Sunday, March 15, 2015

Raygun X

So I spent St. Paddy's day the way any self-respecting Irish-American would; by doing my first solo cuts on the CNC Milling Machine.

And I accomplished all my primary goals, and almost all of my goals for the session:

That's the 2-inch "radar dish" reflector on the front of the raygun. This was only the top passes; I didn't have time to flip the stock and finish the underside.

It was good learning. I leaned quite a lot on other people in the shop, including a few friends I've made there, who helped me with some stuck buttons and where the spindle lockout key was stored and so on. And I asked for advise from staff before purchasing my first ball-nose end mill (for this piece, the roughing pass was made with 1/4" flat end mill, and the finish pass with 1/8" ball nose).

I built a quick 3d model in Carrara, generated g-code in Cut3d, zeroed the Tormach, and did air passes to check my code (and a good thing I did, too!) I edited g-code both off-line and by hand, did a tool-change mid-work, used the feed rate override, and basically got a pretty good workout in setting up and problem-solving on that mill.

What really surprises me is the feed rates and speeds I set up, the tabs I designed in and the rest of the work holding, and basically all the gross physical measurements turned out to be in the ballpark. I guess all that conventional milling/machining time paid off.

(Image from Tormach)

And...right. I've now got a much better grasp of how the process goes; enough to take a second longer look and see if it is still an optimal method for the raygun. first approximation, this will work fine for a Jubal Early; the price point on those is going to be $200-300 and there's less surface to machine away (and more flats that can be conventionally machined, if for no other reason than clean-up). But it is less optimal for this project. One raygun, probably. But the gut estimate now says three or more machine sessions to cut an entire gun, so making multiples -- especially for a price point under $100 -- is less attractive.

And, yes, clean-up will be an issue. The above was just a single pass with a 1/8" ball nose. I can easily add a second clean-up pass, but that one pass was almost an hour of machine time. Cut3d estimates 95 minutes for a dual pass (with some efficiency revisions I already put into the cuts) and I was unable to run at any higher than 90% of the coded feed rate without chatter.


And after writing that last paragraph, I tried a little sanding and hand-polishing on the piece above. Which brought it up to a entirely acceptable shine without too much labor. I had already planned to chuck the reflector, and possibly the main body, in the lathe for polishing. I also had existing concerns that aggressive smoothing might change the curves I'm working so hard to get right.

Lesse. Finishing pass was with 1/8th ball nose end mill with a step-over of about 1/3 diameter. I think I set it to .025 -- 20%. math is not into figuring out the height of an arc drawn from zero to 20% of the diameter, so let me just call it about half of the step-over; the grooves should be one or two hundredths of an inch deep.

It's all about the curves on this prop. Large, parabolic arcs that need to be smooth. I didn't need three days of bondo spot putty and sanding to underline that getting these kinds of curves to look right is very, very time consuming.

Well. Sanding the grip would be more onerous (there's all those grooves in it). But it does seem do-able.

On the gripping hand, I came up with a hybrid idea for one of the alternates. So I had thought of vacuum forming in terms of a lightweight shell, which would be filled with amorphous material (just as I did the Lewis Gun magazines a little while back; glued together then filled with expanding foam).

I also knew quite clearly that if I chose to go molding of a master to make (cheaper) replicas, the molding cost would be added to the existing fabrication costs. And molds aren't cheap. Well, there is one "out" here; the wall thicknesses and other modeling details for the CNC are entirely compatible with 3d printing. So could run off copies using that method.

In any case, here's the idea; pull the shell out of thick ABS, or even acrylic. And laser-cut an acrylic skeleton that supports the structure and provides attachment points for internal and external assemblies (including such important things as a working trigger).

Worth thinking more on. 

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