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  1. #73
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    Default Re: Which bits to use for PCB milling?

    Hi again Craig,

    Hmmm ... your previous post treads many paths as I read it. Both practical and some thoughts about priorities in life and not least hobbies. It made me think yesterday and - in being practical here - what I can say is this:

    About priorities

    - I know quite well my direction in this and additional to what I have written - audio electronics - my use for a CNC is overall "audio related", i.e. also the parts that include transducers, enclosures etc. However, I won't be making e.g. loudspeaker drive units. Thus I am as sure as I can be that steel or stainless steel won't be focus fields for me. Materials to work with will be - as I have written - copper (not just PCBs), brass, aluminum, possibly bronze - and then wood and some plastics. But I guess that wood and plastics are the least demanding so I have not included these materials in my previous post as a "need to be able to do". Should I some day need materials made in steel or stainless steel I would contact a workshop that can work with these materials.

    - What I mainly need is a high precision machine for quite small sizes: I.e. no more than 200 x 250 x 50 mm cubic. What matters most here is that the Z-axis can make leveled surfaces - if possible down to 1-5 um precision over the full size (200 x 250 mm. Absolute precision is not that important but relative precision, i.e. a level surface, is). I will do likely 85% of all work within this size frame and - most importantly - this is where I will be milling metals. Mostly, and most importantly, copper and copper alloys. High milling speeds are not that important. Precision is much more important and typically depths into the material will be no more than 5 mms.

    However, for the last 15% of my work I need a machine that is capable of doing larger sizes - preferably up to 360 mm in the x-axis direction and then up to 500 mms in the y-axis direction. BUT ... if it is a gantry machine the y-axis material length can be longer as the material can be moved. Thus, my main need is to have this 360 mm x-axis work area - and then preferably up to 100 mms in the Z-axis plane. The practical work area therefore can be x, y, z: 360 x 300 x 100 mm. (an additional 100mm y-axis for fastening). ... And if there can be a practical way to move the Z-axis spindle holder e.g. 50 mm up & down then the Z-axis movement length can be kept to 50 mm iff this heightens the precision.

    About economy

    My funds undoubtedly are really limited so I have to make the CNC mill structure myself (here concluding that my current CNC will not work) - something I would guess that I likely can. I have access to a genuine steel NC mill - that is numerical control but not CNC. And I have access to a metal workshop.

    My limited funds also means that I will have to buy wisely - i.e. "best buys" - or make choices that lead me outside of the traditional product/purchase field (new/expensive brands). By saying the latter I also mean that if someone somewhere has found a way to design e.g. a high precision spindle that is not very expensive then this may/might be for me. Or similar approaches in other areas. Again with a reasonable amount of time spent. I notice here that both you and Roger have designed your spindles yourself, however, it is my impression that these are high torque spindles more than high speed spindles, right? In any case: The spindle/router I have now I may either sell or - initially - bring to a new design. But my funds are indeed limited here so a wise choice has to be made - remember here that milling speed is not a high priority.

    What I have

    I think the stepper motors (nema 23, 270 oz), the drivers (DM542), and the PSUs are quite good. 4 pcs stepper motors so can be used for a gantry machine (2 y-axes), or one drive below the milling plate. I won't mind replacing e.g. one of the stepper motors with a higher torque version e.g. for a high weight Z-axis. The drivers can deliver 4.2A and are driven from 38 VDC if I remember correctly.

    I can do 3D drawing and so it should be feasible to draw up a CNC mill to these specifications. FYI I have attached an example of a mill I drew up some months ago.

    What I learned yesterday
    It would appear that you have done some intensive reading of various catalogues and are now starting to pick the good ones from the merely adequate ones.
    Well, actually I had not read any datasheets but given that you appear to have insights into this field I decided to give dy-global another look. And having read your post I decided to study the datasheet for the HSR cars and the Bosch-Rexroth car that I linked to. And I found that two HSR15 cars placed closely together essentially have better load characteristics (static moment) than just one of the HQ Bosch-Rexroth cars of "25" size (the one I linked to). Also that there is approximately a 2x difference going from HSR15 to HSR20 to HSR25 etc.

    I also took a look at cast iron and "machine steel" items on ebay and they do indeed look very, very rigid - and heavy.

