Thursday, June 13, 2013

Yep, I Need Another Breech for a Crosman. Part 1

There's a pile of projects on my to-do list, but this appealed to me the most.  I'd like to make another breech for a Crosman 22XX-type gun.  Something simple this time.  There's no plan to make it a repeater like the last time around

Starting with a piece of 6061 aluminum. 

I finally got around to purchasing a 4-jaw independent chuck for the lathe.

Put off getting a 4-jaw because I couldn't figure out which chuck to order.  (and there wasn't a pressing need)  Eventually, I broke down and just picked out a 6" diameter "plain back" chuck then made an adapter plate-- a "back plate" in machinisty terminology.  The "back plate" adapts the chuck to the spindle of the machine. 

Scribed a center line on the face of the aluminum and adjusted the chuck until the line centered up with a pointed indicator held in the tail stock.

Notice the offset.

Spotted the hole and all looks good.  The offset is easier to see here.  this is part of the advantage of a 4-jaw chuck.  Offsetting work as well as the ability to hold work that isn't round, triangular or hex shaped.

This piece of aluminum is 6" long and I'm drilling a 0.250" hole all the way through.  The flutes need constant clearing as well as lubrication with a cutting fluid to prevent the aluminum from welding to the cutting edges.   It's a very laborious process to drill a hole this deep.  And the smaller the diameter of the drill bit, the harder the job becomes as the bit wants to flex and walk the hole out of alignment by the time it breaks through on the far side.  Slow going, can't force the bit. 

Finally through.  There's 6.25" of drill bit through the aluminum here.  If I make another breech, I'll use a 0.375" drill bit for the through-hole.  The bump-up in drill size make for a more rigid set up. 

It gets easier now.  The .250" hole is partially drilled out with a .4375"  bit--the same diameter as a Crosman 22XX barrel.    I drilled this hole for the barrel just shy of 3.175" deep.  The painter's tape on the bit gave me a quick and dirty depth measurement.

Followed with a 0.4375" reamer.  A spot of dykem on the shank and a scribed line made it easy to hit the exact depth.

As I'm not a fan of the way the "soda straw" Crosman barrel looks, I'm going to add a barrel shroud to bring the barrel up in diameter.   To do this, I drilled a counterbore about 0.600" deep with 0.625" bit.

Not shown:  Finished the bottom of the counterbore with a 0.625" endmill to achieve a clean, flat bottom.

With the holes finished, I faced the piece.

The counterbore for the barrel cover is easy to see here.

Moved to the milling machine and fly cut what will be the right and left sides of the breech.  The fly cutting was used to remove just a couple thousandths and produce a nice surface finish.

A multi-anvil micrometer was used to measure the wall thickness at each side of the counterbore.  One of the sides was thicker than the other by  0.003"--hardly enough to worry about, but I recut the high side until it matched. 

Next is the radius cut along the base of the breech to fit against the gas tube.

Caliper set at 0.3675" to scribe the center line.  I'll use it as a secondary reference and use an edge finder to find true center for the milling.

More soon.