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  • Gear Cutting, yes or no...

    Hi all,

    Ok, so i'm building a large (half-size) traction engine at home, i will soon need to cut or get cut the drive gears. What i want to know is, using my current equipment - Bridgeport and two lathes, is there any conceivable way i can cut these gears myself, approximate details:-

    495mm dia with 76 teeth,
    431mm dia with 66 teeth,
    361mm dia with 55 teeth,
    all blanks are 28mm thick and cast iron, teeth are 4DP. There are others but they are much smaller.

    I am trying to do as much of the work myself, quotes are coming in quite high so there is a good reason for the DIY approach.

    Can anyone shed any light on a possible approach, or is it a definate no-no?

    Thanks in advance,
    Dave
    If it does'nt fit, hit it.
    https://ddmetalproducts.co.uk
    http://www.davekearley.co.uk

  • #2
    Do you have a rotary table or a spindexer?
    Free software for calculating bolt circles and similar: Click Here

    Comment


    • #3
      Dave,
      Email sent.

      [edit] forgot to say yes, Time Leech did a starter ring gear in steel at 12 DP about 22" in diameter on a Town Woodhouse, similar to a bridgy.
      Last edited by John Stevenson; 06-24-2009, 09:59 AM.
      .

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



      Comment


      • #4
        Its a "biggie"

        Dave,

        they are big teeth and big cuts.

        The biggest 4DP gear with 76 teeth has a pitch circle diameter of 76/4 = 19" which has a circumference of 19 x 3.1416 ~ 59.690" with teeth spaced 59.690/76 = 0.785" along the pitch circle and assuming that the width of the tooth and the cut are equal then the cut width ~ 0.785/2 ~ 0.393".

        The depth of cut, neglecting clearance at the bottom, is 2 x 1/DP = 2 x 1/4 ~ 1/2".

        So each gear cut is about 0.400" wide x 0.500" deep.

        That is going to be a big cut if done in one pass - even in cast iron. But as it is in cast iron, I'd guess that 3 or 4 cuts per tooth would be OK - with a "store-bought" good HSS cutter.

        As the mill is a BP, I'd guess that you will be using a stub arbor with and R8 taper which is a cantilever as it will be unsupported at its outer end.

        On that larger gear, because of its 495mm (~19.5") diameter, your dividing head or rotary table and tail-stock centre will need to be at least 19.5/2 ~ 9.75" plus "a bit" (for clearance) ~ say 10" above the table - so a "spacer" may be required.

        The larger gear will be the hardest, but I'd guess it would be do-able.

        As I don't have and have never used a BP or a clone of one, I'd have to leave that to those who can answer those questions for you.

        Don't take my calculations as "gospel" yet (or at all??!!) until some others have done the checks.

        It is certainly not unknown at all for me to cock up a calculation when it needs to be right!!.

        Do have a look at the current thread on mandrels and arbors (for mounting your gear blank for milling) at:
        http://bbs.homeshopmachinist.net/showthread.php?t=35304

        I just noticed John Stevenson's post.

        Comment


        • #5
          Thanks so far guys,

          Yes, i have a 12" horizontal/vertical rotary table that i could likely fit the dividing plates off of my smaller table onto as the ratios are the same.

          Dave
          If it does'nt fit, hit it.
          https://ddmetalproducts.co.uk
          http://www.davekearley.co.uk

          Comment


          • #6
            Originally posted by John Stevenson
            Dave,
            Email sent.

            [edit] forgot to say yes, Tim Leech did a starter ring gear in steel at 12 DP about 22" in diameter on a Town Woodhouse, similar to a bridgy.
            John, no it was on my old Elliott using the horizontal spindle.
            I can picture one snag being the cutter overhang when using the vertical Bridgy spindle, arbor supported one end only.

            Do you have a cutter?
            Tim

            Comment


            • #7
              Originally posted by Timleech
              John, no it was on my old Elliott using the horizontal spindle.
              I can picture one snag being the cutter overhang when using the vertical Bridgy spindle, arbor supported one end only.

