Sharing Knowledge — Grazer

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Metal Moose circa 2013

As I warned in some of my earlier blog posts, many of my future writing will become increasingly mundane, as my build process shifts away from trial and error to pure production. Despite this, I had an opportunity to be constructive in a different way this week and give back to my peers. 

Production has been going smoothly as of late as I become more practiced at the specific milling procedures used for each piece. I’ve decided to make the switch from the handshake joint (the same piece that interfaces with an identical piece) to a more traditional sandwich joint, with male and female (or bread and ham, for metaphorical consistencies sake) ends interfacing with each other. This will provide more stability for the whole leg, and hopefully reduce some of that side-to-side sway I saw in the first completed iteration of the strandbeest leg.

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Ham section of joint.

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Bread section of joint.

Out of an interest to get fluid at a fewer number of milling processes, I decided to focus on one side of the joint for this week. I became proficient at making the “bread” section, a total of three processes, and completed four components during the week (in three 43 minute class periods at that!) since I knew I was going to be away this past ( 😦 ) weekend. It’s been a while since I’ve gone into detail on the actual manufacturing of the leg on this outlet, so I guess it’s about time I write about it:

How to Mill Two Slices of Bread, the Definitive Manual

(Warning: may require looking up specific terms. Do not take my word for anything you see here. Also, I apologize for the atrocious formatting, this WordPress’ post editor has some issues problematic)

  • The start of any milling process, after starting, cleaning, and oiling the machine, is making sure you have the right piece of stock. My designs are made for a 2 1/16” long piece of 3/8” x 3/8” solid aluminum square rod (wow that’s a lot of descriptors), to end up with a 2” long finished component. The stock is cut from a longer rod on a chop saw, sanded briefly to de-burr the edges of the piece, and is then chucked in the jaws of the CNC’s vice with a square to assure a straight fit.
  • The next step is facing the stock. This is probably second most important part of this process, and is also the step that gets rid of the 1/6,”  since a thin layer is taken off both 3/8” x 3/8” face (one face is done at this point, the other is done after the first two mill operations) of the 2” long rod with a flat end mill (“drill” bit). This ensures that an origin can be accurately found using my method of finding an origin. 
  • IMG_3165

    Facing stock with 1/2″ flat end mill.

  • To do this, the end mill is changed to a centering bit (completely makeshift, just a pointy screw in a end mill chuck), and the point of the screw is manually placed as close to one corner of the stock as possible, as dictated by the computer-generated path from Fusion 360. This zeros (verb: to set,a measuring device, such as a scale, so that it reads 0) the x and y axis of the machine. This step, along with facing of the part, lets the machine know where the machines’ tool should be cutting. Should, as in sometimes a flock of ghosts that live in the CNC’s computer messes with the coordinates, potentially destroying a piece or the machine. Tormach (the make of the CNC I use) has since released an update for the driver software with ghosts sold separately. 
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    Changing a tool; done by loosening a draw bar (pictured with wrench) and then hitting it with a hammer.

  • At this point, the Fusion generated pathing can be run. As I said before, this specific part requires three different paths to mill, two for the top inch, and one for the bottom inch (since the vice needs to clamp onto something). Before this can happen, it’s important to do an “air cut,” where the program is run above the actual part so that all of the settings and zero’s can be checked. I’ve come to learn, through a lot of mangled aluminum, that the computer doesn’t always tell the truth, and that I make a lot of mistakes. Once everything is confirmed, metal is put to metal, and an operation is cut, with incredible accuracy. The first operation uses an 1/8” flat end mill to cut away material between the two bread slices, while the second operation uses a 1/2” ball nose (used for contour) to round off the flanges.
  •  

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    Operation #1: Pocket clearing, end mill depicted in black.

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    Operation #2: Contour, x/y/z planes shown.

  • After the second operation is run, the part is flipped, faced, zeroed, and a third operation is run to shave the edges off the component so it can slot into the square tubes that these joints connect. 

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    Surfacing operation to create a slightly thinner section.

As part of my robotics team, I demonstrated this process to the other members. This was the first time I actually demonstrated how to use a CNC machine to other people. It’s been an opportunity I’ve been waiting for, since I’m pretty much the only person in my school that knows how to use this incredibly powerful machine, and I feel that such information needs to be shared. It felt right to be helping people take the first step towards another skill learned.

I’ll take this opportunity as a good time to pitch some shameless self-advertising: here’s a link to the Metal Moose, aka FIRST Robotics Championship Team #1391, aka Westtown School’s very own robotics team… soon to feature freshly CNC’ed parts not made by me.

  

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