What's new
What's new

Are these Bevel Gears DP or Metric Modules Gears, How Do I Figure This Out?

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
How do I calculate if these bevel gears are DP or Metric Module gears? I've looked at the Bevel Gear section of Machinery Handbook and unless I missed it, it only covers DP gear cutting. Ordered a couple more books on gear cutting last night. I've found lots of info on cutting spur gears, not so much on bevel gears. Recently picked up a dividing head and know how to get 56 tooth count using it. Where I'm stuck is on which type of cutter I need. I see the proper gear tooth form on the steel 20T gear, the nylon 56T gear looks like a straight sided V, but it is 40 years old and might be worn.

The nylon 56T gear is 4.540 outside diameter, if other dimensions are needed, please let me know. This is the gear I need to replicate.

I can buy a new replacement gear, but its not cheap. After doing some rough calculations last night making a gear might cost me just as much, maybe more if I screwup too many times, lol. Buying a gear I won't learn anything, I'd rather learn how to do this.
 

Attachments

  • bevel gears 1.JPG
    bevel gears 1.JPG
    1.1 MB · Views: 48
  • bevel gears 2.JPG
    bevel gears 2.JPG
    1.2 MB · Views: 47
Where do these gears come from and what role did it play?
These are the carriage feed gears from my 1981 Yam lathe, pretty sure they would be considered a slow speed application. The machine seems to be mostly metric, with a few imperial parts thrown in just to make it confusing. Cannot see it in pic, but 3 of the teeth have some damage, probably has a little life left, but since its all apart right now I'd like to come up with a replacement, and maybe a spare or 2 to toss in the tool drawer.
 
OK, if you are planning to cut bevel gears in your own shop, without a proper bevel gear machine, there are a couple of things you should know. (And this should be in some of those books.) First off, conventional gear cutters will not cut a perfect involute bevel tooth. They used to make "bevel gear cutters" which have regular involute profiles but are narrower than regular cutters. I don't know if you can find any of these new. I have only seen them in a shop that had picked-over barrels of WWII vintage shipbuilding stuff. The bevel gear cutters also do not cut a perfect involute bevel tooth, but they make it easier to do the manual shop hack.
What you do is get a gear cutter sized for the small end of the tooth. Instead of setting up your dividing head so the cutter path is exactly on the DH radius, you dial in a slight offset. The cutter intersects the small end of the tooth (space) exactly where it should, but when it gets to the large end of the tooth, it is offset slightly from the radius as it cuts the tooth space. Then you dial in the opposite offset when you cut the other side of the teeth. The cutter goes into the tooth space exactly matching the first cut at the small end of the tooth, but offsets in the opposite direction when it gets to the large end of the tooth.
The resulting tooth shape is not a perfect involute. It was routine back in the day (century ago, give or take) to do a little manual filing on the large end of the teeth to flatten the profile towards the correct involute, the smaller cutter cutting a tighter curve that's appropriate for the small end of the tooth.
The books you've ordered should tell you how to compute the appropriate offsets of the cutter path from the true radius of the DH.
Now if you're making a pair of matching gears, you have some additional options. I think Ivan Law's book on gears describes a "parallel depth" technique where the tooth height is made the same from the small end to the large end of the tooth. Gears cut this way won't mesh with proper bevel gears, but they can make a perfectly functional pair.
 
Last edited:
+1 on working from the small end, both sides and then hand fitting. Another option to the correct cutter is to grind a fly cutter to fit. Since you are in plastic and not cutting the 100% real profile, I would go with the fly cutter.
 
Bevel gears look simple but with out the proper machines, which are shapers that cut following a template basically, they are damn near impossible to cut correctly using a standard spur gear cutter because the profile continously and uniformally varies from small end to large. Sfriedberg nailed it but I believe you make three passes, one central and then a left and right offset pass to finish the large end. That is why the cutter is narrower than a standard cutter.
The Cinncinatti milling machine books which have several reprints gives a very good explanation of how to do it. I believe they are on line.
 
Another option to the correct cutter is to grind a fly cutter to fit.
To be brutally honest, it can't be done with a space cutter. You can make something that will work tho, by hand filing.

The problem is, the teeth are tapered. They are smaller all over on the inside than out at the ends, so no space cutter will actually fit properly. The teeth are tapered, the space is tapered.

Whatever you do, making them without a bevel gear generating machine (or profile in an nc mill yes) wlll require a bunch of hand-fitting which is going to take oodles of time.

They don't look bad to me, I'd just wash and/or beadblast and put 'em back in. Unless there's cracks in the roots or something ...

Bevels are a little strange because what they are depends on where you measure them :)

Personally, if the price isn't ridiculous, in this case I'd just buy them.
 
