Yet Another Webster Begins

[minh-thanh]

CFLBob !

How much gap should there be on the sides? I don't have any .001 or .002" shim material, or a feeler gauge that thin.

The drawing for the piston (sheet 6) shows the bottom of the ring grooves as a reference dimension and if you take the reference (0.771) from the nominal dimension of the piston (0.873), that says the groove should be .051 deep. My piston's OD is right around 0.878, not 0.873. Should I try to keep the depth 0.051? I'm using the piston rings from Dave Reed as called out in the drawings. I think the rings were .044 thick

My opinion: it seems that the groove of the ring is not straight at the bottom, this will affect the height of the groove

 

[awake]

Bob, I've not yet tried working with CI rings - I went the easy route and used a viton o-ring - so this is not from personal experience at all. But here is what Malcolm Stride says in Miniature Internal Combustion Engines:

"The ring should be a close but sliding fit in the groove, and the radial groove depth should be approximately 0.003in greater than the ring thickness."

 

[CFLBob]

CFLBob !



View attachment 114838



My opinion: it seems that the groove of the ring is not straight at the bottom, this will affect the height of the groove

View attachment 114837

Minh Tanh - yes, the cutoff tool has a slight "U" shape to it. It cuts deeper in the middle than the very corners. I did a rough check of the angle a ring can lean and I see about 5 degrees, so it looks like I need to widen and deepen them a little.

"The ring should be a close but sliding fit in the groove, and the radial groove depth should be approximately 0.003in greater than the ring thickness."

The ring is now above the piston by about that .003. I will deepen those grooves.

 

[minh-thanh]

CFLBob !
This may be not necessary
Before you put the ring in the piston, put the ring in the cylinder and push the ring along the entire length of the cylinder (by the piston itself or some round bar you can push the ring ) : this ensures that the ring and cylinder are fit
This may be not necessary ... but I always do this before I put the ring into the piston

 

[CFLBob]

I got the grooves much closer to what I wanted, then when I tried to clean out a corner by moving the cutter, it deflected. I pulled it back out of the groove then overcut it.

These are two pieces of a broken ring from last month, not the new rings. The rings are low enough in the slot that I can feel the edge of the groove by dragging a nail across the ring.

Minh Thanh, I'm pretty sure I can do the test you describe, but I can't quite do it while the piston is in the lathe. I think I'm ready to take it out of the lathe, cut off the handle and start machining the back side of the piston on the mill.

 

[CFLBob]

Since Tuesday, I double checked the piston grooves, then took it out of the lathe chuck, turned it around and started work on the back. I just hogged out a little material with drill bits .

Since I don't have small Vee blocks, I thought I'd make a fixture for the piston to hold it 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 in half 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. By itself, when I squeeze this, it doesn't want to close. In the vise, it works as intended.

Today I cut the features on the inside back of the piston and now the fixture's usefulness is done with for the time being.

Now I need to mount the piston for the two cross holes, the .188 (reamed) hole for the wrist pin and the .094 hole for the oil tube. Not sure how I'll hold those yet.

 

[awake]

Nice fixture!

 

[Brian Rupnow]

I have a rotary table with a chuck attached to it. For this operation on pistons, I set the chuck and rotary table so the rotating axis is vertical, and hold the piston in the chuck. I make certain that one jaw of the chuck is exacly on the right hand side, and put a spacer bar of 1/4" scrap aluminum between that jaw and the side of the piston. I use a dial indicator to center the piston under the quill, and machine the slotted hole in the Y axis. Then without breaking down the set-up, I unbolt the rotary table from the mill table, and tip it 90 degrees to the right, then bolt it down again and square it up to the mill table. Again, I use my edge finder to center the piston under the quill. Then I drill the wrist pin hole thru between the two chuck jaws which were originally on the left, thru the piston body and into the 1/4" plate of sacrificial aluminum, then ream the hole. This method insures that the wrist pin hole is perfectly square to the long slot in the piston.

 

[CFLBob]

I have a rotary table with a chuck attached to it. For this operation on pistons, I set the chuck and rotary table so the rotating axis is vertical, and hold the piston in the chuck. I make certain that one jaw of the chuck is exacly on the right hand side, and put a spacer bar of 1/4" scrap aluminum between that jaw and the side of the piston. I use a dial indicator to center the piston under the quill, and machine the slotted hole in the Y axis. Then without breaking down the set-up, I unbolt the rotary table from the mill table, and tip it 90 degrees to the right, then bolt it down again and square it up to the mill table. Again, I use my edge finder to center the piston under the quill. Then I drill the wrist pin hole thru between the two chuck jaws which were originally on the left, thru the piston body and into the 1/4" plate of sacrificial aluminum, then ream the hole. This method insures that the wrist pin hole is perfectly square to the long slot in the piston.

The rotary table was my first thought, but the problem is I don't have a chuck on my RT. I've just attached things to the RT with milling table clamps. I keep thinking of making a plate to attach one but don't really see how to go about making one and haven't started.

I was thinking of putting something into the 3/8" pocket that has a way to ensure that's parallel to the chuck's ways, that are parallel to the table.

