Drawing Gear Teeth for Cutting

Discussion in 'Wood Movement Clocks' started by gleber, Mar 16, 2016.

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  1. gleber

    gleber Registered User

    Jun 15, 2015
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    Does anyone know any good references for how to draw gear teeth for cutting a patch of 3 or 4 teeth? I'm experimenting with a couple of techniques I will share when I get time to photograph or video them. In the meantime, I'd like to find some good reading material and learn the techniques. I started out by trying to outline the existing teeth, but even with the sharpest pointed pencil I could find, the teeth in the drawing were too large. I guess I could just cut the inside of the line, but I wasn't real happy with the shape. Looking at the teeth, they seemed to be straight until the point, and the penciled ones were somewhat rounded. The first "other" technique I tried provided better results, but I think I have an idea I can do even better. Stay tuned...

    Thanks,
    Tom
     
  2. R. Croswell

    R. Croswell Registered User

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    Yes. There was a Popular Menchanics article on building a pillar and scroll clock and movement. There was a link posted here a few months back. It was also reprinted in the NAWCC Bulletin. I'll search for it when find a few minutes. Perhaps someone will remember.

    RC

    try this link http://www.5happy.com/misc/pop_mech_pillar_scroll.pdf
     
  3. gleber

    gleber Registered User

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    Thanks RC. This article looks like how to create a full gear from scratch. I'm trying to transfer a section of an existing gear pattern to stock for cutting a new patch to match the existing teeth as best as possible. I know I could create a CAD drawing, but I'm looking for an easy yet accurate and repeatable method to hand transfer the pattern for any gear.

    Tom
     
  4. shimmystep

    shimmystep Registered User

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    I have used acyrlic spray to do just this Tom.clamp the wheel to the brass, and spray finely, i.e. from a reasonable distance. A couple of passes from different directions.
    The acrylic spray comes off the original wheel easily.
    I now make a tooth cutting tool from tool steel for use on a mill, made to mimick the original teeth, if there are more that a couple of teeth to cut that is.
     
  5. R. Croswell

    R. Croswell Registered User

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    OK, there are a number of ways to do this that can yield an acceptable result. No CAD or special tools required. First, I prefer to start with a solid blank piece of wood. others prefer to cur teeth from a section of a scrap wheel, and others would cast the missing teeth from resin. How one proceeds depends on the resources available.

    IF you have another wheel the same size, go ahead and cut out the bad section and dovetail in the replacement stock. After the glue is set, smooth it to the right thickness and radius. You can do this on a lathe if you have one, otherwise its files and sandpaper. Simply place the "other" wheel over the wheel being repaired and trace the tooth pattern onto the new stock. Use a jewelers saw to rough cut the teeth leaving them a bit "fat". Use files and/or a Dremel sanding disk to do the final shape to match the pattern.

    IF you DO NOT have an identical wheel, go ahead and cut out the bad section and dovetail in the replacement stock as described above. Now the task becomes more challenging. The first step is to determine exactly where each tooth tip must be. You can do this with a caliper measuring the spacing between the "good" teeth. Once marked, check again using the distance between two teeth. Another method is to use a photo copy or paper tracing made from the "good" teeth. If the repaired wheel does not run smooth, incorrect tooth spacing is one of the most probable causes.

    If you have a lathe a divider plate as shown (or some other fancy tool) it is easy to scribe where each tooth tip should fall. There have been a few clever contraptions made to index off of the existing "good" teeth.

    Once you locate the tooth tips the next task is cutting and shaping the teeth to the final profile. That's where the article comes in. You can look up the radius of the two inner circles shown in the illustration and draw the two circles as shown in the article. Using a straight edge, draw a line from the tooth tip tangent to the circle. Use one circle for one side of the tooth, and the other circle for the other side. This will give you the asymmetric shape of the tooth slanting or leading into the direction of rotation. You could also use the straight edge against each side of a "good" tooth to approximate the radius of the corresponding circle to which it is tangent. Once the profile is laid out, rough cut with the jeweler's saw and finish as required with files and/or sanding.


    Obviously the tooth tips have "width" and do not come to a point, so when locating (by whatever means) or measuring tooth to tooth distance, be careful whether you are measuring from/to the leading edge or trailing edge.

    That's the short crash course. Just remember the old saying, easy to cut hair off, hard as heck to cut hair back on!

    Good luck with this project.

    RC
     

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  6. gleber

    gleber Registered User

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    Thanks RC. The photos and all of your descriptions are most helpful. I shall name my first gear repair after you in your honor and send you photos.

