Machining a venturi

Fritz Maurer · May 23, 2019

I have the old one for reference. I can bore the I.D. and make part of the flare in the outlet end with the compound rest, but how to make an internal curve that blends with the I.D. with out any corners? Thanks, Fritz

Oliver 500 Wa · May 23, 2019

CNC is the easy way. On an engine lathe you'll have to do some weird rigging with a radius turner. If you don't have a radius turner then grind a form tool.

unc · May 23, 2019

Fritz: For a one-off piece, you can fiddle with it a little. I'd use your compound with several settings to get the radius as close as you can, and then, as the previous poster suggested, use a ground form tool to eliminate the corners left by this process-OR- knock off the corners with your handy-dandy three corner scraper made from an old triangular file. Use low speed, and check your progress often. When it looks about right, mike your final bore at the venturi, and give it a polish. I think you might be surprised at how easily a scraper will smooth out the surface.
The Marvel Schebler venturis were made from spun or stamped brass- not a terribly accurate process. A lathe turned venturi will probably be more accurate than the factory job. unc

Butch(OH) · May 24, 2019

As they said, for a one off job you make several cuts by adjusting the compound. I would do the finish work with a scraper, then file and finish with emery cloth, Before a proper tool was ground I'd be done, the lathe would be cleaned up and part sitting on the bench,, and drinking a coffee, choice is yours LOL

Oliver 500 Wa · May 24, 2019

As they said, for a one off job you make several cuts by adjusting the compound. I would do the finish work with a scraper, then file and finish with emery cloth, Before a proper tool was ground I'd be done, the lathe would be cleaned up and part sitting on the bench,, and drinking a coffee, choice is yours LOL

If you are making the approximate shape with a bunch of different angles off the compound and then blending by hand, how do you check the contour against the original to be sure it is right?

You can try to "eyeball" it, but how close do you think you'll get? Not very.

You could measure diameters at different depths and get a little closer, maybe.

You could cut a template to match the original contour to use to compare the contours. Results will depend on how close you can cut whatever material you choose to use for it.

A sheetmetal template would be best and give the most accuracy, if roughed with a nibbler then ground to finish.

Of course after doing all the above, one would have spent far less time and had a much more accurate reproduction if you just grind a form tool or two.

Bob · May 24, 2019

Unless you have something unbelievably rare or obscure, you should be able to finx a "NOS" part or a reproduction part on the 'net and move on.

Much like you, a few days ago I was going to machine a complicated "NLA" gas line fitting from brass hex bar stock, but did one more 'net search and was able to find one for $4.68!

Sad part was they've only ship UPS for $12.95, I got it today.

But, no matter, finding it freed up some shop time for me to do something (hopefully) MORE productive.

Machining a venturi

Fritz Maurer

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Oliver 500 Wa

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unc

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Butch(OH)

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Oliver 500 Wa

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Bob

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