Gunsmithing Threading Help?

[woodsy]

Need some advise from the smiths here:

Finally got around to cutting the barrel on my Remy PSS. The goal was to cut 4" off, crown and thead for a flash hider.

Using a Grizzly G4003G lathe I got her cut, dialed in, turned down, crowned, and threaded for a good fit of the hider. Everything fits well and looks prof (for the most part).

However, I'm having a hard time getting smooth crests on the threads. I was threading at 70 rpm and using a sharp HSS bit with good cutting oil. Made 3 cleanup passes but that didn't smooth things out as well as I thought it would.

The fit is good and the hider screwed on snug with no problems. I'm not worried about any problems with the setup so I guess we're only talking about looks at this point.

So, how do I get threads turning out as well as I see here on the Hide? Any and all advise appreciated.

 

[wnroscoe]

Re: Threading Help?

With partial profile tooling it's typical to get "Trash" or metal build up on the thread crest during threading. I get it as well. Simply take a small file and dress down the "Trash" towards the final few threading pass's. Very slight pressure is all that’s needed then continue with your treading. The new thread crest will be silky smooth, as they should be. You could also change your tooling to full profile threading inserts.

Try to make the change over to carbide insert tooling at some point, full or partial profile and increase your threading RPM. It's very easy to thread at 250+rpm on a manual machine, the CNC does it at 1000+RPM. 70 rpm in my opinion is way too slow.

 

[woodsy]

Re: Threading Help?

Ah ha! Never thought about breaking the crests with a file part of the way through the job. I'll try that next time.

Switching tooling won't be a problem as I'm pretty sure that I have inserts around here somewhere. Not sure about threading to a shoulder at 250 rpm though. I need a little more practice before I try that. Ha!

 

[jonaddis84]

Re: Threading Help?

Ive been using a Arthur Warner HSS indexable threader for awhile now, running it at 70 RPM with some good oil coating the tennon I get some pretty nice looking threads. These are straight from the threading tool, no abrasive touch up at all.

 

[Mark Housel]

Re: Threading Help?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jonaddis84</div><div class="ubbcode-body">Ive been using a Arthur Warner HSS indexable threader for awhile now, running it at 70 RPM with some good oil coating the tennon I get some pretty nice looking threads. These are straight from the threading tool, no abrasive touch up at all
</div></div>
Those look like <span style="font-weight: bold">nice </span>threads to me.
I just received a threading tool and inserts from this company. I'm looking forward to trying them out.

 

[turbo54]

Re: Threading Help?

More RPM helps a lot, as does a SWEETLY ground tool very precisely set up.

250rpm oughtta only take a little practice on your part to achieve.

 

[ugsly]

Re: Threading Help?

I have flipped a threading tool upside down in the holder and run the lathe in back gear and thread away from the shoulder. Can thread as fast as you want if you can hit your numbers every time. Just an option, don't know if there's any drawbacks to it or not.

Kc

 

[jonaddis84]

Re: Threading Help?

How are you advancing the tool in the precise amount and engaging the half nut perfectly when threading reverse? That sounds even more difficult than stopping at the shoulder in forward. Unless, are you cutting a relief at the shoulder?

 

[woodsy]

Re: Threading Help?

I thought about threading in reverse last night.

Wondered if a guy couldn't flip the bit and thread in reverse easier using a higher speed. This would eliminate any danger of crashing into the shoulder but would mean that you'd have to:
1. Begin at the same location every time (using a DRO or indicator)
2. Keep your half nut engaged for each pass (one less handle to mess with)
3. Advance your bit a millisecond after hitting "go"

Crazy thinking or not?

 

[KaizenUSMC]

Re: Threading Help?

http://www.youtube.com/user/mrpete222#p/u/52/w2eWqeyO7-4

- Sean

 

[LRI]

Re: Threading Help?

Fundamentals:

First ensure your tooling has the proper geometry. The relief angle under the cutting surface should be prolly around 8*.

