best method to drill a centering hole in a square block of aluminum

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dansawyer

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The project is to drill a centering hole in a square piece of aluminum The pieces are 1.875 by 1.875 by 3/8 thick.
I can think of 2 methods, the first would be to cross mark the squares, center punch the center, and then drill the centering hole.
The second method would be to mount the square horizontal in a mill and use an edge centering tool. First the edge tool is used to find the of the edge of the first side, calculate the center, and move the mill base to the center of the first axis and then repeat for the second axis.
My inclination would be the second would be more accurate.
Is this correct? If there are what are they?
Is there a third better choice?
 
Several ways to do this but much depends on how accurate you have to be. I am assuming the edges are milled square and accurate. If they are rough or as extruded then you takes your chances:
  • Measure the work and determine center, then mark through some layout dye or Sharpie mark with the tips of a dial caliper. Or if you just need to get sort of close, use a hermaphrodite caliper and ruler. Center punch and drill.
  • Use a height gauge on a surface plate or piece of tempered glass to mark center, then center punch it. This will get you within a thou or two.
  • Use your edge finder on the mill with your X and Y handwheels as you stated. This is probably the most accurate way because you can drill after locating the second coordinate. No center punching, which is not that accurate to begin with.
  • Use the lathe as Doc suggested.
 
Hi Dan,

You mentioned “pieces”, plural, so I’d go with the edge finder method on the mill for the first piece for accuracy and set a vise work stop to index subsequent pieces for easy repeatability. If it’s super critical, in hobbyist terms, I’d also spot drill each hole before I through drilled it, even if I was using stub drills, just for that warm, fuzzy feeling one gets from a belt and suspenders.

Tom
 
Be sure to use a stop on the vice to be able to put the subsequent piece in the exact same place as the last. So you take the time to set it once and just put every other piece in and drill the hole on the mill.
 
May not be needed here but remember you can tilt the scale on a parallel piece of work to an even number to easily find center. So you have a piece 1.825 wide, put the scale on the piece and tilt it until it reads 2 inches. Then the one inch mark will be center. Sometimes easier than calculating and having to find half division on an already small scale.
 
For best accuracy, use the mill and edge finder. Make sure to use a spot drill or center drill first, to prevent drill wander
For rough and ready stuff, I just Dykem, scribe and center punch, then use the drill press.
Mark
 
Thank you all. I tried the edge finder path, unfortunately it did not work the first time. The edge finder has a 10 mm shank and I do not have a 10 mm collet. I tried a chuck and noticed a wobble in the base of the edge finder. The videos reference three modes of operation, full wobble, back and forth, and smooth rotating on the axis.
My edge finder did not transition between the three states automatically. I can get it to wobble on start up but not return to wobble during the seeking process. I can get the tip to move in back and forth mode when moving the table out, or away from the center. This will result in entering the smooth rotating state at what I assume is the edge.
I then moved the experiment to the lathe. The edge finder worked better there. The takeaway was the rpm on the mill was too high. I then turned the rpm on the mill down and changed the chuck. I observed the second chuck worked much better. I repeated the experiment. It worked much better at a lower rpm.
Thank you all again.
 
The edge finder is the best way. With careful work, you can locate an edge to within a few tenths of thousandths. If you do not have a DRO. you will have to account for backlash.

Tje beauty of the edge finder is runout in your chuck or collet doesn't matter. The tip centers on the spindle axis; Here is how I find an edge. I will approach the edge slowly and the tip of the edge finder will transition from wobbling to running true as the edge of the work nudges the tip. When all clearance is taken up, the tip will suddenly kick to the side and you have contact6ed the edge. If you back off slightly, the tip will begin to wobble again. At this point, I use my finger nail or a pencil to center the tip and slowly move in to contact the edge.

The key is to move slowly. If you move too fast, a wobbling tip can be knocked to the side resulting in a premature reading or you can overshoot the edge, resulting in a late reading. I like to do my final approach at around .0001"/sec. I always make at least three passes and they have to agree. By backing off around .001" and centering the tip with my fingernail, each pass only takes around fifteen seconds.

If I have multiple pieces and the centering is critical, I won't relay on a work stop for positioning. I locate the edges and recenter for each piece.
 
The edge finder works just fine in a Jacobs chuck'
 
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