In the gear manufacturing world, there are many ways to design and make gears. In the quest to compete, many companies are not satisfied with the minimum. Using the latest and greatest machine tools, we are able to hold tolerances that were unheard of 10 years ago. We cut and grind them, and then check the gear using CNC gear inspection machines that are accurate to millionths of an inch.
Many gear manufacturers routinely check any gear they make in CNC gear checkers. They check the involute, lead, tooth spacing, etc., and generate pages of documentation to prove the quality of the gear. The only problem is that unless the customer wants this information and is willing to pay for it, all you’ve done is run up your indirect labor costs and waste some paper.
For example, if the print calls for the pitch line runout to be 0.001 inches maximum, holds the dimension over wires to 0.002 inches, and says nothing about anything else, that’s all you have to check. In fact, holding the runout to 0.0005 inches maximum just because you can is also unnecessary. If the customer doesn’t have an SPC requirement, then you have to cut parts right at 0.001 inches maximum runout and still ship them. I’m sure many of you are shaking your heads, but if you are tied into doing extra things that only drive up costs, who are you satisfying? Not the customer. In fact, I would suggest that these actions qualify as waste as defined by any lean production system.
This type of over-zealous activity extends to the design side as well. How about prints that hold the dimension over wires to a 0.005-inch tolerance and Pitch Diameter (PD) runout to 0.003 inches, and then hold either or both of the OD and root diameters to the same 0.005-inch tolerance? Sure, you can make these gears, but the designer does not understand the production process and drives up costs with a stroke of the pen (he probably isn’t on a CAD system). What about holding a chamfer to ±1 degree? If this is a functional requirement, price it in and make it. If not, then this is just overkill.
Two points to consider
First, you need to examine your processes for doing things that you are not getting paid for, such as over-inspecting parts, holding tolerances on blanks or during the gear generation process that add no value to the parts, or running parts in inappropriate machines.
For example, I once saw a process for a gear that had a fairly large end-to-end lead crown. The part was routed to be finished by grinding the teeth to achieve the crowning. The part had no gear surface finish requirement, no dimension over pins, and no AGMA quality requirement, and they could cut the crown into the part instead of grinding it in. There was also no heat treatment, so no distortion to worry about. How did this happen? The folks in quoting set up their routing this way, and it was just copied into the job when the engineers made the shop process. I’d rather take five passes in the cutting machine before I’d consider grinding the part; all things being equal. The takeaway here is to question every process step to make sure it is absolutely necessary to the final part quality before finalizing the shop floor routing.
The other related issue is one of customer communication. Make it a point to call them when there are questionable tolerances or incorrect data on the prints you receive, especially when you are quoting the parts. As opposed to calling, you should note these issues on your quote and reserve the right to re-quote upon receipt of complete and corrected drawings.
Also, make it a habit to get friendly with the engineers at your customers’ facilities. First of all, you’ll find that most of them will welcome your input to correct and improve their work. Second, because you are on the inside, there is a good chance that you’ll become the vendor of choice for these parts, and you will not have to survive a quote shootout when the parts are quoted. Third, if you get the problems out of the way before the prints are finalized, your internal processing will be streamlined and the parts can be put into the shop quickly. Failing some other manufacturing problems, the parts will be produced at the projected cost and profit margin and be shipped on time.