Opinions sought on cheap desktop PCB milling machine

[saxonhawthorn]

Grateful thanks to Mike for steering me towards a Proxxon mill. Their new KT150 compound table is the perfect size for milling my PCBs, and looks sturdy enough to hold the work properly. I have settled on:- a KT150 table, BFB 2000 Mill/Drill Stand, and BFW 40/E Mill/Drill Motor and Controller, all at http://www.proxxon-direct.com/index.html .

And to Hank for advice about leveling, calibration, cutters, and warping. All greatly appreciated.

I'll do the CNC conversion myself, and I should be able to machine the stepper motor mounting plates on the mill. Then I'll make a cabinet with extractor to keep the dust under control. So I now have an enjoyable summer ahead, and I'll post the results here when I have something to show.

Thanks again to all for your very helpful advice.

Ian

[HankMcSpank]

Grateful thanks to Mike for steering me towards a Proxxon mill. Their new KT150 compound table is the perfect size for milling my PCBs, and looks sturdy enough to hold the work properly. I have settled on:- a KT150 table, BFB 2000 Mill/Drill Stand, and BFW 40/E Mill/Drill Motor and Controller, all at http://www.proxxon-direct.com/index.html .

And to Hank for advice about leveling, calibration, cutters, and warping. All greatly appreciated.

I'll do the CNC conversion myself, and I should be able to machine the stepper motor mounting plates on the mill. Then I'll make a cabinet with extractor to keep the dust under control. So I now have an enjoyable summer ahead, and I'll post the results here when I have something to show.

Thanks again to all for your very helpful advice.

Ian

As it goes it's the Proxxon BFW/40E that I useas a spindle a nice solid quietish spindle (albeit a bit chunky & low RPMishh) ...I trialled AudioAndy's before buying it....in facr I think Andy had it up for sale a few weeks ago on here for a good price (though probably too late for you now!)

[saxonhawthorn]

Andy had it up for sale a few weeks ago on here for a good price (though probably too late for you now!)

Story of my life!

Ian

[Jonathan]

I also use that cheap PCB from rapid, but tend to need the bigger sheets.

Have you considered toner transfer, where you print the mirrored artwork off using a laser printer onto the correct type of paper, place it over the bare board, iron it on then etch as normal? I've used it quite a lot and you can do very accurate boards with a bit care. The best paper is the type that sticky labels peel off as the toner doesn't stick very well to it. The finest I did was an 80-pin TQFP as a test. PCB was fine but I couldn't solder it!

Anyway, clearly milling PCBs is superior. Whilst selecting the track outlines and offsetting them to create the toolpaths works, if you're trying to make fine board it can be difficult to ensure pieces don't overlap. So instead I convert the tracks to the minimum required lines to isolate each connection.

From this:
(drawn with funny pads/track widths to make conversion easier)


To this:


Unless you're doing fancy RF stuff the odd shape tracks shouldn't be a problem. It's also less cutting, so clearly tools last longer and it doesn't take as long to cut. The latter isn't really an advantage unless you're making several of the same as it takes a while to redraw it.

As long as you don't skimp on the stepper motor and drivers there's no reason to expect long term problems with missed steps. This is particularly important since you seem to have chosen a relatively heavy machine bed with dovetail slides, not rolling balls, so the total force required by the stepper motors (F=uC+ma) will be high compared to a moving gantry router if you want to get decent acceleration and speed, which you do for PCBs since they have a lot of short moves and changes of direction.

[HankMcSpank]

Re what you've done to minimize thecut lines...there's a free app out there called visolate - it's basically a midline voronoi region generator ...it takes a gerber as an input file & creates an NC file. It's poorly supported & the documentation sucks, but it does work ok (you just need java loaded on your PC with JAva's 3D libraries loaded too, it's explained a little better here...

http://blog.makezine.com/2010/07/26/...sing-visolate/


Terrible stray capacitance though so forget it for anything over 1khz or so!

(I'm also in email contact with a chap who after a bit of communication to/fro, is soon about to lauunch a web portal where you upload a dxf & it'll generate the voronoi midlines & give you the file back with the midlines done for you)

Re worring about offsets.....in cambam you can import a gerber, fine tune the offset & see the cut width onscreen...the trick is to get the offset so that the midlines of ajacent pins of your smallest IC pitch so they almost share the same path. Difficult to explain, but look here...



the IC pads are black, the tool cut width is turquoise & the midline for the toolpath is the dark blue thin line in the middle ....it's important to set your offset to get these midlines to almost kiss, but not go over, else you'll be unintentionally cutting into the neigbouring pad.

[saxonhawthorn]

http://blog.makezine.com/2010/07/26/...sing-visolate/

Terrible stray capacitance though so forget it for anything over 1khz or so!

Re worring about offsets.....in cambam you can import a gerber, fine tune the offset & see the cut width onscreen...

Well I think it's a safe bet that I'll be going quite a bit over 1 kHz, and I can do without yet more stray capacitance problems, so perhaps I'll give VISOLATE a miss.

But I take the point, and your pic is helpful. CamBam has probably got to go on the shopping list.

Ian

[saxonhawthorn]

Thank you Jonathan. Yes, I'll make sure the steppers and drivers are beefy enough. I haven't done any CNC yet, but I used to plot PCB artwork on a Roland plotter and I know what you mean about all the stops and starts and changes of direction. Dovetails have more friction than ball bearings, but unless there's a similarly solid XY table out there with ball bearings, my guess is that it's easier to compensate for high friction by beefing up the steppers, than it is to compensate for a less-than-rigid (aluminium) ball bearing table of the kind I have seen being offered by the oriental suppliers, some of which look a bit flimsy to me. But I confess to a total lack of experience.

I hadn't even got as far as worrying about minimising toolpaths, and I'm going to have to print off your two examples and lay them side-by-side to ponder them. Obviously there are things to learn about this CNC business! Is there a good book out there which deals with subjects like toolpath optimisation?

Ian