drilling 3/4 aluminum bar stock

[Rick Sahlman]

I'm looking to drill reasonably accurate 5/16 hole through 3/4 inch aluminum. I have successfully drilled 1/4 hole then followed that with a 19/64 hole now I want to finish the process and i have a choice to use a carbide tipped forstner bit or a hss twist. I'm doing all this on a drill press with the speed set to just under 500 rpm and oil as lube.

I tested my procedure using some hard maple stock and got reasonable good results either way but maple is not aluminum??? Any ideas will be greatly appreciated.

[pat warner]

Isolate the work. (http://patwarner.com/images/close_drilling.jpg)
(http://Isolate the work no.1)Pre-countersink with 120 deg. CS.
Then drill, moving nothing, with 5/16 x 118 deg. short or regular length hss steel drill.
Not a toy drill but something from Cleveland or equivalent.

[Bruce Page]

Rick, it depends on what you mean by reasonably accurate. If you’re looking for dead nuts accurate, e.g. a dowel pin slip fit, then you should drill 19/64 followed with a 5/16 (.3125) reamer. If you want reasonably accurate, typically within .005, then a sharp HS twist drill would be your best option. A twist drill is not a reamer, a twist drill can easily drill a few thousandths oversize depending on grind quality, sharpness, spindle run out, etc. A reamer is more forgiving because it will follow the predrilled hole.

[brian c miller]

Get a Bridgeport.

[Rick Sahlman]

so the carbide forstner bit is not a solution? the twist bit is my best bet unless I want to get a drilling reamer?

[Myk Rian]

Forstner - no

Drill bit - yes, if you don't need precision.

[Phil Thien]

I'd just use a 5/16" twist and drill it in a single shot.

[Bob Wingard]

Use plain old Kerosene as your cutting fluid ... FAR better than oil or Tap-Free.

[Ole Anderson]

No big deal drilling aluminum. Yea if you want super accurate diameter you need to drill undersize and use a reamer. But you said reasonably accurate. As others have said just drill it with a good quality twist drill in one step. WD-40 works well as a lube if you don't have kerosene. I presume you are using a drill press. Primary need for the lube is so the aluminum cuttings don't gall and stick to the drill. Most of the heat will end up in the aluminum, it is a great heat sink.

[Jay Rasmussen]

Rick,
Use a good quality HHS or Cobalt standard jobbers length drill. Clamp you part securely. Cutting oil is OK but I suggest water soluble cutting fluid in a spray bottle. 2500 RPM would be a minimum to start. A somewhat heavy feed only stopping to let the chip break.
Jay

[Bruce Page]

2500 RPM? he's doing this on a drill press, not a machining center. 500 RPM will be fine.

[Jay Rasmussen]

2500 RPM? he's doing this on a drill press, not a machining center. 500 RPM will be fine.

On a machining center it would be more like 8,000 - 10,000.
500 is way too slow to get a quality hole. Your just tearing the material out. Much safer at the higher RPM. I've done it on a drill press.

[Phil Thien]

On a machining center it would be more like 8,000 - 10,000.
500 is way too slow to get a quality hole. Your just tearing the material out. Much safer at the higher RPM. I've done it on a drill press.

Agree w/ this. All my drilling charts say a 5/16" hole should be run at about 2500-RPM.

[Ronald Blue]

I don't know how big the piece of stock you have is other then it's thickness. However you can probably get by with a drill bit but especially in a soft material like aluminum with it drilled so close to size it is going to want to suck into the piece. Securely clamp it down for safety and use a higher rpm but I don't know about 2500. That is a little high in my opinion. If you have multiple holes then you might want to buy a reamer. It wouldn't be to expensive and would give you the best result.

[Ole Anderson]

Aren't we overthinking this just a bit? All he wants to do is drill a reasonably accurate 5/16" hole in a piece of 3/4" aluminum. Bridgeports, maching centers, cobalt bits and water based cutting fluid? In fact it sounds like he has the hole and is just going for the last 1/64" of an inch. Hey, maybe Rick is trolling to see just how complicated we can make a very simple process. Frankly it is easier to drill aluminum than maple, what with the tearout issue, which is a whole other issue when drilling wood.

[Bruce Page]

Ole, that’s exactly what I was trying to get through. I have drilled thousands of holes in aluminum in the 500-1000 rpm range and never had any tearing out issue. You can look at all the Speed & Feed charts in the world and they are all written for a production environment, not for the guy standing in front of his Craftsman drill press with little or no metal working experience.

[Mel Fulks]

Instead of oil try denatured alcohol ,makes aluminum less gummy ,and cut more cleanly. Got this from a master pattern maker ,works well.

[Ole Anderson]

Ole, that’s exactly what I was trying to get through. I have drilled thousands of holes in aluminum in the 500-1000 rpm range and never had any tearing out issue. You can look at all the Speed & Feed charts in the world and they are all written for a production environment, not for the guy standing in front of his Craftsman drill press with little or no metal working experience.

Funny you should mention that. My Craftsman DP has a speed chart on the side and it calls for 2300 rpm (maximum) for a 5/16" bit in aluminum. Handy info.

[Phil Thien]

Ole, that’s exactly what I was trying to get through. I have drilled thousands of holes in aluminum in the 500-1000 rpm range and never had any tearing out issue. You can look at all the Speed & Feed charts in the world and they are all written for a production environment, not for the guy standing in front of his Craftsman drill press with little or no metal working experience.

What advantage would 500-RPM offer over 2500-RPM?

[pat warner]

Given an unhealthy drill press, bad drill bit, and poor centering: slower speed will produce less of an insult to the work and maybe you.

[Bruce Page]

I’m not saying you can’t drill a 5/16” hole at 2500 or 10,000 RPM. If you have the machine, the setup, the tooling - knock yourself out. What I AM saying is you can drill a perfectly good 5/16” hole at 500 RPM, and 500 RPM is likely to be less daunting to someone with little background drilling aluminum.