View Full Version : I love it when a plan actually works

09-16-2007, 06:43 PM
I started to post a plea for help with this little project but I decided to just plod onwards and see how it came out. I guess the experience and knowledge here must be soaking in by osmosis ‘cuz I got ‘er done and feel pretty good about it.

The task was to make a few model airplane venturis for the local distributor of Italian high performance engines called the NovaRossi. Seems like a simple task at first; just a little lathe and mill work from aluminum stock.

Rub #1 comes when you have to precisely position a radiused cut in the side of the of the part. The venturi is held in the intake opening by its sealing O-ring and the fuel supply “spray-bar.” If it fits properly, you can press down hard on the venturi to compress the O-ring, then slide in the spray bar to hold it in place.

Drilling and turning the part to the correct dimension was pretty easy but I struggled trying to figure how to precisely measure and position that pesky little cut in the side. It finally became clear that I would have to measure it’s location in 2 axes accurately and draw it out in CAD. The pic shows a parallel held flat against the top surface and a #46 drill slid snugly between the spray-bar and the parallel. This gave me one dimension and the other was obtained with an inside mike. Between the spray-bar and the opposite wall of the inlet opening. Using all these dimensions I was able to draw it up in CAD and establish the exact offsets required to position the cut, allowing for the squeezing of the O-ring by the S.W.A.G. method.
I cut a V-jig (thanks Roger for that idea!) to go in place of the fixed jaw in the mill vise so the parts can be indexed consistently (gotta make 7 of ‘em)

I used an edge finder, the CAD sketches and my best Jetthro Bodine cypherin’ to move the table to the theoretical position indicated. I spun the mill up and made the quick little plunge with a 5/32” endmill. I held my breath while I slipped the O-ring on, pressed down HARD and pushed in the spray-bar. THE DANG THING FIT PERFECT…HALLELUJAH!!

(See next post)

09-16-2007, 06:44 PM
Rub #2: The elation didn’t last long though because I still hadn’t figured out how to make a nicely radiused curve to transition from the tiny little 4mm I.D. to the large O.D. (This is for some type of spec. racing that allows a max I.D. of 4 mm) I found an old HSS 1/2" radius roundover router bit and a little finagling in CAD showed the radius would work. The small end was way too big though to blend into the I.D., even with the bearing removed so I chucked it up in a Spin-Indexer to hold it steady and worked on it with a Dremel by hand to reshape it. I used a height gauge/pointer to turn ensure the flutes were pretty symmetrical and had some relief ground in.

Again using CAD, I measured the depth needed, chucked the tool up in the tailstock chuck and tried a test cut in some scrap to establish the best speed for a chatter free cut. Again I held my breath and plunged my cobbled up form tool into the part. UNBELEVABLE; it peeled off some beautifulful, wide chips and worked great!

Now, only 6 more to go!

09-16-2007, 07:29 PM
Nice job! Another one of those instances where the planning and setup is 99% of the work and the actual machining is almost an afterthought.

09-16-2007, 07:30 PM
Muy bien. I

09-16-2007, 09:26 PM
Thanks & gracias guys.

I just finished the other 6 and they came out great as well. I kept the tolerances real close during the initial lathe work which took all the grief out of the milling ops. That little v-jig trick allowed me to just place the part down in jig, tighten the vise and that worrysome little plunge cut was automatically positioned perfectly. Now I'll have to go buy myself a new mill since the setup on this one can't be changed.;):D

I've gotta go type up the instructions and clean up the CAD sketch so I can duplicate it easily in the future. (I hope!)

Lew Hartswick
09-16-2007, 09:33 PM
OK Looks good. Now for the 1/2 " roundover bit and, I assume 6061
T6 aluminium what speed and cutting fluid did you come up with?
Always intrested in such for some possible future app. :-)

09-16-2007, 10:33 PM
great photos!

09-16-2007, 10:39 PM
Nice work. Great pics too.

That V for the vise is cool. What type of material did you use? I can see myself having one.


09-16-2007, 10:53 PM
Actually I ended up at the slowest speed my lathe will run which is marked 100 rpm. I've never checked the speed with a tach but it looks pretty slow. I used WD-40 and the Enco thick blue stuff...#1 I think. Both worked well but the WD-40 was slightly better IMO. I tried a couple higher speeds but it chattered during the last part of the cut. Slowing it way down stopped the chatter and even cleaned up the existing chatter marks. Maybe it was the poor router bit gratefully rewarding me for letting it live out it's final days at a nice, unstressful pace.

Since it was abused and blackened with burned pine rosin, I (very uncharacteristically) took the time to work the edges with a fine grit A/O "Toolroom Finishing Stick" 'til it was pretty sharp. I've never had any luck with form tools but this one was a pleasant surprise.

Also being factory ground, I was able to modify the small end to crudely match the profile that was put there by someone that knew his stuff. It still ain't real pretty to look at but the cut has that feel that tells you it's doing what it's supposed to. Nothing like my usual form tool result which ends up sounding like an enraged boar squeeling at the top of his lungs...or maybe l-o-n-g fingernails dragging across a blackbooard really hard.;)

And yes indeedy, 6061 T6 it is.

09-16-2007, 11:01 PM
That V for the vise is cool. What type of material did you use? I can see myself having one.It was made from a scrap of 1/8 x 1 1/4" alum flat stock.

Thanks for the picture kudos! I just try to emulate the example set by others here on the forum. I LOVE looking at the work done by the "gurus.":)

I hope Photo Bucket doesn't blow up someday....sure would be a bunch of little red x's!

Allan Waterfall
09-17-2007, 04:34 AM
Someone posted this link to Novarossi factory factory tour on another forum.

Nice work on the venturi,just wondering if you radius the underside after the spraybar to match the bore of the crankcase,or does the 4mm hole extend to the bottom of the insert ?

Only thing I don't like about Rossi engines is their use of slot head bolts all the time rather than proper hex head allen bolts.Probably a good reason,but always looks cheap to me.


09-17-2007, 08:49 AM
Hi Allan, great link!

Yes, I did radius the bottom as well. I roughed it with a #4 center drill and blended it further by hand with a sharp #11 Exacto blade.

I agree on the slotted head screws completely. Looking at the fits & jewel-like precision in their engines it makes you wonder why they'd use them.