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Which machine to cut a rack ?

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  • Which machine to cut a rack ?

    I want to cut a rack. It happens to be on tube, but apart from workholding issues, that is a side issue.

    I have a lathe and a horizontal/vertical mill.

    I could mount the rack in the lathe, between the chuck and a centre, lock the chuck, mount a cutter on the cross slide and advance the cross slide to cut each tooth. I would advance the saddle to move along the rack. I would either be plunging (method A), and producing curved profile teeth, or running the cutter over/under the teeth (method B), and so having to cut in one go, or readjust the mount and repeat the whole pass.

    To use the mill, I would have to make a fitment to turn the axis of either the horizontal (method C) or the vertical (method D) quill through 90 degrees, so that the spindle that held the cutter was horizontal and parallele to the table.

    The rack by its nature is long, so it wouldn't work to mount it vertically and use the vertical quill, or for and aft and use the horizontal. If I turn the vertical through 90 degrees, the cutter is almost at the end of the table, so I have no movement left.

    Method B seems the most accessible to me, but I would have to make a sort of slow speed toolpost grinder fitment.

    Method C seems the most sensible, but I would have to make a fitment with bevel gears, a spindle, and preferably method to rotate the new spindle out of the horizontal.

    Who has cut a rack, and how did you do it ?
    Richard - SW London, UK, EU.

  • #2
    how's this

    stevenson to the rescue

    http://bbs.homeshopmachinist.net/sho...highlight=rack

    seems to be all about indexing .

    all the best.markj

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    • #3
      I've cut racks on both vertical & horizontal mills.

      On the Vert I used a right angle head and had the vice mounted in the "normal" (aligned with the X axis) position.

      On the horizontal, I aligned the vice with the Z axis (90 degrees to the usual).

      Some pics, and/or drawings, of the working envelopes of your mill and the rack would be helpful.

      One more thought. Do you have a friend with a shaper? That would make it very easy.

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      • #4
        Rack

        Maybe you could grind a 2 flute endmill with the tooth form on it, then just use the vertical spindle.

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        • #5
          Fly cutter?

          http://www.youtube.com/watch?v=ma6kdIHwrFI

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          • #6
            Originally posted by dp
            video uploaded on 14 Feb 2011....rack should be finshed by now

            all the best.markj

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            • #7
              on the lathe perhaps. depending on how long is the work piece. Use a fly cutter or form mill cutter on a long arbor, chucked and tail stock supported. Mount the work on the cross slide, feed across with the cross slide. Index for the tooth spacing using the saddle and appropriate indicator.
              Joe B

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              • #8
                Thanks all for your suggestions.

                I'm looking for a rack about 18 inches long, and I have a small mill with nine inches table travel. I would be quite prepared to do it in a few chunks.

                Toolguy, I've never liked the idea of endmilling multiple slots like this. One or two, maybe, but a hundred or so ?

                Then Dr Stan states the requirement for some kind of 90 degree fitment quite clearly.

                On the lathe front, yes if the rack was only a little wider than the saddle.

                Hmmm...

                I've written out this post in reply at least three times, solving my problem each time, but each time realising my solution was not a solution at all.

                But, this time, I haven't erased it all....

                Mark - I still think any cutting on the mill, whether indexed or using a universal dividing head geared to the leadscrew or whatever needs a 90 degree turn. But...

                Suppose I mount my tube-to-be-racked eccentrically in the lathe, and cut a thread. The eccentric makes sure the thread is only cut over a small part of the circumference of the tube, and if the pitch is quite small, say 48DP, then the helix angle will be small too.

                Then I make the pinion to go with the rack to suit, with the same helix angle. I'll have a little side force on the pinion and torque on the tube when it's in operation, but that can be handled.

                I'll call it a helical rack, and I'll say I did it to reduce noise and enhance smoothness, just like helical gears.
                Richard - SW London, UK, EU.

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                • #9
                  What meant by indexed ..is you do the first couple of teeth off the dial ..then you have something that comes down and engages with one of those first teeth ..each time after ...so from then on..no thinking to do ..right to the end of the rack.

                  also means you move the rack under the milling head ...not move the table ..so enabling you to do a rack that was 6 feet long, if you wanted.

                  these are just ideas in my head ...ive never tried any of them ..

                  and I may be proved wrong ...but to me seems to be the simplest approach.

                  all the best..markj

                  Comment


                  • #10
                    aboard epsilon is on to something. I've used this technique in drilling equally spaced holes in flat bar, but then the tolerances were not as tight as with a gear rack.

                    Maybe you could cut some of the teeth, return the cutter to the center of the last tooth cut, shut of the mill, loosen the vice, move the table making sure to take out the backlash, retighten the vice and continue to cut more teeth.

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                    • #11
                      How big is your lathe? If you have enough distance between centers, you could put a cutter between centers and use it like a horizontal mill. Reposition cross-slide, cutter, and stock as needed.

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                      • #12
                        Here are the details of the profiles of several similar racks:



                        Use Figure 40 for a 20 degree pressure angle rack.

                        Use Figure 41 for a 17 1/2 degree pressure angle rack.

                        The Pitch of the rack is self-evident and is equal to the circular pitch of the gear it is to mesh with.

                        It can be cut with a standard (straight) end milling cutter in a vertical mill with the head tilted left and right by 20 (or 14 1/2) degrees. The "hill" in the bottom of the cut can be removed as a final cut with the milling head set to vertical (ie zero tilt).

                        Pitch is set using the "X" lead-screw on the mill. The cuts are made using the mill cross-slide ("Y").

                        The depth of the cut to get the correct tooth width (0.5 x gear circular pitch at the Pitch Line) can easily be obtained with a single or multiple "wire/s" similar in principal to the thread-cutting "3-wire" measurement system in a lathe. Similarly - as the teeth forms (sides/flanks) are straight and not involute - a wire or wires can be used to have the correct tooth width when the top of the "wire/s" are level/flush with the top face of the rack.

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