It Pays To Be Sneaky...

T

Tozguy

Active User
H-M Supporter Gold Member
Joined
Feb 15, 2013
Messages
2,963
Was cutting a female thread 3/4'' x 40 tpi. Male part was already threaded.
Did the calculations for how much to crank the cross slide for the proper thread depth. Couldn't see very well how the threads were forming so I trusted my calculations and hand wheel dial. I wanted a close fit so I stopped ,020'' short of my calculation to sneak up gradually on final dimension. Figured to do trial fits from there on and after each very shallow cut.
But the first trial fit perfectly. No need to make any more cuts. Sure glad that I stopped early to sneak up on it because I was already there!
 
so nice when that happen, so not nice when it feels sloppy
 
Any ideas why? Cutting many threads I don't remember that worked according to my ciphering. When I get down to the last few thou I cut 1-2 per pass and I still have to trial and error.
 
Boring a 1/2" tap drill hole for an upcoming project. Target was .453, overshot it to .456. (.003" over) :( . It was caused by excess spring in the boring bar; I didn't run a spring pass before measuring and it bit me.
 
Danb said:
Any ideas why? Cutting many threads I don't remember that worked according to my ciphering. When I get down to the last few thou I cut 1-2 per pass and I still have to trial and error.
Click to expand...

The perfect fit to my mind for this job requires very little clearance, say ,0002'' for the sake of discussion. The fit goes from too tight to too loose with a very small change in dimension.
Measuring the 40 tpi male thread to that level of precision was beyond my ability.
Calculating was the easy part but is only as good as the inputs.
Then there is how the cut goes based on everything flexing to an unknown degree.
On my manual lathe, positioning the tool to that level of precision would require dial indicators that I don't have.
Lots of places for things to wander. Thats why I find it most practical to sneak up on final dimensions by trial fit when possible.
 
Last edited:
In retrospect it may have been better to cut the female threads first and then the male threads to fit.
Somehow it seems easier to cut male threads to dimension than female ones.
 
When I'm sneaking up, sometimes I take 2 cuts without feeding (same cut, twice). Just to eliminate any error caused by spring in the boring bar or in the workpiece (especially if it is sticking out from the chuck a bit). Not sure if this is normal or I'm just overly cautious. Sometimes if I see any material taken off on the second pass, I'll take even a 3rd pass without feeding. Check fit, need more, take a few thou, and repeat.
 
In my experience, calculations provide decent estimates only and following them blindly will invariably lead to loose fits. If a close fit (class 3) is needed it is always wise to check for fits early on. There are many factors that affect fits and the calculations do not account for them so when a precision thread is required, check early and check often. Being "sneaky" is also being smart.
 
You must log in or register to reply here.
Back
Top