Need tips on drilling a long thin through hole

mmcmdl said:
When you state " needs to be very accurate " , regarding diameter , true position , parallelism ? All of the above ?
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The holes will need to match up to holes in a larger piece of steel. I will drill those holes as well. I would consider true position to be the most important consideration with parallelism being a close second. If I match the two pieces from the sides I START the holes, that could account for a slight bit off on parallelism. Can't miss it by much, admittedly, but it should work.

Regards,
Terry
 
tjb said:
If I match the two pieces from the sides I START the holes, that could account for a slight bit off on parallelism
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Exactly . If you did need it to be close to perfect , you could drill over size then bore for a bushing from both ends . That's if you didn't need that .1875 diameter the full length of the part .
 
mmcmdl said:
Exactly . If you did need it to be close to perfect , you could drill over size then bore for a bushing from both ends . That's if you didn't need that .1875 diameter the full length of the part .
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I think I'm at least theoretically on the right track. Does that help narrow down my choices for a bit? I'm no expert in making that kind of selection. And I do need the .1875 for the entire length.

Regards,
Terry
 
I found up to 0.002" of run-out when drilling 13/64'ths through 0.5" of 6061.

If you need than more accurate than that, you need something other than a conventional drill.
 
I would drill from both ends with an undersize drill. If there is a slight mismatch where the two holes meet, drilling the second hole tend to follow the first hole resulting in a slightly larger diameter at the intersection. Following with a larger drill will blend the two holes. Depending on the mismatch, I might step up to my final diameter with multiple drill sizes.
 
Looking thru the toolbox in here at work i'm finding solid carbide boring bars a lot smaller then what you need that would easily reach over the 2 1/16" depth to get half way thru . You would still have the mismatch where they meet though unless you were dead nuts on positioning and machine tram . One of the options for going deeper that we used quite a lot were to grind the shanks down on out micro bars and sleeve them for extra length . PITA but the gubmint paid for the R & D and all tooling .
 
I think I'm going buy an extended length bit and try two or three passes at drilling straight through on a similarly sized sacrificial piece of aluminum. I'm guessing I can get by with a thou or two of run out. If not, I'll go to a Plan B.

That gets me back to my original question: Any advice on drill bits? I like Guhring, but what material/tip angle/etc.? And any suggestions on speed and feed rate?

Regards,
Terry
 
Most Guhrings are 130 degree points high helix with a thick web . They make 120 degree also if I remember correctly . For aluminum , no coating . High polish .
 
Terry, you are looking to drill something with a depth to diameter ratio of about 22:1. You need a gun drill if the hole has to be that deep and accurate. A twist drill, parabolic or otherwise, will not drill accurately at that depth. That is, hole alignment all the way through is going to be a big issue.

I wonder if a D-bit would work? After all, a gun drill is a single flute cutter. Might be worth a try.
 
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