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Personally I would just reuse the gear as is, you might find that the gearbox whines more or there is some extra slackness.

I guess the worst that could happen is a gear tooth breaks off, chances are it will just drop harmlessly to the bottom of the gearbox.


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After much cogitation, mulling, and consideration, I have determined that one man's "good enough" is another man's "nowhere near good enough", and yet another man's "way better than good enough".


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Hi All,
Quote
I guess the worst that could happen is a gear tooth breaks off, chances are it will just drop harmlessly to the bottom of the gearbox.

The worst that could happen is a gearbox seizure landing your ass into the path of an oncoming 18 wheeler facepalm

John

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Originally Posted by Shane in Oz
What, no Stellite?
For the record, Stellite doesn't like carbon so the case hardening of the gear would be an issue. Something like that can be dealt with by first applying a thin low carbon steel layer followed by the Stellite, merely doubling the amount of work.

Originally Posted by R Moulding
the pits in the gear would have to be significant or more like craters for them to cause an issue.
Well, even with a generous definition of 'pit', some of them are 'craters'.

If I were to braze the pits/craters I'd have to recut the profile so for that I would need the appropriate involute cutter. Here it gets a little complicated. Three parameters are needed to specify the necessary cutter, which in essence are determined by the diameter of the gear, the number of teeth, and the shape of the profile. In the U.S. we use diametrical pitch (DP) and pressure angle for two of those parameters.

In principle the DP is determined by the outside diameter of the gear divided by the number of teeth plus 2. It actually works for the 18T gear in question since its OD is 2.967" and 18T + 2T = 20T, which means the DP "should" be 10.11. You might think that's a roundoff error, and that it actually should be 10.0, but that's not necessarily the case. Its mating 20T gear is 1.865" diameter so its DP "should" be 11.80. It turns out that when gears are made there are compromises because mating gears must have the same DP, and because standard cutters only come in even numbers. So, we might expect these gears to both be DP 10 or both be DP 12.

As for pressure angle, two variations are in common use, 14½° and 20°. The next two photographs show that a 10 DP, 20° pressure angle involute cutter was used to make the pitted gear.

[Linked Image]

[Linked Image]

The next photograph shows that, although I calculated DP 11.80 for the mating gear, it had to match the 10 DP of the pitted gear and, in fact, it does.

[Linked Image]

Ideally, not only would cutters come in an infinite range of DPs (or, say, steps of 0.1 instead of 2), they would come in different shapes for each number of teeth on a gear. That is, the teeth on an 18T gear should be slightly different in shape than those on a 19T gear. Instead, cutters for a given DP only come in only six shapes, each a compromise that covers a range of several teeth. In my case a 10 DP, 20° pressure angle, #6 involute cutter would be needed since it covers 17–20 teeth.

But, I digress...

Originally Posted by gunner
I guess the worst that could happen is a gear tooth breaks off, chances are it will just drop harmlessly to the bottom of the gearbox.
Originally Posted by chaterlea25
The worst that could happen is a gearbox seizure landing your ass into the path of an oncoming 18 wheeler
I'm reminded of the time years ago I went with a woman colleague to the dealer to drive her back to campus after dropping off her car for some needed service. The service writer noted what needed to be done, then added that although her car's mileage didn't call for it, he had seen timing chains break and cause terrible damage to an engine so did she want it replaced as well. She had a look of panic as she turned to me to ask what she should do it, and I turned to the service writer and said FY, plus additional words along the same line for his sleezy sales ploy.

Not that I'm saying FY to chaterlea25, but there are plenty of other issues a 70-year old motorcycle might have that could send me down the road in front of an 18 wheeler. It's a slippery slope from changing that gear "just in case," to abandoning the project and buying a new motorcycle instead "just in case," to abandoning motorcycles altogether and buying the highest safety-rated car instead "just in case," to staying at home from now on.

I'm unaware of any actual data on probability of gear breakage vs. depth of pitting so it's a judgment call. My judgment is, it's good enough. The next photograph is of the driving faces of two of the worst teeth.

