Making PLT files for circuitboard milling

>>This page gives you an example of how to make your own PLT files for milling prototype circuitboards. <<

After buying and the building of the Zoltar machine I was traying to produce a prototype circuitboard.

I made a design in my circuitboard editor. It produce a kind of PLT file. But there was a problem.

Making the board it cut away the things like paths and wires, away. Just the stuff that have to stay.

After 2 weeks of trying and searching on the internet I found a way to produce the right PLT files.

I'll provide you with some instruction how to create a suitable PLT file for PCB milling.

There are many CAD programs to make circuitboards designs,

and they make a kind of PLT file that my stepper software (PCNC) can't deal with.

I found a CAD program called EAGLE. It has a userfriendly way of working with and a excellent

CAM processor.

I didn't see such a CAM processor in any other program.

You can download a free shareware copy at the website from EAGLE.

For downloading go here

I wish to show with a simple example how to make a working PLT file,

so you also can make a nice circuitboard with your (Zoltar) machine.

Step 1:

Create a design with EAGLE. Below you see a design of two connectors.

The design layout before CAM processing.


Step 2:

Select the ULP (user language program) called mill-outlines.ulp.

This one and may others comes with the download. Now run the ULP.


Step 3:

After running the ULP you get to see this field in wich you can make some chances of settings.

You can select the layer or layers, size of the drills and so on.

Take your time to get familiar with it.


Step 4:

The basic PLT file is ready and this is the result.

You can watch the PLT file in a program called GERBview.

There are more programs but I like this one.

You can also download a evaluation copy of GERBview version 5 ,


Step5 :

Its possible to edit and optimize your PLT file with an userfriendly editor called BOcnc.

A free copy of this wonderful program can be downloaded from

With this program step 4 can be skipped. Bo-CNC can be used to define the radius compensation

aswell the processing order. (it does have many other interresting features)



Load the PLT file in your CAM -steppersdriving- software.

You can see that the PLT file is complete,

first it drills the hole in different sizes and then it mills the wires.

It's great. I hope it works just as fine as it does for me.

If you have any questions or remarks please let me now by e-mail at - -


Have fun with it :< ) John Lammers