Tip angle 60°, flat 4° rake, clearance angle 12°
Page link: https://knoba.wordpress.com/2015/04/...s-for-plastic/
Yes, those bits are used to avoid ("prevent" is probably overstating their efficacy) chipping and cracking. I used special plexiglass bits with similar profiles to drill the canopy on my airplane.
As Ken said.
You can use metal bits but it's wise to take some special precautions;
Use an undrilled area of a sacrificial backing piece (wood) for each hole.
Avoid using pilot holes if process permits.
Clamp the plastic down.
Use slower rpm speeds.
Slow feed pressure down on break-through.
You do some very nice work, btw!
No I haven't reground a set for brass. Do you mean; once the bit grabs into brass it starts pulling itself through without much coercion from the operator?
I would have thought the best geometry for brass drill bits would be to back-off the "lip relief" shown in the following diagram as "x";
Thanks for the compliment. I have similar trait to Ken's toolmaker mentor concerning "finishing the areas that no one else will ever see".
For years I have been temped to reduce the lip relief for some larger nominal sized import drills I use for drilling brass with the tailstock in the lathe. A few times I had 3/4" drills pull the tailstock chuck out while drilling out brass or bronze round stock prior to boring. No serious accidents but something to avoid. I found rolling a small section of printer paper in between the chuck and tailstock Morse tapers seem to make it hold better but doesn't fix the root cause.
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