| Assigning precise values to feeds and
speeds when cutting wood with micro-tools is just about
impossible. The variability in grain structure, moisture
content, and density, make any such assignment at best a rough
guide. Adding to the difficulty are the variations in shop practice,
spindle run-out, and equipment backlash. Nonetheless, bottom
limits can be established as starting points for evaluating bit
performance on your CNC or pantograph woodworking equipment. These
bottom limits represent the boundary between actually cutting wood with
the bits and "spinning your wheels" or grinding your way
through the material. In the following
table, the minimum chip-loads for the entire set of PreciseBITs are given
for three hardwoods.
To use the table, select the diameter bit that you will be
evaluating, and move across the row to the wood column that best
represents the material you want to cut. Multiply the number in the
field by your spindle speed (RPM). The result is the MINIMUM
feed-rate (inches/minute) you should set your controller to when
using this tool in a FULL PLUNGE mode. As you gain experience with
the tool, you can gradually increase the feed rate until you achieve
an acceptable combination of edge quality, tool life and
productivity. To precisely determine the optimum combination
of feed rate and RPM on your CNC equipment, read the PreciseBits
Bit Testing Protocol.
| Diameter |
Depth
of Cut |
Part
No. |
Walnut |
Purpleheart |
Ebony |
| (inches) |
(inches) |
|
Janka
= 1050 |
Janka
= 1860 |
Janka
= 3692 |
| 0.0156 |
0.060 |
M208-0156-006F |
0.00042 |
0.00024 |
0.00012 |
| 0.0200 |
0.090 |
M208-0200-009F |
0.00040 |
0.00022 |
NR |
| 0.0200 |
0.130 |
M208-0200-013F |
0.00019 |
NR |
NR |
| 0.0220 |
0.090 |
M208-0220-009F |
0.00050 |
0.00035 |
0.00012 |
| 0.0250 |
0.100 |
M208-0250-010F |
0.00063 |
0.00035 |
0.00018 |
| 0.0313 |
0.125 |
M208-0313-013F |
0.00079 |
0.00044 |
0.00022 |
| 0.0313 |
0.160 |
M208-0313-016F |
0.00048 |
0.00027 |
0.00014 |
| 0.0313 |
0.260 |
M208-0313-026F |
0.00018 |
NR |
NR |
| 0.0350 |
0.175 |
M208-0350-018F |
0.00056 |
0.00032 |
0.00016 |
| 0.0394 |
0.200 |
M208-0394-020F |
0.00061 |
0.00035 |
0.00017 |
| 0.0469 |
0.200 |
M208-0469-020F |
0.0013 |
0.00058 |
0.00029 |
| 0.0500 |
0.250 |
M208-0500-025F |
0.00080 |
0.00045 |
0.00023 |
| 0.0625 |
0.313 |
M208-0625-031F |
0.0010 |
0.00056 |
0.00028 |
| 0.0625 |
0.500 |
M208-0625-050F |
0.00039 |
0.00022 |
NR |
| 0.0787 |
0.375 |
M208-0787-038F |
0.00139 |
0.00079 |
0.00040 |
| 0.0938 |
0.375 |
M208-0938-038F |
0.00236 |
0.00133 |
0.00067 |
| 0.0938 |
0.525 |
M208-0938-053F |
0.00120 |
0.00068 |
0.00034 |
| 0.1181 |
0.500 |
M208-1181-050F |
0.00264 |
0.00149 |
0.00075 |
| 0.1250 |
0.500 |
M208-1250-050F |
0.00314 |
0.00177 |
0.00089 |
| 0.1250 |
0.625 |
M208-1250-063F |
0.00201 |
0.00113 |
0.00057 |
NR = Not Recommended
Example: Suppose that you want to cut
a pocket 1/8 in. (0.1250 in.) deep in a piece of purpleheart using a
spindle turning at 30,000 RPM. Since you are trying for fine detail, you
decide to use a 1/32 in. (0.0313 in.) diameter bit. To determine the
minimum feed rate, find the tool diameter in the "diameter"
column. Since there are two 0.0313 in. cutters, select the one with the
depth of cut (DOC) that matches your requirements (in this case the tool
with a 0.125 in. DOC). Follow along that row until you reach the column
marked "purpleheart". Write down the number that you find in
that cell (0.00044). Multiply this figure by your spindle RPMs
(30,000).
The minimum feed rate for this
application is:
0.00044 X 30,000 = 13.2 in./min.
|