    What I am thinking

    If possible I would like to make a new smaller & very precise CNC mil. Work area 360 x 300 x 50/100 mm - within the confines of my budget. Considering that milling speed is not that important I reckon the main challenge will be the spindle's cost (unless a creative solution can be found here). I might start with the router I have and then replace it when budget allows.

    I reckon that the most important characteristic of a CNC is that the strength, rigidity & weight basically is localized exactly where the cutting bit meets the material. Thus, a weighty, rigid and precise Z-axis is key. Also that the milling plate is rigid. With this in mind I am thinking that maybe a spindle holding plate of steel or iron may be better than an aluminum plate. That said, I have access to e.g. a 20 mm thickness aluminum plate, or a 15mm plate from cast aluminum.

    For the z-axis maybe an HSR20 or HSR25 size may be better (IF at all available at reasonable cost)

    My limitations
    Time & money.

    The time constraint likely means that I will have to go "a known way", i.e. make assessments & decisions that do not require too much of my time or attention. This is a real constraint, unfortunately.

    It also means that the machine needs be ready in a not too distant future.


    Where I suggest starting

    If tjhere's any chance I can ask you to evaluate 3D drawings I make I would start out by making a 3D drawing of a CNC mill the size I have outlined. I would base it on the HSR15 parts - possibly with a larger one for the Z-axis spindle mount, if available. And 12 or 15 mm ball screws - as you suggest. Main supplier dy-global. I reckon the machine would be either gantry or bed mill - if a bed mill can accept up to 500 mm lengths in one direction (here precision is less important). What do you think about this? The main aim would be a small precision machine - and likely a replaceable spindle, unless a creative solution here can be found.


    Well, whewww! ... many words. Thanks for reading, Craig - I hope you got through in reasonably good shape ... And I would appreciate if you will give me some feedback on this ...

    Cheers from up north,

    Jesper

    Attached Thumbnails Attached Thumbnails Which bits to use for PCB milling?-cnc-1-jpg   Which bits to use for PCB milling?-cnc-2-jpg  


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    Default Re: Which bits to use for PCB milling?

    here that both you and Roger have designed your spindles yourself, however, it is my impression that these are high torque spindles more than high speed spindles, right
    Yes, my spindle is a low speed high torque unit. Industrial DC brushed motors can take a lot of abuse and generate very high torque.
    But no, I did not actually design my spindle, I just stripped it right down and then rebuilt it.

    The size you are suggesting, 360 x 300 x 100 mm, is actually quite large. Many commercial production machines are smaller, although they are likely to have a bigger Z travel - maybe 150 mm.

    Now for the bad news. The design you have is, well, not good enough for a mill. The base is far too light for a mill, although it might be OK for a light router. The vertical arms up the sides will deflect and vibrate badly. If you want to machine metals, you need to go up in mass and rigidity by a factor of at least 4x, although I would prefer 10x. Sorry to be so depressing, but ...

    Cheers
    Roger



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    Default Re: Which bits to use for PCB milling?

    Hi Roger ... Thanks again for your feedback. Just to clarify: The machine I included pictures of is one I drew a couple of months ago ... Today I would not make it not least because I would expect the vertical arms to be insufficient.

    But 4x to 10x .. are you here considering that I am talking about slow milling speeds and removing thin layers in the Z-direction?

    (Cheers) Jesper



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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,
    I think you are getting close to setting the main parameters of your design.

    You need a machine rigid enough to do brass, aluminum etc but is not required to do steel. This suggests to me that a substantially
    built gantry mill would be a good choice.

    Bed mills are appropriate for high rigidity machines but require very large sections of either cast iron and/or aluminum. The axis beds need
    extensive machining to make them useful. The cost of the materials and the time and not inconsiderable cost of all the tooling required to machine
    it count against a bed mill design for you. If however you have 15-20mm aluminum plate you could make a useful gantry mill from that.

    Retain your existing steppers and drives.

    The major and expensive items are:

    1) SPINDLE. If you have a Kress router it makes sense to carry on with it. Were you to replace it you'd want something with angular contact bearings.
    I'm not a fan of Chinese made stuff but have to concede that the high speed (24000 rpm) asynchronous spindle motors with VFDs
    offered by Chinese manufacturers are just too price compelling to ignore. Provided your design allows the room and strength to fit such a spindle in
    future you should be good.

