A new hope: Epoxy granite and steel.

[JAZZCNC]

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.

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.
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. . . . How could you fit a central ballscrew and still have the material move thru the opening.

[hardenum]

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.
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. . . . How could you fit a central ballscrew and still have the material move thru the opening.

I guess some clever positioning? https://scontent-frx5-1.xx.fbcdn.net...58&oe=5FF208F9

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

[JAZZCNC]

I guess some clever positioning?

That's not clever positioning, that's restricting movement and height.!!

[pippin88]

Regards the ballscrews then it only works with one on each column. . . . How could you fit a central ballscrew and still have the material move thru the opening.

I like your new machine, though I think hiding the ball screws like that reduces stiffness too much by cutting into the tubes.

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.

[JAZZCNC]

I like your new machine, though I think hiding the ball screws like that reduces stiffness too much by cutting into the tubes.

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.

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 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.

You could but it will look crap and require you have a high ceiling.
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.

[hardenum]

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?

[JAZZCNC]

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?

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.

[pippin88]

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.

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.

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.


Bellows don't compromise rigidity.

You could but it will look crap and require you have a high ceiling.
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.

Who cares what it looks like?

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.

[JAZZCNC]

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.


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...

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.

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 ££.

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.!

[hardenum]

If I was building a machine for cutting more aggressively and I will be very soon,

Looking forward to that!