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Thread: Accurate drilling - received wisdom

  1. #1
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    Default Accurate drilling - received wisdom

    Preface... I've done a lot of reading here and wanted to summarise it, maybe post it on a blog. But, I figured I should make sure I'm understanding the concepts first. Feel free to comment or point out where I'm going wrong.

    Anyway... Second draft now, implementing the recommendations

    There are 2 common ways to drill accurately placed holes: floating or indexed.

    The floating method:

    1. Layout the part, scribing lines to the accuracy desired.
    2. Prick punch (a fine 30-50deg punch) the scribed intersections. Done correctly, you should be able to feel the intersecting lines with the point of the punch but use a magnifying glass to confirm. An optical center punch is another option here, but again confirm.
    3. Drift the center mark as necessary until it is at the line intersection.
    4. Enlarge the mark with a center punch.
    5. Optionally, scribe witness circles with a divider, using the center punch as an anchor, the same or slightly larger diameter than the desired hole.
    6. Select a very small drill with a tip that fits properly in the center punch mark. Some people prefer to just make the center punch mark larger than the web of the pilot drill they intend to use.
    7. Drill while allowing the work to float on the drillpress table such that the drill will align with the punch mark.
    8. Select the next drill, which should have a web thickness smaller than the original hole or punch mark and have a diameter slightly larger than the web thickness of the final or, if necessary, intermediate drill.
    9. Drill the final size.


    The indexed method:

    1. Clamp and index the part to a known feature. If you index off a punch mark, use an appropriate measuring tool to pick up the mark.
    2. Crank the table to the desired hole location. If using a drillpress, you can index each hole to drill.
    3. Use a short spotting drill our, if it's all you have, a center drill.
    4. Drill to the web thickness of your final drill.
    5. Drill to final size.


    General notes:

    • The floating method relies on a moving work-piece and flexing drill to align with a center punch mark. Rigidity is bad. Do not clamp down the vice or fixture and use jobber length drills. When drilling large holes, remember drill safety and consider indexing instead.
    • The index method relies on rigidity. Clamp and lock everything down. Use stub-length drills where possible. Use spotting drills if you have them.
    • Mixing the two approaches, using stub-length drills when floating to center punch marks for example, will give less accurate results.
    • When using the layout method, if the initial hole has gone off center then you can drift it back a slight amount. Use a cold chisel to notch the far side, the side you want to move away from, and then drill the next size up drill available, possibly repeating several times. Witness circles help with this approach.
    • Layouts on work to be clamped down and indexed will help with error checking but a center punch mark will, in itself, not correctly line up the drilled hole. It's up to the operator to correctly index to the mark.
    • If holes in multiple parts are to align, bolt, clamp, or even tack weld them together and drill all at the same time. Another option is to make a drill guide (jig) that is then clamped to each part in turn.
    • The tables on light-duty drillpresses will often flex down with drilling pressure, thus going out of square. For this reason, deep holes are best done on more rigid machines. If necessary on a light-duty drillpress, block the table up to reduce flex. Optionally, shim the work on the table to compensate for the expected flex for the drill pressure being used.
    • Tramming the table is also important on deep holes.
    • A poorly sharpened drill can drill oversized holes.
    • Accurately sized holes require reaming or boring.
    Last edited by fixerdave; 05-26-2017 at 12:09 AM. Reason: updated with suggestions

  2. #2
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    If the subject is drilling on the lathe, Always bore the first 5 mm depth if the hole location is important.

  3. #3
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    When I am drilling holes on the mill, I do it as you describe, but I also scribe the hole locations first. It is a great way to double check the locations and has saved me from missing the proper location by 0.100" many times.

    Note: Most mills that I have used had 10 TPI feed screws so it was easy to be off by one full turn, or 0.100". Of course, with a DRO this is not necessary.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  4. #4
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    2. First you prick punch then centre punch. A prick punch is sharp, finds the intersection and raises the metal concentrically around the that point. The centre punch flattens it out again to the be right angle for the drill. Its a two step process, centre punches are poor at finding a location and the resulting cone from their use is improved by first raising the material as the prick punch does

    6. the small drill is unnecessary if you've done 2 properly. Unless the drill is very large, in which case, on a manually fed drill press there should probably be a pilot hole.

    7. Whether to float or not a decision mostly based on mass and drill size and how it relates to safety. Most of the time you float either directly on the table or in a vise, however drilling large holes dictates things get clamped. The work is positioned over the drill press's axis using a wiggler so the mark is aligned. When that is the case, you can forgo punching, align the layout lines with the wiggler and spot drill to help the drill bit start at the proper location.
    Last edited by Mcgyver; 05-25-2017 at 07:43 AM.
    .

  5. #5
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    Here's more lore:

    How about a wiggler for acquiring accurate alignment of the drill spindle to the layout? I use them on my 60 year old Craftsman drill press and my turret mill. I get 0.005" of true location routinely if I'm careful. You don't use a wiggler most of the time, only for work you need to be fussy about.

