Easy Shear Lathe Gear Shaft Key

[mattthegamer463]

My craftex 10x18 lathe doesn't offer access to the shear "pins" shown in the drawing without total disassembly of the feed box, and probably feed screw removal too. Pretty dumb. I'm going to assume the pins are steel and may not be prepared to shear before my change gears shed some teeth. I want to figure out a solution that will guard my gears without a major teardown.

Looking at the change gears, there are lots of keyed shafts that I could easily replace keys on, removing the sturdy steel one for some weaker material so it will fail if I crash.

Thing is I'm not sure of a good material that will shear vs. mush and maybe not release the shaft, but also not wear out in a week. I also am not sure which key to replace but seems like the key for the gear on the opposite end of the feed screw should be subject to the same torque a pin would be, probably a sensible choice.

Brass square stock maybe? Any ways to test before the bad day happens?

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[enginuity]

My 10x18 has a shear pin on the leadscrew which is easily accessible. How old is yours? I'll try to grab a picture and post it - perhaps you could just modify yours to match?

[flylo]

Everyone else hated them but I liked the idea of one sacrificial plastic gear in the H_F 9x20 lathes & they sent 1 or 2 spares like they send extra sets of brushes.

[mattthegamer463]

My 10x18 has a shear pin on the leadscrew which is easily accessible. How old is yours? I'll try to grab a picture and post it - perhaps you could just modify yours to match?

Looks like I need to get back to questioning my own sanity again. Looking at a pic on my phone I can see one where I see one on other people's machines on google images.

Guess I'm the dumb one, not the lathe makers.

I'll check it tonight and make sure it's made of suitable material.

[Arcane]

The flywheel key used on many Briggs and Stratton engines is supposed to be sacrificial and is made from aluminum but I don't know what alloy.

[mattthegamer463]

The flywheel key used on many Briggs and Stratton engines is supposed to be sacrificial and is made from aluminum but I don't know what alloy.

My thinking would it would have to be hard so it splits in half cleanly, and also so it doesn't mash into a mangled blob over time.

[Mcgyver]

Brass is a common shear pin material. I'd keep at it and find and replace the shear pin rather than change out the keys. The force required to shear something is a function of the area being sheared.....and I'd guess even those small keys are many times more area than the shear pin itself. Torque also comes into it, if those gears are spinning a lot faster than the shaft with the shear pin on it, they'll be harder to shear. To avoid the tear down in the future you'd have to modify the leadscrew and feed shaft so that they were pinned outside of the gearbox....or make clutches.

[David Powell]

IF you learn to concentrate on the job.

You are very unlikely to end up having a jam up. I had one spectacular jam up some 40 yrs ago, broke shear pins, gear teeth, bent a boring bar. Nearly cost me my job, have never had a major jam up since . Threading to a shoulder or in a blind hole is the most likely to catch you unawares. Its a bit like turning across traffic in your car, you know that if you get it wrong you will be the victim, and you will be the one who gets the blame. Regards David Powell.

[redgrouse]

Hi not familiar with your lathe, don't see them this side of the pond but unless you can get the correct spec start with something weak -- better too weak than too strong, if it shears well try something a little stronger.
My Emco Maximat has a sheer pin on the lead screw [slipping clutch on the feed bar] the pin is alloy and I made some spares from soft alloy rivets --did not like the price of the one supplied by the importers -- think I could ear a couple of hundred pounds and hour making them !!
Hope this helps a little John

[bob308]

I have seen keys made out of two pieces of steel pined together with two pieces of 1/8 brazing rod. if something happens the steel keys shear the brazing wire.

I bought a burke 333 mill some one had put lathe bits in for keys. I replaced them with 1/4 '' brass keys. even use a brass key on the arbor.

[mattthegamer463]

Turns out there was a pin. It had been ground flat and the general gunk obscured it from my view. I bashed it out and replaced it with 1/8" brass.

Thanks for the help gang. I like the idea of pinning two pieces of steel together with brazing rod so the rod shears easier than the whole key trying to split in half.

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[CalM]

Good discovery!

Not so long ago I went through a similar process with a Weiler lathe. The shear pin was "hidden" beneath a fiber gear.
Did that make a mess of threading!

Now knowing the "weak link" It is easy to feel confident that all is well as long as no "crashes" have been experienced.!

[Spin Doctor]

LeBlonde's and others I assume used to use a spring loaded detent on the feed shaft. If the feed pressure was to great the ball would climb out of the detent. I'm familiar with your lathe. I've got the longer version Matt at Quality used to sell. If you want a safety in the gear train I think a system where you have an idler with a shear pin similar to a snow blower might be the best option. The taper pin on the Leadscrew is not meant to be a safety item.