Threading fit issue, tapered?

[mksj]

If you are seeing it both ways of threading then it sounds like a headstock alignment issue. Technically the the rigidity would increase as you get closer to the headstock so when threading toward the headstock you should get less deflection (deeper cut) the closer you get to the chuck. The other possibility is that the chuck jaws are not holding the piece true to center as you move away from the chuck, so there is a little more wobble the further you get away from the chuck which causes the thread to be slightly deeper the further away from the chuck. I like to use Rollies Dads Method to look at both headstock alignment and also +/- swing the as you mover further away from the chuck. I use a ground precision bar and a 0.0001" dial indicator with a flat tip. Example below, the +/- swing remains the same, there is a few ten thousandths swing as one moves away from the chuck. Then do some light test cuts in say 1.5" aluminum rod and measure diameters with a micrometer.

 

[xr650rRider]

The True bore alignment system along with a 6 jaw chuck, is getting the workholding a long way from the spindle. If your cutting a taper and it's larger toward the headstock, than the tailstock end of the workpiece it's skewed toward the operator. I've seen some videos where they are dialing the TBAS in only one spot, but one spot running true does not mean the part is running true. Joe Pie has a video showing that exact thing. Your hopefully indicating on the bore with a long reach indicator. You should be able to dial-in a section 1.5 - 2" apart and get .0002" or less runout or I won't start turning the tenon. After you cut the diameter on the tenon at that point, the OD will be concentric to the bore. You should not be able to measure a taper at that point unless something is moving during the operation. If the tenon has no taper, then threads shouldn't be tapered, again unless something is moving.

 

[jareese]

The True bore alignment system along with a 6 jaw chuck, is getting the workholding a long way from the spindle. If your cutting a taper and it's larger toward the headstock, than the tailstock end of the workpiece it's skewed toward the operator. I've seen some videos where they are dialing the TBAS in only one spot, but one spot running true does not mean the part is running true. Joe Pie has a video showing that exact thing. Your hopefully indicating on the bore with a long reach indicator. You should be able to dial-in a section 1.5 - 2" apart and get .0002" or less runout or I won't start turning the tenon. After you cut the diameter on the tenon at that point, the OD will be concentric to the bore. You should not be able to measure a taper at that point unless something is moving during the operation. If the tenon has no taper, then threads shouldn't be tapered, again unless something is moving.

Agreed - I am using long reach indicators.
I will try some different threading processes and see where I can find improvement.

Like I said - all threads are within spec 3A to 2A, (as far as fitment, not measurement other than OD) it's just me being me about it.

 

[Janderso]

Insert (carbide) tools are rarely sharp on the cutting edge and that results in increased tool pressure which can result in taper. I use HSS tools from Aloris that are sharp and easily sharpen able and adjustable for helix. They are available in several widths and pitches (width of flat on the end). They are sharpened ONLY on the top. and last for years in regular frequent use.

I agree. The Aloris tool is always sharp with a simple touch up. I seem to break the carbide tips so I went back to this.

 

[Janderso]

I use a fixture I made that will grind regular as well as Acmes . Of course you need a surface grinder to do so . Same as the Aloris , if it gets dull , dust the top off and you're good to go . CNCs I did use inserts . I had the surface speed needed to use them and the auto threading cycles made it a breeze with no worry of crashing . If you're using low speed which I'm sure most manuals are running while threading , carbide isn't the choice .

That's why I break the carbide tips. Duh.....

 

[Janderso]

Thats low surface speed for carbide on such a small thread . Also , If you're starting out in an undercut your leadscrew needs time to catch up . This may be the problem . Even on CNC machines , it's common practice to start the thread 3Xs the thread pitch away from the part . You'll get it !

I wish I had half your brain

 

[mmcmdl]

I wish I had half your brain

Half of mine is missing already and I can't remember who got it Jeff .

 

[xr650rRider]

Agreed - I am using long reach indicators.
I will try some different threading processes and see where I can find improvement.

Like I said - all threads are within spec 3A to 2A, (as far as fitment, not measurement other than OD) it's just me being me about it.

You need to be using the 3 wire method or a thread pitch micrometer. Check the thread at both ends and measure to see if there is a taper. If there is, you have alignment issues, if not, you may be experiencing friction from a thread that isn't quite to spec. The further you try to screw it on, the more contact the thread surfaces have. When it's right, it's right. I use carbide almost exclusively and they are even cheap inserts. I use a Shar's left hand holder with 16IR inserts upside down, threading away from the chuck. I've cut 100's of feet of thread and I'm on 2nd corner of original insert. Usually 250-400 RPM or so. Depends on thread pitch but a lot of barrels that are 416SS.

OD of a thread major diameter does not determine if a thread is 3A or 2A, the thread pitch does. In fact you usually file the tops off of the threads.

Making muzzle brakes and thread protectors and relying on fit with what you have in hand is almost a guarantee they may not fit other barrels. A lot of times for internal threads on a brake, I'll cut 50% of the thread single point and then use a tap to finish it up. Then if you fit the muzzle thread to that, there is a good chance most devices will screw on without issues.

 

[Just for fun]

Good reading guys, lots of info here.

Jareese, Good luck figuring it out. I agree with xr650 a nice thread pitch micrometer is the right way to check the thread.

 

[jareese]

Good reading guys, lots of info here.

Jareese, Good luck figuring it out. I agree with xr650 a nice thread pitch micrometer is the right way to check the thread.

Absolutely - thanks everyone for the insight and tips. All well taken and absorbed.

I will be trying different methods and will start measuring with my thread Mic's, just need to get used to them.