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  1. #145
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    Default Re: Which bits to use for PCB milling?

    Hi,
    yes, that is the classic difference between strength and rigidity.

    Bolted joins are unlikely to break, ie they are strong enough, but may move slightly, ie not be rigid.

    In order to make the structure a rigid as possible the temptation is to increase the tightening torque of the bolts at which
    point you could induce a thread failure.

    Craig



  2. #146
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    Default Re: Which bits to use for PCB milling?

    Hi again & good morning as it is here in Denmark ...

    @Zorbit: Hmmm ... I somehow get the idea about dowels but I would really prefer not to make things more complex than they already are. And from searching ebay I notice that although metal dowels may increase the effective "thread diameter" their length appear to not be very long. Which I reckon is less advantageous in this context. But thanks for suggesting ;-)

    @Craig: Hi Craig ... And thank you again for a detailed and helpful reply - and the drawings ... I have been giving the topic some thoughts and what I have arrived at are these "combined solutions":

    * I will make the threads into the CI very long - typically 3-4 times the bolt diameter like you suggested.
    * I will consider using something like a Loctite thread+bolt "lock" glue so as to both lock and "cushion" the connection between the bolt and the CI.
    * I will use 10.9 or 12.9 quality steel bolts wherever feasible ... The idea is that the rigidity/stiffness of these bolts will more evenly transfer any force exerted on the bolt deep into the CI - thus distributing this force deeper into the CI. Hopefully with no/less deformative wear on the CI over time.
    * I will use 12.9 quality threaded rods for the base side mount connections. Again, based on the same reasoning and since these are about 23 cms long I think that would be feasible to do. M12.
    * I will use large size - and thick - washers between the bolts and the CI so as to distribute the force over a larger area.

    And I will omit epoxy everywhere ... Thinking about it (and with e.g. your feedbacks) I just am not sure it will work/last in the long run.

    About the spindle motors

    The motors you linked to from mechatron are indeed impressive, however, I just made a search for their price and - as you hint at - they are quite costly. I think that what would be "ideal" would be a VFD working with e.g. a gearboxed spindle motor, or something "similar", so that at higher speeds the coupling was about 1:1 whereas in order to get a higher torque at lower rpms some kind of 1:2 or 1:3 gearing was available when using lower rpms. I assume, however, that this may be quite expensive - and except for smaller belt driven solutions I haven't seen this offered when searching for spindle motors ...

    So, unless one of you suggests otherwise today I will conclude my search for spindle motors and go for the VFD solution I linked to above.

    Cheers,

    Jesper



  3. #147
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    Default Re: Which bits to use for PCB milling?

    Ooups ... there's a P.S.: ... The Z-axis motor mount plate has a rather long lever arm (I hope this is the correct expression) so I would guess that the lowermost linear rails bolts would see quite some vibration. They are spaced 6 cms apart and although the rail itself is quite rigid it won't be entirely so ... Thus I am considering drilling/milling some additional fixture holes into this z-axis linear rail. E.g. 1-2 holes inbetween the first two pre-drilled holes and then one more hole inbetween the next two pre-drilled holes. Does this sound reasonable - and would e.g. coated carbide end mills do the job at very low spindle rpms? I have access to a manual mill that can do between 26 to 650 rpm ... Weighs a ton (literally).

    @Craig: One more comment on your drawings and the idea of milling solid blocks of cast iron for the CNC axis bases ... It seems like a very good idea and I actually investigated whether I could find such bases. However, Denmark is an earth-quake free country (at least comparatively so) and I couldn't find anything like large slabs of cast iron. I could have bought them though e.g. from Elcas in the Netherlands - but the price would have been three times higher than what I paid now - and these CI bars are already milled which will save quite a bit of time. Also, they are better quality it seems ... I end face milled some of them yesterday and according to an experienced machinist standing nearby the fact that the "metal shavings" were long apparently indicated good quality iron.

    And in relation to your milling: 200 kg left over iron ... My goodness - seems like a bit of work ...

    Cheers,

    Jesper



  4. #148
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    Default Re: Which bits to use for PCB milling?

