Irregular advance of cross feed

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ericc

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Hi. I have a problem with my Craftsman 109 lathe for turning down the diameter of a piece, especially when it needs a precise result. The cross feed screw is 24 tpi, so one turn will take off 0.042" approximately from the radius, or 0.083" from the diameter. This is too much and bad things will happen. It seems to cut well if it is fed about 0.10". The problem is that the feed is not consistent. Sometimes it will not feed at all, then, suddenly, it will feed too much and stall the lathe, dig in the tool, leave a stupid ring or something negative like that :whiteflag:

So, I mounted a dial indicator in the tool post and indicated against the chuck jaw. Most of the time the feed is consistent, but occasionally it will feed almost none, then suddenly catch up with just a slight turn of the handle. A lot of people have criticized the lathe as not having a dial on the cross feed causing accuracy problems, but the dial indicator should solve that and it does not. There must be something else going on. I took a really good look at the screw, and it looks fine. The feed is not smooth. During some parts of the angular position (about 6 o'clock, if this means anything:think1:).

I guess I could work around the problem by using an indicator to monitor the feed and just being really careful near that rough spot. The funny thing is that there is no corresponding rough spot in the screw. Screws tend to wear tapered unless they are run over by a car. The average traverse is right on. If the cross slide ever slows down, it always catches up, even in just one turn. That "catching up" appears to be the source of my digging in problems. Because of warnings on the net, I have kept the belt loose so that I don't break a tool, but I see how it could happen. Most of the time, things are working well, then all of a sudden, the piece is undersized.:panic: Is there something wrong with the screw that I can't see?
 
It sounds like the nut is not secured to the cross slide, or has damaged threads. Could it be that you are not removing all of the backlash by cranking out then cranking back in? I would take it apart and take a close look at the leadscrew and the nut as well as the attaching parts.
 
I get a little of that on my 9x20, but it's flex in the compound/cross and not anything to do with the feed. When I turn the feed dial one tick and no cutting occurs, I can sometimes thump the cross real sharp with my index finger and take off .002 just like that.
 
Could remove the crossfeed screw and check that it's perfectly straight, especially where it fits in the bushings. A 'bump' on the feed handle during moving and transport can cause this 'bent shaft'. Also, ensure that all the axial load bearing surfaces are perfectly straight and true. Sometimes, adding a thin thrust washer or two can help...
 
also check that the screw is not sliding back and forth within the bearings at the ends. you could also have a binding gibs.
 
Thank you for all the suggestions. The problem is not failing to account for backlash. The cross slide screw/nut has a lot of backlash, but as long as one just turns in one direction, that should not matter. Part of the backlash comes from the loose nut, the rest from the slop in the collar and the cross feed handle. The problem is irregular feed. I will take the cross slide apart and take a good look at the nut and screw. I am also suspicious of what SG51Buss mentioned. The surface that the collar bears against may be irregular. But, the collar is round, and some irregularity would just cause it to turn cocked, and not vary in and out with clock position. It seems that the problem always only occurs in the infeed direction, and only at one clock position.
 
Hi. I took the cross feed screw apart and figured out what is wrong. There are multiple things, but I think that the worst one was related to one of the suggestions above. It turned out that the screw was OK. It was not bent, nor was there any asymmetry. True, it was a loose fit. Some online resources said that the nut can be a loose fit in the cross slide. This turned out not to be the case. The main problem with the screw was that it was worn a little in the center and a sloppy fit in the nut all up and down. As I pointed out before, this will not lead to jumpiness in the feed.

The jumping was found to be caused by two problems. First, the inside collar had worn into the hole in the carriage. This won't cause jumping, but the pin holding it in protruded a little, and bit off a piece of the carriage. It was falling into this little divot every rotation, or almost every rotation, and causing the observed problem. Another problem was that the hole in the carriage for the cross feed screw was oversize. This caused the screw to bind and precess (is that the right word? :think1:).

The first problem was solved by making a tiny brass ring soldered together with silver bearing solder. It filled up the little divot worn into the inside of the carriage. The problem, then, was that it was not possible to tighten the handle back on the cross feed screw and pin it. I drilled a center hole and threaded for 8-32. Then, I made a thin wire cotter key, and used it to replace the pin. Then, I inserted the center screw and tightened it into the makeshift cotter pin causing it to bind and forcing the handle down. The screw was still wobbly and now binding, but the irregular feed problem had gone away!

Eventually, I will fix the problem in a better way, but this is much much better than it was previously. Thanks all!!!! :drink2:

In retrospect, an observation I had is that the generally held belief that American iron is superior to import is not always the case. Recently, I saw an import lathe, probably Chinese, at an estate sale. It ended up selling for something like $900, which was out of my range. But, the cross feed screw action was silky smooth and tight, and it had a nice dial on it.
 
Glad you got it sorted out. Pics of what you found and how you fixed it would have been a bonus!
 
I have a 109.20630.....

The cross slide nut is cast into the bottom of the cross slide, so it's not that.

Are you talking about rings when facing a part of turning the diameter?

Rings when facing is a sign of loose gibs, make sure everything is really tight. Including the front headstock bushing.

Rings when turning the diameter is caused either by a bent leadscrew, or the rear carriage hold down bracket. In my case, I epoxied a piece of brass shim stock because the bracket bottomed out. When it's tight is gives a decent finish.

Here is Dean's many solutions to fix the 109s' issues.


http://www.deansphotographica.com/machining/projects/109/109a.html
 
Hi. I did not take any photos, since the problem was difficult to see, but easy to explain once I figured it out. Turning caused rings around the diameter when the tool suddenly dug in. The problem was evident when I put a dial indicator in the toolpost and indicated against a chuck jaw. The advance was irregular. I think that the gibs are OK. I will try with the fix, and see if it does any good.

Attached is a picture of what was wrong.cross.png

cross.png
 
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