Announcement

Collapse
No announcement yet.

lube for tapping cast iron

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • lube for tapping cast iron

    Hello all, working on an L00 back plate that I am tapping for a 5C collet chuck. It requires 8mm x 1.25 threads. I have tried Rapid tap ,Tap Magic, and Anchorlube so far using 3 different good grade taps. that are not very old, (at least I know for sure 1 of them only has a couple of holes in aluminum...) holes were drilled in my mill to 17/64th ( .265") and started in line using the chuck and then hand tapping, but seem to be straining the tap to the point of nervousness. I am backing off every couple of degrees and putting plenty of lube on but something does not feel right. Any suggestions on an alternate lube? I have a brand new tap on order. Thanks, Jim

  • #2
    Last I checked, cast iron is usually done dry. I would suspect the hole size first, never had a problem tapping in it.

    Comment


    • #3
      Dry for me has always worked too.

      Perhaps the lube has held the particles in close and it acted like a lapping compound to jam the works up and perhaps now the edges of the flutes are dull to boot? Clean and degrease and lightly run a small grinding point along the flutes to freshen up the cutting edges and try again? Blow through with compressed air frequently.
      Chilliwack BC, Canada

      Comment


      • #4
        If they're through holes you'll find a spiral point tap is much easier to thread with. It's stronger too.

        -js
        There are no stupid questions. But there are lots of stupid answers. This is the internet.

        Location: SF Bay Area

        Comment


        • #5
          Originally posted by BCRider View Post
          Dry for me has always worked too.

          Perhaps the lube has held the particles in close and it acted like a lapping compound to jam the works up and perhaps now the edges of the flutes are dull to boot?
          I think BCrider nailed it. The one time I tried lube on cast iron, it seemed to dull off the tool instantly because the chips are abrasive, and the lube prevents them from leaving the tool edge.

          Comment


          • #6
            The graphite in the cast iron provides the lubrication. Imagine the mess with oil and CI dust mixed up.

            Comment


            • #7
              Please dont tell me that you are useing a HANSON HAND TAP from the hardware store.i GOOD SPIRAL POINT GREENFEILD tap will tap this easily wet or dry blind hole or thru hole. Edwin Dirnbeck
              Last edited by Edwin Dirnbeck; 05-20-2020, 05:23 PM.

              Comment


              • #8
                Absolutely. NO LUBE, dry for cast iron. Clean it up and start over.
                "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

                Comment


                • #9
                  17/64" (6.75mm) is an acceptable but not preferred drill size for M8x1.25. Preferred size is 6.9mm, and for cast iron I would use a 7.0mm drill.

                  Comment


                  • #10
                    My cheat sheet says a J drill or .277" You're .012" undersize which is why it's tapping so hard. As already been said, tap cast iron dry. There's enough graphite in CI to provide a lube.

                    Comment


                    • #11
                      I should have remembered that cast iron was best tapped dry, but did not. I remembered to clean up and protect the ways when I was turning the spigot for the chuck, knowing how abrasive the bits are. I think I ruined the tap due to lubing as was said above, (no cheap taps for me either...) and I will use a slightly larger drill also. Thanks, Jim

                      Comment


                      • #12
                        It is interesting to note on 3 separate thread charts it lists three different size drills.

                        Comment


                        • #13
                          Yes, the drill size varies by the material and percent thread. For us "Imperial" folks, we convert to what we have. Unless you want to buy a 7.1 or 7.2mm drill bit for a one-off hole tap.

                          Comment


                          • #14
                            Originally posted by jmm03 View Post
                            It is interesting to note on 3 separate thread charts it lists three different size drills.
                            Do these thread charts reference the percent thread for the given drill size? As noted in post#9, 6.9mm drill is recommended. It should give approx. 78% thread which is fine for cast iron. And 7mm would work great too. For future reference, the tap drill formula for metric threads is:

                            hole size = major diameter of thread - ( pitch in millimetres x % thread required x 1.08253 )

                            for the thread you are using: 8mm - ( 1.25 x .78 x 1.08253 ) = 6.9mm

                            Comment


                            • #15
                              I've found the best tapping chart is on the Guhring site, https://www.guhring.com/Support/Tech...l-Sizes-Metric . It gives the theoretical % of thread for different drill sizes. 17/64 is giving 77% thread. Spend a bit of time wandering the support pages above, lots of information.

                              lg
                              no neat sig line
                              near Salem OR

                              Comment

                              Working...
                              X