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  1. #97
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    Default Re: Which bits to use for PCB milling?

    Good morning Craig ... & thanks again for being so forthcoming in sharing your experiences in choosing & designing your CNC. I know I have said it before - but it's really helpful

    Reading your two posts above I am thinking that it sounds as if your machine overall is very balanced/smooth working yet powerful. It also makes me wonder if the fact that you are using planetary gears with a 10:1 ratio allows for less overall vibration while milling - as I reckon the normal step angles in a stepper motor somehow translates into small vibrations being transferred to the end mill. Again likely depending on the overall mass/vibration damping qualities of the moveable system which I guess will dampen this to some extent.

    Considering things, however, I think I will stay with my current stepper/driver solution and then optimize the strength, rigidity and low-vibration qualities of the structural frame. This way I know that the basic CNC is fine and the I may eventually add better steppers/AC servos if I find that this becomes feasible.

    I do, however, need to find a solution to the "drop-heavy" risk of the Z-axis spindle mount. Doing an internet search I found this tip:



    He basically shorts one of the coils on the stepper motor when the driver power is off - apparently it is enough to prevent the Z-axis from dropping. If so then it is indeed a very straightforward solution as it basically can be handled with just the DPDT relay connected to the stepper motor coil (which is what he suggests).

    Regarding the actual material for building the CNC frame I have searched for suppliers of cast iron in Denmark (and a bit in Germany) and it appears that there are some companies selling these goods. Right now I am considering building the entire frame from 20*40 mm cast iron rectangular rods (GG-25). The benefits of doing so appear to be many:

    * the price is accessible I think - ~ USD2 per kilo for these surplus goods rods.

    * I basically can design and build all parts exactly to the sizes I wish to. And place the weight where I see it most useful. I may also build it so that it can relatively easily be taken apart.

    * Using an epoxy glue and filler the vibration level of the machine may likely be very low.

    * Machineability may be very straightforward as the main work process will be sawing the rods ... The way I think about the design I don't even have to consider precise rectangulars angles in the sawing process. And comparatively few holes to drill.

    * Additionally a bit of surface milling and grinding (or the like) for the linear rail mount surfaces. To be done by someone specialized in this.

    All in all I think/hope the design concept is coming together () ... Some process for me - but again very much assisted by you & others here ...


    Two parts that I do think I still need to find are:

    - couplings between the ball screws (specific words to search for? Something to take into consideration?)
    - end stop contacts. I suppose these may be fine:

    https://www.aliexpress.com/item/6pcs...ceBeautifyAB=0

    And then, in time, there is the spindle motor. Reading yours and Roger's posts and comments about this I reckon that a high(er) torque, low TIR version may be desired ... but this may come in due time.

    Wishing you/people here a pleasant day ...

    Jesper



  2. #98
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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,
    my understanding is that you hope to cut cast iron bars and make a lattice by bolting/threading?

    Are you sure that will provide the rigidity you require? It will come down to the design of the bolted connections.

    Have you considered a RHS, say 50 X 50 X 5 welded steel frame. There was an article where a crew made such
    machine and filled the sections afterwards with cement to reduce vibrations, it worked pretty well.

    You would ideally post weld stress relieve the completed structure. Surely there is a foundry in your region with
    a heat treatment oven that could do it for you.

    Epoxy granite is a great idea....have you tried it? I would experiment before I committed to the cost of
    materials and moulds before I did so. My experience of casting in Epoxy is that without the right (expensive)
    grade of Epoxy you are going to have MAJOR exotherm problems.....I did until I bought, at great expense,
    a casting Epoxy.

    I would rather concentrate on a structural design, materials and fabrication process that provided a rigid structure
    without additions like Epoxy granite.

    Craig



  3. #99
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    Default Re: Which bits to use for PCB milling?

    Hey Craig,

    my understanding is that you hope to cut cast iron bars and make a lattice by bolting/threading?
    Yes & no ... I intend to glue the better part of the structure together and then afterwards fill them with the epoxy/granite molding blend. These "blocks" will then be screwed together with bolts & nuts.

    Epoxy granite is a great idea....have you tried it? I would experiment before I committed to the cost of
    materials and moulds before I did so. My experience of casting in Epoxy is that without the right (expensive)
    grade of Epoxy you are going to have MAJOR exotherm problems.....I did until I bought, at great expense,
    a casting Epoxy.
    There's always the possibility of a surprise here.... But I have contact with a person who a.o.t. sells to the Danish Wind Turbine industry (e.g. Vestas) and who has extensive knowledge of epoxies and other glue types. I intend to ask him what may be a feasible solution. Also - from reading another thread on the subject here on CNCZone it seems that the amount of epoxy to be used volume-wise is about 1/6 the volume of the epoxy/granite mixture combined. Also I expect the epoxy to not have very large volumes in isolation so I would assume that the large surfaces and the heat conductivity of the interspersed iron/granite together may lead away the epoxy's exotherm heat generation.

