Grinding drill points intentionally off centre to control hole size
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  1. #1
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    Default Grinding drill points intentionally off centre to control hole size

    Has anyone have experience with this? Watching one of Dons Utube videos on drilling oversize etc, I came across a comment that said the screw machine guys routinely do it to hit a size. After giving it a go im impressed so far, after about 15mins messing with 23/64 stub drill I got it to make a pretty decent 3/8 hole bang on .375", after a 100 or so parts the drill rod at .374" is still a nice slip fit

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    I have done that intentionally and unintentionally. If you sharpen any bit and the lips aren't the same length, it will drill oversize. I have to say however that if I am looking to hit a hole size to plus minus .001", I would not rely on a drill to hit that mark. I would ream it.

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    It would be a great technique for materials that tend to close around the drill.

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    Yeah I did try reaming but had issues trying to use it in a gang tool block, in effect doing it this way saves an operation, drill/chamfer/part off. These parts are going to locate on the drill rod in a fixture and be welded on, im going to ream the assembly to final size after that.

    It would be a great technique for materials that tend to close around the drill.
    That makes sense, things run with a certain amount of clearance behind the business end.

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    I can believe the offset grinding to hit a desired oversize amount, but there must be some skill in it. I'm ashamed to say that selections from a couple of sets of 1-10mm by 0.1mm drills sometimes get used when the reamers don't give the desired diameters...

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    What happens when the drill breaks through? Is the edge of the hole smaller?

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    Quote Originally Posted by Demon73 View Post
    Yeah I did try reaming but had issues trying to use it in a gang tool block, in effect doing it this way saves an operation, drill/chamfer/part off. These parts are going to locate on the drill rod in a fixture and be welded on, im going to ream the assembly to final size after that.


    That makes sense, things run with a certain amount of clearance behind the business end.
    I'm missing something. You need an on sized drilled hole that you are going to ream to the same size after welding. What's the point?
    You don't have undersized drill rod for locating for the welding op? Seems like that might be a workable solution. depending on a lot of things.

    But off set point hand sharpening is NOT something I would want to rely on.

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    Does the drilled hole stay the same size on deep holes? I wouldn't be surprised to see it wander off center.

    Dave

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    How about some shim stock under one jaw in the chuck instead?
    Bill D

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    Used to do this all the time for drilling Ampco bronzes. If you don't, the bronze will often grab the drill in short order. IIRC the CTE for steel is higher than for Ampco, so if the heat starts to build up the drill will grow bigger than the hole. Offset cutting lips on the drill neatly prevents the problem all together.

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    I do it on a regular basis but I'm not doing it for the purpose of hitting a particular size, too many variables in the equation to have a reliable outcome in the hole size. You could after much trial and error with using a drill gauge and measuring flute length establish somewhat of a reliable pattern for hitting a size. IMO you're better off using the correct tooling like you normally would.

    Like already mentioned about drilling DEEP holes in already hardened material and the material collapsing around the drill is my need to do this. Collapsing around the drill is a figure of speech, essentially what happens is the actual cutting corners of the drill wear which reduces hole size that gives the appearance the hole shrunk on the drill. Where size isn't an issue by grinding the drill off center it opens the size of the hole some so as the corners wear it gives you some room to still find the bottom before the hole "collapses" on the drill. Grinding the drill off center causes the drill to cut mostly on one flute more that the other so alternating helps reduce unwanted wear on the outer most outside sides of the drill.

    It's a handy thing to do in the right situations where size isn't much of a concern but being able to hit particular sizes would take much practice and trial and error to get good at size. My 2 cents....

    Brent

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    I have used it unintentionally on "nastyalloys" and while the drilling might seem to work better for a while one of the margins tend to wear down leaving you with tapered drill that grabs and digs in like mofo.

    Undersize holes in Oxidized aluminium bronze is my "favorite" nastyalloy == aluminium oxide dispersed in copper makes it perfect abrasive to hone off the cutting edge from drills, reamers or whatever you have!

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    Mark R:
    Yeah it doesnt take much for change to happen. My approach was to hand grind keeping the flank the same angle, mark the longer flank so i dont get muddled up and trial and error it. Grind/drill/measure hole repeat. It went something like .370 - .390 - .372 - .375.
    The final touch i only caught it near the point but it didnt seem to hurt anything.

    Mark Costello:
    I dont know as im working with bar, id imagine it might though as its the point that locates the position of the drill.

    Cal G:
    Things move around a bit when being welded atm, Im making a hinge. The pin im using is .375 and the reamer leaves the hole a little over .376. The sizing feels nice but i might need to work on supporting the things better in the jig.

    Winger:
    The deepest hole i need to drill is 1.3, have to clear chips near the bottom but this doesnt seem to affect size.

    Bill D:
    I dont think that would work Bill.

    Yardbird:
    Im not sure how I could measure the point position accurately easily atm . Im guessing Moving the point over .001 takes the hole out .002. It doesnt take too long to sneak up on it. Ive got a T&C grinder sitting doing nothing, i need to get it plugged up learn how use it
    Last edited by Demon73; 09-15-2019 at 11:35 AM.

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    I imagine you are power feeding,thus giving a consistent pressure. As above,this is a good idea in materials apt to jam such as copper or phosphor bronze. I recently decided to drill some very long governor sleeves in bronze with a carefully sharpened 3/8" drill-got a lovely fit on the spindles,avoided having to ream it!

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    Did this regularly with beryllium copper on a screw machines,the first couple were oversize then you got consistency.

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    Cheers fellas, sounds like im on a well trodden path

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    Heres a quick look at what im running that drill in. 3/4 mild steel bar running at 320rpm feeding .0075pt

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    Stock reamers can be purchased in 0.001" increments and a number of commonly used sizes between that. Probably similar increments for metric reamers. You are not going to hit a hole size in tenths by grinding a drill bit off center. At least, not very easily or reliably.

    Check out the sizes:

    https://www.mscdirect.com/browse/tn/...vid=4287924615



    Quote Originally Posted by Mark Rand View Post
    I can believe the offset grinding to hit a desired oversize amount, but there must be some skill in it. I'm ashamed to say that selections from a couple of sets of 1-10mm by 0.1mm drills sometimes get used when the reamers don't give the desired diameters...

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    Quote Originally Posted by mark costello View Post
    What happens when the drill breaks through? Is the edge of the hole smaller?
    In my experience the last part is going to be smaller
    (Got a badly sharpened 0.1mm increment drill set and took for a while to figure out why the holes are 0.3 to 0.5mm oversize exept for the final part..)

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    Drill bits are flexible. The reason why a drill bit that has been sharpened unequally will drill an oversized hole is because the two cutting edges will tend to center at the point where they intersect. Then the longer one determines the radius of the hole. This happens because the bit will bend.

    If you move the drill bit off center in the chuck, the tip of the drill will still bend over to that "center" that the two cutting edges determine. If the offset in the chuck is large enough, the bit may break.

    Screw machine length bits are an attempt to prevent or at least limit this flexing as the shorter length will not allow as much bending. A carbide, screw machine length bit would be the ultimate way to limit this flexing and your chuck offset may work with it. Or again, it may break if feed too fast.



    Quote Originally Posted by Bill D View Post
    How about some shim stock under one jaw in the chuck instead?
    Bill D


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