[Linked Image]

At least 80% of the faces are still present to carry the load so I'll gamble that the gear will be fine as long as I don't load it to within 20% of whatever safety margin BSA designed into a gear made to pull a sidecar loaded with the family on holiday.

Originally Posted by GrandPaul
one man's "good enough" is another man's "nowhere near good enough", and yet another man's "way better than good enough".
Truer words were never written.

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Not only pulling a sidecar loaded with the family tyo go on holiday but also used in the heat of competition in scrambles and by learners crashing gears in a way that MMan will never do.
80% of the teeth intact---I would think that the safety margins on these gears would be in the hundreds of %---so I fully agree with your decision, MMan.
Lets hope that we are both correct!

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Another option would be.....

[Linked Image from live.staticflickr.com]CF9194D6-226C-4211-A277-FBC9637C3E26 by First Last, on Flickr

Last edited by Cyborg; 08/14/20 12:33 am. Reason: Remove the somewhat juvenile part
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Just to seek clarification:
  • what possible difference does the condition of the driving face of the tooth make, relative to the strength of the tooth, if the trailing side of the tooth is rife with hollow craters?
  • If it is possible to buy a good replacement gear for $100, surely that would be cheaper than buying, setting up, and using an involute cutter that would likely never be used again?


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Originally Posted by Magnetoman
Originally Posted by R Moulding
the pits in the gear would have to be significant or more like craters for them to cause an issue.
Well, even with a generous definition of 'pit', some of them are 'craters'.
Originally Posted by gunner
a gear tooth breaks off..... just drop harmlessly to the bottom of the gearbox.
Originally Posted by chaterlea25
a gearbox seizure landing your ass into the path of an oncoming 18 wheeler
At least 80% of the faces are still present to carry the load so I'll gamble
This is a DIRT bike, right? You're never going to ride that thing on the freeway. But I do have another comment or two.
Noises and the load carrying surface are but one sort of concern, the stress riser nature of pitting is another. Corrosion pitting is a well researched source of stress concentration leading to cracking and failure, especially when combined with cyclical loading such as in aircraft propellers, connecting rods or……gear teeth! But I still think you should go ahead and use that hard working gear pair for now (all the torque from 1st, 2nd and 3d go through those two), because I have seen gears with wear pitting (spalling) all the way across the thrust face that were quiet and ran for a long time more. Just be on the lookout for a replacement if the pitted one gives out. Cheap and cheerful=good enough for this I say.

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Originally Posted by TR6Ray
If it is possible to buy a good replacement gear for $100, surely that would be cheaper than buying, setting up, and using an involute cutter that would likely never be used again?

I will be cheeky and anticipate MMs reply. (on the assumption that its the same as my reply).

What would be the fun in that?

John

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MOD 2.5 is DP 10.160.

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Originally Posted by TR6Ray
...surely that would be cheaper than buying, setting up, and using an involute cutter that would likely never be used again?
Mighty cavalier of you to use the word "likely" there, mate...


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Originally Posted by TR6Ray
what possible difference does the condition of the driving face of the tooth make, relative to the strength of the tooth, if the trailing side of the tooth is rife with hollow craters?
Even with pits on the back face, the gear tooth is thick enough to handle the force. The problem comes if, say, half the driving face on a tooth has been eroded and if the remaining area is not enough to withstand the driving force (pressure = force/area). If that's the case, the driving face will begin to be knocked way, further eroding it and further reducing the area available to withstand the force.

Although bronze is softer and less abrasion-resistant than case-hardened steel, filling in the pits and resurfacing it would let those bronze-covered areas transfer the force to the steel underneath, while the remaining steel on the surface deals with the tooth rubbing across that of the mating gear.

Originally Posted by TR6Ray
If it is possible to buy a good replacement gear for $100, surely that would be cheaper than buying, setting up, and using an involute cutter that would likely never be used again?
If I had that attitude, I wouldn't have half the tools I have in the garage. I'd also have a bigger bank account, but that's a different issue.

