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Messages - xxtoni

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1
Hello,

I've been a lurker for a while now and I have finally decided to make myself a CNC flatbed cutter.

First a little intro.

I own a signage company and we already have a pretty decent CNC router that we have bought from a local manufacturer and we are pretty happy with it.

Now however we want to build ourselves a CNC flatbed cutter.

In case some of you (probably most) don't know what a flatbed cutter is...it's basically a CNC gantry router that instead of having a spindle has various cutting heads, usually knives, that cut light materials like PVC foam board, lexan, vinyl, paper, rubber and so on. We actually consider/ed making these heads ourselves but it's a bit beyond us at this time so we've decided that for the time being we will buy them from
https://www.damencnc.com/en/tools/tangential-knife---accessories/tangential-knife/1583

We are looking for a travel speed of some 30m/min. I'm actually not sure if the oscillating knife will be able to keep up with it in thicker materials but as said, we will try to develop these tools in the future so we want the machine to be able to travel these speeds.

One advantage of a flatbed cutter is that there aren't as many resistance forces at work here as in a traditional CNC router as for say wood because most of the materials are very light so the cutter doesn't have to be as rigid as it would be if it needed to cut these heavier materials.

There are some things I've decided and the rest I don't really know a lot about and am still researching and hoping for advice here as well.



Size: 300x200cm

This is the standard size of most materials in our industry, I don't see why the size should be a problem in a CNC like this (because it doesn't have to be extremely rigid) but if it is I guess we could cut it down some if it's absolutely necessary.

Speed - Around 30m/min


Those are really our only requirements. The gantry needs to carry the 3 tools, they around 2.5kg a piece so a total of some 7,5kg which I think isn't that heavy.

The rest I am pretty clueless about to be honest.

For the bed we are going to be using a special vacuum foam for holding down stuff, I was planning on using side channel blowers to power the vacuum table but I am not sure what kind of table design to use for the mat, the seller of the mat told me to use a vacuum table with 5mm blind holes that let the vacuum through a 0,6mm opening in, according to him we would get the effective vacuum of a 5mm hole but only a loss of a 0,6mm hole once a cut in the material is made. Ideas ?


Regarding the rest, I honestly don't really know what to do.

I am currently looking at:

- Motors. Stepers vs servos ? Suggestions ?
- Rails - No clue in this regard
- Construction - What design and what material.  I have a team of pretty skilled welders so welding a sturdy construction together is no issue. My current CNC router is made mostly out of steel (the base anyway but a whole lot of steel on the gantry as well)
- Software - I was planning on using Mach3 because of it's wide support as well as having native tangential cutting support.


- Everything else [/B

This is the part that concerns me the most at this point to be honest. I am currently researching but it's a bit overwhelming right now, I do know a fair bit about CNC routers and have been doing research for years but now that it's coming all together it is a bit too much at once.

I would appreciate any advice you can give me, not just answers to these questions but these are some that come to mind right now:

- Belt vs  ball screw

I read somewhere and while doing research on the most prominent flatbed cutters like Zund and Konsberg found that most of the flatbed cutters use belt drives instead of ball screw drives. Any suggestions here ?

Stepper vs Servo

I know that steppers are cheaper and easier to setup because of the lack of encoders but I also know that servos are much better for high speeds so any suggestions here would be much appreciated.


The rest of the parts...

I am a bit clueless here in my mind the major things here are:

- Motors
- Drivers
- Cutting heads
- Construction
- Control Board


What about the rest though, the linear rails, the carriages, nuts, bolts and so on ?


For the end a flatbed cutter that seems to work quite well and seems DIY, I could show you a $200k Zund flatbed cutter but that would be unrealistic so here is a pretty good looking DIY one:

https://www.youtube.com/watch?v=H7s24MxNbDo

https://www.youtube.com/watch?v=a3hVeC1Ub1Y


I really hope that some of you guys can help me out with this.


Thanks in advance! 



2
Alright guys thanks again. Tomorrow I'll try to catch some time and experiment with some of the things you suggested and deliver some feedback.

Thanks for the suggestions! Much appreciated.

3
Once again thanks for all the advice, appreciated!

So I just figured something, it doesn't seem to exclusively be an issue of acceleration but one of velocity as well. Even at lower accelerations but higher feed rates (5000mm/min) the corner stops are so abrupt that the bridge (gantry) shakes when it comes to the stop.

My question now is...does this really matter ? I mean does it affect the cut or wil it in the long term do mechanical damage to the machine itself (rails, bearings, ballscrews, servos even) ?


Also just as a point of curiosity so that one knows for the future, how are machines made more rigid so that things like this don't happen ?

