Z Zero at Machine Bed with Toolpath Cut Depths

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Z Zero at Machine Bed with Toolpath Cut Depths

Postby ShieldIt » Tue Sep 04, 2018 4:12 pm

I want to set my Z Zero Position at the Machine Bed and use 2D Profile Toolpaths. How do I set up my 2D Profile Toolpath Cut Depths when the Z Zero Position is at the Machine Bed?
Are the Start Depth and Cut Depth still measured from the top of my material? Is there a way to get the cut depths to measure up from the Machine Bed instead of down from the top of my material?

Why would I choose my Z Zero position at the Material Surface or Machine Bed?
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Re: Z Zero at Machine Bed with Toolpath Cut Depths

Postby ger21 » Tue Sep 04, 2018 4:32 pm

Yes, Start Depth and Cut Depth are measured from the top of the material, so when setting up the toolpaths, you just specify the depth you want.
In the job setup, you just check Z zero at the Machine Bed, and the code will be output correctly for Z zero at the bottom.


When doing profile toolpaths, and using material where the thickness may vary slightly, Zeroing to the machine Bed will make sure that you are always cutting through the part the same amount, regardless of the material thickness.

Operations that require precise depth from the top surface work best when Z zero is at the top of the material, so that you get consistent cut depth.

In a perfect world, where all of your material is exactly the same thickness, either method would give identical results.
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Re: Z Zero at Machine Bed with Toolpath Cut Depths

Postby 4DThinker » Tue Sep 04, 2018 4:35 pm

I set z=0 at the board top when the depth of my pockets is critical.
I set z at the bottom of the board (bed top), but enter a negative value of the board thickness, to make sure any through cuts get through but don't cut into my bed.
In VCarve if you've chosen the bottom of the board, z=0 will be at the bed top, but the toolpaths will start at the top of your material unless you've set a new start depth.

When doing 3D cuts where you have to change the bit between roughing and finishing passes, sometimes it is easier to have used the bed top rather than the material top as the roughing cut may have eliminated all the original board top surface.

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Re: Z Zero at Machine Bed with Toolpath Cut Depths

Postby IslaWW » Tue Sep 04, 2018 6:16 pm

ShieldIt wrote:I want to set my Z Zero Position at the Machine Bed and use 2D Profile Toolpaths. How do I set up my 2D Profile Toolpath Cut Depths when the Z Zero Position is at the Machine Bed?

Exactly the same as you would when referencing the material surface.

Are the Start Depth and Cut Depth still measured from the top of my material?

Yes, by definition, "depth" is a positive distance from the top surface to the bottom of the cut. When using a "Start Depth" the total depth of cut is the combined total of the start depth plus the cut depth.


Is there a way to get the cut depths to measure up from the Machine Bed instead of down from the top of my material?

If the material has been measured accurately, there will be no difference. In reality, those that do this on a regular basis usually find that since the machine bed usually does not suffer from the variations inherent in many materials find that zero to the bed yields more consistent results.

Why would I choose my Z Zero position at the Material Surface or Machine Bed?


In most cases, assuming material is veriflied flat and measured accurately, there is little difference. With a slight edge of repeatability and consistency going towards zeroing to the machine bed. This is how it was for 30 years with the large commercial (mostly ATC) CNC machines. Zero to the top only became popular with the DIY and small (sub $50K) machines in the late 90's and early 20's. It has been the favorite as it is apparently easier to understand and teach, which is reinforced, even to this day by your questions. There are also volumes that have been written about the inaccuracies and inconsistencies of the conductive z setting devices, and I will not get into them here.

The short answer is that, for the most part, zero to the top is much cheaper to produce parts for and easier for most untrained operators to understand, so in spite of its problems, its very popular.
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