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  1. #1
    Hi,

    I've decided that I need a CNC machine to mill and drill double-sided PCBs (0.4mm track size would be ok). I'll also want to cut fascias in FR4, mill custom PCB mountings (POM or ABS), and engrave casing for prototyping and maybe initial low-volume manufacturing.

    The best combination I can find for my money is a Merchant Dice A4 ballscrew machine kit with 800W Kress spindle, combined with Motion Control 3-Axis Driver Kit-1 from Zapp Ltd (3Nm SY60STH86-3008 motors, PM752 drivers), and Mach3. This appears to be adequate and upgradeable, and will cost me around 1500.

    I'm just looking for a sanity-check. Am I over-optimistic in thinking that a CNC machine costing that amount can give the repeatability and linearity to do double sided PCBs? (layer misalignment over 0.1mm can ruin the board)

    Are there better options for similar amounts of money that I've missed? I'd prefer to buy from UK suppliers. All advice and opinions gratefully received.

    Regards,

    Kevin

  2. Firstly, welcome.

    A good CNC machine should be able to hold an accuracy much better than 0.1mm but you'l need to work on the ballscrew nuts to get down to low levels of backlash to get the repeatability you need, but its certainly feasible The trick with double sided anything is the jig you need to ensure the part is exactly in the right place when flipped. The simple solution is a hard edge to push the board against, the difficulty is getting repeatability as the slightest dirt or swarf will give an offset. The best solution is a 2-part frame into which the board is placed which has locating pins that guarantee the registration from either side. This could be made from engineering plastics (delrin, HDPE) on the machine itself.

    I cant comment on the MD A4 kit as I've never seen one, however I have heard some 'less than complimentary' views. I'd have a think about the Kress spindle, it will certainly do, but runout can be an issue. I beleive there are some simple bearing upgrades to improve it. The 3Nm motors are a bit overkill for an A4 ballscrew machine but will serve for a bigger machine later.

  3. #3
    Hi Irving,

    Thanks for your welcome and reply.

    I had in my mind that I'd use two holes on the board for registration, either drilled just for that purpose, or possibly making use of diagonally opposite pads. However, I expect the custom holder with registration pins will be a much better option. I'm assuming that CAM software such as Mach3 will naturally handle the recalculation of relative co-ordinates.

    Spindle runout may be an issue - it would seem daft going to some lengths for repeatability just to consistently gouge furrows out of the board. The Kress datasheets don't specify runout, and I can't find any reference to people who have measured it. Can you suggest an entry-level spindle that might do as an alternative?

    I've also found it difficult to find any other UK manufacturers of entry-level XYZ tables. I've read mixed opinions on the Merchant Dice kit, but that mix includes positive reports too. If I didn't need relatively accurate PCB tracks, I probably wouldn't think so hard about this - nothing else I need requires much precision. I just want to be sure I don't have copper remaining at one end of an isolation path, or wiggly diagonal tracks, for example.

    I'm realistic about what I might expect from a 1500 machine - as long as I can get milled and drilled double-sided boards that are as good as I get with single-sided toner-transfer and manual drilling, then I'd be more than happy.

  4. #4
    Quote Originally Posted by ksangeelee View Post
    I'm just looking for a sanity-check.
    Okay, sanity check... 1500 would buy you fifty 160x100mm DS, PTH, gold plated PCB's with 2 resist and legend masks, all different and guillotined to size from Olimex with no alignment problems to worry about.

    Presume you want it to do something else or think it looks like fun :naughty:

    Run out in the chuck is not a problem because it still cuts round but oversize. Simply lie to Mach about how big the tool is to compensate.

  5. #5
    Look carefully at what you want it to do. If you want to do any production PCBs - even for very short runs, then pCB milling is a non starter. If you are doing prototypes and you can afford to wait a week or two, then again it isn't worth it. If you are doing prototyping at low speds it is better to use Veroboard or similar techinques.

    Where PCB milling scores is where you need a prototype within hours, or perhaps days but the components are not available in non-SMT, or where you want to investigate high frequency layouts - such as switch mode PSUs, BLDC controllers etc..

  6. #6
    Quote Originally Posted by Robin Hewitt View Post
    Okay, sanity check... 1500 would buy you fifty 160x100mm DS, PTH, gold plated PCB's with 2 resist and legend masks, all different and guillotined to size from Olimex with no alignment problems to worry about.
    Twenty-five boards when you add express shipping, but still respectable. Turnaround time is one reason that I chose not to use a fab-house - I'd need to allow at least one week per iteration, which I could otherwise do in a morning.

    Presume you want it to do something else or think it looks like fun :naughty:
    Both! I'll confess there is something a little 'Christmas Morning' about CNC machines.

  7. #7
    Click image for larger version. 

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    This was done on a KX1 mill as a test for someone who wanted to do roughly the same prototype boards with no lead time.

    Not knowing sod all about PCB's, only JCB's, I asked for a file, this was code generated direct from one of the PCB software packages.
    On the left is the code as sent but the tool depth was too deep so it was edited and the one on the right run again.
    You may recognised the pad layouts but as scale the holes which were in the scrap sample board are 3mm.

    A Krees spindle will cut these tracks with no problems, no bearing update needed, I do finer work than this with no problems.

    .
    John S -

  8. #8
    Quote Originally Posted by John S View Post
    Click image for larger version. 

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    This was done on a KX1 mill as a test for someone who wanted to do roughly the same prototype boards with no lead time. ..... You may recognised the pad layouts but as scale the holes which were in the scrap sample board are 3mm.
    Looks like 2.54mm pitch through-hole, and 1.27mm SMD. The job on the right is close to what I need. Do you happen to remember what tool you used to mill this?

    Just looked at KX1 specs - shame about the exchange-rate, because that would otherwise have been an option for me. The stated linearity and repeatability are well within my needs, and 180mm z-axis travel would be really useful.

  9. #9
    Just a vee shaped engraving tool with 30 degrees included angle, good for PCB's because the less you go the finer the cut and it's not so prone to breaking like a 10 thou cutter.
    John S -

  10. #10
    I tried standard engraving tools from suppliers in U.K., Poland and China in various angles from 30 degrees to 60 degrees, but were unable to get the 0.25mm traces and 0.25mm isolation that I wanted. 60 degrees was better than 30 degrees - cleaner edges and fewer burrs.

    I came across spade mills on CNCZONE a couple of years ago and have not used anything else since - there is little burring at 20,000RPM and the life is about twice the standard type. I can easily get 0.25/0.25mm and have got 0.17mm track and 0.13mm isolation in trials - but not used on working PCB.

    You can get them now from :- http://www.drewtronics.net/

    Mike
    Last edited by leadinglights; 09-06-2010 at 01:38 PM. Reason: add brag about 0.17/0.13mm

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