Author Topic: Worth converting laser with belt system to screw rod system  (Read 1379 times)

Moonie

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Worth converting laser with belt system to screw rod system
« on: August 02, 2017, 01:05:50 AM »
Hi,

just a newbie question ! Is it worth converting the A3 laser to a rod system or not ? a friend is busy building a proper CNC machine in the background and was thinking of just moving the laser head over to that once it is finished or will it not work ? Or maybe replace the whole belt system with proper rods and sliders etc ? or is there a reason why they dont do it ?

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Zax

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Re: Worth converting laser with belt system to screw rod system
« Reply #1 on: August 02, 2017, 04:39:21 AM »
No. Lead screws or even better ball screws are fantastic for CNC machines where you need torque over speed.

The laser has little mass in comparison and speed is much more important. The current design is optimum.

Stonemull

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Re: Worth converting laser with belt system to screw rod system
« Reply #2 on: August 02, 2017, 04:54:25 AM »
You can already buy them like that, more commonly called a CNC engraver than a mill.
I have the eleksmaker A3 and the 30x18 version of this.
The laser is good to around 3500mm/min and the mill to about 500mm/min
The laser is larger ,, putting long lead screws on the a3 would suck I reckon, noisy and the screw length would cause wobbles imo.
On the smaller machine thwn If you are cuttting vectors only then probably fine, speed is adequate then but I think raster images on lead screw would be bad long term in comparison to wear and tear on a belt system.

http://www.aliexpress.com/item/cnc-2418-2500mw-laser-cnc-engraving-machine-Pcb-Milling-Machine-Wood-Carving-machine-diy-mini-cnc/32796872554.html

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Moonie

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Re: Worth converting laser with belt system to screw rod system
« Reply #3 on: August 02, 2017, 07:29:52 AM »
and there i learn something as well !
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Lob0426

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Re: Worth converting laser with belt system to screw rod system
« Reply #4 on: August 02, 2017, 07:39:17 PM »
Another problem with screws versus belts is "Backlash". Backlash is movement caused by the "looseness" between the screw rod and the threaded nut! Belts, when used properly have virtually no backlash. Most CNC controllers have firmware to reduce backlash, or the program that sends the code to the controller takes the backlash into account and reduces it!

There is a dig difference in threaded rod and ball ground screws for backlash, as is reflected in the way different price margins!

And the speed is less as stated before.

That is why most 3D printers use belts for X and Y axis for speed and usually a threaded rod for the Z. Gravity helps fight backlash on the Z axis and the Z axis never moves very fast. The X and Y belts in a 3D printer are way tighter than what is in these lasers! Too "fast" on a belt and then you have to deal with stretch or bounce!

Of course the Laser head will work on the CNC. But you will have to look into the proper software for running it. Something like LinuxCNC or Mach3 to develop the code, as both have both backlash and tool (tool compensation could be useful on a laser but a whole different discussion) compensation. Of course you have to then have a controller that works with one of those or another program that works with the CNC controller.

Yep it can get complicated real fast unless you can find something on the internet about the CNC that is being used! I have looked at making a "Laser Head" for my Sherline CNC mill. The Slow speed just does not make it really worth it!
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mrehmus

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Re: Worth converting laser with belt system to screw rod system
« Reply #5 on: August 02, 2017, 08:50:37 PM »
Belts can be purchased that have synthetic fibers in them that absolutely prevent stretch and ringing. Kevlar and glass fibers are some of the choices. The best belts have round teeth instead of the semi-squared teeth. The round shape prevents cogging which the semi-square teeth can generate, making motion slightly erratic.
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Lob0426

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Re: Worth converting laser with belt system to screw rod system
« Reply #6 on: August 02, 2017, 11:23:22 PM »
Overall belt stretch is rarely a problem if the belt is suited to the task. But it has to be mentioned when you are discussing the accuracy of a CNC style system.

Even the best belt will have loads of chatter cutting Aluminum as the belt loads and unload! You will see it stretch, even with the best belts. A belt system would give you a very narrow speed range and you would have to use very light cuts. And light cuts can cause chatter. Damned if you do damned if you don't!

6mm GT2 belt works just fine on lasers and 3D printers at better than 1500mm/s. 3D printers the limitation is the speed of the extruder. And I have ran it faster in tests, like at 20,000mm/s. But it is just not best suited to milling light metals.

The expense has to be considered in a project also! That is why GT2 belt in so many sizes is available and affordable!

I would love to rebuild my 3D printer with linear rails and cartridges. But the "expense" would about equal the printer price in whole! And even if I did it would keep the 6mm GT2 belt as it works well.

Besides the drive system there is the "rail" system. Dovetailed slides, Linear rails/Cartridges, Linear bearings/ground rod and the Wheel on rail system of these Chinese lasers! And that is the order of best to worse of the four most common too small machines! Even if your belt did not stretch the rail system has a lot to do with rigidity and tracking during operation!

Over the years engineers have worked these systems out pretty well. You just do not see belts driving an axis on machinery that cuts, grinds or machines metal! I am sure there are exceptions, but in general that statement is true! And I have seen
screws on all axis on a 3D printer, but everyone complained about its slow speed! A question of what is best suited to the task!
Richard
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Stonemull

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Re: Worth converting laser with belt system to screw rod system
« Reply #7 on: August 03, 2017, 07:48:42 AM »
I don't think anyone suggested milling aluminium with belts ?

I saw this guys video on reddit yesterday, noticed he is 3d printing with his CNC mill, does a damn good job too though it looks like 100% fill, would have taken a while.

http://www.youtube.com/watch?v=770cSWbmBRg

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Lob0426

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Re: Worth converting laser with belt system to screw rod system
« Reply #8 on: August 03, 2017, 12:01:39 PM »
I don't think anyone suggested milling aluminium with belts ?

I saw this guys video on reddit yesterday, noticed he is 3d printing with his CNC mill, does a damn good job too though it looks like 100% fill, would have taken a while.

http://www.youtube.com/watch?v=770cSWbmBRg

Never meant to suggest anyone did! Just pointing out the limitations of each system, belt versus rod. Key word being "system".
Richard
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Stonemull

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Re: Worth converting laser with belt system to screw rod system
« Reply #9 on: August 03, 2017, 06:08:26 PM »
Ah gotcha.

Not sure if anyone mentioned belt with standard 20T pulley = 40mm per revolution, lead screw is around 4mm or 5mm per rev. Obvious but shoukd be mentioned anyway.

 4 or 5mm, I forget which one my cheapie CNC 'engraver' is now, not going to call it a mill any longer . though it can mill wood  It is like calling outpr 2500mW lasers 'cutting lasers'.

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