Gunsmithing Question for guys threading muzzle 1/2x28

[JJones75]

What do you turn the muzzle down to before you start cutting threads?

I have always turned it down to .501" so after creasting and polishing the threads they end up .5000"-.4990".

I have recently threaded to barrels for a buddy both came out at .4990" OD of the thread and neither would fit any of his three suppressors , so I took his can and make so threaded plugs in progressively smaller sizes and they ended up fitting at between .4970"-.4985"
So for shits and giggles I checked 4 of his guns that came factory threaded 1/2x28 (2 AR's and 2 -10/22's) and the OD of their threads ranged from .4943"-.4962"

 

[mcfred]

Re: Question for guys threading muzzle 1/2x28

Pitch mics are the most reliable way to measure external threads, and just measuring the OD you generally want them a little under size so the thread points aren't so sharp. The very tip of the thread doesn't do much for strength, and having a little corner for crud to go means less chance of binding, galling, etc.

 

[JJones75]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: McFred</div><div class="ubbcode-body">Pitch mics are the most reliable way to measure external threads, and just measuring the OD you generally want them a little under size so the thread points aren't so sharp. The very tip of the thread doesn't do much for strength, and having a little corner for crud to go means less chance of binding, galling, etc. </div></div>

Hence the -.001-.002" from cresting and polishing

 

[skog]

Re: Question for guys threading muzzle 1/2x28

Look up class 2 and class 3 thread dimensions. The crest plays little part in thread fit. The pitch does all the work. Are you threading with a die? Otherwise you would continue cutting with a single point until the part fits. I would definitely not be threading barrels with a die for suppressors.

 

[jbmillard]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JJones75</div><div class="ubbcode-body">What do you turn the muzzle down to before you start cutting threads?

I have always turned it down to .501" so after creasting and polishing the threads they end up .5000"-.4990".

I have recently threaded to barrels for a buddy both came out at .4990" OD of the thread and neither would fit any of his three suppressors , so I took his can and make so threaded plugs in progressively smaller sizes and they ended up fitting at between .4970"-.4985"
So for shits and giggles I checked 4 of his guns that came factory threaded 1/2x28 (2 AR's and 2 -10/22's) and the OD of their threads ranged from .4943"-.4962"
</div></div>

I'm not surprised they didn't fit. You're probably getting interference between the crests of the threads and the internal thread of what you're putting on the barrel. Also, how sharp is the tool you're using? You have to have a pretty small radius to cut these threads. I've only threaded barrels for the YHM supressor mount, but I used their instructions (class 2A as I remember) and this drawing:

Unified Thread

This matches what is in my Machinery's handbook.

It would appear to me that the correct diameter is

0.500 - 2.0*0.125*(0.866*(1/28) = 0.492

I get this from the drawing.

H = 0.866*P = 0.866*(1/28) = 0.0309

Then the Major Diameter is just

0.500 - 2.0*H/8 = 0.492"

Maybe some of the machinists on this site will chime in and correct any mistakes I've made.

Brad

 

[JJones75]

Re: Question for guys threading muzzle 1/2x28

Im using a very very sharp threading tool , so much that i stopped using carbide inserts because of the radius they had , as i though that may have been the problem with the vally not being deep enough

 

[300sniper]

Re: Question for guys threading muzzle 1/2x28

how are you gauging the pitch diameter?

 

[JJones75]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">how are you gauging the pitch diameter? </div></div>

Wire thread gages

 

[300sniper]

Re: Question for guys threading muzzle 1/2x28

did you double check that you are using the correct size wires by actually miking them, not pulling them out of the sleeve marked with the size you need? my guess is the thread pitch is messed up somehow and the crest/root have nothing to do with the issue.

edit:
a 1/2-28 3a has a minimum of .4935" and a max of .5000" major diameter.
a 1/2-28 2a has a minimum of .4989" and a max of .4924" major diameter.

 

[2156SMK]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">how are you gauging the pitch diameter? </div></div>

I have a full set of Starrett thread pitch micrometers.

 

[2156SMK]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JJones75</div><div class="ubbcode-body">What do you turn the muzzle down to before you start cutting threads?</div></div>

I am usually right at 0.495" OD and 0.474" PD.
It makes a nice snug fit.

 

[cal50]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JJones75</div><div class="ubbcode-body">What do you turn the muzzle down to before you start cutting threads?

I have always turned it down to .501" so after creasting and polishing the threads they end up .5000"-.4990".

I have recently threaded to barrels for a buddy both came out at .4990" OD of the thread and neither would fit any of his three suppressors , so I took his can and make so threaded plugs in progressively smaller sizes and they ended up fitting at between .4970"-.4985"
So for shits and giggles I checked 4 of his guns that came factory threaded 1/2x28 (2 AR's and 2 -10/22's) and the OD of their threads ranged from .4943"-.4962"
</div></div>


Once the bore ID is properly indicated in....

Turn OD to .495 diameter.

Chamfer thread lead and undercut thread relief while facing shoulder to thread minor diameter.

Single point cut threads. If its for a flash hider I will thread to near the high PD. If its for a suppressor I will get close to max PD then use the actual mating part for a nice fit.

 

[Randfal]

Re: Question for guys threading muzzle 1/2x28

I agree with undersizing. I usually thread until snug fit though. But .497-.495" is a good starting point. Takes very little to polish off a thou or two if that's too tight, but you can't add it if it's too loose.

