# Breaking Plastic Gears??



## w6br

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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## Ulma Doctor

Hi Ron,
a new lathe sometimes may be a little tight. the plastic gears might not be up to the task.
i would recommend the use of steel gears to remedy your problem.
a general inspection would also be a good place to start too,
since i have no idea the materials you are trying to turn and a whole lot of variables i can only speculate on.
but i'm willing to help out in any event!


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## silence dogood

I would first turn the lathe by hand(unplug it first) and see and/or feel if there is any binding.   To the manufactures,  I realize that plastic gears are quieter and cheaper, but must they also be used as shear pins?


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## RJSakowski

What is your feed rate and depth of cut?  A high feed rate and deeper cut will increase the torque load on the gear.  An improperly sharpened tool bit will also increase the the torque load.  If there are high spots on the bar you are turning or you are making an interupted cut, the momemtum of the drive train could create enough torque to shear a tooth.

Bob


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## royesses

Make sure there is .002-.005 end play on the lead screw, if it is binding it could cause this. Also too much backlash can cause this. I used to keep my plastic gears right up against each other. Make sure the half nuts are adjusted properly and not binding the lead screw. Make sure the apron isn't binding on the ways. I switched to all metal gears for change gears and internal gears and think it was a very worthwhile upgrade. They are noisy but they don't act an expensive fuse. You can stall the motor with them.  Also all the other posters have given great tips on this.


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## Sharky

I'm thinking something is binding on the leadscrew or carriage.  Otherwise maybe gears are set too tight?

http://www.mini-lathe.com/ is a great source for "tune-up" tips on the 7x lathes.
I did quite a bit of adjusting my 7x10 prior to doing much with it, but the pushed it pretty hard.
I took some pretty heavy cuts on 2 1/2" cold rolled bar without any damage.

All was good until I crashed the tool post into the chuck  (yeah, that was bad)
The crash actually broke the plastic gears in the headstock.  A metal gear upgrade and new 16" bed from LMS and I'm pushing it harder than ever.  Still using the original change gears, although I had a little damage byt the keyway slot on the 80 T gear.


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## atunguyd

I would say to the opposite of what royesses said.  I used to find that if the gear mesh was too tight then this would mash up the gears teeth.  Set you gears mesh by placing a piece of paper between the teeth and tighten them up. 

Sent from my SM-N920C using Tapatalk


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## higgite

Hi Ron,

You didn’t say, so I’ll ask just for the sake of clarity, does the plastic gear break before or after the tool comes in contact with the work piece? Does all seem okay when the carriage is on auto feed but not cutting anything? Also, what is the metal, the rpm and the feed rate. In my experience and observation, if you give the guys on this forum enough detail, one or more of them will solve your problem. (Don't ask me how I know that. )

Tom


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## w6br

First of all, let me thank all of you who have so generously given of your time to answer my question!! 

I was turning a piece of Brass approx. 2" dia. and just wanted to clean up the outside surface. The Lathe was running about 1000 rpm and running smoothly. I had run it about 3 times with a feed of roughly 0.010" with the Power Feed on. On the third pass roughly in the middle of the piece I heard a loud pop, and no more power feed. The Lead Screw 80 tooth Plastic Gear sheared off approx. 3 teeth and producing a flat spot. Every time the gear comes around to that spot it stops. I had already replaced one 80 tooth gear due to warpage, and this was the new one. No, I did not use paper to size the clearance, but I adjusted the adjuster so that the Gear ran true and smooth (perhaps not??) After checking everything out I noticed that the reversing lever in the back was not mating well in the little detents to hold it in place. The spring is solid. I did check all of the operations before using the Lathe and cleaned and adjusted and lubricated the Lathe per mini-lathe.com instructions which are very detailed but I followed the instruction very closely. Like everything else, I could have missed something??
With the carriage on auto feed and not cutting anything it did run fine, as well as when cutting. Now, I was using a carbide tipper cutter with little infeed. I feel that I did not use very much pressure on the cutter and part, there again, what is little to me, but may be overload??  What I feel are defective parts, poor finish and  assembly. I will truly go over everything again and double check all adjustments and purchase a set of metal gears. It is a sturdily built little lathe, and I feel with some TLC and some re-assembly patience and correct assembly, I feel that I will have a good running Lathe. What say you??


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## atunguyd

What was your depth of cut.  Remember brass trends to hog in which can cause issues. 

Sent from my SM-N920C using Tapatalk


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## w6br

atunguyd said:


> What was your depth of cut.  Remember brass trends to hog in which can cause issues.
> 
> Sent from my SM-N920C using Tapatalk


My depth of cut was approx. 0.010 or so, not very heavy at all and all was running smoothly. I was trying to get a smooth finish and was being careful??

