Mark Grizotsky of Thornhill, Ontario, writes: We discovered new possibilities in the technology of universal gear milling, as the “dualism” inherent in these machines make them effective for achieving a wide variety of design details via hobbing. These details may include involute gears, gear belt pulleys, ratchet wheels, straight-side spline shafts, and disks with straight splines. Is on aspect of the milling process in which we use hobs as cutting tools.
The calculation of the hob tooth profile requires the use of difficult mathematical formulas and graphical checks. Even with the best calculations, it will frequently be necessary to undertake numerous adjustments during the cutting process in order to achieve the desired result.
The second aspect of the milling machine that we discovered in May 2005 — 100 to 120 years from the time the universal gear-milling process was introduced — involves changing the places of the cutting tool (hob) and the blank.
This “reversible MLG method” will be of interest to those who already have knowledge of the hobbing process. Therefore, I will not go into detail about the actual cutting process or the techniques required for manufacturing hobs and broaches.
The inherent dualism of the machine allows us to use the universal gear-milling machine to create teeth with complex geometric profiles for any hob, automatically, on the machine without any need for complicated calculations. In this reversible MLG method, the cutting tool (hob) becomes the blank and the detail (actually, a broach with the same geometry as the detail) becomes the cutting tool.
The hob that is created will be custom-matched to the shaft; a shaft of a different size will require its own custom hobs.
First step: Instead of installing a hob on the mandrel in the gear-milling machine’s spindle, I mount a blank (usually soft tool steel) with the same O.D. and length as the actual hob. Then, I turn the blank to match the helix of the desired size of the hob. I mount a blank above the broach.
Second step: The broach that is required looks like a straight-sided spline broach for splined holes (round internal, push-type.) The dimensions of the broach must match those of the desired spline shaft. The cutting rake and back angles should match those of the standard straight-sided spline broaches. Unlike a standard broach that is tapered, this broach must be made with a constant outside diameter for its full length. When producing the relief for the edges, be certain to provide enough room for chip clearance between the teeth. The completed broach is installed on the milling machine’s table, in the same manner that a workpiece would be installed.
When I have both the broach and the blank mounted in the machine, I set the feed sand speeds and run the machine as I would do for a typical milling process. After finishing this process, the hob blank will have the correct helical thread coils as well as the correct profile for the teeth. I use standard methods and typical techniques for finishing the blank and producing the hob.
This method also shows the significant technical potential of universal gear milling machines for producing cutting tools and profiling teeth, automatically and without complex calculations.