Last edited:
I made a double bevel gear for a replacement in a lathe apron way back when I was young and had good eyesight and didn't rely on bi-focules as I do today. I also had the original cutters that were used to cut the original from the factory. Also had a copy of the original drawing that called out the angles needed to set the B & S dividing head to. Surprisingly, it came out darn nice!
One thing I have noticed when trying to determine what pitch a bevel gear is, is to take a set of spur gear profile gages. Mesh the profile gage to the outer edge (big end) of the bevel gear and most of the time it will mesh perfectly with the gage.
Need to find someone that has a old Gleason bevel gear generator to make the bevel gear for you. It's a shaping machine designed to cut and generate the perfect profile of the bevel gear. Yeah, I know, that's not going to happen.
 
It's the ideal job if you want to learn why you need a proper gear cutter. Easy material, non critical application and you aren't going to screw anything else up if you don't get it right.
 
Need to find someone that has an old Gleason bevel gear generator to make the bevel gear for you. It's a shaping machine designed to cut and generate the perfect profile of the bevel gear. Yeah, I know, that's not going to happen.
A Gleason can’t cut that gear.
Can only do gears with curved teeth.
Spiral bevels ,“ Zerol Bevels”or Hypoid bevels.
Could redesign that gear to be a spiral bevel, but if you want to replicate what is shown above, it is not gonna happen on a Gleason.
Cheers Ross
 
Last edited:
I only need to replicate the one nylon flatish bevel gear, the steel pinion gear is fine. According to the lathe dealer I spoke with this gear was discontinued in 1982 and there are only 2 left in stock, priced at only $325 each, so if I want a new one on the machine, and a spare in the drawer, by the time we added tax and shipping, it was somewhere around $700. From what I've found so far, yes, it will take 3 passes, the center cut, then a L/R shift to get the profile correct, ish.

Sfriedberg, Ivan Laws book is one of the books I ordered.

I'll be in the shop tomorrow and will figure out face angle and cut angle.
 
If it were my problem, first up I'd use Delrin for the gear rather than Nylon, for ease of getting a crisp result. Second I'd fish out a sacrificial centre drill of suitable size & grind it to nicely fit the small end profile of the spaces, then proceed to do the three passes per tooth trick. Haven't made a bevel this way but have made a helical pinion (about 12 dp) for my T & C grinder table travel. The Delrin one was a trialer for a subsequent bronze version, which I made before fitting the Delrin one. This was 40 years ago the Delrin has maintained nice smooth, almost zero backlash. Couldn't be bothered installing the bronze, as nicely as it turned out. More small area ( 6" x 3" typical) surface grinding than cylindrical or tool sharpening but those too. Back in the day I ground the endmill type tool freehand .. now I'd do it on the Kuhlmann D-bit grinder as a simple radius pair. Could be my eyes but those teeth look tapered, but very little. Should be a breeze.
 
Last edited:
DP and module are just two different ways of looking at the SAME THING. They just look at it from a different mathematical direction.

One is usually used for gears that are specified in English measure and the other for those specified in metric measure. But you can calculate one from the other:

DP = 25.4/m
and
m = 25.4/DP

Where m is in mm and DP is in reciprocal inches (1/in).

It is almost never done and I only say this to emphasize their equivalance, but metric gears could be specified in a metric version of DP (units of 1/mm) and English gears could be specified in an English version of module (units of inches).

Both DP and module are ways to measure the size of the teeth on a gear. And, in reality, those teeth can be any size, including any of the in-between sizes of either of the common scales (DP and module).

When you are talking about bevel gears, the gear teeth will have a DIFFERENT size on the inside and the outside of the cone of the gear. So any size that would be specified would only be correct at one and only one point along the width of each tooth. Each tooth has a variety of sizes weather you measure in DP or module.

It is probably somewhat common to specify the tooth size at one end or the other. I don't know if either end is used more or less. It is quite possible for the size of the same tooth to be a nice, round and often used number in both of the measurement systems at different places along it's width.

As for what cutter to buy, the small end of the tooth is the only place for guidance there as a cutter for the large end would make the inter-tooth space too wide at the small end. But, as others have said, depending on the method you plan to use to cut your gears, you may want a special, reduced width cutter.

Get and read the books. That's plural, bookS!
 
A simple way of cutting lathe apron bevels is to narrow the tooth face down to say 1/2" at the max dia and cut one pass ....replacing the plastic gear with a steel one would give same strength .....I never used a file,but rather a 4" angle grinder with a narrow cutoff wheel to touch up .
 
THE SOURCE for gear cutters and information is: ASH GEAR and SUPPLY in Southfield Michigan.

I am looking at single bevel gear cutters on pages D-6 and D-7 of their catalog no 91

When I needed advice about a cutter for a 16/32 DP 30* PA stub spline cutter , I had a very helpful

conversation with Patrick at (248) 374-6155

petersen
 
Last edited:
To be brutally honest, it can't be done with a space cutter. You can make something that will work tho, by hand filing.

The problem is, the teeth are tapered. They are smaller all over on the inside than out at the ends, so no space cutter will actually fit properly. The teeth are tapered, the space is tapered.

Hmm. This sounds like something one could make with a rotary table fixture on an old-school horizontal shaper. Slowly.

How do the modern generator tools work? Edit: @EmGo answered this question in posting #16. It's a glorified shaper.
 
Last edited:








 
Back
Top