Turning threads off rod fine but now rod advances out of chuck.

[helmbelly]

Ive been turning 1/4" of thread off the end of some stainless 5/16 bolts so I made a coupling to hold them in my 3 jaw chuck by tapping a piece of 1" long hex alum, slicing it on band saw, chucked it up and all worked fine. After turning a handful I got bugged that I had to overtighten the chuck to prevent the hex from slipping back in the chuck when I pressed a bit with manual feed (I'm coming straight down the ways at the threads perpendicular, like I was reducing the OD.) I decided to turn the hex edges off about 2/3rds of the coupling to leave a shoulder to grab the front of the jaws and stop any slip back. Nice I thought just as Murphy came in and sat down.

Take my first cut on the new improved method and the stud turns in the coupling and advances into the tool bit! Goodbye carbide tool bit. So I put a 5/16 nut on the back of the "hex" thinking that would lock it . Nope, bye bye 2nd carbide tool bit.

So I said F it and ordered a brass coupling nut and a couple new tool bits from McMaster. Two questions

1. why did the thread unspool after I turned the hex off the back half? can't figure that out.
2. On the coupling coming tomorrow -better to put two hex nuts behind it. or once again bisect the coupling ?

 

[WobblyHand]

Pictures would be helpful.

 

[markba633csi]

You mean the threads on the workpiece are catching the tool bit and pulling? That is a risk when turning off threads
Changing the feed direction might help

 

[helmbelly]

Pictures would be helpful.

Here is the stud chucked. The coupling I made was working great until as shown in photo I turned off the back 2/3 of it. Since it started back out on me I put a lock nut behind the coupling- maybe I should just put a second lock nut on.

 

[Ulma Doctor]

if you were to take 2 hex nuts and split them instead of a single nut, the grip force would change. The 2 nuts would act like anti backlash devices, preventing easy slippage of the work

 

[helmbelly]

You mean the threads on the workpiece are catching the tool bit and pulling? That is a risk when turning off threads
Changing the feed direction might help

Sorta, the tool bit is pressure is engaging the threads against the socket and backing it towards the tailpiece. Old lathe only safe to run "forward".

 

[mksj]

You may be taking to deep a cut and/or too high a feed and the insert gets caught on the thread and unwinds the bolt. Also the type of carbide cutter and profile. I use CCMT inserts and haven't had any issue with the bolt/rod turning. Ideally you would want something like a 5C collet. Make sure it is free of any oil or lubricant as this can significantly decrease the holding power. I routinely either turn down SS bolts and/or thread cut down bolts w/o any issues. The other approach would be to use a through the spindle stop system.

 

[helmbelly]

if you were to take 2 hex nuts and split them instead of a single nut, the grip force would change. The 2 nuts would act like anti backlash devices, preventing easy slippage of the work

Good thought, thanks Doc, that'd work great for a 1 off, but I need to make 50-100 of these and need something less fiddly - the thing that has me stumped is that the original hex coupling worked well until I rounded it.

 

[helmbelly]

You may be taking to deep a cut and/or too high a feed and the insert gets caught on the thread and unwinds the bolt. Also the type of carbide cutter and profile. I use CCMT inserts and haven't had any issue with the bolt/rod turning. Ideally you would want something like a 5C collet. Make sure it is free of any oil or lubricant as this can significantly decrease the holding power. I routinely either turn down SS bolts and/or thread cut down bolts w/o any issues. The other approach would be to use a through the spindle stop system.

I did notice I had changed the lathe speed so it was running faster. that could be it I suppose.

 

[markba633csi]

Use more than 2 nuts for even more grip