The Big Think

April 24, 2015

JAG 20 and Problems

Filed under: The R2 Project — jasony @ 9:39 pm

Well, I just can’t stop myself. Today at TechShop after a class I checked out the key to the FlowJet (I just love saying that so casually: “I checked out the key to the quarter million dollar FlowJet”) and pathed the JAG20 file in FlowCut. I figured I’d take the file back to the WJ to see how long the cut would take. I’ve done this before and know how it usually ends.

I loaded the file and did a virtual cut. 30 minutes for both pieces. There are 2 JAG20 parts that look like this:

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The part is the body-to-leg flange joint that connects the main frame to the leg assembly. It’s solid 6061 aluminum and 1 1/8″ thick. You can see it located here between the legs and the frame. It’s visible once Artoo is finished so it has to look right:

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I bought a slab of aluminum from Metals4U a few days ago in preparation of this part. This is probably the biggest, most nerve-wracking part I’ll do. Partially because it took me weeks to figure out how to design in Inventor (I finally got help from an expert at TechShop… he solved the problem in 5 minutes), and partly because it is, by far, the thickest part I’ll have to WaterJet in the entire project. The single hunk of aluminum measured 1 1/4″ thick, 11″ wide, and 16″ long. Cost me over $80! So no, I did not want to screw this up.

So I put the hunk of aluminum on the FlowJet table and zeroed out the Z axis to get the mixing tube right down on the metal. At this point you don’t want to smack the tube into any clamps or weights holding the stock down. If you do, you’ll break the tube and owe TechShop $250 for a replacement. Fortunately, I know the guy who teaches WJ (I am the guy who teaches W.J.) and we have procedures to avoid that.

So, the file was loaded, the stock was weighted down, and I even did a dry run without the pump running (the part that costs money). It was a 30 minute cut. Well, why not? I verified that the machine had been working well and went for it.

Part on table weighted down. File loaded. All ready to go!

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I hit ‘cut’ and watched with an eagle eye for the next 30 minutes as the water and abrasive jet slooooowly worked its way around the part. 4 holes, inside cut, outside cut, repeat.

The first part went off beautifully and I could tell that it cut through the material just fine once it was finished. The machine started in on the second part next. The 4 holes went fine (they’re first), but sometime during cutting out the center “shamrock” shape the pump started to lose power. Suddenly the cutting stream wasn’t making it all the way through all 1.25 inches of aluminum. You can tell this is happening because instead of the cutting stream passing through the material, the water starts spraying all. over. the. place. All over you. I got blasted with a muddy stream of grungy water (seriously, I looked like a motocross rider after a muddy ride). The normal procedure if this happens is to manually slow the cutting stream down. As I was already cutting at a pretty slow pace (which gives a better finish) I wasn’t too excited about slowing down further and making the cut even longer/more expensive. But it’s better than the cut being ruined.

So I slowed it down (which you can do mid-cut with a slider in the software) and managed to get the center cut done. On the last cut (the outside perimeter), the jet was still having issues. The solution was to slow it down even further. I think by the end I was cutting at somewhere less than 50% of the max speed and still unsure if it was making it all the way through.

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Here you can see the layers of garnet that get blasted onto the parts during the cutting process. It’s an incredibly messy machine to operate.

Once the machine shut off I checked the parts and discovered that the first part to be cut (the one on the left in the photo above) came out brilliantly. Great finish, cut all the way through. Unfortunately, when I lifted out the part on the upper right I discovered that there were sections of the middle that didn’t get cut all the way through. Fortunately, though, it did lift all the way out so at least it got free. Unfortunately, the finish on the outside was terrible as a result of the gradual pressure loss.

Here’s a pic of the “good” part. You can see the smooth edges with just a hint of the typical jet “lag” on the bottom of the outside curve:

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For the “bad” part, you can see not only where the stream didn’t cut all the way through, but also where the water jet “lag” on the outside is much more pronounced, resulting in a much more ragged and bad looking surface:

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And here’s a side-by-side of the good part and the messed up one (zoom your screen or click for a closer image):

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So, while I’m really happy to get these parts behind me, I am somewhat disappointed in the results, considering that for about the same amount of money I could buy an aluminum part that’s shiny and perfect.

I’m still cogitating on how I’m going to fix this. The good news is that the final piece needs to be 1.125″ thick (1 1/8″). The rough piece in the pictures is 1.25″ thick. So I should be able to reduce the thickness of the piece and “face off” the thin little bridges of aluminum that keep the inside section attached. This will also have the effect of making the faces of the part nice and shiny. I think I can do this on the lathe at TechShop but I’ll have to ask the pros about the best way to go about this.

As far as the outside of the donut shape, I need to see if the part can be a smaller diameter than the 6.375 listed in the drawings. I think it can. If I can safely shave off a few thousands I think I can get rid of those marks on the outside and make the part shiny.

Sorry to be so wordy. This experience was a relief in that the part is somewhat workable but frustrating in that I just don’t feel like I can completely trust that machine. I need to improve my skills on the manual mill and lathe but that takes decades. The WaterJet, while expensive, can let a newbie machinist cut any part they can design in the computer. That’s huge.

The final major part for the frame is the pair of side panels where JAG20 mounts. They’re .5″ thick aluminum (thank goodness) but also contain a lot of little holes that’ll make the water jet turn on and off quite frequently. I may wait a few weeks until the pump is rebuilt to attempt those. They’re only 20 or so minutes each but I really don’t want to mess those up. At least not the small little precision holes.

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