Fixing grinding/interference on bevel gears

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Tillies in a Bottle
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Fixing grinding/interference on bevel gears

Post by Tillies in a Bottle » Wed Apr 07, 2021 10:16 pm

Hello Home Machinist Community,

I'm glad I finally joined instead of just observing.

I am about to take a crack at fixing the bevel miter gears on my milling machine that are doing what I'll call grinding.
These are the gears that turn the elevating screw on the knee of my Nichols horizontal milling machine.
as far as I can tell they've been in this state for a while, certainly for the couple of years I had the machine. What I'm seeing is that the knee is very stiff, and will clack over every tooth between the bevel gears. It's certainly not smooth and easy like it should be. I hardly use the knee for this reason, not wanting to break any teeth.

Anyway I'm going to go about fixing that finally.
I got the knee off and I can see two things right away.
First that the smaller gear that the handcrank would drive is backed off, so that the two gears are not fully meshed.
The other thing is I'm seeing something like burs or spots on the teeth where the metal has smooshed over.
Now that the knee is off the machine and with nothing loading it I'm seeing that the smaller gear will bind a little at one spot. That's the spot where I marked with a little white dot.
The gears
I'm going to first make a bronze bushing to go around the smaller gear's shaft and bring the gears in mesh. I'll then see how it behaves at that point and re-assess the cause of any interference. Maybe I'll stone off some burs if I think they're on the teeth and up to no good.

I know that the knee on a bridgeport is essentially the same, with bevel gears turning the elevating screw. Maybe someone here on the home machinist had a similar problem and can say something about how he went about repairing it.
In any case I'll be posting as I go through with the repair.

the knee
nichols knee.png
An exploded drawing of the knee from a variant of my machine

Bob D.
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Re: Fixing grinding/interference on bevel gears

Post by Bob D. » Wed Apr 07, 2021 11:02 pm

I suggest removing both gears. Clean them up to remove all grease and oil. Use Dykem or a marker to paint up all the teeth. Then spend some time at the bench with a file and work your way around each tooth. The Dykem removal will show your progress. The important thing is to get rid of high spots. Don't go nuts with the file. Just light accurate strokes following the tooth profile to even things up and establish as much uniform contact area as you can. The important thing is getting those high spots as that is what can break a tooth under load. Clean everything again. Put the gears together and get the mesh the best you can. If you put Prussian blueing on one gear you can check the contact area after rotating a bit. The bluing will transfer to the clean gear where there is contact and lighten up on the blued gear where it contacts.
Since nothing is broken you should be able to really improve it. You don't need or want a super tight mesh.
Good luck.
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Re: Fixing grinding/interference on bevel gears

Post by NP317 » Thu Apr 08, 2021 1:12 am

And please let us know what you discover is causing the general binding of the knee movement.

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Tillies in a Bottle
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Re: Fixing grinding/interference on bevel gears

Post by Tillies in a Bottle » Thu Apr 08, 2021 12:12 pm

Thanks Bob, I'll give it a shot, and definetely have some photos of the progress allong the way.

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Re: Fixing grinding/interference on bevel gears

Post by pete » Thu Apr 08, 2021 3:29 pm

Setting the engagement depth between the two gears is similar but a lot less fussy than doing the same on an automotive ring and pinion. Bob's trick of using Dykem to check for high spots and how the teeth are meshing is a good one that I've used a few times. Maybe the assembly drawing isn't that clear or I've missed it, but there doesn't appear to be any method of easily setting or adjusting for tooth engagement. Another trick I've used once you do get the damage filed off and you know there then correctly engaging is to use paper of various thicknesses between the two gears to set the gear lash clearance. You can get paper in increments of .001" on up to set whatever tooth clearance you want. With yours it looks like shims might be required behind each gear to reset them to take up any wear. Also check your bushing to shaft fit where the shaft that turns the gear with the knee crank bears against. If that's sloppy you'll never get a decent and smoothly operating set of gears. It's an often neglected area for lubrication and might have created a large part of the problem.

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Re: Fixing grinding/interference on bevel gears

Post by Tillies in a Bottle » Wed Apr 14, 2021 11:35 pm

Thanks Pete, yes there is some chance that the bearing fit is at least involved in the interference I'm seeing. The "crank shaft" rides inside two bronze bushings, an outer one closer to the crank and an inner one closer to the gear. It looks to me that I would go about setting the tooth engagement by measuring the position of the inner bearing when gears are turning well, and then take it out again and locktite it there. It looks to work well when its sticking out from the casting around 5/16".
bearing in knee.png
The bearing as it determines the gear mesh
The funny thing is that the shaft was turning freely (aside from the gear interference) before I removed it but once it went back in, it was binding pretty severely. A good part of my time was spent stoning and polishing the shaft and looking for any burrs in the bearings, and getting the whole area flushed out. I checked the shaft for run out and I saw a difference of .006 between one end and the other.
Shaft on the lathe showing .008 run out in the same area where gear teeth are interfering.
Aside from that, I went through the small gear's teeth and found the couple of teeth that were binding as they meshed. those couple of teeth lined up with the high spot of the run-out on the shaft. I put some hi-spot dye on the large gear and saw that the tops of the teeth were not making contact, but only the flanks. I'm sure it's a sin to file the flanks on Involute gears, but that did it. It took way more filing than I think sensible too, but the binding spot went from wrench tight to hand tight, though not yet as free and easy as the rest of the gear's travel.

There is still more going on with the knee, and I'll really see it once it all goes back on. the thrust bearing on the elevating screw seems to need a spacer in order to do it's job. The lack of this spacer might just be the cause of this whole mess. Without it, the whole weight of the knee, saddle, table etc is resting on the gear teeth squishing the two gears together. maybe it was enough to bend the crankshaft that .006 I measured.
The knee. There may be a spacer that has been missing for as long as I had the machine.
smaller gear with blueing.jpg
smaller gear with Hi-spot on the flanks

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Re: Fixing grinding/interference on bevel gears

Post by Harold_V » Thu Apr 15, 2021 1:32 am

The tops of teeth should NOT make contact. Filing the face of teeth to ensure that they do is a mistake.

I question your assessment that the bearing location is intended to create proper lash of the gears. That makes no sense.

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