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IndustryArena Forum > MetalWorking Machines > CNC "do-it-yourself" > 14x8x14" Mill Build, Advice Appreciated
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  1. #1
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    14x8x14" Mill Build, Advice Appreciated

    I’m working on finishing up the rough design of a smaller CNC mill I’m looking to make in the near future and I’d appreciate some input on some of the decisions I have to make.

    I’m shooting for travels of about 14” x 8” x 14” (XYZ), a little less would be fine. I’ve put about a thousand hours on a modified Tormach 1100S3 over the past few years so I have a pretty good handle on how much travel I really need. I plan on putting a pair of 4" vises on the table for most jobs. I don’t want to make the mill larger than it has to be, both for size and rigidity reasons.

    Overall, my goal is to build a stout, high-performance prototyping mill for making at most small runs of parts. It will mostly (say 90%) cut aluminum with 1/4” (6mm) and 3/8” (10mm) sized tooling but with some steel and stainless (say 10%). I’m not so concerned with speed when milling the tougher materials, but I want it to be able to cut fairly well with 5/16” (8mm) tooling. To this end, it will be as compact as possible within the constraints of the work envelope. This will allow for both space efficiency and maximization of rigidity.

    Spindle:
    I’ve been looking at ~10k RPM BT30 spindles with servo drives and pneumatic tool retention on Alibaba as they seem to check most of the boxes at around $2k for the package. At the moment I don’t see much reason to size up to CT40 tooling for a mill of this size. (And I recently got rid of all the CT40 tooling I had so there are no economies to exploit there.)

    Rails:
    For the rails I was looking at HGH-25 class rails for the Z axis and either HGH-20 or HGH-25 rails for the X and Y axes. Packaging looks to be a little easier with the smaller rails and they save 10mm of stack height per axis too. I’m just not sure if I’m running a risk of leaving performance on the table by going with the smaller rails.

    Ballscrews:
    I was looking at 5mm pitch, 20mm diameter ground screws as a size that would fit in this package without too much difficulty. I haven’t looked for a source for these yet, but unless I’m missing something, they should be about right for a mill of this size. The longest screw would only be ~600mm long so stepping up to a 25mm screw doesn’t seem necessary, at least from my current perspective. If I’m missing something here, please educate me.

    XYZ drives:
    I’m really only looking at servos for my axis drives. I want to be able to rapid quickly (~500 in/min or ~200 mm/s). I haven’t looked into what’s available in this regard yet, so I’d appreciate any advice.

    Frame:
    I’m looking at something along the lines of 8x6” square tube with 1/2” wall, filled with either just sand or an epoxy-aggregate mix for both the base and the column. My quick sketches suggest that this should be sufficient for the smaller form-factor of this mill. I’m planning on very beefy exterior L-shaped brackets to reenforce the column-to-base joint.

    This will be my first build of this scale so I’d appreciate any and all advice and feedback you have.

  2. #2
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    that is a good size. There is no doubt that the larger the machine the costs go through the roof.

    Of all the parts that you need the costliest and most difficult to get are the ballscrews. If you want genuine ground (C5 or C3) screws from the leading manufacturers like NSK, THK
    expect to pay $1000 to $1500 each for them.

    In another thread some guys have been advocating TBI screws which they reckon are ground and the specs certainly support C5 specs at a fraction of the cost, say $250 vs $1500. Quite frankly I'm
    dubious that the claims are correct......but if you want C5 or C3 at bargain prices....you'll have to decide for yourself.

    Other than that you'll have to buy secondhand or new old stock. Very good parts can be had but you'll almost never find the exact right thing, so you get what you can and make a machine to use what you can get.

    I buy from:

    https://www.ebay.com/str/industrialp....m47492.l74602

    For my machine I bought three 32mm diameter 5mm pitch double nut (BNFN) C5 (genuine ground) THK ballscrews and six THK FK25 fixed mounts including shipping to New Zealand for $1000USD.
    If I had bought new they would have cost close to $10,000USD. It certainly made my project possible....I'd never have been able to afford to buy new.

    Servos are not cheap, but man do they kick arse!!! You are right to get servos. I think 400W would be adequate, but 750W would be supreme. My machine has a 750W Delta B2 series servos on each (5) axis
    and a built-in electromagnetic brake on the Z axis. They cost $438USD a piece (plus shipping) but they sure are good.

    Craig

  3. #3
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,

    This may be useful:

    https://www.ebay.com/itm/22403612340...sAAOSw2lBe2yu0

    A C5 THK screw complete with fixed and floating supports for $279, second hand.......but a small fraction of what you'd pay new. The threaded length is 610mm and the nut length is 56mm for a max
    travel of 554mm.

