Removing Runout and Misalignment - Put Teeth in Their Place!
INTRODUCTION: The following is an article on an important aspect of gear making that, once encountered at the hobber, may be too late. Planning surfaces that a dial indicator "could love" is the purpose of Mr. Lehman's excellent contribution. As he points out, not every situation will necessarily fit into the case study he shows here, but planning for alignment should be foremost in your mind before you commit steel to the lathe's turning tool. Thinking Ahead about Run Out It is not critical that the outside diameter (OD) of a gear be exactly concentric with the axis of rotation - OD is only important in that the tops of teeth cannot bottom in the mating member. The important alignments are pitch cylinder and axis of rotation. This is true for all gears and pinions. Runout is the combined effect of misalignment of pitch cylinder with its axis of rotation. So it is critical that the pitch cylinder be exactly concentric and parallel with the axis of rotation. Since both pitch cylinder and axis of rotation are imaginary, we must employ a "trick". We don't need to "see" either one as long as we align the hobber axis parallel and concentric to the gear blank. Alignment requires special gear blank surfaces that can be used as references that are concentric and parallel to the all-importance pitch cylinder once the teeth are cut. We will rely on these surfaces for all subsequent operations that must be aligned to the pitch cylinder. These surfaces are called "Reference" surfaces and the ability to accurately indicate to them determine the accuracy of the fininshed gear. So, in more ways than one, accuracy begins when you machine the blanks and add machined (or ground) reference surfaces. It should be noted that the usual practice is to grind (possibly machine) the bearing journals as the last step in making the gear. To locate the gear accurately it follows that we must have surfaces we can indicate to. Sound familiar? These are the same surfaces we used to align the blanks to the hobber table. Reading on, you will see examples of "Face" and Rim" reference surfaces for axis of rotation and pitch cylinder. It is tempting to use the OD and Face as reference surfaces but factors can and will conspire to offset/misalign both OD and Face of a gear from its final axis of rotation. You might use the OD to dial indicate the gear blank on the hobber, but once teeth are cut the OD is, for all practical purposes, lost as a reference surface for the pitch cylinder. Having a smooth reference boss/land/groove/step that will be concentric with the pitch cylinder is a tremendous PLUS when moving a gear through its many stages (set ups) of manufacture. So, if at all possible, plan your gear with references that you can use and follow until the gear is finished. Even if these references are not part of the original gear design you can always find a way/place to add them. And, when you plan the references, you need to anticipate each set up of the manufacturing process. Take a look at the first picture: it shows a seemingly perfect reference when viewed on the lathe - but on the gear hobber this reference is blocked by hold-down clamps:
With the reference land blocked, there is no choice but to dial the OD as shown below: The next picture shows the proper location of the reference lands. It also shows how these lands are used to establish the final reference for grinding the journals - making the pitch cylinder concentric with the axis of rotation. In planning the reference(s), one must give careful consideration to how the geometry of the reference(s) will play out in the various mounting configurations (set ups) as the gear moves through the manufacturing process. The diameter of the herringbone pinion in the picture above, is much less than the length (face width) of the pinion. The reference locations (shown in this picture) are perfect for aligning the gear. For a gear with a diameter equal to or greater than the width of the gear, one must use a more suitable reference system. Pictures below illustrate a "face and rim" reference system:
The "face" reference is machined square with the bore, so that when the gear is placed on the hobber table, the axis of gear's rotation will be parallel to hobber table's axis of rotation. You might ask, "why not just place the gear face directly on the hob table?" As is often the case, the cross-feed on the lathe may not cut the face of the gear flat. This may occur for different reasons, one of which may not be so obvious. The surface velocity near the center of the gear face is much less than near the OD. The tool bit will cut to a different depth as it feeds across the face (side) of the gear. Hence, the gear will not lay flat on the hobber table. Even if the gear face were cut perfectly flat you would have two large, hopefully, flat surfaces mating. Any defect on either surface will cause misalignment. The narrow boss (the reference) makes finding and flattening "dings" much easier to achieve a reliable alignment of gear axis to the hobber table axis. (Also, two large flat surfaces can "wring" together making separation difficult - Ed) If your hobber table is true; that is to say, if you dial the top surface of the table as it rotates and if you get zero deflection - then with the reference boss mated to the table one-half of the alignment process is complete. The remaining step is to dial in the rim reference. The picture below shows a rim reference on a herringbone gear: The "face" and "rim" references will follow the gear all the way to completion. The ID (bore) of the gear could be dialed and used as a reference instead of the rim. But later, the center shaft will be installed as a shrink fit. Not only will the shaft cutoff access to the bore but the shaft will bend (deflect) as the gear cools and the OD of the shaft will not be a reliable reference. Most likely the shaft will need to be ground to locate true bearing journals. Since the face and rim references are not changed or blocked by any subsequent manufacturing operations they can therefore be used to set up grinding the journals.
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