Cross Slide / Facing Adjustment

[MozamPete]

I don't know.
Have you got a faceplate? Install it and take alight finishing cut on its face. Traverse the faceplate with a dial test indicator and test for concave or convex. This will take the chuck out of the picture.

I was thinking something similar - will try that tomorrow.

 

[British Steel]

I would think that if the chuck jaws weren't coaxial with the spindle axis, the diameter cut would stay parallel, as the tool will cut around the spindle axis - this is how we can cut eccentrics in the 4-jaw, by offsetting the work, after all.

When you machined the two rings in the pic, did you have the bar supported by the tailstock centre? over that length of bar, for anything other than an incredibly light finishing cut, I'd expect to see a bit of diameter variation (smaller at the chuck end) due to flex in the bar from tool forces.
BUT if the spindle/bed alignment was out such that it cancelled the diameter variation, I think it would face concave...

Just a thought, facing to the centre the surface speed drops off close to zero as you approach the centre, so the tool isn't going to cut as effectively? Is it worth bringing the spindle revs up as you approach centre?

Dave H. (the other one)

 

[MozamPete]

Dave,
No I don't support the test bar in the tail stock. It is two aluminum collars on a steel bar I made just for testing and setting up the lathe - I only take light cuts, just enough to ensure I'm taking some metal off all the way around the diameter - 3 to 5 thou typically.

 

[MozamPete]

OK, I'm not sure what I was doing on Saturday but I repeated the chucked up parallel test as described in Connelly and have now confirmed that the cross slide is off from perpendicular to the spindel axis, and in the direction that will cause it to face convex. I think I just got the geometry mixed up in my head the first time and once I thought the the cross slide was off in the wrong direction for it to be cutting convex started looking for other possible reasons, but every other test was pointing back to it being the cross slide that was off.

End result is it is actually a lot worse than I suspected:
Clucked up a parallel and adjusted it so both ends were reading zero when I rotated the the chuck


Then traversed the cross slide




End result - cross slide moves about 0.55mm away from the work over the 150mm travel, which calculates to about 0.2 degrees off perpendicular.

Will have to do some more investigations and see if this is something I can possibly fix.

 

[Mac1]

I've always read that Myford made a decent product. .55 mm sounds like about .0216 inches and to my untrained mind that sounds like an awful lot of wear. Does the rest of the larhe show a lot of wear? I think before I started removing metal, I would dissasemble the carriage and clean it up and make sure all of gib adjustments were good. I guess its possible the lathe has always been out from the factoty.
I just realized the Myford is a flat bed lathe , so you won't have the problem of scraping vee's.

 

[MozamPete]

Did a bit more investigating tonight.
Measured the wear on the front way (which is the one the carriage in guided on on an early ML7) and the worst I got was 0.1mm narrower than at the tailstock end - so that was not enough to be causing such a large deviation in the cross slide angle.
Then had a play with the gib and trying to shim the carriage with feeler gauges to get an idea of what it would take to get it to face square, and eventually just pulled the carriage off for a look - I have owned this lathe for over 2 years now and this was the first time I have removed the carriage.


Problem was pretty evident then:
The sliding/bearing surface on the carriage had very uneven wear with a noticeable ridge a one end. From nothing at the headstock end of the carriage to approx 0.2mm at the tailstock end.




I think it would be easy enough to mill that back to parallel using the unworn lip as a reference surface and get a substantial improvement (still have the wear in the bed ways though).
The other alternative is a conversion to 'wide guide' which I have read about being done on the ML7 when you change the bearing surface to the rear of the carriage and run along the previously unworn rear bed way. Will think about it some before I start removing metal.

 

[Mac1]

There is a product called "Turcite" for building up wearing surfaces. It may be a possible solution.

 

[MozamPete]

Took the plunge last night and cut some metal off my lathe to square up the carriage.
Indicated off the unworn lip, put a brand new carbide cutter in and took off just enough to bring it all down to the most worn end in a number of light passes - about 0.25 mm in total had to be removed from the unworn areas.



Reassembled the carriage and gib on the bed and it is smoother at traveling along the bed - still tightens slightly towards the tail stock but to be expected as I measured about 0.08mm of wear in the front bed way width (which is just over the 0.003" magic figure where Myford recommend a regrind).



Still need to reassemble the apron and cross slide before I can quantify the improvements, but suspect it may still face slightly convex due to the bed wear. I could 'over correct' the carriage to allow for the bed wear but I would rather fix the bed if possible instead of introducing an intensional error to compensate.

I know the purest will be horrified, but I'm considering a DIY regrind of just the front bed way vertical surfaces to tidy up the alignment. More on that later once I have though it through some more.

 

[cathead]

Took the plunge last night and cut some metal off my lathe to square up the carriage.
Indicated off the unworn lip, put a brand new carbide cutter in and took off just enough to bring it all down to the most worn end - about 0.25 mm had to be removed from the unworn areas.

View attachment 125769

Reassembled the carriage and gib on the bed and it is smoother at traveling along the bed - still tightens slightly towards the tail stock but to be expected as I measured about 0.08mm of wear in the front bed way width (which is just over the 0.003" magic figure where Myford recommend a regrind).

View attachment 125778

Still need to reassemble the apron and cross slide before I can quantify the improvements, but suspect it may still face slightly convex due to the bed wear. I could 'over correct' the carriage to allow for the bed wear but I would rather fix the bed if possible instead of introducing an intensional error to compensate.

I know the purest will be horrified, but I'm considering a DIY regrind of just the front bed way vertical surfaces to tidy up the alignment. More on that later once I have though it through some more.

Nice post and detective work as well. I will have to do some measuring on my stuff and see what the readings say.

 

[Mac1]

Good work. You probably need to check the alignment of the lead screw since I think you moved the carriage by the amount you took off.