    Craig

  4. #4
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi Callum - You are travelling down the right path. My suggestion is that you use plate for the whole structure and not use SHS or RHS. This gives you the ability to use the correct thickness as needed and make things the correct size. Once you use hollow section you get trapped into geometry that may not be optimal... Peter

  5. #5
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    see a mistake in my earlier post. The threaded length of the 20mm screw I linked to is actually 528mm with a spanner flat ground into one end, making the usable threaded portion 510mm and with the
    nut length of 56mm the max travel is 454mm (17.9").

    With regards your question about optimum size of the ballscrew, the short answer is the bigger the diameter the stiffer they are. The downside of over large screws, aside from the obvious mechanical considerations
    is the rotational inertia. Rotational inertia increases to the fourth power of diameter. So a 32mm screw has 2.7 times the rotational inertia of a 25mm screw....quite dramatic for a modest increase in diameter.
    When I was looking for my screws I found the 32mm screws I mentioned earlier. I did not, at that time, know to much, or rather allow for, rotational inertia, but rotational inertia dominates
    the momentum equation. I had already selected 750W servos so the extra inertia was not too big a drama, but if I could have gotten 25mm screws they would have been even better, not that the 32mm are wrong per se.
    The short answer is that you can go too much oversize....and therefore require high torque servos just to overcome their inertia.

    Having said that 400W servos (1.27Nm @ 3000rpm) will accelerate 25mm screws very VERY rapidly indeed. That a 400W servo would accelerate 20mm screws even faster is a moot point, anything over 0.2g
    is over the top, at least in a hobby machine. In fact 400W servos would be fine on my 32mm screws, but with 750W (2.4Nm @3000rpm) the axes accelerate at 0.27g....more than enough. I deliberately detune
    them to 0.15g and that is still any amount fast enough.

    Just to reiterate: the momentum of a moving axis is commonly dominated by the ballscrew.....NOT the axis mass. This is counter intuitive but is none the less true. In my machine the ballscrew is 80% of the momentum
    while the armature of the servo is a further 12.5% and the 150kg cast iron axis is only 7.5% 150kg of cast iron slamming back and forth is ONLY 7.5%!!!!

    Craig

  6. #6
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Quote Originally Posted by peteeng View Post
    Hi Callum - You are travelling down the right path. My suggestion is that you use plate for the whole structure and not use SHS or RHS. This gives you the ability to use the correct thickness as needed and make things the correct size. Once you use hollow section you get trapped into geometry that may not be optimal... Peter
    That's an interesting thought that hadn't occurred to me. Working with flat stock would allow for a number of conveniences.

  7. #7
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Quote Originally Posted by joeavaerage View Post
    Hi,
    that is a good size. There is no doubt that the larger the machine the costs go through the roof.

    Of all the parts that you need the costliest and most difficult to get are the ballscrews. If you want genuine ground (C5 or C3) screws from the leading manufacturers like NSK, THK
    expect to pay $1000 to $1500 each for them.

    In another thread some guys have been advocating TBI screws which they reckon are ground and the specs certainly support C5 specs at a fraction of the cost, say $250 vs $1500. Quite frankly I'm
    dubious that the claims are correct......but if you want C5 or C3 at bargain prices....you'll have to decide for yourself.

    Other than that you'll have to buy secondhand or new old stock. Very good parts can be had but you'll almost never find the exact right thing, so you get what you can and make a machine to use what you can get.

    I buy from:

    https://www.ebay.com/str/industrialp....m47492.l74602

    For my machine I bought three 32mm diameter 5mm pitch double nut (BNFN) C5 (genuine ground) THK ballscrews and six THK FK25 fixed mounts including shipping to New Zealand for $1000USD.
    If I had bought new they would have cost close to $10,000USD. It certainly made my project possible....I'd never have been able to afford to buy new.

    Servos are not cheap, but man do they kick arse!!! You are right to get servos. I think 400W would be adequate, but 750W would be supreme. My machine has a 750W Delta B2 series servos on each (5) axis
    and a built-in electromagnetic brake on the Z axis. They cost $438USD a piece (plus shipping) but they sure are good.

    Craig
    Thank you for the detailed insight. That's given me a lot to think about. I may spring for those screws; they look like they could suit me fairly well.