              Do you have a cutter?
              Tim

              No, not yet, in fact i've never cut a gear in my life although i have wanted to for some time to further the education a bit.

              Could i not brace the lower end of an arbor back to the knee ways on the column?

              Or would a horizontal bridgy attachment be any better. I have even thought about getting a horizontal miller for doing it on.

              Dave
              If it does'nt fit, hit it.
              https://ddmetalproducts.co.uk
              http://www.davekearley.co.uk

              Comment


              • #8
                If I had to do this job I would use the Bridgeport horizontal attachment with the arbor support that fits in the ram dovetail. I would mount the wheel blank horizontally on the rotary table and with a support between the rim of the blank and the machine table (a long bolt screwed into a T nut with a lock nut would do) as near to the cutter as possible, raise the knee to apply the cut. The will be a tedious pain in the arse to do but should be quite rigid.

                Malc.

                Comment


                • #9
                  Originally posted by Davek0974
                  No, not yet, in fact i've never cut a gear in my life although i have wanted to for some time to further the education a bit.

                  Could i not brace the lower end of an arbor back to the knee ways on the column?

                  Or would a horizontal bridgy attachment be any better. I have even thought about getting a horizontal miller for doing it on.

                  Dave
                  Here it is, the pics are still out there although I gave up the ISP a couple of years ago







                  It's a flywheel of about 150 to 200 kg, rotary table hooked up to my CNC Bridgy but the job is on a manual mill.

                  Tim

                  Comment


                  • #10
                    Thanks for the pics, it looks so much easier on a horizontal machine


                    Its lloking quite do-able so i'll look into the setups a bit further i think.

                    Dave
                    If it does'nt fit, hit it.
                    https://ddmetalproducts.co.uk
                    http://www.davekearley.co.uk

                    Comment


                    • #11
                      Davek, those are large gears to be cutting on a mill, especially if you are using castings and are inexperienced at gear cutting. That said, it can be done if you are patient and careful.

                      Before you jump in, contact Phil Goetz somewhere in Michigan. He has a bunch of gear cutting machines, and has done gears for many years before he retired. Send me a PM for his phone number if you want it. He used to advertize in Gas Engine Magazine, and has done a good bit of gear work for me, and at very reasonable price I might add. I'm pleased with his work.

                      I don't get anything for recommending Phil.

                      Put up some pics of your project as you progress. It's always an inspiration to see the process of a big project.

                      Neonman

                      Comment


                      • #12
                        Profile

                        I don't know if your BP has sufficient spindle to table clearance or not.

                        If I were doing it I'd certainly be considering doing it with end milling cutters - possible self/home-made as it should be a much better set up with maximum rigidity and none of the "cantilevered arbor" problems.

                        The clearance required would be at least 20" (see previous) plus the cutter protrusion from your (presumed) R8 collet.

                        It is not difficult to draw that gear profile either by hand on a drawing board or in CAD and print it out. The "drawing" can be used as a "master" by cutting it out very accurately and using it as a template to grind your tooth-cutter to.

                        As the gears are cast iron where a zero top rake (aka "hook") angle is required, it should be OK - ands it is easier to sharpen and to maintain the profile.

                        The cutter only needs to have one cutting edge. I'd make it out of the shank of a HSS milling cutter.

                        A separate cutter will be required for each gear.

                        I'd run several preparatory cuts with end mills so as to minimise the load on the "home-made" cutter which should be left until the final cut. There is no real reason why you can't cut/finish one final cut/side at a time

                        I'd guess that the slight errors would be of no real consequence as it I recall correctly, those gears are made to operate in the open air with low speeds and low loads. Provided that there was neither too little nor too much "back-lash" they should be OK. Depth of cut is not too important but the finished width of the teeth at the pitch circle is important as that more than depth of cut will determine your "back-lash". There is absolutely no need for "gear calipers" (unless you have a set) as it can be quite well measured accurately enough with your every-day set of vernier or digital calipers (no need for "wires" either). I can work it out for you if required.

