Lapping machine for piston rings

Hi everyone
after trying the manual grinding of piston rings , on the 2 faces in contact with the seat on the piston with this tool and sandpaper , i did not get a perfect parallelism between the 2 surfaces (red arrows )


My manual tool and sandpaper

So , I decided to build a lapping machine , trying to copy some machines seen on Youtube
I had to replace the brake discs in my car and the idea was born at this time, use a brake disc as a lapping plane.
I asked a friend of mine to create a perfect plan on the brake disk through its grinding machine .

I built a flange, a shaft with 2 bearings, a tube for bearings

I bought a used small electric motor , cost 2 Euros

this is the initial sketch

That's cool. I've seen what I suspect are similar industry videos - what look like bearing rings or similar. I assume there must also be some way to reliably control down force (lapping pressure) when parts are very light. The parts I recall seeing maybe had enough of their own natural weight o were held in a fixture, maybe for that reason? Please show us some assembled pictures of yours & how the results turn out when the time comes

Is the cast iron mostly because you have it available or the properties that embed lapping medium? Reason I ask is other lapping videos show CI but also series of relief grooves cut in there. I've been experimenting with simple split or 'clam shell' lapping blocks for round things things like valve stems. Some were aluminum, some brass. But what seemed to make a difference was grooves in the surface. I can mention some other links but I don't want to plug up your post.

I don't know why your rings need to be perfectly parallel. As long as the ring groove is wide enough to fit the ring's max thickness plus a couple of tenths or so, who cares if the clearance is not the exactly same all the way around the piston. As long as the downward facing ring surface is flat to fit the bottom of the piston ring groove closely, it will seal properly.

I love to know how you are planning to make that little undercut. It is completely unnecessary IMHO.

WOB

Probably more important is that the ring is not twisted like a spring washer and that goes back to the heat treatment process .

Hello Peter
I intend to lap 3 piston rings simultaneously on 1 fixing tool and build 3 fixing tools, total 9 piston rings being processed
I'll put weights on the 3 piston rings

Regarding the material of the lapping disc, I looked for a spare disc for lapping machine and many manufacturers supply the disc built in cast iron , i think because cast iron holds the grinding dust and embed lapping medium.
I do not have a chance to make grooves cut , my piston rings are about 1 inch in diameter and thickness 0.031, max 0.04 inch wide grooves would be required , i have no tools with this small thickness.
I will try without grooves , although the lapping machine is less efficient, i 'm interested in quality.

you can see the brake disc, the 3 piston rings (blue color), the brass fixing tool (red color) and the rotating ring (pink color)
the weight will be inside the rotation ring

Hello Wob
I'm not an expert in piston rings but IMHO the parallelism/ flatness with the piston groove and perpendicularity with the cylinder liner are important to get a good compression in top dead center phase and a good intake in bottom dead center
My low manual ability does not allow me to get planar and parallel surfaces with a manual tool and sandpaper
when I measure the thickness of the piston rings with micrometer i always find small differences, 0.01/0.03 mm (0.0004/0.0012 inch)

Hello XD351
I tried to build the piston rings following the Trimble method but about 30% of the rings are scraps because i didn't get a good circularity, even after grinding the external diameter, i think because of the twist like a spring

This time I try to build the rings for the Howell v4 engine without heating so as not to deform them and using the lapping machine to get a good planarity and parallelism.
After which i grind the outer diameter to the same size as the cylinder liner with a grinding tool post and also the inner diameter.

Ah, I get it now. Those pink rings kind of loosely confine the parts to be lapped. Yes, that principle is just like the high end commercial lappers I saw videos of. The only thing I don't quite get is if you have 3 different thicknesses and a weight applied on top, I don't see how it can make consistent thickness. Consistent flatness & surface finish yes, but each ring is actually going to see the same removal rate. Or is that the magic of the weight where the thick part sees more weight & gets lapped more whereas the thin part more or less floats until they are similar height? But that would have to be a close fitting weight within the confining ring that is kind of constrained in a piston motion?

In terms of the piston rings themselves, I had not planned on anything more than your hand fixture on abrasive plate as others have done, but I am new builder. I did buy some commercial (OS Engines) rings as insurance just in case my home made ones turn out to be too challenging to begin with. I will take a close look under magnification now & report back thickness variation.

Your method has great merit as that is very similar to how we lap flats for Optical work...

An alternate method would be to use a surface grinder with a magnetic chuck...

Best Regards,

Preston

I turned 20 piston rings, all of the same thickness +/- 0.01mm (+/- 0.0004 inch)
3 + 3 + 3 lapped each session with 1 + 1 + 1 weights of about 650 grams (22.9 ounces) each
I think if the piston rings have different thicknesses among them 3, the machine can not correct their thickness, only a small part.
In a few days I'll try the machine and I'll make a report

Foketry said:

Hello XD351
I tried to build the piston rings following the Trimble method but about 30% of the rings are scraps because i didn't get a good circularity, even after grinding the external diameter, i think because of the twist like a spring

Click to expand...

