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I am sure you have all read the excellent tutorial on lathe grinding
written by Sherline.
Recently Frank Hoose has put up a page with some very good pictures
explaining what to do. As a beginner, I could not picture the process
in my head. Frank's photos made that much easier.
Now my only problem was to get the tool at the appropriate angle
to the grinding wheel. The Sherline pages suggest an angle of 7
degrees for all faces and the instructions would have you adjust
the rest so that the appropriate angle is made with the wheel.
My (cheap) grinder wont let me adjust the rest in that way. It
wont even line up perpendicular to the wheel. I messed about with
wedge shaped bits held on the rest but that was not very satisfactory.
After a bit, it occurred to me that all you actually have to do
is change the height of the tool while keeping it level and
the angle will appear by the magic of geometry. As ever, a picture
may help..
Note that nothing is meant to be to scale.
The lower (red) tool is held at an angle but up to the horizontal
centerline for grinding. The upper (green) tool is simply held horizontally
but elevated above the centerline of the wheel. both ways give the
same angle. The advantage is that it is much easier to control the
height of a horizontal tool than the angle of a tool that must point
at some arbitrary spot in space.
OK, so now we have to worry about how high to mount the tool. In
case trigonometry was not among your very best subjects at school,
I have made a table to let you see how high to place the tool.
The table is in inches and shows the correct height of the top of
the tool above the wheel centre for the angle in the left column.
Clearly, the curvature of the wheel will mean that only the very
edge will have this angle but that's just what you get from grinding
aginst a wheel edge anyway. Conversion to metric is left as an exercise
for the reader :)
As an example, suppose you have a 1/4" tool to be ground at
7 degrees on a 6 inch wheel. I am assuming you have the rest set
up horizontal at wheel center height. The table shows a height of
0.366 inches. Well, this is nearly 0.375 or 3/8". You can simply
fix a 1/8" flat spacer to the rest and you are all set up.
| |
Wheel Diameter
(inches) |
Angle |
6
|
8
|
10
|
12
|
| 5 |
0.261 |
0.349 |
0.436 |
0.523 |
| 6 |
0.314 |
0.418 |
0.523 |
0.627 |
| 7 |
0.366 |
0.487 |
0.609 |
0.731 |
| 8 |
0.418 |
0.557 |
0.696 |
0.835 |
| 9 |
0.469 |
0.626 |
0.782 |
0.939 |
| 10 |
0.521 |
0.695 |
0.868 |
1.042 |
| 11 |
0.572 |
0.763 |
0.954 |
1.145 |
| 12 |
0.624 |
0.832 |
1.040 |
1.247 |
| 13 |
0.675 |
0.900 |
1.125 |
1.350 |
| 14 |
0.726 |
0.968 |
1.210 |
1.452 |
| 15 |
0.776 |
1.035 |
1.294 |
1.553 |
| 16 |
0.827 |
1.103 |
1.378 |
1.654 |
| 17 |
0.877 |
1.169 |
1.462 |
1.754 |
| 18 |
0.927 |
1.236 |
1.545 |
1.854 |
| 19 |
0.977 |
1.302 |
1.628 |
1.953 |
| 20 |
1.026 |
1.368 |
1.710 |
2.052 |
| 21 |
1.075 |
1.433 |
1.792 |
2.150 |
| 22 |
1.124 |
1.498 |
1.873 |
2.248 |
| 23 |
1.172 |
1.563 |
1.954 |
2.344 |
| 24 |
1.220 |
1.627 |
2.034 |
2.440 |
| 25 |
1.268 |
1.690 |
2.113 |
2.536 |
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