Not a bit surprised, Kent. I've run enough machines to know that they are not jig borers. I expect minor deviations when moving the knee, but only a few tenths at most over several inches of swing.
Two things can cause inconsistent reading when moving the knee, one being a loose gib, and the other tremendous wear on the knee ways. A third one to consider would be when a quill is well worn and extending it farther to sweep the table causes the quill to tilt slightly in the head. Again, we're not talking about jig borers here, they're mills, and in many cases by the time they're relegated to the home shop, any useful life they may have had is long gone, especially if they have been rusted.
One thing to consider. If your habit is to sweep the table with the lock set on the knee, but to operate with the lock free, it's entirely possible that the locking action alone is part of the problem. I'm sure you've taken note of the half thou of movement (deflection) you get when locking either the table or the saddle on a Bridgeport. That's the nature of the machines.
I agree, the vise will cause deflection, but I will never be able to measure it. Next chance I get, I'm gonna use my B&S .0001" test indicator (sorry, best I own) and see if I get any movement by just lifing the vise (6" Kurt) off and on.
Um, probably because you'd have to lower the quill further to tram the head. I had the quill extended less than 2". That is about my "general rule of thumb" because I tram were I work (90% of the time).
...either that or the knee ways are not ground straight up and down on their machine. In this situation, you could tram the head to the table and when you move the knee, it would actually move on the Y axis (even the X too!) and your true positions would never be right if you moved the knee.
Clean the mill table and vice surface, spray both surfaces with a solvent or wd-40 then immediately use an extra fine coarse file to gently rub over the table surface in order to feel a slightest imperfection not visible to the human eye. Wipe both surfaces clean of dust. Look at the mating surfaces before suspect any machine problem.
Thanks. Good advice. I've been known to deburr my mill table and vise on a few occasions.
I think though that you misunderstand. The side that was out by .008" was a slow, smooth, gradual change in reading. Over a 180 degree sweep, there was the .008" TIR. It wasn't a sudden jump of the needle. I think I would have figured out my problem right then and there :-p I also did move the table a half inch or so and got basically the same error.
Remove the vise. Place a good plunger type indicator vertically in the spindle, and touch off straight down on the table and lock the knee.
Traverse the table from one extreme to the other and see what the dial says. I think eveyone here would be interested to see what happens.
Jim
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That's sure as hell what I think should be done. It sounds like the table isn't parallel to the ways. Now to discover the reason, if so. That test would tell the tale, but it could also indicate the entire knee assembly tipping, but ONLY WHILE it is in the process of tipping, if even then.
The other suggestion: With an indicator on the table, located away from under the spindle, perhaps above the edge of the saddle, lift and push down hard on the end of the table to see if you get movement. If so, then look to see what's moving. Either the saddle is tipping, or the knee is if the table is parallel top the ways. Could be a combination of all conditions. You'd have to mount the indicator on a rod (from the spindle) or use some other means, it can't be touching the table or any part of the knee by anything but the indicator tip.
A good magnetic base will allow you to indicate various places while stuck to the base casting. I do this regularly.
Gunner
"As my father told me long ago, the objective is not to convince someone with your arguments but to provide the arguments with which he later convinces himself." David Friedman
Ok, For all the enquiring minds (me included). I took off the vise, cleaned and deburred the table top then ran my B&S .0005" test indicator full travel on both the X and Y axis. Stylus was set about 15° and had about .008" worth of travel. The quill was extended about 2" and the indicator was attached to the spindle nose with an "Indicoil" All the locks where locked except whichever I was moving.
Nothing exceptional happened on the Y axis. Maybe .0005" over the entire 8" of table travel. I got about .006+" TIR over the entire travel of the X axis, ~24". I could get a bit more travel, but the power feed stops were set and I didn't want to move them. Sounds about right based on my tram experience last week. So I check the tram on the right side and all four quadrants were close enough to zero. Move to the left end of the table and got almost the same .008" TIR reading as last time. I guess the extra .002" worth or movement was from the "extra" travel of spinning the indicator around vs. it being stationary during the first test.
Even though I had not yet found my problem, I decided to tram the middle of the table (where I previously had a vise) and do some "real" work. After all, I was getting paid "consulting" wages and felt obligated to make some chips. I moved to the the table center and decided to really crank down on the locks. Well HOLY CRAP! I put some "weight" into the Y axis lock and heard a snapping/cracking noise! I scraped my thumb and got way lube all over my arm as my hand slipped off the handle and my arm went by the bottom of the table. The Y axis lock could now make another full revolution! It was never tight in the first place! I didn't disassemble the locking screw, but I checked the left end of the table again.
TIR was now .0015 on the left side with the table locks *definitely* tightened. I released the Y lock and the indicator moved about .005". Tighten the lock and watched the needle go back to .0015!!! The entire table over it''s entire travel now has a TIR of .0015. I can live with that. And that amount of movement sounds about right for a machine in this good of a condition (practically new).
Problem solved. Some junk in the threads of the table lock causing it to not really lock. Probably been there since day one. I'm a little concerned as to why the Y axis lock could have such an effect, but I don't care anymore. I'll just keep the exeperience knowledge for future reference.
1.5 thou over the entire travel is reasonable. The five that moves when you apply the lock is a teeny bit on the big side - you might consider snugging the gibs up a tad and you could reduce that. It's not so much that the movement is a problem when locking, but that much play in the sliding ways will reduce the machine's rigidity and make it perform poorly. If you tighten the gibs up a bit it will take a heavier cut without chattering, and will be more accurate on the lighter cuts.
Glad you found the smoking (well, oily) gun!
Jim
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