I suspect your cast iron rod is poor quality and might also need stress relieving before machining. I have made about 50 rings by the Trimble method, and with a good quality cast iron rod my scrap rate was always less than 15%. I never needed to grind the OD. Simple turning to about .001" over and then polishing with SC paper yielded a mirror finished and on size tube ready for parting off the rings. My rings were typically 1" OD and .023" thick. As cut off the tube with a narrow cut-off blade, they were uniform thickness within .0001" . After heat treating on Trimble's jig, hand polishing on a 600 grit diamond bench stone using a small rubber block to apply pressure to the ring produced a nice smooth finish on the ring faces and there was no problem maintaining uniform thickness within .0003". That is more than close enough for model engines IMHO. Circularity was never a problem. After finishing the rings, inserting a bare ring into a cylinder bore and holding it up to a strong light showed only a tiny amount through the ring gap, and nothing around the ring circumference. Again, I think your cast iron is not good quality. Cast iron is brittle and tends to be nonuniform in composition on a microscopic level. That's why the best quality is essential and must be stress relieved or it will change shape after machining. Good luck.

WOB

Foketry said:

Hello XD351
I tried to build the piston rings following the Trimble method but about 30% of the rings are scraps because i didn't get a good circularity, even after grinding the external diameter, i think because of the twist like a spring

Click to expand...

How long did you let your rings soak in the fixture. I soak my rings at 1100F for 3 hours and have not had any trouble with deformities. Got the temp and time from a very well respected metallurgist. I also buy gray iron instead of ductile iron. Read somewhere that it is a better choice. Personally I couldn't tell you because I have never tried ductile.

For Holt 75 rings i followed the Trimble method , i used ductile iron (centrifugally bar ) , gray iron for sleeve, but too many scraps.
Good compression after 10 minutes of engine running on the lathe ,i reached 5 - 7 bar (72-101 psi)

For engine i am building now, Howell V4 i will follow the method described in this video :


I don't own a surface grinder and then i make a lapping machine with a really low cost, all recycled material

First test, the electric motor has little power, I have to replace it
I can only lap 3 rings at a time, I have to lap 3 + 3 + 3 rings together

Foketry said:

For Holt 75 rings i followed the Trimble method , i used ductile iron (centrifugally bar ) , gray iron for sleeve, but too many scraps.
Good compression after 10 minutes of engine running on the lathe ,i reached 5 - 7 bar (72-101 psi)

For engine i am building now, Howell V4 i will follow the method described in this video :


I don't own a surface grinder and then i make a lapping machine with a really low cost, all recycled material

Click to expand...

Mr. Villarreal's ring procedure is way too much work, is mechanically unsound, plus more steps means more possibilities for failure.
He apparently never heard of Mr. Trimble.

Foketry, if you use good quality gray iron and follow Trimble's method exactly, you will make good rings with a low scrap rate. There will be some scrap, but less than 30%. OK, what ever you do, good luck and have fun.

WOB

Foketry said:

Good compression after 10 minutes of engine running on the lathe ,i reached 5 - 7 bar (72-101 psi)

Click to expand...

Were you 'running in' the rings on the lathe just by spinning it with the lathe or with fuel and ignition supplied to create combustion? Rings will bed in quite quickly (and create a much better seal) when subjected to combustion pressures rather than just spinning the engine over. In fact, the rings may never create a 'proper' seal if they've been 'run in' without combustion pressures.

I know this post is more about lapping but I wanted to make sure you were aware of this link where it discusses ring making in more detail, the Trimble method & other related input.
http://www.modelenginemaker.com/index.php?topic=3794.msg68928#msg68928

One thing I noticed on the BR2 video is he appears to be using a slitting saw to part the rings. Maybe I'm off base but I thought the objective was to make a clean a break as possible (ie. equivalent to zero kerf as opposed to width of saw), then mount on the gap / heat treating fixture which is sized on this assumed basis? Then final gapping. Its been a while since I looked at this stuff so don't hold me to it. And maybe some of the subsequent turning operations on BR2 are compensating somehow. Something similar gets mentioned in the attachment post#6 which deviates from Trimble method & he explains why. Above my pay grade but I do intend to make my own rings.

Cogsy said:

Were you 'running in' the rings on the lathe just by spinning it with the lathe or with fuel and ignition supplied to create combustion? Rings will bed in quite quickly (and create a much better seal) when subjected to combustion pressures rather than just spinning the engine over. In fact, the rings may never create a 'proper' seal if they've been 'run in' without combustion pressures

Click to expand...

I run 10 minutes the engine with head on the lathe, but without fuel, just a little oil,
after which I do a compression test with a pressure gauge equipped with a block valve
If the pressure is less than 4 bar, i disassemble the motor and control the piston rings




valve leakage is being checked previously with vacuum pump