Nylon milling machine gears

[J Tiers]

Is it axial? I don't know what is under the cover in there, but most spur gears will push apart under load. My Logan has a factory latch to hold it in back gear, preventing the gears from popping out of mesh.

[Willy]

Is it axial? I don't know what is under the cover in there, but most spur gears will push apart under load. My Logan has a factory latch to hold it in back gear, preventing the gears from popping out of mesh.

You're likely right, lots of radial thrust between two spur gears.
I do have to question the engineering behind a gear set that requires band aid solutions to keep gears in engagement. It may work but fundamentally it's only bad design practice to rely on such methods.
Can you imagine requiring such methods on a manual transmission in a car to keep the counter-shaft and the main-shaft in engagement under load?

I suppose it's cost effectiveness for the intended market and anticipated price point that is the determining factor.

[J Tiers]

Well, they kind of DO keep everything together that way.... Although the gears slide together, the case is designed to hold the gears in mesh and hold them in mesh at the power the tranny is designed to handle.

Almost EVERY back gear on common lathes will use an over-center mechanism to hold the gears in mesh, because they are moved in and out of mesh radially. I assume that some such system is inside the box on Brian's mill. The issue is that either the over-center is poorly designed, or they never put one in, and rely on friction to hold the gears.

I suppose it's possible that the dial turns a screw (pretty steep pitch, though) that moves gears axially, or a lever that does the same. in that case, it may only need a latch to keep it from vibrating out of position.

That last seems pretty possible, given the way the gear looks as though the teeth were ground up all but right at the one end of the flanks near the bigger gear. Maybe the "top" end wears first, and then it gets worse as there is less tooth to carry the load until the last part fails.

I admit to being pretty curious just how it IS set up. Maybe Brian can get a photo when he replaces the gear.

[Willy]

Exactly, the case is strong enough to hold the the supporting shafts together despite the anticipated radial thrust forcing them apart. That's good engineering.

Gear sets that are forced apart due to an environment that isn't designed to accept those loads is called bad engineering.

There are more user friendly alternatives to a mechanical fuse than a deeply embedded set of costly gears that are doomed to failure.

[J Tiers]

.............

There are more user friendly alternatives to a mechanical fuse than a deeply embedded set of costly gears that are doomed to failure.

Totally agree on every possible level.

Always possible that the plastic was for cost, or noise, and not intended as a fuse. It seems to me that those also are bad design, especially since Brian's issue with them is not actually the fault of overloading, but an unrelated mechanical fault in the design.

If the gear is not allowed to fall out of mesh in whichever direction is does so, maybe it will then last the life of the machine. We don't know that.

[Video Man]

On the issue of $70 gears, if you can create an .stl file of the gear from your cad program, you can have them commercially printed very inexpensively. There are any number of vendors, I am using Jawstec for glass-filled nylon gears for a project I'm working on and the printed parts are very affordable. (No personal connection to the vendor.) I'm using FreeCad because, well, it's free, and it has an excellent gear production module.

[psomero]

I too have the same machine.
Once the gear stripped out the first time I converted the machine to belt drive with a 3HP three phase motor and VFD.
No problems since then.

A 3HP motor is what I had on my old RF45 clone. I quickly concluded that the machine was not stout enough for that much torque and the column would deflect if I pushed it hard using larger holesaws in chromoly tube.

I can't imagine what would happen on this class of machine, being like 1/3 the iron...

[Boats69]

Does it have to be a plastic gear?

I'd think that an aluminum on would be easy enough, and would hold up better. If the plastic meshes with other plastic gears, then I suppose it would be more work. I am not a big fan of plastic gears used as a "mechanical fuse", but a lot of manufacturers seem to be, or else they have discovered that plastic is cheap.

Different teeth count but similar design and problem. CI replacement gears solved that - there is still a more accessible plastic gear in the gear train, if a fusible gear is really required.

[rdfeil]

Another option for machine safety would be a lower strength key or a simple shear-pin on a shaft interface. A more complex system that requires no disassembly to reset is what is used on my LeBlond lathe lead screw. It has a spring loaded steel ball in a hole in a sleeve pressing into a radius pocket on the shaft. This allows the shaft to stop rotating in the event of a crash and the sleeve continues to rotate from the gearbox. Stop the machine, uncrash the carriage, and restart the machine. The ball will reseat and all works again. The tension on the spring holding the ball is adjustable and sets the force needed to unseat the ball and stop the lead screw. This is much more complicated, but very cool .

