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  1. #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



  2. #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



  3. #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



  4. #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.








  5. #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



  6. #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.



  7. #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



  8. #108
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    Default Re: Which bits to use for PCB milling?

    @Craig ... I wonder if you have time for a quick question? I'm right now considering whether I should choose 20mm ball screws C7 precision or possibly 15/16mm screws with higher precision. Thinking about it I won't say that the absolute precision is the most important to me ... rather it is the relative precision between sections of the milled part that is close to each other that matters (within a few centimeters distance). Thus also low backlash may be relevant ...

    With this in mind I would guess that the extra rigidity likely achieved with the 20 mm ball screw would be a better solution. What do you think?

    BTW I am considering a "full-monty" machine. It seems to me that the price/work effort difference between a structure that can handle most all sorts of materials or a structure that can handle copper/aluminum materials is not that big. And then I'll keep the spindle & stepper motors "basic" for a start and then possibly replace them in time.

    Cheers,

    Jesper



  9. #109
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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,
    there was a pretty good description of the different accuracy grades in one of the NSK catalogues. I've deleted my copy.

    C7 means an accuracy of 50um in 300mm.
    C5 means an accuracy of 18 um in 300mm.

    What is not so plain is the variation in one revolution. Its called cyclic error. C7 (which are made by rolling) can have as much as 30um of error over one turn
    whereas ground C5 screws have and error of less than 7um over one revolution.

    When there is a variation in pitch such as occurs in rolled screws then the preload varies as well, thus depending where in the revolution you are you might
    have 8% preload, ie very tight but reducing to 0% at some other part of the turn, ie have backlash.

    Having said that rolled screws are very much less accurate than ground screws (true) they are still pretty good and certainly good value for money.

    If you want a machine accurate to within 0.05mm then rolled screws are OK, if you want to get closer to 0.01mm then C5's are required and if you want better
    then its C3's and for semiconductor manufacture special order C1's.

    There are two aspects to the required diameter. Either 15/16mm or 20mm are actually strong enough, your machine will require less than 500N force to accelerate
    the axis back and forth and counter the cutting forces. What a larger diameter offers is stability in rotation. If you have a long screw and you spin it fast as you
    would with a servo, say 2500 rpm, then it can 'whip' which is bad news. It is unlikely that with your small machine (short ballscrews) with steppers (which are pretty
    slow) you will encounter 'whipping'. There again the NSK catalogues are a great design resource.

    The other issue with the size of the screws is the size of the bearings. The motor end bearing block has an angular contact bearing pair in it. It has to
    transmit the thrust of the screw to the bearing block, thus the rigidity of the machine is dependent on the rigidity and quality of the motor end bearing block.
    Larger screws mean larger blocks which mean larger angular contact bearings which mean lower pressure for a given level of thrust. As I recall there is a very
    good discussion in one of the SKF catalogues about the bearing pairs used in bearing blocks, they are similar to regular angular contact bearings but tend to have much
    greater contact angles, 300 and 450.

    As for your choice its a case of 'pay your money for your choice and live with the consequences'.

    I commented earlier that finding the right ballscrews for your project is going to be a hurdle, especially if you want C5's or better, they are just plain rare and
    expensive even second hand. Rolled screws are easier to find and cheap enough that buying new is not out of the way but are about 1/5 as accurate as ground
    C5's.

    Craig



  10. #110
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    Default Re: Which bits to use for PCB milling?

    Have you considered using a cheaper ballscrew but driven by a servo with linear encoder on the axis ? On the machines I'm re-fitting I'm using the standard ground ballscrews, DC servo driven, with two encoders on each axis - one rotary on the motor and one linear on the axis. Drives are Machdrives BRA dual encoder type. They claim good performance with a dead stop on position. Time will tell.



  11. #111
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    Default Re: Which bits to use for PCB milling?

    Hi Zorbit,
    linear scales to 0.01mm are expensive, probably more than a second hand C5 ballscrew, and then OP has steppers not servos and lastly
    you'd need a realtime controller to close the loop. It would achieve a result but I think the complication and cost outweigh the benefits.

    Even if you used a linear encoder to linearize your screw you still have the difficulty of cyclic error causing fluctuations in preload.

    If Jesper determines that the accuracy he wishes to achieve is in the region of 0.01mm or better then ground ballscrews are the way to go.
    To make them affordable buying second hand is the way to go but then you just have to be patient and wait for the right items to come up for sale.
    Once they do you have to nab them pretty quick, good ballscrews are sought after....you can't afford to mess around once you've found some good ones.