    Deep threads on their own are not effective, stretch in the bolt means that only the threads nearest the surface will be doing the work. In an overload situation the surface threads will give, and the resultant stretch in the bolt will mean that the joint will be loose. You can counterbore the threaded holes, to put the load deeper in the metal, but the real solution is to use accurate dowels / pins to do the work locating the pieces, and use the bolts as clamps.



  5. #149
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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,

    Thus I am considering drilling/milling some additional fixture holes into this z-axis linear rail. E.g. 1-2 holes inbetween the first two pre-drilled holes and then one more hole inbetween the next two pre-drilled holes. Does this sound reasonable
    Those rails are hardened, you'll not be able to drill them.....grind.....yes.....drill no. Further they are strong as hell you don't need more holes!. The
    manufacturers of these things have invested hundreds of millions of dollars in R&D, if they needed more holes they would have put them there.
    If you absolutely have to drill some holes then you'll need to take it to someone who has an EDM drill.

    The continuous cast slabs are almost extruded out of a graphite die. As a result the cooling rate is controlled, there is little or no scale
    or chilled layers. Most of the companies will sell you the blanks, commonly 3m long as is, or you can get them to run it through their four sided
    mill at extra cost.

    The elevator weights I got cost me $90NZD each and they weighed 190kg each, about 50cent per kg or in USD about 40cent per kg.
    The prices from various mills making these slabs sell them for about $3 per kg and a slab 210mm x 75mm x 3m weighs 380kg so about
    $1100 per slab.

    That works out a bit cheaper than big chunks of 6061 or 7075 aluminum. The downside is that the mills that I found were in the US, the UK
    and Europe. All a long way from here which adds to the cost.

    If I make my own patterns I can get them cast for about $4 per kg. Given that with casting there is very much less waste despite being more expensive
    per kg it works out cheaper than importing big slabs and then turn over half of it into swarf.

    My high speed spindle cost me just over $1000NZD by the time it was in New Zealand and my high torque spindle cost over $2000. You can see where this
    is going....as a proportion of the overall cost of my machine the frame is only a modest fraction of it. The bottom line is that the frame of the machine determines
    how good it can be.....you can have all the flash rails, ballscrews and servos but in a flexy frame it's all just a waste. This is my experience....I wish now that
    I had taken a lot more care in the design, materials and construction of my machine. Its not bad....but it could have been so much better.
    If I want to improve my machine significantly then I'll have to make a new frame and not try an do it 'on the cheap'.

    Make some enquiries about getting the components cast, I can promise you it won't be cheap but you'll end up with a HUGELY SUPERIOR
    result.

    Craig



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    Default Re: Which bits to use for PCB milling?

    Hi again,

    Thanks again to you for commenting and suggesting. A couple of comments from up north:

    @Zorbit: I may consider using dowels for some of the holes on the z-axis - I guess they would see quite some stress. Would you by any chance have a link to a dowel that you consider good? They need be long dowels ...

    @Craig: I will then omit making more holes in the linear rails. That is also easier!

    Make some enquiries about getting the components cast, I can promise you it won't be cheap but you'll end up with a HUGELY SUPERIOR
    result.
    Well, as you may remember I have previously said that CNC machine building is not my main focus ... I need this machine to make some of the parts that I otherwise need/would like to make, and although perfection here (whatever that may be) could be intriguing it is not my focus. And now I have bought the G40 iron, I have sawed it and I have begun milling it, so no ... And I do think that it will cope with what I need it to do - and not be shaky in this context.

    BTW - and FYI if you have not tried this material before - yesterday I did some drilling and thread making in this G40 material. And comparing with what I have heard from others about cast iron (presumably G25 quality) it seems that there may be some difference. When making a thread in the G40 material e.g. the "metal shavings" (not sure this is the right word) do not break .. I finished a 30mm M10 thread and ended up with two ~ 10 cm "metal shavings" that were unbroken. Also I tried to tighten a bolt in this M10 30 mm long thread and even when using a 20 cm long wrench - and tightening as much as I could - there was no hint of the thread giving in. Probably relevant to say that I am a runner type body build so only very moderately strong.

