# Breaking Plastic Gears??



## w6br (Dec 12, 2015)

Gentlemen,
I am having some frustrating moments with my new G8688 Lathe!! It seems that when I attempt to do some cleaning up of round metal and I lock it on the Lead Screw, I wind up shearing the 80 tooth Plastic Gear for the Lead Screw. I have replaced the Plastic Gear twice now and it still continues to happen?? I know there are metal gears available, and these are not on my priority list, there are other truly needed things that I want to purchase. Perhaps that is the answer??
I have made certain that I have adjusted the adjuster properly, I think, according to the manual and other reviews.

Any suggestions and help??

Thank you and have a Merry Christmas to All, and to All a Good Night!! 

Ron, w6br


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## tmarks11 (Dec 12, 2015)

The plastic gear is there to prevent real damage from occurring when you crash the cross slide.

If you are stripping the gear (and haven't crashed it), then I suspect you are taking a cut to heavy for the machine or have a feed rate too fast for the material.

1. What material are you turning?
2. How deep of a cut are you taking?
3. What feed rate do you have selected (determined by the change gear arrangement)?
4. What is the spindle speed?

Generally Feed & Speed selection is where most people who are starting out in this hobby run into problems.


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## w6br (Dec 12, 2015)

Ok, I have a Bronze tube basically that is rough, and all I wanted to do was to clean it up. I was using a Carbide cutter at high speed with not very much pressure and just very little infeed, say 0.010. I managed to do two or three passes and on the last one I heard a POP, and no more feed on the Lead Screw??

Thank you for your help!!

Ron, w6br


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## tmarks11 (Dec 13, 2015)

So bronze requires about 200 sfm (HSS) or 700 sfm (Carbide).

If it was a 1" tube, that equates to a spindle speed of rpm = sfm*4/D = 200*4/1 = 800 rpm (HSS) or 2800 rpm (Carbide).

Since you aren't using coolant, I would cut that in half to start (400 or 1400 rpm).

Still, none of this explains why you have trashed two gears, especially if you were only taking a 0.010" cut.

Can you easily manually traverse the carriage the whole range of the bed without any binding? Also, take a look at the leadscrew near where the half-nut was clamping it each time you stripped the gear.  Maybe the thread is deformed there and locking up on the half nut when it tries to pull the carriage by.

Just a note: it takes a good amount of pressure for carbide to cut, and your mini-lathe will (in general) work better if you use HSS instead of carbide (takes a lot less cutting force, and will give you a better finish on this machine).  If you don't want to grind your own HSS bits, look into the indexible ones you can get from AR Warner.


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## Bill C. (Dec 13, 2015)

I never understood why any manufacturer would use plastic gears. I have sheared key stock before, that used to be the safety in the most of the machines I had used.  

Good luck


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## w6br (Dec 13, 2015)

Bill C. said:


> I never understood why any manufacturer would use plastic gears. I have sheared key stock before, that used to be the safety in the most of the machines I had used.
> 
> Good luck


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## w6br (Dec 13, 2015)

Well, I finally figured out what caused the Plastic Gear to Break - the damn thing was out of round!! It was egg shaped so it was binding on the high spot where it sheared. Cheap assed parts anyway!!
Well, as I said earlier, no more plastic I am ordering Metal Gears from LMS, or does anyone know of a cheaper place??

Thank you all again for your help and thoughts and you ALL have a Very Merry Christmas!!

You guys are The Greatest and what a joy to be a member here!!

Ron, w6br


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## wrmiller (Dec 14, 2015)

I had no complaints about the gears from LMS. As others have said, they make a bit more noise, but you can at least get on to more important stuff and quit worrying about stripping a gear.


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