The reason I'm asking, among other things, is that I've seen some flatbed cutters equipped with spindles that route at pretty high speeds (10m/min+) but they most certainly don't seem sturdy to the naked eye.

Case in point:

https://www.youtube.com/watch?v=4Z7gDIzx5Rk

 

4
Alright so I just tried cranking up the acceleration 20 fold and this is the result - https://www.youtube.com/watch?v=Ox4c9hCxApU

The whole machine shakes pretty violently when it comes to a stop, you probably can't see it in the video but you can hear it very well.

As one might guess the shaking is more intense on the longer axis, Y in my case (3m long) than on the X (2m) but you can still feel the vibration with your hand on the table when you move the X as well which probably isn't a good sign.

The velocity of X and Y is 12000mm and the original acceleration for both was 250mm, in the video it's 4000. At 1000-2000mm it shakes a bit so that you can hear it to some degree and feel it but it's not bad, at 250mm obviously it accelerates and decelerates very slowly and there is no shaking.

To me the logical reasoning is that the machine is shaking because it's too light, but I'm probably missing something else as well.

Any advice on what my next step should be, I'm guessing that any vibration at all is a deal breaker, especially when the machine has to perform a lot of turns and "twists" it would probably be shaking quite a bit (still have to test this one).

If any of ya folks have experience with this issue I'd appreciate some advice, I'm gonna do a few short tests like routing small stuff at high speeds and at the normal accelerations to compare the radiuses and then return it to the original accelerations until I have more info.


5
If your machine is really solidly built, then I don't think that 1.1Kw servos are overkill at all.

As Terry said. it's all about the acceleration.

Just hope that they built it as solidly as you think, because when you increase acceleration, the forces the machine will see will get a lot larger.

Eh, how would someone with limited knowledge go about testing that without doing damage ?

6
You need to set the acceleration values higher to account for the higher speeds . The faster you go the higher the acceleration requirements are to help prevent corner rounding.  For what you are doing I would keep raising them until I started getting position errors from the servos.

A machine that has LOW acceleration values HAS to round corners it has no choices to OTHER than to run exact stop mode and THAT slows down the cutting down  sometimes dramatically as it has to start and stop at each segment of code.

I think the real problem was your machine designers were not really machine designers just parts assemblers.

(;-) TP

Thanks for the reply, one question though. I did have that option in mind but I had one worry, when I'm running the machine at high speeds and if it somehow does come to an abrupt stop, for example if I do a stop command instead of a pause, you can hear a fairly loud impact like sound when the machine comes to a sudden stop. Will this happen during acceleration/deceleration and if it does could it result in damage to the machine like the ball screws, rails and such ?

Thanks again!

7
Hi everyone,

So last year we bought a CNC router with a 3x2m table from a local manufacturer, when we ordered the machine we specified our requirements to the manufacturer in detail and he told us that he would make a top of the line machine for our application, which is signage. One of the requirements was that the machine be stable because we route a lot of thick acrylic (4cm) so that there would be no vibrations and that the cut would be as good as possible so that we don't a lot of manual labour with the finish of the acrylic. We were told to achieve that they would use top of the line components that are totally overkill for our application but just to make sure there would be no skimping on the parts. They used 25mm ballscrews (if I remember correctly) and the servos used are made by Schneider Electric with: 1.1kw of power,  IO 5.55 arms, MO - 3.4 Nm nN - 4000 rpm, lmax 17.84 Arms, nmax 8000 rpm

All in all it's a pretty nice machine, works well enough but there's a rub. For some reason the machine can only cut "well" at 2-2.5m/min. If you increase the speed any more than that you will get extremely rounded edges where they should be a straight edge (excluding the radius of the bit of course).

For example when routing the letter K at 2m/min with a 4mm bit you would get a 4mm radius but if you were to route at 5m/min with the same bit the radius is much larger.

Now here's my main question, as I don't understand as much about CNC mechanics as I probably should but why does this happen ?

I've never felt that the machine doesn't have enough power in the servos, I mean it has no problem jogging at 10-15m/min around the table when it's not cutting and these are all lightweight materials like 3mm acrylic and foamex so it's certainly not a issue with the spindle (3kw Teknomotor air cooled spindle with 24k RPM, not that it matters for this specific issue.)

I also have a theory as to why this happens, my guess is that the router can't change direction fast enough or rather that it can't stop abruptly and then start again to make the "sharp" edge and that it sort of just does a continued motion (if that makes sense) and that's why it leaves the very curved edge.

