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Flat bottomed drill holes?

Eric U

Hot Rolled
Joined
Feb 26, 2003
Location
Eastern AL
I've got some holes I'm drilling that should really be flat bottomed, and accurate, with chamfer or radius in the bottom corner.

I'm doing this on my lathe right now. The material is 4140 RC 44-45 and I'm trying for a 3/8" diameter hole 1.375" deep. I'm pre-drilling with a cobalt 11/32" drill. I then follow up with a carbide four flute 3/8" endmill with .030" corner radius. The finish is beautiful, and the diameter is right at 3/8", plus 0 minus .0005". The problem is the end of the endmill isn't flat. There is a "cone" left at the bottom of the hole that is about .015" higher in the middle than on the edges.

Any other way to do this easily, and hopefully cheaply? Any source out there for flat bottomed drills? Flat bottomed end-mills with radius or chamfer? Any such thing as a bottoming or end cutting reamer? I can live with a small (1/16") center hole in the bottom of my bigger hole for clearance or non-center cutting purposes. My cobalt drill is already dulling, after only 9 of these holes, so I'm not convinced that I sould be using HSS or cobalt cutting this material. I'd prefer carbide if possible.

Thanks,
Eric
 
They have spade drills with flat-bottomed insert tips. Sorry, not in carbide, maybe an insert drill?
 
Look in the phone book for a tool grinder near you and call them. Tell them you want the end of the endmill to be ground flat to finish a hole bottom flat. They will be able to do it and probably have done hundreds of them if a big shop.

You can also hand grind a drill bit to do the same thing if you are good at hand sharpening your drill bits. If not maybe a nearby shop can do it for you.

If you have a surface grinder and spin indexer you can do the flat end accurately and then hand grind the back relief.

Are you using cutting oil when drilling? A cobalt drill should last longer than what your getting. I used cobalt stub drills to drill out turbo studs on diesels and that was the only thing short of carbide that would drill them after they had been in service for a while. My drills stayed sharp for more than 9 studs and the studs were hard stainless steel.
 
Drill 3/8 just past depth and go back in with a 3/8, 4fl, center cutting, Corner R'd, endmill - preferably in a floating holder.

For 4140 you may want to pony up for cobalt.


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Think Snow Eh!
Ox
 
I've had good luck with half round drills. Just grind down to the point where the flat reaches the outside diameter. They are easy to modify and you get a flat all the way to the center. Use as a second operation.

Regards,

Stan-
 
In a recent thread, I asked about drills that were sharpened with no point... flat on bottom like a "slot drill".

The replies confirmed my suspicion that they were indeed for flattening hole bottoms, i.e. deep counterbores.

Might be a plan, except that it would require a full diameter hole made first, and then running the flat drill down to do the shoulder. Of course, it would work as well to clean up the slight cone left by the EM, but a third op vs a drill and finish.

I doubt any drill would get the size "on" as you want it though.
 
I personally think the milling cutter is best for this, with a slightly smaller drill roughing it out first.

But if a flat bottomed drill with rounded corners was used, it would probably be self centering just like a drill with an angled tip if the slightly smaller rough hole is drilled first.

I think the key to a good finish on the sides is probably keeping the chips to a minimum while drilling/milling the final diameter. It is the chip accumulation in the flutes of a common drill that produce the scoring commonly seen inside drilled holes.
 
owner Expert Die Inc

We can make a tool that will do what ever you want, e-mail a print or hand drawing of your spects and we will quote you on a tool made out of Carbide. Thanks Eric
e-mail [email protected]
ph 706-277-4854
 
I second the half round drill, or D-bit. They act like a reamer as well, giving good size and finish, and you can make your own if you can grind a flat across a suitable pre-ground carbide blank.

Google "D-bit"
 








 
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