    I've also written up my instructions for creating a template to draw the teeth. I plan to take some photos and will post all of that info. You can see a sneak peek preview here http://rovdoc.com/clocks/data/misc/gear_teeth_template.pdf. Pictures will probably help immensely, but I tried to be as explicit as possible and hope it makes sense without them.

    Tom

    P.S. I used to live in Bowie and frequently traveled to NJ via the Eastern Shore (avoiding I-95 at all costs). I see you are on the Eastern Shore. How is life there these days? It is a lovely area and I hope it's not completely overgrown with sprawl and shore traffic.
     
  7. R. Croswell

    R. Croswell Registered User

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    Tom, I tend to be a visual learner - like pictures. I read over the description but would need to actually do it each step at a time to be sure. There is more than one way to accomplish this task so what ever works with the resources one has. The most critical thing is that the tooth tips all have the same the same spacing and width. In the end you want the loading to be transferred smoothly from one tooth to the next.

    Life on the Eastern Shore is not like it was in the '50s when we left the keys in the car so they wouldn't get lost and no one even knew where the key to the house was. But still a pretty nice place where I live. Traffic from DC and Baltimore going to the ocean beaches can get pretty awful. Ironically, I-95 is now the easiest way to get from here to NJ. The Wilmington / Memorial Bridge area is awful where 95, 40, 13, and a Del. toll road all come together to go over the bridge to NJ. It's about 8 or 10 lanes wide but I-95 is a fairly straight shot through the mess. Too many people, too many regulations, too much government...........better stop before I get in trouble and say something political!

    RC
     
  8. gleber

    gleber Registered User

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  9. R. Croswell

    R. Croswell Registered User

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    The layout errors accumulate the more times they are transferred. If I were going to cut the teeth before inserting the new wood I would simply lay the original wheel over the wood and trace around a section of good teeth. Your difficulty will come when you try to fit the dovetail into the wheel so the spacing between the first and last teeth of the insert are precisely the correct distance from the nearest tooth of the original wheel. You will be working to control two things at once and most likely either the dovetail or the tooth spacing will be compromised. It is possible and some cut a good section from an extra wheel and fit it this way but it must be done with much care. I prefer to insert a blank piece of wood first so I only need concentrate on getting a good dovetail. If one has to lay out the new teeth anyway, I would lay them out on wood that's already solidly in place. Your plastic template may be one way to locate the tooth tips, keeping in mint that the teeth do not come to a sharp point so you really need to locate the leading and trailing edge of each tooth. Approximate won't get it - the leading edge of each tooth tip must be the same distance from the leading edge of the tooth next to it, same for the trailing edge.

    What I think is that you need to practice your method on a few wheels and take the process all the way to completion and demonstrate a working movement. Only then will you know how well it works and issues, if any, you will encounter.

    RC
     
  10. gleber

    gleber Registered User

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    #10 gleber, Mar 20, 2016
    Last edited: Mar 20, 2016
    Congratulations, it's a boy... Nutjob I just finished my first attempt, and as promised, it is named, "RC."

    20160320_164453.jpg

    I certainly appreciate all of your guidance. I did try to lay the gear on top and trace as my first attempt, but the drawn teeth were oversized and when I tried to cut them I found it hard because I like to cut and leave the line on as a reference. Cutting the line off is much harder for me to control.

    I'm quite happy with my result without having a lathe and divider plate. It is much better than the patch that was in the wheel when I got the clock, which was quite poor. The tip radius and the tip to tip spacing are equal as are the faces, but the root is a little deep at the left end. A little extra glue there should fix that. It runs smoothly when tested with rest of the wheels, but as you say, the proof will be in the pudding when the clock is running.

    Thanks again for all your help. One more wheel with two more patches required and this phase will be done. Maybe I'll try the dovetail first for one or both of those.

    Then it's on to creating a verge...

    I'll post updates as this project progresses.

    Tom
     
  11. R. Croswell

    R. Croswell Registered User

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    Don't think I've ever had anything named after me before! Don't for get to put a little load on the next wheel when testing. Sometimes these will seem to spin freely but you may feel a rough spot under a light load. Will be interesting to see the final result.

    RC
     
  12. gleber

    gleber Registered User

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    A good mentor should be rewarded and recognized. Having your initials on a gear hidden in some clock may not be very noticeable by many, but I certainly appreciate your effort and advice.

    With the train in up to the escape wheel and a weight attached, it took off fast and ran smoothly. I'll try adding a little resistance and see if it behaves differently when the new teeth pass the pinion.

