1/5 Chenery Gnome Rotary: Piston fit with cylinder

[albd5a]

Almost have one aluminum piston so far: everything is just about perfect except the wall got very thin at the large opening at the bottom of the skirt. The wall here is probably about half as thick as it should be as per the drawings. Not sure if it’s usable or not, there are a lot of steps to machining just one piston. This is my third one too. I made a couple of dumb mistakes on the others. I found that the 0.625 root diameter on the crown of the piston is a critical measurement. Otherwise it doesn’t fit right into the slave fixture and the gungean hole will be off… Also, again, I’m having trouble with the thin wall at the opening of the piston. My first one was perfect, but I screwed something else up, I’m using a 11/16” End Mill four flute cutter here rather than a boring tool…

 

[Jasonb]

It's a job for a small boring bar, you are probably getting chatter on the large 11/16 cutter which is causing it to cut oversize and it may well also be bulging the skirt making the piston OD larger.

 

[albd5a]

It's a job for a small boring bar, you are probably getting chatter on the large 11/16 cutter which is causing it to cut oversize and it may well also be bulging the skirt making the piston OD larger.

EXACTLY, chatter mainly with the lathe and wobbled eccentrically . First attempt with large 11/16 End mill on milling machines went well - amazed me; cut like butter, minimal chatter. But since then, having trouble getting concentricity with milling machine…. You’re right, boring bar would be safest and best precision. Thanks.

 

[albd5a]

EXACTLY, chatter mainly with the lathe and wobbled eccentrically . First attempt with large 11/16 End mill on milling machines went well - amazed me; cut like butter, minimal chatter. But since then, having trouble getting concentricity with milling machine…. You’re right, boring bar would be safest and best precision. Thanks.

Finally got a good one; not perfect, but good. I don’t have DRO, but I can see now how useful that would be.

 

[albd5a]

Finally got a good one; not perfect, but good. I don’t have DRO, but I can see now how useful that would be.

 

[Jasonb]

It would be easier and quicker to put the flat bottomed hole in and bore the skirt on the lathe then just use the mill for the slots

 

[albd5a]

It would be easier and quicker to put the flat bottomed hole in and bore the skirt on the lathe then just use the mill for the slots

Nice, helpful photos. I would need to drill quite a sizable hole in order to use the boring bar. I was kind of plunge cutting with my end mill cutter in my mill. I would get perfect concentrity using the Lathe - boring bar. You didn’t plunge cut this skirt bottom hole, did you? Also, what is that ‘fixture’ that you show on your slot cutting photo? I would like to get one.