    2) BALLSCREWS. 12mm is too small, you should be looking at 16mm or better yet 20mm. Remember that the main bearing block will have to contain the axial
    forces of the axis to which it is attached. The next thing you have to decide is whether you want 'ground' or 'rolled' screws. A rolled screw will
    commonly be C7 grade, ie 50um in 300mm whereas ground screws start at C5, 18um in 300mm, and go upwards in accuracy and price up to
    C1. The highest grades are made to order so I doubt you'll go there. A rolled C7 screw 20mm in diameter 500mm long with bearing blocks
    might cost $150 new. The same sized screw but C5 ground will cost you $1500 new. Ground screws are roughly ten times the price of rolled
    screws. If you want the accuracy of ground screws then buying second hand is the only reasonable way to do so. I got four C5 20mm (5mm pitch)
    450mm long with bearing blocks for $150US each plus shipping. My mill was essentially designed around these items. Rolled screws on the other
    hand are very much cheaper and readily available. With such a wide selection you will almost assuredly find the size you require whereas the ground
    screws you take what you can get.

    3)LINEAR RAILS/CARS. You have already been investigating those. The best rigidity is achieved by two rails placed parallel to each other with two cars on each rail
    and the axis bed/gantry/spindle attached to the four contact points. If you rely on a single car to mount your spindle than any flex in that car on its
    rail will result in flexure of your spindle, it represents a poor design. Provided your design uses matched rails/cars with sufficient separation then 15mm
    rails are adequate. By all means use 20 or 25mm sizes if they are available but generally they cost considerably more and the extra strength is not
    required. A good design with 15mm rails will be rigid enough for your purposes.

    I would start by deciding what ballscrews you want and/or can get. They are expensive and yet they represent the ultimate in what can be accurately achieved.
    If going for ground screws then what's available will dictate the size of the machine to a large degree.

    The next step is as Roger has suggested to decide whether you do a gantry type router or whether you do a bed mill design. I'm thinking a well built gantry design
    is most likely. If you want better results then a bed mill is the way to go.

    Craig



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    Default Re: Which bits to use for PCB milling?

    Hi Craig ... Once more thanks for your very helpful and detailed feedback. I'll be off for the weekend now but will get back on Monday. Likely things may clarify at least somewhat over the weekend.

    Wishing you both a pleasant weekend

    Jesper



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    Default Re: Which bits to use for PCB milling?

    Just thought I'd post here quickly on a success.

    I've recently had to go sub-200 micron detail in a circuit, where the spacing between component legs can be from 0.4mm to 0.8mm and the biggest pads are spaced at 1.0mm, The 0.8mm is through-hole too. Just to make it difficult. Generally, Thats about the tolerance limit of my milling process, but I tried a 30degree 0.1mm triangular bit...

    The results were incredible. Clean cuts close to 0.1mm across the board. Solder tins and flows right up to them perfectly. I acidentally sanded some circuit material from the PCB and thought it was still there as the clean lines were still in the base material.

    Here's a pic ( I'm also really starting to understand the value of a good microscope and am using an Andonstar ADSM201 with a 1080P capture card ) - For reference, the through-holes are 0.4mm dia. Spacing of holes is 0.8mm.


    Which bits to use for PCB milling?-win_20180914_23_00_21_pro-jpg

    The bits seemed kind of new, so I wasn't sure how they'd work, but so far I'm pretty happy with the results. 11000 RPM brushless spindle, 6 inches/minute feed, on a 6040, using FreePCB, FlatCAM and Autoleveller. Manual change of bit to drill.

    Attached Thumbnails Attached Thumbnails Which bits to use for PCB milling?-win_20180914_23_00_21_pro-jpg  


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    Default Re: Which bits to use for PCB milling?

    Hi evalon

    Yep, all things considered.

    Have a look at the specs for commercial machines. Sure, they are more than you want, but the specs will be educational.
    Then have a look in the epoxy-granite stream here at CNCZ and see how they talk about weight and stiffness.
    Milling any metal is worlds different from routing wood and engraving PCBs. For a mill, weight and stiffness are king.

    As a rule of thumb, if you want to reliably mill to within 0.1 mm, you really need a machine good to 0.03 mm, although I would prefer 0.01 mm. Yep, an order of magnitude better. Imho.

    Cheers
    Roger



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    Default Re: Which bits to use for PCB milling?