  8. #8
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    these may be very useful as well:

    https://www.ebay.com/itm/19624598367...MAAOSwAaRlzfhX

    They are 25mm diameter and 10mm pitch. I personally think 5mm pitch is better but there is nothing wrong with 10mm. These are new old stock NSK screws with a double length ball nut for
    extra stiffness. Note they do not have either fixed or floating end mounts....those you'd have to buy separately. The threaded length is 493mm with a 15mm spanner flat and a 109mm ballnut for
    369mm (14.5") travel. These are $349USD each, and there is two of them.

    Given that they don't have support bearings these are considerably more expensive than the 20mm THK's I linked to previously, having said that 'crispy new' ballscrews...even if you have to
    buy the mount bearings is still very appealing.

    Normally I would suggest you do not buy parts until you have completed your design, otherwise whats the bet your design will change and thereafter the parts no longer fit. With second hand
    or as in this case new old stock ballscrews that no longer holds. If you like what you see you get them and then design your machine around them.

    If these 25mm screws appeal then nab them. Just be aware that in so doing you have defined certain dimensions of your machine.

    Craig

  9. #9
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    my own machine (pictured above) has travels of 350mm x 350mm x 350mm (13.7" x 13.7" x 13.7") and is very similar to your design.

    I have 32mm C5 ballscrews which are great, very stiff. It did pretty much dictate 750W servos to accommodate the ballscrew momentum though.
    25mm ballscrews would have been perfect, still adequately stiff and yet could have used 400W servos.
    Personally I think 20mm ballscrews would be underdone.

    My machine axes are cast iron and when they start 'rocking and rolling' I rather think 20mm ballscrews would be too compliant (flexy). That is of course only my opinion.

    Craig

  10. #10
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Quote Originally Posted by joeavaerage View Post
    Hi,
    my own machine (pictured above) has travels of 350mm x 350mm x 350mm (13.7" x 13.7" x 13.7") and is very similar to your design.

    I have 32mm C5 ballscrews which are great, very stiff. It did pretty much dictate 750W servos to accommodate the ballscrew momentum though.
    25mm ballscrews would have been perfect, still adequately stiff and yet could have used 400W servos.
    Personally I think 20mm ballscrews would be underdone.

    My machine axes are cast iron and when they start 'rocking and rolling' I rather think 20mm ballscrews would be too compliant (flexy). That is of course only my opinion.

    Craig
    I don't understand you on this point. Ballscrews take load axially and are meant to be rigid axially. They're not a rigid part of the mill in the radial direction. How are you attributing flex of the ball screws to a perceived lack of rigidity of the mill itself?

  11. #11
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    you are correct, the screws need to be rigid axially as the thrust of the screw is transmitted that way. The radial rigidity is really only so the screw does not flex under
    rotation.

    When my machine is in maximum tune the 5mm ballscrews are direct coupled to the servos, and again in max tune they do 5000rpm. The axis therefore tops out at 25m/min or 416mm/s.
    At rated torque of the servos the axis accelerates at 0.27g. Thus the axis can accelerate to max speed in 154ms, continue at max speed until its time to decelerate , another 154ms to a stop
    at the opposite end of the axis, 350mm distant. The entire move takes 1.48s, ie it can go from one end of the axis and back five times in 15 seconds! The axis weighs 115 kg, the trunnion/fifth axis on top of that
    weighs 85kg, the saddle 12kg...so with the vice and part a round 250kg. This thing flies around......it plain scary. Its still slow by comparison to an industrial production machine but it too way scary fast
    for me.

    When you have that sort of mass lurching around you'll ask yourself..... 'is that 20mm ballscrew rigid enough or will that axis burst off the end and keep on going?'

    Within the exception that my 32mm screws have a lot of angular momentum I can say that I do not regard them as overkill. 25mm balscrews would have been enough but 20mm would not, in my opinion.

    I had axis beds cast in iron. It was always my intention you use cast iron, then precision machine to accept top quality ballscrews, linear rails and cars with good servos. These I determined I would make as good
    as I could possibly afford, and they would form the basis of my machine for the next twenty years. The frame to which the axis beds are bolted is small beer by comparison. It may well be that I update that frame
    with a more rigid construction, but the axis beds would remain constant. The axis beds, including casting, machining, the linear motion components and the servos cost $16000NZD ($9600USD).
    Simply there is NO opportunity to replace the ballscrews at a later date, they are at the heart of the machine and everything is made to fit them.

    I would suggest you consider this very closely. Ballscrews are at the heart of your machine, to replace them is likely to scrap a large part of the machine. Ergo you should use ballscrews that will last the life of the
    machine. Given also they are the among the most expensive parts of your machine they deserve intense study and concentration.

