I am new to machining and don't have a lot of experience. I'm using my Harbour Frieght mini lathe to make a spring plunger that is used on the Savage 10-ML muzzleloader.

I can do most any of the lathe's functions fairly well except cutting threads. I have the timing down (I think) and the gear selection but the threads I cut come out a bit rough. I thought it might be my HSS tool I crafted from a block but the purchase of a turning set that had a 60 degree tool as a part of the package didn't help. I believe I've centered the tooling properly but other than that can't say I know proper set up.

Do you mind giving a novice some hints on how, what, and why of cutting smooth threads on a mini lathe? I shoud say the threads I'm cutting are 1/4 28 common screw threads.

RBinAR -
You will need to set the compound slide to 29.5 - 30 degrees and be sure the tool bit is exactly parallel with the chuck face, then feed with the compound.

Hello,

http://www.mini-lathe.com/ has a very good tutorial on threading on the Mini-Lathe and if you have never seen one of them, Jose Rodruigez http://www.homestead.com/tool20895/jose7x10taig.html has some excellent tapes that cover many aspects of Mini-machining. I highly recommend them.

Alan

Did you grind a proper side relief angle? Don't forget your tool travels much faster than when normal cutting, so it needs the normal relief angle plus the thread pitch angle. If too small, the tool will scrub. This angle must be larger for low pitch threads. The one in the turning set usually is good only for high pitch threads. If you hold the tool by hand with the left side relief against a good thread, the top should be tilted 5-10 degrees to the right.

Ken Shea's advice ensures that the tool will only cut on the edge facing the chuck. Set the dial on the cross slide to zero so it's easy to find the same setting for each pass. If you feed with the cross slide it will cut on both faces, which gives the result you describe. The last shaving should be done with the cross slide though to take off any steps on the trailing flank. This must be a very fine cut.

Use cutting oil.

Good luck. It's satisfying when you get there.

TNX all for the advise. I have to admit I have a million more questions. But to not rush into more than I can absorb in a short time let me ask just one more. Is it possible to know how much force a common screw thread (given a common typeand construction) will take before it will break? I'm not talking about the shank of the bolt I'm talking about the thread holding power itself. I'm trying to decide what thread size I need for a given application.

RBinAR -
The strength of a bolt may rely more on the material then the thread, I.E. soft no carbon steel=weak bolt, fine threads do however offer more holding strength. I am sure there are specs for this but I am not sure where. Do a search and see if anything comes up.

Well that was easy :)
http://dodgeram.org/tech/specs/bolts/SAE_bolt_strength.html

RBinAR -
The strength of a bolt may rely more on the material then the thread, I.E. soft no carbon steel=weak bolt, fine threads do however offer more holding strength. I am sure there are specs for this but I am not sure where. Do a search and see if anything comes up.

Well that was easy :)
http://dodgeram.org/tech/specs/bolts/SAE_bolt_strength.html
Thank you so much for the reference that's exactly what I needed. What I needed was a thread strength to hold 9000 lbs. load with some margin for error. Acording to the site any good grade 1/2 X 20 bolt with sufficient length threads will handle that.

I'm no expert...but man a 1/2" diameter bolt to hold 9,000 pounds? I wouldn't want to be standing under that! :eek:

-Chris