machine an eccentric

[John Rudd]

Guys,

I need the wisdom of the collective...

I need to machine a cam  as per the crap-o-cad sketch...

Basically, I want to machine an 8mm dia stub on a piece of bar that is 12mm in diameter...I've checked in my TC handbook on how to machine an eccentrc but it is as clear as mud...
So I bow to the knowledge of the collective to tell me how to do this please...

[Chuck in E. TN]

I have made eccentrics with a 3 jaw chuck by putting a shim under one of the jaws . Kinda trial and error way of doing it until you figure out how thick a shim  you need.
I have also done it in a 4 jaw independent chuck. Much easier.
Chuck

[John Rudd]

I have made eccentrics with a 3 jaw chuck by putting a shim under one of the jaws . Kinda trial and error way of doing it until you figure out how thick a shim  you need.
I have also done it in a 4 jaw independent chuck. Much easier.
Chuck

Chuck, I've tried the spacer trick in the 3 jaw just as a trial...Seemed to work but I ended up with a spigot about 6mm in diameter..

There must be an empirical way of doing this rather than trial and error.......

[mklotz]

If you must use shims, there's a program on my page (ECCENT) that will help you calculate the required shim size.  Also included in that distribution is another program that uses a far safer technique than the redneck shim approach.

For a 4jaw, spot the center of the tenon you wish to turn on the end of the stock, then use a pump center to get that spot running true, then cut away.

[Lew_Merrick_PE]

John,

Your sketch leaves a lot to be desired.  You have an ø8 mm spud sticking eccentrically from the end of a ø12 mm round, right?  The edge of the ø8 mm spud should be tangent to the ø12 mm round, right?

The centerline of the ø12 mm round is 6 mm from the edges (duh).  The centerline of the ø8 mm spud will be 4 mm "inside" the OD of the round (right?).  Therefore, you need to offset the ø12 mm round by (6 - 4 =) 2 mm to make everything line up.  An independent 4-jaw chuck is the way to go if you have such a beast.  Otherwise, you stick a 2 mm shim inside one jaw of your 3-jaw chuck, tighten everything down, and walk through all the alignments to make sure that nothing is tipping.  If my assumptions (stated above) are correct, this should work for you...

[andyf]

Is it something like the top end of this you're after, John?



I made that in the 4-jaw by centring up a round bar a bit bigger than the diameter at the fat end with the lever on it. The middle bit and the bit which would eventually form the off-centre stub was turned to the diameter of the middle bit (16mm in my case), and the end was faced. With a threading tool set dead on centre height, but with the toolpost at an angle to the lathe bed, I touched on the 16mm diameter, zeroed my cross-slide dial and then moved carriage to the right, taking the tool off the end of the work. Using the dial, I ran the cross-slide in by 4mm (half the diameter of the stub) and used the tool to scribe a light circle with the tool on the faced end. I then centre-popped a mark on the circle (the scribing located the punch). I could then use a wobbler located in the tailstock and a DTI to centre up the punch mark in the 4-jaw, and turn the peg down to 8mm. As I recall, I then reversed the job and gripped it in a 16mm collet to tidy up the fat end before adding the lever.

Its function is to lift a plate carrying a motor and idler gear in and out of mesh with a gear on my mill's feedscrew, so absolute accuracy wasn't needed; the idler was positioned later with the lever in the "meshed" position, and it didn't really matter where the idler ended up when the lever was turned to "unmesh".

Andy

[Chuck in E. TN]

a far safer technique than the redneck shim approach.

Marv, I resemble that remark...
Chuck

[John Rudd]

Lew,

Your answer makes perfect sense to me...I need to find a 2mm thick shim..else go the 4j route.

Andy,

The cam shaft is for a cam lock arrangement for a mini lathe tail stock...I'm trying to replicate the one on my Sieg...

Marv,
Thanks for the info and the approach using a 4jaw with the pump centre...might give that a try...

[Pete.]

I used my 4-jaw. Turned the OD true using a light cut as the jaws were lightly snugged, then put my dial indicator on the No1 jaw and wound it out 1mm. Spun the chuck round and wound the No2 jaw in 1mm. Now I had an eccentric with 2mm throw with some very minor adjustment using the DTI.

[Lew_Merrick_PE]

John,

The attached document may help you....

[Jasonb]

Otherwise, you stick a 2 mm shim inside one jaw of your 3-jaw chuck, tighten everything down, and walk through all the alignments to make sure that nothing is tipping.  If my assumptions (stated above) are correct, this should work for you...

No a 2mm shim will not work as the other two jaws also move, thats why you need Marvs program to calculate the shim thickness.

Personally I would do it in the 4 jaw. Mark out and punch the end 2mm off center, clock the punchmark with pump ctr and dti and then turn as required.

J