Tapping Question

Discussion in 'General Engine Discussion' started by davidyat, Oct 8, 2018.

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  1. Oct 8, 2018 #1

    davidyat

    davidyat

    davidyat

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    When I started to learn about model machining about 15 years ago, I think it was about a 4-40 tap I broke off in a part I was machining. I was too far along in the part build, that I ponied up the $$$ to have the broken tap EDM'ed out. I would like the more experienced here to give me some basic tips on how NOT to do it again. That experience rattled me so much that nowadays I just go up a few sizes on the pilot hole that the charts recommend when I'm tapping holes that small. Then when tapping, I close my eyes and FEEL the resistance while turning the tap. If I feel that I might break the tap, I go up another size. I turn the tap 1/4 turn, back off, 1/4 turn, back off and so on. Every 1 to 1 1/2 turns, back completely out and blow out the chips and continue. Just looking for experienced advice on tapping really small holes.
    Thanks,
    Grasshopper
     
  2. Oct 8, 2018 #2

    ShopShoe

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    It sounds like you are doing all the cautious things to avoid breakage. Another factor might be the choice of tap. In the small sizes the quality of the tap and the appropriateness of the tap to the job really do matter: Not a place to save a few bucks. Lubricant also becomes important. Starting straight can be a factor: Do you use a tapping guide? Also, sometimes the three-tap approach is better (taper, then plug, then bottoming [US terms]).

    --ShopShoe
     
  3. Oct 8, 2018 #3

    tornitore45

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    I stopped breaking taps when I got through the idea that you drill with a size equal to the root diameter. BAD IDEA
    Now I drill for 75% thread as a starting point.
    In aluminum I may go one drill size smaller, only if I feel the strength is needed.
    For those SS that work harden 50% thread is plenty.

    Use tapping oil. I have a plastic pipette that delivers a small quantity of oil, without wasting.

    Alignment. A drill bit started on a punch mark off center will manage to drill a crooked hole rather than breaking.
    A tap started at an angle will tap a few thread but then is faced with cutting deeper and deeper on one side while unsupported on the opposite side. Breakage is guaranteed.
    Always have part and tap aligned in the drill or mill, preferably immediately after drilling without returning to the position. If one has many holes to drill and tap changing tool may be undesirable in that case be sure to record the coordinate of each hole, returning to them in the same original direction to avoid the backlash error.

    I built enough confidence now, after many clenched teeth experiences, to power tap down to 4-40.

    Something to know
    #6-32 taps are weaker that #4-40 taps due to the fact that the larger core diameter does not make up for the extra torque to cut a coarser pitch.

    Use spiral point taps. Their 2 flutes make them stronger.
     
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  4. Oct 8, 2018 #4

    doc1955

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    I think the biggest thing is to keep the shaft of the tap supported and aligned. As far as power tapping here is a video I did quite awhile back tapping aluminum 75% threads in a blind hole on the lathe.

     
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  5. Oct 8, 2018 #5

    davidyat

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    Usually taps have a drill dimple at the top part. When I was at the NAMES show last April, I got from Little Machine Shop a spring loaded tapping guide that I'll put in the chuck to keep the tap aligned. Now with that said, there probably isn't a dimple in taps that small. Any advice on where to get a tapping guide for these small taps? And thanks for all the responses.
     
  6. Oct 8, 2018 #6

    cheepo45

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    Use a small T type tap handle with a hole at the end of it for the tip of the spring loaded guide.
    As others have stated-tapping the hole straight is important.
    Scott
     
  7. Oct 9, 2018 #7

    Cogsy

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    I try, whenever possible, to thread holes that I drill on the mill. So I drill the hole, then change out the drill bit for the tap and turn the chuck by hand as I start the thread, for perfect alignment. Once the tap is sufficiently engaged that alignment won't change, I release the chuck and use a tapping handle to finish the job. I do this regularly for 2-56 and 0-80 sizes and, as yet, I have not broken a tap using this method, in steel, brass, cast iron, aluminium and drill rod. Of course I always use a decent tapping fluid as well. I do have some good brand name taps in some sizes but my small ones are from a cheap Chinese 'watchmakers' tap set that cost all of about $40.
     
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  8. Oct 9, 2018 #8

    tornitore45

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    Tapping #2 and smaller.
    Clamping the tap in the mill chuck and turning the chuck by hand does not provide enough feel. There is too much machinery that needs to be turned.
    For smaller threads I made a knurled driver 3/8" diameter. The not knurled portion is held in collett or a mill holder of the same size, loose enough to turn freely still providing alignment. Is easy to drive a small tap with two fingers.
     
  9. Oct 9, 2018 #9

    tornitore45

    tornitore45

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    Small taps do not have a dimple, they have a conical point. They can be kept aligned with a spring loaded plunger that has a female conical hole. You can make one easily with a center drill, but then the question is: How do I drive the tap?
    See the post above. All small taps have the same shank diameter, so one driver can cover them all. Two set screws engage the square portion of the shank op into the hole.
     
