Cam for Elmer's open column twin

[Sshire]

Thinking about Elmer's open colum twin for next engine. Everything looks pretty straightforward except the two cams.
I've read Elmer's cam instructions about 50 times and still don't understand the procedure. I've got a 5c indexer and a rotary table.
Best
Stan

 

[Mainer]

Can you scan or photograph the cam instructions and post them? There should be somebody who can explain the procedure in an understandable way.

 

[Sshire]

Here are the drawing and Elmer's instructions

Any help will allow the beginning of this build
Thanks
Stan

 

[mzetati]

Coating the piece with well diluted layout blue helped me visualize the finished item (if my picture is correct): it does not look that much difficult to make anymore.
I suppose You could scribe that 5/16 arc on the lathe itself with a pointed tool or scriber held into the toolpost.
Bet the istructions are written so to make them cams in a 'lathe only shop', without using a mill.

Marcello

 

[Troutsqueezer]

Bet the istructions are written so to make them cams in a 'lathe only shop', without using a mill.

Marcello

Yep.

 

[Sshire]

I think my confusion is with the shape of the part. Having never made one of these (this would be my 3rd engine) I'm assuming that the radius starts at 5/8 and continuously increases to 5/8+0.30.
Procedurally I get using the indexer and (assuming once again here) increase the radius by 0.30/36 every 10 degrees.
If I'm completely incorrect here, let me know this when you stop laughing.

Best
Stan

 

[cfellows]

I

I think my confusion is with the shape of the part. Having never made one of these (this would be my 3rd engine) I'm assuming that the radius starts at 5/8 and continuously increases to 5/8+0.30.
Procedurally I get using the indexer and (assuming once again here) increase the radius by 0.30/36 every 10 degrees.
If I'm completely incorrect here, let me know this when you stop laughing.

Best
Stan

I agree, the cam looks like a spiral. Seems a little strange for this kind of engine, but it must work.

 

[miner49r]

Stan,
This post caught my eye because I've been thinking of building #44. Either I am thinking of the wrong engine or I'm missing some pages. Which Open Column Twin are we talking about? (13 or 44)
Alan

 

[arnoldb]

Hi Stan

Yes, the cam is an approximation of a spiral - Elmer used three circle arcs to approximate it, though he does not refer explicitly to its construction in the text. The process he describes is basically an exercise in "milling to the line.

I did a quick exaggerated drawing construction drawing last night, but was sidelined and didn't post then:

And without the construction base circles:

The red part of the image can be machined at the same setting.
While machining the magenta arc, the cutting bit needs to be moved slightly away from the center line for each "facet"
The blue section does not get machined - that corresponds to the OD of the workpiece originally.

Hope this makes things a bit clearer...

Kind regards, Arnold

 

[lensman57]

Hi,
This looks like an involute to me if I am not mistaken.
The procedure that has been discussed also sounds a lot like the one used for making a multi tooth involute gear cutter from a disc.
Very interesting and thanks for posting.

Regards to you all.

A.G

 

[Sshire]

Chuck
"Spiral" is the word. If Elmer had used that in the description it would have helped.
Alan
This is the #13 Open Column Twin with Poppet Valves. Troutsqueezer built a beautiful version and his video is not to be missed.
Arnold
The three circles are a wonderful technique for laying this out. I'm at home today and expect to go down to the shop and try it as soon as I reassemble the Grasshopper ( a few parts are humming away in the vibratory polisher so 48 hours should be enough for them, ready or not). BTW, I must thank you for your excellent build log on the Grasshopper. (I'll be posting mine next week). I doubt that I could have done it without many of your techniques and photos.

I report here on the cam progress.
Best
Stan

 

[miner49r]

Yep. I am missing some pages to #13. Luckily JohnTom still has them for download.