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lube for tapping cast iron

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  • #16
    Already been said but cast iron is cut dry. Ive always heard that if you add lube to cast iron dust, you make a pretty effective lapping compound. Not the kindve thing i want near cutting tools!


    • #17
      A little bit of a cheat to get away with not having to buy a special size of drill is to drill as best as you can undersize then ream with a taper pin reamer, Measure your reamer and mark with magic marker where the reamer reaches the diameter you need, Stop when you get to your mark ! IF at all possible do the same from both sides. I know this is not a " proper" way of doing things but it has saved my ass and avoided broken taps once in a while.
      Wife says I am obsessed with drill bits, she says there are only a few sizes that matter ( Too small too big and the right size!???) For the price the Canadian Tire sets of several hundred drill bits on sale at about 30$ Can are good value, Most are well sharpened and cut just a thous or two over nominal.
      Regards to all David Powell.


      • #18
        std 8mm thread = 6.9mm hole for tapping.
        cast iron - tap/machine dry only.


        • #19
          75% and above is pushing the limit for home shop tooling. If you plan on sending parts into space (or building a crane), the formulas may be fine but they still don't account for H values or what/why a fastener is being used. I've gotten so used to power tapping I rarely, if ever, go above 75%. There's no need. But if you do, you better have some darn good and sharp taps.


          • #20
            Spiral point tap if you have it. A new one if at all possible. Older well used taps will not cut as well. You will find the new tap cutting edges will be dulled by the cast iron depending upon the number of holes.
            I built the MLA Loop 5C collet chuck about 7-10 years ago. Required some good lathe work and tapping holes. Did everything without lube and a HEPA filter on a vacuum. Worked out great


            • #21
              Originally posted by tom_d View Post

              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
              And the formula for imperial is

              hole size = major diameter of thread - (1/TPI)

              For a 3/8-16: (3/8-(1/16)) = 5/16.

              Gives 75% engagement IIRC.
              21" Royersford Excelsior CamelBack Drillpress Restoration
              1943 Sidney 16x54 Lathe Restoration


              • #22
                Metric tapping no tap drill chart required. Subtract the thread pitch number from the major diameter. Example.....8mm - 1.25mm = 6.75mm. 6.75 x .03937 = .2657. I'm guessing you have a common .265 drill in your fractional index. Tapping cast iron wet or dry? Nobody all will work just fine if you are using a decent tap. Cast iron is normally soft


                • #23
                  lube for tapping cast iron

                  Bacon grease. JR

                  Why? Very thin with heat. Just about bleeds off what is not needed.

                  I fine hone many metal knives. Not with Bacon Grease LOL

                  Anyway. JR
                  Last edited by JRouche; 05-22-2020, 03:53 AM.
                  My old yahoo group. Bridgeport Mill Group



                  • #24
                    well, the tap i thought was sharp obviously wasn't because a brand new one went through a 17/64th hole with no problem. Appreciate all the comments. Regards, Jim


                    • #25
                      Thanks for the follow up! it helps all of us learn and get better. And yep I've been bitten by taps that I thought were sharp, its easy to do that.