Swapping parts, rather than spending the time necessary to repair them, might be necessary in order for a business to make a profit, but I'm not a business. If I only had one bike, repairing it ASAP might be necessary, but I have more than one bike. Spending the time to teach oneself how best to braze gear teeth and reprofile them with an involute cutter that might not (or, then again, might) be used again could be a waste of time, except for someone with a life-long obsession for learning new things. Besides, what's the fun, and where's the satisfaction, in simply pulling out a part and replacing it with a new one from the shelf?

My behavior over the last half-century shows that if the choice is between buying a replacement part for $X and installing it, or buying a special tool for $X and having to spend two days using it to refurbish the old part, I'll almost certainly go for the latter. That choice certainly is no less unreasonable than my choice to build an Alloy Clipper from 2000 separate used parts. Anyway, if you're obsessed with learning, have more than one motorcycle, and aren't in it for a profit, my approach may make sense. Otherwise, maybe not.

Originally Posted by kommando
MOD 2.5 is DP 10.160.
The humiliation I feel from knowing Cyborg has involute cutters and I don't may force me to go metric. Modern machinery has standardized on gears with a 14½° pressure angle so, of course, BSA used 20°. Unfortunately, sets[*] of Imperial involute cutters in the old 20° profile are hard to find, but they are available in metric. Aside from the slight difference in profile of 10 vs. 10.16, the problem with metric cutters is they're, well, metric, so an appropriate metric arbor is needed as well.

[*] The gears in my M-type gearbox run from 16T to 29T, and in a "modern" BSA gearbox they run from 14T. What this means is profiles from #4 through #7 would be needed to cover all possibilities[**], which are five of the eight total in a complete set. So, of course, this means I "need" a complete set.

[**] The teeth on the kickstarter and shifter mechanisms were cut with different DPs, but we'll ignore that. For now...

[Linked Image]

[Linked Image]

Last edited by Magnetoman; 08/14/20 4:32 pm. Reason: added two photographs
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Well to be honest... I figured your were “mistaken” when you said you didn’t have the cutter. It couldn’t be true and I thought perhaps you simply forgot where you had put the them (probably in triplicate and made in Switzerland) or it was some sort of ruse to get chaterlea25 to just send you the gear. Maybe waiting to see if GrandPaul or NYBSAGUY would start a GoFundMe page.....
If you wanted to braze the gears, any man worth his salt would just simply make the cutter. Had I believed you at the time, I probably would have made a necklace out of mine and posted a picture of myself looking rather smug.

I edited my previous post with the picture of the gear because it sounded a bit juvenile, so I may have come back later and completely delete this one.

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Originally Posted by Cyborg
I edited my previous post with the picture of the gear because it sounded a bit juvenile, so I may have come back later and completely delete this one.
Too late, I already downloaded your post.

Originally Posted by Cyborg
Well to be honest... I figured your were “mistaken” when you said you didn’t have the cutter.
Not too long after I bought my mill I was convinced by people who might know better that it was very hard on the splines of the quill to cut gears so I didn't pursue involute cutters after that. Clearly, in the context of the Tool Wars™ (but possibly not in reality...), that was a huge mistake. It's not that I'm thinking of cutting gears now, but rather that it might be nice to have the possibility to trim excess material from gear teeth, whether that material is brass, Si-bronze, Al-bronze, or steel. Whether or not I ever make use of that possibility is of secondary consideration to recovering from my humiliation in the latest skirmish in the Tool Wars™ ...

On a different topic, several years ago I bought spray bottles of Meguiar's Hot Rims Aluminum Wheel Cleaner and Mothers Aluminum Wheel cleaner for possible use on my long-stalled Spitfire Scrambler project. The former contains a half-dozen organic chemicals and the latter oxalic acid plus two organics. However, I never used either of them.

