooth shape correctly at any point. It requires fudging to get anywhere close to correct. By fudging, I mean that you will have to rotate the blank a fraction of the tooth space, and offset the table sideways a little in order to make the tool continue to enter the small end of the tooth space without cutting the tooth too wide there, while engaging the blank more and more as the tool moves towards the large end. This is because the tool profile is correct only for the large end, but at the same time, it is too narrow to cut both sides of the wide end of the tooth at the same time.

At the small end of the tooth, the addendum will not be adequately shaped either, because the scale of the tool's profile does not introduce enough shape to cut the involute, at that lesser depth of cut. So, the machinist is supposed to file the small end addendum to create some involute shape. It is not hard to file some clearance, although it does not look very pretty.

The $117.00 is not a bad price, if the ready made gear is completely finished and ready to be put into your assembly.

I've got some form relieved bevel gear cutters. Never used one, but they're marked the same as standard cutters, and stamped "bevel". If I can remember, I'll take a look tomorrow and see which various ones I have, and see what the difference appears to be between a bevel cutter and a standard one for the same pitch.

JT.
That's not a stub tooth.
You are correct with what you have worked out.
Go with the Colvin and Stanley descriptions or better still the ones in the Brown and Sharpe book Practical Treatise on Gearing.

John S.

Bevel gear cutters are in fact thinner than normal. Actual cutting involves moving cutter off center both ways. I have cut one bevel gear in my career. It actually worked. Was a replacement for the bottom band wheel on a Marvel (Armstrong Blum) #8 roll in band saw. The details of this 1969 adventure are somewaht vague , but I do remember getting the rotab up on the proper angle by putting blocks under one end.

John

That must be an Atlas or early Clausing drill press. T gear you are talking about I thik is the same as part number 9-48B for an Atlas 10 f series lathe. Call Clausing service center in Goshen Indiana and see if this part is still on the shelf.
Bruse

That must be an Atlas or early Clausing drill press. T gear you are talking about I thik is the same as part number 9-48B for an Atlas 10 f series lathe. Call Clausing service center in Goshen Indiana and see if this part is still on the shelf. 517-533-0371
Bruce

A number of years ago, Live Steam magazine published three series of articles about making miniature geared logging locomotives -- Shay, Heisler, and Climax. All three have been republished as books, and are currently in print. The author showed how to cut the Climax gears with fly cutters (the other two locos used heavily modified stock gears). Check with your local library. If they don't have the book "Building the Climax" by Kozo Hiraoka they can probably borrow a copy through interlibrary loan. The book is available from Caboose Hobbies http://www.caboosehobbies.com for about $43 plus shipping -- a 15% discount from list price. Caboose Hobbies has other metalworking books, so if you order $100 or more by October 17 shipping is free. No financial interest in the publisher or Caboose Hobbies, just a satisfied customer. Charles

P.S. the Climax loco uses skew bevel gears...

as written by Johnoder:
I have cut one bevel gear in my career. It actually worked.

Click to expand...

Hey John, one must be the magic number, because that's all I ever cut for bevel gears, as well! Once is enough

Seriously though, with OneCNC XR2 soon coming up with 4th axis machining, in combination with planar toolpathing, it is feasible, I think, to make a surface model of the bevel gear and then machine it with a ballend mill. I have not yet tried it, but likely will at some time. I believe the results will be better than using a special bevel gear cutter on the mill, and much quicker, and more accurate.

Hu,
This does look interesting.
Taking JT's problem as a model I calculate that the small end will need a 0.087" cutter to get in.
Does OneCNC give any indication on how long it would take to cut an 18T gear of this pattern?

John S.

John,
The cutting time would be just a matter of stepover and feedrate, so the desired finish would be subject to the machinist's whim to some degree, as well as the mill's top rpm.

Say 10 IPM feed, 1" path length, 30 passes per tooth, maybe 3 minutes per tooth to finish? Allow a couple of minutes per tooth for roughing with a couple of heavier tools? Apart from the programming and setup, the cutting time should not be too bad.

HuFlungDung & JS :
The reason I figure it as a stub tooth form is because that's what it works out to per Colvin and Stanley.

Sure, and it IS a bevel, but that says not much about the form of the teeth, aside from that they taper.

If you look at the CP, and look at the total depth, it is just about right for a 12/14 stub tooth form, per above numbers as I posted, unless you think they work out differently.. I got it from Colvin and Stanley's gear tables.

Stub tooth 20 deg .... 12/14 CP .2618 depth 0.1607.... Lines up perfectly with what I measured

The CP is 12 and teh depth is 14.... I thought that was nearly the definition of a stub tooth...... Pitch for the one and depth equal to that for a bigger pitch.

Using the regular formulas for getting at the equivalent DP for a bevel, using the back cone distance and the tooth at the OD, it comes out to a DP of almost 9, which is ridiculous for a gear 1.5" diameter.

