To those who might want to make these gears!
What follows will be long yet will hardly say as much as should be
said. Also, the order of what I write will be a little "free-form", based
on what comes into my head as I write.
On wooden teeth, there should be no problem. With the right construction
wood makes some rugged teeth and has been used for gears for
centuries (Harrison made precision clocks with wooden gears [I believe he
was a carpenter by trade!] ) and they are still in good condition. I
believe some big, commercial machinery has also relied on wooden gears,
with good success.) The problem is that wood is not isotropic, i.e., its
properties vary with direction. Harrison knew this and already made his
gears from laminated wooden blanks. If I were making a rack I would
consider using many layers with the following structure, maybe
even repeated several times-
H + - H - + H where "H" means this layer has the grain running parallel to
the length of the rack, while "+" layers have their grain tilted 30 degrees
relative to the H layers and the "-" layers have the same 30 degree but
tilted in the opposite direction with respect to H. I picked 30 degrees
because it puts the grain roughly in compression along the direction of
greatest strength in these layers. (20 degrees might be better, but my
feeling is that it is less good for the average location and direction of
pressure on the tooth!)
How you deal with the wooden pinions is beyond me and I look forward to
reading about good solutions! In a larger gear the problem does not look
to difficult but in a small one I see trouble.
Selection of a good wood is important. Tensile strength in the cross-grain
direction is valuable and some of the better woods for strength along the
direction of grain are rather poor in the cross-grain direction. Experts,
please advise!
What really brought me to the keyboard is a message asking how to make a
form cutter. First of all; a short lecture. It seems as if a search of
the Internet failed to provide any prescription for doing this. Yet there
are other sources, like books and magazines. I would not like to search
magazines but books are available with titles clearly suggesting that they
could be good references. Of course, we must pay a little for them and
slowly I have the feeling people look at the Internet as THE source where
EVERY bit of intellectual effort is available for FREE. It is not yet
so! And I hope it never is because the motivation to write down all the
knowledge we want will slowly disappear in many cases if the receivers
don't cough up a little hard cash as a return for the considerable effort
of putting all those wonderful things down as printed words. Perhaps it
is precisely this lack of motivation which results in the shortage of the
information on the Internet on making gear cutters!
To get a wider understanding of the construction of gears you can turn to a
number of books. One I found on my shelves is "Gears and Gear Cutting"
from Ivan Law, printed by Nexus Special Interests. It costs about $10 if
the price printed on mine still applies.
Mr. Law describes a procedure in which you can approximate the
theoretically correct involute with a very good fit by a circle. A true
involute is rather hard (but interesting to consider because it is not very
hard!) to machine exactly but the approximation will serve the purposes we
discuss here. Let me assume you have a lathe to make the cutter and that
we are now making a special tool for this lathe.
Take a piece of strong steel, but it need not be hard. Form a slightly
tilted, flat surface (about 5 degrees) at the front of the raw tool. This
will receive some buttons which are the actual cutting edges of your new
tool. The tilt provides the clearance at the cutting edge so the tool does
not drag on the work piece (the new gear cutter). You will be drilling two
holes in the tilted surface to receive the buttons, the holes spaced by the
amount given below as "Center Dist."
The buttons are simple, very short cylinders turned to diameters I will
list below. Each has a reduced diameter part which goes into the holes I
mention above. After hardening the cylinders they are glued (Locktite)
into the holes of the tool. After hardening, temper only the slightest
amount; you will be cutting steel with these buttons and they should not be
too brittle but none-the-less they must be hard.
The following data are quoted from Mr. Law's book and are for gears with 20
degree pressure angle. It might be better to go to 30 degree pressure
angle for wood and small diameter pinions. The book tells you the values
for this angle also (down to 10 or 11 tooth gears).
TOOTH COUNT Diam. Center Dist. In Feed Width
135 -
Rack 51.3 49.6 17.79
4
17 -
20 7.8 8.7 3.44
4
I hope this looks similar to what I now see on my screen, or at least that
you can sort the numbers into the correct columns (I fear other text
styles will ruin the columns.)
I give only the tooth counts which seem to apply to your problem; the book
also supplies values for all other tooth counts. What are these;
Diam.= Diameter of the buttons on the place where they cut the cutter!
Center Dist.=The spacing between the centers of the buttons, i.e., the
spacing of the holes you will be drilling to receive the reduced diameter
part of the buttons.
In Feed is the amount you feed the tool into the new cutter when making it
in the lathe, measured from the front edge of the buttons to the uncut
(outer) diameter of the new cutter.
Width= How wide the blank disk for the new cutter should be (I guess wider
is no problem, within reason.)
Grind the buttons flat on top, so the flat is parallel to the original
surface of the tool (not the slightly tilted surface on which the buttons
are mounted.)
Make a disk to become the gear cutter (steel which you will later harden)
with a center hole to mount the disk. Feed the tool into the cutter by the
given amount to generate the "involute" and harden and temper a bit.
There is a problem with your new gear cutter. It has no relief at the
cutting edge and as such it will cut badly if at all. Proper gear cutters
are generated with a procedure which makes this relief and Mr. Law
describes a device in detail so you can do this. But that is complex if
you are not going into gear cutting in a big way. Instead, cut a big gash
into your nice new cutter. This gash should have a straight edge which
will be the new cutting edge and this straight edge should be parallel to a
center line draw (or imagined!) on the disk, but with the cutting edge
being formed by removing material even beyond the center line. If you gash
just to the center line there is still no relief (please read below about
how you will be using this cutter) but by continuing a little farther than
the centerline, the part of the cutter coming after the cutting edge will
be sloping away from the newly formed gear tooth instead of dragging on the
wood. The cut will be cleaner and the heating far less.
