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View Full Version : Accuracy of milling PCBs for surface mount?



logik
06-20-2009, 02:21 AM
I started looking at building a home CNC for simple PCB drilling by salvaging parts from old equipment. In the process have discovered that I could potentially do many more useful things like cutouts in plastic cases, light aluminium work etc if I built a semi decent one. Prices for parts from online shops seem quite reasonable, but what sort of accuracy can I expect? One site I've seen uses a fairly expensive CNC to mill PCBs, like the one below.

http://timeguy.com/cradek-files/cnc/pcb/dscn6119.jpg

(Sorry for image size)

This sort of accuracy is pretty astounding, would a decent DIY CNC be able to do the same? Being able to mill SMD pads like that would be very neat, but if that's too small then I'd probably just build an automatic PCB drill instead. A 60 degree cutter is apparently used to do the above. I don't need a huge model, a table top 30cm square working area would probably be fine.

Any thoughts from people who have built machines previously? I've seen other threads about PCB milling, but don't know whether people regularly mill SMD pads.

Jason Marsha
06-20-2009, 07:03 AM
Check out link below

I cut those on my JGRO machine and they came out pretty good. I milled and drilled with the same bit.

http://www.cnczone.com/forums/showthread.php?t=53717

mcphill
06-20-2009, 09:50 AM
Certainly doable!

http://public.fotki.com/mcphill/mcpi-inc/fireball/pcb-work/pcbmilling2/

pminmo
06-20-2009, 01:42 PM
Same discussion is further along in this thread: http://www.cnczone.com/forums/showthread.php?t=81987

logik
06-20-2009, 03:05 PM
Thanks all, there is some really interesting information here that I hadn't found on Google.

I am most interested how accurate it would be at milling SMD pads, but it does look promising. Time to think about building one, which will take a few months I'm sure!

mcphill
06-20-2009, 03:54 PM
Thanks all, there is some really interesting information here that I hadn't found on Google.

I am most interested how accurate it would be at milling SMD pads, but it does look promising. Time to think about building one, which will take a few months I'm sure!

If you want to build one, just to build one, that is fantastic, and I hope you enjoy the process. If you would rather "get started" using it, and shift the learning curve from how to make one to how to USE one, then follow the link:

http://www.probotix.com/FireBall_v90_cnc_router_kit/

You can't beat this machine for the price, and I think you'd be hard pressed to to make one cheaper yourself! While it's not the most robust or stoutest machine out there, it will get the job done at a very attractive price point.

No, I am not affiliated with Probotix, but I am a fervent fan of the machine. The support forum is also fantastic:

http://groups.yahoo.com/group/Fireballcnc/

Zangetsu57
09-21-2010, 01:42 AM
I'm glad to see that accurate milling is possible on a good (relatively) inexpensive DIY/kit CNC!

Not to threadjack, but I'm also a newbie to milling machines, at least when it comes to building them. I have a good amount of experience in using the LPKF S100 as a student to fabricate various through-hole and surface-mount PCB's. I'd like to build a PCB CNC machine to do the same, ideally for less than $500 (cheaper is better). It is purely for hobbies.

How does the Fireball compare to this Zen Toolworks CNC kit: http://www.zentoolworks.com/product_info.php?cPath=14&products_id=74 ? Does anyone have experience with both?

mactec54
09-21-2010, 10:04 AM
mcphill

The Fireball would not be able to cut the SMD pads accurate enough, you need a tight
linear rail/ballscrew machine & a very good spindle, to do the fine work needed for those boards, regular boards yes it could do it, but will not do SMD boards very well

mcphill
09-22-2010, 09:06 AM
mcphill

The Fireball would not be able to cut the SMD pads accurate enough, you need a tight
linear rail/ballscrew machine & a very good spindle, to do the fine work needed for those boards, regular boards yes it could do it, but will not do SMD boards very well

NEED, or want... Sure better tolerances are desired, but are they needed? Not for my work. SMD is also a very broad topic.

