• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.
  • Guest,  We want to wish You and Your Family a Healthy, Happy Thanksgiving! Click the "X" at the top right corner to remove this notice)
  • PLEASE: Read the FORUM RULES BEFORE registering!

4

Lathe is cutting a taper

3
Like what you see?
Click here to donate to this forum and upgrade your account!
10

Metal

Active Member
Active Member
Joined
Sep 30, 2015
Messages
231
Likes
113
#1
So my dad has a grizzly lathe I was turning down a shaft on yesterday

The thing cuts fine, a little bit of squeaking and intermittent cutting at the beginning, but it doesn't cut straight.
One end of the shaft ended at 7.35 or so, the other at 7.55 (about 12" of shaft, so its waaay off)

For the life of me I can't imagine how that could be so, the only thing I can think of is the head and center are inline with one another, but not inline with the bed. I suppose it could maybe be the dead center being off as well (I will try and indicate it) but I had no signs that the 1" shaft was being bent by two hundreths or more, and the center holes I drilled seemed good.

Any idea how to look at and adjust this?
 

Dave Paine

Active Member
Active Member
Joined
May 10, 2014
Messages
396
Likes
279
#2
Have you confirmed the head and tailstock are aligned? If not, I would start by checking this alignment. The tailstock should have adjustment and could have moved over time.

There are many videos on lathe alignment. An example is MrPete222 aka Tubalcain.

 

Metal

Active Member
Active Member
Joined
Sep 30, 2015
Messages
231
Likes
113
#3
I'll check the tailstock like I said, assuming it is straight, what is the next step?
 

darkzero

Global Moderator
Staff member
H-M Supporter-Premium Member
Joined
Nov 27, 2012
Messages
2,993
Likes
1,713
#4
3 major things could cause taper, bed twist, tailstock alignment, & headstock alignment. I would check in that order. Usually leveling the lathe first (not level to the world, for bed twist) then aligning the tailstock first should do it. But if for some reason the headstock is out of alignment I would align that first & then the tailstock. But you first have to check bed twist.
 

homebrewed

Active Member
Active Member
Joined
Jul 28, 2017
Messages
46
Likes
26
#5
Do you know if the lathe _ever_ cut straight? Depending on the model, some of the less-expensive Chinese lathes are well known for needing a lot of adjustment before they perform well. The tailstock often is the worst in this regard. My 7X12 certainly fell into this category.
 

Dave Paine

Active Member
Active Member
Joined
May 10, 2014
Messages
396
Likes
279
#6
If the headstock and tailstock are aligned, the lathe is level and it is still cutting a taper, then you may have to tweak the level to add a slight twist to offset for what may be wear in the lathe bed.

Keith Rucker had to use a slight twist in his LaBlond lathe restoration to offset wear in the lathe bed.


A recent thread on how to get metal stock for machining talked about the drive rods in printers/scanners having good tolerance so can be used as a poor-mans test bar. Another source is the shafts of shock absorbers and hydraulic cylinders.

A decent piece of metal for a poor-mans test bar will be useful to diagnose the root cause of remaining problems.
 

Silverbullet

Active Member
Active Member
Joined
May 4, 2015
Messages
2,366
Likes
1,068
#7
I have a feeling it's push over from tool bit pressure I bet a follow rest would prove it. I'm not saying the head can't be off but I'm betting its either he didn't use the tailstock between center and chuck or its the material flexing during the cut. If you want to Ck use the centers and drive dog method. Tapers can be from the tailstock or head stock.
 

Glenn Brooks

H-M Supporter - Premium Member
H-M Supporter-Premium Member
Joined
Nov 14, 2014
Messages
651
Likes
665
#8
One simple inspection you can make while checking alignment of your tailstock, is to examine the bottom of the tailstock, where it slides along the bed. If your ways have wear, often you will find equivalent wear on the bottom of the tailstock. Wear usually causes the tailstock to dip down and to the left - causing the lathe to cut a taper if positioned out along the ways in the areas where the tailstock most resides during turning operations.

