nuclearcricket wrote on Jun 10^th, 2015 at 7:00am:


Sam,
I think it is an optical illusion with the close up pictures this often occurs. The Pinion wheel being closer to the camera lens than the rack makes it pinion look larger. I've had similar problems myself.

Ron, 
A very nice piece of work, I hope the steel version comes out as well.

Harry.

IIRC, A while back, Mike Hunter had a post about cutting a gear using a tap. Wonder if that method would work to cut the rack and pinion. Would take some fixtures.
Guess I am asking: How would cut the rack and pinion?
Chuck

To cut your rack and pinion you'll need two different cutters as the 14 tooth gear and the rack are at opposite ends of the gear spectrum The gear appears to be about a 62 Diametral Pitch. I doubt that any cutters are available and you'll end up making them. I haven't worked with gears for several years since I've retired but I had to reverse engineer a lot of gears in my career, none this small.

I've seen what you can do Otto, and I think you're up to it.

Being well aware of the challenges that present them selves when trying to take pictures up close and in great detail, this sight project looks damn good. You did an excellent job.
Congrats
Sam

This aroused my curiosity also, particularly as to how the rack was cut.

This is my first look at this thread as I was on vacation when it was started. The 3-D printing aspect is particularly interesting. 

At a recent model engineering show, a gentleman was displaying a 1/3 scale 1911 Colt auto he made by 3-D printing cores for wax casting and then casting the parts. Minimal machining was required for final fit up and assembly.

Ron,Chuck,&NC
Thank you for your kind compliments.
Chuck and JCH
Machining the rack and pinion was an interesting project in itself. I'll describe how I cut the rack but should first state that I'm an amateur machinist, who knew nothing about gears two weeks ago. I certainly have no expertise in machining gears and would appreciate any corrections by professional machinists, on the methods described below.
The first step was to calculate the DP (Diametral Pitch) of the Hadley pinion. The # of teeth (14) +2 divided by the outside diameter of the gear (.271" to .260") gave a DP of 59-61 so I chose 60.
The second step was to calculate the center to center distance of the rack teeth. 3.1416 divided by 60 gave a distance of .052"
The third step was to determine the depth of cut. 2.157 divided by 60 (DP) +.003" gave me a depth of .040"
The preceding formulas are readily available in many machining reference books, YouTube videos, and the Internet.
The next challenge was finding or making the proper cutter. The rack cutter is easy to make but the tooth cutter is a form cutter and the making of this cutter would be a project in itself. This is further complicated by having 8 cutters per DP. So for this project I needed to make the two single cutters or purchase a 60 DP #1 and #7 cutter. As luck would have it, I found a 64 DP #1 cutter on EBay, which I thought would be satisfactory for this project.
The Maynard tang sight staff required moveable supports in the center section, in addition to the attachment at each end. I had to make an extended range cutter holder to mill the rack in the vertical mill and it may have been easier to do the job on the lathe or horizontal mill. Once the holding fixture and tooling was made, it took very little time to cut the 45 grooves .040" deep.
I think that if I were going to make another Hadley R&P sight, I'd select a DP for which cutters were readily available and use commercially available gears. The gear would be a little smaller than the original and the number of teeth on the staff would be different but it would work as originally intended and the differences would be hard to see.
The R&P mechanism could be printed in steel from the 3-D model. An estimate for the pinion portion alone was $215 which would make the sight expensive using this method.
Below are two additional close up pictures showing the rack and pinion portion of the mechanism.
Otto 

Thanks for the information. For a low speed/low power application of this sort, a bit of fudging with standard involute gear calculations can be tolerated. There are a couple of approaches that can be taken that don't require the use of involute cutters.

A tang sight has been on the back burner after a couple of less than optimum trials. The rack & pinion provides a simpler approach to elevation than the double start threads of other types.

Good Job Otto! I'm impressed. I knew you had skills but having the determination to accomplish this is a rare trait. You are indeed a talented craftsman.