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  • Normal day at the office.

    So about two weeks ago I was sent a short piece of brass rack 3/8" x 7/8" by 24 DP and asked if I could get a 800mm long piece [ 31 - 1/2" ]
    Shopped round but not in this size and length so said I'd have to make a length. It also had to have a 2 - 1/8" rad on the back as it screwed to a mast to raise and lower an arm in a ultrasonic crack detector.

    They would send the mast down later for fitting.

    Ground a simple D bit up and using a long clamp arrangement on the CNC mill table I did the rack in two passes and waited for the mast before doing the back radius in case it differed from the sample piece.

    So today the mast was delivered but it stood higher than me, looked on the side and it was tapped all the way up.
    rang them to ask where on the mast did they want the rack fitting ? Top, middle or bottom ?
    Reply was it needs to go top to bottom, Uh ? rack is 800mm mast is 1300mm.

    Oh well start again........................



    Close up of the teeth being cut.



    Over view of the whole rack before moving up in the clamp.



    Over view of the radius cutting.



    Close up of the radius cutting. Using the standard generic boring head to get the radius.

    If you set a boring head vertical and say 2" diameter and take a facing pass, provided the tramming is correct on your mill you will get a flat swathe 2" wide.
    Now set the head so the boring head is horizontal and if you were able to take a facing cut you would get a 2" diameter groove.

    So it stands to reason that at a given angle and radius you can get any radius you need. Just off vertical and you have a radius approaching infinity.

    There are formulae to work all this out but in typical baler twine for boot laces mode, I guessed at 25 degrees and adjust the diameter of the head so it scratched to sample part all over.

    No doubt Evan can Google us up the formulae, correct to 236,453 decimal places. <g>

    Too lazy to move the jig over from the CNC to the Bridgy so just did about 7" at a time and moved up.


    Finished rack.
    http://homepage.ntlworld.com/stevens...iles/rack5.jpg
    .

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




  • #2
    Sir John, tho I'm just a rank amateur, and thus more easily amazed than many I'm sure, you never cease to amaze me!

    You're worth the price of admission here, all by yourself.

    Lynn
    Lynn (Huntsville, AL)

    Comment


    • #3
      Beautiful work John,pity it's all for naught,hopefully they will aknowledge that it was their mistake and not yours.
      When did you clean up the shop...is that daylight that I detect through the doorway?
      Home, down in the valley behind the Red Angus
      Bad Decisions Make Good Stories​

      Location: British Columbia

      Comment


      • #4
        Interesting job. It was generous of the customer to provide you with the opportunity to make a practice rack.

        I think Marv Klotz has the calculations for radius generation to save Evan some Google time.

        I can't help but notice how you have tidied up your shop.
        Jim H.

        Comment


        • #5
          John,

          I won't bore you with the equations. However, the next time you want to do
          this, download the RADIUS program from my site and it will do all the
          calculations for you. As an example, here's a partial output from the program
          using your dimensions.

          ==========================================
          Allowable error in desired radius [0.01 in] ?

          You must specify either the depth or width of the cut.
          If you don't know the depth, enter zero and the width will be requested.

          Depth of cut [0 in] ?
          Width of cut [0 in] ? .875
          Depth of cut = 0.0455 in
          Width of cut = 0.8750 in

          Minimum inclination angle = 6.3007 deg
          Cutter diameter at minimum angle = 0.8762 in

          Cutter diameter to use must be greater than 0.8762 in and less than 4.2500 in
          Cutter diameter to use [2.56312022 in] ? 2

          Angle from vertical to cutter axis = 26.8537 deg

          Using Steve's second method, we get, as a check:
          Angle from vertical to cutter axis = 26.8537 deg
          (Steve's first method yields 28.0725 deg)

          Resultant error in radius when using 2.0000 in diameter cutter = 0.0005 in
          ==========================================

          As you can see, your guess of 25 deg was pretty damn close.
          Regards, Marv

          Home Shop Freeware - Tools for People Who Build Things
          http://www.myvirtualnetwork.com/mklotz

          Location: LA, CA, USA

          Comment


          • #6
            Yes, Impressive set-up!

            Comment


            • #7
              Most excellent work John!

              I don't see a problem with butting a shorter rack up to your 800mm rack to achieve the desired 1300mm length. The pinion gear won't mind a bit

              Tess

              Comment


              • #8
                Marv,
                Thanks, never though to check your page, you never cease to amaze me what's on there.
                So 25 was close, it's uncanny just how good baler twine is

                JC.
                Not really tidied up but it has been windy here.

                Tess,
                Problem is the end user on this jig is Rolls Royce so even though a butted rack will work OK they won't like it.

                .
                .

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



                Comment


                • #9
                  A man can learn a bit hanging around here...

                  I should be able to log time on this site towards my continuing educaction credits.
                  James Kilroy

                  Comment


                  • #10
                    John,

                    You do know I presume that the cutter doesn't produce a true circular section when tilted? It will be an elliptical section which shouldn't matter in your application but if it does matter it is good to be aware of it.


                    Projection of the sections produced between vertical and horizontal:
                    Free software for calculating bolt circles and similar: Click Here

                    Comment


                    • #11
                      Well done as usual John I have a meddings drill which I have just relaced I use in my wood shop as it didn't have a rack. I got hell for not making my own from Thrud however I simply did not know how to,but you seem to make everything look so easy to me anyway, although I am sure I would botch it unless I saw it being done first.I cannot understand why meddings which has such a good name made a model with no rack anyway good job pal regards Alistair
                      Please excuse my typing as I have a form of parkinsons disease

                      Comment


                      • #12
                        [quote=Evan]John,

                        You do know I presume that the cutter doesn't produce a true circular section when tilted? It will be an elliptical section which shouldn't matter in your application but if it does matter it is good to be aware of it.

                        quote]

                        It was high tensile bailer twine.

                        .
                        .

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



                        Comment


                        • #13
                          Round rack

                          I had to do a similar round rack some years ago, overnight.

                          I didn't have a cnc, or a "d" bit.

                          This rack was in a 2" bar with large teeth maybe 6 pitch or so?

                          I set a slitting saw in the right angle head on 14 degrees and used the readout to space the teeth.

                          Obviously had to cut both sides of the teeth so it was a two setup job.

                          That job made us hero's and landed a lot of work.

                          I didn't have a digital camera (not invented yet) either, so can't grace you with a photo.

                          Nice write up Sir. John.

                          Kap

                          Comment


                          • #14
                            John,
                            I've stumbled on to this doing some research on rack cutting. I got here from your suggestion on how to make an involute cutter using a shop made tool.

                            What I'd like is a shot or dimensioned drawing of the D-bit tool you made. Specifically, I'd like to know how much relief you put into the non-cutting side of the tool. Also, what size rod did you make it from?

                            Thanks.

                            Comment


                            • #15
                              John here is the formula I use.
                              Divide the diameter of the cutter By the diameter to be cut .This equals the sine of the angle to which the head is set off vertical. Works for me .
                              Every Mans Work Is A Portrait of Him Self
                              http://sites.google.com/site/machinistsite/TWO-BUDDIES
                              http://s178.photobucket.com/user/lan...?sort=3&page=1

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