Thank you all

This cam design is copied from my 3cc vertical engine. It is the forth version I made for that engine, I haven’t gone back to try the previous 3 again to see how they work with the engine in it’s final form.

The designs were made using the cam calculation programme on the MEN website:

http://modelenginenews.org/Go to the menu on the left and open up ‘Resources’ and then ‘Design Centre’. This is a Java programme so I don’t know how well it will work on modern browsers.

One of our members has produced a version based on Excel which has a similar interface but I have not used it.

The design was based on values given in various books on model petrol engines.

Valve lift is 1.5mm (based on ¼ of the valve diameter)

Theoretical opening period is 260°

The angle between the lobes is 114°

This gives a theoretical overlap of 32°

The output of the programme is a table of angles and lifts for one lobe. I put this in a spreadsheet and shift the angles by the angle between lobes and also invert the lift values due to the way my milling fixture is set up.

This design programme does not allow for any clearances so I have reduced the base circle by 0.1mm. The transition was worked out by plotting each cut in a cad package and extrapolating.

I have attached the spreadsheet I used as well as the base circle drawing. The green circle is the base circle used in the calculation. The next black circle is reduced by 0.1mm. The ‘cuts’ for half a lobe are shown, the red line being the last from the cam design. The blue lines are then another three cuts to reach the reduced base circle.

George, I made this fixture before I had a rotary table and have kept using it. I can see benefits in a RT especially when cutting the base circle