Think & Tinker, Ltd.
P.O. Box 1606, Palmer Lake, CO 80133
Tel: (719) 488-9640, Fax: (866) 453-8473
Sales: Sales@thinktink.com, Support: Support@thinktink.com
PreciseBIT Feeds and Speeds
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 Think & Tinker Bit Testing Protocol.
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.