Need Help! Lathe Cutting A Taper!!!

Blaser;
I had nearly the same issue a few weeks back with my Precision Mathews 11X27. It turned out that my headstock had become misaligned with the bed of the lathe. I found a technique called Rollie's Dads Method for checking the alignment. http://www.neme-s.org/Rollie's_Dad's_Method.pdf
It worked great for me. I used a plunger style indicator with the tip removed so that the reading was being picked up on the horizontal and not affected by being a little above or below center. I had a piece of 1" stock about 20" long. I was cutting .001" per inch and after tweaking my headstock got it to .0015" in 18".
 
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I must have overlooked any mention of cutting tool sharpness and it's shape. The height of the cutting tool could be a factor as well.
Have a good day.
Ray
 
I need some input and or suggestions. I have a 10x22 king lathe, ( I know , sorry for your luck) all kidding aside. It seems that all of a sudden I am cutting a taper .002" over about an inch, two inches out from the headstock. I am using an ER 40 collet chuck that I built. The work is slender so I am expecting some deflection, but even with spring passes the taper continues. I have made a test bar and re adjusted my tailstock to less that .001 run out turning between centers. I checked for chips under the mounting register and face of the chuck, re adjusted the headstock bearing tightness and can see no appreciable movement there I have done the compound hold down mod for added rigidity, any suggestions where I should be looking. The only thing that I haven't looked into is leveling the lathe bed, and do not know if it can even be done as the lathe is in my basement and not anchored to the concrete ( yet ). As I had mentioned this is a new problem to this machine and I have been running it for 3 years with very little in the way of issues. Any help is greatly appreciated and thanks in advance.

How sure are you that your ER40 chuck is holding tight enough?
 
The material is o-1 1/2" diameter with 2" protruding from the collet too small for a live or dead center and definitely to short for a follow rest! will have to try and slow down the cut and use HSS see if that helps, like I had mentioned this just seems to have arisen as of late.

Do you mean that you have done a similar turning job in the past without getting a taper?
 
Yes, I have turned many progects with no perceivable taper, I plan on spending the day cleaning and inspecting today . I will report my findings. As before ( good cuts ) I was using ccmt 2151 tooling @ 1200 rpm and .010" depth of cut.
 
A day of cleaning and stewing over the miss alignment issue! I tried cutting with 1" diameter aluminum shaft 6" long protruding, .005" depth of cut @1200 rpm with a brand new ccgt ( aluminum cutting ) insert on a holder that presents a 75 deg. approach angle at the lowest rate of advance per rotation possible . The cut is the same .001"per 2" run or slightly better towards the head stock. I also ran a test indicator along the side of a ground HSS reamer blank and had the same measurable result. My next test will be to take a facing cut and indicate across the end of the piece to see if there is a " taper " there. I talked to the machinery service dept. @ Quality Cutting Tools here in town today and he suggested indicating the side of the 3 and 4 jaw chuck to find if I get similar readings, thus indicating the headstock was actually angled to the side.
 
From what you have described, it may well be that your headstock is pointing slightly toward the rear side of lathe. That was my measurable conclusion (starts about post #17).
http://www.hobby-machinist.com/threads/tail-stock-alignment.53621/#post-446343

Traversing across the chuck face should theoretically tell you the same thing, however its a shorter distance so more prone to error measurement. It also presumes chuck face is ground perpendicular & sitting 100% accurate on spindle nose etc. Maybe facing off a face plate & measuring dish or V might be more indicative?

I just received a precision ground test bar, MT3 on one end, ~8" constant diameter on the other. Its supposedly very accurate ...if I believe all those 0.0000 zero's in the ebay advertisement from India :) My lathe headstock is MT5 spindle & it came with a ground MT5/MT3 adapter sleeve. I dry assembled the parts & they stuck like glue so I'm going to install the assembly & traverse down the ground surface bar sticking out with DTI. No chuck or tailstock involved. If that confirms the same taper amount I last saw from my last cutting test, I think this will be a good & quick way to check validate the lathe every so often. MORE often! Its probably not a replacement for actual turning test, but I also think turning can introduce some degree of ambiguity depending on material, diameter, cut, cutter, unsupported length, diameter measurement etc.

Personally, at least after my own experience, I think this whole lathe jacking thing should be the second thing not the go-to initial fix. For sure the lathe should start out level (meaning without twist) with best instrument you can lay hands on. But a likely culprit is headstock rotation & it only takes a smidge to achieve taper. That was also how the story ended in the 2 YouTube videos I referenced in my post from pretty experienced guys. Put another way, if lathe bed was actually 100% level (non-twisted) & unbeknownst to you the headstock was the culprit, then shim jacking the lathe bed to impart compensating twist now means you have 2 problems fighting one another, not just one root cause. And its probably not doing the bed casting any favors either being twisted outside of how it was initially ground.

After adjusting mine back I've also been wondering about the set screws that adjust the head in position prior to locking down with vertical bolts. I kind of viewed them as jacking screws t assist rotation which is probably what they are. But if for whatever reason they backed off or loosened on their own over time, then thousands of repeated cutting pressure traverses might slowly serve to swivel the headstock over time? (viewed from top). Who knows on that one, but I would for sure check after moving the lathe. I'm still not finished my alignment tune up but now I'm glad I have at least an appreciation for what's going on.
 
I just came up from the shop, trying to eliminate possibilities. I removed my collet chuck and installed my 3 jaw and repeated the prior tests and measurements. The results were the same larger diameter away form the headstock and smaller near to it. I guess I know what I will be doing tomorrow. Wish me luck.
 
All lathes "cut a taper' regardless of cost or age, the question is how much is acceptable to you.

If your answer is ZERO then you are in the wrong game, however there are several ways to get happy here, one is to purchase a CNC lathe and program the taper out, this is what I do. I never achieve ZERO taper.

A cylindrical grinder is an option as well, much better then a lathe for this work..

Good Luck
 
Realistically I am not expecting 0 but 1 thou over 2 inches an inch out from the headstock when added in a linear way adds up to a lot more than 1 thou over the length of my bed 22" .
 
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