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Need to make a thread in cast iron

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  • Need to make a thread in cast iron

    I've never created a thread this large or threaded cast iron for that matter. I need to bore a hole in a piece of cast iron that is 3" inches long and 3" in diameter for a 5/8"x11 thread. I have to verify that the thread is a 5/8"x11. Is a tap available that long? If so, anything I need to know (what to buy, technique etc.)? I have an old J-head BP.

    Would something like this work? It doesn't specify a lenght ...

    Last edited by Smokedaddy; 09-18-2010, 09:49 PM.

  • #2

    As its for a BP and if the thread is a "Vee" form, it is probably a 5/8"-11tpi- UNC thread.

    I'd buy a tap and be done with it.

    Tapping size is 13.50mm (~ 17/32")


    • #3
      Assuming no challenges like chill spots that'll wreck any cutting operation, cast iron is very easy to tap. It machines easily and the flaky nature of the chips means you don't have to back up to break the chips. your choice machine or bench, I'd probably start it in the mill and finish on the bench...its a biggish tap and i don't like to push my like spinning something in the mill. Lathe would be very easy with a centre and tap wrench handle against the carriage
      in Toronto Ontario - where are you?


      • #4
        I have an old EMCO Maximat Mentor 10x30. I don't mind doing the operation on the lathe whatsoever. I just haven't done a tap this large, nor used a lathe to do so. Not sure how to do it on a lathe either, nor what you suggesting? I have done a ton of taps by hand, just never on a lathe or mill.



        • #5
          Enco has several grades of 5/8-11 taps from 3 13/16" in the usual grades of taps ($30-60) to a HSS "extension tap" at 6" in length for $40.98 Plus they have that 10% plus free ship til the end of the month...

          As mentioned, the act of tapping cast is - usually - pretty easy, it can occasionally have that 'hard spot' Mcgyver mentioned, like a ball bearing that didnt quite melt! I agree, in the lathe is probably the best way.

          Edit -- I see you were asking about the lathe while I was typing -- use your straight tap handle (may have to use an extension to make it reach a hold spot) and run the dead center up to the back of the tap while held by the wrench, using heavy hand pressure on the tail stock rotate the chuck by hand and start the tap. Once its got a bite, release the tail stock and continue rotating the chuck - I use the chuck wrench - to finish, with the tap wrench butted against the lathe bed or tool rest or ?
          Last edited by Bill Pace; 09-18-2010, 10:56 PM.
          If everything seems to be going well, you have obviously overlooked something........


          • #6
            Originally posted by Smokedaddy
            I have an old EMCO Maximat Mentor 10x30. I don't mind doing the operation on the lathe whatsoever. I just haven't done a tap this large, nor used a lathe to do so. Not sure how to do it on a lathe either, nor what you suggesting? I have done a ton of taps by hand, just never on a lathe or mill.

            When you offered you have a bport, I thought you were going to power tap it on the mill, something that is very handy and done all time. BUT a big tap like that takes a lot torque to turn. If something takes a lot torque, you want to be careful about subjecting a machine to that torque - spindles, collet's, drill chucks etc all can/will be damaged if the torque is too great and something slips. Thinking you were planning on tapping it in the mill, I was suggesting the lathe might be more solid. With a hefty chuck holding the work and the tap handle against the carriage and centred by the tailstock the setup can take a lot more torque than a tap in an R8 collet

            Bills method is excellent, just turn the lathe over by hand. Your's is a quality lathe on the lighter side and we want to keep it that way. I'd get it started as Bill describes and when I knew it was going straight I'd probably finish at the bench just because its quicker. You could probably do it under power, but why over stress things (unless you have 100 to make!)?

            Tapping by hand absolutely works as well. I use short cylinder of metal drilled and faced as a tapping guide and away one goes....but on a large size like this its harder/trickier/more time consuming to get the tap started straight by hand; a little big for a tapping guide and both hands are needed on the square. Of course its doable, but starting them in the horizontal tap starting fixture that's just sitting there (lathe) is easier.
            Last edited by Mcgyver; 09-18-2010, 11:24 PM.
            in Toronto Ontario - where are you?