 

[awake]

Bob, it is too late now, but I stumbled into a way that seemed to work pretty well - I drilled the wrist-pin hole before milling out the slot. To do that, I held the piston crosswise in the vice, resting on parallels, with the top and the bottom of the piston contacting the vice jaws. I centered and located for the hole, center-drilled, drilled, and sort-of-reamed (that's another topic).

Then I could set the piston up in my v-block in the vise, resting on a rod through the wrist pin hole. This not only ensured that the piston was absolutely vertical, but also gave me a reference for the wrist pin; I adjusted until the rod was perfectly parallel with the vise jaws, and then I could mill the slot at a right angle using the y-axis.

None of which helps you now, since you have the slot but need the wrist pin hole.

But here's my thought: make a jig that fits the slot and lets you set it horizontal, something like this:

The piston should just fit over the center stub. Put the piston on the jig into the vise so that the piston is aligned along the y-axis, resting on parallels; use a parallel or DTI or dead reckoning by eye to make sure the long part of the jig is perfectly parallel with the base of the vise (really, with the bed of the mill). Then center and locate and drill.

Yes, it would be something of a pain to make the jig as shown out of metal; you could make it easier by drilling two .25" holes .313" apart in the flat piece:

Then turn two pins, .375" diameter with a .25" stub on the bottom, and cut a flat about .040" deep on one face of each:

This lets the pins sit face-to-face in the base piece, forming the jig:

Of course, if you have access to a 3d printer, it would be relatively easy and quick to print up the jig as originally shown above.

I don't know if any of this will help - just my $0.02, off-the-top-of-my-head thoughts!

 

[CFLBob]

What I was thinking of was like yours except instead of the long thin base, perhaps cutting it off of a 1/2" aluminum bar. It would look like this

I think this could be made from a 1-1/2" long and 1/2 thick aluminum bar. This would go into the piston's 3/8 slot, like yours, and this fixture would be held sideways in the jaws so that one of those large flat surfaces was the vise's bottom.

I have some parallels that might be useful to keep it spaced well vertically in the jaws.

No 3D printer - it's more machining time.

 

[awake]

Hah - great minds think alike. Are you planning on holding the jig in the vise jaws, with the piston sticking out beyond? If so, how will you hold the piston in place - maybe Loctite?

That was part of my thinking with the jig that I show - it holds the piston between the jaws, but not in an orientation that could distort the round shape.

 

[CFLBob]

No, I was planning on holding it in the vise jaws. I just realized that 1/2 bar can't be 1" wide. It has to be smaller than the piston's diameter so that the jaws would grab it.

 

[awake]

Right, that was why I was thinking you were extending it out to the side.

But if your vise opens wide enough, consider turning it 90° and aligning the piston with the Y-axis. No chance of distorting the cylinder of the piston that way.

 

[CFLBob]

Am I overcomplicating this? I mean making a custom fixture to drill two holes?

I just put the piston in the vise on a pair of spacers and eyeballed it in place. I could use a 6" scale for a vertical reference, but that hole for the pin is twice the diameter of the setscrew holes. I don't think I could get the angle far enough off to matter

Yes, it's in mid-air. I had two parallels in there and took them out because I don't want to drill holes in them. I'll put some scrap aluminum there to drill into.

 

[Brian Rupnow]

Bob--If the bore for the wrist pin isn't exactly 90 degrees to the milled slot in the piston, the piston will rotate in the bore a small amount as it travels from top to bottom. Probably a bit of this "twist" is tolerable, but the closer it can be kept to 90 degrees, the better it will be.

 

[werowance]

Bob, how I did it was I have a 7/8 oversize er32 collet. I put it in my rotary table to do both the slot in the bottom for the rod and then changed my table from vertical to horizontal and did the cross drill

 

[werowance]

and I see that you don't have a chuck for it. do you by chance have a collet chuck that fits your head stock? my lathe headstock with chuck removed accepts an mt3 collet chuck. which also fits my rotary table so I swap them around. but I did have to buy an over size collet for it as 7/8 is larger than standard sizes in er32 I am told. picked one up on ebay

 

[CFLBob]

and I see that you don't have a chuck for it. do you by chance have a collet chuck that fits your head stock? my lathe headstock with chuck removed accepts an mt3 collet chuck. which also fits my rotary table so I swap them around. but I did have to buy an over size collet for it as 7/8 is larger than standard sizes in er32 I am told. picked one up on ebay

No, mine is pretty much factory stock with just an MT2 bore in the center, and I use a set of tee-nuts and clamps to hold things to it. I bought an MT2 end mill holder to hold a gauge pin and use that to center the table under the quill, that's the only other thing I've bought for the RT. I switched my big lathe over to a 5" chuck from the 4" it came with, and I could use the 4" chuck on the rotary table if I made a mounting plate.

What I've been overlooking is that this is talking about my 6" rotary table on the big mill. I have my Sherline "micro mill" with a 4" rotary table and I have both 3 and 4 jaw chucks for it. I tend to think of using the Sherline for small parts and it's really great for that. A 7/8 diameter piston would work on the Sherline. I need to go look at that.

 

[minh-thanh]

one more solution :