My lathe is turning a taper

Pcmaker

Pcmaker

Registered
Registered
Joined
Jun 12, 2018
Messages
724
I have a PM-1127 lathe and my current project is making a replacement drawbar for my mill. It's 14 inches long, the longest thing I've ever had to turn on the lathe.

The difference is around .015 from end to end. That's a big difference. I have the work piece supported with a live center.

How do I narrow down what could be causing the taper? I lined up the tailstock with the headstock not so long ago. The ways is pretty level when I checked it a month or so ago. The difference between the ends of the workpiece is so large, that I don't think it's unlevel ways.

I"m thinking maybe my center drill hole isn't dead center? How do I find out? I wanted to stick out the round bar without the live center support and indicate it, but I"m worried that it sticks out long enough that the round bar will not be straight.
 
How did you line up the head and tailstock?

I have found that sometimes the tailstock alignment is best set by using an alignment bar or by turning and tuning on the current project and once it is right turning to the final dimension before moving anything.
 
Is the tailstock side larger or smaller then the headstock side? Depending on which side is bigger determines which direction to adjust the tailstock, forward if its larger and to the rear if smaller ..... If im not mistaken your lathes tailstock has an adjustment screw on the bottom rt side (standing in front on the lathe).
 
I haven't done it with steel, but aluminum deflects enough over that length that I got all sorts of weird on a long cut like that. It makes sense, it's pretty easy to bend 10 thou in the middle of thinner aluminum bar. Put an indicator in the middle and push on the bar. It's easy to deflect it that much.

The good news is your lathe came with a tool for that. The follow rest.

The bad news is that it takes practice to get it to work right. I took a few long cuts before I got the hang of it.

The question becomes, do you need it to be that accurate over the whole length? For a drawbar, I would say that you don't. I mean, it's a nut on one end and a inch or two of threads on the other, right? Perhaps I'm missing a reason why, if so, I hope to learn. I made one for the lathe spindle that is just a piece of all-thread from a box store, and a couple nuts and washers with a bushing I made to center it.
 
Yup, sounds like your tail stock needs to be adjusted, front to back. The other thing that can cause taper is the tailstock height is not on the spindle center line. But you can compensate to some degree by adjusting front to back.

Getting a little deeper into the weeds here, is your head stock aligned to the ways? Probably is, but doesn't hurt to check.
 
MontanaLon said:
How did you line up the head and tailstock?

I have found that sometimes the tailstock alignment is best set by using an alignment bar or by turning and tuning on the current project and once it is right turning to the final dimension before moving anything.
Click to expand...

I took off the chuck, put a dead center in and live center on the tailstock, aligned the tips by eye

Latinrascalrg1 said:
Is the tailstock side larger or smaller then the headstock side? Depending on which side is bigger determines which direction to adjust the tailstock, forward if its larger and to the rear if smaller ..... If im not mistaken your lathes tailstock has an adjustment screw on the bottom rt side (standing in front on the lathe).
Click to expand...

the tailstock side is smaller, and gets larger as I get closer to the headstock.

Also, could a live center cause it by not being ridgid enough? I got a cheap live center from eBay for $35

I'll align the headstock to the tailstock again later, maybe it's shifted from the last time I did it.

The taper doesn't matter much to the project I'm doing, but it's annoying when the taper can be noticeable visually.
 
You said that you aligned the tailstock by pulling the tailstock to the chuck end and aligned center points. You should check alignment where the tailstock will be at 14” away. From what it sounds your tailstock is closer to the operator.
Have you tried doing it in steps. Stick out 5” from the chuck cut your OD, then pull out another 5” and repeat till you get your length.
 
Cadillac said:
You said that you aligned the tailstock by pulling the tailstock to the chuck end and aligned center points. You should check alignment where the tailstock will be at 14” away. From what it sounds your tailstock is closer to the operator.
Have you tried doing it in steps. Stick out 5” from the chuck cut your OD, then pull out another 5” and repeat till you get your length.
Click to expand...

That's what I was thinking of doing. I'll work on lining up the tailstock first, though. I was thinking of putting an indicator to the tailstock and move it away from me about .0075
 
I wonder if I can use a round bar with center drillend both ends, hold it with dead center and live center and indicate along the length of it. I don't have anything more precise than 1018 CR round stock
 
You must log in or register to reply here.
Back
Top