The piston has a handle, a half-inch diameter, couple of inch long piece barely visible on the left in this picture, held between the chuck jaws. That's not part of the piston design, it was just there to make it easy to work on. Now that I was willing to accept the piston, it was time to turn the piston around, part off the handle and start to work on the back side, which has a few features. While it was turned around, I took out a bit of metal with a drill bit and then the biggest end mill I can put in the lathe's tailstock.
Next, I needed to make a fixture to hold the piston tight in my vise without marring it. I took a piece of scrap aluminum and made a hole in it that's about .003 bigger than the piston, then slit the whole fixture in half lengthwise so that the two halves would be perpendicular to the vise jaws and clamp onto the piston. I had the piston wrapped with a piece of soda can, also aluminum about .004 thick. When I squeeze this just by hand strength, it doesn't want to close. In the vise, it works as intended.
This is the back of the piston with a groove for the piston's connecting rod to rotate and work. There's a pair of 4-40 holes on either side that hold a pin that the connecting rod moves on - called the wrist pin (Wikipedia tells me the real name is a Gudgeon pin).
The piston is now close to being finished with its machining with the exception that it needs two holes drilled through it. If you take the horizontal axis, where the fixture is split, as one axis, a hole gets drilled fully across the piston a little forward of where the piston is held in this fixture parallel to the top of the fixture. That hole is 3/16" or 0.188 diameter, and has to be precisely sized (a reamed hole, not just drilled). Perpendicular to that hole, along an axis from the top of the frame to bottom, is a smaller hole, 3/32" or 0.094, for a small oiling tube. That hole is on only one side of the piston.
In other words, I need a way to hold the piston in two orientations in the vise jaws with it lying parallel to the floor of the vise. The piston will be oriented more like the picture on the top here, probably except for the solid top being on the left in the vise, not the right like on the lathe. The first orientation is with that slot visible in the middle of the piston parallel to the floor of the vise to drill the 0.188 hole, and then rotate it 90 degrees so that slot is perpendicular to the floor of the vise, for the small tube
I've thought about this and don't have a real feel for how accurate those angles have to be. I know that for any deviation from perfection in the wrist pin's holes the piston will rotate in the bore a small amount as it goes back and forth in the cylinder. That wobble would add to losses or inefficiency in the engine. Some playing around in 3D CAD gives a fixture that could be made from a scrap of 1/2" thick aluminum bar. The section that plugs into the slot in the piston is obvious. Those surfaces on the rectangular section become reference surfaces that I can count on to align the piston for drilling in the right places.
On the other hand, it might be acceptable to just eyeball the alignment and the position of the pin with respect to that slot will be fixed by the setscrews anyway.
Don't you have your wrist-pin jig already made?
ReplyDeleteSeems like drill-n-tap the corners for tension bolts, open a slightly larger cross-hole, and ream'r out.
MN Steel
To be honest, I not only don't have one, I've never even heard the term "wrist-pin jig" before. The drawings I'm building this to don't have such a thing, so I'm completely ignorant.
DeleteI did a web search and saw some being used on real engines (full-sized, if you prefer) not these little models.
Sorry, what I meant was your middle picture IS the jig.
DeleteThe drill-n-tap for the holding jig would allow you to clamp it in the perpendicular.
MN Steel
Is the .188 (.1875) hole for the wrist pin? Is it locked in place by the 4/40 screws? IE, the pin hole intersects the threaded holes at 90 degrees? Is it supposed to be a press fit?
ReplyDeleteI would blue the piston, including the top, set it up in some v blocks held in the vise, and use a test indicator on the flat of the connecting rod recess to get it parallel to the vise/table. Then use a machinist square and scribe to make a vertical line on the piston top at 90 degrees,continue it along the piston side. That is the oil line center location. Use an indicator to find the high spot on the piston, that is the wrist pin line. Double check it measuring from one side of the part to the other and halving it. Now the two axis are established.
Good place for a screw machine drill, 135 split point. That assumes the hole is radial to the centerline. If not, you might have to spot face it to get a accurate hole start.
