pre drilling for holes

Discussion in 'Tools' started by Luiz Fernando Pinto, Sep 30, 2019.

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  1. Oct 3, 2019 #21

    john_reese

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    I think the diameter difference is the root of the problem. The larger drill probably screwed itself into the pilot hole destroying the cutting lips on the drill. For a pilot drill pick one just larger than the web thickness of the larger drill. If you are enlarging a hole by 1mm or less the proper tool is a reamer.
     
  2. Oct 4, 2019 #22

    Cogsy

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    1mm is about double of what I would normally leave for a reamer to remove but it is less than I normally leave for a drill to remove as well.
     
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  3. Oct 4, 2019 #23

    BaronJ

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    Hi Guys,

    If I'm reaming a hole I try for about 5 to 10 thou, much more than that, upto a millimetre or so and I go for a "D" bit. Most metric drill sets seem to come in 0.5 mm increments anyway so only 20 thou between sizes.
     
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  4. Oct 4, 2019 #24

    Charles Lamont

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    I can't make sense of the shape of that tip. Did you check that the drill had a correctly ground point in the first place? Sometimes the grinding can go completely haywire and the drill escapes inspection. I often drill a few thou under size before putting the final drill through. Never had a problem.
     
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  5. Oct 4, 2019 #25

    goldstar31

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    I'm raising several questions.
    The first is whether today's drills need lubrication to avoid damage. I keep seeing warnings about this
    Again, about reamers, I always understood that new reamers were machined oversize so that a subsequent re-grind was 'on spec'

    Thirdly, I 'm ancient but recall the tale- possibly an old wives one about the competition BEFORE WW2 between the UK and Japan drilling holes and then getting another smaller hole and then another and so on.
    Again, my copy of Sparey, the Amateurs lathe book and reference to drilling and then finish drilling. Was Sparey wrong as he made a lot of designs -- which you are all copying.

    Lastly, drills heat up without lubrication and I recall that with long carbon spade drills in very hardwood wood that they both expanded and burned and -- went soft in consequence.

    I doubt that my recollections are correct. Please comment

    Norm
     
  6. Oct 4, 2019 #26

    tornitore45

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    Self-feeding or grabbing happen with plastic, brass, or at the breakthrough.
    It happens when feeding with the lever, you are pushing and all of a sudden supposed to reverse your force direction.
    The problem can be eliminated by feeding with a "screw mechanism" like raising the knee or using the fine advance.
    The lever feeding has other shortcoming. In some particular conditions it allows the bit bounce up and down like when trying to dill out a broken tap with a carbide burr. The bouncing ruins the cutting edge.
     
  7. Oct 4, 2019 #27

    Charles Lamont

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    Phosphor bronze is particularly bad for grabbing. If resources run to it, it is a good idea to have some drills dedicated for 'brass' work, with the top-rake ground or honed off at the tip. Straight fluted drills are supposed to exist, and I think I may even have seen one once.
     
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  8. Oct 4, 2019 #28

    Deholby

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    Whats the chance you drilled through part and hit a hard steel parallel under it on the edge ?
     
  9. Oct 4, 2019 #29

    tornitore45

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    That would not explain the flutes "superior" finish.
    By the way, I know how a parallel look like when something like that happens. The parallel is the rack and the drill is the pinion.
     
  10. Oct 4, 2019 #30

    mcostello

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    I have drilled 52 holes to tap 5mmx.8. Material is 304Stainles steel. The tap drill chart said to use a #19 drill, .166", tapping was horrendous. Increased the size to #18 ,.169" now doable without taking butt bites out of the chair. This is taking .0035" out of the hole or .00175" per size, theoretically. No problems so far.
     
  11. Oct 5, 2019 #31

    xpylonracer

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    Tapping drill size for M5x.8 is 4,2mm (for metric tap size drill deduct the thread rate from the diameter) is the reason for poor tapping performance with a #19 drill.

    xpylonracer
     
  12. Oct 5, 2019 #32

    mcostello

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    If I have typed correctly, Google says 4.2mm is .1654, #19 is.166, not much difference.
     
