Archive for the “Cnc” Category

In Part 1 we verified the touch plate being connected and configured right. Now it’s time to get it working.

 

Credits

Without the information in this thread on cnczone.com I wouldn’t be able to do this so credits to cnczone.com and especially to author Spokes who provided a tutorial and bundled the files needed on page 11 of the mentioned thread. Here is a link to zip file with his files and tutorial. While I’m, setting up my CNC with the touch plate I provide this step by step overview. I’m using LinuxCnc 2.5.0 2.5.1. 2.5.3

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Configuring LinuxCnc to accept my touch plate (also known as a touch-off plate) was not easy. Based on the information form cnczone.com I compiled this overview.

Part 1: preparation

  1. Add Touch Plate to your system
  2. Configure the Pin in LinuxCnc
  3. Verify the basic functionality works

Step 1: the actual touch plate

My system didn’t have a touch plate nor did it have a connection prepared for this. This article is not about adding a touch plate interface to your controller board, but about configuring LinuxCnc to be able to use it. Nevertheless 2 photo’s of my work to modify my controller to accept a touch plate.

 

Driver-board-empty-socket

Driver board with empty connector. Section A is for the limit switches. Section B will contain the wires for the touch plate.

 

Driver-board-full-socket

Section A has the limit switches wired up. Section B has the new wiring for the touch plate.

 

Step 2:  Configure the Pin in LinuxCnc

LinuxCnc needs to know to which (input) pin number of the driver board the touch plate is connected. In my case it’s pin number 13, consult your driver boards manual or other provided documentation to find yours.

Configure LinuxCnc for using the touch plate. Start the Stepconf Wizard.

Select modify your configuration.

Select the existing configuration file.

Press forward on the Basic Machine Information screen.

Press forward on the Advanced Configuration options.

Now you arrived at the Parallel Port Setup screen.

Select the correct pin number, mine is 13, and select Probe In from the dropdown menu (See A). After verifying my setup (See step 3 further down) I found out that I had to invert my signal hence the invert box is ticked (See B).

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At some point you need to do the math: the theoretical milling speed of your milling machine running a milling bit at high speed. And guess what we just mentioned the 2 variables to te equation:

  • Spindle speed
  • Milling bit

So you have to measure 2 things and then you’re able to calculate te IPM. Coming up: theory versus practice.

Spindle speed

Tachometer

Tachometer

Fair enough most spindles will ave ample documentation describing te maximum RPM. Mine as well but I wasn’t all that convinced so I bought a tachometer.

Theory versus practice.

In theory the spindle would run at 13000 RPM maximum. Measurements show a maximum of 11400 RPM. So the spindle speed was exagurated by 15%.

 

Theoretical Feed rate

Fl = R/min * Ft * n

So here it is:

Fl = 11400 * 0.3 * 1 = 3420 mm / minute = 57 mm /second

 

From a pcb milling forum (it off course depends on the type of bit used) : Advised speed on milling a pcb is 20 to 30 mm/second on 40000RPM. Proportional reduced to 8 mm/second for my 11400 RPM spindle.

 

From pcbgcode: As a rule of thumb, a safe feedrate for very small milling bits (under 20 mils diameter) is about 1% of the diameter per revolution.

Using the rule of thumb approach. The bit is 0.3 mm V-tip and the RPM is 11400:

0.3 * 0.01 * 11400 = 34 mm / minute = 0.5 mm /second

 Conclusion

“Your milage will vary”

The calculated speed seems very high. From other sources and depending on the bit used te feed rates are muc lower. The deafult on the pcb-gcode setup screen is 254 mm /minute (4mm /second). It appears that te spindle speed cannot be entered there so I assume it will be higher then my 11400 RPM.

First I’ll stick to the 4mm / second but I might have to reduce that, after giving it a go.

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