    In any case: I may also have access to some 50x50 mm & 60x60 mm rectangular bars, also cast iron. I could glue these together so that e.g. 4 of them made up the Z-axis vertical supporting structure - a 100x100 cross section area. Or two 60x60mm glued together to make a 120x60mm structure (thick material in the y-axis?).

    Alternatively I have access to a 100x100 mm "normal steel" rectangular bar that I could have cut to length for the purpose. Either of these probably would be easier (and also heavier ) to do but the weight would be more than about 40 kgs. I'm simply not sure I can lift this .... I'm a "runner type" body build so lightweight and adroit but just limitedly "strong" in a traditional sense ... But what would you choose from a strength/damping point of view? The price difference is limited.

    I think that for the actual plates that hold the items to be milled, and the plate that holds the spindle I will choose the cast iron 20x40 rods and glue them together with a suitable high strength epoxy. This would give high strength, some damping due to some constrained layer effect, and high weight where the vibrations are generated.

    And then - not to repeat myself but just to make sure this is of less importance:

    - couplings between the ball screws and the steppers (specific words to search for? Something to take into consideration?)
    Would you say that there is anything in particular to look out for here ... Or can I just use any coupling?

    Cheers,

    Jesper


    A P.S.: If I can ask you to just - in short is fine - to give a feedback on the "structure options" and the coupling choice I think it would be simpler if I draw up a 3D drawing of the structure including assembly bolts and then post it here so that we have a common reference. This likely will take a couple of days ...

    Jesper



  4. #100
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    Default Re: Which bits to use for PCB milling?

    Epoxy granite would be good.

    I have two commercial made PCB routers, by Lenz, they weigh 1200kg each because they have "torsion box" type construction made from cast iron. Each machine has two 60,000 rpm spindles in order to cut two PCB at a time. The spindles are 1.1KW, model Precise SC53. These spindles are "direct chucking", the collet is part of the spindle and they pick up a bare tool. Each spindle has 9 tools to choose from, the collet is air driven, the spindles are water cooled.

    The machines are moving bed, the gantry is cast aluminium and substantial.

    I believe spindle runout is less than 3 micron.

    This gives an idea of what was considered necessary by Lenz.

    I'm currently replacing the electronics on these machines with Machdrives BRA double encoder servo drives, it will probably take me a month to get around to it, but I can do some tests on thick PCB then.



  5. #101
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    Default Re: Which bits to use for PCB milling?

    Hi,
    the epoxy may make up only 1/6 of the volume of the mix but you will have roughly 10l mixes, that's still a lot of epoxy.

    I have used epoxies in boat building, model aircraft making and potting high voltage electronics. All epoxies exotherm to a certain
    extent, its a question of the thermal properties of the mix and the volume of the mold relative to the surface area.

    I'm sorry but I think you are going overboard here, epoxy granite with embedded cast iron for mountings......is a very extensive/expensive
    and time consuming project, rather more than is required to mill 'easy metals' as is your goal.

    A welded structure of hollow rectangular section steel suitably stress relieved will be more than adequate. If you wish to fill the sections
    for vibration damping afterwards so much the better. The material is cheap and readily available and very quick to fabricate compared to
    epoxy granite.

    - couplings between the ball screws and the steppers (specific words to search for? Something to take into consideration?) Would you say that there is anything in particular to look out for here ... Or can I just use any coupling?
    I bought some el-cheapo aluminum couplers with a spiral cut slot. They are inclined to flex somewhat under torsion, rather more than I thought.
    They account for more 'lost motion' than the backlash of the gearboxes. As it turns out I've had two decent crashes with my machine, in both cases
    me doing something dumb, and in each case the coupler 'twisted up like a carrot'. Because of the gearboxes the steppers can generate over 20Nm
    stall torque, enough to do real damage. The aluminum couplers are a bit like a mechanical fuse, they shear off before anything really expensive gets
    broken.

    Bellows type couplers are much torsionally stiffer but way WAY more expensive too. The lost motion I get from the flexure of the cheap couplers I use is
    about 4um....I live with it! Theres all sorts of things I can spend money on without getting hung up on couplers. Good microswitces for home switches are
    a much better use of my money.

    Craig



  6. #102
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    Default Re: Which bits to use for PCB milling?

    Hi again Craig ... & thanks once more for a constructive feedback ... I will just consider what to do in terms of the cast iron/epoxy granite moldings - but apart from that I think I now have sufficient information to begin draw the mill. Looking forward to it!

