. .
Page 7 of 7 FirstFirst ... 567
  1. #61
    Quote Originally Posted by pippin88 View Post
    I can't see how the stiffness loss with a slot is worth it.

    The below shows two identical columns, except one has a slot cut in it. There is 2.36x as much deflection with the slot.
    Attachment 29240

    Bellows don't compromise rigidity.
    Like I said the first time, it depends on the application if it's worth it or not. This machine has a specific usage (which I can't mention because of customer confidentiality) that doesn't require high strength so deflection won't be any concern even if it deflected 10x.
    This is not my first rodeo, I wouldn't build something that wasn't more than up to the task it's been designed to do by a large factor.

    If I was building a machine for cutting more aggressively and I will be very soon, then you'll see it's designed very different with much more bracing and thicker wall tubes, but it will still have the slots because they very much protect the screws. Also, you are making a big assumption that there is nothing inside those tubes.!!

    I'd also be interested in the forces you applied to those tubes.? Are they realistic or would we need 30Kw servos with 2mm pitch 30mm ball-screws to repeat...


    Quote Originally Posted by pippin88 View Post
    Who cares what it looks like?
    More than you think do care, also usually IMO when something looks wrong, it usually works wrong or awkwardly.



    Quote Originally Posted by pippin88 View Post
    Racking is a consideration. Depends on width between rails and bearing spacing along the rails.
    Racking can't NOT happen IMO when it's plunging into hard materials for operations like drilling, it's just by how much.?

    The extra expense of making it more massive, etc to counteract this far outweighs buying another ball-screw as it doesn't stop at just the width, spacing, etc. As it gets heavier, wider, etc, it requires larger and more costly everything.
    Doubling up means you share the loads so can use a smaller size and less expensive components which cancels out any cost offsets but gives a much better-balanced machine.!. . . Which looks like it was designed by someone who knows what there doing and not a tight-arse who's trying to save a few .

    Quote Originally Posted by pippin88 View Post
    I agree, ball screw expense is not the biggest consideration (though starts to get much more when you use high quality bearing blocks etc). I'm thinking about screw mapping and linear encoders - much harder with two screws.
    Belt stretch is a concern for a long enough belt to link those two ball screws.
    Well first I wouldn't use belts on something any wider than that little machine I did and I wouldn't use belts if I was chasing accuracy to the degree of mapping ball-screws as it defeats the point and like you say makes it very difficult.

    Regards Screw mapping then very few do that at the DIY level and if using linear encoders rather than rotary encoders then you shouldn't need to map the screw. Your only concern then is keeping the two sides in sync and I'd assume if using linear encoders the control system will be fully closed-loop so will handle that side.!
    -use common sense, if you lack it, there is no software to help that.

    Email: dean@jazzcnc.co.uk

    Web site: www.jazzcnc.co.uk

  2. #62
    Quote Originally Posted by JAZZCNC View Post
    If I was building a machine for cutting more aggressively and I will be very soon,
    Looking forward to that!

  3. #63
    Quote Originally Posted by JAZZCNC View Post
    Like I said the first time, it depends on the application if it's worth it or not. This machine has a specific usage (which I can't mention because of customer confidentiality) that doesn't require high strength so deflection won't be any concern even if it deflected 10x.
    This is not my first rodeo, I wouldn't build something that wasn't more than up to the task it's been designed to do by a large factor.

    If I was building a machine for cutting more aggressively and I will be very soon, then you'll see it's designed very different with much more bracing and thicker wall tubes, but it will still have the slots because they very much protect the screws. Also, you are making a big assumption that there is nothing inside those tubes.!!

    I'd also be interested in the forces you applied to those tubes.? Are they realistic or would we need 30Kw servos with 2mm pitch 30mm ball-screws to repeat...
    The absolute value of the deflection (and the absolute load) is not the point. The point is the comparison between slot and no slot under the exact same conditions.

    Click image for larger version. 

Name:	Slot 2.png 
Views:	134 
Size:	264.5 KB 
ID:	29255

    You can double the thickness of the tube and you will still be behind. Better just to get rid of the slot.
    (If you put the slot in the opposite side of the tube, away from the rails / load, deflection is roughly in the middle between 'no slot' and 'slot')

    If you want to cover the screw, put it within another tube attached to the side, but don't compromise the stiffness of your main structure.

