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Author Topic: one day, ill go cnc :) spherical boring/turning  (Read 2767 times)

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one day, ill go cnc :) spherical boring/turning
« on: June 15, 2013, 08:58:21 AM »
i had a mission, to make a high compression head, with a squish band, and a hemispherical combustion chamber!

well, gemoetry simply ended up smacking my bottom and telling me i was naughty. that was basically the end result. silly law of volume versus area! you cant get a squish band in a 50cc head while retaining a decent volume on the hemisphere.


i have a mill with cnc fitted, thats cool.

i dont have a lathe with cnc.

so boring a hemisphere posed a challenge.

well, i rose to the challenge :) i stuck my thinking cap on and looked at things upside down.

1 32dp rack.
1 small lathe
1 (or two) lumps of alloy
1 piece of brass
1 piece of 70mm steel.
1 32dp gear from an RC starter(HSP or HPI?)... never counted teeth :(
1 lunatic

and i maded it! vanquished!

i have a rack on the cross slide.

gear is mounted against rack in alloy blocks, with brass bushes for bearings, vertically under spindle.

70mm lump of steel was turned down, and hexed partially to drive hex inside small gear, forming a turntable.

very dodgily drilled and tapped and used mill clamp on badly sharpened tool just to test it out.

works perfectly :)

the turntable is going to be thinned down somewhat, with some sorta toolpost. later on, one day...tomorrow!

unfortunately, as its fixed on the centre line, its not suited for spherical TURNING, only boring really.

but it was the only way i could think of to get the pivot in far enough to do a 25mm hemisphere, for instance.

for which it will need a toolpost, cus the clamps getting in the way currently :( and i can only go in so far before hitting chuck jaws!

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Re: one day, ill go cnc :) spherical boring/turning
« Reply #1 on: June 15, 2013, 10:02:53 AM »
Your squishband should be 50% of the AREA of the bore. There should be 1 mm squish clearance between the head and the piston when assembled. The angle of the squish should match the top of your piston. Compression ratio to suite your application. The higher the CR the harder it is to tune the jetting and prevent detonation. Start low(about 9:1)  and work you way up.

(;-) TP
Re: one day, ill go cnc :) spherical boring/turning
« Reply #2 on: June 15, 2013, 10:59:57 AM »
i know that. 50% area with 0.6 to 0.8 mm clearance on a 50cc.

and then, i just did the maths, hoping to prove myself right again.

and somehow, i didnt.


 i got the same final CR on the same stroke length despite changing cylinder diameter... as long as ratio between hemisphere and bore stayed constant. poop..

i sure didnt get them results the first time. overlooked something i guess. i swore there was something akin to the volume versus surface area idea going on.

now maths is punching me in the eye for going anywhere near geometry!

so i just gotta make these hemispheres more hemispheres than segments. meaning i gotta set the radius better. meaning i really gotta finish this thing...

« Last Edit: June 15, 2013, 11:02:00 AM by HeadSmess »