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Posted by BillS on February 11, 2009 at 19:45:17:

In Reply to: SETTING UP ATERNATE SPIRAL BLADES posted by Shahzad on February 10, 2009 at 03:40:31:

The summary should tell you:
(1) Spread-blade point width for cutting gear in one pass. Outside cutter blades form the concave tooth surface while inside cutter blades form the convex tooth surface. Point width is measured from outside tip of outside blades to the inside tip of inside blades. Don't confuse point width with blade point width. Individual blade point widths only have to be large enough so that outside blades can overlap inside blades in the cut.

Since the gear tooth space is formed in one finishing operation, any correction to tooth contact patterns is done by making adjustments to pinion setups.

(2) In pinion single-side cutting, one operation uses all outside blades to form concave side of pinion. A 2nd cutter with 2nd machine setup with all inside blades forms the convex side of tooth. Point width for these cutters does not determine final tooth thickness as would be the case for a spread-blade (gear) cutter. Instead, point widths for the pinion cutters should be large enough to remove any fin that might occur at the widest tooth space, and small enough to pass without interference through the smallest tooth space. Other than that, tooth thickness is determined by the use of a stock dividing gage used to remount a pinion for the 2nd operation. Judgement of appropriate point width is made during summary creation since that is where tooth spaces at heel and toe are determined.

If the summary is for an offset-axis hypoid pair, then things are slightly more "customized" when setting up the cutters. The gear cutter has inside diameter blade points and outside diameter blade points that are specified by the summary, and that's how you set the blades - using shims to get correct diameters. Pinion is cut single sided just as before, and two cutters are used in two separate setups. Cutter blade-point diameters (concave and convex) are made to match the gear.

Things can get confusing in a hurry. Just keep in mind that in most methods, the gear is formed in one machine operation, and the pinion is formed in two machine operations.

A method such as Versacut also specifies blade letters (e.g. A, B, C, etc), which basically determine blade pressure angles, not to be necessarily confused with gear/pinion pressure angles. To oversimplify the purpose of the Versacut method is to say that a small range of cutters can be used to generate a wide range of gear/pinion pairs. This method is perhaps popular with job shops since it uses a relative few number of cutters with a Gleason No. 26 machine.

And then there is a cutter numbering system which defines the inside/outside variation in blade pressure angles, where a number "0" cutter is balanced; i.e. inside/outside blade PAs are equal. There is more on this cutter system in the FAQ. As a summary is generated, two cutter numbers (one for gear, one for pinion) are selected so that the sum of gear and pinion cutter numbers falls within a range defined by summary calculation.

And don't get me started on asymmetrical tooth pressure angles, which increase tooth loading capacity in a prefered direction of rotation, e.g. auto, truck, and tractor differentials.

FYI, Spiral angles differ between members as a function of offset, but if there is no offset, spiral angles are equal (but opposite).

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