Anyway, I realized the Clipper's main gearbox case was a good candidate for testing whether or not these were any good. The following photograph doesn't do a good job representing how the cases look -- both look better in real life -- but they do show the relative improvement in the looks after I applied the Mothers after having first used the Meguiar's (right), with the Meguair's-only on the left.

[Linked Image]

The dull grey areas in the photograph are due to the lighting. They're actually "aluminum colored." Using both products in series did result in a noticeably shinier case than using only the one.

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HI MM and All,

OMG what have I started ???

Stuart is on the same page as me,
Quote
(all the torque from 1st, 2nd and 3d go through those two)
That is the reason I would not use that gear

Privately I sent you links to a couple of gear clusters/ boxes which would work out a lot less than the suggested $100 per gear
or search out a gear set over here for you
You also said that you were unlikely to pursue the rebuild of the M21, which means you have a gearbox to hand that may be in a lot better condition, 30 minutes of your time will tell you

My motive was and is purely one of safety and of your well being,
I would not use a gear that is that pitted , remember you are now powering that B31/3 gearbox with a much more powerful BB engine !!!
I know BSA gears are tough but pitting leads to fatigue cracks and without x ray the depth of corrosion cannot be seen.
Having survived in my youth a seized gearbox I am lucky, but a friend and work colleague did not survive a similar event on a trials bike shocked

John

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Originally Posted by chaterlea25
OMG what have I started ???
OK, OK, I give up. I caved to the peer pressure and ordered a NOS gear from Australia. While waiting for airplanes and ships to connect that remote island with the world again, I'll put the rusty gearbox aside and plan on installing the M21 gearbox without rebuilding it if I get that far in the overall rebuild. However, I'll at least have to find a replacement seal since the rubber in the present seal has had at least 45 years to harden.

I'm also looking into involute cutters. Why? Because Cyborg has them and I don't, that's why. Should I find one for a 28T gear I'll experiment with reconstructive surgery on the allegedly-lethal gear.

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Originally Posted by Magnetoman
Originally Posted by chaterlea25
OMG what have I started ???
OK, OK, I give up. I........ordered a NOS gear from Australia.
Have you looked closely at the slippery slope* it mates with? It might be just as bad.

*(gear)

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The truth is I haven't changed my opinion about the suitability of the gear for my purposes, but that damned Cyborg sent me down an involuted rabbit hole. As a result, I want to experiment with reconstructive surgery on gears and for that I need a victim. And the appropriate involute cutter.

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MOD gear cutters 20 degree, MOD 2.5 is in there, select 42 per page and go to page 2

https://www.toolmex.com/CatSearch/231/involute-gear-cutters-20-degree-pressure-angle

and this guy has used cutters in stock at times but not listed on website

https://www.homeandworkshop.co.uk/

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There's a couple of interesting videos on you tube where a guy repairs broken gear teeth, the first one Here shows the missing tooth being built up by braze and then machined to form the tooth with the correct form.

The second video shows how a broken tooth is replaced by a new one silver soldered into a pre milled slot see THis Link. Its claimed that the silver solder is extremely strong. The new tooth is then cut to the correct form on the mill.

Presumably MM you will just be building up the pitting by brazing and then reshaping on the mill. However I thought I would share the above in-case you decide to completely remove a tooth and start with a new one.


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Originally Posted by Magnetoman
Originally Posted by TR6Ray
what possible difference does the condition of the driving face of the tooth make, relative to the strength of the tooth, if the trailing side of the tooth is rife with hollow craters?
Even with pits on the back face, the gear tooth is thick enough to handle the force. The problem comes if, say, half the driving face on a tooth has been eroded and if the remaining area is not enough to withstand the driving force (pressure = force/area). If that's the case, the driving face will begin to be knocked way, further eroding it and further reducing the area available to withstand the force.

Although bronze is softer and less abrasion-resistant than case-hardened steel, filling in the pits and resurfacing it would let those bronze-covered areas transfer the force to the steel underneath, while the remaining steel on the surface deals with the tooth rubbing across that of the mating gear.
What happens on the overrun when the bike is slowing against the compression of the engine, and the driving face becomes the trailing face. Does not the pitted side of the tooth have to shoulder the load?