Ground HSS tooling should be sharp. Best way I've found to maintenance this is to stone the top surface. Wipe towards the back of the tool. Sharp means sharp. Your cutting. Your not using the tool the same way carbide inserts work.

Assuming this is all good to go, onto setup:

Again, fundamentals. Ensure your tool is at a right angle to the center of the part. Not above/below center. The way I've always done/did this is with a 6" scale. Squish it between the part and the tool. If it rocks away from the tool post your over center. If it rocks towards you its under.

Next, getting the bisection of your form tool (which is what a threading insert really is) square to the Z axis of your part. A thread fish does this fairly well. Unless the tool is ground on a surface grinder, checked in a comparator, and indicated on the cross slide, there's no really good way (at least that I'm aware of) to know other than with a gauge. The thread fish does a pretty good job.

Barrel steels are generally classified as free machining. Some recently are a little more prone to work hardening. That being said you should shoot for 30-50 SFM (Surface Feet per Minute) at your tool.

Formula: SFM = 0.262 x D x RPM

On a 1.0625" thread at 70 rpm your only at 19SFM

Buzz that biche up to somewhere between 140-160 rpm. The shoulder will come at you a bit sooner, but with some practice its easy stuff.

Practice, practice, practice.

Cutting fluids:

Your big challenge with HSS tooling is heat management. Oil is a terrible coolant. (why its used to quench steels after heat treat, it's not as fast as water) Water based coolants are much more efficient.

So, if you have flood coolant, whip up a mix of around 9% solution and have no fear.

If you don't have coolant, then switch to a cutting oil. The stuff with lots of sulfur works pretty good. Mobile is what I used at Dakota Arms and it wasn't too bad. Pipe tapping oil is pretty good too.

The oil isn't designed to manage heat. It's reducing friction by lubricating the cutting surface. This does manage heat I guess, but its bigger purpose is to make life easier on the tool. It'll help with mitigating chip weld (sulfur does this) too.

Compound angles.

If your machine is robust, run your compound straight in. If it's got some slop, try the 29.5* trick. I'd shoot for a .01" depth of cut on your tool to start following up with .005" finish passes.

Light cuts with HSS can lead to work hardened material. Then you have a mess. You want to cut steel, no rub up against it in hopes of peeling a little layer off. At full depth you have a fair amount of tool/material contact which invites chatter. Make sure gibs are tight, tool reach is kept to only what's required for the job, and that your loading both the compound and cross slide towards center.

Hope this helped.

C.

 

[hero's machine]

Re: Threading Help?

Woodsy, Are you cutting your tennon to exact major or giving a couple thou to allow the top of the threads to not come to a point?

Chad, you must have some mad keyboarding skills, that was a mouthful, and I agree.

 

[woodsy]

Re: Threading Help?

This was for a FH threaded 5/8x24 so I turned major to .625. Got it right on the money but maybe I should have left it a thou or two proud. I actually thought about doing that and finishing it up with a die but didn't want to add an extra step.

Chad - thanks a ton for taking the time to write the great info! Looks like I have my setup nailed pretty well so I think the problem could be taken care of with more rpms. I still have the stub of the barrel that I cut off so I'll try spinning this thing up and see what I get.

All great info guys and I appreciate the help! Keep 'em coming.

 

[jonaddis84]

Re: Threading Help?

Try taking your major down to .622 or so, unless your tooling is perfectly sharp and setup on the money youre likely to build up a bit of material on the crest of the thread and the major will grow slightly. Dont forget, your minimum major diameter even for 3A threads is .617. I think you are better off with a smaller major than having to take your tool in deeper to get a fit.

My procedure is with 29.5 compound, first pass set at .010, then .015, then .020, then .022, after that I advance the crosslide .001 and check each time until it fits.

 

[2156SMK]

Re: Threading Help?

Looking at this drawing may help:

I use 0.620" major diameter on the thread tennon for 5/8-24 threads.
You can't go to a perfect point on external threads because the internal threads have a radiused root.