Finishing gear teeth

[DianneB]

I have a new project that uses 4 gears, all 12 DP. Three gears are 27 tooth and one is 60 tooth.

I printed them in 3D and had them investment cast so the teeth are a pretty good approximation of the proper shape but they need to be cleaned up a little to remove the cast iron roughness.

I looked at the price of involute gear cutters (OMF!) I also looked at other methods of cutting gear teeth and a fly cutter in a vertical mill looks more economical but I am not sure about grinding a tool bit to the right profile. (Major tools I have available are: lathe, vertical mill, rotary table.)

(I WISH someone made a (hand) file shaped to the space between teeth! I suppose I could file them smooth free-hand with a small file....)

Does anybody have a cheap and easy way of cleaning up gear teeth?

Thanks gang!

[drmico60]

These might help:
http://mikesworkshop.weebly.com/making- ... tters.html
http://mikesworkshop.weebly.com/improve ... tters.html
http://mikesworkshop.weebly.com/designi ... tters.html
Mike

[Harold_V]

A couple things jump out at me.
Cast iron is known to have sand within it's outer surfaces. If the gears in question do have, a HSS cutter won't serve long enough to do the job intended. In such a case, it is very desirable to get beneath the layer of sand---with a typical recommendation of a .05" depth of cut.

Cast iron, in thin sections (like gear teeth) have the potential to come out of the mold chilled. If the teeth are hardened from chilling, they may not cut well at all, so a file wouldn't serve the intended purpose. One could enjoy success using a form dressed wheel on a cutter grinder, assuming (proper) indexing could be accomplished.

I am aware that large gears used to be cast and run without alteration to the teeth. In defense of that practice, the gears were large enough with coarse enough teeth that they could be gapped enough to compensate for the irregularities. Small gears may or may not enjoy that same ability.

H

edit:
Assuming the iron lends itself to machining, there's nothing wrong with the idea of hand grinding the required tooth profile on a fly cutter, then cleaning up the gears using the tool. You can use a similar gear (commercially machined, and not worn) as a gauge. That's what I've done in the past. I've machines spur gears by that method with exceptional results. The one negative is that the tool likes to chatter in the cut. Keep it as short as possible, and held in a rigid fly cutter. Indexing is still a problem, however.

[DianneB]

Cast iron is known to have sand within it's outer surfaces...

That is quite likely.

One could enjoy success using a form dressed wheel....

BRILLIANT! I just happen to have a skate-sharpening machine I built many years ago that uses an 8" diameter wheel 1/4" thick. I could "re-purpose" that to replace the head on my vertical mill. I would just need a diamond dresser to profile the face of the wheel.

I am aware that large gears used to be cast and run without alteration to the teeth.

I did that when I built a 1/2 scale traction engine in 1995/96. The gear teeth were cut into the wooden patterns and were run "as cast" - worked fine!

Thanks for the stroke of genius H!

[Inspector]

Dianne I'm no expert by any means but I believe if you want to grind cast iron you'll need different wheels than the ones you sharpened skates with.

Pete

[DianneB]

The problem went away. No sand in the cast iron - it was investment cast - and no chilling. The teeth clean up with a file and the as-cast profiles are accurate enough that the gears run together just fine (for a low speed drive).

Thanks guys!

[Harold_V]

Dianne I'm no expert by any means but I believe if you want to grind cast iron you'll need different wheels than the ones you sharpened skates with.

Pete

A good observation.
Gray iron is best ground with silicon carbide (not a green wheel, however).

H

[Harold_V]

Thanks for the report, Dianne. I hadn't considered that the gears might be investment cast.

When in the infancy of my training, some investment cast electrical chassis came though the shop. Features of the casting were held to ±.005". It's a wonderful way to create a high degree of precision when castings are required.

H

[DianneB]

I hadn't considered that the gears might be investment cast.... It's a wonderful way to create a high degree of precision when castings are required.

I had started making patterns for this engine, which is a tremendous P.I.T.A. when you are only doing one-off. I had ventured into 3D printing a few years ago so I did a 3D print in ABS plastic and took it to the foundry and asked if that would work for investment casting and they thought it would. So, after about 15 hours of printer time I took nine ABS patterns to the foundry.

If I had known the detail that would be captured in the investment castings, I would have taken a LOT more time cleaning up the patterns before casting! In the final product you can see every 0.010" dia strand that the printer laid down LOL!

The only problem I have run in to is that the ceramic casing stuck in some of the small crevasses in the iron in some places and that stuff is WAY harder than high speed steel tool bits I had to employ the angle grinder to get some of the ceramic off and a carbide cutter where I didn't get it all.

All in all 3D printing worked amazingly well for investment casting. Nine (small) castings cost me $200 and it was much easier for the foundry to do than sand casting.

[earlgo]

If you could make a setup with adjustable center distances, you could run the gears together with lapping compound in the teeth and in a crude sort of way, lap them smooth.
Just another thought.
--earlgo

[earlgo]

I mentioned this to my clockmaker friend and he said that clockmakers make a wooden gear to approximately the right tooth profile, load it with polishing media and run it against the gear to be polished. This way the compound imbeds in the wood and not the metal gear.
My suggestion would work, but you'd have to be very careful to scrub away ALL the lapping/polishing compound as it will stick to both metal gears.
Good luck.
--earlgo

[DianneB]

That's not a bad idea! It might be worth the setup time