Gearhead assembly gig question

[malegault]

Hi,

we have two metal gears that needs to be mounted with a high level of coaxiality on a machine (~0.0001 or less). To do so we are developping an assembly jig with a shaft that will be machined with the coaxiality requirement need. This shaft will be used as a guide to align the two gears before being fixed.

Do you have any suggestions regarding the steel we should use for the fabrication of this alignment shaft?

Regards,

Tony

 

[drawoh]

malegault,

Material would not be the first thing I would ask about if you wanted me to design your jig.

We need more information. How does the jig work? You want a material that is hard, mindbogglingly rigid, has low friction and is easy to machine.

If your jig positions your gears as you clamp them to something else, your fixture will strain elastically as you clamp your gears. Your gears will not wind up in the right place. Either, you need a non-disturbing clamping system, or you need your machine to centre your gears.

JHG

 

[malegault]

Hi JHG, thank you for your reply.

The two gears we want to align are actually screw with together (bolt circle). Friction between coincident lateral faces of gear (spur gear) will garantee gears wont move axialy relative to each other when operating. To ensure good axial location of each gear axis when they are being assembled, we want to machine some sort of shaft (or should a say guide pin) to do so.

Gear a made of steel, dont know exact steel and hardness at the moment. The question is what material could be used since we have many of the gear assembly to do..?

Regards, Tony

 

[drawoh]

malegault,

Is there any way you can modify your gears to have a shoulder and a matching centrebore? This would solve your problem without the jig. If your jig is going to centre within .0001", your bores are going to have to be significantly more accurate than that. I do not think you are doing this with off-the-shelf gears.

Your clearances will have to be less than that. Look in your Machinery's Handbook for Limits and Fits. An ANSI RC1 fit, the tightest sliding fit in the book, requires a minimum clearance of more than .0001" for any size larger than Ø.12".

Once you assemble your gears to the jig, you have to remove the jig. My remarks above do not account for maximally decentred gears.

If you can do very accurate bores, you could design a three pin fixture in which one pins moves precisely in and out from the centre. This would allow you to engage your gears' bores, and then disengage after you tighten screws. Your .0001" accuracy will be a challenge.

For any kind of a rigid fixture, you need to match the thermal expansion coefficients very accurately. Almost certainly, this means using the same material as your gears. I hope your gears are not made of a galling material like stainless steel.

JHG