How close is close enough?

T

Tomas

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I was having some issues with my lathe and nearly had myself convinced the tailstock was out of alignment (In the end, I think most of the problem was a loose quill gib)

As part of the troubleshooting process, I turned down a piece of 1/2" steel rod. Across about 8" the diameter varied by about .005" so I started to wonder. Is that pretty typical for a basic 7x12 metal lathe (Grizzly)? It's probably good enough for the limited tinkering I'm apt to do with it.

Thanks in advance for the advice.
 
The potential is there for to be some problems when making parts that must assemble with specific fits. As Bill said, no, 0.005 is unacceptable for that. If all you need is some loose fitting, or "visually" good parts, then the problem may not be a problem. My opinion is that the lathe is capable of better, and it's nice to be able to rely on a machine to do accurate work, because sooner or later, it will become a factor.

Besides, you will get more intimately familiar with the lathe if you go through the exercise of adjusting it to its most accurate potential.
 
Thomas,

I wouldn't settle for .005 taper over 8".

Yes, it's probably good enough for many less precise applications, but what if you want to make something that requires tighter precision? You should learn how to turn a diameter to a tighter tolerance with your lathe now so if you need to do so at a later date, you'll know how.

First of all, does the diameter of the shaft you turned vary evenly from one end to the other, or is it smaller on both ends and larger in the center? If it tapers evenly then the fix is simple; all you need to do is adjust the tailstock to get rid of the taper. If the diameter gets larger in the center, your tool could be improperly ground or the feed rate set too high and the work is pushing away from the tool in the middle where it's unsupported. Make sure your tool bit is on center (not above) and that it has proper clearance angles and a small radius on the tip. Also, the depth of the finish cut should be around .005".

Your lathe is certainly capable of better performance. As a point of reference, if I were to turn that same part on my lathe, I would expect a diameter consistent to .0005 over 8" and better with a little tweaking. Granted, I'm using a Hardinge, but pretty much any lathe in good condition should be able to hold at least .001-.002" over 8" with little trouble.

Try it again and see if you can do better.

Tom
 
How close is close enough?
Well, that depends on you really, and the work your doing.
Alot of good infro allready posted, so Im not about to repeat any of it. :whistle:

I know that the Co-Axe indicators are not a favorite tool by many . But they are now reasonably priced now, in the 50-60 buck range. The co-axe indicator makes short work of dialing in your tail stock quill. Sweep the quill, or the centre you have mounted up.

Many of these smaller machines dont have taper attachment, so off setting the tailstock is a normal procedure. Many folks will be shy to ajust the offset on the tail stock because the read all the horror shows to getting it back in on centre. With a co-axe is so quick and easy, its going to take you all of a few mins to re ajust.

Getting your lathe to cut without a taper can be a combo of a few things. Certainly the tailstock being on centre is a good start. Having the quill retracted on the housing as deep as possible can make the differance too. Also having the quill locked in position can make the differance. Some lathes will show the reading change when the quill is extended out, or locked, or unlocked. These little things are much easyer to spot using a co-axe indicator.

Ive had friends borrow my co-axe for setting up the tailstock, and say, wow! That was so easy to get it set up, Im going to get one for myself. Once you have one, checking where you are with the quill extended out, or locked, can be checked in mins. Some of the guys that borrowed mine found that they need to do some shimming to get the tailstock right, as the bore has worn over time, and the co-axe showed them this.

Dont be scared to tweak your machine, strive for the most accurate parts you can. You may say, thats good enough now, but time will come, when you will want the best it can be.

Good Luck
 
That's one difference in hobby vs job. I had a tendency to try for nominal all the time at one shop I worked. Even if the cut fell in tolerance, I'd take another cut to bring it closer. It's not practical in a commercial shop, but for home shops, why not??
 
i had the same problem on my 9x20 it seemed to be getting worse with time
ended up my plywood bench had saged on the tailstock end
I leveled the lathe and everything was good again with no adjustments
steve
 
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