My threads is riding up!

Casull

Casull

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Okay, I need more threading advice. When I am cutting threads my diameter is growing and not just because I like cutting threads so much! Say, I start with 1.1", by the time I get them cut and looking good, my diameter is up to about 1.15" or thereabout. Obviously I am pushing metal up. Sharper threading tool? Or do you guys just start with a reduced diameter and accept it like 1.05" or something?
Also, how deep is considered proper for a thread, say a 1" 60º 16tpi depth? Should I be cutting out something like .050" or so?
Any advice is appreciated. Please don't quote my numbers, I am just using an example.
Rob
 
Are you locking your slide and compounds??
A sharp tool should remove the same depth across the entire cut, if your tooling is moving that will alter your cut depth. Set a mag based dial indicator on the opposite side of your barrelshank and see if your cross is moving away from the barrel during threading, same thing for your compound and tool head, that way you can eliminate which if any is causing the problem.
Are you chucking muzzle and using steady rest and live center in tailstock or just grabbing the shank?
Also how tight is your lathe? If you lead screw is worn that won't help either.
Another possibility is your live center, see how tight it is.
Many possibilities, you have not provided enough info to be more specific

I cut my male threads .001 to .002" less than the female depth to get as tight a tolerance as possible.
 
The only way that the diameter is getting larger is if metal is being displaced, rather than cut. I would check the geometry and setting of your cutting tool. If you examine the thread with a magnifier, can you see if a burr is being thrown up - or some other distortion? My practice is to turn the shank to diameter, and then cut the threads until the barrel screws into the receiver with a good fit.
 
ATR, I don't mean the thread is riding from one end of the shank to the other, I mean the diameter of the top of my thread is larger when I'm done than when I start.
I am not locking my cross slide, I will check that. My lathe is nice and tight, but I don't know how much it would take to do what I mean.
Tiriaq, this is the way I have been fitting them and sure it works, I just thought there was a number to start with or I was doing something not quite right and maybe I am.
Cutting the male threads slightly smaller makes sense to me. What I am trying to ask is, is this normal or am I buggering something?
 
The depth of thread changes with the number of threads per inch(TPI) There are formulas to calculate this, but if you pick up a machinists pocket reference, it will have tables listing everything you need to cut a specific thread. It has lots of other handy info as well. If you want real fun, try cutting square threads, and grinding the proper leading and following angles on the toolbit to cut them.

Pat
 
Casull , possibly you are not at true 90 degrees to your center line, thereby displacing metal as Tiriag mentions, how sharp is your tooling and what sort if any of chip breaker have you cut into it? Sorting these thing out by internet is even harder than by phone, wish I could see what you had going on so I could be a tad more helpful
 
Casull,
I am only an apprentice machinist therefore maybe not too much help. But i do a fair amount of single point threading some days i may do 10 or 15 different threading jobs . One thing to remember is that your thread pitch will basically dictate the depth you need to cut your thread . To get pitch you divide 1" into the # of threads EG 1 into 12 + .083333 . And that aplies to all threads that are 12TPI whether they are 1" or 4" or whatever . By the way for internal threading to figure the diameter to start with before threading all you do is take the thread size say a 3.00" 12tpi and subtract the pitch EG 3.00 -.08333 =2.9166 would be your starting size.
If you have a Starret 60deg. center gauge or similar tool for setting up/aligning your threading tool on it you will ussually find rough calculation for depth of cut for the various pitches .That is only a guideline as a lot depends on how the female thread was cut and to what tolerances , some threading tools have a very sharp point some have round some are squared . So there are many variables . I have learned from my boss and also in discussion with some guys who build benchrest rifles , do not get your threads too tight , there is no need for it. The strength in a thread is on the side of the thread 30 degs. not on the top or bottom.
As far your threads getting bigger the only way i can see that happening is metal is being displaced instead of removed , i have had that happen a couple of times (well maybe more :( ) and a few times it was a dull tool and /or bad relief angles the only other thing that i can think of and i may be way off base here but is your cutting tip "on center"? i've made that mistake also.
Any how I hope i have been a bit of a help and i am hopeing my info is correct , if not someone please correct me .
If you do not already have one pick up a copy of the machinist's handbook it is a must have for machining and you can get older copies fairly cheap .
Happy Threading
 
Well, it's never been claimed that my tooling was that sharp to start with.
I've got lots of new carbide bits but to be honest, most of what I've been doing is with a 55º cutter that I ground myself to do Mauser threads. My wife printed a pattern for a 55º angle in AutoCad. Yes, I married well above myself. I don't even know what you mean by a chip breaker. I have lots of books and my threads are much better for sure and steadily improving. This growth of the threads seems consistent in my work though. First off I guess I put in a new 60º cutter and try like hell to make sure it's 90º to the work and see what happens.
Where would I pick up one of these pocket machinist's references?
Winchester 070, that formula makes sense to me, this I like! I do have a 60º gauge but have had no luck finding a 55º gauge.
Guntech, my threading has been with HSS. I can't say about the paper, I haven't tried that. Interesting that two of you say to go looser than tighter, I had been told exactly the opposite and was trying hard to get them tight.(Threads)
 
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Are you threading with a sharp HSS bit or carbide?

Carbide requires more pressure...

I think what you may be measuring is a small burr developing at the top of the thread.... it only take a couple of thousands and you have twice that on the diameter..

Does this "burr" remove easily with 320 grit paper? If it does don't worry about it.

How deep I cut my threads on a barrel..... until the action threads on fairly easily by hand. A little loose is better than a little tight. The 60 degree threads center correctly if a little loose rather than too tight.
 
You can see if your threading tool is dragging on the backside by putting some black marker on the cutting face of your threading tool. Take a pass and observe where the marker comes off.

For O.D. threading, your tool should be slightly below centerline of your workpiece. For I.D. threading, your tool should be slightly above centerline of your workpiece. This applies for all tooling except centerline drilling which should of course be on the centerline.

Usually 70% thread is considered tight. I think the standard is little more than 60%. Indeed the more surface area of the side of the thread you have, the more friction (holding) ability you have. It just makes it easier to assemble/disassemble without binding if you have flats on the top of your male thread.

It's hard to tell over the internet but if you have a machinist buddy to come by, I bet he could tell you right away after looking at a pass being made what is wrong. Major O.D. shouldn't grow with threading. Good luck.
 
As many have already pointed out, your cutter is displacing metal probably due to insufficient clearance on the leading edge/face. Another possibility is that you have your cutter sitting above center. Regards, Bill.
 
Lots of great advice, and as far as depths go pick up a Machinery's handbook. Its a great reference for all this sort of stuff...

h ttp://www.industrialpress.com/en/mh.asp
 
Casull, remeber that 'rolled' threads are stronger than cut threads, so maybe you are just ahead of your time! Maybe you are pioneering a technique to cold-forge the barrel threads and one day we'll all be using the "Casull Technique" to do our barrels.

;-)
 
I would suggest swinging your compound off perpendicular by 29 degrees. Keep the tool perpendicular to the workpiece. By feeding the threading tool at 29 degrees, it will cut only on the leading edge, where it it intended to cut.

Major diameter increases are comon, and easily removed with a single cut lathe file.

If you want accurate thread measurements, use measurement over wires method as explained in the machinery handbook.
 
I use 29.5 degree setting, this is for 60 degree v threads NOT 55 degree whitworth threads for a mauser. 29 DEGREE'S IS THE WRONG SETTING IF YOU ARE USING A 55 DEGREE THREADING BIT. You should be using 27 degree's or so for 55 whitworth.
 
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