This was done on my fireball:

http://images42.fotki.com/v1372/photos/1/435091/7662505/pennyscale2-vi.jpg

Those are SMD pads, so you cannot say that it "cannot" be done on a Fireball. If you want to work with something really small, like QFN packages, you are right, not a good fit, but that is at the VERY small end of SMD for the hobbyist...

mactec54
09-22-2010, 05:04 PM
mcphill

Sure it can be done on almost anything, but in logik photo is what he was looking for not
what is in your photo

Zangetsu57
09-22-2010, 11:18 PM
mcphill

The Fireball would not be able to cut the SMD pads accurate enough, you need a tight
linear rail/ballscrew machine & a very good spindle, to do the fine work needed for those boards, regular boards yes it could do it, but will not do SMD boards very well

I guess it all depends on your definition of "accurate enough". This guy milled this board on a Fireball CNC: http://www.millpcbs.com/index.php?option=com_content&view=article&id=25&Itemid=67 . The results are pretty damn good imho, but obviously not as good as, say, a LPKF S100.

These are the size of components I'll want to be able to mill. Does anyone have any suggestions for CNC kits or good DIY guides that can do this type of work? Does anyone have experience with the Zen Toolworks 7x7 kit?

Thanks!

RomanLini
09-23-2010, 04:04 AM
mcphill

Sure it can be done on almost anything, but in logik photo is what he was looking for not
what is in your photo

I don't think there's any significant difference in accuracy between the 2 photos, Logik's photo looks pretty held up to the light but there's nothing special about it. In fact it seems to have used a cutting tool with a wide point and/or lots of spindle runout.

McPhill's photo looks messy up close and hasn't had the burrs sanded off yet, and was cut a little deep into the pcb but accuracy is fine you can see by the double cuts lining up together with no problems even when it changes direction.

I expect any of those small machines should be good enough for this job provided you snug up the travels so they have no slop and likewise snug up the leadnuts.

mactec54
09-23-2010, 07:40 AM
RomanLini
Quote
I expect any of those small machines should be good enough for this job provided you snug up the travels so they have no slop and likewise snug up the leadnuts.

This is what I'm saying, to take the slop out you would have to rebuild the machine, with different rails & ballscrews, you can't take the slop out with what they have been made with

mike2768
09-23-2010, 09:15 AM
Here a machine that offers great accuracy and the fit and finish is second to none. See link below. I believe this machine start at $1000. The pictures for the second link is a PCB that was done with these machine, scroll down to Post #237

http://www.m5050.8k.com/

http://www.cnczone.com/forums/showthread.php?t=109757&page=9

microcarve
09-23-2010, 05:41 PM
Fireball machines...as well as my new small machine...which is yet
un-named do not have "slop" in them.

It'd be pointless to spend countless hours making machine with "Slop"
in them.

Doing ANY sort of extra fine traces like these examples has every bit as much
to do with operator experience....which means choice of spindle, bits, software
settings, and leveling the table. Speed, Feed, Depth of cut.

My machines...and those I've made in the past will certainly position and well
have enough power and accuracy to make these examples. But no inexpensive
machine will do them without some trial & error and practice. The problem
with the whole thing is the soft buttery copper laminated to the rigid, brittle
and highly abrasive fiberglass.

That's 2 separate kinds of material being cut at the same time. Bits wear
at different rates depending on what they're cutting. That's why PCB bits
have to be changed so often....there are different levels of wear on it after
even a few uses.

The right bit angle....the right depth of cut....High Quality bits. All those
will make a big difference. Again...speeds, feeds...etc. etc...

Operator experience.

John

mactec54
09-26-2010, 12:34 PM
logik

The other thing is to see what other people are using, to do the job that you want to achieve

http://www.voodooengineering.com/index.php/pcbprocess/milling/66-my-cnc-pcb-mill