            • #7
              I'd probably finish at the bench just because its quicker.
              I'd completely forgotten/overlooked that you were gonna be doing three inches! Doing that turning the lathe chuck by hand with a chuck wrench -- well -- it'd be better to finish up on the bench... And like Mc says that size tap and at that depth and the coarse thread --- it'll be a workout on the bench...
              Last edited by Bill Pace; 09-19-2010, 12:08 AM.
              If everything seems to be going well, you have obviously overlooked something........


              • #8

                I agree as regards cast iron being easy to tap - very.

                A cheap carbon or "tool steel" tap will do it but a good HSS taper tap is better. The taper tap will self-centre and is easier to "drive".

                Do be sure that the tap is not only long enough but that the shank of the tap is not larger than the tapping size of the tap - so that you can use its full length.

                I don't like tapping in the lathe. If the tee-bar handle is resting on the saddle or the lathe bed it causes a very unbalanced load.

                I prefer all-hand tapping as I can "feel" how the tap is going and if there is likely to be a problem. I have power tapping on one of my mills as well as two very good tapping heads and I rarely use them unless there are a lot of holes to tap.

                I prefer to tap under a drill press (pedestal drill is fine) or a mill (vertical mill and mill-drill are fine.

                I put a scrap bit of steel with a 60* point on it (same as a lathe centre) in the drill chuck.

                I use the centre hole in the end of the tap or tap-wrench ("T" type) or the four sides of the jaws in a "bar" type tap wrench.

                I centre the job under the mill/drill quill (a bit of wire in the drill chuck is good enough), put the tap in the tap wrench, put the tap in the hole to be tapped, lower the drill quill until the 60* point engages the tap or wrench, hang a weight on the drill handle and start tapping. The weight will put enough thrust on the tap to start it and keep the point engaged and keep the tap centred. The weight will allow the quill to rise and fall as the tap is reversed from forward>back>forward etc.

                I've done it this way since forever - and it works well. I've never need a tapping guide either for this sort of work.

                The work needs to be pretty well secured or bolted down to the mill/drill table or in a lathe chuck that is clamped to the mill/drill table.

                This is a spring-loaded version of my "60* point". I bought it from for the princely sum of US$10. Its worth all that sum and more.

                search for item/part number 1963 at:


                • #9
                  A long time ago-----

                  I was asked to tap some giant steel "C" clamps. From memory they were flame cut from 2" plate. The threads were about 1 1/4" dia The lathe is an ancient flat belt job ex Montreal Loco Works ( About 4' swing)and is driven nowadays by an hydraulic motor. The tailstock quite definitely does not really accurately point at the headstock, nor ever will again.I made a holder that fitted the #5 morse, it held the taps with set screws on the squares and had a couple of loose( about 10 thous down) changeable guide bushes for the various size shanks of taps available. The holder was stopped from rotating by a bar through it sliding on the bed. I think I got about 50 of these things done in a long day, the hardest part was loading them into the 4 jaw.The nominal drill size gave a hole about 1/16 over and that gave about 70% thread. Plain hydraulic drive, especially where the pressure is provided with a gasoline engine with worn out governors, is exciting! Success came from the very slow speed the lathe has ( Triple backgears about 20 rpm to tap with) and LOTS of cutting oil. Trying to hold the tap firmly in the ancient tailstock chuck,which itself was out a lot merely resulted in very poor threads or broken taps( only broke one when testing, good job the washroom is near!) Hope this is of interest, David Powell.


                  • #10
                    Originally posted by Smokedaddy
                    ...I need to bore a hole in a piece of cast iron that is 3" inches long and 3" in diameter for a 5/8"x11 thread.
                    Why does the threaded length need to be so long? It won't give you any extra strength and just makes the tapping more tedious. Have a little look around under the theme of 'thread engagement' for the science.

