Re: Auto-Level a PCB ????

jeff.birt
 

>> My experience tells me that not all PCB material is of consistent thickness.

 

I have seen this mentioned several times so this morning I sat down with a micrometer and several different scraps of PCB material from cheap stuff to expensive Rogers HF board. Most of it is newer stuff but I do have some stock that was 20-year-old coated PCB stock that I dissolved the coating off so I could use it for mechanical etching (so it sat in a slight caustic cleaner for 24 hours or so.)

 

I made several measurements on each board and found less than 0.001” variance on every piece of PCB stock. I have never seen any PCB stock vary more than this. Then I wondered how much this 0.001” difference would really make to the width of the cut. Given a 60-degree V bit with a 0.005” tip and cutting 0.004” deep the DOC would be 0.0096”. A DOC of 0.003” would yield a DOC of 0.0085”. If you are using a shallower angle bit the difference would be less.

 

My point is not to pick a fight but to point out that unless you are getting the worst quality PCB stock in the world I don’t see how PCB thickness is making any real world difference. The three biggest problems I find when talking to folks are: 1) machine set up, 2) PCB stock hold down, 3) bit run out/poor quality bits. All of these things will still interfere with making good PCBs even if you use something like the auto-leveler software. I’m not making an argument against using auto-leveling software rather I’m saying it is the last in a line of things you should look at.

 

To give you an example. When I started engraving the end panels on my control boxes I would on occasion have one where the engraving depth would vary noticeably. The end panels are pre-punched to shape and I have a special fixture to hold the stock flat. This puzzled me for a while as I was certain the thickness of the material was consistent over the 4”x10” stock size. What I finally discovered was that while I was deburring the bottom edges of the stock the rounded corners, which looked fine, had small burrs that would lift the edge of the stock very slightly. Now I deburr all four corners and the engraving comes out fine.

 

Maybe this will be of some help?

 

Jeff

 

From: pcb-gcode@... [mailto:pcb-gcode@...]
Sent: Sunday, December 13, 2015 9:55 AM
To: pcb-gcode@...
Subject: Re: [pcb-gcode] Auto-Level a PCB ????

 

 

Hi Bent,

Jeff Birt's advice is sound and applies to all cnc machining.
However, it is not the whole story. Even if you take Jeff's advice and the bottom surface of you PCB is PERFECTLY lined up with your XY plane, you may still have problems with Depth Of Cut (DOC).
This is because what matters in PCB track cutting is not the bottom of the PCB but the top.
My experience tells me that not all PCB material is of consistent thickness. You cannot even rely on the copper being of consistent thickness.
I always surface a pocket jig prior to the etch pass (more for the location in XY) and use vacuum hold down but can never get consistent results without using Autolevelling. The free version is fine.
These days, most of my boards are for SMD components with track widths of 0.3mm.
FYI my settings are
Cutter 0.2mm 30deg
DOC 0.045mm
RPM 25000
Feed 250mm/min

To be honest, with the Autolevelling software, you don't have to bother getting the board 'flat'. Just make sure it cant flex during etching and it will be fine.

I hope this helps

Regards
Martin Marriott
mlm Solutions

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