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Originally Posted by kommando
MOD gear cutters 20 degree, MOD 2.5 is in there,
Thanks for that. I've now looked into it and this is a case where close isn't good enough. There is enough difference between the shapes of a 10 DP and an M 2.5 (=10.16 DP) that I need the Imperial version. Ideally, once adjusted, the HSS cutter will skim 0.0001" above the case-hardened surfaces of the existing parts of the teeth and only encounter the projecting, softer filler material. To the extent the profile isn't identical to that of the cutter originally used to cut the teeth it will make contact with those teeth, and HSS doesn't do well against case-hardened steel.

Originally Posted by gunner
There's a couple of interesting videos on you tube where a guy repairs broken gear teeth,
I looked for videos, but I didn't find any that were useful for the task I have. Several of them had people filling in missing pieces of teeth with weld, then "profiling" the repair with angle grinders. While that technique might be good enough for a Triumph, I'm working on a BSA...

Originally Posted by TR6Ray
What happens on the overrun when the bike is slowing against the compression of the engine, and the driving face becomes the trailing face. Does not the pitted side of the tooth have to shoulder the load?
True, but the deceleration on the overrun is less than the acceleration on full throttle so the force on the gears are less. As a reminder, BSA made all of the gears in this gearbox strong enough to handle the toque required to accelerate and decelerate rigs like this, whose total weight is ~2× that of the Clipper with rider:

[Linked Image]

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Originally Posted by Magnetoman
... I'm working on a BSA...
Quite.
That's what makes all this talk of measurement to the nearest Angstrom so risible.

[Linked Image]
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Not sure about those gentlemen that advised you not to get cutters. There’s always someone trying to apply logic to tool purchases! My gear cutting has been extremely limited and have mentioned before that the gear cutting section of the Machinery’s Handbook makes my head want to explode.
Personally I wouldn’t worry about the quill, it takes far more abuse with other forms of interrupted cutting, self excited vibrations and my mastery of feed and speed.
I know you are a devout follower and full time member of the insert clan, but tool steel can be you friend. Don’t see why you couldn’t use a fly cutter to remove whatever ultimately gets used to fill the pits. With light cuts and slow feed, I don’t think the mill would object too much. Just for the record, I think it’s a good idea to replace the gear. Add to that the likelihood that it will just haunt you if you don’t replace it. Just for sport, guys like me would start using avatars displaying pitted gears. Actually I did consider an avatar with the correct involute cutter worn on the appropriate finger, but that would probably be considered bad form.

There is also the option of mounting your dividing head on the lathe and use the carriage like a shaper.... it would probably take less than a month to set up. You could grind the correct profile on that piece of tool steel and just move the carriage back and forth by hand. Excruciatingly monotonous, but ultimately worked ok on my knockoff Lucas breather cap. Not a gear, but same general idea. Rigidity is a bit of an issue, so would likely need some final dressing by hand or lapping. Could explore using a tool post grinder too I suppose? Either way it falls into the category of “life is too short” for sane folk, but hey.... there wasn’t a cutter available that I know of and now I have what I think is a “reasonable” facsimile of a rather rare part.

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Originally Posted by Cyborg
Don’t see why you couldn’t use a fly cutter to remove whatever ultimately gets used to fill the pits.
I certainly could form a tool to an acceptable approximation of in involute shape, but that still would leave me without the actual involute cutters that you have, and I don't, and that would be completely unacceptable.

By the way, I'm not worried about the quill when trimming excess this way. The worry is the possible damage that could result when hogging 28 teeth from a thick piece of steel to make an entire gear from scratch. For that I would need a horizontal mill. Say, I wonder if I have room ...

Although I haven't found a used involute cutter of the right specs as yet, I have queries out so there's reasonable hope that one is likely to turn up soon.

Last edited by Magnetoman; 08/15/20 8:30 pm. Reason: deleted a question
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