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For various reasons today I decided to check the spindle alignment on my 7x12.? I have a 12 inch 3/4" diameter polished Thomson guide rod I bought awhile back just for this purpose.? I removed my headstock, installed a 3/4" MT3 collet and secured the rod in the collet.? I measured the runout at the headstock and it was less than .001", using a QCTP-installed dial indicator.? The DI was in contact with the side of the rod.? Running the carriage to the left I noticed something interesting.? The dial indicator reading started bouncing up and down and near the end of carriage travel I was seeing about a 0/.005" variation on the indicator.? Twisting the saddle would cause the same magnitude of variation.? At the halfway point the variation was around .0025" when I messed with the saddle so it seems to be sort of linear.

This variation can't be related to the spindle, it has to be the saddle.? Since saddle twist should be constrained by the bed vee, it looks to me like the issue is a gradual change in the match between the bed and saddle vees..? Since this variation is?least in the region where the bed sees the most wear, I'm wondering if the bed and saddle have sort of been use-lapped to better match each other near the headstock.

Deliberately lapping the less-used portion of the bed vee might work, but it's not clear to me that the vee is the issue.? Maybe it is the flat part of the bed that's worn down and thereby increased the contact area in the vee?? What sort of measurements would provide more insight into the problem?

One thing:? I have not measured the height variation along my test rod.? That will a good thing to do, based on my theory that bed wear is the issue.

The other thing to note is that if my theory is true, a new lathe should exhibit this kind of sensitivity to twist, but all along the bed.

Comments?? Suggestions?? Should I just be happy that I'm only seeing .005" variation over about 10 inches?? If this was just taper, maybe; but, due to this effect I will see a periodic variation down the length of a work piece, not a uniform taper.? I had noticed periodic variations in the cutting sound produced as I travel down a longer workpiece, and now I think I know why.

I'm also curious if others have observed the same thing.

-Mark

The top of the bed is surface ground at the factory - it's the? reference surface for everything on the lathe. The mating surfaces on the carriage & HS are NOT ground & may well be misaligned. Check the vertical alignment & see if also varies along the length of the bed. I had to clean up the underside of the HS & deburr things for stability & add a small shim to get decent alignment in both planes. Similarly, the underside of the carriage is worth refining. I did away with the jack & clamp screws by shimming the slide plates. I was able to get things close enough that I had to file off the raised ridges from the serial no. being stamped on the bed.

Roy

Mark,
I think .005” variation is a lot over 10”.
I think it sounds as if there is a issue with the fit of the saddle as you eluded to. Is the saddle pivoting or twisting on the ways? If so, do you think there is a possibility the saddle retainer plates are loose fitting and not holding the saddle down? It could also have something to do with the rack and pinion creating the twisting movement. You probably have to go through some process of elimination.
Dick

Some good comments!? ? ?Give me more homework :)

I hand scraped all the sliding surfaces of the carriage and cross slide on my 7x16. I was also able to get the cross slide scraped square so I am able to face square with .0002” concavity over 1”. ?I did no filing or lapping. It has not needed any further attention in the ten years since. A scraped surface also holds oil well in all the minute little pockets. The underside surfacesof the ways were well within .001” of parallel from the factory. That was a great help right from the beginning.

Mark,
I ran into saddle slop and found it was caused by ridges in the carriage "V" on my lathe. It is worth checking for this since it is easy to detect and fix.? See:


Once the carriage slop is settled you might look at RDM as a method to check headstock alignment.? It is unclear from your description exactly how you made the measurements where the DTI reading was varying but it sounds much like the measurement used in RDM except that you weren't averaging to remove the effect of the test bar being held at a slight angle in the collet. The theory for this method of alignment is here:


If you poke around on that page you'll find more info on RDM and how it applies to use with your Thomsen guide rod.

John

John,
I was in the process of performing RDM when I found the issue described in my original post.? Since the saddle is unstable there was no point in continuing with RDM until I figure out what's going on.

Based on your and other similar comments I definitely will check the carriage V.? Since the problem could be related to differential wear along the bed I will examine it as well.?

Thanks,
Mark

Update.

I found some adjustment issues with my saddle's shimmed slide plates (say that 3 times fast!).? Their mounting screws had loosened up a bit over time, permitting the saddle to ride up a bit over the Vee as the saddle was twisted.? Tightening them up eliminated the indicator variation due to that, so I reinstalled my 3/4" test rod and performed RDM.? I saw about .0005" variation at the headstock end and zero to .004" as I rotated the spindle, so I believe that comes to misalignment a bit less than .002"? over a distance of about 10 inches.? The polished guide rod is very smooth so it's pretty good for this kind of test.? I didn't observe much evidence of bed twist as I moved the indicator along the bed.

I haven't performed RDM on the vertical axis yet.? I really need to switch over to using one of my DTI's for these measurements, now that I know the spindle is fairly well aligned.? I have a 15-0-15 and a 4-04 (.0005" and .0001") DTI I can use for this.

Visual examination of the saddle V revealed that about 25% of the V really is in contact with the bed V (based on the polish marks on the V).? Rather than scraping to improve the contact area, I'm thinking about using PSA-backed sanding paper strips attached to the bed to refine the saddle "V".? Pretty much as described by a document in the Files section.? The procedure doesn't alter the bed surface.? But as it stands the lathe does pretty well on steel so why fix something if it ain't broke??

You make a good point, Mark!
As long as it is performing to your satisfaction all is well. Over time the saddle should settle into a comfortable spot on it’s own.