    Craig



  12. #112
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    Default Re: Which bits to use for PCB milling?

    Good ballscrews, plus good, properly engineered, double end bearings and good GT2 drive belts. With a single encoder on the motor. Extra encoders serve no useful purpose.

    Cheers
    Roger



  13. #113
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    Default Re: Which bits to use for PCB milling?

    Quote Originally Posted by RCaffin View Post
    Good ballscrews, plus good, properly engineered, double end bearings and good GT2 drive belts. With a single encoder on the motor. Extra encoders serve no useful purpose.

    Cheers
    Roger
    Some would disagree, the idea being that more accurate acceleration and position is achievable. I'm aiming for a very accurate micro-mill, so for me it's worth a try.

    https://www.pmdcorp.com/resources/ge...ion-case-study
    "Dual Encoder Configuration

    To maximize positioning accuracy, a dual encoder configuration (dualloop control) is used. This powerful technique allows one encoder (on the motor) to be used to stabilize the motion, while the second (on the stage) is used for positioning accuracy, with all of the control details handled by the Magellan chip. Dual?loop corrects automatically for mechanical errors from backlash, lead screw or table assembly non?linearities"

    Two encoder options are to either measure output-axis motion through the ballscrew shaft with a rotary encoder or directly measure linear motion through a linear encoder. Some applications — such as a milling machine, for example — need the latter. That’s because one rotary encoder can’t compensate for thermal expansion (top image). Here, a linear encoder also helps track any displacement (bottom image). Image courtesy Heidenhain.





  14. #114
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    Default Re: Which bits to use for PCB milling?

    Hi & good morning ..

    @Craig: ... You really are a treasure-trove of knowledge & information about this ... Thanks again for clarifying!

    there was a pretty good description of the different accuracy grades in one of the NSK catalogues.
    Thanks for the tip. As it is I have actually been looking in THK's catalogue. They also have a section about the various "C" class accuracies as well as - as I saw yesterday - a small section on the rotational precision of these C classes.

    Rolled screws are easier to find and cheap enough that buying new is not out of the way but are about 1/5 as accurate as ground C5's.
    Hmmm ... I remember earlier in the thread you mentioned that your experience is that the used parts have 1/4 the accuracy of new parts. Thus, if this can be taken "verbatim", then a new good quality C7 screw would be very close to the quality of a used C5 - or so ... Isn't that correct?

    In any case I think you are right that it may not be that easy to find ground ball screws in the sizes I need. Not least, if they are to be of good quality, and at the price I can pay. I have searched in the last couple of days and there's not really a large selection on ebay. I did find this one although it likely is too expensive. But C1S quality:

    https://www.ebay.com/itm/KURODA-C1S-...AAAOSwjexbkqBZ

    I have also been drawing a bit more on the CNC mill. And even if it is still being conceptualized I think it is overall getting to look as I would like it to. The z-axis profiles are 40x40mm cross-section with 5mm material thickness. I intend to fill these with a combination of a suitable cement (as you have suggested) and small granite chips. I reckon it will likely be as good as epoxy (?), have less environmental impact, and be easier to work with. Can I ask you what you think of this design?

    Personally I think it will be a very solid machine - probably more solid than what I need for copper & aluminum but I am thinking that it is the process of making the actual structure of the CNC mill that takes time - replacing a spindle, or a stepper motor, or a ball screw is comparatively much less effort. Thus making a rigid frame from the beginning is well worth it.


    @Zorbit:

    Have you considered using a cheaper ballscrew but driven by a servo with linear encoder on the axis ? On the machines I'm re-fitting I'm using the standard ground ballscrews, DC servo driven, with two encoders on each axis - one rotary on the motor and one linear on the axis. Drives are Machdrives BRA dual encoder type. They claim good performance with a dead stop on position. Time will tell.
    Now ... I can see that both Roger & Craig have answered while I have been writing ... It sounds like an interesting approach but wouldn't it - at least if using the linear scale in connection with the milling table movement - require some very precise fixed x,y,z references - e.g. based on optical lasers? If that could be done - and the algorithms in the MachDrives you link to are well-designed - then I would actually guess that it could be extremely high precision ... And a very attractive solution since when considering ultimate precision it would not require very expensive ball screws - maybe not even very precise linear rails (?) ...

