MetalworkingFun Forum › Machinery › Lathes v
[HELP] Threading Troubles

Thread Modes
Help Threading Troubles
EdK Offline
Whipping Boy
*******
Posts: 8,859
Threads: 319
Joined: Feb 2012
Location: Arizona/Minnesota
#1
10-22-2013, 02:54 PM
OK, this is embarrassing but I've hit a brick wall so it's time to swallow my pride. I've cut a fair number of threads with my lathe, not a ton, but a decent amount. It's been awhile since I've cut one but it's not rocket science so I'm at a loss as to why I'm having this problem. I'm cutting a 1/4-20 thread and I do the first scratch cut to check that the TPI is set up properly, and it is. Then the next pass is off a little bit even though I engaged the half nuts on the same number as the first pass. According to the chart on the thread dial housing, for a 20TPI thread I can engage the half nuts on any of the 8 numbers. I tried number 2 the first time and assumed I screwed up on engaging the half nuts so I started over and paid attention to engaging the half nuts on the number 7. Same problem. The second pass is off by a bit but it's still 20TPI. I'm perplexed. 17428

In case it might help, the lead screw is 8TPI.

Ed

   

       
Reply
Thanks given by:
doubleboost Offline
Super Moderator
******
Posts: 1,459
Threads: 159
Joined: Feb 2012
Location: Northern England
#2
10-22-2013, 03:18 PM
Try leaving the lead screw engaged I do on short threads
Just revers the lathe
John
http://www.youtube.com/user/doubleboost
Reply
Thanks given by:
EdK Offline
Whipping Boy
*******
Posts: 8,859
Threads: 319
Joined: Feb 2012
Location: Arizona/Minnesota
#3
10-22-2013, 03:31 PM
(10-22-2013, 03:18 PM)doubleboost Wrote: Try leaving the lead screw engaged I do on short threads
Just revers the lathe
John

John,

I have no doubt that would work but it shouldn't be necessary cutting an imperial thread pitch with an imperial lead screw. I've never kept the half nuts engaged while cutting threads with this lathe unless I'm cutting a metric thread. The perplexing part is that I've cut this exact thread before with no problems. I'll go ahead and try keeping the half nuts engaged. I'll be even more perplexed if that doesn't work. 17428 17428

Ed
Reply
Thanks given by:
doubleboost Offline
Super Moderator
******
Posts: 1,459
Threads: 159
Joined: Feb 2012
Location: Northern England
#4
10-22-2013, 03:38 PM
The only part that can "slip" is the job in the chuck / collet
Check all the key ways ect in the gear train
John
http://www.youtube.com/user/doubleboost
Reply
Thanks given by:
chucketn Offline
Junior
****
Posts: 375
Threads: 53
Joined: Jun 2013
Location: Jonesborough, TN
#5
10-22-2013, 03:43 PM
Ed, looks to me (a real novice) that there is some backlash or slop somewhere in the gear train driving your leadscrew. Possibly a sheared key? I too, would advise leaving the half nuts engaged and reverse the lathe. I do it for all threading, metric or imperial, and my lathe has a 16 tpi imperial lead screw. Just try it and see if you're results improve.

Chuck
Micromark 7x14 Lathe, X2 Mill , old Green 4x6 bandsaw
The difficult takes me a while, the impossible takes a little longer.
Reply
Thanks given by:
Highpower Offline
JAFO
******
Posts: 4,460
Threads: 184
Joined: Feb 2012
Location: Missouri, USA
#6
10-22-2013, 03:53 PM
You have the correct change gears loaded up for doing imperial threads, yes?
Swapping them to do metric threads and forgetting to put them back will do that to you.

Might want to check that the gear on your threading dial hasn't come loose as well. Been there - done that.
Willie
Reply
Thanks given by:
DaveH Offline
Child of the 60's
******
Posts: 4,513
Threads: 139
Joined: Feb 2012
Location: Kempton Park. South Africa
#7
10-22-2013, 04:25 PM
Ed,
Just wondering if you set the compound slide over to 29/30 degs to do your screw cutting, or do you go straight in?
Smiley-eatdrink004
DaveH
Reply
Thanks given by:
EdK Offline
Whipping Boy
*******
Posts: 8,859
Threads: 319
Joined: Feb 2012
Location: Arizona/Minnesota
#8
10-22-2013, 04:25 PM
John,

Nothing I can see is slipping, but obviously something has to be and if it is, it's doing it very precisely and consistently. I'll investigate that some more.

Chuck,

All lathes have some backlash but it would be taken up once the tool starts moving towards the headstock.

Willie,

The change gears are correct, otherwise I wouldn't be getting the correct TPI. I did triple check them though. My lathe does not have separate gears for imperial and metric. You swap places of two large gears to change from imperial to metric and I did check those. I also checked the threading dial gear, that's one of the first things I suspected.

I did try cutting the threads with the half nuts engaged at all times and it did cut the thread correctly, but I already knew it would. So I'm still perplexed as to what is going on. I think I'll sleep on it and maybe I'll have an answer come in a dream. Chin

Ed
Reply
Thanks given by:
stevec Offline
69 yr. old senior
*****
Posts: 2,685
Threads: 29
Joined: Feb 2012
Location: Nova Scotia, Canada
#9
10-22-2013, 04:27 PM
I hate to say this but, If the threading dial sports an "INDICAOR TABLE" any number of other things could be wrong. 17428

Willie's thought about the thread indicator gear having loosened is worth checking out.
Busy Bee 12-36 lathe, Busy Bee Mill drill, Busy Bee 4x6 bandsaw, Homemade 9x17 bandsaw, Ad infinitum.
Reply
Thanks given by:
EdK Offline
Whipping Boy
*******
Posts: 8,859
Threads: 319
Joined: Feb 2012
Location: Arizona/Minnesota
#10
10-22-2013, 04:28 PM
(10-22-2013, 04:25 PM)DaveH Wrote: Ed,
Just wondering if you set the compound slide over to 29/30 degs to do your screw cutting, or do you go straight in?
Smiley-eatdrink004
DaveH

Dave,

I have it set at about 29 degrees. Angled or straight in shouldn't cause it to cut two threads though.

Ed
Reply
Thanks given by:




Users browsing this thread: 4 Guest(s)