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moving camera over Z-axis  

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diep
Posts: 7
 diep
(@diep)
Active Member
Joined: 4 weeks ago

Most interesting!

if you want to scan at 0.3 - 0.4mm accuracy that'll work ok.

 

To me interesting is a build where the scanner that is as accurate as the human eye can see - so 0.01 - 0.02mm we speak about.

blindfolded i would guess then that one would want to limit the Z-movement as much as possible and lift as little as possible weight over the Z-axis. In that case i do not see a problem in 10mm smoothrods over a distance of 200mm and add a little - bearings are 29mm high and we take then 4 bearings at 2 smoothrods which means we need 58mm extra. So that's say 260mm smoothrod exposed in the open and to also attach it that's say 270mm smoothrod (tapped hole at 1 side and fastener at the other side for adjustment).

 

Now getting to 0.1mm is a big struggle already. A hard requirement i would guess is a more accurate turntable then if i consider the math needed to determine how the object looks like. Just 3d printing that will give you a pancake that doesn't turn around very accurately.

 

Stiffness of a structure is a directfunction of distance to the power 3. So if you move the camera over a distance of 500mm then you need more like 16mm diameter rods and a very stiff structure. I would call that an over the top effort going from zero Z movement to 500mm.

 

Now i do not know whether it is an advantage or disadvantage to also move lasers over the z-axis. I would put the smoothrods at a small as possible distance from each other and use 2 and 4 LM10UU bearings. The LM10UU bearings are in itself not so accurate so you need 4 of them in a square to get some sort of accurate smooth movement with them.

 

The leadscrew - and what you drew there looks like a German company called ISEL that delivers ballscrews with square nut that are notoriously inaccurate (c7 quality is the highest quality that ISEL can deliver - i have that on paper from ISEL at questions mine) - so a good leadscrew from Germany for handful of euro's is just as accurate there. Yet there is a huge price difference between the 2 and the leadscrew nuts are much smaller and easier to fit into a design making the design smaller. Backlash is not an issue either as during the scan the camera moves in a single direction - it's not going backwards.

So that saves big cash.

 

In short i would make it all much smaller and not go for 570 mm length smoothrods of 10mm - as that's too long for 10mm smoothrods. Pure theoretic spoken it would only be possible with an extremely stiff structure yet at such huge distances such structure would simply cost too much. Think of a cube made out of 40x40mm aluminium extrusions then that's encapsulating such leadscrews.

 

What object would you want to fit on a 150mm diameter turntable that's 500mm high? 

 

As for scanning statues from artists - forget about that - it's not gonna be accurate enough. You would need to scan those at 0.01  - 0.03 mm type accuracy.

 

Would get a very expensive project and build. 

 

Better is first build a first careful version that has a very limited Z-range and see how accurate you manage to scan that - and whether artists are happy with that accuracy.

 

At a distance of 500mm then using 10mm rods which kind of are free in the air is asking for troubles from a stiffness viewpoint. From a stiffness viewpoint seen ballscrews that are free floating in the air kind of or hardly attached to any sort of structure one would be dividing by the constant '3' at a crabble paper calculation whereas a stiff strong structure where they would be attached to you use the constant 192. 

 

A relative weak structure where the smoothrods get attached to at both sides, we could see as a bridge where the bridge is free laying onto 2 points - so there stiffness constant is 48.

 

Interesting would be a movement of 150mm to 200mm vertical and then build a very accurate turntable and then see whether it's possible to get accurate scans out of it. The time the software needs is less relevant as long as it's less than a week - after all this is free software made in a high level languae that's factor 10k+ - 1M+ slower than the sort of software i program for prime number crunching. We cannot expect miracles there on a r-pi there and be happy and grateful that this great software is there for free and simply give it enough time to do its job!

 

We also would need to train the software to compensate for leadscrew errors.  Now if those were tiny i wouldn't even mention is yet C7 we soon could see errors that relative to the absolute correct path can be 0.07mm off easily or slighly more.

 

Now in itself it doesn't hurt the smoothness of the object, as the error happens very slowly and gradual, just hurts the absolute positioning of the object scanned. So it's possible some just would want to ignore that error. Not me :)

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diep
Posts: 7
 diep
(@diep)
Active Member
Joined: 4 weeks ago

Please note i do not have a legal license of solidworks. I'm using a combination of design software.

OpenSCAD, FreeCAD and TurboCAD (is autocad compatible) i have a legal license. mainly that means that i mainly design currently in FreeCAD and only when something is far too complicated or slow to do in FreeCAD, i'm using usually TurboCAD to finetune objects.

Especially take complicated fillets and such there or correct complicated forms.

 

Factories i have objects produce from different sorts of metals i usually communicate with using STEP format. That imports, exports and modifies everywhere very easily.

As for the mount of each laser i would suggest something very simplistic. That's U-shaped or tube shaped aluminium and only the last few millimeters you attach a 3d printed object onto it to attach and adjust the laser with. Yet ideally you would want to hang them not too far away from the superstructure (from aluminium) of the  scanner. So the main idea is to use aluminium everywhere for the scanner structure and to attach things use as little as plastic as possible so that thermal expansion difference of plastic versus aluminium isn't a big deal.

 

Now of course steel would be cheaper and even better there - yet that's difficult for most here to machine. That my machines can do it is not a good excuse to build it from steel.

 

As the hat  supports 2 lasers i would suggest 1 laser at  the bottom and 1 laser at the top of the scanner.  Exactly how to position the lasers i do not know yet. No experience at all with scanners there.

 

Using 10mm smoothrods to connect the turntable to the laser part - get rid of that. Simply an aluminium cube or similar made out of extrusions which connects to everything. Away with smoothrod nonsense.  

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