Unless you own a machine shop (or have access to one) at some point you will have to hand off your blueprints to a machinist. Following the best practices will save you time and money. To learn how to make drawings a machine shop can actually use, I spoke with Rick Mitchell of R&R Machining
. Rick is Bob Twigg's (Cubesat co-inventor) main source for Cubesat and PocketQube parts, I figured he knows a thing or two about manufacturing. Find out how much information you really need to put on your blueprints and what you can leave out. Does the machine shop need to be ISO certified?
Resources Discussed During the Interview
Improve electrical conductivity using Alodine Plating Guide
Machine Shop Interview
What are some of the common issues or mistakes that you’ve seen. Somebody drops off their blueprint and now you have to go make it. What are the common questions that you’ve seen that you have to go back and say, “Hey, I have a question about this.”
Rick: There are so many things. It could be dimensions, it could be locations. There are a lot of different things it could be. If I have questions on drawings, most of the times all the drawings I get are pretty clear. What are some things that as a designer I need to make sure that I’m incorporating and putting in there so that when I give it to the machine shop, there’s not a lot of questions. Or I guess, what separates the good blueprints that you see from the bad blueprints that you see.
Rick: I don’t know if you know flatness and parallel and perpendicular and you got concentricity and some guys don’t put that out in their blueprints and it needs to be.
Describe flatness and parallel
Rick: Well flatness is, I could cut a piece of material and they have flatness call out at the bottom of the drawing in 0.001’s, if it says 0.001. I cut it and I lay it on the surface plate in my inspection room and it can’t rock, no rock, zero, none. That’s within the 1000’s, it can’t rock. Parallel, if I cut two sides, it has to be parallel within one thousands so I can use a micrometer and it has to be at least two hundred, it could be 199 or 201 parallel. What about tolerances? I know that was an issue when I was making drawings. The tighter the tolerance you have, the harder it is to make and the more expensive it is.
Rick: Yeah, yeah, you got to plus or minus a tenth or two tenths or five tenths, the more the price goes up, that’s true. Plus or minus five is, for me, plus or minus two at three thousandths is easy but then you start to get into tenths then things start changing a little bit.
What kind of tolerances and flatness do you require for a 1 or 2 U or a 3 U CubeSat?
Rick: It’s actually kind of open. I mean, when Bob sends me some print it is kind of open, like he would have 1/16th of a fraction. So when you see a fraction, that’s like plus or minus 0.030 on a fraction. What are most CubeSat’s made out of? Aluminum?
Rick: Yeah. All aluminum. 6061 T6. The parts you use are very thin, they are 31000th so 130 second . They are really thin. If don’t care about flatness or I don’t care about parallelism, should I note that in the drawing or should I just leave it blank?
Rick: If it’s not on the drawing, then I’m assuming that you don’t really care about the flatness or parallel, if it isn’t on the drawing. How long does it usually take to make 50 Cubesat structures?
Rick: It took me about a week. Is ISO certification important for a machine shop building Cubesats?
Rick: No, not at all. I’ve been through all the ISO and I’m not ISO myself but I know, it’s just a ton of paperwork and a paper trail. So no, I wouldn’t be concerned about the shop being ISO. Some of the launchers that you’ve built, have they actually flown?
Rick: Made a bunch of PocketQubes, and really small ones, satellites, and, he said they were up in space now, I think he said he was doing a cancer research. And, he said they are going to be up there 25 years. As a designer, should I take stock material into account when I’m designing a part. Are there some standard sizes?
Rick: They have all like a quarter inch, an 8th inch, 3/16th, 1/2 inch, 3/4, one inch. That’s all standard. So if I go by 8th inches or 16th inches I should be okay most of the time.
Rick: Yes, yeah you’ll find that easy, for sure for any kind of material. As long as you have all the dimensions and they are clear to read and I can see it and understand it, it should be good to go. Image - Flickr via Tobo