Help- Lathe cutting a Taper.

[rwm]

My lathe is cutting a slight taper that I cannot seem to figure out.
The issue: it cuts a taper of about 2.5 thou over 6", wider at the tailstock end, measured with a Starrett mic. Yes, I am using a 3 jaw at the headstock but that should not cause a taper even if there is runout. The measured TIR of the chuck with a dowel pin is .0005"

Investigation: I figured the lathe level had drifted slightly. When I checked (Starrett machinist level) it was off slightly so I dialed in the feet to get as close as I am capable. This did not fix/change the issue. My second thought it was play in my live center. I tried again with a carbide dead center and got the exact same taper. So I figured the tailstock is off towards the back splash. I put my test bar in with the carbide center in the tailstock. I used a DTI in the tool post. I get the exact same reading at both ends of the test bar. Based on this information it seems like the lathe could not physically cut a taper. I am in a quandary.
I know we have some pretty smart people here. Hopefully someone can explain what I am missing.

 

[RJSakowski]

Depending upon the diameter of your stock, cutting a taper that is larger at the tailstock is not uncommon. Any turning operation necessarily creates some tool pressure which deflects the work. The further from the headstock, the greater the deflection. The smaller the diameter, the greater the deflection.

However, if you are using the tailstock with a center, the tailstock has an adjustable offset, specifically to allow turning tapers. There should be two adjustment screws at the base of the tailstock for tha purpose of adjusting the offset. Since you are turning a greater diameter at the tailstock, you need to back out the front screw and tighten the rear screw. This will bring the tailstock closer to you. Loosen the tailstock before adjusting. It will be easier if you use a dial indicator to monitor the adjustment.

Runout won't affect a taper.

 

[benmychree]

A Starrett #98 (assuming that is the level that you have) is not sensitive enough to level a lathe, it will put you in the general neighborhood, the sensitivity is .005" per foot per graduation, the levels for machine alignment are .0005" per ft. such as the Starrett #198, that is it has ten times the accuracy of the #98. Assuming your lathe did not cut a taper formerly, a bit of twisting (or should I say untwisting) of the bed could cure it. A test of a lathe for cutting taper should not be done with tailstock support, the best way is the so called two collar method, which does not make necessary long fine cuts where tool wear might enter into the results. Having said that, as RJ points out, your problem could easily be simply be a tailstock adjustment.

 

[rwm]

Thanks guys!
My test bar on the DTI reads 0 difference at the tailstock end. Are you saying I should move it anyway about 1.5 thou to make up for deflection? So have it read 0 at the headstock and .0015 at the tailstock?
Yes I have a Starrett 98. Honestly I had trouble getting both ends level with that. If I had one 10 times more sensitive it would be impossible. Also the adjustment I made to the level did not seen to affect the taper in any way. I do not actually know if my lathe ever cut straight. I never really had the need to dial is in so carefully. I used the 2 collar method with a test bar and a DTI. That reads 0 difference (maybe a few thenths.) When I actually machine a bar I get the taper described.
I have two other ideas. I will check the tailstock height vs the headstock. I will try machining a heavier steel bar and see if the taper persists. If it does, I can deliberately put one end out of level to see what happens.
I think I confused you with improper use of the term "test bar." It is not a straight bar like an MT coming out of the headstock. It is a 2 collar bar that has support at both ends.

 

[rwm]

Well I put my two collar bar between carbide centers and I double checked everything. The tailstock is about .001 higher than the headstock. That's close enough and I don't think that's the problem. I did find an offset in the tailstock that I did not see before. After some playing around it appears that my tailstock ram is not bored perfectly. The offset varies depending on extension of the ram.
I decided to set the offset to 0 with the ram extended 1". I don't have time now, but I will try to cut some metal and report back. I don't plan on cutting the whole bar, just a small collar at each end to make measurements.
Wow, measuring down to tenths is difficult. Every time you touch something you get deflection and movement!

 

[Jake M]

Wow, measuring down to tenths is difficult. Every time you touch something you get deflection and movement!

This doesn't really change anything, just a reflection based on what I think would be a reasonable indicator stand for the job There are larger and smaller, but I'm going with a 15 inch path from the attachment point to the tip of the indicator. Just for perspective on how careful one must be with fine measurements. It varies by who you are and what you do, but at some point EVERYONE has measurements that go from practical to ideal to theoretical to hypothetical... This is more a perspective on the care that would have to be taken.

If you're using a dial indicator to measure 2X4 lumber to build a wall, and need to be inside an eighth of an inch, the indicator stand is 120 times the length of the measurement.

If you wanted to measure a work piece to +/- 0.005, the indicator stand would be something like 1500 times the length of the measurement.

If you wanted to measure a work piece to a ten thousandth of an inch, the indicator stand is more like one hundred and fifty thousand times the length of the measurement.

So, if you picture the steadyness and repeatability of a tenths indicator on a 15 inch stand (all included), that would be kind of the same (in relative terms) as holding a stated tolerance of holding a 0.010 indicator (for that +/- 0.005 inch tolerance) on an indicator stand that's (all included)) 15 feet long a 1/8 inch indicator on a stand that is (alll included) about a third of a mile long.

Care and attention to detail goes a long way when you're reading little numbers.

Or depending on how you're holding your indicator, the number might come out different, but the proportions are gonna stay the same.

 

[rwm]

With some food in my belly, I took some careful test cuts with the tailstock ram at 1". I am now getting a taper of about .0015" over 11 inches in Z travel. I'm calling that good. I need to remember to keep my ram at 1" stickout.