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Author Topic: Machining by strata (Thick 3D model)  (Read 1013 times)
CNC Jedi
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« on: September 13, 2018, 02:40:11 am »


To machine thick 3D models, it may be useful to use machining by strata .

This consists of creating strata by cutting the part into "sheets" of a more manageable thickness.

We want to cut this part into strata of 20mm high to get something like this.

It is not easy to do this cutting in a 3D soft, and it is not only necessary to slice the part but also to close the empty surfaces at the top of the cut.

All these operations can be done very simply in CamBam.

By very simple, I mean that it is not even necessary to physically cut the 3D model, we use directly the complete model.

Method 1 - moving the 3D model "through" a limited Z machining area.

In this case, only one 3D machining operation is used and a Gcode file is created for each stratum at a time.

On the image below, we see a complete machining on the whole height of 100mm. I intentionally used a single pass of 100mm depth so that the image stay clear.

The top of the model is at Z = 0 and the bottom at Z = -93.2.

In Cambam, the machining will be done between Stock Surface and Target Depht ; all that is outside these Z boundaries is ignored. The yellow box show the stock.

A Cutviewer simulation

Now, we will create a file for each stratum with minimal modifications to our file.

The principle is to limit the machining depth to 20 mm instead of 93.2 mm for the complete model.

The value of Stock Surface remains at Z=0, so we will always machine from 0 to -20 mm, it is the 3D model itself that we will move in height to generate the Gcode of each stratum.

For the first stratum, only the Target Depth has been changed from the starting file. (as well as the size of the stock which is now 20mm thick for the simulation)

Here I use the Bounding box boundary.

here is what it gives with Shape Outline boundary.

Now we move to the next stratum without changing anything in the machining operation. We simply move up the 3D model of 20mm.

Anything above Stock surface will not be taken into account for the toolpaths generation.

For this we use the alignment function: Transform / Align. (you can also use the NumMove plugin)

The X and Y axis are disabled (none) and we ask for an alignment where the top of the 3D model (upper) will be at + 20mm on the Z axis.

Our 3D model has moved 20mm to the top, and after regeneration of the toolpaths we can see on this side view that the top of the model is at 20mm above the Z=0 but that the toolpaths remain in the Z area 0 to -20

A 3D view with hidden 3D object (hidden layer) allows to better see the toolpaths.

Then we start again, but this time with a shift of 40 in Z for the part, then 60 etc.

For each stratum, remind to generate the Gcode and save it with a unique name for each stratum.

If you want 3D fasteners, just create 2 tubes with the extrusion function, then set them to -20. They must be selected with the 3D machining operation, in addition to the 3D model itself, but of course we will not move them in height with the 3D model.

In the following image, we see 2 bars completely crossing the model, and located at -20mm. We also note that they are selected in the machining operation (Primitive Ids = 2,6,7).

In order not to machine to the end of the bars acting as fasteners, it is necessary to limit the machining area. The circle (circle [8]) is used as a boundary using Boudary method= SelectedShape and the number of the shape used as a boundary in Boundary Shape Id.

For the next step, same thing ; of course only the 3D model must go up, the fasteners remain where they are.

Note: all simulations are done with a ball nose mill, which explains the "grid" of material remaining.

Method 2 - Using multiple machining operation at different strata levels then move the toolpaths for each stratum at the required Z position so the cut always start at Z=0

The principle is to create as many 3D machining operations as there are strata to cut ; the beginning of each machining operation start at the end of the other.

Example with 4 strata of 8mm.

In the first machining operation for the top most stratum: Stock Surface = 0 and Target Depth = -8, for the next stratum: Stock Surface = -8, Target Depth = -16 ... etc

the Clearance Plane remains at the same value for all machining operations (here +5)

Fasteners must also be created for each stratum; the fastener bars corresponding to the correct stratum must be selected with the 3D object for each machining operation.

Then, to distribute the 4 machining on the same plate, we can shift the toolpaths in X, Y and Z for each operation using the transformation matrix of each machining operation.

For example, the 3D mop # 1 is not shifted, # 2 is shifted x=120 y=0 z=8, # 3 is shifted x=0 y=80 z=16, and # 4 is shifted x=120 y=80 z=24 (see the "Transform" property of each machining operation)


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« Last Edit: September 13, 2018, 02:46:52 am by dh42 » Logged
CNC Jedi
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« Reply #1 on: September 13, 2018, 08:54:52 am »

Nice work and a lot of dedication!
Thanks, David!

Before asking a question do some effort and walk through all menus and options in CamBam.  Maybe the answer is there. Please.
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