Posted by BillS on September 13, 2007 at 22:53:48:
In Reply to: new hobb angle posted by Mike Prida on September 13, 2007 at 18:11:19:
The angle etched in any hob (shank or shell type) is the lead angle of the hob itself. Think of hobs as worms with gashes ground into them. Every worm or screw has a lead angle, which is the angle between a tangent to the pitch line and a line perpendicular to the worm axis.
You always need to know the cutter's lead angle when setting up the hob machine. The machine cutter table is set to that angle to cut a spur gear so that the hob teeth "line up" with the gear teeth as they are cut. This provides the necessary side clearance in the cut.
As for direction of the table, I like to think of looking at the cutter from the top (where the gear will be) and seeing which way the table needs to swing from 0 degrees to get the cutter teeth to line up with the workpiece axis.
For a helical gear, the table is set to the helix angle plus (or minus) the lead angle, again to provide clearance in the cut.
Plus (or minus) is determined by hand of gear and hand of cutter, and whether the helical angle to cut is larger or smaller than the lead angle of the cutter. To visualize the process of setting the table angle, start with the table angle set to align the cutter with spur teeth. Now think of this as the new 0 degree table setting. Of course you can't, but imagine you can move the table scale without moving the table and the scale is now set to 0. Now from this new "0", swing the table by the exact amount of the helix angle in the direction to align with the helical gear. For a LH gear, the direction would be opposite to the direction for a RH gear. To do this, you had to mentally add (or subtract) the original setting (which was the cutter angle) to (or from) the helix angle.
In other words, if the second setting (helix angle) is in the same direction as the first setting (lead angle), then the table setting is helix angle plus lead angle.
If the two settings were in opposite directions, then one is subtracted from the other. The usual case is that the helix angle is greater than lead angle, so the table setting would be helix angle minus lead angle.
But what if the lead angle happens to be greater than helix angle? This is not so rare when cutting worm gears especially with multi-start cutters. So, if the lead angle is greater than helix angle, set the table to lead angle minus helix angle.
For spur and helical gearing, this cutter angle is only required to provide clearance. In other words, the exact setting of the table is not critical for spur and helical gearing. This angle has nothing to do with the straight formation of spur teeth, and nothing to do with the helical angle. But that's another story.
For a throated worm gear, the table is set to the lead angle of the worm gear plus (or minus) the lead angle of the cutter. Shank type cutters are typically used for throated worm gears, so that the diameter of the cutter is slightly larger than the OD of the worm intended to run in the worm gear. This assures a contact pattern in the middle of the gear teeth. Otherwise, if the cutter OD was smaller than the worm OD, the worm would contact at the both faces of the gear which is something to be avoided. A throated worm gear, unlike helicals or spurs, requires an accurate setting of the cutter table. You may need to use trial and error with the table angle to get really good alignment and contact pattern with the worm that runs with the gear. Also, the alignment depends on contact at the pitch line, so check at near full depth.
Also, if the cutter is removed and ground, this reduces the OD and increases lead angle of the
cutter, which will likely require a very small adjustment of the table angle. How much depends on
pitch, OD, and number of leads in the cutter.
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