I got 2.6" PD using the back cone radius of 1.13", and its 18 teeth.......

I must be doing OK, I have cut TWO bevel gears before, and did it on a shaper at that. No more of that nonsense.... took too long and was way too touchy on the downfeed when I got down to cutting nearly full form at the bottom of the tooth.

I do have some cutters that are close, but they aren't close enough, it looks like, And a fly cutter is going to need sharpened a couple times in mid-cut....

I know about the set-over, etc, etc. Colvin and Stanley a re quite clear on that. But I don't see a #5 cutter doing the job.... doesn't look right. But that's what their table says.

Jerry

I hate to bring this up, but what is the nature of the problem with the existing gear...I detect some light chipping from the photo but no major trashing or teeth missing.

I know we all cuss Zamak and rightfully so....possibly I am missing something else?

-Matt

The existing gear is worse than it looks. It has teeth distorted so badly that it locks with its mating gear. The zamak is flaking in spots, from teh galling it has had. The teeth are considerably thinner than the mating gear, and no longer have a form close to original.

Aside from that its fine......

The main thing is that I believe it is close to failing, stripping teeth, and becoming a real pain to repair. I had to take the machine apart to replace bearings etc, and now is the right time to fix this.

It is a part number 10F-82A, per the diagram, and Clausing wants $117 for a replacement. The replacement is steel, by the way.

As far as the stub vs normal deal..... I'm going by the measurements.

Here is a pic of the true tooth form, on teh steel gear of the pair. It mates fine with similar gears, and has no distortion.

You can see by comparison that the zamak gear in teh background has been worn badly.



I'd make the replacement out of brass or bronze if I had a chunk teh right size, because the existing steel gear would really be a pain to make. A somewhat sacrificial gear in the zamak gear's location would make sense.

I have almost the same situation as far as making a bevel gear. The steel gear you have looks to be both a bevel and a spur. On my gear the bevel and spur are both the same pitch and number of teeth. This is good because both can be cut without interfernce. In your case, it seems the spur is a different pitch than the bevel. I can't think of any easy way to make that gear. The cutter for either the spur or the bevel will take a bite out of the other's teeth. Is your steel gear actually two parts? A spur and a bevel? If so it might be way easier to buy two off the shelf bevels and modify them.

Rick

Jerry,

You could be right about the stub tooth form, but I would not sweat that detail too much, the reason being that a form cutter does very little to give you the correct contour except at the large end of the tooth. All the rest of it is wrong anyway.

It sounds like you have a good selection of cutters available. Just sort through them, and size up one that fits the contour pretty good and just use it. Even a rack cutter would probably be serviceable, given that you have to interpolate the form with rotation and side shift.

If it is a stub tooth, perhaps you can opt for a standard cutter one or two DP sizes smaller than the normal DP calculation, in order to get the thinness required at the small end, without creating a step in the tooth face at the large end.

It sounds like you have a good selection of cutters available. Just sort through them, and size up one that fits the contour pretty good and just use it.

Click to expand...

There is the rub. I DON'T have a good selection, so for this one I need to make a good guess at the one to buy.

Without them on hand, I can't simply hold them up against the undamaged gear of the pair (the steel spur/bevel combo gear) and see what looks close.

While I am in favor of more tools, at $25 a pop for cutters that I may or may not need in future, I would like not to make too many guesses.

Hence the original nub of the question, which was how would I get at the correct cutter for this WITHOUT being able to try a visual comparison?
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RJibosh:
Yes, steel gear is one-piece.

I think the bevel portion of the steel gear (which is fine, btw) could be cut OK. The cutter pass would miss the spur teeth.

The spur portion would have to be cut with a shaping type cutter, for the reason you mention. Any Fellows or similar gear shaper would do it, but hobbing etc could not be done.

You can see in teh pic above a small relief between the sections, to allow the cut to complete.

Matt isserstedt:

Lest you think I am exaggerating, here is another picture of the lovely piece of zamak.....



At top, bent teeth. At left, teeth with significant pieces missing... All teeth are somewhat thinner looking than the steel gear with which it is supposed to mesh.

Oh!

Calculate or measure the width of the bottom of the tooth space at the small end of your good gear. Then, compare that to the tip thickness of standard DP cutters, one, or maybe two sizes smaller. The tool has to fit through the small end of the tooth without cutting it too wide.

Like I've said before, I wouldn't buy a form cutter to cut a bevel gear because it is really impossible for it to cut the tooth in one go, as would be the case for a spur gear. You would likely be as far ahead to buy the rack cutter and try to interpolate the tapered faces with that, as buy a small tooth number involute and still have to interpolate the tooth faces.

How about using a slitting saw to cut the tooth space to depth, at the proper angle? Then grind up a flycutter to match your good sample at the large end. There should be no need to resharpen the flycutter half way through the procedure, if you rough the tooth out first. I would recommend a horizontal mill arbor with outboard support to support the flycutter.