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  1. #1
    elanjacobs is online now Apprentice gear maker and machine doctor
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    Unhappy Tapping tiny threads sucks

    My job at work today was to drill and tap a bunch of M2x4.5mm blind holes in 4140. This is an ongoing job, so we have various jigs to hold the parts how we need them. We power tap the first 2mm in an old Brother Hi-Tap to make sure the tap goes in square and finish the rest by hand. We have 3-flute spiral taps bought specifically for the job, but even with the started thread, plenty of oil and frequent backing out to clear the chips, I still managed to snap 3 of them over 40 holes. No one was particularly surprised or upset, but I feel like I can do better. We're going to try drilling the next lot at 1.7mm instead of the normal 1.6mm in the hope that it reduces the cutting pressure just that little bit.

    I had no problem feeling resistance, just found it tricky to tell if it was normal cutting force or that tiny bit extra that was more than the tap would handle. Are there any tricks to keeping small taps alive, or is breaking them pretty much inevitable with a combination of tiny thread and fairly tough material?

  2. #2
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    Default Hard metal

    drill and tap a bunch of M2x4.5mm blind holes in 4140
    Ah yes, a known problem. Sure, the tap says M2x0.4, but that only works in aluminium or soft brass. Steel and SS - ha!
    Try a 1.65 mm drill bit if you have one, or go to 1.7 mm if you don't. And of course use some tapping fluid.

    It might be worth you while buying some 1.65 mm drill bits for this. I recommend Dormer bits, from Bolt&Industrial - their prices are a LOT better than most small retailers. And maybe buy some new good brand M2 taps while you are at it: blunt taps are fine in plastic, but not in steel. They are CONSUMABLES!

    Cheers
    Roger

  3. #3
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    Default

    Surprised that they hadn't considered increasing the tapping drill size before.

    Did the taps all break after tapping about the same amount of holes? If so you may want to look at changing taps a couple of holes early.

    Are you using a tap wrench or Tee handle wrench?

  4. #4
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    Default

    Hi Elan,

    M2 ! no problem

    Seriously though, it really is very difficult to learn the needed feel. I've done m07 in brass a couple of times, luckily they were through holes in a pocket watch plate.

    I have a stand that I use for these small taps and use it to ensure that they go in dead square, the handle is a 3/4" inch diameter knurled knob. A conventional tap handle is just too hard to control. The cutting fluid also makes a difference, you don't want the chippings to stay in the flutes, with fine taps a re-cut chip will break it.

    Fortunately I've broken very few taps, though I used to break 1/4" BSF taps in aluminium with great regularity in blind holes. At least I did until I learnt to stop when I hit the bottom of the hole.
    Best Regards:
    BaronJ.

  5. #5
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    Default

    I would say that tap selection is very important at small sizes. Are you using a spiral tap, that way the chips get evacuated out the back. A few months back I had to do 40 or so mixed M2 and M3 holes in o1 and I used one OSG spiral tap in each size, all done with no tapping head just held solid in a collet but turning the drawbar by hand with a big knurled knob. O1 is a bit easier than pre hard though.

    Sent from my Nokia 8 Sirocco using Tapatalk

  6. #6
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    Default

    Does anybody actually read the full content of what people post or just guess what may be written and then add their comments?

  7. #7
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    Default How to not break the tap

    tricky to tell if it was normal cutting force or that tiny bit extra that was more than the tap would handle

    It is not that hard to tell whether there is too much torque on the tap. You just watch the tap very carefully to see how much it is twisting.

    Um - on spiral flute taps this is HARD. I use straight flutes taps, even at M2, and I can see them twist. And I do back and forth a bit with them as well, partly to clear out the swarf.

    Cheers

  8. #8
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    Default

    I've nothing to add but I find tapping any more than 1 hole a drag.

  9. #9
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    Default

    Sorry I didn't read the bit about having spiral taps. I'm assuming they aren't spiral point or gun taps as they can't do blind holes.