                    Or use a bit of common sense and consider a 5/8" nut and bolt (grade 8?). How deep is the nut in terms of of the bolt diameter? Make an assumption for the relative strength of your cast iron female thread (50% is conservative) and proportion accordingly.


                    • #11
                      A long thread

                      I occasionally - here included - see mention of "power tapping" using a mill or a lathe or a pedestal drill.

                      Most of it refers to using what is a "hand tap" (of which there are usually three - taper, intermediate and "bottoming/plug" - taps which are intended to be used by hand with the progressive "turn forward - stop - reverse - forward" etc. procedure.

                      Quite often tapping using power works with "hand" taps - but not always!!!

                      There are a range of taps - "power" and "gun" taps included - which are made for powered tapping - some "straight through in one plunge/go" and others with a reversing power tapping head.

                      Here is a guide:

                      I use spiral/gun pointed taps in my tapping heads.

                      Tapping heads are made to run continuously so that the motor on the drill is not reversing or starting under load. Both of my tapping heads are the reversal type. The smaller one does not have an adjustable "slipping clutch" but the larger one does.

                      So - I suggest that in the interests of safety that you tap that job by hand using the "60* "point" that I referred to earlier.

                      The length of that tapped hole seems very long. Is there a reason for it? Can it be shortened with the rest of the hole being a 5/8" clearance hole?


                      • #12
                        I'd second the notion that you may not need 3" of thread length if the function is maximum strength in bolt holding. An inch of thread engagement with the rest clearance hole would be more than enough.

                        If you need the full 3", say for an adjustment mechanism, then buy the long tap.

                        Not sure it was noted -- don't use cutting oil while tapping cast iron.


                        • #13
                          A standard 5/8-11 tap will get you within one thread of your three inch depth... a little deeper if you grab the tap by just half the square drive for the last turn or two.

                          Tapping is no black art. Start it on a machine - whichever machine you feel comfortable with. Once the tap is in far enough to ensure that driving it by hand won't set you off course then run it in the rest of the way by hand with a tap wrench (really at that point you could use an adjustable wrench).

                          Your chips will be falling out the bottom of the hole as you go, so there's no need to even worry about that. I say don't sweat it, just run the tap in straight and you'll be fine.


                          • #14
                            Ditto on it should'nt have to be so deep. Alternatives include just using a nut, Or clearance drilling the first 2 or last 2", 1" should be plenty of thread

                            PS: When we say use the lathe or mill, we do not recommend at any point turning the power ON, The lathe/mill methods are strictly used to align the tap precisely, because if not aligned precisely the tap WILL break when trying to tap such a deep hole.. Even shallow holes if its not aligned precisely its very bad for the tap. (The tap will NOT self correct and will just keep trying to thread/cut a hole along the axis its started at)

                            PPS: While some say they do power tap with the lathe/mill (ie apply power), they have learned to do manual taping first and know exactly whats going on, And 99% of the time use 'spiral point' taps on throughholes only, These taps can be easily power taped without backing out, other taps will fill up with chips and jam/snap.
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                            • #15
                              i agree with whatever

                              whatever makes you feel safe and comfy. just my $0.02 here.
                              i feel, if you blow that tap drill through on a lathe (if the lathe can handle it)for example, i'd place the tap (i love gun taps) in that same tailstock chuck and loosen the rail clamp. w/ oil on the rails so it slides nice and smooth. of course, run the cross slide. under the headstock chuck,out of the way. with the quill cranked out and if you can find cherry oil (very good on cast). under low speed power, push the tailstock, gentley to the piece and let it bite.keeping your hand on the foreward and reverse. if it makes you nervous, just reverse the spindle and do it in a peck fashion. make sure tap is sharpe and you should be good to go. done this stuff so much, it's actually boring to me. no punn or science intended here. again, whatever