[DrMike]

I must be missing something here.

The operator of this benchtop mill reports that it works fine except when he occasionally drills 1" holes in aluminum. When he does this, he strips the expensive and hard-to-replace nylon gears in the transmission. The first indication that the gears are failing is that the transmission won't stay in the low-speed position.

And the suggested "fix" is to mechanically force the transmission to stay in the low-speed configuration, and increase the damage on the gears???

How is the solution to this problem not "stop drilling 1" holes in aluminum"?

It's clear that this mill, for whatever reason, can't perform that operation without tearing itself apart. It doesn't matter if one thinks the mill "should" be able to handle the job, or that other methods take longer. The mill doesn't seem to care that anyone, including the operator, thinks that this is a reasonable thing to ask of it. Since it has happened more than once, it's perfectly clear (at least to me) that this mill cannot handle this task. Adding a mechanical "third hand" to force it to perform this operation will lead only to more frequent, and perhaps even more costly, repairs.

My suggestion? (1) Fix the mill with the gears shown. (2) From now on use another way (and there are many) to make these holes that won't have the mill tearing itself apart. Either that or find another larger, stiffer, more powerful mill that will perform this operation at the speed you are looking for without damaging itself.

[brian Rupnow]

I have never seen an article or a "how to" on replacing the gear drive with a belt drive on my particular machine. If anyone has that, I would certainly appreciate a link or referral to it.--Brian

[brian Rupnow]

This is the shape of the nylon gear I currently have. The tooth form is an approximation
The gears are measured with a Vernier caliper.

[J Tiers]

I must be missing something here.

The operator of this benchtop mill reports that it works fine except when he occasionally drills 1" holes in aluminum. When he does this, he strips the expensive and hard-to-replace nylon gears in the transmission. The first indication that the gears are failing is that the transmission won't stay in the low-speed position.

And the suggested "fix" is to mechanically force the transmission to stay in the low-speed configuration, and increase the damage on the gears???

How is the solution to this problem not "stop drilling 1" holes in aluminum"?...........................

It COULD be that "solution".....

But the issue seems to be that the thing partly drops out of full engagement, putting the full stress on only part of a weak plastic gear. There is no suggestion that it breaks "reliably" as soon as the drill gets to full diameter, based on the description of the problem, and the picture posted.

Nothing wrong with using a cutter that takes less force to drill. That's entirely reasonable.

But from the info presented so far, it seems that the problem is more with the mechanism of engagement, and not so much with the strength of the gear. The "operator" is quite familiar with mechanical things, which he actually designs as a business. His comment is:

"When it first decides to eat that gear, it jumps out of gear into neutral. If you can spare a hand to hold it in gear, it works just fine, but sometimes I run out of hands to do that. It doesn't take much force to hold it in gear, and I'm sure that if it was held firmly in gear so that it couldn't jump out, then it wouldn't wreck the gear."

[DrMike]

But the issue seems to be that the thing partly drops out of full engagement, putting the full stress on only part of a weak plastic gear. There is no suggestion that it breaks "reliably" as soon as the drill gets to full diameter, based on the description of the problem, and the picture posted.

Perhaps this failure is an issue with gear engagement, in which case one would expect it to occur whenever the gear train is placed into the low speed range and is stressed, and not just with this one particular operation. That possibility, however, is directly contrary to what was reported, to wit

"This milling machine does everything I want, except drill 1" holes in aluminum. I don't do that very often, but when I do, it's mostly ornamental holes thru flywheel webs. And this is the point where my mill lets me down."

[brian Rupnow]

All right!!! You fellows have convinced me. Time to make a raid on the Rupnow Fortune and buy Three annular cutters, a 1", a 15/16" and a 7/8". These things come with a 3/4" shank, and I don't have any R8 collets that are big enough, so I will buy 3 dedicated R8 collets which will take a tool with a 3/4" shank. I will still go ahead and fabricate the aluminum arm which will prevent the mill from jumping out of gear when in low range under heavy load. Thank you for your help.---Brian