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-bard 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.

Monday, May 4, 2015

Making a Bulk Gas Tube for a QB Rifle -- Part 2

Picking up where I left off....With the tube threaded for the bulk fill cap and cut to length, I wanted an easy way to add the holes and slots for the breech, striker and trigger components.

A square collet block will provide for easy 180 degree indexing to align holes top and bottom.

Not shown--I painted the tube in layout fluid then using a caliper, transferred all the hole and slot locations to the new tube.

If I had to do this again, I'd set the collet block to the left of the vise and clamp the tube directly rather than use jacks for support.  This did work, it just wasn't as rigid as I'd have preferred.

Edge finding.  Once I found center, I started spotting holes, drilling and milling using the original tube as a guide.


Had to make a tap run.  Didn't have the oddball M5 x 0.5mm tap on hand for the forward trigger mounting position.

Tube finished.  Pretty straightforward for once.  Not shown:  As expected, I had to dust 0.003 to 0.004" off the valve body and striker to fit into the ever so slightly smaller bore of the  new tube.

The only problem was the extended tube length meant my old valve tool was far too short to reach the valve face.  I did a test assembly anyway without tightening the valve.

Everything fit together and functioned correctly. 

Now for that valve tool.  My old valve tool consisted of a flat strip of steel.  Wanted a better tool this time around.  Used a slitting saw to slot the end of a 22" length of 5/8" drill rod.

Flipped and through drilled the opposite end 5/16".

Cut the end off my old valve tool, squared it up then press fit it into the new extension.

Cut and finished the ends of a piece of 5/16" drill rod.  Even did a quick coat of cold blue.

A bit nicer than what I was using before.  I stripped the action back down, tightened the valve and added a bit of CO2.  It held pressure for 24 hours, so I'm calling it good.  Need to anchor the valve into the tube a bit more securely, make or source a new barrel band then begin working on the stock.

More in a few days.

Sunday, April 26, 2015

Making a Bulk Gas Tube for a QB Rifle -- Part 1 -- The Tube.

Finally got it together for a new project.  I have several rifles based on the Crosman 160 air rifle--including a Crosman 160, Crosman 180, Tech Force TF79 (AKA: Industry Brand AR2078A) and an early QB77 (AKA: Industry Brand QB78/79). --Click on any of the models to see past blog posts covering repair/modifications.   The QB77 had at some point, been stuffed into the same target stock as the TF79. Aside from caliber, there really wasn't any difference, and the QB didn't get used much.  A couple years ago, I'd purchased a QB walnut stock blank from Dennis Quackenbush with the intent of finishing and fitting it to the Crosman 160 action.  So far, that hasn't happened and I think I'm now going to use that stock for a QB77 project.  That's the back story. 

Before I start the stock project, there are a few modifications I'd like to make to the QB.  Rather than use 12g cartridges, I run the gun on bulk CO2.  It's easy and cheap to fill the gun from a paintball tank. I was thinking about making a gas tube extension for additional on-gun CO2 capacity, but it seems cleaner to make a longer, extended gas tube.  Wanna run the tube full length to the end of the muzzle. 

The QB rifle, being built in China is metric with corresponding metric sized tubing.  The gas tube is 22mm in OD.   This became the first hurdle.  Metric tubing, while available, is pretty limited in the US.  It made sense (at least to my squirrel brain) to use an imperial sized tube.  7/8" (0.875") is pretty common and just slightly larger in size than 22mm.   Then there were wall thickness and corresponding tube ID dimensions to consider.  I was concerned primarily about fitting the valve and hammer components into the tube and achieving a gas tight seal.  Comparing ID dimensions to the stock QB tube, everything I found online looked 0.010" too big or 0.005" too small.   Eventually settled on a piece of tubing at the bottom end of ID figuring that it would be easier to turn down the valve body a couple thousandths rather than shim it up to make it fit tighter.  That made it 0.875" OD with a 0.065" wall thickness.

Got this from McMaster-Carr.  Three feet of 4130 Cro-Moly for $18.

The hardest part of the project--for me--will be threading the tube for the gas tube end cap at the muzzle end.

I had a really awful M22 x 1mm die that I bought for super cheap--it was about $8--shipped.  When it arrived, it wasn't the nice looking split die pictured in the listing.  I sent the seller a few helpful listing tips without mentioning words like "mail fraud".  At least I got positive buyer feedback.  No extra charge for the tetanus.

Lacking a tailstock die holder and not being interested in setting up the lathe gears for metric threading, I turned the end of the tube to be a snug slip fit into the die threads then cut a taper to start the thread.  Idea was to let the tube center itself in the die.  Aluminum sheet pads the steel tube in the chuck.

Die slips over the turned down section of tube before it engages the taper.

I didn't have a proper die stock for this thing.  The die has an OD of 45mm.  The machine tool supply place I visited a mile from the house had never seen or heard of a die with a metric OD.  SO, that meant no dice on a handle and I wasn't about to spend half the day making one.  That's OK, I wasn't exactly enthused with spending more money on the mail fraud die.  Improvised (barely) with a strap and pipe wrench. 

But it was enough.  Here, the guide has been cut off and faced leaving just enough full size thread for my end cap.

Astonished that the die worked this well.

Test fit:  The bulk fitting from the QB fits into the tube and the thread locks down tight.  Nice surprise --the bulk CO2 fill cap didn't have to be turned down to fit into the tube.

Now it's a matter of cutting the tube to length.  This is a high precision tape measure kinda cut.  27" with the bulk fill adapter installed.

The Ultra Fine Point Sharpie makes the mark.

Cut the tube, faced and beveled the end.

Now, I need to disassemble the QB77 down to a bare gas tube and start transferring hole and slot locations.

More in a couple days.  I've got a gun to strip.