    If you're drilling accurate holes of any depth remember to previously tram your drill press table and that drilling thrust will deflect the table a bit. If the table is out of tram there's little you can do about it except shim the work off the table to correct it. Or map the error and remove the table for machining out the error using the mapped figures for setup true to the reference plane of the mill or lathe.

    If you determine the drill press table deflection (under a 3/8 drill's thrust in steel for example) you can figure in about half that deflection when you correct for tram error. You won't eliminate the deflection error but you will have reduced it to some acceptable level. My drill press table is pretty good, I never bothered to machine it to tram figures but the deflection for a 3/8 drill feed is about 0.005. If I need really square hole axes I move the work to the turret mill.

    Steps 3 and 4 of the layout method where the hole location is center punched from layout:

    First prick punch. A prick punch is long and slender and has a sharp point of 45 degree total cone angle. Further the prick punch is sharpened with the point held up against the grinding wheel (like you lick a candy cane) so the finish striations are in the axial plane. This finish when impressed in the work is brilliantly reflective making re-acquisition for the starter drill or the center punch that much easier. The prick punch is driven with a peck from a 4 oz hammer. if the mark is larger than 1/16" dia you over-did it. BTW, only rubes sharpen prick punches by laying them on the tool rest and grinding round and round like you eat an ear of corn.

    The center punch is a heavier more robust tool having an included cone angle of 60 to 90 degrees depending on the severity of service. A center punch may be driven by as heavy a hammer as desired using the prick punch mark to set its point. I have one with a handle welded to it - a 3/8 rod furnished with a file handle - I use with a 32 oz hammer for outlining cuts in plate i intend to OA cut.
    Last edited by Forrest Addy; 05-25-2017 at 08:15 AM.

  6. #6
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    Quote Originally Posted by Forrest Addy View Post
    Further the prick punch is sharpened with the point held up against the grinding wheel (like you lick a candy cane) so the finish striations are in the axial plane... BTW, only rubes sharpen prick punches by laying them on the tool rest and grinding round and round like you eat an ear of corn.
    Interesting. Seems like somebody forgot to tell Starrett, or maybe Starrett just forgot how to do it properly. Just received an 816B, and the grind marks on the tip are not axial. You can just see it in their enlarged photo http://www.starrett.com/metrology/product-detail/816B

    Also came with a nice burr on the point, not up to their usual standard. No red box or inspection certificate either. Just a little plain brown envelope with the punch inside. Sold and fulfilled by Amazon, so should be the real deal.

  7. #7
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    Same applies to tungsten electrodes.
    There's a further way, use a jig and drill bush, fairly accurate, one jig can have several bushes, you set the jig relative to the datum edges of the job, aka jig drilling
    Mark

  8. #8
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    Good info...

    I believe a prick punch is typically 30 degrees included.

    on edit I see the Starrett above is 50 degree included... so seems they can be 30-50 degree included and still be a 'prick punch'.
    Last edited by softtail; 05-25-2017 at 01:11 PM.

  9. #9
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    Quote Originally Posted by pinstripe View Post

    Also came with a nice burr on the point, not up to their usual standard. No red box or inspection certificate either. Just a little plain brown envelope with the punch inside. Sold and fulfilled by Amazon, so should be the real deal.
    It should be yes, but often isn't. I've purchased the exact same item from Amazon LLC (not 3rd party) at different times and gotten the "real deal" in factory packaging the first time - and a fake knock off in a plain brown box the next time. And the kicker was I had paid extra to receive the factory clam shell packaging instead of Amazons cut rate "Easy opening packaging". Guess which one I got the second time.

    Amazon reviews are chock full of complaints from people that have received phony merchandise that is "shipped from and sold by Amazon LLC."

  10. #10
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    Quote Originally Posted by pinstripe View Post
    Interesting. Seems like somebody forgot to tell Starrett, or maybe Starrett just forgot how to do it properly. Just received an 816B, and the grind marks on the tip are not axial. You can just see it in their enlarged photo http://www.starrett.com/metrology/product-detail/816B

    Also came with a nice burr on the point, not up to their usual standard. No red box or inspection certificate either. Just a little plain brown envelope with the punch inside. Sold and fulfilled by Amazon, so should be the real deal.
    Rubes are rubes even when they work for Starrett and Starrett clones. Don't take my word for it; nor Starrett's. Experiment for yourself. See which method of prick punch sharpening leaves the most visible mark. And lasts longer for that matter. On the scale of a prick punch tip the grinding striations are en effect stress risers. An axial ground prick punch (or center punch for that matter) outlasts the circumferential ground punch several times.

    I can't believe I sucked my self into defending a trivial detail like the proper way to sharpening prick and center punches but everything counts - to a point.
    Last edited by Forrest Addy; 05-25-2017 at 03:02 PM.

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