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Shaper form tool for spur gearing

metalastic

Plastic
Joined
Mar 18, 2019
Hi,

First up I am NOT a qualified machinist so please forgive my ignorance. I have an repair that to me anyway seems achievable.

I have two double spur gears around 13" diameter on the large gear and 7" on the small gear. There were originally "as cast steel" with minimal clean up. So the tolerances are very generous.

I want to build up the smaller gear and re-machine it as it is very badly worn, but can't use a Hob or Gear cutter as both gears are cast as one so the shoulder of the large gear will prevent this.

So the way I see it is that it could be re-cut on a CNC multi axis mill or possibly on a large shaper with a dividing head (which I have). Maybe a specialized gear generator? Now the thing is I wanted to build up the teeth myself, but a few firms I approached said that was not an option for them, and wanted to make gearing from scratch.I think they are worried about destroying expensive tooling on the weld build-up which is fair enough.

I am on a tight budget so I want to keep my costs low and if I can do it myself so much the better, so I was wondering if there was such a thing as a shaper form tool for largish DP gear form? tooth depth around 1.5" - 2" I could have a go at grinding one but if such a beast exists off the shelf that would be ideal.

Maybe some of you may have other thoughts?

Cheers

Ian
 
I do cut gears on the shaper and it is quite straightforward. The tool profile is the same as the teeth gap and when repairing or duplicating a gear it can be ground to fit an existing tooth. In your case when the cut must stop at the large gear face you will need to cut a small groove (if it does not exist already) between the two to have a bit of a clearance for the cutter and the cutting. If the tool holder gets in away, one must mount the cutting tool on some extension, akin to a boring bar shape.
 
That's an option as I could probably secure it to the other gear with studs- thanks, something I had not really considered.
 
For a gear that size you could carve the teeth out with a 1/4" diameter end-mill even 1 inch deep fairly quickly. if the gears were indeed "as cast" then you could get away with hand programming the g code, something like 10-20 straight line segments approximating each side of the involute shape should be good enough.

For a tooth depth of 1.5 to 2 inches but only 7 inch diameter that sounds like 9 teeth, (about the lower limit). you may want to do better than 10-20 line segments approximating the involute curve.

why did the gear fail? why was it as cast? if you can redesign the whole machine and use properly cut and hardended gears they should last 10+ times as long as "as cast" gear would.
 
This gear is 108 years old on a steam traction engine. I believe that these gears were often as cast, as were the differential pinions. It has simply failed because of use and the fact that it attracted a lot of dirt adhering to the grease over that time, all other gearing on the engine was machine cut. Once repaired it will certainly see me out and probably my grandchildren as well :)37790605_1888271091234968_7051481721799180288_n.jpg
 
might try plywood soaked in epoxy!

replace both gears with 20 or 25 degree pressure angles cut from yes.. plywood might work, cut from a template on a band saw. those gobs of cast iron look like they are ready to break off. can you double the face width of the gear (aka depth) ?


the advantage of higher pressure angles is lower tensile stress in the root of the tooth at the cost of higher thrust loads pressing the shafts apart. plastic gears share the load between 2 or 3 teeth with ease compared to steel or cast iron. 3d printed PLA is comparable to cast iron for example because the ratio of modulous of elasticity to tensile strengh is low enough that you can guarantee the oil/grease film thickness combined with the elastic deformation can guarantee the force is spread out on 2-3 teeth even if the tooth form isn't that great. you can't guarantee that with cast iron or cast steel gear teeth unless the geometry is exceptionally good, because the gears are stiffer than the geometry and or the oil films. problem with PLA and some plastics is the fatigue limit.


anyhow.. i would again replace that gear especially if it is open lubrication situation with a 20 or 25 degree pressure angle gear of more teeth.
 
Unfortunately the depth or face width is not changeable, and they are 14.5 deg. That pic is really not that good as the wear is extreme though it doesn't look like it from that pic. Probably a good 1/2" backlash between the teeth.
 
Ian, I think the greatest difficulty you face is building up the gear teeth without something cracking. I'd say a lot of general preheat as on a charcoal fire, gear flat on its back,&, not being facetious, build up the teeth with nickel bronze using oxy torch. You have the template in place, the ends of the teeth appear to be as original so, in the spirit of the age of manufacture, I'd cold chisel (sharp chisel)the build-up close to shape/size then finish off with probably the biggest flat riffler you can find. Likely turn out better than new.
 
Unfortunately the depth or face width is not changeable, and they are 14.5 deg. That pic is really not that good as the wear is extreme though it doesn't look like it from that pic. Probably a good 1/2" backlash between the teeth.

haha.. no i took one glance at the gear and thought.. no that don't look right at all. your mention of 1/2" of backlash is about what it would take to make that photo look like a 14.5 degree gear.

and that 1/2" of backlash is how much material you would have to add to the gear to fix it.
 
Hi,

First up I am NOT a qualified machinist so please forgive my ignorance. I have an repair that to me anyway seems achievable.

I have two double spur gears around 13" diameter on the large gear and 7" on the small gear. There were originally "as cast steel" with minimal clean up. So the tolerances are very generous.