Be careful with reamers- slow is good, I have been using WD 40 as an aluminum fluid,look for a chart on stock removal- I printed off one from Rock River Tool that has pre-ream sizing for 2 and 3 percent removal. This is a good place to try test reaming a scrap- and do not load up the reamer, the swarf can jam up and score the heck out of the bore. Back it out and clean it. A test cut also confirms sizing- BTW, the number of machinists who hate reamers is numerous.....try a search on reamers on PM and read the responses! Yikes! I always thought they were magical problem solvers...apparently not..
Sorry- I can't edit- on the above, I confused the oil line hole with the wrist pin pin hole.
ReplyDeleteI like your idea about just using this as pretty much any setup problem. I don't like the idea of special fixtures for everything. I was extra cautious about the piston because I didn't want to mar the body after literally months being stuck on it. The marring caused by the lathe jaws is especially bad, but making the soda can soft jaws (well wrapped around the piston not the jaws) took care of that.
DeleteIn the mill vise, the fixture you can see in the middle picture was useful and fairly easy. I didn't want the piston to move while it's being cut and the cutting forces are pretty big. Remember the cylinder on my flame eater that slid downwards during the cut and had to be scrapped? I kept that as a pencil holder about arm's length from me right now.
With drilling, the forces trying to rip the thing out from a drill bit are much smaller than from a 3/4" end mill hogging out a 1/4" deep hole so what I need to do is more to keep it from sliding around and changing the coordinates of the hole while drilling.
Reamer story - I had to ream a hole to 1/4 and grabbed a 5/16 reamer by mistake. That created some interesting scrap! The hole actually came out irregular, kind of like a stop sign but with seven sides instead of eight.
Was that reamer a six flute? When you drill a hole, it will not really be round, but will have lobes that amount to one more than the drill has flutes. In other words, a typical two flute drill bit will leave a three lobe hole.
DeleteYes, it's six flute (straight flutes). That's really amazing and thanks for passing that on. The reamer works fine if the hole is undersized by maybe .007 radius (1/64" diameter) but when I put it through a hole that was 1/16 small it came out with the lobed appearance.
DeleteNow I'm going to be trying to think of how the physics of that works to always give one more lobe than the cutter has! Can't depend on speed or relative speed...
Can you cut down your holding fixture and turn it 90 degrees in the vise?
ReplyDeleteThat would eliminate the risk of the V blocks marring the piston.
Do you mean turn it vertically - so the piston is horizontal? I wouldn't use V blocks, though. It will be on some scrap metal to keep it off the vise ways and sit on something that I could drill into.
DeleteYes. Use it so the piston is horizontal, and cut it as necessary to do the job. Might have to bore a hole in it to get the drilling location on the piston clear.
DeleteSig, There is no rotational movement that comes from the wrist pin. If it exists it is because the crank pin is not parallel to the cranks axis of rotation. However if the wrist pin is not perpendicular to the wall of the piston there will be a degree of interference where the skirt contacts the bore. Also the big and little end of the rod bores must be perpendicular to the beam.
ReplyDeleteThe mandrel you show would work for locating in that plane, but locating the piston in a “V” block that holds it 90 degree to the mill/drill axis is the most important. I reamed my wrist pin holes because in a semi floating assembly they were the bearing surfaces. In your engine the small end of the rod will be the bearing surface. Cross bore accuracy is not as important for your engine within reason. Full size pistons have offset wristpins.
Model Engine Builders magazine has a description of a fixture I made to expand rings using a tapered pin and a retaining ring to avoid breaking them when mounting them on pistons
Thanks for that, anon. I pulled that verbatim from an engine builder I read, but I just couldn’t see how that piston wobble would happen.
DeleteMy mill’s headstock is perpendicular to the table and the vise is parallel to the table, so pushing the piston down on the vise and clamping it in place gets the at a hole right angle to the piston. I think I could eyeball it close enough.
Ooops that's supposed to read - "gets the hole at a right angle to the piston"
DeleteIt was autocorrect, I swear.