How to drill flat bottom holes?

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rcr22b
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Joined: Thu Sep 14, 2006 7:17 pm

How to drill flat bottom holes?

Post by rcr22b » Fri Oct 13, 2006 12:01 pm

I want to drill a blind hole with a flat bottom and don't know how to do it.

My drills have 118 deg points and don't leave a flat hole.

Any suggestions?

Roger
Last edited by rcr22b on Fri Oct 13, 2006 4:04 pm, edited 1 time in total.

Jake davis
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Post by Jake davis » Fri Oct 13, 2006 12:47 pm

You could drill the hole just shallow of the depth you are trying to acheive then finish the rest of the hole with an end mill bit. This will provide you with the flat bottom your attempting to acheive.

-Jake

Skeeter5000

drill blind hole

Post by Skeeter5000 » Fri Oct 13, 2006 1:06 pm

You could use a brad point drill bit. This will have a small point, but for most applications this works.

The only problem is that most brad point drills are made for wood. I still use one now and then.

Hope this helps.

Good luck.
:)

Harold_V
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Post by Harold_V » Fri Oct 13, 2006 1:54 pm

Drilling flat bottom holes is no big deal, assuming you understand how a drill cuts, and you are able to hand sharpen. All the easier if you have a cutter grinder at your disposal, but certainly not necessary.

Sounds difficult, but it's dead easy.

Using a small square to insure that you get the drill flat, grind the end off the drill until you have a full diameter, then back off both sides so the drill has relief. You then grind a slight positive rake, enough to move the cutting edge to the center of the web, half from each flute. That operation is necessary if you expect the drill to be able to cut to center, and leave a flat bottom, not one that has a configuration of sorts, created by the flute. Be careful to not grind past the margin of the drill, otherwise it will cut tapered at the tip. It's OK to hit the margin, even to reduce it somewhat, but make sure you grind both sides equally, and that you have positive rake when you're finished grinding.

Drill your hole close to depth with a standard drill bit, then finish off with the flat bottom drill. Don't start holes with such a drill, they tend to cut well oversized until the margin pilots the drill well. That's why counterbores use pilots.

Store the flat bottom drill in an index for future use. You'll find they come in quite handy, particularly if you work with socket head cap screws.

I strongly suggest you avoid using an end mill for such an operation, especially if you have a light mill, or are using a drill press. End mills have the margin sharpened so they cut, unlike drills, which have the margin circular ground. The slightest irregularity in drilling and the end mill cuts the hole oversized. Use one only as a last resort.

Harold

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Big Pete
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Post by Big Pete » Sat Oct 14, 2006 4:51 am

you could also make a D bit if you just need to flatten the bottom of the hole. A piece of drill rod turned to size the ground so that half the end is gone (so the end view looks like a D) and a bit of relief on the back face. Harden and temper and tidy up with a stone or diamond lap. drill the hole with a normal bit then finish with the D bit. The D bit isnt good at clearing swarf so you cant take out a lot of material at once, but its good to get a flat bottom hole or an internal shoulder.
Theres no such thing as too many tools.

rcr22b
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Joined: Thu Sep 14, 2006 7:17 pm

Post by rcr22b » Sat Oct 14, 2006 6:06 pm

Thanks to everyone for their replies.

All were good suggestions but the last two by Harold_V and Big Pete seem to meet my needs the best.

Harold_V,

You lost me after the part about grinding the tip flat. I sort of understand about relief but it is hard to visualize what you are saying about positive rake.

So unless I have a vision I think Big Pet’s D-bit is most practical for me.

Thanks again.

Roger

Al_Messer
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Post by Al_Messer » Sat Oct 14, 2006 9:15 pm

The nice thing about Big Pete's suggestion is that a "D" bit can be made without special grinding tools and they have been in use for probably over a hundred years.
Al Messer

"One nation, under God"

Harold_V
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Post by Harold_V » Sun Oct 15, 2006 3:15 am

rcr22b wrote:Thanks to everyone for their replies.

All were good suggestions but the last two by Harold_V and Big Pete seem to meet my needs the best.

Harold_V,

You lost me after the part about grinding the tip flat. I sort of understand about relief but it is hard to visualize what you are saying about positive rake.

So unless I have a vision I think Big Pet’s D-bit is most practical for me.


D bits have their place, but this task isn't one of the better choices. They don't have a good ability to discharge chips--and take considerable time to create, very unlike altering a drill bit-----a drill bit that is created to cut and remove chips via helical flutes. In all truth, you could alter a twist drill and have your job completed before you could get a D bit roughed out-----let alone heat treat and grind it so it functions properly. It's simply too much work when there's a much easier and better alternative at your disposal.

I'm attaching three pics of what a flat bottom drill looks like in the hopes that they will help with the vision you need. The positive rake mentioned is nothing more than a minor alteration of the helix angle of the flute, whereby you grind the lip of the flute straight to the center of the web, so it becomes center cutting (although the one pictured isn't well executed----cutting to center was not a requirement). You have options of angle when you grind this alteration----so your choice should be to reduce ever so slightly the helix angle of the drill, not to grind it negative. Hope this helps.

Harold
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rcr22b
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Joined: Thu Sep 14, 2006 7:17 pm

Post by rcr22b » Mon Oct 16, 2006 1:39 pm

Thanks Harold.

I would like to use the method you describe if I can understand it.

How do I see the pictures you refer to?

I can't find any posted.

Roger




Nevermind. Now I can see them but I'd swear they weren't there before.

Roger

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