No announcement yet.

Checking Gear Tooth Diminsions

  • Filter
  • Time
  • Show
Clear All
new posts

  • Checking Gear Tooth Diminsions

    Checking gear tooth diminsions in one of those things that should come up with all of the gear cutting threads around here. The following is options that may be available to the HSM

    1) The much vaunted and prized Gear Tooth Vernier. IMO a highly over rated and error prone device that isn't worth a Tinkers Dam. The vernier on the Starrett ones is a .020" scale which can lead to accidental errors and even in my younger days I had a hard time reading the small scale.
    2) The size over pins. Simply the best way to check gear sizes for the HSM provided he has the pins he needs. The process either requires three hands or the use of rubber bands to be be effective.
    3) Checking the backlash diminsion over the required number of teeth. This is the option that most likely is going to be available to the HSM. In the Machinery's Handbook it is called Checking Spur Gear Size byChordalMeasurement over Two or More Teeth. In my copy (19th Edition) it is on pages 971 to 973. Mostly used for checking the amount of backlash in the gear it is the simplest method I know to find if your gears are cut deep enough. The table on page 972 gives the Chordal Diminsion for 14-1/2 and 20D pressure angle gears in 1DP. To obtain the diminsion in another DP, say 10, merely divide the number for that number of teeth and subtract 1/2 the amount of desired backlash. Not subtracting backlash will give you the theoritical size over the number of gear teeth called for. For example, a 1DP gear with a 20D PA and 30T has a Choral Diminsion of 10.7526 over four teeth (from tables 10 and 11). 10.7526/10=1.07526.. If .010" backlash is desired the diminsion is 1.07026.
    Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

  • #2
    It's a bit empirical but I put my small gears on my flat bed scanner and compare the images against other gears (and CAD profiles):


    • #3
      Gear-tooth measurement- another way.

      You've got it SD.

      Gear tooth calipers, vernier or digital - are a PITA as you've got about four things to get right together. The caliper must be set parallel to the pitch points. It must be square to the axis of the gear. Trying to get calipers to read accurately when you are only using the points and not the inside faces etc. etc. is prone to errors and inconsistencies.

      Measurement over wires - 3 if odd number of teeth, 2 if even (I need to check that) is a damned side easier than 3-wires on a lathe as the wires on a gear sit into the cuts nicely. Measurement can be by a good set of calipers or a micrometer. Chordal measurement is ideal for checking back-lash, the solution is even better - just increase the depth of cut!! Measurement of the chordal distance too can be done with a set of good calipers (might have to grind a little off the backs of the jaws but won't affect the calipers. Otherwise use a "disc micrometer" (~ US$35) - which are a damn side better than a gear-tooth caliper - and much cheaper.

      This process is directly related to to the "numbered sets" of gear cutters that most HSM-ers will use.

      I have (had?) it in mind to run a thread on this in the next month or so. I intend to "walk people through" the process using "Machinery's Hand-book".

      This may well set off the "must only use gear calipers" brigade, but I think I can handle that/them!!

      It should help a lot of people with gear-cutting and measurement.

      I will pick up on this thread if you don't mind so as not to hi-jack it.


      • #4
        Hell, if it was easy, anyone could be a machinist. You can make good gears with a gear tooth vernier provided you pay attention and perform your calculations correctly. I cut lots of generated gear using gear tooth readings and so long as you don't expect precise baklash or center distances they can work quite well. The 0.020 vernier snake has bit many a good machinist me included. Calculate the chordal height and adjust it to correct for actual measured gear OD. Mkae the chordal height setting and lock it in. When taking the thickness reading wiggle the vernier about a radial axis as you adjust the tooth thickness jaw. When the lost motion just disappears you're about right on. Take several readings without looking at the vernier. It's easy to wish in a measuerment.

        However size over pins really is the best way to go - if you have goo gear measuring wires. Not everyone hase a set. Drill shankks are a poor substitute. That said pin size over gear teeth numbers cut with milling cutter near the edge of the cutterrange may not mesh quite at the calculated center distance. The backlash can be off.