Aligning centers with the tailstock up against the headstock only tells you you are aligned at the headstock. Move the tailstock back, with bed wear, taper can be a problem. So checking for worn contact surfaces on the bottom of the tailstock will either eliminate bed wear, or be a possible indictor wear is a problem.

If you can feel a notch, or ridge, on the bottom of the tailstock, with your fingernail, figure wear is a major culprit contributing to cutting unwanted tapers. If so, it is possible to shim back up to factory height.

Glenn
 

mikey

Active User
H-M Supporter-Premium Member
Joined
Dec 20, 2012
Messages
3,048
Likes
2,966
#9
3 major things could cause taper, bed twist, tailstock alignment, & headstock alignment. I would check in that order. Usually leveling the level first (not level to the world, for bed twist) then aligning the tailstock first should do it. But if for some reason the headstock is out of alignment I would align that first & then the tailstock. But you first have to check bed twist.
Metal, I would listen to darkzero's advice. Get the ways leveled (an entire discussion all by itself) and align the tailstock, then go from there.

And when you do your 2 Collar test, listen to @Silverbullet and minimize deflection by taking light passes (0.002-0.003" deep) with a sharp HSS tool. This is Machinist 101; basic, but you have to do it right.
 

Wreck™Wreck

Active User
H-M Supporter-Premium Member
Joined
Sep 29, 2014
Messages
1,919
Likes
1,463
#10
Said machine has an adjustable tail stock? If so check this first as it is the easiest thing to change, many have index lines but unless it is a very highend machine I would not trust them.

Every part is a test bar unless you always finish the part in one pass, skim the length then measure the ends and center then adjust the tail stock accordingly, do so until it reaches a taper that is within your idea of what it should be. This is the so called 2 collar test in place on the part being turned, it doesn't get any better then that. If a long thin part it will also tell you how much the part is bending away from the tool in the center which is more difficult to easily overcome.
If a NC lathe just program the taper out, this is by far the easiest way (-:
 

Metal

Active Member
Active Member
Joined
Sep 30, 2015
Messages
231
Likes
113
#11
Yep it appears to be the bed, now I'm back to screwing around trying to get my own lathe to cut, thanks guys :)
 

Doubleeboy

Active User
Active Member
Joined
Dec 3, 2014
Messages
625
Likes
328
#12
Unless that lathe has tens of thousands of hours on it and was never lubes or had ways clean I have a hard time believing you have enough way wear to cause .200" variance end to end on work piece. In my way of thinking that lathe would have to have soft ways and been seriously abused to be that bad in the wear dept. I would do the two collar method, dialing tailstock in to get both collars to be within a thou or two, nothing fancy there. Then turn the whole piece end to end with a light cut , positive rake, sharp HSS bit, and see how big a saddle or hump there is in middle if any. .200" is a ridiculously large amount to be bed wear or saddle wear. I have seen more than my share of clapped out lathes, and turned on a few, never seen one that bad, I am betting you have a seriously out of position tailstock, or wrong tailstock, or bent tailstock ram, something is way out of wack and I would bet money its the tailstock.

Squeaking and intermittent cutting at beginning of cut would lead me think you may have the tool bit too high, maybe dull or very negative rake. You may have a combination of issues. I would rule one out at a time, make sure you have a nice positive rake tool bit, set on center, (use the trapped ruler method to check if that is all you have it works fine) after you know you have a good cutting tool then tackle the tailstock dialing in.
 

Glenn Brooks

H-M Supporter - Premium Member
H-M Supporter-Premium Member
Joined
Nov 14, 2014
Messages
651
Likes
665
#13
+1 for what doubleboy said. .200 taper in 12" is more than a lot. Bed wear may be an issue, but Iam guessing something else is the real culprit.