    Craig

  12. #12
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Larger ball screws do have higher axial rigidity

    Longer and double ball nuts have higher axial rigidity
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  13. #13
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Quote Originally Posted by pippin88 View Post
    Larger ball screws do have higher axial rigidity

    Longer and double ball nuts have higher axial rigidity
    Yes, of course. The question was what performance issue was being attributed to a lack of ball screw rigidity? What evidence was being followed that lead to the conclusion that the ball screw rigidity was at issue?

    Broadly speaking, I'm not a fan of upsizing everything because 'bigger must be better' and that kind of mindset in general. I like to understand the forces, components, and design parameters that make up the system so I can chose appropriate components for the design and intended application.

  14. #14
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,

    Larger ball screws do have higher axial rigidity
    Yes they do. Firstly the section is thicker therefore more force to cause a given extension/compression. Secondly larger ballscrews have larger fixed mounts with
    larger, axially more rigid, bearings.

    I like to understand the forces, components, and design parameters that make up the system so I can chose appropriate components for the design and intended application.
    I understand....but I'm the one standing in front of a 800kg machine that can lurch all around all over the place when I push it to its max...and that tends to make you wonder if parts are rigid enough......not strong enough,
    nearly all parts we would use are ten times stronger than they need be, but rigid is another whole argument.

    Craig

  15. #15
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi Callum - In the rail direction the ballscrew provides the axial rigidity as in the rail direction the bearings are compliant. So take a ballscrew thats D20 and 500mm long. Its stiffness is around 125N/um which is about the same as a commercial VMC. So a D40 screw will be twice as stiff. In the cross direction to the bearings the structure provides the stiffness and that needs to be maximised as well. The ballscrew manufacturers publish the assembly stiffness of the ballscrew if you look it up. Same as the bearings, so designers can make a clear decision if they are under spec'ing the stiffness. Peter

  16. #16
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    this is from the THK catalog:

    A 2005 BNFN (20mm diameter, 5mm pitch, double nut 2 circuits of 2.5 turns/circuit) is 760N/um
    A 2505 BNFN (25mm diameter, 5mm pitch, double nut 2 circuits of 2.5 turns/circuit) is 910 N/um
    A 3205 BNFN (32mm diameter, 5mm pitch, double nut 2 circuits of 2.5 turns/circuit) is 1110 N/um

    The bigger the screw the stiffer they get.

    Craig

  17. #17
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    Quote Originally Posted by joeavaerage View Post
    Hi,
    this is from the THK catalog:

    A 2005 BNFN (20mm diameter, 5mm pitch, double nut 2 circuits of 2.5 turns/circuit) is 760N/um
    A 2505 BNFN (25mm diameter, 5mm pitch, double nut 2 circuits of 2.5 turns/circuit) is 910 N/um
    A 3205 BNFN (32mm diameter, 5mm pitch, double nut 2 circuits of 2.5 turns/circuit) is 1110 N/um

    The bigger the screw the stiffer they get.

    Craig
    Thank you. This is more useful. If I do some back of hand calculations, a 200 kg table (way more than mine will ever be, even with vises, etc.) accelerating at 0.2g gives a force of 400 N which would be a roughly 500nm deflection in the 20mm screw.

  18. #18
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    that calculation predicts the axial compression/extension of the screw but does not accommodate the fixed mount.

    Elements that are logically in series, for instance the ballscrew, the fixed mount, the axis bed: the reciprocal of the stiffness of the combination is equal to the sum
    of the reciprocals of stiffness for each part in the series.

    1/Scomb=1/Sballscrew + 1/Smount +1/Saxis

    Lets say that each of the individual stiffness's is 600N/um then the series combination of the three results in a stiffness of 200N/um.

    Thus any individual component may seem adequate, every component degrades the stiffness of the combination or assembly.

    Craig

  19. #19
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    Re: 14x8x14" Mill Build, Advice Appreciated

    Hi,
    consider also that a 400w servo (1.27Nm@3000rpm) or a 750W servo (2.4Nm@3000rpm) will exhibit a great deal more thrust than the acceleration alone.

    A 5mm pitch ballscrew exerts 125kg (1250N) thrust per 1Nm applied torque. Thus a 750W servo will easily exert a thrust of 2500N. Acting on a 32mm screw results
    in 2.25um flex whereas the same servo operating on a 20mm screw results in 3.3um flex.....and that's the screw alone, not counting the fixed end bearing mount.

    Craig

  20. #20
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    Re: 14x8x14" Mill Build, Advice Appreciated

    BK mounts are only about 100N/um stiffness
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

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