    However, in my context I think it will be too expensive ... I will need the driver, a brushless DC motor, a rotary encoder, a laser linear scale encoder (or something better - e.g. laser based), and still a ball screw ... So unless there are less pricey solutions available then it is likely out of my reach for now ..


    Cheers,

    Jesper

    Attached Thumbnails Attached Thumbnails Which bits to use for PCB milling?-cnc_3d-jpg   Which bits to use for PCB milling?-cnc_side-jpg   Which bits to use for PCB milling?-cnc_bag-jpg  


  15. #115
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    Default Re: Which bits to use for PCB milling?

    Oh ... I can see that zorbit has replied also since I was writing .. To me it sounds like an interesting idea ... one thing, though is how it will be possible to make both the x & y axes precise at the same time as the one moves relative to the other and thus the reference for one of the axes - appears - to depend on the other axis' precision ... Or am I missing something here ... ??

    BTW a linear scale 450mm is USD69 ...

    https://www.aliexpress.com/premium/0...ewCP=y&catId=0

    Cheers,

    Jesper



  16. #116
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    Default Re: Which bits to use for PCB milling?

    I haven't milled a board in a while but i think last time i had a perfect result with a 0.8mm two flute Kyocera bit



  17. #117
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    Default Re: Which bits to use for PCB milling?

    Hi Zorbit

    the idea being that more accurate acceleration and position is achievable.
    That is true, at least in principle. I think Galil offer suitable drivers for this.

    However, there are several problems to be dealt with.
    1) Temperature effects. Unless you use a laser interferometer, you will have non-matching TCs all over the place.
    2) Temperature stability. You will probably need to air condition the room and water cool the machine.
    3) Linear encoder resolution. If you want better than 0.01 mm accuracy and resolution, plus stability, you are going to be paying a lot of money for the linear encoder. A LOT more than a hobbyist might wish to pay.

    Cheers
    Roger



  18. #118
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    Default Re: Which bits to use for PCB milling?

    Hi Zorbit,

    To maximize positioning accuracy, a dual encoder configuration (dualloop control) is used
    That might have been true for DC servos where you had a tacho to linearize the velocity loop and an encoder (linear or otherwise) for position, but
    it is not true for AC servos. The common entry level is 20 bit absolute encoders and they wipe the floor with any but the most expensive
    interferometric glass scales. Increasingly common and coming down in price is 24 bit multiturn absolute encoders and they are streets ahead.

    Semiconductor machinery has a laser interferometer as its linear scale. If I could afford a three axis laser interferometer I would not be pissing around
    with an hobby CNC stuff!

    Craig



  19. #119
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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,

    your experience is that the used parts have 1/4 the accuracy of new parts
    I don't recall saying that... but may have done. The C5 ground screws I bought are superb, better than I can measure despite being second hand.

    then a new good quality C7 screw would be very close to the quality of a used C5 - or so ... Isn't that correct?
    No, that is not correct. What you are trying to do is talk yourself into believing that rolled screws are as good as C5 ground screws....they aren't.
    Why would anyone buy ground screws at $1500 a pop when you can get a rolled screw for $150?

    SKF don't, or at least didn't, make ground screws. What they do instead is roll them and then grind. They claim accuracy approaching C5 but at lesser cost.
    A close read of their catalogue tells you that they aren't bad, about half as good as a genuine ground C5, but about four times the price of a rolled screw
    or about 1/2-3/2 that of a ground C5. The cost benefit didn't stack up to my way of thinking.

    BTW a linear scale 450mm is USD69 ...
    Buy one and see....if they can make a good 1um glass scale for $69 I'll eat my hat. Have a look at Heidenhan or Reinshaw and tell me how many corners the
    Chinese have to cut to sell them at this price.

    Your choice is simple....decide on the level of accuracy you want THEN buy the components suitable for your choice. If you want 0.01mm accuracy then ground
    C5 or better. If you are happy with 0.05mm then C7 rolled screws are fine.

    If you decide to go for C5's then you will have to be patient....I searched for two months before I found a reasonable deal. That link to the Kuroda screw is superb.
    If you want it nab it now.

    Craig



  20. #120
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    Default Re: Which bits to use for PCB milling?

    Some good points made on this thread. I have no idea if my current re-fit will be as fast and accurate as hoped for, but I will post results eventually.



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Which bits to use for PCB milling?

Which bits to use for PCB milling?