New CNC laser Build

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Re: New CNC laser Build

Postby terjeio » Sun Jan 11, 2015 8:58 pm

I am building a similar machine, also based on linear rails - just got the axes moving today.

Having now completed my PCB exposer, which is running at least one order of magnutide faster than the CO2 laser will be doing, I must say I am a bit confused about the need for heavy duty 0.9 deg steppers. There is a tradeoff between resolution and speed, which is ultimately dependent on the reponse time of the laser itself - some say that running the tube at around 3ms per pulse is optimal. At 1200 dpi this calculates to about 7mm/sec (423mm/min), fairly slow when compared to my PCB exposer which is running at approx. 200mm/sec. You have to reduce the resolution to some 42 dpi to get comparable speed on the 2.x... For the PCB exposer I have used 1.2A 1.8 deg steppers, I think the X-axis load is about the same as for the 2.x so a stepper of similar spec should be adequate? I am planning to implement my PCB epxoser controller for the 2.x as well, I will use this for engraving and Mach 3 for cutting. Since GT2 belt in conjunction with 17 teeth pulleys provides a nearly perfect match for 1 step per pixel at 1200 dpi (at 8 microsteps) I will try this for my 2.x as well. The error is only 100um/inch which is acceptable for my use, I can then output one pulse per microstep which simplifies the controller a great deal.

Ok, over to the experts - I am a novice having just started out on this journey, there is a lot to learn - no easy answers I think because we have different backgrounds and different expectations of what to achieve with our machines.
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Re: New CNC laser Build

Postby bush flyer » Mon Jan 12, 2015 6:23 pm

The more I read the more confused I get, When I built my CNC router It was just a case of getting the most powerfully and largest you can afford With the laser its a trade off between power and weight acceleration and speed.

When I done the calculations for the XL belt with a 20 tooth pulley and 10 micro steps it works out at only 500 steps/inch But when the calculations for the GT2 2mm pitch at the same 20 tooth pulley and 10 mirco steps you get 1282steps/inch that's a big difference.
Or I'm I doing this wrong? Would be the first time and certainly not my last.
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Re: New CNC laser Build

Postby cvoinescu » Mon Jan 12, 2015 9:49 pm

XL pitch is much larger than 2 mm. It's 0.2", or 5.08mm. So yes, 20-tooth GT2 pulleys are less than half the size of 20-tooth XL pulleys, so you need more than double the number of rotations (and hence steps) to move the same distance.

Maybe you wanted MXL belt, which is 0.08", or 2.032mm -- nearly the same pitch as GT2?
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Re: New CNC laser Build

Postby bush flyer » Tue Jan 13, 2015 11:54 am

Lets start again sort of. The reason I went looking for other belts is because when I looked at the Gt2 belt the teeth are only 0.76mm height, the MXL belt has smaller teeth at only 0.46 mm. I was worried that the belt would slip on the pulley. But perhaps I'm looking for problems that aren't there. If someone said the Gt2 pulley and belts are fine and would not slip and jump teeth, then I would be happy to fit them, but what I don't want to do is fit the belt and six month down the road having to replace them with a different set of pulleys and belts. The same goes for the number of teeth on the pulley I need someone to say fit 20 or 17,12 tooth pulleys, Having never built a CNC laser I have nothing to compare if 20 or 10 tooth pulleys are the best option.
When I built my CNC router I had to change some parts one was the router, I had bought just a wood workers router and I had to wear ear defenders when using it, then the bearing ceased and I replaced it with a water cooled spindle and VFD, Had I known about the noise and how little time the router would last I would never bought it and went straight to buying the spindle. This is why I don't want to go down this road building the laser I want to get the best part I can afford first time.
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Re: New CNC laser Build

