Download Plans
A simple device, but one that can save a lot of effort setting items to run truly on the faceplate. You can buy (or make) a 'wobbler' for holding in the drill chuck which fulfills a similar function, but this home-made version offers a few advantages. The magnification of movement off-centre is about 4:1 (depends on length of shaft) so it is easy to get pretty close without resort to the DTI.
My own version uses a twin race self-aligning ball bearing as the pivot point, which I obtained from a scrap motor. It was (believe it or not) 0.75" OD x 6mm bore. An odd combination seems to me. The wobble shaft was a piece of silver steel 0.185" dia (it's important that the shaft is really straight - check it with a DTI in the lathe).
First thing to make is the mounting bracket. Without a suitable piece of heavy angle I fabricated a length 3" x 3" x 1" x 5/16" from two pieces of bms. The ends were squared with an end mill and two 2 b.a. socket screws and a dose of Loctite 601 used to hold the two pieces together. In the centre of one side a 5/16 was drilled for the mounting t-bolt, and either side of this hole on the longitudinal axis 3/8" holes were reamed to take locating dowels to fit the t-slots. Short lengths of 3/8" silver steel were Loctited in to form the dowels.
A knurled thumb screw was produced to mate with a short t-bolt so the device can be quickly mounted/dismounted. The angle was attached to the lathe cross slide and the lathe centre brought to coincide with the vertical centre of the angle in order to bore the seating for the bearing (a 0.751 reamed hole). A bushing was turned from brass which allowed the 3/16" indicator shaft to be mounted in the 6mm bore of the bearing. The shaft was 8" long, turned to a point each end (a little less than 60 degrees included angle) and when in position 2" protruded one end (headstock end) and 6" out the other. The bearing was then fitted in place, again with Loctite 601 retaining compound.
The bearing was just 0.25" thick which allowed a 32 thou shoulder either side. This was convenient because collars are used to limit the movement of the indicator shaft, and these could seat nicely in the 3/4" bore. The collars were turned up from 7/8" dia brass rod, about 5/16" long, hollowed out on the inside with a large diameter drill (11/16") and finally drilled through 1/4" clear. This gave sufficient clearance for the pointer shaft to move without it flopping around. Although it is not strictly necessary a collar was turned for both sides - at least they help keep muck out of the bearing.
On the top edge of the vertical side of the angle a short stub of 1/4" silver steel (1/2" protruding) was screwed in to provide a mounting point for a DTI - for when accuracy needs to be the best.
The device hardly needs description for use, but in it's simplest mode just insert the short pointed end in the centre pop in the work and use a centre mounted in the tailstock. Rotating the work will cause the tailstock end of the pointer to orbit around the tailstock centre - adjust the work until it lines up. For more accurate setup use a DTI against the pointer.
A useful addition is to drill a 1/2" ball bearing (see how under workshop tips) so it fits snugly on the end of the pointer - can be used for centring holes in workpieces.
(c) Chris Heapy 1996.
|
Back to previous page |