Piston Head: Day 2

The lathing of the piston head came out great. Hopefully I can do as well milling the arm slots.


The slots themselves were kind of a pain, but went pretty smoothly. I’ve never had to mill anything using the dials on the handles before and I was worried about the accuracy. I made a test slot in a piece of scrap before this though and it came out perfectly, so I went into this with some confidence.

I cut the first 2 slots .008 undersized, thinking it would be best to take off the last hair with a file for a perfect fit. I tried a the fit afterwards and the arm didn’t fit by exactly that little bit, so I just said what the hell and milled the other 2 to the exact size. It turned out fine and they barely wiggle at all.

While the piece was still lined up and clamped in the vice, I drilled the holes as well. Even that took a while. Align, center drill, tool change, drill, tool change four times. Then I tapped the holes.


The fit came out nicely, but I had a hard time getting rid of all the burrs. I’ll have to find a new cleanup method. ..later.


I quickly cut out drilled and rounded 4 arms for the piston head. A this point, my plan was to carefully band-saw then file them down to the right length and curvature. They felt very rigid though, so I decided to take a change and cut them with the lathe.


This setup is kind of scary.


I was seriously gritting my teeth as I creeped the tool into the path of the spinning arms. It worked great though.Β It didn’t stall or explode anything at all. It cut off nice chips with a steady tak-tak-tak-tak-tak.


I turned the arms down until the outer diameter was 3.74 like the other pieces. The fit was a bit tight though, so I put it back in the lathe and took off another .02, now it has a little wiggle room, which is good.


And, the finished part. Not my prettiest, but very cool.



Piston Head: Day 1

I think I’m ready to tackle what’s probably the most difficult part. The piston head. It should be pretty straight forward, but it’s a lot of operations to do without making a mistake.


Here’s the cad model. The piston head is highlighted in red. As you may have noticed in the exploded view, the arms are separate pieces that are attached with screws.

The jaw center on the lathe I’m using is a little off. Rather than fuss with the 4 jaw chuck, I used a piece of stock big enough for me to machine all the important parts without flipping it around, this way the alignment doesn’t matter.

A couple of hours later:


…I forgot to bring a drawing to the shop, so I redrew it on some graph paper and recalculated the dimensions.


I faced off the chuck of extra stock and viola. Perfect fit. I’m getting pretty good a creeping up on the target dimensions. πŸ™‚

Woot. Screw holes worked out!

Here’s the template I made.


For some reason, it came out about 1/16th too short when wrapped around the pipe. Maybe the paper thickness? Anyway, I just adjusted the grid size and reprinted it. After Getting it all lined up and square, I punched through paper at the crosshairs. I also slid the paper cylinder back just a hair and marked with a sharpie the bit that protruded. I filed the marked area off with a hand file the get the end perfectly square to the holes.Β  ..well that’s the theory. All I know is that it’s square now as far as I can tell.


The drill press didn’t seem accurate enough for this job, so I cleared off the mill table and drilled the holes on there. The only V-Blocks I could find in the shop were rusty and un-even, so I made some wood ones quickly on the table saw.

As per the plan, I fixed the end cap on, the center drilled, drilled and tapped the holes. It took most of Saturday night, which my girlfriend wasn’t too psyched about.

I probably should have been more careful about squaring and securing the end cap in the pipe. It may have gotten slightly skewed. I’m not going to worry about it though, since perfect squareness isn’t critical here.

I also noticed another mistake.. When I reprinted the template, I accidentally made it with only 7 screw holes, not 12. I think it’ll still be strong enough though. I’ll have to do a little FEA before I test fire it.


Here’s the finished assembly. I made more mistakes on this part than I’d like, but eh, live and learn. It should work perfectly, and it looks awesome. πŸ™‚

The hard part

Pre-post today. I’m going to try to drill the radial screw holes today.Image

I don’t have the proper tools to do this (a large indexing head). On top of that, if I mess up, I’ll ruin my back plate. I’ve got an idea for how to do it though, and I’m going to go for it today.

I can’t fit this pipe on the lathe at all, so even scribing an accurate circumference line will be difficult.

My plan is to polish the OD, and carefully measure the circumference. I’ll then print out a template to wrap around it. If I pull the paper flat and line up the ends, it should sit fairly square on the pipe. I’ll punch through it to mark the holes.

I’ll also use the edge of the paper to mark a circumfrence line on the back edge of the pipe. That way I can square it up with a file.

I’ll fix the back plate on somehow and drill it at the same time. That way even if the holes aren’t perfect they’ll still line up. If I really nail it, the end cap might even fit in any orientation. ..We’ll see how that goes. Wish me luck.

Annnnd another.

Nothing too tricky, but I left to go make this one at 10pm and didn’t get home until 5am. Towards the end I was terrified that I would make a mistake and ruin it. Didn’t though πŸ™‚


I did hit one snag. I didn’t have the right fixture to hold a boring bar. Luckily I was able to reach in just far enough with a regular tool.


Day 5: Another Part Done


I made the back of the piston tonight. It took about 5 hours. Turned out perfectly though.

The o-ring guide I’ve been looking at recommends about 20% compression of the o-ring (.110 for my 1/8″ o-rings). That fit was a bit tight on my last part, so I decreased the compression to 10% (.124). My pipe is 3.745 ID, so I turned the bottom of the o-ring groove to a diameter of 3.497. It still felt too tight, but I applied some oil and now it slides pretty easily.


I had to take off a lot of material…

2013-12-11 01.51.45

Another thing I just noticed. This 4″ PVC pipe fits perfectly over my 4″ OD aluminum tube. This might come in handy later.

Day 4: Got My Feet Wet Machining

So here goes nothing… I’ve used a lathe, but never to make something that needed to be this precise. I’m going to make the back of the cannon first.


There will radial screw holes around the circumference eventually, but I’m putting that off for now.

I ran into a problem right off the bat. I only have access to a small lathe, and the 3 jaw chuck (which is self centering) won’t fit this disk. There’s a 4-jaw chuck that would fit it, but that one’s not self centering.

To find a rough center I used a caliper to scratch some half-diameter arcs on the disk. Where they converge is a pretty good center point. I punched the spot and then used a drill press to make a hole there.



I centered the piece by putting the live center in the hole and pining the piece to the chuck. I then very gently tightened each of the jaws. After that I incrementally tightened them down, slightly tightening one apposing pair, then the other until they were all tight. (The pic’s from the second time I did this, after I finished the first side).


Here’s the finished part after I turned and faced the other side, and cut the o-ring groove. To cut the groove, I took the advice of some people on the practical-machinist forum and ground an old broken tool down to so it had the same profile as the groove. The tool was wide with a flat nose, so I didn’t think it would cut well, but I used a lot of cutting fluid and it actually cut a perfect groove without too much trouble.


It came out really nice. No alignment issues at all. I kissed the edges with a file to take the sharpness away.


It fits perfectly too. I’m so happy.

I decided to try to make the piston back next.


The centering and facing went well. (Look at that awesome surface!)

…Unfortunately, while I was turning the diameter I accidentally made too many passes without measuring. (It’s supposed to be 3.74″)


I’ll have to start over.Β  ..And that’s when I decided to call it a night.