                        Given a choice, I'd mount the larger gear/s directly onto the face-plate of my rotary table for rigidity and the minimise "flutter" or "chatter" on a mandrel/arbor. Leave the arbor/mandrel until its needed for any gears that are a lesser diameter than that of the face-plate of your rotary table.

                        The rotary table should be "stiffened" or "backed up" by an angle plate or struts back to the mill table as the rotary table is at its weakest when set vertically.

                        I don't know what the gear pressure angles are - as that may well need to be a consideration as well.

                        Neither do I know if the gear profile needs to be a pure involute or hypo-cyclic or a mix as is the case with the shop-bought cutters which are involute at the pitch circle and epicyclic and hypo-cyclic above and below the pitch circle. It sounds worse than it is. I'd probably settle for an involute form.

                        If you have the head-room and the rotary table and are willing to make or have the end-milling cutters made, you should be OK.

                        Even if you do use the dividing plates for indexing the cuts, I'd suggest printing out the actual angular settings "just to be sure" and as a check-up that your indexing is correct. Its all too easy to either not set the indexing correctly in the first place or to "bump" the indexing arms etc. and to "get lost along the way".

                        For short runs, I use the print-out and set it on my hand-wheel and vernier and don't bother about the indexing arms. I use the indexing arms for longer runs.

                        DO remember to always approach the cut from the same direction on your rotary table (to counter back-lash in the worm and wheel in the rotary table) and to ALWAYS clamp the rotary table when taking a cut as it too adds to to better end result and rigidity.

                        I am NOT "sticking my oar/nose in" - just trying to help. If I am intruding or being a smart-ar$e - just tell me to "rack off" - and I will understand and do it - gladly.

                        Comment


                        • #13
                          Making "out"

                          Originally posted by neonman
                          Davek, those are large gears to be cutting on a mill, especially if you are using castings and are inexperienced at gear cutting. That said, it can be done if you are patient and careful.

                          Before you jump in, contact Phil Goetz somewhere in Michigan. He has a bunch of gear cutting machines, and has done gears for many years before he retired. Send me a PM for his phone number if you want it. He used to advertize in Gas Engine Magazine, and has done a good bit of gear work for me, and at very reasonable price I might add. I'm pleased with his work.

                          I don't get anything for recommending Phil.

                          Put up some pics of your project as you progress. It's always an inspiration to see the process of a big project.

                          Neonman
                          That's good advice and caution Neonman.

                          I've always had my "better" gears made "out" by a specialist as they have the very expensive tools and metrology to "get it right". Costs - as expected - were "high" but in my case, well worth it, as the results were first-class.

                          I merely pointed out a possible "Home Shop" option for the OP to consider.

                          Comment


                          • #14
                            Thanks Guys, good advice.

                            It's looking "possible" but full of difficulties. I will look into it further as soon as i get the mill clear.

                            I am in the UK so that compounds the troubles more as we have bugger all access to affordable companies any more that are willing to even talk to HSM'ers like me

                            Yes the gears run in the open with very little cover, they could likely even be hand cut and filed if i had nothing else to do for a couple of years.

                            Dave
                            If it does'nt fit, hit it.
                            https://ddmetalproducts.co.uk
                            http://www.davekearley.co.uk

                            Comment


                            • #15
                              Get slotted!!

                              Dave,

                              I had a look at your web sites - great work!!

                              Another option which should be a lot easier would be to use a single tool in a good solid slotter - aka vertical slotting machine - (like a vertical shaper) with a good big rotary table on the slotter table. It is a solid as hell and solves most of your concerns. They are absolutely marvelous for internal keyways and internal gears etc. and like shapers, they are "tops" for male and female dove-tails. If I had a chance at both but was limited to either a shaper or a slotter - I'd take the slotter.

                              I recall one that would be just the right machine in an old pic of John Stevenson's shop.

                              I'd suggest PM-ing or faxing John Stevenson as he will not only know all about that machine of his but where there might be others that you could see or use.

                              Are there any slotters in the shop's of hobby club members?

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