The more you know...

...the more you realize you don't know.

Finished four M40 grenades yesterday. Six hours at the lathe. Now, some of that was just me being a little rusty, but it is still a labor-intensive prop.

This morning cleaned them up for shipping. Here's what has to be done after all the machining is complete;

1) Fit the primer and superglue it in.
2) Clean off the excess glue.
3) Break the edges and remove scratches and jaw marks with 400-grit and 800-grit paper.
4) Hand-buff the whole thing with 0000 steel wool.
5) Apply a light coat of sewing machine oil.
6) Wipe down carefully with clean rags and paper towels.

And of course:

7) Trim a mocap (HDPE pipe cap) to .740"
8) Cut off the label.
9) Sand the trim and top flat
10) Buff the entire cap so it will take paint
11) Apply two coats of Krylon Fusion for Plastic (or Tamiya Olive Drab 2, depending)
12) Carefully cut a 3/32 wide strip from a roll of white vinyl tape
13) Apply the tape 3/32 from the edge and trim to size
14) Spray the entire cap with two coats of Krylon Crystal Clear gloss

I figure that adds another two hours per grenade. Meaning it basically takes a day to make each one.


Anyhow.

Here's a typical step; drill a 15/64" hole, lathe a piece of brass rod to the same dimension, and part off. Simple enough?

Not really. Drill sizes are nominal; it may or may not be .234" as advertised. Worse, drill bits chatter. The outside of the hole will be a larger dimension than the deeper parts, by several hundredths of an inch.

And when you part, the parting blade pushes up material. So even if you try a test fit (which I did), the primer may -- well, did bind as I tried to insert it into the finished grenade.

This is why I'm now lathing the bodies to 2-6 thous larger than the finished dimension until all the grooves are cut and the knurling done (which is even worse for materials displacement). Then a final pass to take off those little raised edges. It is more difficult to do with the primer, since that is a mere .2" long. But I need to figure out how to modify the process in order to add this step, or compensate for this problem.

And this? This stuff here? This is the meat. This is the difference between learning a tool or a method, and getting practical experience. This is the mostly undocumented ground knowledge that goes away when a generation retires; the reason why having the "blueprints" to a Saturn V doesn't allow you to just start building a new one. The difference between being a graduate of Google U and being able to struggle through a mostly-right answer, and being an actual professional in the field who solves questions like that daily (and has gained a gut instinct on what is right and what is wrong and where the traps and pitfalls lie).

Oddly enough, I had some of this conversation as I was cleaning up last night. Which started when I was sharing laments with another TechShop member about not following up on the CNC SBU.

TechShop gives you an SBU (Safety and Basic Usage class) before you are allowed access to a tool, but this is like a 101 class, a survey; it orients you to the field. You need to spend a lot more time before you actually achieve some competence.

But here's a thing -- like a lot of learning, you need to follow up. The SBU includes a practicum, and you finish feeling like you at least know how to turn the tool on. Well, you do -- at that moment. Give it a couple of days, or a couple of weeks, and that knowledge will fade.

This is why it took me a full day on the lathe just to get back to making cuts. And why I expect the same thing will happen on the mills (CNC and manual). Because I didn't jump back on immediately, which means I'll be in the same situation of trying to remember what seemed so easy to remember during that four-hour introductory intensive.