    I have another question... If the dovetail is tight, how do make sure the mating surfaces are properly glued? Do you just put a light coat on both piece and then slide them together? It seems like most of the glue would just get wiped off if the fit is tight. Is the remaining glue enough to hold the piece? I guess there is not a lot of load on the glue and all it really needs to do is keep the piece from sliding in or out of the dovetail?

    Thanks,
    Tom
     
  13. R. Croswell

    R. Croswell Registered User

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    If you coat both parts with glue there will be a good bond. "Tight" is usually not perfect fit down to the last molecule. I usually clamp it in rubber jaw bar clamp across the wheel to make sure it is seated while the glue dries. A good glue joint will be as strong as the wood anyway.

    You can test whether the replaced teeth are right with the verge removed. You want only hand pressure driving the great wheel because you need to "feel" if there is any difference as the teeth pass over the pinion. You won't likely notice and difference in operation unless a tooth actually binds and stops the clock. Also if the pivot holes are a bit sloppy a poorly shaped tooth can hobble over the tight spot and easily go unnoticed, but turning by hand with a little loading (just enough to take up the backlash of the gears) will allow you to feel any issue. You have to experience a poor fit to really get the feel for it.

    RC
     
  14. gleber

    gleber Registered User

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    Hey RC,

    So, I've taken your advice to heart and tried doing the dovetail first. I then tried to mark the teeth using dividers and the results were abysmal. I tried setting the tips from one direction and then checking from the other and I was always off, not by a little, but enough to know it would never work. I also can't draw lines with out a template.

    So, I put my template thinking cap back on and came up with another method that I think is superior to the last. You can view the instructions here: http://rovdoc.com/clocks/data/misc/gear_teeth_template/gear_teeth_template_1.pdf

    And here is a photo of my results, once again named in your honor! :Party: :clap::thumb: You've been a great inspiration and mentor.
    20160323_213055.jpg

    Tom
     
  15. R. Croswell

    R. Croswell Registered User

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    Well it looks like it worked out OK for you. Aside from some parallax distortion in the photo, it looks like in your wheel the leading edge is radial and the other face tangent to the circle shown. The yellow lines from your replaced teeth appear to be tangent to the same circle and radial, like the original.

    As for the method and the jig, whatever works. Keep in mind that the next wheel will probably have a different size center. The one problem we face with any method is that the original tooth tips have width and are usually rounded and somewhat worn. That makes it very difficult to mark a single point as the tip, and even attempting to mark the leading and trailing edge of the tooth tip with a pencil, even a sharp one, leaves a lot of eyeball work. That of course is why I leave the tooth a little fat so it can be worked down to the final shape.

    This drawing also illustrates something to think about to refine the process a bit more. If one extends the two radial lines they converge at a finite point. A bit of geometry and one can determine the circumference of the gray circle. Now the circumference can be divided into segments corresponding to the number of teeth. A little more heavy math and you can calculate the cord or the arc across one segment which will be the exact distance between the points on the gray circle.

    Now if you really like jigs, take what you have and rig a "peg" that will fit between two of the good teeth (at the tight in your picture). Drawl that side of the tooth, lift the wheel and position the next good tooth over the peg and draw the next tooth. Set your straight edge with a good tooth in position. You won't need to locate the tooth tips or make a template. The wheel becomes its own template. To be practical, the device should be adaptable to various size wheels. I keep telling myself to make something but then I need it and don't have time to make it so just end up doing it "the old way".

    RC
     

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  16. gleber

    gleber Registered User

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    Thanks for taking the time to create that image. Not to sound too cocky:hat:, but I fully expected that the template would keep those lines consistent. It's cool to see it did work with an independent external QC verification:cop:.

    But now you have me thinking(i) I could create a jig that would allow me to skip the drawing phase altogether and just go right to cutting the teeth on a band saw (which I have, but no lathe). You're right, the teeth edges project to a point and using a jig you could cut straight pointy teeth, take off anything outside the outer radius and then finish rounding the tips by hand.

    Hmm... Seems like as good of a reason as any for me to get another wooden movement since this was the last wheel in this one needing teeth repairs (knock on wood).:whistle:

    Tom
     
  17. gleber

    gleber Registered User

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    I got the wheel repairs finished and the movement reassembled. It seems to run smoothly, but I still need to make a verge (new thread coming...). The only adjustment I had to make was the strike release (not warning) was a little too high and every time the stop pin went around you could hear it clicking. I bent the release down a little and it seems fine.

    Here are some photos:

    20160325_081341.jpg 20160325_090204.jpg 20160325_100738.jpg 20160325_100847.jpg 20160325_100820.jpg


    Tom
     

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