Once the magic smoke comes out, it won't work any more.
John Kasunich
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Tue, 15 Apr 2008
More Production - Tecumsah Intake Manifolds
A friend of mine is very into minibikes. These bikes use a Tecumsah engine that is identical in most respects to a snowthrower engine. However. the engine is mounted on an angle, and needs a non-standard intake manifold to keep the carb level. My friend had patterns made and got 18 manifolds from an aluminum foundry, and then came to me for machining.
Holding the raw casting was a bit tricky, but eventually I came up with a setup that located the part using some features at the carb end flange. The photo below shows the setup I used to machine the carb end. One program with one tool change, to mill the end flat, clean up the inside of the manifold to make the opening round and concentric, and drill the screw holes. The second pic shows 11 of the 18 parts after the first program. The final step at the carb end was to thread the holes. That went rather quickly with a tapping head on the drill press.
The block end had a lot of material to be removed. Instead of nibbling at it with the Shoptask, I screwed each manifold to a steel block, clamped the block in a tilt vise at the proper angle, and let my Van Norman make a single pass with my 5-1/2" face mill. A slow shutter blurred the cutter - it is only doing 140 RPM or about 200 SFPM.
The cores were pretty far away from the desired shape at the block end, so the ports needed a lot of work. I used the same steel block and angle vise to mount the parts with the port facing straight up on the Shoptask. Jeff Epler let me use some alpha-stage offsetting software that he has been working on. The resulting g-code matches the port in the manifold to the one in the engine block, and blends that profile down into the as-cast interior of the manifold. The program used a 5/16" end mill for roughing, a 1/8" mill for finishing, and a drill to make the mounting holes. Because of machining time, most of them were done with a 0.050 stepdown. But I did a few with a much smaller step size, and they came out very nice.
Both ends of the manifold needed tool changes during the program run. My machine has totally manual toolchanging. However, I'm using the "Tormach Tooling System". which works quite nicely. Each tool has a 3/4" straight shank that goes up into a collet in the spindle. As the collet draws tight, it pulls a 1-1/2" diameter shoulder up against the spindle nose for a very repeatable Z position. It is quite fast and easy to change tools - I was getting 30 to 40 seconds chip-to-chip, with very repeatable results.
Tormach sells a variety of tooling for the system, but it is really pretty simple to convert other tools. For example, I'm too cheap to buy the Tormach collet chucks at about $80 each. But I found some very nicely made 3/4" straight shank ER20 chucks from MariTool Inc. for about $45 each. I made 1-1/2" diameter rings with a 0.749 bore and shrunk them on to the 3/4" straight shanks, and they work perfectly. The photo below shows my current collection of spindle tooling - the collet chucks with the homemade rings are on the right.
(posted: 15 Apr 2008 22:15) (permalink)