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Machining steel after torch cutting

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  • Machining steel after torch cutting

    I am making some 3" discs out of 1/2" plate. They don't have to be 3", just close. I have a sheet of 1/2 plate and I am cutting circles out with an oxy acetylene torch. I have tried to machine after letting it air cool and while it was still fairly warm. ( I have tried on lathe and mill). The steel has some very hard spots in it that will tear up an end mill or a HSS lathe bit in short order. Once I finally make it through those hard spots it machines fine. I know this is not the ideal way to do this, but it's what I have laying around. As you can tell, I am in no way a machinist. The machines I have to work with are an old round ram Bridgeport and a Craftsman(Atlas) 12x36 lathe. Any ideas on a way to make the steel easier to machine?

  • #2
    It sounds like you're getting some local hardening with the torch heat and rapid enough cooling. The solution would be to anneal them. If you've got a wood stove or fireplace, put them in the coals of a good hot fire, then let it burn down and out overnight. Basically, the steel should be brought up to transformation temperature (good red hot) and cooled slowly. You might be able to do it with the torch if you lay them on a bed of ashes for insulation, heat them up red, then cover them to cool overnight.
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill


    • #3
      Grind ALL of the mill scale off. Flame cut steel is hard at the cut as you found out. Once you grind it ,it will cut quite well. Jim
      JIM : You don't get a 2nd chance to make a 1st impression.


      • #4
        Kingweld is right. Get thee an angle grinder, Harbor Freight has 'em sometimes for $20, and take off the heat affected zone.

        I also flame cut and have an Atlas lathe and this works just fine for me.


        • #5
          I have an angle grinder. I will try that. Thank to all for the replies.


          • #6
            Carbide inserts work for me. Run at medium fast rpm and deep cut but slow feed. Try to cut under the torch cut part.


            • #7
              Maybe try trepanning (sp?) the circles instead of torching them out.

              4 Jaw chuck + Squarish plate + cutter = Round Disc



              • #8

                Keep in mind that the OP has an Atlas lathe, which has about as much mass as a 6-foot sub sandwich and ways that are about as rigid as a bamboo scaffold. A cut of even .050 at any speed is asking a lot from this light-duty machine. Turning a 3-inch dia. disk at a speed and feed appropriate for carbide would bog mine to a standstill.


                • #9
                  If they don't have to be close just use a hole saw on the mill


                  • #10
                    Hey, gregl, you might want remember where you are "Home Shop Machinists" not a Big Pro machine shop forum. He has a bigger and better lathe than the average home shop has and just needs a litte friendly advice. Not cut down for his equipment. If you was half the machinist you think you are you would have a decent answer for him. If not your on the wrong site.

                    I would rather have tools that I never use, than not have a tool I need.
                    Oregon Coast


                    • #11
                      What the others have said.
                      I usually drill a hole in the center for a bolt and nut, then stick the bolt into a drill, mount the drill between a couple of pieces of wood in a vise, and run at slow speed. With an angle grinder cutting the heat affected zone while the drill is spinning it hardly requires a visit to the lathe if dimensions are not critical.
                      If a through hole is not wanted simply tack a nut to the center of the disk, you can knock it off later.
                      Home, down in the valley behind the Red Angus
                      Bad Decisions Make Good Stories​

                      Location: British Columbia


                      • #12
                        What's worked for me (repeatedly) is Step 1. to use a circle cutter with the torch - this cuts down (no pun intended) on the amount of slag, etc you deal with in the next step. Step 2 is to use the angle grinder (or any other rough grinder) to remove all the cutting dross and slag down to clean steel, even if you have to grind a groove to get it all. An interrupted cut is easier on your tooling than the slag or cutting dross will be.

                        Then mount in the lathe and start the clean up. Your lathe will tell you how heavy a cut to take. The first few passes should be done wth light cuts and under fine power feed. The interrupted cut should "tick" rather than pound or slam. Patience in prep and machining is the key.

                        If there are hard spots after all the cutting dross is removed, then annealing as posted above is in order. Regardless, you will not regret the time spent with a grinder before taking to the lathe whether you have to anneal or not.

                        My $.02 cdn.

                        P.S. I'd post pics, but they're on a different computer, sigh.
                        Design to 0.0001", measure to 1/32", cut with an axe, grind to fit


                        • #13
                          lugnut, your out of line with that statement because he is giving him honest advise. What good is it to lie to him and tell him he can do what his lathe probably can't.

                          gregl, is right, his lathe may not be able to cut .050" DOC to get under the hard part of the disc. What he needs to do is burn them out slightly oversize and grind the burned area away.

                          HSS tool bits get eat up by cutting torch surfaces and carbide is the best thing to use but you have to get under most of the surface burn. Using insert tools is not good because he would destroy to many inserts. the best thing is brazed carbide cutters and resharpen after each pass on a small lathe.

                          You don't have to run carbide fast, just not have interrupted cuts.

                          What I want to know is how is he going to hold the 3" disc in a chuck and turn the outside of it? Has none of you seen the problem there? I have a way but it requires welding the disc to a stub shaft.

                          Long ago I made an attachment to lay the cutting torch in with a point to set in a center punch in the center of what I want to burn. With that I can burn almost perfect circles when everything goes right. It's called a circle cutting attachment.
                          Last edited by Carld; 02-05-2010, 12:06 AM.
                          It's only ink and paper


                          • #14
                            Flame cut the circles bigger by .25 inch on the radius. Set your tool bit so that the front, leading, cutting edge is set at a negative rake angle, (tip trailing), and take a deep enough cut so that the tool tip is in the soft, (i.e. not flame hardened)part of the steel, using a slow spindle speed and hand feeding. This keeps the tip from getting buggered up by the hard spots which are better absorbed by the full leading edge. I use those cheap brazed cemented carbide tool bits in a C-5 grade, (Enco 3/8 X 3/8 X 2 1/2 @ about $1.50 each).


                            • #15
                              Agreed, Flamecut, Grind (Draw a circle on the part so you only grind whats needed) and then mill to perfection (If even needed! grinding to a line can get supriseingly accurate if a little bit of time is taken)

                              Just use the tip of the 1/4" grinding wheel like a cutting disk to do the roughing (higher angle = fast stock removal, 90 degrees = fastest), then use it at a less agressive angle to smooth it out once you get near the line.

                              PS: using a peice of stock as a clamp in the headstock(chuck), and a cover for your tailstock live center, can produce a 'disk clamp' as I like to call it, letting you turn material into a disk without welding to it or cutting into it or drilling it. I used it to make some UHMW cup coasters, One problem however is it has next to no drive torque, so anything more then thin cuts will stall the work (on plastic, it just stalled and would resume as soon as you removed the cutter, never flunge the work out, but that was a LIGHT peice of plastic, steel might get flunge out when you hit too high a cutting force, stay OUT OF LINE with the work, and maybe cover your ways!)
                              Good news is your passes are incredabley short length, and with the right tool you can turn left and right, so each pass is only seconds if your manualy carriage feeding (Not worth the time to flip the levers!)
                              Last edited by Black_Moons; 02-05-2010, 12:18 AM.
                              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.