In a case like this (say, on a barrel) you want the threads to extend right to the bottom, or as near as possible. A bottoming tap will work once the hole is drilled but how do you create a flat-bottomed hole? I read that you cut the drill bit off flat, then grind a relief angle on the back of each flute. Perhaps not so hard on a 1/4" bit, but pretty difficult on a #31 (6-48). Maybe possible to grind on a milling machine if you can get the speed up enough?
Drilling a flat-bottomed blind hole
- Thread starter josquin
- Start date
Pull out a number 60 or 70 drill bit and stare at it a while, and a number 31 drill won't look so bad.
Drill the hole with one new drill bit, flatten the end of another drill bit and then add some relief to each cutting edge. Not as tough as you are making it sound. Besides, even if it only cuts on one side, the hole won't wallow out (guided by the hole already made), and it'll still end up flat on the bottom.
Drill the hole with one new drill bit, flatten the end of another drill bit and then add some relief to each cutting edge. Not as tough as you are making it sound. Besides, even if it only cuts on one side, the hole won't wallow out (guided by the hole already made), and it'll still end up flat on the bottom.
Or just buy a carbide endmill bit and use that. Carefully, making sure everything well secured with minimal to no runout on the bit. Go slow, check often. Endmills are great for this sort of thing. Of course grinding your own out of a drill bit can work too, been there done that. In a pinch some good light and a cutoff wheel in a Dremel can do some fairly fancy whittling in drill steel.
Problem is, endmills aren't available in such specific sizes. (0.120 for #31, 0.14 for #28). And I may have to do a 3mm for a #4BA hole. Otherwise, yes, that'd be the way to go.
@trevj: #60 or 70? Pfft! Let's have a real challenge: wire drills!
Yes, I had assumed starting with a regular bit, then finishing with a modified one. I'll just get out the eye loupe and magnifier and have a go. I have a fixture to hold my Milwaukee "Dremel" so with good light (and not much coffee) I'll see what I can do. I have bits of scrap to practice on before going anywhere near a barrel
I use the 1/8 carbide end mills from flea bay ,chuck them up on the lathe ,using tool post grinder turn to size .I do 10 at a time as setup is the time consumer .Start hole with centre drill and finish hole with end mill ,,,should have said something sooner would have tossed one in the box
Not that hard to make flat bottom drills, practice on old dull drill bits. My old machine shop teacher wouldn't let you near any machines till you could properly sharpen drills or lathe bits. A good grinder with a good wheel is your friend.
Don't overthink it. Converting a hole from a #31 drill to flat bottomed requires removal of only a tiny amount of steel, from both the drill and the hole..
I just chuck the drill bit in a drill, and while it is spinning touch it to a grinder. There is the flat end. Add a bit of relief.
I just chuck the drill bit in a drill, and while it is spinning touch it to a grinder. There is the flat end. Add a bit of relief.
Problem is, endmills aren't available in such specific sizes. (0.120 for #31, 0.14 for #28). And I may have to do a 3mm for a #4BA hole. Otherwise, yes, that'd be the way to go.
@trevj: #60 or 70? Pfft! Let's have a real challenge: wire drills!
Yes, I had assumed starting with a regular bit, then finishing with a modified one. I'll just get out the eye loupe and magnifier and have a go. I have a fixture to hold my Milwaukee "Dremel" so with good light (and not much coffee) I'll see what I can do. I have bits of scrap to practice on before going anywhere near a barrel
Grinding a flat bottom drill is super easy.........but, you can totally buy 3mm (.1181") endmills if you were inclined, and for what you need to do, .1181" will be close enough to .120". Your tap is still going to have a little bit of lead at the bottom of the hole and the couple thou smaller won't matter
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Problem is, endmills aren't available in such specific sizes. (0.120 for #31, 0.14 for #28). And I may have to do a 3mm for a #4BA hole. Otherwise, yes, that'd be the way to go.
@trevj: #60 or 70? Pfft! Let's have a real challenge: wire drills!
Yes, I had assumed starting with a regular bit, then finishing with a modified one. I'll just get out the eye loupe and magnifier and have a go. I have a fixture to hold my Milwaukee "Dremel" so with good light (and not much coffee) I'll see what I can do. I have bits of scrap to practice on before going anywhere near a barrel
You were aware that the Number sizes were ALL wire size drills, weren't you?
The smallest drill bit I have here is 4 thousandths of an inch in diameter. I won't be sharpening it on the bench grinder....
If you don't mind the wait and the price, end mills are available in about any size you want, and as mentioned, a 3mm end mill is close enough for that work, and about as hard to find as common dirt. Cheap too. Small carbide tool blanks are pretty cheap, they start to get pricey at around the 1/2inch/12mm size or so.
Welcome to the rabbit hole of hobby machining, where you learn that you need a tool, to make a tool, to make the tool you need, for a simple five minute job.... LOL! And then someone shows you that you could have made a different tool and done the whole job in three minutes....
Long AFTER I posted, yes. I was thinking of the couple of #104 (0.0031") I have, although I have no idea why I bought them, or where.
As I am contemplating the purchase of a milling machine and reading various books (Mollentrecht, Krar, Hall etc.) I can see the rabbit hole popping up frequently in my peripheral vision.
What you are looking for is a "D-bit" - hard to find because they are ridiculously easy to make yourself.
You could also use a counter bore,
mmm use a 135 degree bit to minimize the amount of material at the bottom of the hole then grind the taper off your bottoming tap ....
if you grind the relief off a chucking reamer that may work as well (I have never tried sacrificing a reamer for this).
if it is something you need repeatedly, having a center cutting end mill made to spec may be worth it.
your question seems academic so not sure if this comment is really welcome, but "why bother" Your fastener is probably not reaching the very bottom of the hole anyway - you just need enough thread engagement to make your application secure. (and if your fastener *is* hitting the bottom of the hole you may not be able to torque it up enough)
As the diameter to depth increases less material in the bottom becomes more important - but at the sizes you have there 1 or 2 threads is probably (!?) trivial.
Yes, I read about those online. Of course, as you suggest, I'm being excessively persnickety, but unfortunately that's in my nature. The more I pootle around on projects like this, the more I try to refine the process, given that I'm not a trained gunsmith or machinist. The Red Green approach is OK up to a point but only when working on a beater, and the gun in question here is a semi-sporterized M-S 6.5 x 54 rifle I've had sitting around for several years.
I just remembered that I did the same thing on a rebored Martini Cadet a few years ago and used a drill bit that I'd cut off (no relief cuts) to finish up the hole and it worked OK. I think I also used some Loctite 380 (Black) under the ramp, rather than soldering it on. But this time I was thinking that I'd try to do it as "properly" as possible.I already have a 6-48 bottoming tap so I can get pretty much to the bottom of the hole and it's no problem to grind one down even further as I have a couple.
