Printed Circuit Board

[paul_z]

I needed a PC board and made it with my bot.


1874

The disc under the board is a quarter.

Paul Z

[magic]

That looks like you've got some tight tolerances.. Is that all engraved?

Second question, I need someone who can program some pic micros and/or stamps, what do you know about it?

[dana_swift]

Paul- what bit did you use? Did you start from a Gerber file? Or just draw the layout in PW.

Magic- I have done a lot of embedded CPU work, including PCB layout. Usually I use Renesas CPU's if I get a free choice.. let me know if that has any interest.

D

[paul_z]

Guys,

I used a good 60 degree bit to cut the traces. For some reason the image shows the traces to be thinner than they really are. Probably a jpg compression thing.

The PC board is quite flexible but the V bit forces the PC board against the table.

The board is a shaft encoder which will be hooked up to a PIC microcontroller. I've written several thousand lines of PIC code. It's interesting (but not fun).

Paul Z

[ted]

For others who might be interested in PC boards, I have been puttering making some very small boards for surface mount stuff. Not perfect success yet, but promising.

You might find these two 1/64" cutters available from MSC of interest:
- MSC#04468567 ROBBJACK Solid Carbide mini endmill
- MSC#05836036 SGS 4Fl Carbide Namite coated endmill

But what I really find promising for cutting thin grooves and leaving thin traces is a 60deg ZrN Mech Etching Bit (.005 tip 1/8" shank) from Think&Tinker (www.precisebits.com (http://www.precisebits.com)). These have the potential of doing some really detailed work.

With the 60deg etching bit, I haven't done anything other than a couple of simple sample circuits. I brought the bitmap of the board into PartWorks where for testing, I manually drew over the circuit and then tool-pathed it for v-carving. Seemed to make a nice test path.

Two issues with this kind of work: 1) The irregularities in the thickness/height of the board can really be a problem for v-carving. I'm hoping to get someone to help create a surface mapping routine (say, touching-off with the cutter) that we can then use to locally correct the height of the cutting [I'm bothering Bill Young and Gordon Bergfors about this, but don't have a system yet]. 2) The little bit of backlash in ShopBot's gearboxes does come into play at this level. We'll probably need to spring load each axis a bit for this type of cutting so that the gear slack is always taken up in one direction, or plan cutting so that positioning slack is always taken up from the same direction.

I'd enjoy hearing from anyone who makes progress doing tight circuit boards.

Ted Hall, ShopBot Tools

[paul_z]

Ted,

What if ... one had a mechanical contactor (like the touch probe) that was attached to the Z axis near the bit. It could be used to detect the surface of the board and dynamically adjust z up or down to follow the surface of the board. It could even detect flex while cutting and control the Z axis in a closed loop.

Machines designed to do this type of PC board cutting actually have a spring loaded cutter (the motor and bit) and a foot that rides on the board. I wonder if a dremel tool has to much run out? It could be spring or gravity loaded and have a foot attached. Hmmmmm ...

Paul Z

PS I'm going to get some of those bits. Thanks for posting the info!

[stevem]

Paul, most Dremel type tools and wood routers have enough runout to break small bits as soon as they touch the material.
I have been cutting aluminum recently and found that HSS bits are much more forgiving of errors in feed rates and spindle runout, at least while learning to use small bits.

[dana_swift]

Ted- thanks for the info. I will probably write a gerber to SBP translator at some point as I make quite a few PCB's a year. Gerber files are in RS274X format, which is also the format of GCode files so widely used in the CNC industry. I have written the RS274X file reader, just not the translator.. the first time I want a prototype pcb that will probably happen.

Being able to make single sided prototypes would be a great capability. Most of my boards are 4-6 layer with plated through holes and very small pitch surface mount components. It would take a small bit to cut between the traces, but a downspiral end mill should work. Double sided boards should be possible, as long as component leads can work as via's.

The cutting I have done in wood with .020 bits has resulted in short bit life, so cutting FR4 with a 2 ounce copper surface seems even worse, but I haven't tried it (yet).

Too many projects- too little time! The thing that keeps me from getting too excited is the $33 price per two layer boards from advanced circuits. That has taken care of a lot of boards at a price I don't expect I can match by doing it myself.

Its still intriguing tho-

D

[dingenis]

paul zank,
just a thought,
there are anti-backlash gears, springloaded in the gear design.(I have no idea if these are available for the 'shopbot range gears')
Berg manufacturing, have catalogue from pre-internet .(precision drives and components)
establishing proper z-depht needs an extra axis, and can be done.(without installing a second Z axis, just a simpel spring operated device with a probe would be able to handle that. Some programming involved .
grt

dingenis

[knight_toolworks]

here is what you need but it is not cheap. You can use the depth stop nose to control the depth.
http://www.2linc.com/spring_loaded_engraving_tool.htm

[ted]

ahhh ...

Interesting stuff ... maybe we'll have to go deeper into this. The dynamic z compensation techniques sound good. I think I'll push a little more on the simpler stuff first.

Paul Zank also forwared me a note on an interesting air die grinder from Harbor Freight (47869-2VGA) that might be useful for these purposes (fitted on the Z).

But have to agree with Dana ... it sure is easy and (relatively) inexpensive to order a board from the internet. It comes with great thru-hole vias and labeling (at very good resolution and complexity) ... and you can even get it quickly for a bit more of a price. I guess we all are still just a little attracted to the quick d-i-y version ...

There are several small commercial CNC systems available for PCBs. I think the Fab Lab (see my web column) guys use the Roland Modello(?) at the moment and seem to make some pretty reasonable boards with it (they use those 1/64" bits). This would also suggest a certain broader interest in immediate gratification because these boards also suffer from thru-hole (though there is a system for this) and multi-layer limitations.

Ted Hall, ShopBot Tools

[dana_swift]

I thought it would be interesting to look at my most recent PCB and see what its requirements were.. the track-track spacing is 0.008", but could be laid out with a 0.010" requirement to make it "shopbot compatible".

Here is a typical 8pin SMT IC, it uses 0.010 spacing, but could be laid out so "traces between pads are not legal" and that would make it possible with the 1/64" bit.


1875

Almost all of my boards have one or more component like this one:


1876

That has a hard requirement of 0.010 due to the lead density, and this example was laid out with a 0.008 allowance. (This is a CPU footprint.)

I looked at some typical SMT areas, almost all of my discretes are 0805 parts, so that would be easy to do with a 1/64" bit.

It certainly indicates shopbot PCB's are practical for some of my boards, and I will probably experiment with this.

The first problem I will need to address is going from Gerber files, where the Gerber specifies "what not to cut". The toolpath is an area clear between the traces. Some interesting programming for sure!

A 1/128" bit to make the smallest cuts is really really tiny, I can see a two pass strategy. Use the 1/64 (or similiar bit) to clear the bulk of the copper, then switch to a "detail" bit and finish up with the fine areas around the pads where the other bit wont be able to get into.

FYI the above board from a PCB house costs $33 each as shown. Custom PCB's are actually a commodity due to CNC technology and the standardization of file formats in the industry.

Very interesting stuff to think about-

Thanks

D

[jseiler]

Before we started shipping boards off to expresspcb, we used one of these for fab.

http://www.lpkf.com/home.htm

It had ballscrew x and y axes, with a solonoid z. the cutting depth was set by an adjustable collar. it worked pretty well for its day.