I thought it's pretty strong, stronger then what I'd get with a U plate and a spindle bolted to it. I based it on dmu 340. In which direction is it weak?Quote:
Originally Posted by JAZZCNC
That is what I meant.Quote:
Originally Posted by jarjar
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I thought it's pretty strong, stronger then what I'd get with a U plate and a spindle bolted to it. I based it on dmu 340. In which direction is it weak?Quote:
Originally Posted by JAZZCNC
That is what I meant.Quote:
Originally Posted by jarjar
Pro's and cons to both but IMO the cons for having the slightly longer length end machining the price is a too high price to pay in strength terms. There are other better ways. . . For instance.!Quote:
Originally Posted by jarjar
For high clearance requirements, I have a design that will work and provide high clearance under the gantry with minimal loss in strength. I've just built a small version with a cutting area of 500 x 500 x 350 that could easily have the clearance and strength increased if required without any impact to it's cutting ability.
https://www.youtube.com/watch?v=M-hbeujeuRg
I did a mockup of this design and I had concerns about the depth of cut. It does solve the Z axis problem you mentioned. There's actually this start up that's doing the exact design called Vulcan, that's meant to be sold for around 8k. Here's their structure:
https://scontent-frx5-1.xx.fbcdn.net...d2&oe=5FF33F20
https://scontent-frx5-1.xx.fbcdn.net...58&oe=5FF208F9
Why did you have concerns about DOC.? It doesn't get any better than with this design for a machine with a larger cutting area as the Z extension is never any longer than the tool length, and that is at any Z height.Quote:
Originally Posted by hardenum
Some of the largest Gantry mills in the world use this exact design. If you build it strong enough the only limit on DOC will be spindle power.
The main DOC concern was that when I put different pieces with different height onto the vacuum table I'd hit the higher ones with the vertical beam. Like machining the bottom and top piece (http://www.jt-precision.com/uploads/...7293225867.jpg) one after another without actually having to go and change the piece. Sure I could put the higher one to the front, it would work, but knowing me, I'd switch the 2 unconsciously and mess some part of the machine.Quote:
Originally Posted by JAZZCNC
Anyway, do you recommend a ballscrew on each column or one thicker in the middle?
I suppose it's possible gantry brace could hit if your having very different height materials but I don't think it will be a massive issue and how often are you likely to do that.? It certainly won't be an issue cutting the part you show.Quote:
Originally Posted by hardenum
Also there is nothing stopping you having both a lifting gantry for very high parts and fitting a conventional Z axis to this. This way you have the best of both worlds. Again this is commonly done on Large gantry mills, and when I say large I mean something that could mill a full size Car from a billet of steel.!!
Regards the ballscrews then it only works with one on each column.:cower: . . . How could you fit a central ballscrew and still have the material move thru the opening.:stupid:
That piece would not be machined assembled. It would be done in two pieces and then the hinges connected.Quote:
Originally Posted by hardenum
Your design won't machine that work in one piece anyway. Your cutting tool will be say 5mm, but 50mm higher in Z you will have your full spindle cartridge/Z axis to hit the piece standing up.
How much experience do you have machining / making parts?
Some of the issues you are trying to solve with machine design are actually fixturing issues.
No machine design will save you from forgetting to do something correctly.
I guess some clever positioning? https://scontent-frx5-1.xx.fbcdn.net...58&oe=5FF208F9Quote:
Originally Posted by JAZZCNC
Anyway you have me convinced(https://www.k-mm.com/wp-content/uplo...s_picture4.jpg).
Gonna take a long deserved brake before I dive into the new design. Thanks
I like your new machine, though I think hiding the ball screws like that reduces stiffness too much by cutting into the tubes.Quote:
Originally Posted by JAZZCNC
You can have a single central screw on a lifting gantry machine. The screw moves up and down with the gantry. Nut it attached to the fixed top cross beam. Either rotating but with servo attached to fixed cross beam, or rotating screw with Z servo moving up and down with Z.
Pros: Single screw (screw mapping, servo tuning, cost). Cons: A bit trickier to implement. Will have screw sticking right up high when Z fully up.
That's not clever positioning, that's restricting movement and height.!!Quote:
Originally Posted by hardenum
Well that depends on application and what stiffness is required. In this application it doesn't need high stiffness but does require the height. However, I can tell you it's still very stiff.Quote:
Originally Posted by pippin88
I will be building a much larger version that will work the same with lifting gantry but with much more bracing using 10mm wall tubes rather than 5mm used on this.
You could but it will look crap and require you have a high ceiling.Quote:
Originally Posted by pippin88
Also it won't work very well because the gantry could and IMO will definately rack when plunge cutting at the outer edges. Disaster waiting to happen IMO.
The design I've shown only uses a single motor connected to the screws with belt/pulleys so it's only one extra screw so no big expense in the grand scheme.
Was just about to ask whether I have to switch from belts to direct drive. Do I need brakes on the vertical servo's? It's gonna be lifting 150kg after all?Quote:
Originally Posted by JAZZCNC
You have to design to your own requirements but lifting 150Kg then yes I would go with dual motors with brakes, doesn't need to be servo motors thou, Closed loop steppers will do the job perfectly well enough.Quote:
Originally Posted by hardenum
Retracted.
Quick servo question
Can I mix and match servo sizes? e.g size 60 for X and size 80 for y etc. ?
Of course.Quote:
Originally Posted by hardenum
yesQuote:
Originally Posted by hardenum
It's not looking good, the change to new design doubled EG by volume. I'm gonna have to look into doing some FEA to know where I can shave off some material.
Question
Are there any handbooks that talk about forces acting on a machine during machining that I can use as a guide?
What do you all think about this setup? Moving the bed to the side(for easier side milling)? I'm not sure if this is a machining center or a grinding or whatever though. By the way that's not a workpiece but the fixture block. Can I mount a workpiece to a vacuum table sideways?
http://www.pillevaran.com/images/rezin02.jpg
https://i.ibb.co/1Mvc6bx/asdff3ws.png
So who's slapping their forehead?
I can't see how the stiffness loss with a slot is worth it.Quote:
Originally Posted by JAZZCNC
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.
Who cares what it looks like?Quote:
Originally Posted by JAZZCNC
High ceiling - maybe. Not that high. Depends on what level the base of the machine sits...
Racking is a consideration. Depends on width between rails and bearing spacing along the rails.
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.
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.Quote:
Originally Posted by pippin88
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...:joker:
More than you think do care, also usually IMO when something looks wrong, it usually works wrong or awkwardly.Quote:
Originally Posted by pippin88
Racking can't NOT happen IMO when it's plunging into hard materials for operations like drilling, it's just by how much.?Quote:
Originally Posted by pippin88
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 ££.
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.Quote:
Originally Posted by pippin88
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.!
Looking forward to that!Quote:
Originally Posted by JAZZCNC
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.Quote:
Originally Posted by JAZZCNC
Attachment 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?
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?Quote:
Originally Posted by JAZZCNC
The risk of racking is about the proportions / aspect ratio between the spacing between the rails, and the bearing spacing along the rail.
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.Quote:
Originally Posted by JAZZCNC
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.Quote:
Originally Posted by JAZZCNC
(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.
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.Quote:
Originally Posted by pippin88
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.
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
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
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.Quote:
Originally Posted by pippin88
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)
Attachment 29261
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.!Quote:
Originally Posted by pippin88
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.
Solved.
Are these regular fixture clamps you'd use to hold the workpiece or some special types?
https://i.imgur.com/gdaktuO.png
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