To get the exact depth, you can always drill the hole with the flat bottom, measure the depth, and then trim the hex bar until the hole is the exact depth. Does require starting with stock longer than 2 1/2 inch long and trimming both ends. One end to get the hole the exact depth and the other end to end up with a 2 1/2 inch long part.
spaco on Wed, 17 Nov 2010 09:15:20 -0600 typed in rec.crafts.metalworking the following:
Flat Bottom Girls - err drills.
Use an end mill, or boring bar. Drill a 'pilot hole' smaller than final size needed, say .500 dia , where depth of the hole is measured to the of the drill. Then use a boring bar to "clean up" the last
0.039" (diameter of final hole - drill size)/2. You could thread it the last bit, I'd have to look up the min/max values for a 5/8-18 thread.
at least that is what comes to mind. I'm sure there are easier (and more complicated) means
Like using a 9/16 (36/64") drill and a 37/64" end mill to clean up.
Or drill a sequence of .125 holes in a circle pattern, and knock the center part out. Then weld the hole section back onto the stock, and finish truing the hole.
Instead of putting the drill in the tailstock, mount it like a boring bar, use the carriage feed dial to measure depth from first contact to your target depth as you advance the drill.
Then using a small endmill or a real boring bar, touch bottom in the hole at the center, and bore from center to the sidewall. There may be some problem if the flat has to be dimple-free in the center, though.
You might also consider an endmill in the tailstock for the finish (but the diameter and depthfinding are trickier).
Why not run the hole slightly deeper and focus on the flat bottom first. Then measure the depth after facing off the end. It's more accurate to use the compound infeed to remove the correct amount to stay within depth tolerance, eih? The end needs to be faced off anyway....do it second op. Then do the other end as was mentioned here earlier to get overall length. phil k.