I want to build up the smaller gear and re-machine it as it is very badly worn, but can't use a Hob or Gear cutter as both gears are cast as one so the shoulder of the large gear will prevent this.

So the way I see it is that it could be re-cut on a CNC multi axis mill or possibly on a large shaper with a dividing head (which I have). Maybe a specialized gear generator? Now the thing is I wanted to build up the teeth myself, but a few firms I approached said that was not an option for them, and wanted to make gearing from scratch.I think they are worried about destroying expensive tooling on the weld build-up which is fair enough.

I am on a tight budget so I want to keep my costs low and if I can do it myself so much the better, so I was wondering if there was such a thing as a shaper form tool for largish DP gear form? tooth depth around 1.5" - 2" I could have a go at grinding one but if such a beast exists off the shelf that would be ideal.

Maybe some of you may have other thoughts?

Cheers

Ian

I think it is kind of a waste to try to buy or even to try to make a full form tool that large. The setup in the shaper ain't gonna like it when that massive edge enters the metal. KACHUNK. Even in cutting a small gear, there is fair impact when the tool enters the part (on a steel gear, anyways). The resulting deflections shows up as a little ripple at the entering side of the gear. Not that that would matter for this particular case :D

I think it is big enough to profile mill. But, it is most likely going to require hundreds of dollars worth of nickel rod to build up a machinable deposit. More practical to get a new gear cut on waterjet or a good plasma cutter or even oxyacetylene outfit and worry about fastening it to the old one some way, some how.
 
I would look into waterjet with abrasive for the smaller gear And then a shrink fit or loctite it into the bigger gear Plenty strong enough
I would not shrinkfit CI onto something So if shrinkfitting onto the big gear happens to be the best way I would make the new gear out of some steel
You could even remachine the mating gear and make the teeth of the new gear a bit thicker
But then you must know something about gears

Peter
 
One might single-tooth cut a new gear as a ring in steel/ci, turn off some of the old gear, shrink fit new bit onto it.

The single-tooth cutter could be hand filed to shape in hss.
 
Repairs like this are something we need to do from time to time at our museum. The latest case was a barring gear for a big beam engine that shattered (big bang!) due to an operator error. This is a large cast iron gear with teeth as cast, but with a large 'bendix' cam to push it out of engagement when the engine takes over. We looked at all the alternatives for repairing or remaking this gear, including machining from solid out of a very large chunk of continuous cast bar, but in the end we chose to do it the traditional way - i.e. make a pattern and have it cast.

We are fortunate in that we have a very experienced pattern maker on the volunteer team and he made us a superb pattern for this job. The cast gear weighed about 68 kg (148 lbs)even though we cored the bore. The bore and the end faces needed turning and some milling was needed for the driver slot, but the teeth we just dressed, not fully machined. Gears like this were not often gear cut when the engines were new. At best you might have a fitter or a team of fitters filing the teeth to fit a template.

I investigated the tooth form of this gear. It was not involute form or anywhere near it. There was a simple rad at the top of the tooth and in the root, with straight sides blending between the radiuses. I suspect that your traction engine may similarly be a bit 'unconventional' in tooth form. The general wear that occurs on open gearing like this tends to make it more difficult to establish the original form, but there are usually sufficient clues.

Big mill engines can have flywheels that are enormously heavy. The engine in question has a flywheel 18 feet diameter and weighing about 30 tons. The broken gear engages with the flywheel rim. This is very far from the largest engine of its type. To generate accurate gear teeth on something that large can be done, but it would be very very expensive and there are only a handful of companies that can do this scale of job. At the time when big steam engines were first introduced, accurate gear cutting machine tools (hobbers, gear shapers etc) were just being developed and certainly not for mill gears of this size. This was even more the case when large bevel gears were used - there were no really practical way of making accurate, very big bevel gears until some time later. Teams of fitters could do a good enough job when labour was relatively cheap, and they did so frequently.

To take a look at the gear I describe, go to our museum facebook site: Bolton Steam Museum - Home | Facebook
and scroll down a bit.
 
Say tooth count and actual OD and maybe we can home in on DP or CP - (circular pitch equally common in the day)

A for instance 7" OD and 7 teeth would be near 1 1/4 DP or 2 1/2 CP

9/7 = DP (you add two to tooth count for known OD)
 
This is a way to do big gear teeth, see:- YouTube
On something that size then maybe finish the faces with abrasives somehow like a tiny flap wheel or disc sander. Even a fine sharp file would make it pretty good without too much struggling.
 
Not for the job in hand but this hydraulic copy attachment along with a dividing head would cut your gear:- YouTube
I wonder is a lathe copy attachment with the slide replacing the shaper's tool slide could be adapted to work like that.
 
There's no way a shaper could do this, the stroke would have to dead-end into the flank of the larger diameter gear.

+1 on turning down the teeth on the original and then silver soldering, locktite, or pinning a ring gear to the stub.

Ring gear made with an involute gear cutter or any of the other suggestions mentioned.
 








 
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