Thank you,

Ron


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## Baithog

Brass should turn in the range of 200 to about 300 sf/min. That would put the spindle speed for 2", even with carbide, at 500 rpm. Brass also needs tool angles different than most off the shelf cutters. At that speed, and with a GP cutter, it wouldn't take a lot for the cutter to hog and strip some gear teeth. Also, running smooth is not necessarily running tight and smooth. The little lathes and mills are notoriously sloppy. Any lack of rigidity can muck things up right quickly. I had my 7X lathe for 5 years. I turned mostly steel under 1.5". I never lost a tooth. I know others that could strip a gear just by looking at it wrong. You may be one of them. But keep in mind that if those teeth didn't strip, then something else, something much more expensive, could have broken. Even the bigger bench lathes like the 10X22 have a plastic gear in the gear chain to sacrifice itself in the event of a crash.


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## 54hawken

Baithog said:


> Brass should turn in the range of 200 to about 300 sf/min. That would put the spindle speed for 2", even with carbide, at 500 rpm. Brass also needs tool angles different than most off the shelf cutters. At that speed, and with a GP cutter, it wouldn't take a lot for the cutter to hog and strip some gear teeth. Also, running smooth is not necessarily running tight and smooth. The little lathes and mills are notoriously sloppy. Any lack of rigidity can muck things up right quickly. I had my 7X lathe for 5 years. I turned mostly steel under 1.5". I never lost a tooth. I know others that could strip a gear just by looking at it wrong. You may be one of them. But keep in mind that if those teeth didn't strip, then something else, something much more expensive, could have broken. Even the bigger bench lathes like the 10X22 have a plastic gear in the gear chain to sacrifice itself in the event of a crash.


When you said it happend on the third pass with your speed what it was I would guess your part probably gained .005 in diameter, thus increasing your depth of cut by approx. 50%.. Also if your insert was zero positive rake angle or chip breaker it tends to "suck" into part, and even more so when more pressure is applied due to diameter growth by heat. Using inserts with a negative rake angle should be used cutting brass. ( negative will appear to the eye like you are scrapping the metal off,  positive will appear to your eye that it is cutting the metal off)   If this fixes your problem on brass try steel same size stock using proper rake insert, proper feed and speed  and reduce depth of cut 50%.  I worked for Leblond as a field service technician for 10 years among other things I did for a living.  Been retired a few years but still in my workshop machining several days a week at 71   Good Luck to you  Don


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## w6br

Well Folks, Duh!!
I feel like a real Dummy!! I re-read all of the postings that were so greatfully replied to me and decided that there must be something amiss, so I tore back into the Lathe. Well I took the Carriage apart again, and took out the Split Nut, cleaned out all of the grease and guess what, some of the threads were buggered. Evidently, when it was molded some of the threads came out not formed and when the half-nut was threaded the threads didn't turn out so good. Eventually by using the Power Feed it worsened the threads and caused the Lead Screw and Split Nuts to bind shearing of the 80 tooth Plastic Gear!!
Now, in addition I am going to inspect the Lead Screw with a fine tooth comb!!  I hope nothing else is bad. Anything else you Gentlemen suggest that I do.

Ok Grizzly here I come. I guess a number of your suggestion are correct in inspecting and THOROUGHLY cleaning the equipment before operation!! A word to the Wise!! Well, with diligence, WE finally found the problem and my heartfelt Thank You to you All.

That's what happens when you are a Newbie!! Thank God for this Site and all of your help!! 

Merry Christmas,

Ron, w6br


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## royesses

w6br said:


> Well Folks, Duh!!
> I feel like a real Dummy!! I re-read all of the postings that were so greatfully replied to me and decided that there must be something amiss, so I tore back into the Lathe. Well I took the Carriage apart again, and took out the Split Nut, cleaned out all of the grease and guess what, some of the threads were buggered. Evidently, when it was molded some of the threads came out not formed and when the half-nut was threaded the threads didn't turn out so good. Eventually by using the Power Feed it worsened the threads and caused the Lead Screw and Split Nuts to bind shearing of the 80 tooth Plastic Gear!!
> Now, in addition I am going to inspect the Lead Screw with a fine tooth comb!!  I hope nothing else is bad. Anything else you Gentlemen suggest that I do.
> 
> Ok Grizzly here I come. I guess a number of your suggestion are correct in inspecting and THOROUGHLY cleaning the equipment before operation!! A word to the Wise!! Well, with diligence, WE finally found the problem and my heartfelt Thank You to you All.
> 
> That's what happens when you are a Newbie!! Thank God for this Site and all of your help!!
> 
> Merry Christmas,
> 
> Ron, w6br


Ron,
         Thank you for letting us know what you found. Now to store that in the memory banks for future use. Check the alignment of those gears. Mine takes a .062" flat washer between the banjo and mount stud. Also I use hardened flat washers under the hold down nuts as the soft washers will deform when torqued and allow slip. The banjo moves around on these little machines and causes excessive backlash under load.
Merry Christmas and Happy Machining.
Roy


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