  10. Oct 9, 2018 #10

    dkwflight

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    I bought a tapping fixture a number of years ago and discovered the big factor in tapping small threads. The big factor is alignment with the hole to be tapped. The second is the hole size along with the hole diameter.
    Going for a 75% thread is very sensible especially in a model where the loads on a given screw or bolt are small.
    I often tap a hole in my drill press right after drilling the hole. Making no movement of the part. This costs time by removing the drill and substituting the tap in the chuck. I do not try to tap under power but move rotate the chuck by hand. If the tap is big enough the require a lot of strength I use a bar inserted in the chuck key hole to give some leverage. You can also start the tap in the hole several turns and loosen the chuck and put a tap handle on the tap to finish the operation.
    I don't use the tapping fixture very often now.
    Dennis
     

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  11. Oct 9, 2018 #11

    GreggA

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    I agree with Shopshoe...I have been tapping 4-40 threads for years and rarely break a tap. Use a quality tap, Take some time and make a tap guide, Just a round cylinder with a hole thru it, such that the tap shank fits smoothly and both ends faced accurately and flat. I have one for each tap size I use, really helps to keep things straight. I have found breaking taps usually has more to do with getting off to a bad start. Drill straight, tap straight, have a good sharp tap, and follow your instincts. if the tap starts to squeal and complain better check your hole size and alignment. the tap generally tells you when its not happy.
     
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  12. Oct 9, 2018 #12

    doc1955

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    For hand tapping I use a tap guide block and a thumb wheel tap handle. Using a tap guide block assures the tap is held straight.
     
  13. Oct 10, 2018 #13

    popnrattle

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    Tap drill (decimal equivalent) is always Pitch(P) subtracted form Major Dia.(MD) for 60 deg. V-threads, Metric included. (Closest drill from a selection of Letter, Number, or fractional drill indexes) and depending upon material,(SS,or alloy) the next size up. For number(#) taps the MD = (#)(.013)+.060. Tap quality very important too. I've had best luck with OSG taps. I prefer the old black cutting oil, stinks but works best. For small tapping I leave the drill chuck shank loose in my collet(or drill chuck if I'm tapping in the lathe) to turn with thumb and middle finger for "touch" tapping. Later, rt.
     
  14. Oct 10, 2018 #14

    10thumbs

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    Something that has not been mentioned is to use break-free for tapping oil when using tiny taps. Not cutting oil per se, but with it, the taps don't get stuck.
     
  15. Oct 10, 2018 #15

    CFLBob

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    Thanks for that. My "DAMHIK" moment is that there's still part of a broken 6-32 tap inside a piece of my CNC controller box I built about two years ago.
     
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  16. Oct 10, 2018 #16

    colby15642

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  17. Oct 10, 2018 #17

    davidyat

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    Thank you all for your experienced advice. I'm almost at the end of building a Cirrus 2X compressed air V-8 by Chuck Fellows. I followed Bazmak's 2 times dimension build. At this stage I don't want 2 years of building, for me to end up with a rather expensive paper weight on my desk. :eek::eek::eek::eek::eek:
     
  18. Oct 10, 2018 #18

    colby15642

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    I haven't broken a single tap since adopting a few rules 20 years ago:

    1. Avoid free-hand tapping at all cost.
    2. Use only new USA-made Greenfield taps on critical jobs.
    3. Always start with a taper-style tap. Finish with a plug tap or bottoming tap if necessary.
    4. Use the same single work-piece setup for both drilling and tapping. The first drill used is ALWAYS either a short rigid spotting drill or a center drill.
    5. Mount the tap in a T-handle tap wrench having an integral tail-piece and matching sliding-fit tail-cup which is captured in the drill press or mill chuck. These tap handles are available in at least two sizes from McMaster-Carr and others.
    6. Tap cast iron dry. Use the most expensive tapping fluid specifically formulated for other materials. A lifetime supply will cost you $20, half the price of one case of decent beer.
    7. If the tap is not cutting like the work-piece is butter, use a different tap. I have encountered new USA-made Hanson taps that will not cut. Save your Chi-Com taps for thread-chasing rusty tapped holes in manure spreader parts.
    8. Tapping some stainless steels and drill rod may require a larger tap drill and special prayers.
    9. Believe what you know is true. The best taps wear as they work and must be replaced periodically. Resharpening does not address wear on the peaks of the threads behind the cutting edges on the tap OD, which causes the tap to wedge tight in the hole, possibly causing breakage.
    10. To reduce required torque when tapping tapered pipe threads, first taper the hole with the appropriate pipe-thread tapered reamer.
     
  19. Oct 10, 2018 #19

    davidyat

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    OK, advice on this problem I'm going to be up against. I want to tap an 8-32 hole that is only 0.150 to 0.165 deep. Hey, it is what I'm up against. I plan to drill the hole depth with a number 29 (0.136). Find zero on my Bridgeport and raise the table, 0.160. Then I have an extra 9/64 end mill that I ground flat on my diamond wheel grinder and use that to flatten the bottom of the hole. I do have a spare 8-32 tap that I ground flat also that I've been using with good results after starting with a taper tap when possible. I have a lot of spare taps that if a taper tap needs some taken off, I can do it. Then a plug tap, then my bottoming tap. I haven't done anything yet as I want to get as much advice before I start this tapping of 16 holes!!!
     
  20. Oct 10, 2018 #20

    abby

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    Your hole will only have about 4 threads , no room for a taper tap start , I would grind the tap right back flat and hand tap , the feel will be better and there is almost no chance of breaking the tap.
    What is the thread doing ? can the part be better designed ?
    Dan.
     
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