ShopBot Feeds and Speeds and Chip Load
To make good and efficient cuts, we need to feed the bit along the cut at an appropriate feed rate while it spins at an appropriate speed. There are a wide range of feeds and speeds that can be achieved by the machine, but there are constraints that need to be followed to make good cuts and not break the bit, cause machining artifacts to appear, or burn the material.
We calculate this by looking at chip load, which is the amount of material removed by a single cutter flute in a single revolution of the bit.
Chip Load = Feed Rate/RPM x # of flutes)
This gives a starting point for setting our tool parameters. Because we're dealing with wood or other inconsistent materials, this is only a starting point - I've run toolpaths with a chipload well under the suggested range, in order to not chip the wood I was working with, Going over the recommendations leads to scalloped edges because the bit travels too far along the cut before taking another bite.
Carve, in the tool settings in the database, helps you with this calculation, there are also chip load calculators in the ShopBot Control Program and available online. Many tool catalogs supply the chipload for each bit in their catalogs, and using a chipload for the same or similar tool is perfectly OK
For any given chipload the feedrate varies directly proportional to the spindle speed. On a variable speed machine like our ShopBot, we can vary the speed to avoid secondary issues like chatter.
The following chart is based on the Onsrud cutter catalog:
| Chip Load | 1/4 Inch | 1/2 Inch |
|---|---|---|
| Softwood | .002-.020 | .003-.030 |
| Hardwood | .005-.020 | .005-.030 |
| MDF | .005-.020 | .005-.030 |
| Plywood | .005-.020 | .005-.030 |
| Laminated Chipboard | .008-.020 | .010-.030 |
| Soft Plastic | .004-.012 | .006-.014 |
| Hard Plastic | .006-.012 | .008-016 |
- If depth of cut =2x diameter, then reduce chipload by 25%
- If depth of cut = 3x diameter then reduce chipload by 50%
There is considerable overlap in the range of parameters and you may need to test for the best settings for your particular application. The feed rate and RPM increase together maintaining the same chipload. You should vary the feed/speed to minimize chatter of either the bit or the material, and realize that higher feed rates puts more lateral force on your workpiece that might cause it to move.
