Gear Size Identification for Missing Worm Wheel Gear

[ChandlerJPerry]

I am pretty unfamiliar with gear specs and sizing. I have run into this problem with a newly acquired milling machine in the quill feed gearbox. Unfortunately someone took a grinder to this gear for the manual fine quill feed and removed all the teeth. I would like to restore the function if possible, however there is no manual or parts available for this machine. Therefore I'm in the predicament of needing to identify what the appropriate gear is and try to replace it. I am also uncertain of whether it would be a helical or spur gear. Would it be possible to measure the worm somehow to determine what the correct choice is?

 

[KellyJones]

Yikes! What a mess.

the short answer is, yes, you might be able to back engineer the gear from the worm.

First, the missing gear is most likely to be a worm gear, not a spur or helical gear.

The general procedure will be:
1. measure the pitch of the worm. That is, the distance from tooth to tooth, just like measuring a thread. If possible, lay two gage pins in two adjacent tooth spaces (such that they tough the worm teeth at approximately the pitch line) and measure the center distance between the pins. If that's not possible, then measure whatever you can and you will be close.
2. From the worm pitch, calculate the diametral pitch of a mating spur gear. The DP is related to the tooth pitch. It will most likely be a standard DP, whether imperial or metric, so if you are off by a little bit, round to the standard DP.
3. Estimate the pitch diameter of the worm gear. The pitch diameter of the gear should be assumed to be tangent to the worm. Keep in mind that the OF of the gear and the OD of the worm will overlap by approximately 2/DP inches or millimeters, depending on the system.
4. Measure the center distance between the worm and gear for checking your math.
5. Using the estimated pitch diameter and DP of the gear, calculate the number of teeth. It must be an integer number.

Keep playing with your measurements and numbers until it works out. Double check that the measured center distance is equal to the calculated radius of the gear plus the calculated radius of the worm.

I left a lot of details out here, but this should get you started. The formulas for gears are easily found on the internet.

A better question would be, once you figure out the gear data, how are you going to repair it?

 

[ChandlerJPerry]

Yikes! What a mess.

the short answer is, yes, you might be able to back engineer the gear from the worm.

First, the missing gear is most likely to be a worm gear, not a spur or helical gear.

The general procedure will be:
1. measure the pitch of the worm. That is, the distance from tooth to tooth, just like measuring a thread. If possible, lay two gage pins in two adjacent tooth spaces (such that they tough the worm teeth at approximately the pitch line) and measure the center distance between the pins. If that's not possible, then measure whatever you can and you will be close.
2. From the worm pitch, calculate the diametral pitch of a mating spur gear. The DP is related to the tooth pitch. It will most likely be a standard DP, whether imperial or metric, so if you are off by a little bit, round to the standard DP.
3. Estimate the pitch diameter of the worm gear. The pitch diameter of the gear should be assumed to be tangent to the worm. Keep in mind that the OF of the gear and the OD of the worm will overlap by approximately 2/DP inches or millimeters, depending on the system.
4. Measure the center distance between the worm and gear for checking your math.
5. Using the estimated pitch diameter and DP of the gear, calculate the number of teeth. It must be an integer number.

Keep playing with your measurements and numbers until it works out. Double check that the measured center distance is equal to the calculated radius of the gear plus the calculated radius of the worm.

I left a lot of details out here, but this should get you started. The formulas for gears are easily found on the internet.

A better question would be, once you figure out the gear data, how are you going to repair it?

Thank you! I will have to take some measurements to try to narrow it down. As far as the repair goes, it's difficult to tell but upon close examination I believe the gears may be separate pieces that can be removed from the shaft. If that is the case and the gear corresponds to a "standard" size my thought was that I could purchase a premade gear and modify the shaft hole/gear width etc to fit this application.

Otherwise I would probably end up paying someone to hob a new gear for me if the cost isn't too exorbitant, as I have no gear machining equipment or experience. Currently my mill is only tooled with a rotary table, I don't own a dividing head or any gear cutters, so I would be looking at a pretty significant investment to cut a single gear, as it's not a job I find myself needing to do regularly.