Monday, February 24, 2014

Surface Rough and Surface Finish Toolpath Setup

This post documents the process of setting up a 3-axis surface roughing and surface finishing of sheet material using Mastercam.

Setting up the machine and stock, as well as backplotting and simulation, is exactly the same as is shown in the Mastercam Toolpath Setup post. This post will cover adding a surface roughing and surface finish toolpoaths.

Surface Roughing Toolpath

Surface roughing toolpaths typically use larger tools, multiple stepovers, and multiple step downs to quickly remove larger volumes of stock and leave an even amount of stock for finishing. The roughing toolpaths you choose for your part depend on the shape of the part, shape of the stock, and machining situation.
Check out the Mastercam help file for details on the various roughing strategies shown below. This post will use the Parallel toolpath.


You'll be asked to select the surfaces to cut to via the selection dialog:

  • Drive: The surfaces, solid faces, solid bodies, or CAD files that will be cut.
  • Check: The surfaces, solid faces, or solid bodies that you want the tool to avoid. 
  • Containment: A closed chain of curves that limit tool motion. 

 Click the arrow under Drive and choose the surfaces to cut. Once selected exit the dialog.

As with other toolpaths you need to choose a tool. For small MDF panel surfacing projects a good choice is a 1/2" downshear rougher.

Once selected change the tab over to the Surface parameters options.

Here you set the Retract and Feed Plane settings. Retract sets the height that the tool moves up to before the next tool pass. Select the check box to activate the retract plane, then click the button and select a point on the geometry or enter a value. This option is off by default. The retract height should be set above the feed plane. If you do not enter a Clearance height (clearance sets the height at which the tool moves to and from the part), the tool will move to the retract height between operations.

Next set the Stock to leave on drive surfaces. This will leave the extra amount that the finishing pass removes. Leaving 1/8" or 1/4" is fine for MDF. This lets the smoother finishing pass clean down to the actual drive surface height.

Next change the tab to set the Rough parallel parameters.

The Maximum stepover controls how far the tool moves over for each cut in XY, and the Maximum stepdown controls how much material is removed in Z for each cut. You can safely use 80% of your too width for the step over provided you are not stepping down to much in each pass. When surfacing MDF I like to step down no more than twice the tool width, and step over about 50% of the tool width on each pass.

Use the Plunge control parameters to control where the tool is allowed to make plunges. Use the buttons to set cut depths, control motion between adjacent surfaces, and motion on surface edges.

Change the Cutting method to Zigzag. Under Plunge control select the Allow multiple plunges along cut. This makes multiple entry moves along the part. This will significantly speed up the cutting time.

Check both the Allow negative Z motion along the surface and Allow positive Z motion along the surface. This will maximize the speed of the roughing as the tools we are using can cut while plunging and while retracting.

Surface Finishing Toolpath

Surface finishing toolpaths typically finish a part down to the drive geometry (or to the stock to leave amount if one is specified). These are usually using a small stepover and don't cut very deep. To surface a 3D form a typical stepover is 0.05". The typical amount of material being removed might be 0.25" or 0.125".

To add this toolpath right-click in the Toolpath group and choose a Surface Finish toolpath. In this example I'm using Parallel.

You'll be asked to select the Drive surfaces as in roughing.

Next you'll be asked to select a tool. For surfacing choose a ball end mill. Choose the largest tool that can still get everywhere into your parts form. Using a larger tool gives a shallower scallop making for a cleaner surface. In this example I'm using a 1/2" ball end mill.

Next move to the Surface parameters tab.

Here you set the Retract and Feed plane values as above. Make sure the Stock to leave on drive is set to 0.0 so it mills all the way to your surface.

Next move to the Finish parameters tab.

Finish parallel toolpaths cut the drive geometry using linear zigzag or one-way motion. In this example we'll set the Cutting method to Zigzag. 

The Maximum stepover controls how far the tool moves over for each cut in XY. Using a smaller value will result in a smoother surface but take more time. For a 1/2" ball bit I like a value of around 0.05" as the stepover. 

Simulation

Make sure to carefully simulate your roughing and finishing cuts. 
Control of Simulation is the same as was shown in the Mastercam Toolpath Setup post. 

3D Contour Toolpath (Waste Trim)

If you want the router to trim the waste from your panels you can use a 3D Contour toolpath to do so.
Note: Since the toolpath will need to cut entirely through the material, doing this will compromise your form for future vacuum forming of panels. Therefore, make sure you have an extra form for trimming, or have enough panels already cut!

Right-click in the toolpath list and choose Router toolpath > Contour from the menu.

The Chaining dialog will appear and you select the chain to cut. Careful setup in Rhino to create a single closed curve will make this setup process easier in Mastercam.

After you select the Chain the Contour toolpath dialog will appear. First select the tool. I'm using a 1/4" straight downshear.

Next go to the Linking Parameters branch. Set your Retract, Feed plane, and Top of stock values.

Next go to the Cut Parameters branch. Make sure the Contour type is set to 3D. You'll also need to set the Compensation direction to right or left. After you simulate you can easily tell if you need to switch the setting if it is cutting on the wrong side of the line.

You may also want to set a small Break Through amount using that branch. Here you can set a low value to cut through the material to make sure it is fully free from the waste.

You can then backplot or simulate to make sure the Contour is cutting as expected.





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