Wednesday, April 30, 2014

Step #7: Lapping, back-facing, drilling and tapping the steady rest finger clamps

While I was working on the frame, the finger clamps arrived. They were beautiful and equally as accurate as the frame parts. Little bits of suspension compound remained in the grooves but 2 minutes work with a scribe and they popped right out. Any of the compound that was left deeper in the holes just disintegrate when the drill comes through. It's soft stuff.
I went to work lapping the front faces to great success. So much fun to see things come along so quickly!
Raw finger clamps with suspension compound in grooves

finger clamp clamping screw guide hole
Finger clamp lead screw guide hole
I lapped the face first just removing enough metal to bring up a good shine. Fun to see the "M" monogram on the parts. I then held the clamps in a chuck and centered the guide hole on the rear stem for accuracy of turning and drilling. A four jaw chuck would have worked as well. I then turned the shaft down to .225, a free fit in the cast barrels in the frame. I also faced off the back of each clamp removing about .050 in depth to the front face. You can remove more later if need be and this dimension is not critical as long as the back face doesn't touch the frame when the finger is in place leaving enough room to clamp.
facing off the back of the clamp and turning the shaft to dimension
 With the clamp still centered in the chuck, it's an easy matter to drill out the shaft in preparation for tapping it for a 6-32 thread. I'd recommend making the hole just a tiny bit oversized. Bronze infused Stainless likes to grab drills and heat up fast. It likes to grab finished threads too. When drilling, go slow, drill in very short duration plunges, use lubricant and clear chips often. I can't stress this enough. you'll turn the metal blue before you know it.
Drilling out the finger clamp shaft.
Setting the tailstock depth ring so as not to drill through the face of the clamp

Tapping the finger clamp shaft
I almost forgot to mention that in order to fit the clamps into the frame, you'll need to lap, file or machine a small amount of metal off of the sides of the front face of each clamp. I'd recommend doing this in small increments so as not to make them too loose in their channels. a smooth but close fit here helps the clamps to remain oriented when you are finger tightening the lead screws thereby lessening the chances of binding. I did this to a snug fit at this point since I was going to remove a little material off of the channel walls in a following step. Notice that the clamps should fit proud of the surface at this point. You can always remove more material off the back side later if need be. Conservative and cautious makes for fine fits.

Clamps look amazing slid into place
Good fit in frame with proper gap for brass fingers
It's time to drill out the clearance holes for the 6-32 screws in the extended barrels seen in the back of the frame. Here again, the guide holes make super easy work of this and align perfectly. A few more brass fillet washers and the clamps are installed. These washer stand .215 tall and I simply matched the bottom diameter to the outside dimension of the barrels. I like the look.
Clamps, washers and retaining screws installed

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