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Anyone turn your own crankshafts? on a lathe?

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  • Anyone turn your own crankshafts? on a lathe?

    I am pretty good at building chevy motors.. Problem is the machine shop costs eat me alive. I got a 55 gallon barrel to soak them out in, Just wondering if anyone knows what the tooling to set a crank up on a lathe looks like? Got any pictures? The local guy who used to do mine had a belt sander with a stainless strip, I wasn't interested so I didn't pay any attention to the jig to hold the crank.
    Mostly, I think 305-350 chevys.. and 5.0 fords.. hate to diversify too much from the motors I know.

  • #2
    The ones I have seen use a big grinding wheel.


    • #3
      A crankshaft grinding lathe is a pretty large and involved chunk of machinery.

      If your lathe can swing enough, you could get away with working just the mains on a regular machine, assuming you compensated for droop and sag.

      But to do the throws requires some wild equipment- The one I saw essentially had a large faceplate with a movable head that slid sideways on a micrometer track kind of like a boring head. On this slider was a 3-jaw chuck which held the nose of the crank. The "tailstock" end had something similar but with a "center" rather than a chuck.

      With a little care and an understanding of how the adjustments worked, one could center any of the throws just like you dial in a part in a four-jaw. I wasn't familiar with the procedure but there has to be something about dialing both ends, so the crank isn't twisted.

      More importantly, one doesn't just turn the throw with a tool like machining a bar of cold-rolled. Surface finish is critical on cranks, and they're almost always done with a grinder. The wheels on the unit I saw were fully two feet in diameter in order to clear the throws.

      The "belt sander" you saw was very likely a final-stage polishing process.

      Doc's Machine. (Probably not what you expect.)


      • #4
        The crank grinder that I have watched work was made by Landis and it too had a very large wheel that turned fairly slow. The guy that ran it had been doing it for as many years as I had been alive.



        • #5
          I served my time running big Churchill crank grinders grinding the cranks on Rolls Royce Merlins.
          These took a 2" wide by 30" diameter grinding wheel. They had hydrostativ head bearings and it took at least 1/2 an hour to get these warmed up enought to get them to run true.
          All seven mains were ground at the first setting with wooden finger steadies on either side of the main being ground.
          Next the six throws were ground in three lots of two pairs.
          To get these to run true the centres holding the crank were wound down on a set of slides so the whole crank was out of centre but the two throws were clocked up true. The next two sets were done at this same setting but the crank rotated in the jig to bring the next set in line. When the throws were being ground you had to fit balance weights to the driving and tailstock ends to compensate for out of balance forces. These were very old machine but still deadly accurate.
          Final polish was put on with 1000 wet and dry paper run wet in a massive pair of wooden nutcrackers. Crank ran at about 50 to 70 revs and you stood there holding these nucrackers until you had a polish you could see your face in.

          Later on I saw a modern smaller crank grinder what worked by some elaborate compensation mechanism whar the crank always ran true. The wheel head advanced and backed off as the throw was rotating so the wheel was always following the diameter.
          I spoke to the guy using it but he wasn't as keen on it as the older machines but it was quicker to setup.

          As for doing grinding them on a lathe - forget it.
          Polishing is OK

          John S

          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


          • #6
            Droop and sag.. I guess that explains why the pictures I have found of Model A crankshafts being ground were spinning on thier end. (a good challenge to engineer?) So.. Spinning would complicate matters with inertial load.
            My large old dusty 24 inch 4 jaw lathe I think can swing about anything I put on it. The 2,000 dollar grinding wheels, well.. I'll have to figure a way around that. I am hunting something to fix the crank so setup won't have to be done on each rod journal. The mains.. well no problem. I too have made clothes pin type polishers with tacked in emery cloth.
            Spinning homemade indexers? in a adjustable dovetail slide? On each end? Perhaps I need to measure the droop on a v8 crank?

            [This message has been edited by ibewgypsie (edited 04-20-2003).]


            • #7
              Several months ago I posted story about when we broke down in Scotts Bluff Neb in maybe 1938- 40. Rebuilt the engine under a sign board. The engine used shims to set bearings clearance. Man came out and polished the crank in the engine. Had a machine that mounted on top of the block, sanding belt ran around the journal, top was on springs so it could follow the throws. The engine was rotated and belt was moved at same time. I do not remember how the belt was joined to move from throw to throw. It took out the marks that dad was afraid would destroy the bearings. The machine was used to re-do farm equipment on a routine basis. I was under 10 years old so memory may be faulty. But it was done. I doubt it would meet modern specs- for one thing rods were probably babbited. I know as late as 1955, many shops had machines to cut inserts to size- which would not work on modern multilayer bearing materials.


              • #8
                Hi guys.I need an excuse to buy a digital camera and learn to post pictures.If enough of you want to see them I'll post pics of my crankshaft grinder and the steadies,dressers,cross slides on the chucks(they move in two directions) etc...Of coure I'll have to clean it up really good so it won't look so bad.A video would be better but I don't know if I can pull that off.Thanks,Robert.


                • #9
                  I remember seeing a crank turning attachment for South Bend lathes in one of their old advertisements. The attachments didn't look too complicated. You could probably fabricate something. But it looked pretty involved to set the crank up in it.


                  • #10
                    You might be able to get around buying a 30 in. wheel,by using an abrasive belt setup. I've seen crank grinding lathes go pretty cheap at auctions.


                    • #11
                      Not necessarily. The belt can't always assure concentricity, and typically can't be set to grind a rolled fillet where the throw meets the counterweight.

                      Belts would be fine for final finish/polish, but I sure wouldn't want to turn a crank down .010" with one.

                      Doc's Machine. (Probably not what you expect.)


                      • #12
                        Instead of grinding you can hard turn the journals and throws with CBN inserts. You need a very ridgid machine for this.

                        The crank can be milled in a CNC mill with a 4th axis table.

                        In europe production cranks are made from hardened forgings in two steps - a roughing mill with an oscillating "D" shaped cam that has hundreds of inserts along its arc and then hard turned to finished shape with CBN - no grinding required.

                        No easy way to do it quick without CNC.


                        • #13
                          John S:

                          When did those things go out of production? I sort of assumed they were gone by about the time of the Korean war.

                          I suppose spares might have been made for quite a while though.


                          • #14
                            Here's a photo of a crankshaft grinder like one I ran a long time ago. The head and tail stock centers were adjustable to center them for the main journals or offset them for the crank pins. The large wheel at the front center moved the entire bed including headstock, tailstock, and crank from side to side. The crank to the left of the wheel moves the grinding wheel into position, the small wheel at the top to the right of the grinding wheel is the fine feed. The two knobs in front of the crank are a steadyrest. there is a dial indicator to the left of the grinding wheel with three jaws that clips on the journal and shows your progress as you grind.
                            THAT OLD GANG 'O MINE


                            • #15
                              I know someone who did made a crank on his lathe - OK it was a small, flat plane, 4 cylinder & 2 main bearing crank but it was a crank.
                              His comment was that everyone has a crank in them - that was his crank, and he wasn't going to do it again!
                              Fifteen years later it's still going fine.