  13. Oct 5, 2019 #33

    petertha

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    I've purchased a few regular HSS Dormer's to fill in worn drills & they were decent quality. Can we see the cutting edge?
    On your one pic, is the coating coming off, or that is remnant of swarf?
     

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  14. Oct 6, 2019 #34

    xpylonracer

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    Perhaps the drill lost some diameter drilling so many holes in 304 stainless ? or the tap got blunt ?
    As you say #19 is almost the same size as 4.2mm.
    You didn't state thickness of the material but many people drill slightly up on the tapping size and rely on 60% engagement.

    xpylonracer
     
  15. Oct 6, 2019 #35

    mcostello

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  16. Oct 6, 2019 #36

    tornitore45

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    Stainless steel is not tap friendly, for that reason a 50% tread is usually done.
    A M5 screw into a 0.350 (0.25 engagement) is plenty strong. the screw will break long before it strips.
     
  17. Oct 6, 2019 #37

    Ozwes007

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    Drills and drilling. Today’s drills usually come in 5 flavours.
    1 cheap and nasty - Carbon steel.
    2 cheap - M02 (straight HSS)HSS with or with out coatings- coating only lasts as long as the moment you wear through it, then it’s just a M02 HSS drill.
    3 M35 HSS ( 3-5% cobalt HSS) with or without coatings.
    4 M42 HSS (5-8% cobalt HSS) with or without coatings.
    5 Carbide Drill bits.
    Most people will buy #1 or #2 drills.
    Stainless is a soft material, except if you overheat it! Then it becomes super hard and will destroy the chiselpoint and lands on all #1 and #2 drills and some #3 drills.
    Slow down your speed and it will cut it like butter. I mean really slow it down.
    Tapping stainless is best done with sharp taps, check your taps if you can see a reflection from the edge they are blunt and throw them away. Use gun taps or spiral taps for stainless and you will have no problems. Make sure they are at least M35 grade HSS.
    The general rule of predrilling is to make the predrill slightly larger then the chiselpoint. Chiselpoint thinning can remove the necessity to predrill upto about 25mm drills. When drilling brass, bronze, thin steels etc, just run a fine grind or hone along the lip of the cutting edge parallel to the drill to produce a neutral or slightly negative rake angle will help greatly.
    Metric taps use the rule diameter minus pitch to give you the tapping drill size for an 82-85% thread profile. The #19 is perfectly fine for drilling a M5 tapping hole, the problem will be twofold in stainless if you are having issues:
    1. You overheated the steel and hardened it.
    2. Your tap is blunt(usually this one)shiny edge, bin it! (Yes they can be reground)
    Also lubricant is always used for tapping Stainless(make sure it’s a high pressure one suited to Stainless), never for brass or bronzes or cast iron.
    Kerosine or lard kerosine mix for Aluminium.
    Just from my 50years in the trade this is what works.
    Wes
     
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  18. Oct 7, 2019 #38

    Luiz Fernando Pinto

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    It is a remnant of swarf, definitely.

    Look at the cutting edge.... it's almost identical to the photo you posted.

    upload_2019-10-6_20-26-29.png
     
  19. Oct 7, 2019 #39

    petertha

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    The cutting edges (red) look a little ratty. Degradation? Or maybe just picture resolution.

    But the (green arrow) silver stuff does not look good. I assume that is not the drill coating coming off but a smear of metal buildup. That can happen on gummy sticky alloys like certain grades of aluminum. Usually some combination of cutting fluid, speed, feed & cut depth is required. But regular carbon steel I cant say I have seen similar, myself. Strange. Is it making regular spiral chips? Are the chips silver or blue/tan? Assume the chuck is true & work is solid mounted, all the fundamental stuff?
     

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  20. Oct 8, 2019 #40

    Harry.

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