How to Measure and Model a Curved Gear Rack

Hello,
I have a gear rack to reproduce that has a few broken teeth. I have next to no information on the original, so it's going to take a fair bit of measurement and figuring out in order to get it right. It's a little unusual, however, in that it's curved like a section of a ring gear. I've looked around for guidelines on trying to measure profiles on ring gears, but I haven't found much information. Does anybody have any tips on how to do this correctly?

Photos of the pieces in question

That's just molded plastic, so likely not super-precise. I'd be tempted to scan it, then import the scan and fiddle with it in CAD to mock up the profile.

What's the part do? What sort of loads and speeds?

Milland said:

That's just molded plastic, so likely not super-precise. I'd be tempted to scan it, then import the scan and fiddle with it in CAD to mock up the profile.
What's the part do? What sort of loads and speeds?

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It's a gear that meshes with a potentiometer for the purpose of measuring how far a foot pedal is depressed in an race arcade cabinet. People stomp on the gas, and the 30 year old plastic gets brittle and fails. Nothing that's super critical to get exactly right, but I'd rather have a clean profile that I can reproduce at will, and it's probably best to keep wear away because of excess slop in the system, especially if I use something less compliant than injection molded plastic. I'm planning on making it out of something a little harder and with a longer life so that it won't need replacing again. OEM doesn't make these anymore, and the primary source seems to be cannibalizing other cabinets in worse repair. I'll likely have to replace the gear on the potentiometer at the same time if I go with steel like my brain wants me to, but I can reproduce the outside gear profile just fine.

I'll likely start out by scanning it in with our laser table, but I typically confirm measurements where I can to use as a guide for my models.

I'd not use steel, how about Delrin? Steel may just mangle the mating part, and the extra weight may throw off the "pedal action".

Measure the spur gear, then it should be fairy simple to infer the ring profile.

I’ve been having stuff printed out of nylon for some prototypes and it is nearly as good mechanically as the same part machined from 6/6 and takes no work to make. Im positive that gear would last nearly forever if 3D printed in nylon. With proper stops on the pedal I don’t know why the gear would see much load.

Milland said:

I'd not use steel, how about Delrin? Steel may just mangle the mating part, and the extra weight may throw off the "pedal action".

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Only issue with steel I can think of is that it'll wear on the mating part as it exists, yeah. That's why I mentioned that if we go with something that hard we'll need to also replace the mating gear at the same time. Client stated that he prefers metal, but I told him I'd look into what the right material might be. Looking at the teeth that have been torn off the existing part, I think there's a good chance they're made of delrin or nylon as is. There's a fair bit of plastic deformation surrounding the shear area. As for weight, I don't think that parts this small are likely to mess with inertia that much. If it's a problem and he's dead set on metal pieces, there's always the jump to titanium...

Nmbmxer said:

Measure the spur gear, then it should be fairy simple to infer the ring profile.

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Already called him to see if he could pull it off the machine. We'll see what he can do.

Nmbmxer said:

Im positive that gear would last nearly forever if 3D printed in nylon. With proper stops on the pedal I don’t know why the gear would see much load.

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I mentioned 3d prints and he kinda balked, so I'll do my due diligence and see what he thinks. First thing I'm giving him is the CAD models I make out of the existing parts, so I'll suggest it again when those are done as a stopgap measure to just put the things back in service and see how the design functions. Otherwise I guess I get to play with the EDM and charge him a fair bit of money.

Lotaxi said:

Hello,
I have a gear rack to reproduce that has a few broken teeth. I have next to no information on the original, so it's going to take a fair bit of measurement and figuring out in order to get it right. It's a little unusual, however, in that it's curved like a section of a ring gear. I've looked around for guidelines on trying to measure profiles on ring gears, but I haven't found much information. Does anybody have any tips on how to do this correctly?

Photos of the pieces in question

Click to expand...

Well can you measure the center to center distance of the Two gears? from that and the pitch of the gear you should be able to calculate everything else you need to make the parts.

I’m a huge proponent for 3D printing in the right application. Reverse engineering is one of those applications, I’d print one whether you were going to make one from steel or not just to confirm it fits correctly before you spend a ton of time on the edm. The accuracy of newer printers is quite good and the engineered plastics are getting really good.

Lotaxi said:

Hello,
I have a gear rack to reproduce that has a few broken teeth. I have next to no information on the original, so it's going to take a fair bit of measurement and figuring out in order to get it right. It's a little unusual, however, in that it's curved like a section of a ring gear. I've looked around for guidelines on trying to measure profiles on ring gears, but I haven't found much information. Does anybody have any tips on how to do this correctly?

Photos of the pieces in question

Click to expand...

Take a hi-res photo of it with a measuring scale in the photo. Bring it into your cad at full resolution and trace the perimeters. Then scale the drawing until dimensions match the actual part and the scale in the photo.
Then 3D print. This is a perfect application for 3D printing

APD said:

Take a hi-res photo of it with a measuring scale in the photo. Bring it into your cad at full resolution and trace the perimeters. Then scale the drawing until dimensions match the actual part and the scale in the photo.
Then 3D print. This is a perfect application for 3D printing

Click to expand...

That's what I would do as well. I'd scan it in a flatbed office paper scanner along with a good ruler to confirm the correct scaling.

If it is curved, it's not a rack.

I used to grind a part like that in hard steel on a one-up basis for a matching set of two with the teeth set on a radius arc. They were straight across teeth in three different angles 3, 5 and 7*, yours would be easier because the teeth are straight across, and yours seem to have a radius on the opposite, mine were square on the opposite side.
I dressed the wheel to the V angle and then with a magnifying glass aimed each V straight up and down-ground each V individually to a witness of the existing V notch on the teeth that still had a little V left, those that did not have enough to witness I used a depth gauge the I had made. It was a slow job that likely took an hour to do each one because each tooth V had to be re set and clamped there, and they came in matching upper and lower halves. likely could only hold about .001 but being difficult to inspect the inspector could not check them much closer than .oo1ave made a gauge at the radius arc and used feeling a wire like from a 3 wite thread checking set to get closer than .001, and because when the job came to me it was boiling hot with a machining line down untill I finished the set.
Agree plastic can be a bugger with loading a suface grinder wheel. Milling the part might need a cutter with the included V notch angle.

EG is right. That is simply a section of internal gear.

or a section of a regular old spur gear- not sure the OP ever stated what side of the curve the teeth were on.........

That was precisely my first thought. They are segments of internal gears. Knowing the correct terminology will make the process easier.

Those more experienced with making gears can correct me, but if you are going to cut them, then you can not use standard gear cutters. The wire EDM with a good 3D model may be the way to go.

But when you create a 3D model, you may be able to turn the 3D CAD tooth curve inside out if the software allows that. Or do any 3D CAD programs allow internal gears to be modeled? Again, others may have more on this.

EmanuelGoldstein said:

If it is curved, it's not a rack.

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