        • #5
          Have at it. Having cut more gears and splines on real gear cutting equipment* than I care to think about I sometimes find that I have a hard timing holding my typing finger in check. As to the single tooth methods JS's button method works. But in reality only for that number of teeth and maybe a couple each way. Then you need a different cutter The same goes for the numbered sets IMO. Also the chordal measuring method also works on helicals. The semi-hob method that came up here about 1-1/2 years ago shows some promise but it has compromises in the tooth form. Compromises that could be overcome to some degree with additional passes at 1/2 or 1/4 the Circular Tooth Thickness as described here

          *Barber-Coleman's for splines, spur and helical and Fellows Gear Shapers for internal and external.
          Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.


          • #6
            Depthing tool

            Something I use regularly in clock and watchmaking is a depthing tool. Basically it positions a gear being made against a reference gear to check form, engagement and backlash.

            For something a little more accurate, I like to use a reference rack gear on a milling table under the gear on an arbor. Making the reference rack as accurate as needed is much easier and less expensive than purchasing an accurate reference spur gear and the rack does a good job of measuring the full width of the tooth rather than on a single point as the gear caliper does. With the rack oon the mill table and a good protactor on the arbor you can fairly easily get pitch diameter on the gear as well as be able to accurately measure backlash through the full range of engagement.

            I suppose that best reasonable dimensional check would be against a template on a 50 times magnification optical comparator.


            • #7
              Gearing up for gearing

              Originally posted by Forrest Addy
              Hell, if it was easy, anyone could be a machinist.
              I suspect that was a bit tongue in cheek Forrest - I hope so.

              I won't say its falling off a log easy, but for the purposes of many HSM-ers who are over-awed by the "mystique" that seems to attach or be attached to ordinary run-of-the mill everyday shop gears it isn't all that difficult. Its one of several things that are unduly and all too often unnecessarily "talked up".

              I'd reckon that the vast majority here could make a very passable gear as a stand-alone gear or as part of a meshing or mating set using just a few basic tools, the required "number" cutter, a reasonable mill (just about any from mini-mills will do the smaller stuff in aluminium or brass and it goes upward from there). A dividing head or horizontal/vertical rotary table (4" will do much of the smaller gears, a 6" will do most of it and an 8" will do just about everything). A good tail-stock is needed too.

              There is very little that is better at encouraging people to put a toe into the water and "have a go" and do more of it than successfully navigating Machinery's Hand-book and using a few relatively basic and cheap tools.

              Getting it done is a big a "kick" as getting your first screw-thread done well.

              They are not only good feelings but good confidence-boosters too.

              They will be my target audience - others are welcome to come along.

              Any group or forum should be geared (sorry) toward "users/members helping each other" and not hindering each other.


              • #8
                The "poor mans optical comparator" method seems good. A good scanner has _many_ dots per inch.


                • #9
                  Originally posted by Teenage_Machinist
                  The "poor mans optical comparator" method seems good. A good scanner has _many_ dots per inch.
                  The value it has for me is I only make gears to match existing gears, so I can test them quickly this way. Actually what I'm testing is my cutter profile. I cut a piece of plastic round to form a few teeth then scan it with it's mate to see if it fits. I can also place the cutter in the tooth space of the existing gear and scan that. The clarity and magnification is rather good with this method. I've also compared my cutters (think fly cutter) against cad generated profiles with good success.

                  I use 1/4" square HSS cutter blanks. It makes for a noisey interrupted cut when making the gear (actually a spline that I part off into gears), but it works.


                  • #10
                    Some additional data.

                    Evg all,

                    Gear tooth verynears, with aerospace bevels it was all there was until the master gears were established to inspect the production gears, these days multi million Inspection machine.

                    For the home shop bevels, sandwich a bit of thin stock, mill a notch, chordal addendum deep, by required chordal thickness wide. This is your Gear tooth "VeryNear!

                    As noted M.O.W. is the best for spurs, splines, as long as the actual diameter of wires is known, it is not essential to have actual gear wires, but keep close to the gear wire series. Also by using smaller and larger wires, one can figure fairly close as to how close the form is. The size over wires changes greater than the tooth thickness change, the sample gear below it is 2.70 to 1, which accounts for the accuracy of the method. The larger wires would reduce to 2.33 to 1, and the smaller wires increase to 2.84 to 1.

                    Les H.
                    The Impossible Takes Just A Little Bit Longer!