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…
1/5 Chenery Gnome Rotary: Piston fit with cylinder
- Thread starter albd5a
- Start date
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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.
Finally got a good one; not perfect, but good. I don’t have DRO, but I can see now how useful that would be.
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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.
fixture ER32 Collet blocks www.amazon.com/Xetilmo-ER25-Collet-Chucks-Block/dp/B0CYFLS1K8
collets are typically sold in sets or separately. ER32 collets have more "range" than a 5C collet. for example 1/2" ER collet will clamp from ~0.460 to 0.500 (usually will grip up to 1mm undersize - but never grip bigger than the stated collet size). If you acquire a set, read this link to learn how to properly fit the collet in the collet nut.
https://tormach.com/articles/er-col...cpKopoq3r7n3YHvDfTBn8RqEfWNfY_gNve45us9WSBSly
Yes it's an ER 32 collet block ( square one) the advantage is that you can hold the work vertically to mill the slot then move the block so it is horizontal and then then drill the wrist pin hole and as you are referencing the square face of the block the hole will be an exact 90deg to the slot ( don't remove piston between the two operations. I also have them in ER25 and 5C
That particular piston the slot goes all the way down so I just drilled a hole to depth to make putting on a cut for the slot easier as well as making room for the boring bar. That piston is 20mm dia so about the same as yours. If it were your piston I would do that hole on the lathe, drill just undersize and then finish with a flat bottomed cutter, then do the skirt with the boring bar so keeping it all concentric.
That particular piston the slot goes all the way down so I just drilled a hole to depth to make putting on a cut for the slot easier as well as making room for the boring bar. That piston is 20mm dia so about the same as yours. If it were your piston I would do that hole on the lathe, drill just undersize and then finish with a flat bottomed cutter, then do the skirt with the boring bar so keeping it all concentric.
