Counterboring

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bahowe1

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Hello all,
First let me say 'thank you' for the wealth of information I have already learned from you guys, but I needed to finally ask a question I couldn't find an answer to, so I registered. I admit, I am very new to machining, but have been in the engineering field for years, so I appreciate your help.
I have 1/4" aluminum plate that will be cut with water jetting, including some holes. The holes will be .410", but I need a counterbore on top of the hole that's 11/16th" and a depth of only 1/16th" - this is where the machining part comes in. A rubber washer is placed in this recessed counterbore, so the counterbore bottom needs to be flat.
I was 'planning' to use a jig with a pin that's just shy of the 0.410" diameter hole, and the pin would be centered under the drill press. I would place my plate with the 0.41" hole on the pin for alignment, and then use the drill press to cut the counterbore. This is how I was going to do it, in my mind. However, as I look around, I am not sure this is the best way to do it. Tolerance doesn't need to be tight, the head of a carriage bolt is what is compressing the washer and resting on the top surface of the aluminum plate, creating the void for the rubber washer.
What would be the best and quickest way to do this function? I need to do a lot of them, and I actually don't have the drill press, or any of the tools yet, but would be looking to purchase whatever I need. Any thoughts/recommendations?
Thank you in advance!
 
If the tolerance is loose, that will work.
After you get the plate located, bolt some stops to the table.
That is how I align parts at work, use a known good part, put a pin in a collet or chuck, line it up, run a few thousand parts.
 
Thank you Jim, so what exactly is the tool name I am looking for? An “11/16ths flat bottomed counterbore”?
 
One way to cut a counterbore is with an end mill. However, a drill press is not the machine to use as it lacks the rigidity and the end mill will tend to skate.
Counterbores are made which have a pilot and can be used with a drill press but the are usually used to cut counterbores for socket head cap screws and are limited in the diameter of the counterbore. Here is one that is close to your requirements (.438" pilot an 21/23" counterbore).
 
That looks like a great option, but that would also mean that I couldn’t use a lower pin to center the hole (thickness is only .25” for the plate). I guess this means I would use the pilot to center it and then clamp the piece each time?
Also, you mention a drill press is not the right tool. I also have access to a mill. It’s a older mill, and I don’t have the specs on it, but would this be a better option than a drill press?
Since I will have so many to do, I’d like the most efficient process.
Thanks again!
 
A mill is definitely the better option. You can use an indicator to locate your counterbores and use either a boring head or an 11/16" end mill to cut the counterbore.
 
Using a piloted counterbore, you don't really need to locate the hole. You can let the plate ''float'' and it will self center. Then all you need to do is provide depth control via the depth stop on whatever machine you are using. Low RPM, with almost any kind of oil works the best. WD-40 is a good cutting fluid for aluminum.
 
When you make your jig with a pin, make it with a slip fit pin / dowel that is an inch long or so, maybe more. Any hardened dowel of size should work for a pin. That way you could install the pin in the fixture, clamp down your work piece, remove the pin then drill with a conventional counterbore as RJ is showing. Your fixture will need to be thick enough to allow some relief so the pilot of the counterbore will not trash your hole, maybe a recess of .125 or so should do it. Using a quill stop should make things work pretty well.
 
Any 1/2 way decent drill press will be fine when using a counterbore with the proper size pilot. The pilot is the 'locator' so no jig is required. An 11/16 (sharp) counterbore will cut your aluminum like butter. If the workpiece is large enough, a simple torque stop could suffice. If too small, individual clamping is in order. Like Jim said, low RPM and a squirt of WD-40.

If you can't buy the 'perfect' diameter pilot, buy the next larger diameter, chuck it up in a drill motor and spin it against your bench grinder. Many pilots are hardened. Spinning the pilot, and being careful, you can grind it to within a few thousandths of round and the desired diameter.

You will find it simple to do and it will work very well.

PS: End mills, often, do not have flat ends and will cut slightly conical counterbore.
 
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