Friday, February 28, 2014

Round 2

So I abandoned my first attempt at the plug and primer for the M40, and went in today to cut a new set. Which took a lot longer than it should have, because the lathe was seriously run out.

I was a bit run out myself this week what with wet weather and a cold.

This was the potentially trickiest bit of lathe-work. I was doing press-fit, which meant the pieces had to be within a few thousands of an inch in precision. The original attempt I cut the primer to one or two thou big, heated up the plug with MAPP gas, and hammered it in. More-or-less. Then I picked up a couple of coffee cups from upstairs, filled one with ice water and the other with boiling-hot tea water.

See, aluminium expands when heated. Heat it enough, and you can slide the part into the hole. After the assembly cools, it isn't coming apart again.

But the piece refused to go in. I gave up for that day, went home, and read up on the actual coefficient of thermal expansion for aluminium. Which is 21 x 10^-6. Which works out to, with a hole diameter of around 3/8", only about a thou per hundred degrees delta-T. That's about a third of what I'd been estimating, And my Starbucks Special setup was giving me fifty degrees if I was lucky.

Unfortunately the pieces had to be cut off after being turned to diameter, and it was impossible to re-chuck them in order to adjust the dimensions.

So back in today with a new plan. Here's the setup; there's a hole drilled through the grenade body. The button is inserted from the back, along with a spring, then the hole is plugged. And in the center of that plug is where the primer goes (a tiny bit of 1/4" brass rod). To keep the button from falling out the front there's s shoulder on the button. Starting from the estimate from the screen shot of about a 5/16" button, adding the shoulder and clearance brings the plug to almost 3/8". I'd ordered 3/8 stock, drilled the next size down and needed to take just a few thou off the stock in order for it to fit snug.

But the lathe was not cutting straight. One of the shop guys checked and the headstock was running out of true by 8 thous. By the front of the jaws, 12 thou or more, and by the time you got an inch or two out from the jaws the stock was wobbling so bad you could see it. I was able to shim the jaws of the three-jaw chuck with cardboard ripped from the front of my binder, and that got the hole reamed out to the new dimension. Sure, you can establish a new lathe center on any stock, but there wasn't enough metal to get there; by the time I was centered again my diameter was too small.

Well after all, this was picked as a learning project. So I didn't get a lot of metal cut today, but I learned how to replace a chuck, use the dial caliper, and set up a four-jaw chuck. And with that last, I was at last able to get cuts within the required tolerance.

I'd also come up with a better idea. Instead of forcing in the primer first and seating it to depth, then seating the plug and hoping I didn't cut too deep in the final facing, I'd shove the plug up to a shoulder. That way, I could pound the primer in last, without risk of the plug falling inside (and since there's no mechanical load on the primer, it didn't have to seat all that well...could even be a little loose and held in with a drop of glue).

Drilled a bigger hole to make that shoulder. At which point I realized that although the tailstock wheel is in inches, the markings on the tailstock spindle are metric. I'd confused centimeters with tenths and cut the hole way too deep. So had to go up a couple more drill sizes, and at that point the hole was much, much larger than the 3/8" stock.

So chucked my 1" bar in the four-jaw, trimmed it up to a runout of under five thou just because, and hogged it down to .405" in diameter. Cut the tiny nub that retains the spring, parted off, shut down the lathe and vacuumed up.

Yes, of course. The new part went right into the vacuum. Fortunately it was lying right on top when I opened up the shop vac to search for it. I cleaned it up, oiled it lightly, assembled the grenade and cranked down on the arbor press.

It seated. It is in there well enough I'm fairly confident I can do the finishing cuts now. But it was a long day, and a lot of struggling with a piece of equipment that needs a little more maintenance than it has gotten this week. And I'm on 12-hour days for show call until Monday.

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