How to drill a blind hole?

[sam dupont]

I need to drill a 3/4" diameter hole, 1/4 inch deep in a 3/8" thick, flat piece of cast aluminum. I have a drill press, what do I need for a bit?

The hole gives clearance for a bolt

 

[pnora]

It will cost a lot to find anything that will work. Is a bolt going through that hole or is it just a pocket hole to go over a bolt head?

 

[fesser]

I am sure that there is some sort of mill bit that would do it, but if it was mine, and I had some practice material, I would try a Forstner bit. Will likely take a lot of pressure and some cutting lube. Also, Forstner's don't really clear out their chips as they bore so back it off and blast the hole clean as you go.

 

[pnora]

Looks like this guy uses the wood forstner bit in aluminum. New sharp bit and WD-40 to lube and keep it cool. If the bolt is to pass through this hole I have another idea.

 

[padam]

Use regular drill bit 1st, start small and step up. The less you have to cut in the center the better. So go a little deeper with, say a 1/4 drill first.

 

[padam]

Then if you’re handy with a bench grinder, take an old drill and grind it flat bottom.
Better clamp the part down pretty good in the drill press though.

 

[Paul_B]

If you need a flat bottom hole, you could potentially use a 3/4” diameter 4 flute end mill in your drill press and use it like a drill bit. Be sure to clamp your workpiece as things will want to move. Also use some oil and you may want to adjust your drill press speed.

 

[padam]

If you need a flat bottom hole, you could potentially use a 3/4” diameter 4 flute end mill in your drill press and use it like a drill bit. Be sure to clamp your workpiece as things will want to move. Also use some oil and you may want to adjust your drill press speed.

Most 4 flute end mills are not center cutting.
2 flute usually are, but have 3/4 shanks, may not fit in drill press

 

[sam dupont]

A video following Nora's video post showed a wood paddle bit. Never would have guessed a guy could use one of those. Seems to work better than the Forstner. I have a quality set of Forstner's but I hate to sacrifice one. Will buy a cheap one or I have a bunch of old paddle bits.

My drill press is commensurate with my machining skills. It is a 1930's John Deere press with a 1960's Sioux 1/2 inch dril.

I'd like to do it today, is there any box store that would have the 4 flute bits? Harbor Freight has router bits only. I might have to drill a number of these, but for now, I only have to do 6 of them, so I want to start out cheap. I'll buy a quality bit if I end up needing it,

 

[dadsbee]

You can use any wood working tool to cut aluminum, including your cut saw.

That said I'd see if you can get a counterbore. I have a full collection that accept a pilot bit.

 

[Davefinan]

You could probably take piece of 3/4 inch thick wood block , drill through it with a 3/4 inch hole saw , remove the pilot bit afterwards, clamp the wood over the spot on the aluminum piece, drill to the depth needed and then hog out the rest of the material with grinder. Crude but should work I think

 

[YY1]

How about an end cutting carbide bit?

Pretty sure I have one close to that size.

All sorts of uses for it, too.

 

[gkent]

The problem is your thickness of material. The pilot on a paddle bit will come through the other side before you reach the desired depth. I don't see wood being a good pilot but a piece of 1/4" steel plate with a 3/4" hole would be. You cold then get rid of the paddle blade pilot. You'd still need to pre-drill a pilot hole with a regular bit to remove the metal normally removed by the paddle's pilot.

Piece of cake. Oh, spend the extra couple bucks on a better paddle bit.

 

[gkent]

Drill presses usually don't run true enough to do anything without some sort of guide or pilot.

 

[sam dupont]

It's to make space for the head of a bolt, so a little wandering probably won't hurt me. My blade bits are vintage, so they have good metal in them. I have some that go to a Brace.

Dadsbee, is that a wood bit? I found this on eBay, HSS should be a good burr:

3/4” Dia. 3 Flute HSS Counter Bore 4 1/4” OAL Pilot Hole .216 Shaft 3/8” | eBay

<p>We do combine Shipping so Purchase multiple items and wait for a corrected invoice with adjusted shipping charges.</p>

www.ebay.com

 

[dadsbee]

Dadsbee, is that a wood bit? I found

No it's a metal bit ment for the mill.

 

[1 Wild R/T]

Router bits cut aluminum fine... Router bit in a router will cruise through aluminum like butter.... Won't hurt the bit... As Dads Bee mentioned if it'll cut wood it'll probably cut aluminum... HSS, Carbide, Cobalt...

 

[sam dupont]

Thanks for the help. A ball end burr gets me what I needed. Now I want to enlarge an opening. It's 2 3/4 inches down inside of the casting. I was about to go pawn shop hopping to find a Router as the Internet says they will work fine on my aluminum. Would like to clean up the bottom too. Thinking this end mill in a router:

 

[padam]

That looks delicate

 

[gkent]

1/4" bit is just fine. Too big of a bit might present a problem because of the distance between the flutes or cutting edges.

Be interested to see what you're actually working on.

I once gasket matched an intake manifold with a carbide burr in a router !!