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RE-NEW MACHINE & MAINTENANCE, A Division of Repair Parts, Inc.
2415 Kishwaukee Street, Rockford, Illinois 61104
Phone: (815) 968-4499 Fax: (815) 968-4694
The One-Stop Shop for
Barber-Colman Machine Tools
- Machine tool reconditioning
- Repair and replacement parts
- Factory trained service
- Training -- operator, set up, or maintenance -- in your facility or ours
- Gear Machine Sales -- We have reconditioned hobbers & sharpeners in stock -- can be seen under power
- Hob sharpening
- Consulting and appraisals
The "Tips & Techniques" section offers helpful hints on the use and maintenance of your
Barber-Colman gear cutting and hob sharpening machines. We have a wealth of information available on
this kind of equipment and we think that this would be the ideal forum to share some of it with you.
We update these monthly, so be sure to bookmark this page and check back often.
This month I thought that we would
discuss HOB WEAR. Hob wear is such a complex subject that there is probably a different answer to
each individual problem. Some of the many items that effect wear are feeds and speeds, material of
the blank, material of the hob, pitch coatings, coolant, direction of shift, and the condition of
the machine. I will attempt here to show only a method by which various types of wear can be
checked and compared. Wear is caused, in general, by the continued use of a tool's cutting edges
during a machining operation.
To define the types of wear that we
will refer to in dealing with high speed steel hob teeth for involute forms.
1. Abrasion Wear - a continuous dulling of a cutting edge similar to that which would be produced by
filing the edge until it became an area whose width extended back from the face.
2. Flank Gouge - the formation of a groove or cavity on the side of the tooth. This is usually just
below the top corner of the tooth.
3. Cutting edge breakdown - a crumbling away of the original contour of the cutting edge.
4. Periphery wear - the wear formed on the peripheral surface(top) of a hob adjacent to the cutting
edge.
5. Corner wear - the wear, abrasion or breakdown produced on the corners of hob teeth.
6. Cratering - a pocket or groove on the face of the tooth near the periphery and adjacent to the
edge.
Wear is normally measured as the distance back of the cutting face, not as the change in thickness
or height of the hob tooth. To obtain the amount of wear, a microscope with cross slides equipped
with micrometer barrels is used. By adjusting the micrometer screws in the cross slides, the
operator can observe and measure the wear by using the cross hairs in the microscope as a
reference. This can also be accomplished on a comparator.
Recommended Maximum Hob Wear
| Diametral Pitch |
Semi-Finishing |
Finishing |
| 4 through 10 |
15 to 20 |
10 to 15 |
| 11 through 19 |
10 to 15 |
8 to 12 |
| 20 through 48 |
6 to 10 |
4 to 8 |
| 48 and finer |
5 to 7 |
3 to 5 |
NOTE: All reading in thousandths of
an inch.
It is my experience that the
operator that has been properly trained can pretty much judge by eye when the proper amount of
wear has been achieved on the hob. If the hob is used beyond the above recommendations it is
common that the involute form and finish on the gear teeth will suffer.
Next month we will discuss the finer
points of shifting the hob to determine proper wear as well as positioning.
If you have any questions or have a
particular topic you would like to see discussed here, please contact us.
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