HONING GEAR FACES and BORES

One of the major issues in dealing with finishing the faces of pinion gears, timing gears, pressed metal gears or even the outer ring of cam phasors is finding and locating the true centerline of the bore to the spindle center and being able to finish the face perpendicular to it. This has been an ongoing issue with machine builders and fixture designers. How to get the true centerline at the top and bottom of the bore.  Many different, albeit complicated and expensive methods have been used with some success.

The processing of pinion gears faces and bores has long been a very difficult and diverse operation. The range of processes includes rough locating the bore and grinding the datum face, then grinding the bore and running a dual honing process. The other methods include skipping the bore grinding but adding a triple honing process. All these are to make sure the center bore is square to the end faces of the workpiece without taking too much stock off the ID and without changing the relationship between the center bore and the gear teeth. Another issue is to keep the end faces as true to the gear teeth as possible. All these variable have an effect on how the gear will run and perform in the assembly it was designed for.

THE PROPER ALIGNMENT

The typical process is to locate and clamp on the bore of the workpiece and make sure this is running true to the spindle centerline. This is not something that can be completed simply. Different types of fixturing designs have been tried with varying results. A simple method is to utilize an expanding collet on a portion of the bore. This section would be targeted to the workpieces “best” spot. Meaning the most stable and consistent portion of the bore. The typical process is to locate and clamp on this section of the bore and make sure this is running true to the spindle centerline. Diaphragm and sliding type chucks also have been used with success in the squaring of the straight bores but these methods again need to have the most stable portion of the bore to lock onto. Different types of fixturing designs been tried with varying results. Another design utilizes an expanding bladder or thin wall membrane expanded with oil to clamp on the bore. This method has a better chance of spanning and “finding” slight irregularities of the bore. This also relies on a pliable bottom for float so as to use the bore for the datum.

All these methods utilize the most accurate portion to reduce the amount of material removed in the faces of the workpiece. There are cases, as with the powdered metal camshaft phaser, which do not finish the ID at all. Here, there needs to be a more reliable and intuitive method of fixturing.

The bottom of most all fixture designs need to have a pliable surface to be able to float and thus use the bore as the datum. This would be only on the first face finished. The second face would of course use this qualified surface.

REDUCING STOCK REMOVAL

The stock removal on the faces can be reduced to only what is needed by first aligning the bore to the spindle centerline. This means not an arbitrary point somewhere along the bore length but at prescribed locations at the top and at the bottom of the bore. This will add a stability to the fixturing, a compliance to specifications and a continuity to a final finishing of the bore. It also gives the consistency of inspection and final measuring. Once the center of the bore is established, the faces can be machined square to this established centerline with only the required amount of stock being removed. In some cases the finishing of the first face surface can be accomplished with minimal stock removal. This can then be sent to a secondary machine such as a single disc grinder for the other face.  With this completed the bore finishing is simply set on the finished face and since the bore is centralized and the centerline is square a minimal amount of stock has to be removed.

In the assembling of stacked pinion gears, the process includes press fitting, heating and swaging and other operations which can impart taper in the bore diameters. Again, as in previous methods, this can generate variability in the clamping of the bores and increase variation and even instability due to diameter differences. They may still be round but not straight and true.

NOT-SO-NEW FIXTURE

The method we will described below is a very basic and rigid arrangement using a tried and true fixturing process. The previously described fixture designs require the center bore to be relatively true in respect to taper and diameter. In the finishing of, for example, the camshaft phasers or any powdered metal component the ID bore of the workpiece is usually extremely round but has a distinct taper due to the draft angle used in the powdered metal sintering process. This taper can change from mold to mold and a fixture designed to fit one mold angle is not accurate for another. This design sets these aside from most other workpieces. Another difference in the phaser clamping is that the center bore is not a full diameter surface and needs to have separate “pads” to clamp on these surfaces. These pads are not usually machined as they can be cast as near net shape and size. The machining of these pads would be another issue onto itself. The powdered metal design allows for the cost effectiveness of these phaser assemblies and additional machining would be unacceptable.

The device shown above utilizes a simple and time tested approach to clamping on a taper or clamping on different diameter bores. The separation of the upper clamping ring and the lower clamping ring allows the fixture to find the center of the bore at the upper section and at the lower section and establish a true centerline through them. The unit shown is using a simple set of springs for clamping.

FIXTURE OPERATION

The fixture starts out as both wedges are returned (raised). The lower wedge drops and expands the lower set of jaws. This centers the workpiece and establishes a true center. The second (upper) wedge lowers and expands the upper set of jaws. This will align the upper diameter to the lower diameter and align the two to the same centerline. The inside wedge is guided by the outside wedge and maintains the centerlines to each other. This unit can be spring loaded (as shown) or actuated by any number of methods. The clamping method will maintain the clamping force during the machining cycle.

THE FINAL FINISHING

By clamping the workpiece in this manner the top face can be machined accurately to the centerline of the bore. Once this is completed the part can be flipped and loaded onto the second face where any number of simple bore clamping fixtures can be used. The second face can then be finished accurately and at a much lower cost. With both these surfaces true to the bore and to each other, a common floating fixture can be used to hone the bore. In the case of a camshaft phaser, the part would be completed without touching the center bore.