Wednesday, March 11, 2015

Raygun IX

Many of the designs I'd been tinkering with implicitly incorporated construction strategies. The one that got approved, however, does not.

This means the next task is working out how best to build it. This is another one of those multi-variable problems where every choice is a trade-off. Construction time, cost, infrastructure cost (aka additional tooling and training for new techniques), risk management, etc.

Here's several pointers to keep in mind whilst working through the construction design phase;

1) Everything goes over estimate.

2) You can only know the known unknowns. Leave yourself space to solve what are currently unknown unknowns. (Make this space bigger when ever dealing with unfamiliar materials or methods!)

3) Watch out for hidden assumptions. Write out everything in plain English so you can look at it and realize you don't actually need forty watts of power or a high silver content or whatever. Or realize you do; and need to weight that aspect higher in the trade-offs.

My big accomplishment for today was estimating costs and time for several different fabrication strategies.

Here are a couple of the big things I'm worried about:

1) The round form. I have some experience in hand-carving compound curves, and I did that in scale for the mockup. But I know that it is quite time consuming, and difficult to be truly accurate; enough to want to look into any way of mechanically achieving those curves. That would be lathing, or various options in computer fabrication.

Lathing has a couple of issues; I haven't any wood lathe experience, and these curves are difficult to achieve on a metal lathe.

Computerized fabrication falls into roughly three methods; slicing (turning the desired shape into a stack of pieces which are glued together and smoothed), printing, and CNC machining. Out of all of them, the CNC requires the least clean-up.

2) Access to the inside for electronics. This has an assumption wrapped in it that I need to expose properly; that the internal spaces are actually planned around the electronics package. It may be incorrect; it may be that a slush-cast, and potting the electronics before final assembly is just fine.

3) Duplication. Almost certainly there wants to be more than one. Fortunately, the majority of the construction methods I have considered so far aren't feasible for the finished model; meaning a molding stage is necessary in any case.

4) Metal. This is a very much questioned assumption. I want the look of metal. I'd like the weight and solidity. I'm pretty much ruling out vacuum forming as viable, but a metallic look is still achievable with paint or cold casting. So, again, 3d printing and resin casting are still on the table.

More data is needed. Using simple volume calculations taken from the mockup, here's how a couple of competing technologies break down;

Molding: Doing a box mold of the whole thing would consume about 100 in^3 of silicone. Smooth-On's Oomoo-30 is available in trial packs of approximately 75 in^3. Given that I'd really want to mold several parts separately for ease in mold design, clean-up, and final assembly, it works out to a minimum of $50 of silicone. So molding is within reason.

Printing: the cheapest Shapeways print is priced by bounding box for models under a certain density. In any case, it works out to around $50 for just the main body...with careful nesting of the parts for most efficiency, could probably come under $100 for the total print.

(As a reference, my V150 model -- thin walls, under 5" long -- costs $45 to print.)

CNC machining: I have much less experience with this technique, but working off listed feeds and speeds from both Tormach and various end-users at CNC and machining forums, it seems capable of doing a roughing pass over the body of the raygun in about five minutes. Finishing pass should be at least 4x that, and there are of course more parts involved, but it seems plausible that even with work holding and tool changes to get the whole thing fabricated in one machine-time reservation.

Cost of the metal; raw stock big enough to form the body with space for clamping works out to about $20 worth. Given all the various parts, wastage, and errors, can assume $80 of metal, and less for repeated fabrication.

So. CNC machining in metal is very competitive. It achieves the desired shape in one step (instead of fabricating a master then molding and casting that), so savings in materials and time. It seems the next step in the planning process is to work up a projected CNC approach and see if that is as feasible as it appears at first glance.

Fortunately, there's a perfect experiment that needs to be made. The raygun, as I currently envision it, breaks down into several elements:

1) Body shell (probably two-piece body, handle may be integral to the shell and likely also two-piece -- the alternatives here are all about being able to chuck the body in a lathe for final polishing).

2) Emitter. Probably turned acrylic, which is also largely unexplored technology for me.

3) Trigger. Another CNC'd part -- metal in any case -- with pin and spring and some way to engage the electrical switch.

4) Internal electronics; have purchased a bag of 3W Cree in pink -- which my new control board will PWM just fine even without the revised version. LED in the fin as well (which leads to questions about how to lead the wires up). And of course it has to "pew pew pew!"

5) Selector knob. Not sourced. Current thinking is to CNC the basic knob and design the way it integrates to the body and is held in place as part of the fabrication. Behind it would be either an encoder or, more likely, a potentiometer. Have not thought about indicator dial, however....and I have some leanings to work out a lighted inset to the side plate, too.

6) Reflector. I tried to source a dish shape in a chrome finish but the closest I came to were some Lego parts. I've also contacted a chrome plating company but I suspect the run is too small for them to do and/or me to afford (minimal order for chroming tends to run in the low hundreds of dollars).

The current best thought for the reflector is to CNC it, smooth it further on the lathe, and then polish it up to the best finish 6061 aluminium alloy can achieve with a buffing wheel. I did this long ago with some Cyberpunk claws and I know it is achievable. The major downside is aluminium oxidizes, and essentially all protective coatings dull the finish.

So my next step is going to be to work up a new model for the reflector I can then try to run off on the Tormach mill.

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