Postby greenvandan » Tue Jan 13, 2015 1:27 pm

I think the majority of builders here have used MXL belts and I don't recall seeing a single post about belt slippage. If you are concerned that might be an issue, you could use GT2. I definitely would not use XL. The pulleys spec'd for the laser are 20T and have also seemed to work fine for most builders, again I can't recall any discussion about pulleys or belts not behaving as they should.
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Re: New CNC laser Build

Postby TLHarrell » Tue Jan 13, 2015 5:16 pm

MXL belts work fine and are extremely hard to get to slip, especially if tensioned properly. Get them just tight enough to produce a low tone when plucked, not a guitar string tightness, and not so loose they don't twang.
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Re: New CNC laser Build

Postby bush flyer » Wed Jan 14, 2015 4:43 pm

That's just what I wanted. I now know that the XL pulleys and belts are to large, and I will get MXL or Gt2 and also to order 20 tooth pulleys you have put my mind at rest.
I have to say This is one of the best forums that I have had the pleasure of been on, all replies have been constructive and friendly, My thanks to all for the help. I have ordered some Aluminium profile today so it should not be long now, and I can start building which I will post here. :D
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Re: New CNC laser Build

Postby bush flyer » Thu Jan 15, 2015 10:20 pm

Now that we have sorted the drive belts and pulleys out lets see if we can get the stepper motors and drivers sorted out, we have decided that 0.9deg stepper motors are over kill after reading terjeio post, and will get just 200 step stepper motors but should I get Unipolar or Bipolar I would have liked to get bipolar and wire in Parallel to give me the best in torque and speed same as my router, but that's a 8 wire stepper motor and I could only find them in the larger stepper motors. So what is most desirable better torque or speed for a laser also I will be using linear rails so the will be slight more resistance ( not much more when settled in ). What Holding Torque will I need. Then there the question about nema 17 or nema 23 I suspect that will be answered with the holding torque. Another thing is I have to watch is weight, I would not want a heavy stepper motor on the gantry X axis.
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Re: New CNC laser Build

Postby cvoinescu » Fri Jan 16, 2015 1:46 am

Most drivers need bipolar 4-wire motors, which are the most common. You can use 6-wire unipolar with some performance degradation, and 8-wire unipolar/bipolar motors too. A 4-wire bipolar motor gives the same or slightly better performance compared to an 8-wire motor wired as either bipolar series or bipolar parallel. Very few drivers work with unipolar 5-wire motors -- avoid those.

As for 1.8° (200 step-per-rev) vs. 0.9° (400 step-per-rev), it's all a matter of speed vs. precision. The 1.8° motors are likely to be faster (both because they need fewer, larger steps to move the same distance, and because they're generally somewhat more powerful than similar 0.9° motors), but 0.9° motors have twice the resolution. With the usual belt pulley sizes, say 20-tooth GT2, giving about 0.1 mm per step for 0.9° and 0.2 mm per step for 1.8°, the decision isn't obvious, one way or another. With a laser cutter, the forces involved are small, so microstepping is likely to be effective in increasing the resolution; and speed can be important when raster-engraving or when cutting very thin materials -- so I would probably opt for 1.8° motors.
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Re: New CNC laser Build

Postby bush flyer » Fri Jan 16, 2015 7:40 pm

Thanks for replying cvoinescu I have just spent the whole afternoon reading about stepper motors from sites that sell stepper motors and different cnc forums, and its just about fried my brain, there is a lot of opinion, some very confusing. Anyway I'm glad cvoinescu posted a reply. As cvoinescu and terjeio have replied I see no advantage in going to 400 step stepper motors especially as I will be doing raster-engraving and cutting thin balsa, also there is no point in having stepper motors that have 439oz-in of torque as any torque capability in excess of what the application requires comes at the high cost of unnecessary motor heating.
Excess torque capability beyond a reasonable safety margin will never be used but will exact the penalty of an oversized power supply, drive stress and motor temperature.
So Ideally the motor should deliver sufficient torque at the highest speed the application requires and not much more.
That's if I read it correctly. Of course I need to know how much torque does it take to accelerate X and Y axis?
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