    What is inside the tube that negates the loss of the major load path?

    Quote Originally Posted by JAZZCNC View Post

    Racking can't NOT happen IMO when it's plunging into hard materials for operations like drilling, it's just by how much.?
    Any axis where the cutting force is not directly centered and you are only using one screw is subject to some degree of racking. Every cartesian design has one axis where the cutting force is not centered. Your machine above has the Y axis (table axis) with one single screw. Are you concerned about that racking?

    The risk of racking is about the proportions / aspect ratio between the spacing between the rails, and the bearing spacing along the rail.

    Quote Originally Posted by JAZZCNC View Post
    The extra expense of making it more massive, etc to counteract this far outweighs buying another ball-screw as it doesn't stop at just the width, spacing, etc. As it gets heavier, wider, etc, it requires larger and more costly everything.
    Doubling up means you share the loads so can use a smaller size and less expensive components which cancels out any cost offsets but gives a much better-balanced machine.!. . . Which looks like it was designed by someone who knows what there doing and not a tight-arse who's trying to save a few .
    Adding a second servo etc becomes a non-trivial expense. Tuning two servos with two different screw errors (unless you are paying $$$ for very high precision screws) is non-trivial.

    For my router I have used dual drive and would never go for a single drive on a wide gantry. For a wood router driven by steppers and cheap ball screws I totally agree with dual drive and strongly encourage everyone to drive both sides of the gantry.

    Quote Originally Posted by JAZZCNC View Post
    Well first I wouldn't use belts on something any wider than that little machine I did and I wouldn't use belts if I was chasing accuracy to the degree of mapping ball-screws as it defeats the point and like you say makes it very difficult.

    Regards Screw mapping then very few do that at the DIY level and if using linear encoders rather than rotary encoders then you shouldn't need to map the screw. Your only concern then is keeping the two sides in sync and I'd assume if using linear encoders the control system will be fully closed-loop so will handle that side.!
    Knowing the error and compensating before it happens (screw mapping) is generally better than chasing it after it happens.

    (Obviously the machine I have in mind with AC servos, linear encoders, 45mm roller rails is more than the average hobby build).




    Machine design is always a series of compromises. I am not saying your machine design is wrong, or that mine is right. I am merely pointing out possibilities and weak points. For me, the slot in a tube to hide a ball screw is not worth it at all. There are other ways to protect ball screws. Less experienced people than you may not understand the design choices / compromises you have accepted and copy that feature without understanding the substantial impact it has.

  4. #64
    Quote Originally Posted by pippin88 View Post
    The absolute value of the deflection (and the absolute load) is not the point. The point is the comparison between slot and no slot under the exact same conditions.

    Click image for larger version. 

Name:	Slot 2.png 
Views:	134 
Size:	264.5 KB 
ID:	29255

    You can double the thickness of the tube and you will still be behind. Better just to get rid of the slot.
    (If you put the slot in the opposite side of the tube, away from the rails / load, deflection is roughly in the middle between 'no slot' and 'slot')

    If you want to cover the screw, put it within another tube attached to the side, but don't compromise the stiffness of your main structure.

    What is inside the tube that negates the loss of the major load path?
    I understand what your saying and I'm not arguing that you are wrong, my point is that it doesn't matter if the loads are never going to be high enough to deflect the tubes.
    The application of the machine shown is cutting plastics with minimal cutting forces, so even at the top of travel, there will be no significant cutting forces that will come close to deflecting those tubes.
    End of the day it's about the application and if I was chasing strength then I would take measures to cancel out the effects of cutting slots in tubes, but in this application, which is the only application this machine will do all it's working life then it allows me more scope to full fill other design requirements. Like, compact a design, etc.


    Quote Originally Posted by pippin88 View Post
    Any axis where the cutting force is not directly centered and you are only using one screw is subject to some degree of racking. Every cartesian design has one axis where the cutting force is not centered. Your machine above has the Y axis (table axis) with one single screw. Are you concerned about that racking?

