Gunsmithing And Yet Another Thread on Threading and Methodology

[Bully]

So, I've cut a couple of threads. I'm certainly no expert, but I do understand the concepts fairly well. That said, I know that folks will debate the 29.5* compound angle vs. the 30* compound angle for single point thread cutting until the end of time. And that's all well and good. However I recently watched a video by a fairly well respected 'smith on chambering and threading and his compound was set at 90*. I'm wondering if anyone can explain it and if/why it may work for him but not be recommended. I do believe he is using a carbide insert/full profile tool, which may be part of it and not something I'm at all familiar with.

I'm hoping we can all avoid a pissing match. I'm simply looking for information and hope we can all respect each other's methods.

Thanks in advance.

 

[Bugholes]

Threading at 90deg with a full profile tool is fine if your lathe is up to it
I'm not a machinist but you can reduce tool pressure and deflection by cutting at an angle closer to 30 degrees

 

[Supersubes]

As bugholes said, tool pressure will be much higher since you’ll be simultaneously cutting on both flanks. I have have one machine that will easily take that, and one machine where I wouldn’t even try it, as it just doesn’t have the rigidity.

 

[Bully]

Thanks y'all.

 

[RonA]

Even on a CNC where rigidity isn't an issue, a G76 threading cycle asks for thread angle so it can feed in at an angle to reduce tool pressure. Listen to a G92 cycle versus a G76 and you can hear the difference. Even on a 8k# machine. Tool life is greatly improved as well. Valenite makes a grade of laydown insert(grade 5010) that is amazing on stainless and mild steel. I machine stainless almost exclusively and the 5010 stuff is all I use for turning, boring, and threading.

 

[Rubicon Precision]

Even on a CNC where rigidity isn't an issue, a G76 threading cycle asks for thread angle so it can feed in at an angle to reduce tool pressure. Listen to a G92 cycle versus a G76 and you can hear the difference. Even on a 8k# machine. Tool life is greatly improved as well. Valenite makes a grade of laydown insert(grade 5010) that is amazing on stainless and mild steel. I machine stainless almost exclusively and the 5010 stuff is all I use for turning, boring, and threading.

This is why I also use G76.

 

[Bully]

Even on a CNC where rigidity isn't an issue, a G76 threading cycle asks for thread angle so it can feed in at an angle to reduce tool pressure. Listen to a G92 cycle versus a G76 and you can hear the difference. Even on a 8k# machine. Tool life is greatly improved as well. Valenite makes a grade of laydown insert(grade 5010) that is amazing on stainless and mild steel. I machine stainless almost exclusively and the 5010 stuff is all I use for turning, boring, and threading.

This is why I also use G76.

I have no idea what either of you is talking about but thank you for the input.

 

[RonA]

I have no idea what either of you is talking about but thank you for the input.

Basically, CNC's use two methods of threading. One feeds straight in, the other at an angle. The one that feeds in at an angle has less tool pressure and less chance of tool breakage than the other. Generally I touch off the tool on the od of the turned section and zero x axis with the compound zeroed and ful forward. Then I multiply the pitch of he thread I am cutting by the cosine of 30 degrees. Multiply that by .70(75% thread). Multiply that by 2 and subtract that from the full major diameter. From X dial in the amount calculated and re-zero the x axis. Using your compound set at around 29.5 deg off perpendicular, start taking progressively smaller cuts till your cut a zero on the compound. At this point you should be around .005 to .010 big on the pitch diameter. Dial in how much you need to go on X and re-zero. I usually shoot for the top of the pitch diameter tolerance, then dial in the x axis for the last .001- .002 for finish. Usually 2 final cuts at finish is good.

 

[Bully]

Basically, CNC's use two methods of threading. One feeds straight in, the other at an angle. The one that feeds in at an angle has less tool pressure and less chance of tool breakage than the other. Generally I touch off the tool on the od of the turned section and zero x axis with the compound zeroed and ful forward. Then I multiply the pitch of he thread I am cutting by the cosine of 30 degrees. Multiply that by .75(75% thread). Multiply that by 2 and subtract that from the full major diameter. From X dial in the amount calculated and re-zero the x axis. Using your compound set at around 29.5 deg off perpendicular, start taking progressively smaller cuts till your cut a zero on the compound. At this point you should be around .005 to .010 big on the pitch diameter. Dial in how much you need to go on X and re-zero. I usually shoot for the top of the pitch diameter tolerance, then dial in the x axis for the last .001- .002 for finish. Usually 2 final cuts at finish is good.

I think my head may have exploded.

Yup.

Cleaning up the mess now.

I'm a hairdresser. This is all VERY foreign to me. I play on a very manual machine. No DRO. So what you just typed, and typed clearly, is a lot for me to digest. I'll have to re-read it tomorrow. And probably the day after. Thanks.

 

[RonA]

I was afraid it might sound complicated, but it really isn't once you've done it a few times. This gives you a safe rough calculation for setting an X axis zero so you don't have to keep checking with a thread mic until the last few thousandths.

 

[Bully]

It's all good and I appreciate the feedback.
I'm just gonna go in at a 29.5ish angle and color it done. I'm not sure why the gent I watched goes in at a 90 (perpendicular to the work) but he does and it seems to work very well for him. He is on a manual machine as well, fwiw.

 

[ptm121]

One nice thing about feeding in with the cross-slide is that you can use the DRO to check DOC. Or, you can set the cross-slide to 90 and then use the dial with no trig conversion. I don't care about that, because I do it this way:

1) Cut the relief
2) Touch the threading tool off the OD, and zero the DRO/dial on cross-slide and compound
3) Plunge (into the air of the relief, no metal contact) until you're at the minimum DOC for that thread.
4) Lock the cross-slide
5) Dial the compound back out to clear the part - when you thread, just dial it back in until it hits "0". That should have you perfect or slightly oversized.
6)Measure over wires and finish to dimension with either compound or cross-slide.

I prefer to run the thread at ~29deg with the compound, and then take the final fine cut to dimension with the cross-slide. I find this gives the best finish using my tooling. But depending on the setup, I might thread away from or towards the headstock, with an upside down or right side up tool, or even a boring bar on the backside of the part with a threading insert.

I also prefer to do the math and use a single pitch wire when using a partial-profile tool. This only works if you know your OD precisely, because the mic rests on the thread crests on the other side of the part. I take the extra time to hit my OD to within a couple tenths, and then DOC and a single-wire let me nail the PD.

But, I normally use a full-profile insert, and as there are really only 5-6 common thread pitches, I have the numbers worked out in advance for all of them. I cut a few thou oversized, cut at 29deg to minimum thread depth (per steps above, but finishing a little oversized because the OD was oversized), then plunge straight in until I hit the OD I want (usually 1 thou under max). Then I check the PD, but it's always perfect with good (ISCAR) inserts.

The only time I cut threads to fit (instead of to spec) is when I'm mating to something out of spec. Some 'smiths like to cut to a really close fit, but it doesn't really buy you anything. Thread form and alignment is the key, it'll self-center and align off the shoulder.

That brings me to another point - the shoulder is where the magic really happens. Take the time to cut a really perfect shoulder, with appropriate relief or radius at the inside corner. Be sure you're not getting a little play in the carriage or compound when you cut it - I prefer to set a hard stop and keep a little pressure on the carriage handwheel while I face the shoulder off.

There are a huge number of ways to skin this cat. Find a set of techniques and tools that work for you, and stick with them.