Please tell us how you set up the round stock to be cut. For example, Did you hold between centers, and use a dog to drive the shaft? Or hold one end in a chuck, supporting the tail stock end with a center?? If the shaft was held askew in a chuck, the far end of the shaft could easily be out .250" at the tailstock.

Also, for example, has the headstock ever been removed and re-bolted back on? The machine rebuild guys will say the headstock needs to be carefully tightened back down with uniform torque - or the headstock axis will tilt - Imparting taper. So lots of other things could be contributing to your taper. Most are easily fixed, it's just discovering what's going on that can be the hard part. Flustered

Glenn
 

homebrewed

Active Member
Active Member
Joined
Jul 28, 2017
Messages
46
Likes
26
#14
More info about the setup that caused the taper is needed. Was the stock held in a chuck, collet, or driven by a dead center + dog? If a chuck, what kind? What end was the large end of the taper -- the headstock end, or tailstock end? Is the taper reproducible if you turn a test piece or two?
 

Metal

Active Member
Active Member
Joined
Sep 30, 2015
Messages
231
Likes
113
#15
Like I said guys, it was the bed, my dad just dropped the thing off in the corner of his garage over a large crack and it was slightly twisted, between that and the tailstock being offcenter appears to have done the job.
 

homebrewed

Active Member
Active Member
Joined
Jul 28, 2017
Messages
46
Likes
26
#16
Like I said guys, it was the bed, my dad just dropped the thing off in the corner of his garage over a large crack and it was slightly twisted, between that and the tailstock being offcenter appears to have done the job.
Thanks for the update, which could help others with similar problems. Closure is good.
 

Metal

Active Member
Active Member
Joined
Sep 30, 2015
Messages
231
Likes
113
#17
I got the taper down to like 5 thou, the threads on the adjustment for the tailstock are really course so I ran out of patience, i'll hit it again in a few days :)
 

tq60

Active Member
Active Member
Joined
Jan 11, 2014
Messages
512
Likes
262
#18
Search on making a spool for a 2 collar test.

Simple to make and you adjust as you make it.

Once made you use a dial indicator in place of cutting tool and adjust until zero.

You just need a chunk of round stock about 3 inches long by 2 inch diameter.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 

homebrewed

Active Member
Active Member
Joined
Jul 28, 2017
Messages
46
Likes
26
#19
If the bed is twisted, isn't it possible you could adjust the tailstock end to match the headstock end, but still have variations in diameter along the length of the piece?
 

Metal

Active Member
Active Member
Joined
Sep 30, 2015
Messages
231
Likes
113
#20
If the bed is twisted, isn't it possible you could adjust the tailstock end to match the headstock end, but still have variations in diameter along the length of the piece?
Yeah that is exactly what the story was, the tailstock was lined up but it was still cutting a taper which was a head scratchier.
just shimmed up the feet and all set! (after redoing the tailstock)
 

Doubleeboy

Active User
Active Member
Joined
Dec 3, 2014
Messages
625
Likes
328
#21
If the tailstock has poor adjusting feel, it can be easier to do with two wrenches or screwdrivers with them fighting each other so to speak. Glad to hear you got the bed twist taken out.
 

Asm109

Active Member
Active Member
Joined
Aug 29, 2017
Messages
73
Likes
80
#22
To adjust the tailstock on a SB Heavy 10 I mount a test indicator so it touches the side of the tailstock ram.
Loosen one screw, Tighten the opposite screw and watch the indicator move. Snug up the other screw once the indicator is in the correct location.
I do this with the bed clamp snug not super tight. It does not have to be loose.
 

tertiaryjim

Active User
H-M Supporter-Premium Member
Joined
Mar 3, 2014
Messages
519
Likes
218
#23
I find that the cast iron might slowly move after leveling and cause an overshoot in adjustment.
Always recheck after a couple of days until it's settled.
 
[6]
5 [7]