You will be using the cutter as a fly cutter. These things have only a
single tooth but a lot of clock gears of high quality are cut with fly
cutters. It is a quick but effective way to get a gear and for one-off
jobs there is much to recommend it. Mount the fly cutter so it rotates on
a diameter greater than the diameter of the cutter blank and so the cutting
edge flies around the circumference of a circle. In this way the back edge
comes nowhere near the gear in production. A carrier for these flycutters
should be very easy to construct and probably needs little description from me.
The cutter can be resharpened many times by just grinding away enough metal
while following the rule "parallel to a center line but a bit behind it to
produce clearance", so its lifetime will be great if overheating is
avoided. However, flycutters are best run at high speeds and some judgement
is needed to avoid overheating.
Please see some book for additional details. I have written too much and
worse, I believe drawings are not wanted on these sites (say "Jpeg") and I
do not maintain a site where I could post any drawings so you will have to
sketch what I have left out from my text. Sorry.
One other point; it is possible to make cutters with a single button which
is used to form both sides of the cutter by just moving it over. Another
point; the values given for dimensions of the tool can be scaled with a
pocket calculator for other gear pitches. Mr. Law tells us these
dimensions are for diametral pitch= 1, that is, the gear has a tooth for
each inch of diameter at the pitch circle, where the dimensions given are
taken as inches. (You could as well read millimeters and the results would
be fully as valid; you would just be scaling by 25.4.)
Bob
PS I hope you do not have to read too often to get the idea. I will try
to deal with questions if I did not make much sense!
At 10:45 25.10.01 -0600, you wrote:
>Bob,
>
>Basically this system will be an indexer with the tooth shape pre-cut so
>that a person could cut the teeth with hand tools or a shaped router bit.
>I'm trying to work out the most versital design so we could make gears of
>different sizes (I would like to make a clock with all wood gears as well)
>but we do have several limitations. First, of course, is that complexity
>and size = high cost. I want this to be !well! under $100 per unit if
>possible. The system could be built out of wood (see Fine Woodworking March
>1986 reprinted in Small Woodworking Projects, Taunton Press) but then you
>wouldn't get exact tooth spacing without a lot of time, patience and scrap
>(at least for me) and the jig would have a limited life. Making it out of
>steel would get us jig that could last a lifetime. Also, as I said earlier,
>the more of us that get one will drive the cost down as well. Also, the CNC
>machine will do a great job of cutting the shape of the teeth to the correct
>dimensions so we won't have to get too detailed about that side of the
>construction although I can say that the gear teeth will be typical involute
>gears that can be used in rack and pinions. Obviously, you won't be cutting
>helical or worm gears with this. So far it looks like the list is myself
>and Phil McCourt. I'll keep the list informed of the plans and as I get
>them into Power Point I'll post it on my website so you all can see what I'm
>doing.
>
>Dan Rhoades
>www.rhoadescameras.bizland.com
>
>----- Original Message -----
>From: "Robert Mueller" <[EMAIL PROTECTED]>
>To: <[EMAIL PROTECTED]>
>Sent: Thursday, October 25, 2001 2:14 AM
>Subject: Re: [Cameramakers] Rack and pinion template system
>
>
> > Would it not be easier to mill the teeth (router) with a shaped cutter
>and
> > just some device to advance the cutter by one tooth spacing after each row
> > of cuts. Making cutters should be quite easy. The rack can have straight
> > sided teeth and it is not hard to make an involute cutter for the
> > gear. Metal workers have all the details worked out.
> >
> > Bob
> >
> >
> >
> > At 21:47 24.10.01 -0600, you wrote:
> > >I was wondering in anyone would be interested in a template/jig to be
>able
> > >to shape and cut a rack and pinion out of wood. I'm working with a
> > >engineer with a good CNC metal shop to build a jig system for cutting the
> > >pinion and rack by hand using files and chisels. I think African
> > >Blackwood, Cocobolo, Hard Maple or other hard stable woods would be great
> > >for gearing since there isn't a lot of pressure on the gear teeth. The
> > >design we are working is based on hardened steel runners in which you
> > >would place the piece of wood in between two jigs, clamp the wood tight,
> > >drill the center hole using a predrilled hole in the steel that would
> > >guide the bit straight, and then cut and file each tooth. The rack will
> > >be a similar design but won't need the center hole.
> > >
> > >Now, I'm going to build one for myself since I like having rack and
>pinion
> > >focusing on my cameras and a wood system is so much cooler than adding
> > >metal to the system. My friend (Kirk Kovel of Kovel Precision Machine in
> > >Colorado Springs) can build the system cheaper in production than one at
>a
> > >time so if others want this type of system we can all benefit from a
> > >cheaper price. He is also great since he knows that this is just a hobby
> > >and none of us are out to make money doing this so he charges a
>reasonable
> > >price. I have no idea at this time what the cost per unit would be since
> > >more units will bring down the price, but the price would be at cost.
> > >
> > >My current camera is using a 16 tooth system and is quite stable with no
> > >stress on the gear teeth. I think a 32 or even a 64 tooth system would
>be
> > >quite reasonable and workable but I want to see what any other interested
> > >parties would like. The other size requirements we need to work out
> > >amongst ourselves is the size of the pinion and the length of the
> > >rack. My current system has the pinion at a 2" diameter and the rack at
> > >16" long and that seems to work well.
> > >
> > >Alright, let's see if anyone is interested and we'll work out the details
> > >and see if anyone has good ideas to make this system work.
> > >
> > >Dan Rhoades
> > ><http://www.rhoadescameras.bizland.com>www.rhoadescameras.bizland.com
> >
> > _______________________________________________
> > Cameramakers mailing list
> > [EMAIL PROTECTED]
> > http://rmp.opusis.com/mailman/listinfo/cameramakers
> >
>
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