    Cheers & have a good weekend ...

    Jesper



  7. #103
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    Default Re: Which bits to use for PCB milling?

    @Zorbit: Hi ... Sounds like very capable machines you have ... When you get around to it I'd be interested in hearing about your results with milling thick PCB (i.e. thick copper). Maybe can be a reference as to what can actually be done ...

    Cheers & also a good weekend to you,

    Jesper



  8. #104
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    Default Re: Which bits to use for PCB milling?

    Quote Originally Posted by evalon View Post
    @Zorbit: Hi ... Sounds like very capable machines you have ... When you get around to it I'd be interested in hearing about your results with milling thick PCB (i.e. thick copper). Maybe can be a reference as to what can actually be done ...

    Cheers & also a good weekend to you,

    Jesper
    I'll try to update the thread when the re-fit is done. I can't help feeling that your PCB needs are not compatible with the other requirements - brass, bronze etc., The tiny carbide tools used on the PCBs need very good high speed spindles. My spindles were refurbished by Vikingtest in the UK, who specialise in PCB equipment, to quote their website:

    "
    Collet taper is reground if total indicated runout exceeds .00005?. "

    "
    All armature shafts are balanced to less then .5mg/inch. on our German built Schenk balancing machine."

    Perhaps a specialist would be worth employing.

    Incidentally, the old-fashioned way to increase the current capacity of a thin PCB track was to simply lay a bead of solder along it.








  9. #105
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    Default Re: Which bits to use for PCB milling?

    Hi Zorbit,
    those machines sound really nice.

    If you have followed this thread I have a German made spindle that fitted with a Rego-Fix collet is claimed by the manufacturer to have
    a run-out of less than 5 um. All I can say for sure is that it has less run-out than I can measure! It is capable of only 24000 rpm, it was all
    my budget could afford.

    It certainly would be nice to spin the tiny endmills faster, but its what I have got and what I can afford and it does a good job if somewhat slowly.

    I started responding to this thread when Jesper indicated that he wanted to use heavy copper PCB, up to 0.5mm. The heavyist copper PCB I've
    ever seen is 0.42mm copper, and that I bought from the US. While it might be traditional to put solder on top of a trace to increase its current handling
    I can assure a copper trace 0.42mm thick can handle loads of current, way more than the same 1oz copper trace topped with solder!

    My project uses TO247 MOSFETS and have peak currents of 48A and 15A continuous with up to 500V between traces. Given that it was always my plan
    to isolation route the board and therefore not have the ability to electroplate additional copper thickness (added current capacity) required I start with
    a board that had adequate copper thickness from the start. As it turns out it is much harder to find than you might have thought. Then even once you've
    gotten it you still have to work out techniques to route it.

    I have done so to my satisfaction. My boards are SMT in the main with 0.6mm minimum spacing and minimum 3mm creepage distance around the TO247's.
    The continuous rated current density in the 3mm width source and drain tracks is 11.9A/mm2 and so runs cool hour after hour.

    Craig



  10. #106
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    Default Re: Which bits to use for PCB milling?

    Craig, sounds like you've got it nailed good. I doubt I'll ever use copper that thick myself, don't even think I've ever seen any. Old boards I worked on in the past often had solder beads or simply a point-to-point wire.



  11. #107
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    Default Re: Which bits to use for PCB milling?

    Hi Zorbit,
    I had to search far and wide before I found any.

    I work on welders for a living and yes I've seen all manner of means for manufacturers to increase the current handling
    including solder overlays and extra copper wires soldered in parallel.

    Most of the better US and European manufacturers seem to use 2oz PCB, which is fairly common and when the through holes are plated
    an extra 2oz gets deposited on the board. Thus if you took to such a board with a microscope you'd find 4oz, approx. 0.15mm copper.

    Another strategy is that copper traces are cut from copper sheet of the required thickness and then glued to the board. Such boards will
    often have 1oz signal traces but 1mm copper conductors bonded to the board for the power traces.

    When you come to discrete semiconductor devices like TO247 that the pins are not even 6mm apart. Thus if you have a 3mm trace centered on
    each leg of the device then the clearance between trace is a fraction under 3mm. The potential for flashover at 500-600V is plain. However if you reduce
    the trace width to improve the spacing then current density in the narrow trace that remains will means it heats up and the glue retaining it to the fiberglass
    fails. My solution was to use heavy copper board.

    Now that I have it I find it very useful for other less demanding boards as well. I have made some speed control boards for the wire drives of welders. I anticipate
    no more than 5A, and more regularly 1.5A in the motor circuit. With heavy copper board you can easily accommodate that current in a trace 0.8mm -1mm wide.
    It makes designing boards easy!

    Craig



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Which bits to use for PCB milling?

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