# Help diagnose surface imperfection - possible leadscrew related issue?



## compsurge (Jan 31, 2015)

Hi all,

I acquired a SB 10L a few months ago and have been learning about how to operate it. In my use, I have noticed surface imperfections. At first, I would describe it as resembling this quite a bit: http://www.deansphotographica.com/machining/projects/109/rebuild/109b.html

This image best shows the imperfections on the left-most portion. This is a "trial piece" of aluminum that I all but broke as I abused an HSS tool bit as I was playing around. You can see the remnants of the too-high-feed areas. The very smooth areas in the right half still have a trace pattern in them that matches the imperfection in the first few clean areas. For reference, I was using a Korloy CCGT at the recommended feed rate. I put it close by in the video for reference.





I used the website for helping identify that my compound and cross slide leadscrew nuts were both loose and the gibs on the rear of the saddle could be tightened down to help minimize the rocking. I have play in my compound, and I have run it all the way out to lock it in place.

I took the saddle and apron off of the lathe and did an inspection. For the most part, the gears look good. The ones that operate the power feed are in great shape. The handwheel wobbles slightly and the pinion gear that runs on the rack is worn. The gear on the handwheel shaft is not too badly worn. Upon futher inspection, I found the worm gear has some axial play and the bushings inside the casting will move along the axis as well. I would guess there is about 0.005-0.010" of clearance between the pinned collar and the bushing after moving the worm gear axially.

I have posted a video here from after I had run the lathe with the above test article and after I made some changes:
https://www.youtube.com/watch?v=BG3cSyQnIWc (ignore where I say the patterning may be normal  )

Here is a video from after removal of the saddle and apron:
[video=youtube;bozK3RhR-m4]https://www.youtube.com/watch?v=bozK3RhR-m4[/video]

Here is a photo of the key (the other side is less worn):




Any thoughts on the key and the worm gear leading to that surface imperfection? If so, does anyone know where can I get a replacement key or should I get some key stock and make my own by milling a slot out of it? If making a replacement, what material should I use?


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## 4GSR (Jan 31, 2015)

Put your apron back together and put back on the lathe for now.

I see several things in your picture that could be causing the effect shown. First, I don't like using CCMT tooling on smaller lathes unless you can push the proper chip load to make it cut.  Most of the time you can't do that on a small lathe.  I know people use them.  Going back to the problem you are having.
I had the same problem on my 9" SBL a couple years ago when I was turning a larger OD of about 3".  Wasn't taking a heavy cut, just a light cut, using triangle insert tooling, or maybe DCMT style insert.  I don't remember exactly, I do use both.  Anyways, I was getting the "lines" in the work about every .060".  I know my lathe is in very nice shape.  I have gone through it 12 years ago and reconditioned it.  What I found out and determined what was happening, the saddle was riding "high" on top of the vee ways on the bed.  Wasn't much, maybe a .001" at the most, but it was enough to cause the problem similar to that in your picture.

I went in and deepen the relief groove in the bottom of the vee on the saddle with a 1/8" endmill.  Then I took impressions with blueing and found where I needed to do a little scraping for a better bearing fit.  Once I did all of that and put it back together, its been doing fine ever since.


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## compsurge (Jan 31, 2015)

Interesting. I'll put it back together for now and see if I can repeat it. I had the same effect using ground HSS, but I may need to hone it up. FWIW, the patterning is worse near where the ways are worn more. Your suggestion might bear some direction as to the physics of what is going on. Thanks!


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## compsurge (Feb 2, 2015)

After some more reading, I'm starting to think my initial suggestion of the worm gear movement is likely key. I'll try verifying the leadscrew is set right as well.


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## stupoty (Feb 2, 2015)

Have you tried it back together without the feed screw and run the cuts using the hand wheel?

if you get bad finish patterns still at least you have eliminated the lead screw and if it disappears then some thing in the drive is making it jump (maybe the lead screw)

Remember most importantly don't lose any bits 

Stuart


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## 4GSR (Feb 2, 2015)

compsurge said:


> After some more reading, I'm starting to think my initial suggestion of the worm gear movement is likely key. I'll try verifying the leadscrew is set right as well.



*You key is not the problem*.  In fact, it is in very good shape!  Compared to many I've seen in my past.

My 13" Sheldon lathe has a key that is worn so bad, the last time I checked it, it was only about .045-.050" thick!  Still running fine!


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## compsurge (Feb 2, 2015)

@4gsr, I apologize for the confusion. I was stating the worm gear axial movement is likely the issue as shown in the linked post. I was simply stating that was _key_, not the actual key was the problem 





stupoty said:


> Have you tried it back together without the feed screw and run the cuts using the hand wheel?





stupoty said:


> if you get bad finish patterns still at least you have eliminated the lead screw and if it disappears then some thing in the drive is making it jump (maybe the lead screw)
> 
> Remember most importantly don't lose any bits
> 
> Stuart




If memory serves, I only observed the issue under power feed. I will try it again without power feed to verify. 


Thank you for the help and ideas.


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## 4GSR (Feb 2, 2015)

Axial end play in the worm could but doubtful. How much endplay are you talking about?


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## compsurge (Feb 5, 2015)

Axial play of the worm gear and bushings in the casting is measured at 0.028". In line with the post on Practical Machinist (one similar issue occurred with just 0.019" play), I am going to put some shim stock in place to bring the axial play to about 0.004-0.005" clearance.


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