Centering odd shaped pieces in the 4 jaw

[John Stevenson]

Plenty of posts about setting up in a 4 jaw when running square or round centrally but thought after setting up an odd sided plate today to take some pics and show a very simple tool to assist.



Not to great a picture but that holder fits in a tailstock chuck making it able to be moved from machine to machine. The hole in the hexagon is about 1/2", enough to allow the 1/8" pointer to move about. The pointer id held in the round bit that fits into the chuck.
Then all there that remains is the dial gauge that bears on the pointer.

In use you offer this pointer to the centre punch mark in the part and rotate the chuck by hand and work two jaws at a time, top and bottom which is determined by the movement of the dial gauge. I have posted this on another forum and the anally retentive's have jumped on the fact that the dial gauge won't read correctly because of the cosine error.

SO WHAT? it could be calibrated in whores per square metre, all you are interested in is no difference in the reading.

http://www.stevenson-engineers.co.uk/files/P1000896.MOV

Crap hand held movie showing quarter of a thou variation after only 4 revolutions [ having a bad day, usually do it in two ]

John S.

[mklotz]

Well, John, my whole family tells me I'm anal retentive but even I can see that the cosine error just doesn't matter.  "Whores per square meter"?  Yeah, I think I've been in that neighborhood of Amsterdam too.

What I don't understand is why you don't mount the thing with the clock facing forward, i.e., facing the 9 o'clock position.  That way you would be sensing the fore and aft movement of the part as you adjusted the two jaws you can reach simultaneously with the two chuck keys you should have.  That approach works better for me than having to rotate the chuck to get at that jaw at the 6 o'clock position.

[John Stevenson]

Marv,

No reason why not to put it in the tailstock chuck at 9.00 o'clock.
I have heard of the two chuck key method before but I can work faster just holding one instead of picking one up all the time from the lathe tray, i have to rotate the chuck to check the runout so all the jaws are accessible on the turn round.

John S.

[andyf]

I made a super-sophisticated telescopic version, as below. Inside the tube, a spring from under the flint of a discarded lighter holds the thing engaged between a centre-punch mark at the pointy end and a centre in the tailstock at the blunt end. A stop soldered into a slit in the side of the tube engages in a long D shaped section filed into the steel rod, to stop the rod coming out. In use, the friction on the point is much less than the friction against the TS centre, so it doesn't rotate while the chuck is manually rotated and the jaws adjusted. Thus, there was no particular need to keep the device concentric from tip to tail. Soft solder holds the parts together. Crude, but effective. 


Andy

[websterz]

Crude but effective is my middle name! Well, it's Alan actually, but that is still a cool tool!