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Milled hardware don't match software preview
« on: April 08, 2009, 12:56:06 PM »
Hello,

when i mill an smd pcb the edges wich are straight on the preview arn't straight at all... here is a picture:

The milled region is 18x24mm... so it's realy small...
I use a zigzag motion to cut out the PCB because there is backlash on my X-axis. Normally the G-code file isn't the problem since the preview is ok.
Here is the g-code file:
http://www.basicfox.com/toplayer.zip

What could be the problem? I use the PICStep boards.

Offline Hood

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Re: Milled hardware don't match software preview
« Reply #1 on: April 08, 2009, 01:14:08 PM »
Please attach your code.


Hood
Re: Milled hardware don't match software preview
« Reply #2 on: April 08, 2009, 01:17:39 PM »
http://www.basicfox.com/toplayer.zip thats the G-code file, if i put it online as .tap i can't download it

Offline Hood

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Re: Milled hardware don't match software preview
« Reply #3 on: April 08, 2009, 01:28:56 PM »
Sorry never saw your code, for some reason there was no text at all below the pic when I first replied, either that or my eyes are going bad ;D

Ok so which edges are you saying are not straight as it looks similar to the toolpath to me. Looks like you have a blunt cutter to me as it seems to be ripping the copper more than cutting.
Hood
Re: Milled hardware don't match software preview
« Reply #4 on: April 08, 2009, 01:39:56 PM »
I can make a gap of 0.4mm i think, its a engraving (looks like this: http://www.2linc.com/images/New_Equipment_Digest_0803.jpg)
don't realy know what tool is the best for these kind of cnc work, its all new to me. And I have covert the coppersurface with thin. But indeed its like you say the tool rips the copper off... don't cut it nice...

But if u look at the top of the image you can see that edge is not straight. It's like the more copper is between them the longer the path is.

Offline Hood

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Re: Milled hardware don't match software preview
« Reply #5 on: April 08, 2009, 01:57:43 PM »
I see what you are meaning now at the top, do you have any flex in the cutter or spindle?

What is the resolution of your machine?


What spindle speed are you running? The ripping could be because of that being too slow.

Hood
 
Re: Milled hardware don't match software preview
« Reply #6 on: April 08, 2009, 02:41:47 PM »
Thanks for all the fast reply's

I don't really know what you mean with flex? Flexability? no, but i think its a little bit out of center, so my tool shakes a little bit. Maybe that the rough copper cutting.
The resolution is 400steps/rotation and 4mm of movement/rotation so 0.01mm, but my cnc is running on 1/8 microstepping does that increases my resolution?
The spindle rotates at 6000rpm, but i can go 15000 is that a better speed?

lot's of questions, but for the original question, either the problem is with my drivers or mach 3.

Offline Hood

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Re: Milled hardware don't match software preview
« Reply #7 on: April 08, 2009, 03:16:31 PM »
Microstepping does increase the resolution, its not totally equal but its near enough to count it :)
Ok so resolution is not your problem, the shake wont be helping the cutting but shouldnt be causing your overcutting in those places.
I would tend to go a bit faster on the spindle, I have found that for my mills to spindle speed of 4000rpm I get the best results at 100mm/min feedrate, so if you are moving at 4000 I would say 1500 would be close. I use carbide engraving cutters so if yours is HSS you maybe better to slow the feed down a bit rather than increase the spindle.

I am not sure where your problem is, looks more mechanical to me but then again why it would just be at these locations I am not sure.
Hood

Offline Jeff_Birt

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Re: Milled hardware don't match software preview
« Reply #8 on: April 09, 2009, 10:59:09 AM »
There are a few things to keep in mind when milling/routing PCBs.

The first is your PCB must be mounted perfectly square to the spindle. Your mills table will probably be off 0.002" or so end to end so you'll need to use your mill to machine a surface plate to mount your PCB material to. This is essential as your only trying to take off the copper which is very thin.

Second trying to do mill the board in a raster pattern will not work well. What I use and suggest is using Eagle PCB layout software (http://www.cadsoft.de/) and PCBGcode (pcbgcode.org) to convert the Eagle board file into Gcode for Mach. This combination works very well. I have some pictures here on the forum of PCBs I have created this way.

Third you need the correct bit geometry for isolation routing. What works best is a 60 degree 'V' bit with a flute design for this application. Something like this: http://soigeneris.com/shop/Precise_Bits_86618.aspx . Look about 1/2 way down the page for the type of V-bit I'm speaking of.  At the bottom of the page you'll see 'Starter' kits that I put together to help folks get the most of the bits they will need to start milling PCBs. (Yes, I'm biased that is my web page. There are lots of other places that make similar bits. But I know the URL to mine really well  :) ).

Isolation routing does not try to remove all copper between traces. It just takes multiple (you can set how many) passes around each trace to isolate it electrically from the others. This works very well and produces very good results. If you need to/want to remove large areas of copper you can put this in the rub-out layer in Eagle and use a small end mill (far less precise) to do so.

Hope that helps...
Happy machining , Jeff Birt
 
Re: Milled hardware don't match software preview
« Reply #9 on: April 09, 2009, 12:36:39 PM »
thanks, been looking for new carbide engraving bits and found 0.1tip with a 10° angle, is that better than 60°?
here is an image: http://images.virtualvillage.com/007807-105/003.jpg?s=500&q=95&c=uk&z=en&f=0

I zigzag because i have backlash on my x-axis... (its a Deckel G1L where i put steppers on) gonna try to remove the backlash later, but zigzaging is my current solution
I've created a tool for this that strokes each path x-pixels or until it reaches an other path, then it only leaves a pixel between them to mill on. Here:
http://www.openservo.com/forums/viewtopic.php?t=931

Thanks Ben