Posted by Ben on December 21, 2005 at 17:00:05:
I have an interesting time at work sharpening hobs.
I'm not sure of the exact Kapp model #, but I am using a 10 inch borazon 15 degree wheel. The machine has a chart to compensate the grinding wheel angle or (head setting) to correct the convex rake or crowning effect while sharpening through the given flute lead.
With larger diameter hobs the error of compensation in head setting will be greater. This is also creating a rolling negative crown rake which can be seen while set up in machine. With course pitch hobs, the chips are very blue to black at the tip, but very silver along the side, as if rolling up the cutting edge and generating tool pressure at the tip before shearing off the blank being hobbed.
In reading the chart, which I do not have in front of me at this time, a generic ex. would be as follows
4dp 25 deg pa approx. hob o.d. 3.883
4 deg 28 min
The machine angle setting with the chart would be approx. 5 deg. 15 min remember cannot dress a crown in wheel. There is a minor error in the hob (crown), but itís very minimal.
Other co-workers on other shifts when sharpening use values for old Barber Colman 10-12 which I have used in the past. On a BBC 10-12 offsetting the rake to compensate the crown is used (moving wheel in or out). I found that this chart also maybe used on a BBC in lieu of moving the dressing unit to the side of the head which I have done in the past.... To some up this posting no matter how many drawings I make or visual comparisons to new hobs we make some how I am wrong in there eyes.
I have done base pitch comparisons to figure out the approx. change in pressure angle for various jobs and used different diameter pins to plot out tooth forms and I still seem to be speaking a different language.
I you have a technical article that I may refer to and or comments on this subject please respond.
I have found through experience that maintaining the quality of tooling will eliminate variables and deviations for the end result. I just wish more Owners, and Engineering Depts., realize these values rather than shoot someone down for doing a good job.
P.S. for Hobman. You may post those pictures we are all waiting for on ebay, then attach to this link. Ya never know, you may even make a few bucks.
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