Spur Gears and Racks for a 14x17 inch camera

[jon.oman]

I am planning to design and build a 14x17 inch field camera. With that in mind, I need gears and racks to focus it with. I have found the brass gears and racks that McMaster-Carr sells, but I do not know what size I should order. They do have a range from 24 pitch to 64 pitch. I was thinking that the "24 Pitch—1/4" Face Width" gear could be used, with the correct rack to match. But, how about the number of teeth, they have gears from 12 teeth to 48 teeth? Looking at the gear diameter (0.58 inches), would the 12 tooth gear provide enough focusing accuracy?

I found the brass gears and racks under the "20° Pressure Angle Spur Gears and Racks" selection on this page:

http://www.mcmaster.com/#racks-and-pinions/=mjre6j

Can anyone tell me if I am headed in the right direction?

[gambaphoto]

Jon- I've been down this road myself, what I found was that the more teeth and the larger the gear, the more control you'll have. keep in mind that weight of your standards with the lens and film holder included to make sure that the gears will still move smoothly. I am planning a similar gearing focus for my next build, I decided that my first build was to remain simple so I opted for 2 tripods and a macro focus head for my fine focus on just the rear standard. It works so that what gave me the same idea that you have here

[Leigh]

McMaster-Carr has a basic selection. I've purchased many gears from them.

A much wider selection is available at Stock Drive Products http://www.sdp-si.com/
Their site is a bit difficult to navigate until you get used to it.
Click BUY ONLINE in the top row, then on the left side click Gears, below that either Racks or Spur Gears.

The basic dimension for a spur gear is its Diametral Pitch (DP).
This is the number of teeth on a gear that's 1" in diameter (along the pressure circle, not outside).

For example:
an 18 DP gear with 18 teeth is 1" in diameter, while
an 18 DP gear with 36 teeth is 2" in diameter.

A side-effect is that gear widths (and rack widths) are standardized for lesser DPs.
As the DP decreases (fewer teeth per inch) the width of the gear increases.
There are more options for small gears like the ones you need than for larger gears.
I don't know at what point the "standardized" width kicks in.

For the assembly to work properly the gear and the rack MUST have the same Diametral Pitch.
This seems incongruous for a rack, since it has no diameter, but DP defines the size and shape of the tooth.

They must also have the same pressure angle. Common PAs are 20° and 14½°.
It does not matter which you choose, as long as both parts are the same.

To simplify your design, choose the rack first, then choose a mating gear.

Point of terminology: A "pinion" is just the smaller gear of a pair, and is used in "rack and pinion".


- Leigh

[jon.oman]

Thanks for the responses, but I'm still confused as to whether I should use a set with a 24 pitch, or a 32 pitch, and whether to use a gear with 12 teeth, or more. What have people used?

[Leigh]

Do you know what diameter you want the gear to be?
A 24 DPI gear with 12 teeth would be 1/2" pitch diameter, slightly larger overall diameter.

What material (brass or steel probably)? I suggest brass.
If you use steel it will require some corrosion preventive finish like chrome plating.

How long do you want the rack to be? That defines the distance the front standard can move.
Are you doing front focus only or both front and rear? That will affect the rack length.

Do you have access to a machine shop? There is some significant machining to be done.

- Leigh

[Curt]

What diameter is the shaft going to be? Is there a design limitation restricting the gear diameter OD.

[Steve Smith]

For my current camera, I am using 12 tooth 32/Mod 0.8 pitch pinions as used by model car makers and CNC cutting my own racks.


Steve.

[Steve Smith]

For example:
an 18 DP gear with 18 teeth is 1" in diameter, while
an 18 DP gear with 36 teeth is 2" in diameter.

Am I right in assuming this is the pitch circle diameter rather than the outer diameter of the pinion?


Steve.

[Colin Graham]

Not sure if this will help, but I've always used 20 degree sets; 48 pitch - 1/8" face widths for everything up to 5x12. To help keep the bed thinner, and the weight down, I use 24 tooth spurs- the hub is just big enough to bore for a 3/16" shaft. Since most of the gear is housed in the bed, there are design limitations to the size of gears used. Also, if your bed/ focusing extensions will have tongue-and grooves milled into them, the practical size of the racks used can be design-limited as well, as deep and wide dadoes to house the racks can weaken those parts considerably.

Admittedly, my experience is limited, I've only made relatively small cameras, and a 14x17 would have considerably more load on the gears. I'll seriously consider using 24 or 32 pitch gears if I ever build a camera larger than 8x10. Maybe someone with an 14x17 will measure the gears for you- the gear sets are pretty pricy to experiment with.

[E. von Hoegh]

Keep this in mind: The larger the diameter of the driving gear - the pinion in this case - the more rapid will be the focussing. A pinion of 1" diameter (as an example) will move the rack 3.14" in one turn. "pi" is your friend here.