Tuesday, May 12, 2015

Making a Bulk Gas Tube for a QB Rifle -- Part 3 Final

With all the holes and slots in the tube finished, I wanted to add a couple more screws to better anchor the valve.

Installed the valve, spotted, and drilled right through the gas tube into the valve body. 

Tapped M5 x 0.8mm.

Used a center cutting endmill to counterbore the screw heads into both the tube and part of the valve.   The idea is to get the larger diameter head of the fastener to bear the load rather than the smaller diameter thread.

Yeah, the extra valve screws are tiny.  Here's one I made from a turned down M5 button head.

And installed.  They barely stand proud of the tube.

Another view.

I took the tube into work, and with the owner's OK, polished it at lunchtime on our Rand-Bright BH-50.  This is an industrial polisher used to finish shafts and hydraulic tubes (among other things).

It's a great machine but I don't get to run it as often as I'd like.  A large sanding belt rotates while a feed wheel spins and draws the part under and across the belt.  You can vary the pressure of the belt on the part, as well as the speed of the feed wheel.   Of course, there are a large assortment of various sanding grits available. 

Took longer to install and adjust the feed guide platens than to do the actual polishing.

Started with a 240 grit and did about three passes.  Then a 400 and about the same. 

I think this pic is after the very first pass.  The smudged looking spots  are non clean up areas.

Final passes were done on a diamond coated belt.  I could've done a few more passes, but I didn't want the gas tube to look substantially better than the QB's barrel and breech. 

Four coats of Van's Instant Gun Blue had it looking pretty good. 

Overall, I'm pretty happy with the Van's bluing.  The only drawback, like all the cold blues I've tried, is lack of durability.  Handling the steel seems to wear the bluing off pretty quickly.  If it becomes an annoyance, I'll either epoxy coat the tube, send it out for hard chrome or have it hot blued. 

Here's the QB reassembled with the extended gas tube.  The action now has strong overtones of a Benjamin Discovery.

I see that I still need to figure out a replacement barrel band--the original plastic band cracked in several places due to age.  The walnut DQ stock is probably up next, though I do have a piece of aluminum on the workbench for that barrel band.  We'll see which feels more inspiring next.

Man, almost forgot.  The entire purpose in making this tube was to increase the on-board CO2 capacity.  I haven't had nearly enough time to do the shooting, but this more than doubles the original gas tube's volume.  So, I guess I'm hoping for at least 100--110 good shots.

Check back soon.