    The risk of racking is about the proportions / aspect ratio between the spacing between the rails, and the bearing spacing along the rail.
    Again your correct and no argument from me. But there is a big difference in the direction of the cutting forces and like you say proportions/aspect ratios working together. So in this direction and at this width then the racking will be much higher unless the bearing spacing was proportional and this isn't practical for a Z-axis as it would need to be huge so increasing height or lowering travel, take your pick. Using two ball-screws takes all this away and reduces any risk of racking.



    Quote Originally Posted by pippin88 View Post
    Adding a second servo etc becomes a non-trivial expense. Tuning two servos with two different screw errors (unless you are paying $$$ for very high precision screws) is non-trivial.

    For my router I have used dual drive and would never go for a single drive on a wide gantry. For a wood router driven by steppers and cheap ball screws I totally agree with dual drive and strongly encourage everyone to drive both sides of the gantry.



    Knowing the error and compensating before it happens (screw mapping) is generally better than chasing it after it happens.
    Erm.!! tunning and mapping are IMO two completely different things, but if you are using Linear scales then screw mapping isn't required so no chasing is being done. This is why all high-end machines use linear scales ( and Very expensive ballscrews)


    Quote Originally Posted by pippin88 View Post
    Machine design is always a series of compromises. I am not saying your machine design is wrong, or that mine is right. I am merely pointing out possibilities and weak points. For me, the slot in a tube to hide a ball screw is not worth it at all. There are other ways to protect ball screws.
    Again we mostly agree, but we will have to agree to disagree about the relevance of the slot because while you may see weakness from your Armchair in the VIRTUAL example you provided I can tell you the REAL WORLD example I provided doesn't deflect to any significant degree and is a magnitude stiffer than is required to do what it as been designed to do.
    My test of every router/mill I build is to cut the Aztec calendar in 10mm Aluminum because of the very fine detail in it. Then straight after I cut it again over the top to make sure there are no errors. I do this because if there were any errors or significant deflection then the second pass would destroy the fine details cut in the first. (See the poor pic)

    Click image for larger version. 

Name:	Aztec.jpg 
Views:	170 
Size:	493.0 KB 
ID:	29261

    Quote Originally Posted by pippin88 View Post
    Less experienced people than you may not understand the design choices / compromises you have accepted and copy that feature without understanding the substantial impact it has.
    Well first I haven't compromised anything, Also I can't be responsible for others' assumptions and if like you they assume I've just cut slots in the sides without knowing or caring about the effects of doing that then that's on them not me.!
    Anyone who is building should design based on their own application and research or ask questions based on those needs. If they just copy from a single picture or video without asking or getting in touch with the designer then more fool them.
    -use common sense, if you lack it, there is no software to help that.

    Email: dean@jazzcnc.co.uk

    Web site: www.jazzcnc.co.uk

  5. #65
    Solved.
    Last edited by hardenum; 17-12-2020 at 10:33 AM.

  6. #66
    Are these regular fixture clamps you'd use to hold the workpiece or some special types?

    https://i.imgur.com/gdaktuO.png
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	354073_107605.png 
Views:	184 
Size:	468.6 KB 
ID:	29290  

  7. #67
    After much consideration I've decided to no longer build a cnc, any cnc for that matter.

    The reason is the large discrepancy between what I want(EUR20k) and what I'm willing to pay(EUR10k).

    You can go ahead and close this thread. I better find myself a cheaper hobby.
    Thanks

Page 7 of 7 FirstFirst ... 567

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Big Epoxy Granite machine, Which frame is better ?
    By EZELab in forum Gantry/Router Machines & Building
    Replies: 27
    Last Post: 18-09-2020, 10:18 PM
  2. BUILD LOG: Epoxy granite vertical mill build log
    By Nick in forum DIY Mill Build Logs
    Replies: 60
    Last Post: 08-06-2020, 05:30 AM
  3. Large Format Epoxy Granite Gantry CNC - Looking For Feedback
    By Max Maker in forum Gantry/Router Machines & Building
    Replies: 5
    Last Post: 23-08-2019, 03:57 PM
  4. Epoxy granite or mineral casting
    By mike mcdermid in forum Gantry/Router Machines & Building
    Replies: 46
    Last Post: 20-02-2014, 05:21 PM
  5. Epoxy Granite Molds?
    By gavztheouch in forum Moulding Machines
    Replies: 8
    Last Post: 18-02-2014, 06:28 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •