View Full Version : Closure Recommendations

02-02-2012, 07:37 AM
I'm making an end cap for an aluminum container. I have not cut anything yet. I want to get all my planning completed before I put tool to metal. I figure there are two ways to do it. First with a screw thread and secondly a tight fit with locking set screws through the side.

If I do the threaded route I don't know what thread I should cut. There will be mating external and internal threads. The outside diameter of the container is 1.5" and I would like to have at least 0.125" wall thickness in the thread area so the diameter of the threaded prortion would be in the 1.25" area. What pitch would be recommended? The entire length of the threaded portion on the end cap will be 1/8" to 1/4" long.

The other alternative is to make a tight fitting end cap and secure it with small, perhaps four set screws through the outside walls to lock the end cap in place.

The set screw method is faster and easier but the screw thread would look a lot nicer. Appreciate any guidance. Thanks.

Your Old Dog
02-02-2012, 08:03 AM
You don't mention how much stress the end cap will have to endure. If this cap is only on one end and low stress try using the coefficient of expansion of aluminum to your advantage. Turn the cap so it is close and then chill the cap and heat the tube, drop it in and let it stabilize temperature. I've had real good luck with the process and I'm not much of a machinist.

Ian B
02-02-2012, 08:50 AM
As YOD says, how much load does it need to resist? if not too much, machine the cap and the end of the tube to take an O ring. Think of a ball race, but with the balls & cage replaced with the O ring.

Neat (as you don't see anything), seals too. Quicker & easier than threading. Would that be strong enough?


02-02-2012, 12:28 PM
I'm going to be making a barrel shround with a removable end cap so the inside of the shroud can be cleaned. The shround will be mounted to the receiver and not readily removable. This is why the end cap is to be removable. There will be no load on the end cap as it is decorative.

The owner of the gun wants the shroud added over the barrel so there will be places to mount Picatinny Rails. He want to mount "things" on the gun and the Picatinny Rails will permit him to do that.

If I choose the thread mounting of the end cap, I'm envisioning something like a screw-on lens cap for a camera lens.

02-02-2012, 01:19 PM
I don't recommend threading on unanodized aluminum. It has a strong tendency to gall.

Make a threaded plug from plastic using a fairly coarse thread and fasten that to the inside of a plain cap that is a close fit on the tube. If you make the lid thick enough you will be able to fasten the threaded cork to the inside from the inside so no external fasteners are visible. Use two screws to fasten the cork so it won't spin in the cap. That won't gall. Naturally you will have to thread the inside of the end of the tube to match but it will not be at all critical.

By putting a flange on the end of the cork it will centre itself in the cap and also act as a gasket.

You can also make the cap so it is flush with the tube so there is no lip to catch. Some nice knurling would also look good.

Jim Shaper
02-02-2012, 01:29 PM
Make a threaded insert out of brass or such, and loctite it in the end of your aluminum shroud. Then make a cap with a knurled shoulder that threads into that. Now you're tool-less for removal and there's a visible indicator that the barrel is covered from the side and the knurling looks nice.

02-02-2012, 01:36 PM
I was still adding to my post when you posted that Jim. Good idea. :D

Jim Shaper
02-02-2012, 02:45 PM
Great minds... :p

02-04-2012, 05:05 AM
All are great ideas and I appreciate the help. Now, all I have to do is to work them into my design.