Wednesday, February 5, 2014


So I got checked out on the metal lathe at TechShop.  I've been wanting to lathe for the longest time, but even if one of the little benchtop models was within my price range, it wouldn't be a good fit for an apartment.

Now I need to practice. And the 30mm "pulse rifle" grenade from the James Cameron Action/SF film Aliens seemed a natural. It's basically a cylinder with some grooves cut in it. A little looking around, and I even found a scale drawing.

(The famous grenade scene in the air vent.)

Except. Except the drawing I found was in millimeters, and the lathe I'm learning on is in thousandths. And you can convert, but as I did so I realized it also wasn't a good fit for available stock metals. And then some of the dimensions didn't seem quite right.

And that ends up making it a better project; research, dimensioning, planning around the stock, planning the cuts. And it isn't entirely unexplored territory because I still have Steven Pasek's wonderful drawings to fall back on when I get confused.

So let's do it the Volpin way. Screen shots are hard to come by, but I did manage to find a good picture of a screen-used prop. One difficulty in researching a popular prop like this is that a lot of people have made their own approximations (some better than others). So if you aren't careful, you end up with pictures of some-one else's mistakes.

Of course photographs and screen shots introduce distortion of various kinds. Foreshortening will change the apparent diameter to length, and cause spacing to appear different at different ends. I layered, rotated, and scaled two different reference pictures to try to zero out some of these factors, and made tick-marks both down centerline and down the sides.

And I compared what I was getting from these screen shots with the dimensioned drawing I had originally intended to work from:

Not that bad, really. Could even be acceptable. Except that I need to re-develop the dimensions anyhow (as well as convert them).

Unlike many movie props where the only good reference is an actor's hand in shot, the M40 HE grenade was built from a specific real-world object; 12 gauge snap-caps. (Revealed by the movie's armorer in the Special Edition DVD extras).

(Wikipedia Commons, uploaded by Magicmatt)

Turned out to be a pain to find the dimensions of shotgun brass, (most of the discussion online is of choke.)  But I finally found the SAAMI specs, (and eventually got my calipers on actual brass to confirm), and I can be pretty firm with .800" as a base dimension.  And that means I can scale the rest of the drawing to that.

So the next step is to combine the different references and bring them where plausible towards standards; standard shotshell dimensions, available metal stocks, and dimensions that are convenient to tool.

In this drawing, each grid line is a hundredth of an inch (here's what I did; I turned on the grid in PhotoShop -- yes, PhotoShop; I'm not quite up on any of my CAD software yet -- reset the dpi until it was in the ballpark, then scaled the image layers until they landed on the grid properly).

The blue is my combined references, the black is the new shape I'm developing, and the green indicates standard sizes for metal stock available from OnlineMetals. The red here is a reference to standard shotgun primer size, which I may or may not use as a dimension. (Would probably require shaving 1/4" stock, under-drilling, then carefully boring the hole to press-fit tolerance. Which I'll be doing far enough of already with the spring pin!)

Or did I mention, I'm going the full distance; spring-loaded trigger, plastic protective cap?

Possibly on this drawing, or more probably in the sketch form it is now, is the order of operations. Machining is quite sensitive as to which cuts you make in what order. As a general outline, I'll bandsaw about a 5" chunk of probably 1" stock, chuck it with less than three inches showing, and the first pass will take down everything but the rim to .8"

The SAAMI standard for the rim on a shotshell is slightly larger than the next-smallest standard aluminium billet, and after correcting to the lathe center I'd be a good tenth smaller than that, too. So starting from 1" gives me some headroom. That removes a full hundred thou from the body of the grenade, and when I was checked out we didn't go over 60 thou, so that comes out to two rough passes and one or two finishing passes.

The grooves in the front are close enough to be scaled to 3/32 -- an available parting bar size (and hopefully I won't have to order tool steel from McMaster-Carr before I can finish this thing!) I haven't decided whether to drill the end and use a live center, but by the time I've milled it down it will be past the 3:1 rule for extending out of the chuck. And of course the most complicated cuts are on the "nose!"

Live center might also help with the knurl (I'm assuming we have a straight knurl in stock; I did see one knurling wheel in the box). But at some point I need to drop the center and bore. Since the bore has to be opened up with a shoulder to retain the trigger pin, I need to swap the grenade in the jaws after parting it. But that's fine, because I'll also be parting with at least 20 thou waste and coming back to face the other end.

And so it goes. I fully expect to wreck several of these trying to do this, and probably break a cutting tool, too. But if I avoid damaging the lathe or myself I will be more than satisfied.

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