The reason I'm asking this is that it's pretty frustrating that the machine can't go faster accurately, I was expecting that a new machine with strong servos and good components would be able to do 10m/min. It's not a tragedy, we do good work with the machine but I'm trying to figure out if anything can be done to make it go faster and remain accurate.


Thanks in advance for any tips!




8
Nevermind I mixed something up.

Just re-read Tweakie's post. I thought that you used two lasers for getting the cross instead of one. Mea Culpa.

9
I didn't have time to perform the cut tests today but will do so early tomorrow so it doesn't get away.

Tweakie (hopefully you're still following this) the 2 laser setup to get a crosshair seems a bit daunting to me because of the need to angle them and well...use 2 lasers.

Would a single laser like this work or am I missing something important ? - http://www.ebay.com/itm/New-650nm-5mW-Laser-Cross-Module-Diode-w-driver-/260839502067?pt=LH_DefaultDomain_0&hash=item3cbb3ef0f3

10
Ian,

I would very much like to pick up where you left it at that conversation and I promise I won't disappear any day soon.

To address the question of the distortion here's a more in-depth view.

The technology that we use, and most others do is solvent inkjet printing. What basically happens in the printing process is that a piezo-electronic printhead shoots tiny droplets of ink pigment in an acetone solvent. The solvent "burns" into the material and leaves sort of a relief into which the pigment gets into and gets stuck there.

The printing process itself doesn't necessarily stretch the material, we are talking about self-adhesive vinyl here since it will be applied to a rigid substrate, but it leaves it susceptible to the stretching during application since the vinyl is much softer now.

If you leave the vinyl to gas off for 24 hours or so the ink will dry, the solvent gases will fume out and the stretching effect from the solvent is mostly gone.

The other stretching though comes from the mechanical forces that it endures during application to the rigid substrate.

If you have never seen solvent vinyl without the liner on it is almost rubery.

This is what the application looks like - http://www.youtube.com/watch?v=-u4TxTBU80g#t=51

As you can see the vinyl is quite stretchy so the distortion really depends on the direction you are applying the vinyl in. There may be some distortion in other directions as well but I think that it would mostly go in the direction of the application.

Now in this industry when you are doing serial cutting either via punching as is done in offset or via hand you leave some bleed. Bleed is basically some extra space or color around the image which is a tolerance so that if the cutting misses a bit that you don't get a white part there.

This image explains it perfectly:





To see just how much of a real-world problem the distortion really is tomorrow I'm gonna do a few prints, apply them straight out of the printer, ungassed, to a substrate and try cutting them based on a single cross-marker and then we can proceed from there.

Am I correct in thinking that the non-linear distortion is introduced when the print is affixed to the substrate? i.e. every piece will be "differently non-linear"?

OR

Is the non-linearity introduced at the print stage i.e. A batch will be non-linearly distorted with respect to the original but the individuals within a batch will be identically distorted?

The majority of the distortion would happen during application but I have reason to believe that either each or at least some of the pieces would be differently distorted. The reason for this assumption is that the distortion (stretching) is a result of mechanical forces applied on the vinyl during application. So for example you could apply some parts with less force and you could maybe pull a bit harder on a certain part and that could cause the distortion to increase on that part but not on the others.


Here is one thing I don't understand though, optical registration of print marks and cutting based on them is nothing novel in our industry. Pretty much any plotter cutter you buy has this option, even those shabby Chinese ones so it can't be that hard...I mean we are talking about a $500 or so and they cut the printed vinyl without fail every single time.

Here is an example of how that works - http://www.youtube.com/watch?v=jV1v8dRdsn4#t=158

It prints 4 of these marks (the black dots) total, one on each corter of the print then the cut head scans each dot and based on those it starts contour cutting the print based on the vectors you defined in your design software.

Same goes for the Zund and iCut system. You have to print these reg marks out and the camera scans these and based on them it cuts the print, no matter the distortion.

What I can't understand is how do they account for the distortion based on those registration marks ?

Demonstration of how the iCut system works - http://www.youtube.com/watch?v=s3MeHLluzC0

An even more impressive example, an iCut system on a laser cutter - http://www.youtube.com/watch?v=F4OLzJsakag
That must be less than a millimeter in precision and from the description:

"Printed signs, displays or POS materials can be cut on an individual basis using laser technology and i-cut®. The cutting line therefore follows the exact printed specification. With other methods, slight distortions of the printed design create unsatisfactory results. However, i-cut® is an intelligent, easy to use system which recognizes any distortions in the printed design. No matter whether it is a linear or non-linear distortion or a rotation, the cutting path is adjusted automatically and dynamically. The cutting lines therefore always perfectly match the printed design on both flexible and rigid materials."

EDIT: Added the iCut demo

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