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How to keep a ballnose cutter above the table

Prevent the cutter to travel below the bottom of the part

ballose cutter, correct and incorrect Z
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 will damage the machine's working table.

DeskProto screenshot showing an insole with toolpaths
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. 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 !

DeskProtopart parameters, tab Ambient
Setting the Ambient area level to Custom, in the Part parameters.

This 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 "Equal to bottom level of segment", a ballnose cutter will travel R mm below the bottom of the part, 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.
The image below 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 this example the edit box shows a Z-value of 0.0 as the designer of this insole has made the bottom of the sole 0.0 in his CAD system: the value shown here is in un-translated CAD coordinates, as present in the STL file (scaling, rotation, etc have been applied though). So in case your CAD design has a different position in Z you will see a different value here. The same trick of selecting Custom will nevertheless work !

When exporting the toolpaths the Translation (Part parameters) will be applied. For insoles in many cases the Workpiece zero point also needs to be set at the bottom of the block, as then for all insoles the same zero point can be used. This can be achieved by selecting "Make bottom of part zero" on the Translation tab of the Part parameters.

DeskProto screenshot showing an insole with toolpaths
Same insole in DeskProto, now with correct toolpaths.

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, in other cases the value that is present can be increased by 1.0 (or any desired value).

It will be clear that for these insoles some manual labor will be needed after machining: the rest material that was described now remains present and needs to be removed. For insole makers this is a very simple operation though. In most cases also a piece of thin leather will be glued on top of the insole to complete the product.