How to CNC machine custom orthotic insoles
Also: keep a ballnose cutter above the bottom of the block
1. Why a ballnose cutter needs to travel below the bottom of the block
Machining a simple cube: rest material (in red), unless the cutter travels R mm deeper.
When using a ballnose cutter, the tip of the cutter needs to travel below the bottom of the part. This is needed in order to create vertical (or steep) walls at that position: otherwise some rest-material would remain present due to the tip radius of the cutter.
See the image above: the rest material is shown in red, and the part is correct when the cutter travels R mm deeper (R being the Radius of the cutter).
It will be needed to use a "wasteboard" below the part, as of course it is unwanted that the ballnose cutter would damage the machine's working table.
2. Why this can be a problem, for instance for custom insoles
Toolpaths for an insole, with the contour toolpaths below the bottom of the insole.
In some cases however, such wasteboard is not possible. A common application example is the creation of custom insoles, where a block of flexible foam is 'clamped' on the machine using a vacuum table. And this vacuum table (directly below the foam) may of course not be damaged.
The image above shows an operation for the insole's top surface (strategy Offset) and a second operation for the outer contour of the insole (strategy Contour only). A ballnose cutter with a 10 mm diameter is used, so this second toolpath is done at a Z-level 5 mm below the bottom of the insole. Which in this case is not good, as it would damage the vacuum table !
3. How to solve this problem
Setting the Ambient area level to Custom, in the Part parameters.
The problem can be solved by using a hidden feature that DeskProto offers: the part parameter Ambient has a different behavior when selecting a custom Z-value:
- When the Ambient is set to the default "Equal to bottom level of material", a ballnose cutter will travel R mm below the bottom of the block, as described.
- However, when the Ambient is set to "Custom" (with the same Z value in the edit field) the tip of a ballnose cutter will not travel below this level.
In this example the edit box shows a Z-value of 0.0 as on tab Zero-point the Translation for Z has been set to "Make bottom of block zero". Most podiatrists use this setting, as it means that the same zero point can be used for all insoles: for blocks of any thickness.
When you uncheck the box "Display translated coordinates" a different Z-value may be shown, as in the CAD software a different zero point may have been used.
Same insole in DeskProto, now with correct toolpaths.
The image above shows the result of this new Ambient setting: the contour toolpath now is done at Z=0 mm (bottom of the part) instead of at Z=-5 mm. In the screenshot the blue cube shows the position of the Workpiece zero point.
4. Adding a safety margin
Setting a Custom Ambient level that exactly matches the bottom of the insole will let the cutter 'scratch' the working table, which is not good. In case the table is not perfectly horizontal even a groove may be machined. So it is better to apply a safety margin here, and set the minimum tool height 1 mm higher. In the example this can be achieved by entering 1.0 in the edit field for Custom Ambient level.It will be clear that for these insoles some manual labor will be needed after machining: some rest material now remains (as described at the start of this page) that needs to be removed manually. For insole makers this is a very simple operation though.
You can find more information on insole machining in this DeskProto tutorial video:
The best CAM software for custom insoles