    Cheers,

    Jesper



  7. #151
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    Default Re: Which bits to use for PCB milling?

    Hi Jesper,

    Well, as you may remember I have previously said that CNC machine building is not my main focus ... I need this machine to make some of the parts that I otherwise need/would like to make, and although perfection here (whatever that may be) could be intriguing it is not my focus. And now I have bought the G40 iron, I have sawed it and I have begun milling it, so no ... And I do think that it will cope with what I need it to do - and not be shaky in this context.
    I thought the same thing about my machine, and as I have said....its not bad....but could have been much better. The advice I have
    given is what I learnt by making my own mistakes. Whether I would have accepted an acted on advice like this had I been given it
    at the time....I suspect not!

    Cast iron has a range of properties.

    Grey iron, the most common type, have low tensile strengths, about 30000 psi, about 1/2 of mild steel but great vibration damping. It casts well, machines
    well forming small chips.

    There are other grades, still nominally called grey iron but with tensile strengths up to 1.5 times that of mild steel, about 90000 psi. Such iron
    is often called semi-steel. It has great impact strength but it rings (vibrates) as bad as steel. You'll often encounter it in vices and lathe chucks.
    It forms much longer swarf than lower strength grey irons.

    There are yet more grades, SG (sphereoidal graphite) with good strength and impact performance. Used for hard working trailer couplings etc.
    SG forms very tightly curled small swarf,
    Then there are white irons, they can be heat treated and can get remarkably hard. Its not uncommon to see a modestly hard white
    iron used in lathe beds where the beds can be flame hardened.

    Machine builders tend to use modest to moderate strength grey iron. The higher strength grades offer no more vibration damping than steel.
    The low tensile strength is accommodated by making the sections bigger. Even grey irons will flame harden a certain amount but if a genuine hard finish is required
    that where you see white iron.

    For your purposes you want a moderate strength iron so the threads don't pull but if you don't want it to ring under impact don't go overboard
    on the tensile strength. It sounds like the G40 grade is ideal for your machine.

    Craig



  8. #152
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    Default Re: Which bits to use for PCB milling?

    Hey again Craig,

    & once more thanks for giving a detailed feedback

    I thought the same thing about my machine, and as I have said....its not bad....but could have been much better. The advice I have
    given is what I learnt by making my own mistakes. Whether I would have accepted an acted on advice like this had I been given it
    at the time....I suspect not!
    Hmmm ... the thing is that I probably could have made detailed calculations on torques, loads, stresses etc. if I had had the time. I remember doing these calculations during my studies but eventually I went in the electronics direction and undoubtedly it would take me some time to dig into the "lower layers" of my memory to bring this knowledge out in the open. So I admittedly have decided to listen to what you - and others here - have said and then twist this information in a direction that suits my needs. Which - as you may remember - only just crystallized relatively late in the process when I decided to go for two main objectives: A rather big work cube - and at the same time a quite rigid CNC machine.

    Looking at stress & shear strength specifications for various materials on the internet, and watching this video:



    I decided that grey iron (cast iron GG25) was the material to aim for, however, as I wrote earlier - when I arrived at the company selling this GG25 iron the offer of some GG40 (in Danish called "sej-jern" with directly translates into "tough iron") iron that was pre-milled came up while being there. When tapping this material it is a bit more lively sound-wise than GG25 but also much more rigid and "firm". I reckon that an even better solution might have been their GG50 (or GG60, I don't exactly remember) grade iron which is an annealed version of the GG40. I suppose it would be somewhat softer but likely also less "ringing". Apparently the company owner can hear a difference when he taps the GG40 and the GG50/GG60 materials and compare the sound.

    I any case I am now getting things together and working on the machine - I hope it will be fine!

    BTW: Some days ago - inspired by you coming from New Zealand, and Roger from Australia (IIRC) - I Google Earth'ed New Zealand and was among other things impressed by its varied nature. Quite remarkable IMHO - .... I know it is a bit off topic but might I ask just approximately where you live in New Zealand ... ?

    Cheers,

    Jesper



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Which bits to use for PCB milling?

Which bits to use for PCB milling?