Making a cam

Gentlemen,

I have to make a new cam for my Lister H, the cam operates the fuel pump and is basically like a thick washer, 10 mm thick x 30 mm dia, with an off centre hole, 15 mm, through it.

My question is how to machine the hole, my thoughts are turn the OD and cut and face both sides, use a centre finder and scribe a line across the flat then measure from the edge of the dia along the scribed line to the point of the off centre hole, centre punch the point.

Fit my four jaw chuck and roughly adjust the jaws and clamp the cam, put a pointer into the tail stock chuck and bring forward to the chuck and adjust the jaws so that the centre pot mark lines up with the pointer.

Once happy drill and bore hole.

Does the above make sense.

One more question please, I have to reamer a tapered hole through the cam for a taper pin to attach to the engine, how do I decide on what size pilot drill I need to start the hole.

Thank you.

Martin P

Hi Martin, your method sounds good to me for the off centre hole.

Ref the taper pin hole size, it should not be greater than the small end diameter of the taper pin when the pin is fitted.

Emgee

That's the standard way to turn an eccentric so you should not have any problems.

I usually pic a drill that is just above the small end of the pins diameter and enough so the reamer will enter the hole.

Gentlemen and Andrew J,

Thank you for your thoughts, as said at the beginning I've never made one and wanted to make sure I had it right on both counts.

Martin P

Martin

There is a table/graph in "Machinery's Handbook" that details the suggested pilot hole sizes for the various sizes and lengths of taper pins. Basically the initial hole should be the size of the small end of the pin, but if the pin is long the hole should be step drilled with two or more larger sizes especially if using a straight fluted reamer as opposed to a spiral flute one. If you don't have access to the book if you let me know the dia. of the pin at both ends and it's overall length I can tell you the appropriate drill size(s).

John

Edited By John Purdy on 24/01/2018 19:26:42

John P,

Thank you for that nugget of info, I will now go and find my Machinery's Handbook, I rarely use it to the point I'm currently no sure where it is.

Martin P

Martin

If you can't find you book let me know and I'll get you the info.  It's on page 1654 of my edition #20.

John

Edited By John Purdy on 24/01/2018 20:56:50

Tubal Cain gave a handy formula for a spacer to put under one jaw of a three-jaw chuck to offset an eccentric.

T = 1.5E[1 - 1/2 E/D - 3/8 [E/D]^3]

Only an approximation, but it works.

Neil

If you left a concentric stub on one the faces of the cam you could use a dial indicator in conjunction with your 4-jaw to set the offset from the centre-line precisely correct before facing the stub off and boring through. There'd be no need to scribe and mark up the face first. The TIR is simply twice the offset.

Of coarse if the cam is big enough to project from the jaws then there's no need to use the stub at all.

HTH

Jon

Neil,

Thank you for the equation, all I need now is it explained. I assume T is the spacer size, D is the cam diameter, E is the offset, what does ^ mean and do I work out all in the brackets first.

Martin P

^2 means squared ie 3^2=9

^3 means cubed ie 3^3=27

yes work out E/D first then work out [E/D]^3

then work out [1 - 1/2 E/D - 3/8 [E/D]^3]

then multiply the result by 1.5*E

I recently had to drill offset holes in some brass. About 4mm offset in 16mm dia rod. I used the 3 jaw with packing at one jaw. Just did it by trial and error, could not remember that formula, did not need great accuracy but worked well. However I thought the set up did not seem very stable and think that a bigger dia. might be a bit dodgy. Depends on chuck size I suppose, mine is 100mm dia.

Gentlemen,

Thank you, I've never seem N^2 expressed as that so its something else I've learn't.

Martin P