Threading a barrel for Brake

[bsekf]

Disclaimer: Last shop training I had was 67 yeas ago in engineering school machine shop.
Tool maker friend has a lathe and is a excellent operator. Going to have him cut and crown my barrel and cut a 1/2x28 thread for a Harrells brake and I have some questions.

6mm barrel so we have to open the brake up to .263 (0.02 bigger than bore). The brake bore will be in the center, the barrel bore may not be. How important is being perfectly symmetrical between bore of barrel and brake? Could we use a 4 jaw chuck and a live center and be close enough? Or a 3 jaw and not worry about it?

Should the shoulder of the brake fit snugly up against the crown of the barrel? Or is a .002-3 gap OK? I am thinking about heat locking the brake on the barrel.

Haven't been sleeping well and I am running out of things to over think!

 

[Walt Krafft]

Suggest you chill out and let a gunsmith install it. This is how I would do it. The threads need to be concentric to the bore not the od of the barrel. The brake should shoulder on the on the barrel at the back of the thread, not on the crown. A simple drop or two of blue Locktite is all you need to hold the brake.

 

[ridgeway]

How I do it...I chuck between two spiders, indicate bore true, then turn/thread. Fit brake to shoulder up. I thread to the brake for a nice close thread fit. There will be clearance between crown. The internal threads only go so deep. .5 or so, depending on MFG. Once brake is fitted, tighten it to shoulder then bore ID .020 over diameter and face. I usually put a 11 degree taper on face. Then indicate OD true and turn OD to match barrel OD. Light Emory to polish for seamless look.

 

[GenePoole]

If your friend is a toolmaker, he should be able to work from a print. Make it to the specs in the AAC Thread Guide. Concentricity, perpendicularity, parallelism, etc. are specified on the prints. 1/2-28 threads start on page 4:

AAC THREAD GUIDE REV 01

 

[skippermdj]

Ridgway has the correct approach. It's also important that the bore near the muzzle be in line with the axis of the lathe. Bores frequently not perfectly straight. A long-reach 10ths dial indicator like the one in the photo can be inserted in the bore so it runs true both at the muzzle and an inch in, ensuring the bullet will enter the brake straight and centered.

 

[tobnpr]

Bore or ream the brake in-situ after threading/clocking as mentioned. Can't be anything but right assuming the bore is running true after setup.

 

[Carl S]

Threads and shoulder square to the bore.screw brake on and bore brake to desired diameter..can't go wrong.