    You could use a form tap, although I don't know how they go in hard materials. They give beautiful threads and you could just do them in one in the brother no hand work needed. I feel like with a bit more thought there would be no need to do them by hand.

    Sent from my Nokia 8 Sirocco using Tapatalk

  10. #10
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    Default

    I have found using either a tap wrench or T wrench on small taps you loose the sensitivity you need.
    What you need to make first is a guide to keep the tap square... a piece of aluminium 20-25mm diam 20-25 long with a hole 0.05-0.1mm bigger than the tap you are using.
    Then from the same piece of alloy put a straight knurl on the outer edge... drill an hole 0.15-0.2mm smaller than across the corners of the square end of the tap...part off about 3mm thick...put the tap in reverse in a drill press and press the tap into the center of the disc and you have very sensitive driver for the tap.
    Hope you can make sense of all that.

    Robert

  11. #11
    elanjacobs is online now Apprentice gear maker and machine doctor
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    Default

    We've got proper spiral taps from Walter, they bring the chips out really nicely and only have about a 0.5mm tapered end so we can get nearly full depth. Using the smallest T-handle we've got and holding it as close to the middle as I can to try to limit how much torque I'm applying. Didn't really notice a pattern to the breakage; one broke when I was backing it out, one was just me trying to go a little faster than I should. Keeping it square is not a problem, we specifically start it with the Brother to have a square guide for the hand tapping.

    We use carbide drills for stuff like this, usually Guhring or Titex, might have some 1.7 lying around, 1.65 would have to be ordered.

    I'll mention form tapping and see what they say, if it means being able to do it fully under power I reckon they'll give it a go. The hole centres are 1.5mm from the edge of the part, not sure if form taps would push material too far out or not, although we could just give them a rub on some wet/dry if they do.

  12. #12
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    Default

    I'm not sure how deep your holes are but the chances are they are only quite shallow. If your holes do have a bit of depth, you could try serial taps. For those who are unfamiliar with them they come in sets of 3 normally, with the rougher taking the first cut, where a shallow cut is made and this is followed by the second or intermediate tap which deepens the first cut made by the rougher tap. The final cut is made by the finisher tap which cuts the thread to final size. The cutting forces are thus lessened, because each tap takes a finer shave than the conventional taps which cut to the final size, providing of course that it is a through hole or a hole deep enough to allow the tap to cut threads to the section of the tap above any tapered lead in on the first few threads of the tap. I had a quick look on eBay and this link shows an example of serial taps.
    https://www.ebay.com.au/itm/TiN-HSS-....c100677.m4598
    As others have said, if you can afford to have a lower percentage of thread depth, that can make a big difference when tapping with small diameter taps or when tapping tough materials.
    Here is a useful thread cutting calculator, you will notice that for any different thread depth %age, two drilling sizes are given. One is for cut threads, and the other for formed threads like Caskwarrior referred to. Hope this is of some use
    Tap drill size calculators

  13. #13
    elanjacobs is online now Apprentice gear maker and machine doctor
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    Default

    Quote Originally Posted by Ropetangler View Post
    I'm not sure how deep your holes are but the chances are they are only quite shallow.
    Holes are drilled to 5mm deep, tapped to 4-4.5mm deep

  14. #14
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    Default

    Quote Originally Posted by elanjacobs View Post

    We use carbide drills for stuff like this, usually Guhring or Titex, might have some 1.7 lying around, 1.65 would have to be ordered.
    Seeing you are using carbide drills, could they be work hardening the holes causing strain on your taps?
    Just asking.
    Rgds,
    Crocy.

  15. #15
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    Default

    Quote Originally Posted by Ropetangler View Post
    Here is a useful thread cutting calculator, you will notice that for any different thread depth %age, two drilling sizes are given. One is for cut threads, and the other for formed threads like Caskwarrior referred to. Hope this is of some use
    Tap drill size calculators
    Thanks for this, hope you don't mind, but I've added it to CONVERSIONS & CALCULATIONS, for future reference of all.
    Kryn

    To grow old is mandatory, growing up is optional.

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