HOW do you make a master lobe ??

[James A. Lee]

I hope some knowledgeable soul out there can clear this one up for me...I've researched this for months until I'm crosseyed and blue in the face and still have no definitive answer. How in H*** do you go about making a master cam??

Gene Switzer in his design for a grinder glosses over the question with hardly a mention. except, "Make the master cam lobe four or five times larger that the lobes on the finished camshaft". (To me, that sounds as though you make it the exact shape as the finished cams, merely scaled up. that and sounds sensible to me) "The machine will reduce it in grinding.' (I am assuming that means you draw the master to a known larger scale, and grind to the needed final dimension.)

Ken Hurst says, "Making the masters is easy. I used aluminum plate, used a belt sander to shape the lobes and hand worked flanks and crowns. With good layout tools it doesn't take long." Oh! That's helpful!

In SIC, Don Bell says, "Master cam design is not a scaling up exercise. It is not an issue of adding the desired rise to the cam's base diameter. It is an adding activity".
WHAT??!! Bell shows a normal-looking cam together with his master, which is a very slightly flattened circle, goes on with the simple arithmetic used to derive the shape and then tells me, "A small defect on the master cam is magnified 12 times on the desired cam". That really blows my mind!! The concluding statement is, "Happy master cam designing".

And the 21st Issue of MEB, in the first part of a cam grinding article, the author shows the same oddly-shaped master, sort of a pumpkin-like thing which in some occult fashion results in a nicely shaped cam. using a rocker grinder.

OK, it's easy to see that I am good and confused by a lot of (to me at least) apparently contradictory info, and am really IGNORANT. I ADMIT IT, GO AHEAD AND DUMP ON ME ALL YOU WANT...I can take it!. But please,somebody, help me out here! I'm going bonkers!
Many, many thanks in advance. (And apologies to whomever I may have unjustly maligned)
Jim

 

[tel]

Not that I know much about it, the only cams I've made were for automatic spring coiling machines many moons ago, but;

"A small defect on the master cam is magnified 12 times on the desired cam".

Surely he means - A small defect on the master cam is reduced 12 times on the desired cam?

 

[Blogwitch]

Jim,

I was going to make a cam grinding machine, then came to my senses, but before I did, I had everything worked out.

I would have made an exact profile of the cam, only scaled up 4 times.

Where I think the pumpkin shaped ones turn out as they do, is because they were using just a flat beam follower rather than a pointed follower as I was going to do.

You will find that a flat follower just resting on the master cam flanks won't reproduce the correct profile if you use a true profile shape, especially around the top areas of the lobe, as under certain conditions and profiles, it can be in contact with more than one position on the master cam at any one time, whereas a pointed follower will follow the cam as a true profile.

I hope you understand what I am on about.

If not, I will do a C-o-C when I am not so tired.

Bogs

 

[gbritnell]

Bogs explained in brief where the profile differences come from. It all depends on the type of follower used on the grinder. It could be flat or it could be a roller type follower. In each case the shape of the master would be different because of the tangent point of the follower against the master.
gbritnell

 

[James A. Lee]

Dear George and Bogs;
Aha! That would seem to clear up much of the fog here....but I read also that several people have used a flat piece of delrin as a flat follower (to avoid stylus bounce) with no mention of having to change the master made as as a scaled up lobe. Hmm...

Making the master as a straight scaled up version of the contours just intuitively feels right to me.

Thanks for your input. (I enjoyed the part about coming to one's senses before making a grinder!) but I'm committed to doing just that...I'll post on progress later.

Jim

 

[Lakc]

The simple way I draw it, which would be the same way I would make one.

Dye up a plate of whatever material you want to use for the master cam. Scribe a centerline across it.

Take your desired base lobe diameter, multiply it by 12 or 4 or whatever scaling factor is correct for the machine. Scribe a the circle on the centerline for your base diameter.

Now comes where you have a choice. The majority of cam designs quote only the lift and duration, some quote duration at XX lift. For your first cam, I personally would stick with simple duration, mark the beginning and end of the valve opening and closing points on the base circle. Now pick a nose radius, smaller gives a pointyer (that a word?) cam and bigger nose radius a more rounded one. Pick something between 20-33% of the base diameter just to start.

Take your lift, multiply by your scale factor, subtract the radius of the nose diameter. Measure that distance along the centerline away from the base circle, scribe your nose circle, double check the distance should be your scale factor times your desired lift.

Draw lines tangent to the nose circle and your duration marks on the base circle, depending on how aggressive you want to lift the follower, use a french curve for that. Flatter sides are least agressive in lift.

Note that I had to swap back and forth between diameter and radius several times, thats where I usually screw up my first attempt. : Making scrap is how you learn and I have done a lot of learning. Still have a lot of learning to go, however.

Obviously, I left a whole lot of camshaft design out of that explanation. Another method would be milling a master lobe, and for that I would recommend Rons model engine news website. In the design section, he has a calculator that I liked enough to drop my own ideas about building a cam grinder. http://www.modelenginenews.org/ Still, with a cam grinder, you can make pre hardened cams without the fear of warpage if you deceided to harden a milled camshaft after cutting. I did briefly think about bidding on Bob Washburn's grinder when his shop equippment went up for auction a while back.

 

[steamer]

You can make a cam with a mill and an index head and harden it after.

Look about for my "Seadog" build....I used Ron's spreadsheet that is on the web site you mentioned.

Dave

 

[James A. Lee]

Thanks for your great input, guys! I think I have it pretty much under my belt now. Though I did pick up somewhere that the flanks can be drawn as tangent arcs by extending the lobe centers outward and along those lines scribing the flank curves using the base circle diameter as the radius. Think I will include that and see how it all goes. Thanks again for all the help. Much appreciated!
Jim

 

[gbritnell]

Hi James,
We have some threads out there covering cam design and some mention has been made of the radius on the cam flanks. This radius is very large in proportion to the cam and it's purpose it to accelerate the lifter without having it slap against the sides of the lobe. Now we're talking about a full sized cam that has an overall size (base circle to top of lobe) of up to 2.00 inches at times. When building a miniature cam that is maybe .312 overall there's not going to be much of a flank radius on it. Personally I have never added it into my designs and I haven't had any problem with wear.
gbritnell

 

[Blogwitch]

Jim,

Really, 'coming to my senses' was incorrect.

I decided that making one wasn't the right way for me to go, purely because I had already made cams using E.T.Westbury's method of using a jig on the lathe, and they were perfectly satisfactory for what I wanted to do at the time, and then realised I would never make enough engines or do enough development to warrant me making one.

In fact, if I rooted deep enough in my stashes, I already have the drive and grind motors and speed controllers ready wired up and modified to make one.

I suppose one day, if I have the need to grind a lot of hardened profiles, I just might get the urge again, as they are not as difficult to make as some people think.


Bogs

 

[James A. Lee]

Thank you all for helping me to understand this material better. And George, your point is well taken...it's easy to forget that splitting hairs in the making of these small cams is not as important as it may seem.

Thanks again,
Jim