    The cuts look pretty good! The only thing I would do different is clear some unused copper between the traces to make soldering easier.



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    Default Re: Which bits to use for PCB milling?

    @Roger:

    Yep, all things considered.
    Then have a look in the epoxy-granite stream here at CNCZ and see how they talk about weight and stiffness.
    Milling any metal is worlds different from routing wood and engraving PCBs. For a mill, weight and stiffness are king.
    My goodness ... I just took a look at a couple of the granite mill threads I could find here at cnczone. And I have to admit that it humbles me ... probably better early than later and now more a feeling or "state" than acknowledged and work-able in thoughts and thinking. However, one thought that comes up is if it would be a good solution to include some kind of epoxy/granite into a design. E.g. a "constrained layer" assembly of maybe a steel/aluminum plate and then a cast epoxy granite plate ... I reckon it would really reduce the vibrations of the mill (of course depending on how it is done), right?

    @Craig: Thank you once more for elaborating and explaining. You are really, really saving me a lot of time and likely suboptimal choices! Thanks, Craig ... One question to your post above: When you mention the ground screws you bought I get the impression that those were not bought from dy-global. If this is the case do you then have a suggestion for a good vendor of ball screws?

    I will draw up a crude sketch of a CNC machine today and post it later in the day. Am somewhat inspired by the bed mill type but will see if I (with my current knowledge ) can make it work out ...

    @C-O-D: I never got to give you a feedback on this comment of yours earlier in the thread, but here it is:

    Well, I can tell you the first thing that would happen: you will buy a preheater and the most powerful soldering iron you can find.
    I think you are right, however, as it is I already have a 90w weller soldering iron, a heatbed, and a 700w desoldering station. Hope it is enough - and an additional benefit to giving heat to thick PCBs may be that I may also otherwise appreciate the extra heat now that autumn and winter is approaching here in Denmark

    Cheers to you all,

    Jesper



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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,

    When you mention the ground screws you bought I get the impression that those were not bought from dy-global
    You are correct, I got them off a guy in Malaysia, he had a swag of them so I got four. I don't have a goto supplier, ground ballscrews are
    rare and precious, you take what you can find when you can find them.

    Craig



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    Default Re: Which bits to use for PCB milling?

    Mass and thickness . . .
    all very good.

    Be warned: I incline towards the precision end of things. For a router that might be a bit over the top - but for making PCBs for modern fine-pitch chips, not so much. You end up needing the precision - until you eventually go photo-etch.

    Cheers
    Roger



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    Default Re: Which bits to use for PCB milling?

    Hi all,

    So I have been drawing for some hours now on a design that came to inspire me: I reckon it is sort of a bed mill although - well, one never knows ... I don't know the exact criteria for a bed mill. But I hope I can ask you to take a look and gently comment ;-)

    I have attached a couple of pictures of the mill so that it can be seen from two angles. A couple of comments:

    * It is a sketch as I prefer to clarify the basic design before going into details.
    * The usable movement is x,y,z: 250 x 200 x 120 mms. And then if larger items are to be worked on then they must be moved.
    * The dark grey plates are made of 40mm steel. The light grey rectangular rods are made from 50x30 mm aluminum.
    * Except for the vertical distance of the cars on the Z-axis the spacing between the cars (maximum distance between the distant edges of the cars) is either 200 or 250 mms. The vertical spacing between the z-axis cars is 180 mms. All rails are HSR15 rails & cars.
    * The brownish cube are epoxy granite as described in this post:

    https://www.cnczone.com/forums/epoxy...ml#post1996146

    The framework at the back of the mill is to be filled with this epoxy granite as well.

    * I don't yet know how much work is involved in making such a mill nor what the cost is so there are at least these two reservations in terms of actually making it.

    That said my guess is that it can be really rigid, and also well damped due to the epoxy granite fillings.


    Can I ask you what you think about this? One thought I have had myself is if the HSR15 rails+bearings will survive such a placement in the long term if they are not covered ... ?

    Cheers & thanks,

    Jesper

    Attached Thumbnails Attached Thumbnails Which bits to use for PCB milling?-mill_v4-jpg   Which bits to use for PCB milling?-mill_v3-jpg   Which bits to use for PCB milling?-mill_v2-jpg  


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Which bits to use for PCB milling?

Which bits to use for PCB milling?