 

[wnroscoe]

Re: Question for guys threading muzzle 1/2x28

I turn the tenon down to .498" - .500" and thread to fit a 3A ring gauge, same as I would 5/8x24 (.623"-.625") or 9/16x24 (.560-.5625")

Suppressor threads are all over the road and is why I've chosen to use ring gauges. If you used thread wires and went by the numbers and the suppressors still don't fit, I'd say the suppressor threads are out of spec.

 

[cal50]

Re: Question for guys threading muzzle 1/2x28

Some low end suppressors look like a simple tap was run through the mounting end (similar to a flash hider). My AWC cans have what looks like a CNC cut thread. No run out and precise on the small side for pitch diameter for a wobble free mount.

Threading is not rocket science but can be a lot more refined from cranking a button die over a barrel stub or running a tap into a hole. Do it right the 1st time and its good for life or until Murphy screws you.

 

[crumpmd]

Re: Question for guys threading muzzle 1/2x28

JJ
I do as you do. I turn to precisely .500. I cut to the intended size and check fit. Usually too tight. And after working off the sharp edges of the crest and polishing it will get started. With my Omega I always have to cut a little (.002) deeper that I expected.
I may try starting at .495 next time.

 

[LRI]

Re: Question for guys threading muzzle 1/2x28

Jim Borden and I just recently had a conversation that sort of runs with this topic.

The fractional size of a thread is nominal. As with anything related to machining, threads have a tolerance window. In a case like this it'd likely be + zero/ - whatever class.

If I were cutting a thread for a part, I look at the nominal size and subtract a couple .001's to start with my outside diameter. In this case, a .500" thread would end up at .498".

Then we start making the wrinkles. I use thread wires. Simple, proven, and cheap. Thread micrometers are awesome. So's the price.

If your using a full form insert (one that dresses the crest of the thread) then it's really not an issue. You just keep making passes till the pitch diameter catches up to what your after. Where the OD ends up is where it ends up. In theory you could have your cylinder +.05" and it wouldn't matter. The insert would whittle away at the OD as it cut threads. Your shoulder would look a little funky, but you get the idea.

Pitch diameter is where its at when it comes to thread fits. A sharp crest shoved into a shallow root on the opposite part just invites galling and permanently attaching two parts. There must be clearance to ensure a long and dependable service life. With SS it should be a standard operating procedure to allow clearance.

In your case having .002-.005 extra clearance on the diameters won't hurt a thing. There's still plenty of thread contact on the flanks to adequately align/support the brake/can being installed. It's not half assing if the pitch diameter gets its due attention. If you look at the fancy drawings about threads the crest (top of the peak on a thread) typically is shown with a radius. If your using a full form insert then its handled for you. If your not, then the -.002-.005 on the OD effectively creates this when your threading. A non cresting insert will just leave the flat at the top. (think of it as a sort of acme thread, only really, really small)

My point is regardless of how you get there, the clearance needs to be present if you plan to remove/install the device with any frequency.



Hope this helps.

C.

 

[2156SMK]

Re: Question for guys threading muzzle 1/2x28

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: C. Dixon</div><div class="ubbcode-body">I use thread wires. Simple, proven, and cheap. Thread micrometers are awesome. So's the price.

If your using a full form insert (one that dresses the crest of the thread) then it's really not an issue. You just keep making passes till the pitch diameter catches up to what your after. Where the OD ends up is where it ends up. In theory you could have your cylinder +.05" and it wouldn't matter. The insert would whittle away at the OD as it cut threads.</div></div>

I picked my starrett thread mics up on ebay.
It took a year or more to chase them all down, but I got most of them for $40 to $50 each.

Go splurge and at least buy yourself the ones for 16 and 24 threads.
Once you start using thread mikes, you will dread the wires!

Full form inserts are growing on me.
I been threading and cleaning up the crests with a thread file for a long time, but every time I use a full form insert, it makes me want to go buy inserts for all the other pitches I don't have yet...

eta: you know you want this one for barrel threads:
http://www.ebay.com/itm/1-2-STARRETT-SCR...=item19ca37fe82

 

[dunes450r]

Re: Question for guys threading muzzle 1/2x28

i have always used single point threading, made a thread protector using a tap from a local distributor first then just threaded until i got a nice fit with the TP

 

[Hooper]

Re: Question for guys threading muzzle 1/2x28

.498" with last pass being a finish cut that provides a smooth finish. All profile work relating to customer/drawing requirements such as thread relief, o-ring grooves, etc.. are done pre threading.

I have found that to prevent crest growth during threading, it is best to make .005" incremental cuts on the compound for a few passes, make a non adustment clean up pass then start with .003" cuts till I see where a measurement is needed.

Another trick is to setup the compound dead nuts 60 degrees setting making sure the tool holder is perfect square to the ways. Then lock the tool post down. Next, adjust the compound .5 degrees back to where only the front edge of the insert is cutting.

JJones, there are tool holders that use an insert that is made for mini threading. Here are some examples:

NICOLE or NIKOLE can be purchased from Victor Machine out of New York. It uses a side mounted upright insert specifically made to run up close to a shoulder. A small kit can be bought that has both grooving inserts and threading inserts for about $100. It has a 1/2" shank. Inserts have two ends and are about $6 each. They last a longtime and are ti coated carbide.

McMaster Carr sells some nice lay down insert models that have 3 cutting sides. They are made in Israel and of great quality.

I have/use both and like the NICOLE best, but both are a lot better than a HSS tool that requires sharpening often.

I used hand made HSS for years. Once I went to thread specific inserts, things are a lot better.