View Full Version : Another taper issue

08-20-2009, 08:08 PM
Hi Guys,

I know this has been beat to death on this forum and others, but! I did so a search on this subject (on this forum) and read all the posts I could find. (their were quite a few of them). I still did not find the answer I was looking for.

I have a jet GHB 1340 lathe. I have leveled the lathe to 0.1* over the bed length (as measured with a sears electronic level). I used a piece of schedule 40 rigid pipe for a test bar and turned off about .002 over an 8" length.

When I checked the diameter over a 6" length with a micrometer, I had about .001 difference in the diameter. Is this reasonable for a HS machine? How much taper is acceptable for a HS machine over this distance?



08-20-2009, 08:15 PM
Tool sharpness, material type, and spring passes are critical for these tests. 0.001 is pertty good though.

08-20-2009, 08:18 PM
.001" is about as good as it gets. dont change a thing. you can chaise .0005" forever and never catch it.

Run it, you are good to go. :)

08-20-2009, 08:30 PM
I checked the Griz site, and their G4003G Gunsmith lathe has a spec of .0008" per 12". Sounds like you are very near that.

I agree with others, it's good enough for most work. Typically longer lengths require support, at that time you will need to use a center. Adjustment of the center will determine the accuracy over the total length.

08-20-2009, 09:29 PM
Thanks guys,

I felt like it was ok, as a newbe I just wanted to be sure.

The pipe I was using for a test bar was about 2-1/2' long, I had the tail stock end supported on a pipe center. The section I turned started about 2.5" from the end of the chuck and extended 8" towards the tail stock.


08-20-2009, 09:37 PM
0.1 inch bed level end to end isn't very level... I try for much better than 0.001 per foot...

And.. I assume you mean level in both planes?

John Garner
08-20-2009, 09:42 PM
Tim --

The Bearer of Bad News here . . . when checking for lathe bed for twisting by cutting a test piece, it's important that the test piece be supported ONLY by the headstock spindle's collet or chuck . . . no tailstock, steady rest, or follow-rest allowed.


08-20-2009, 11:05 PM

That was 0.1 degrees or .0698" over 40". I only checked level across the ways.


Dang, I will retest tomorrow using the prescribed method (ie. not tail end support)

Thanks guys for your responses. you are all a big help.


08-20-2009, 11:39 PM
Did you have the end running on a center??? 8" hanging out of the end of a chuck is bound to deflect. A more accurate test would be with a length of brass, aluminum or free machining steel such as leadloy, 1215 etc.
Try it with the tailstock center and with out. I would use at least a 1" dia. round or larger.


The Artful Bodger
08-20-2009, 11:42 PM
Deleted by me... it was a dumb comment! Duh!!

08-22-2009, 03:05 PM
Hello again guys,

I missed my chance yesterday, but this morning I cut my schedule 40 pipe test bar down to 10" in length and chucked it in the 4 jaw. There was about 6.5" left sticking out, left the TS end unsupported.

I turned 6.25" of the pipe down about .005 to get it concentric, then skimmed of .001 as a finish cut. When I checked the taper over 6" I found a difference of .0015 as measured with my old Starrett vernier mic (using MK1 eyeballs to split thousands).

As I said in an earlier post there is about .068" of twist in the bed over 40". I have ordered a Starrett 98-6 level (should get here next Friday) in hopes of improving the leveling of the lathe. From what I can glean from your posts, this may also improve the taper in this lathe.

Guys, thanks for you advice, I appreciate all of you.