Questions about Hobbing Machines

Any metal cutting tool must be relieved in order to work properly. The lowly metal chisel shows this in a simple manner when it is properly ground and properly held. Another simple reference is the point of a properly-sharpened twist drill.

When the hob tooth form is relieved, it means that a slight eccentricity exists so that only the cutting edge makes contact with the surface to be cut. Each section (between the flutes) of the hob is relieved evenly. The amount of the relief is quite small, and if one treated it as a line tangent to the periphery one would see that it is perhaps from 5 to 10 degrees.

I am pasting a link that shows the relieving of the simple endmill: http://www.endmill.com/pages/training/design.html Pay particular attention to the primary and secondary relief angles. These allow the cutting edge to do its work and keep the area behind the cutting edge from dragging on the work.

Where endmills have noticeable relief angles, gear hobs often have eccentric relief and this can be difficult to see. Please look at Figure 3 of this web page: http://www.maxwelltools.com/tips.htm and you will see the eccentric relief angle. The cutting would be done by the surface near the letter "B".

In days gone by, engine lathes were sometimes equipped with "relieving attachments" that used a gear and cam arrangement to move the tool towards and away from the diameter being machined in order to make milling cutters with an eccentric relief. The turned part would appear to have a series of lobes, and the flutes were later cut after the turning process.

I hope this helps you understand the concept of cutting tool relief. A gear hob can be a most difficult tool to understand how the relief works since it is difficult to see.

Best Regards,

Ing. Robert Forbus