Robinhood
02-12-2017, 02:44 PM
If you are reading this you probably know what a modified case looks like. You may have even purchased the tap from Amazondotcom. But now you are having trouble with the drilling part of it.
A vice in a Drill press with some slotted wood to hold the case with a rubber jar lid gripper in between to keep from damaging the case. Sounds elaborate but once you have it your in business. Just enough pressure to hold the case while the holes are being drilled.
Start with clean fired unsized case, a #2 center drill or something small enough to pilot on the flash hole then a little larger to reduce the load on the cutting bit. Too much of a cut and the holding force needs to be increased. and that could potentially distort the case. Last drill needs to be a .290 Letter L drill bit. Use a cutting fluid. Just about any lubricant will work but a drilling and tapping fluid is best.
Now the reason I started this thread is a lot of guys at the club ask me how I do this without having the drill bit grab and destroying the case. The trick is to modify the drill bit. The flutes on the drill bit act like a conveyor to remove chips from the cutting area. With a metal like brass those spirals accelerate the feed rate and cause the drill to feed so fast it either spins the case or otherwise makes it difficult to finish that part of the operation with a desirable outcome.
So we all went to one of the guys houses who had the "L" drill and a bench grinder and I put on a quick little seminar on how to modify the drill to make it easier for the case to be drilled tapped. If you already knew this then don't judge me, my disclaimer is the edge of the grinding wheel had a big radius the light was poor and I did not have my cheaters on.
What has been done here is to put the cutting edge flat with the axis of the drill. This eliminates the way a new drill will grab as it feeds into the brass. Make sure you do both cutting edges as uniformly as possible to eliminate a chance of oversizing the hole. You only have .011 depth of thread per side. You can use a slightly smaller bit if you want but this thread is not a fastener so it is not really necessary.
Good luck.
http://i1243.photobucket.com/albums/gg552/Proneone/stuff/mod%206_zpsrkzi02vt.jpg
A vice in a Drill press with some slotted wood to hold the case with a rubber jar lid gripper in between to keep from damaging the case. Sounds elaborate but once you have it your in business. Just enough pressure to hold the case while the holes are being drilled.
Start with clean fired unsized case, a #2 center drill or something small enough to pilot on the flash hole then a little larger to reduce the load on the cutting bit. Too much of a cut and the holding force needs to be increased. and that could potentially distort the case. Last drill needs to be a .290 Letter L drill bit. Use a cutting fluid. Just about any lubricant will work but a drilling and tapping fluid is best.
Now the reason I started this thread is a lot of guys at the club ask me how I do this without having the drill bit grab and destroying the case. The trick is to modify the drill bit. The flutes on the drill bit act like a conveyor to remove chips from the cutting area. With a metal like brass those spirals accelerate the feed rate and cause the drill to feed so fast it either spins the case or otherwise makes it difficult to finish that part of the operation with a desirable outcome.
So we all went to one of the guys houses who had the "L" drill and a bench grinder and I put on a quick little seminar on how to modify the drill to make it easier for the case to be drilled tapped. If you already knew this then don't judge me, my disclaimer is the edge of the grinding wheel had a big radius the light was poor and I did not have my cheaters on.
What has been done here is to put the cutting edge flat with the axis of the drill. This eliminates the way a new drill will grab as it feeds into the brass. Make sure you do both cutting edges as uniformly as possible to eliminate a chance of oversizing the hole. You only have .011 depth of thread per side. You can use a slightly smaller bit if you want but this thread is not a fastener so it is not really necessary.
Good luck.
http://i1243.photobucket.com/albums/gg552/Proneone/stuff/mod%206_zpsrkzi02vt.jpg