# Thread: Funny shaped Cam lobe

1. ## Funny shaped Cam lobe

I am working on an engine designed by the late Bob shores. In his design sheets he details a cam fixture to make a cam but the shape of the lobe concerns me.

The base circle is .350 inches.
The flank radius is .696 inches.
The nose radius then is .115 inches.
Lobe lift is .065
Duration is 260

That makes the nose radius almost the same size as the base circle. Seems to me the flank radius should be reduced to make the nose of the lobe smaller.

Is there a "rule of thumb" that would give a better shape to the lobe? I know the cam "as is" will work but I would like to improve the design to make the cam action less violent.

Here it is!

2. You could look at hobby or full size cam lobes for a rule of thumb. Obviously you are trying to achieve a certain lift and duration at manageable accelerations.

I posted some info about calculating acceleration from lift, duration; base, nose and flank radius at http://www.cnczone.com/forums/showpo...48&postcount=9 (Cam Grinder)

What acceleration is OK? Consider the inertia and strength of the parts. Also, achieving low jerk, as in smoothly changing acceleration is important, not just low peak acceleration.

NC Cams has a lot of wisdom to offer on the subject of home made cams in many threads. Just read past him not seeing why a hobbyist would want to learn to make cams.

3. Originally Posted by dynosor
You could look at hobby or full size cam lobes for a rule of thumb. Obviously you are trying to achieve a certain lift and duration at manageable accelerations.

I posted some info about calculating acceleration from lift, duration; base, nose and flank radius at http://www.cnczone.com/forums/showpo...48&postcount=9 (Cam Grinder)

What acceleration is OK? Consider the inertia and strength of the parts. Also, achieving low jerk, as in smoothly changing acceleration is important, not just low peak acceleration.

NC Cams has a lot of wisdom to offer on the subject of home made cams in many threads. Just read past him not seeing why a hobbyist would want to learn to make cams.

First off, thanks for the reply.

I tried the calculator and was asked for cam action angle. for a 260 degree duration i used 130 CAM degrees.

Anywho, by changing the flank radius from .696 to .500 the G's dropped from 145 to under 85. There is a trade off im sure but it seems to me that this change would be easier on parts.

The other link would do me no good. I dont use a mill or cam grinder to make cam lobes. I use a simple fixture on the lathe. I have to have my lift, duration, base circle, and flank radius set before i make the fixture. The fixture is not adjustable but easy to make.

As for NC, that is just his way. Most people in the hobby arena give freely of what they know. They also dont depend on it to feed there family.

Well my next part was going to be the cam but I need more info so I'll do some lifter bushings or something.

Thanks again.

Steve

4. Steve, nice job machining that crank from solid. Did you make it on a lathe or mill, and from what material?

5. Originally Posted by dynosor
Steve, nice job machining that crank from solid. Did you make it on a lathe or mill, and from what material?

I used both machines to make the crank. I started with a 3/4 X 1 inch piece of 1018 CRS. With this being just a display engine, it will work fine. I turned the outside diameter. Then i turned both rod journals. Then the center diameters were turned. I went to the mill to cut the counterweights out and put the flat on the front shaft. Then i cut it to length and threaded the rear shaft.

6. Looks beautiful. If it was under heavy use, what material would you have used instead out of interest?

7. Originally Posted by LongRat
Looks beautiful. If it was under heavy use, what material would you have used instead out of interest?
It should be hardened and ground.

8. Well I cant find anyone who can advise on the cam info so i will build the cam as designed by the designer. I can't wait any longer. I am itching to get at it.

Photo of the cam blank.

9. Got er done!

10. Originally Posted by stevehuckss396
Got er done!

Beautiful job Steve! Did you grind the cam profile, or polish in the lathe?

11. Originally Posted by dynosor
Beautiful job Steve! Did you grind the cam profile, or polish in the lathe?

Polished in the lathe using some 1000 grit paper.

Never did get any straight answers so i made it to the specs in the drawings. Well except for the extra lift and duration. Hehe

12. You managed to create a good finish without distorting the shape. Did you use a hard backing behind the 1000 grit paper?

I still don't understand how you cut the base circle on a lathe without removing the cam lobe... Can see how to make the whole thing on mill with 4rth axis.

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