Noob sizing issue with MeshCAM and FabCAM

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Noob sizing issue with MeshCAM and FabCAM

Postby rjfoley2006 » Wed Jan 21, 2015 6:33 pm

A few weeks ago I received a mydiycnc 3 axis machine. I have found MeshCAM and FabCAM to be the most accessible software for people like me only because I can at least accomplish the goal of taking a design, creating the code and then sending it to the machine. I have been utterly baffled by just about all the other software available, especially the free and open source software. My problem which I have detailed here every step of the way involves strange sizing conversion from MeshCAM to FabCAM which comes with the mydiycnc machine.

Notice that the code produced generates an odd axis numbers and does not show a G21 code to designate mm specs. Should I assume these numbers are all inches??

Here's how I came to get this code with MeshCAM:

Step one:
Create simple block in autocad123 50mm x 50mm x 10mm
export to .stl file

Step two:
Open .stl file in MeshCAM
Define stock - it automatically shows the dimensions as x50, y50, and z10 as MM. XY and Z positions are all zero.
Change stock size to x60mm, Y60mm and Z20mm which results in automatic XY and Z positions as +5 mm all centered.
Click OK and.....

Step three:
Set retract height - left as default of 2.54 mm

Step four:
Program Zero
Chose bottom of Z position so it reads -5.000 mm
XY Position is x-55 mm and y-5 mm
Note - I'm not entirely clear on what Program Zero screen does other than set where the starting point will be and/or how far down the bottom of the stock is. I don't really know what these perameters mean.
I have however confirmed that the "part" is now clearly defined in the MeshCAM screen and the zero point is in fact where "I believe" it should be.

Step five:
Edit tool and confirmed that I have an endmill with perameters that seem like they make sense.
Overall Length 37 mm - (Is that the length of the mill where from where it comes out of the spindle?)
Shaft diameter 3.16 mm - (I assume this is also the diameter of the cutting part ie the fluted part)
Flute diameter 3.16 mm - (I assume this is also the diameter of the shaft?)
Flute Length 10.00 mm - (I used a digital caliper on this and rounded for these initial tests)
no corner radius
Taper angle 0 - (I have no idea what this is)
Tool number 1
Spindle Speed 7000 - (This is the lowest speed on the spindle I'm using and it's manually adjusted anyway)
Toolpath settings - Feedrate 127 mm, plunge rate 127 mm, stepover 2 mm, depth per pass 1 mm
(These were all default settings which I left as is due to uncertainty about what the proper parameters should be although I do feel that i know what these parameters mean)
Click OK

Step six:
Toolpath Parameters screen
Set Tolerance to .025 mm
Machine whole stock selected (I have no idea what this is as apposed to "Machine geometry +)
Enable arc Fitting selected
Roughing pass enabled
Tool selected and parameters shown match with the tool as previously defined (All show MM)
Use 3D Roughing selected (I have no idea what this is)
Use Parallel Path selected (I have no idea what this is)
Finish has been disabled. (I'm just trying to focus on the basics)
Click OK

Step seven:
model on the screen looks great - I can see the tools paths and I get the popup window allowing me to save toolpath - everything seems in order!

Step eight:
When I import into FabCAM the model on the screen is enormous!
Viewing the gCode I get the following:

'(STOCK/BLOCK, 60.000, 60.000, 20.000, -0.000, -0.000, -0.000)
(TOOL/MILL, 3.1600, 0, 10.0000, 1.0)
M6 T1
G00 X0.000 Y0.000 Z22.540
G00 X60.000 Y1.00 Z22.540
G01 X60.000 Y1.000 Z19.001 F127.0

I don't see any description M21 which defines mm as the measurement specification - Why?
z 22.54 is way off the mark as far as I can tell. Is this inches or mm? In my retraction height I set it to the default of 2.54 mm so what is this z22.54?? it's adding another 10 millimeters(or inches) and the Z motor tries to extend past it's physical limit. Does this have something to do with the 10 mm I have set for the flute length??

I'm baffled :-(
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Joined: Wed Jan 21, 2015 5:17 pm

Re: Noob sizing issue with MeshCAM and FabCAM

Postby rjfoley2006 » Wed Jan 21, 2015 10:04 pm

I did find an answer here:

1. there were some obvious parameter errors I made but those were irreverent to the main problem of the inch to metric conversion
2. FabCAM has a button on the main screen called "Metric" (go figure) that you press to specify metric conversion. I assumed this was done in gcode but this doesn't work with FabCAM

Posts: 2
Joined: Wed Jan 21, 2015 5:17 pm

Re: Noob sizing issue with MeshCAM and FabCAM

Postby Randy » Sat Jan 24, 2015 5:23 pm

Hi rjfoley2006, and welcome to the forum. I'm sorry for the delayed reply.

Which MeshCAM postprocessor did you specify? The metrc posts should put an M21 as part of the setup lines, if not it is easy to edit the postprocessor (it is just a text file that tells MeshCAM how to format the gcode output) and add it yourself.

About the tool paramaters--the flute diameter is the actual cutting edges diameter of the cutter. The shaft diameter may be larger than the flutes on very small cutters. If you put a shaft diameter larger than the flute diameter, MeshCAM will pull the cutter away from the surface on deep cuts (deeper than the flute length) to avoid hitting the shaft. I've found it best to set the shaft diameter the same as the flute diameter and just ensure myself that I pick a cutter deep enough for the cut. There are tapered cutters where you would set the cutter angle (think a chamfering cutter), but straight cylindrical cutters use an angle of 0.

The Program Zero is the 0,0,0 point that the gcode is written relative to. You specified a stock 20mm high, with the Program Zero at its bottom. So the top of your stock was at Z=20mm in program coordinates, and so the retract height was at Z=22.54mm.

All opinions in this post are mine alone. I am not a MeshCAM employee, I do not have a financial interest in MeshCAM, nor do I speak for MeshCAM. MeshCAM user since Beta 5 in 2003. viewtopic.php?f=11&t=15333 :ugeek:
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