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How do you tell when a tap is dull?

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  • How do you tell when a tap is dull?

    Prior to the last year, the only taps I had used were to chase threads in an existing threaded hole.

    After reading your collective wisdom ( and other sources) on tapping techniques / do's / don'ts / etc, I started using some taps to cut new threads for various projects. My latest is tapping 3/8-16 holes in a 3/8 mild steel plate welding table surface for fixturing various items that I weld together.

    I've broken 1 tap so far, and I think I understand why ( tap not started perpendicular to the plane of the surface, and I turned it too hard when it started to bind )

    I acquired a large machinist toolbox a number of months ago which included a lot of high quality taps. How do I tell if a tap is "sharp" ? Is it a feel thing?
    Is there a way to sharpen them? ( If so, is it wise to so?)


  • #2
    Originally posted by 67chevelle
    How do you tell when a tap is dull?
    It breaks.

    Before breakage it rubs instead of cuts...JRouche


    • #3
      A Tapguide to get started straight did wonders for me and about 300 4-40 holes I had to do by hand. Made the guides out of wood. Your Old Dog deserves the credit and I've been grateful ever since.



      • #4
        Have a real good look at the cutting edges and look for worn cutting teeth, rounded cutting teeth or small pieces that have broken away from the sharp cutting teeth.
        Another way is to scrape the cutting teeth over your thumbnail/fingernail and if it cuts or leaves small tracks in your nail, it"s sharp.
        DO NOT try and use a worn tap as it is just courting disaster, the only place for a worn tap is in the rubbish bin, not back in the tap draw.
        If you want to make sure that you are starting the tap off dead square to the surface, drill a hole slightly larger than the tap in a block of metal , place the tap down the hole and then start the thread of square. Remove the tap from the thread, away with the block and back into cutting a square thread, instead of attempting it free-handed and getting a drunken thread which can and does/will break taps.
        regards radish


        • #5
          If they're thru holes, tap them with a spiral or "gun" tap. This angled cutting edge of the spiral tap "spirals" the chips with an axial component around the gullet and sends them out the bottom. You can simply spin the tap in without backing away every 1/6 turn - provided you get a square start.

          A regular hand tap sends the chips around the gullet in line with the thread forcing it against the wall of the hole. The chips if in ductile material form an almost unbreakable arch stopping further rotation of the tap. Forcing the tap will cause breakage. Thus you have to stop and back away to break the chips.
          Last edited by Forrest Addy; 02-22-2007, 02:05 AM.


          • #6
            Slight correction Forrest. That is a spiral point tap. A regular spiral flute tap winds the chips up out of the hole and is for blind holes.
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            • #7
              Originally posted by Evan
              Slight correction Forrest. That is a spiral point tap. A regular spiral flute tap winds the chips up out of the hole and is for blind holes.
              Thanks Forest and Evan good advise. I worked with a guy with 25 years machining experience that insited spiral flutes were for through holes.
              I worked in another shop that the "head" machinist only ordered hand taps because he did not know any better. Also they were all taper taps if you needed a finishing tap for blind holes you had to grind it. I buy only gun taps for the home shop even though most of the tapping is one or two holes by hand. But if I decide to pull out the tapping head I do not have to scrounge. POI the larger catalogs often have tool selection guides that can help selecting the right tap for the job. A good cutting fluid also reduces wear and tap breakage.The factory rep from greenfield recomends Castrol molly D. espesialy for tough materials.
              Ad maiorem dei gloriam - Ad vitam paramus


              • #8
                Tapping mild steel

                Tapping "mild" steel plate-aka boiler plate-is harder to do than tapping better grades of steel. In the plate the tape "grabs & sticks". Got to be careful when it grabs and turn it backwards! DO NOT FORCE IT! Also use taping oil!
                Good luck learning how to do hand tapping. I takes awhile for new guys to get past being called "tape destroying experts" in the shop I work in!



                • #9
                  Just to clear it up some more on the tap types here is how they look for those unfamiliar with them.

                  I use spiral point taps on a regular basis. They are designed for power tapping and I use a tap holder I made to make them easy to use in a power drill.

                  Spiral point taps do an excellent job of forcing the chips ahead of the tap. Spiral point taps can also be used on a blind hole if there is plenty of extra space in the hole below the maximum thread depth. It can be very difficult to remove the swarf but if it doesn't matter then you can just leave it in the hole.
                  Free software for calculating bolt circles and similar: Click Here


                  • #10
                    A 3/8.16 thread in 3/8 plate should not be a problem, make sure you drill the correct hole size (5/16 or 8mm) use a taper tap, these usually align OK, take light cuts and as stated before, use a lubricant preferablely a good cutting oil (have got by with engine oil many times).


                    • #11
                      I started buying spiral point taps for power tapping but also have found they work just as well for hand tapping.

                      I currently don't see any advantage of a traditional hand style tap over a spiral point tap for hand tapping other than in shallow blind holes, does anyone have any points that counter that opinion?

                      Paul T.


                      • #12
                        I've got a hodge-podge assortment of taps. All the sae fine and course sizes through 3/4 and in both taper and plug so i'm super happy and i use them a bunch. I think its fun to be able to make overly-complicated mechanical do-hickies with lots of hand tapped holes. It looks impressive to me since only a few years ago i had no means of cutting threads

                        That being said, i also have to say thanks to Evan for the pics. Turns out quite a few of the taps that i just assumed were the ordinary taper actually were spiral point taps. I did notice just the other day while threading a 3/8 24 through hole how nicely the chips were flowing out the bottom of the hole. I thought, "hmm i need to do more through tapping, this is nice" but it turns out the tap was a spiral point tap. I guess that had a quite a bit to do with it.

                        I also have noticed a difference in small taps between HSS and coated cobalt steel taps. The coated taps seem to cut better with less binding besides just staying sharper longer. Usually i equate coating with longer life and good for production stuff but i never really think about the reduced friction of the surface coating. It really makes a difference when doing it by hand too. In fact, i was tapping some 8-32 blind holes and after i got started with a taper i switched to my plug as soon as i had about three good threads cut because the plug one is coated and its much easier to turn. I guess the alternative to the coating would be tapping fluid................

                        p.s. lest you all think i horrible abuse my taps i do use tapping fluid when doing most jobs but these were for a miniature distributor i made for my jacobs ladder project and i didn't want oil all over.


                        • #13
                          Tap quality alone makes the most difference to ease of cutting, much more so than any coatings. I prefer Dormer brand but there are several other excellent brands as well. This is another case where the Chinese haven't caught up to American and European quality yet.
                          Free software for calculating bolt circles and similar: Click Here


                          • #14
                            You can tell when a tap is dull when ...

                            No more beer comes out of it. Time to change the keg, except for the gluten intolerant.


                            • #15
                              Yup, spiral POINT tap is is. My bad.