New CNC laser Build

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Re: New CNC laser Build

Postby cvoinescu » Fri Jan 16, 2015 11:25 pm

bush flyer wrote:there is no point in having stepper motors that have 439oz-in of torque

That would be absurd. When I was comparing 0.9° and 1.8° motors, I was thinking about 48 mm long NEMA 17, not anything bigger than that. Typical torques are around 60 oz in for 0.9°, and 70 oz in for 1.8°.

Think of it this way. Acceleration is force divided by mass. You know the mass of the gantry and that of the X carriage. Force is torque divided by arm; in this case, the arm is the pitch radius of the pulley. Take the 60 oz in motor: that's about 0.42 N m. However, that's the holding torque; at higher speed, torque decreases substantially. Take the top speed you're trying to achieve, see what that means in terms of rpm for the motor, and look up the torque in the torque by speed graph from the datasheet of your motor. Let's say it's 0.1 N m. A 20-tooth GT2 pulley has a pitch circumference of 20 teeth x 2 mm/tooth = 40 mm, so a pitch radius of 40 mm / 2 pi = 6.37 mm approx, or 0.00637 m. The force is then 0.1 N m / 0.00637 m = 15.7 N. Assuming 1.2 kg for the gantry, that gives a maximum acceleration of 15.7 N / 1.2 kg = 13 m/s^2, or about 1.3 g. For the X axis, the carriage is much lighter (but don't forget to include the belt and the motor's own rotor inertia); say 400 g to be on the safe side. That gives 39 m/s^2, which is almost 4 g. Of course, there's friction, so the actual acceleration won't be as high; but the nice thing about a laser cutter is that there's no variation in load -- so it will work reliably even if tuned fairly close to the maximum capability of the motors and drivers.

Pro tip: if you want to redo the calculations, Google is very handy. For instance, if you type "15.7 N / 1.2 kg", units and all (but without the quotes), it says "13.0833333 m / s^2". Want that in gees? "13.083 m / s^2 in g", and Google responds "1.33409472 g". The only thing that doesn't work is converting oz in to N m, because oz in is technically wrong -- it should really be ozf in, but Google doesn't know ozf. You need to do it in two steps: "60 oz in lb" gets you "3.75 lb", and "3.75 lbf in in N m" results in "0.423693109 N m". Enjoy!
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Re: New CNC laser Build

Postby r691175002 » Sat Jan 17, 2015 2:47 am

Never use XL belts for linear positioning. Technically all trapezoidal belts (including MXL) are awful for positioning as the teeth are actually smaller than the corresponding slots in the pulley. Having extra space is great for reducing friction during power transfer, but introduces error for applications that require positioning.

Image

People still use mxl belts for 3d printing because poor frame rigidity masks other sources of positional error. The GT2 belt is more than sufficient, in fact it is used in many laser cutters ( http://www.lightobject.com/X-Y-Stages-C64.aspx ).

Cvoinescu covered the math. Pretty much any stepper motor will work fine. I'd recommend using something that is popular since it is likely to have a reasonable inductance. Note that for stepper motors of similar size/weight, increased torque comes at the expense of lower top speed.
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Re: New CNC laser Build

Postby bush flyer » Sun Jan 18, 2015 10:06 am

That's a very good demonstration on those pictures r691175002 of how the teeth run in the pulleys. I have already ordered the Gt2 belts with 20 tooth pulleys, so it looks like with all your help I have bought the correct belts.
As for the stepper motors I will order Nema 17 Stepper Motor 17HS 8401 I will not need much power to move the laser head on X axis, but for the Y axis I need a dual shaft to move the gantry, which I'm having trouble finding in nema 17 that I can buy just one, they seem to sell these in threes or fives but since this will not have to move I was thinking about using a small nema 23 dual shaft, but the smallest that I can find dual shaft is this Nema 23 Stepper Motor Dual Shaft 56mm 57BYGH56-401B which has 174 oz.in holding torque which is double the size I was looking for. And as for the drivers I will use CW5045 as I have them in my cnc router and have given no problems. As for the controller I will be using either the AWC604c or AWC704c still have to do some more reading on them first.
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Re: New CNC laser Build

Postby cvoinescu » Sun Jan 18, 2015 12:51 pm

What about this one, for instance? http://www.ebay.com/itm/180701254135

Or go to their site, alltronics.com. I've bought motors from them before and they're good.
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Re: New CNC laser Build

Postby bush flyer » Sun Jan 18, 2015 4:04 pm

Thanks for pointing that one out cvoinescu. So have put that stepper motor in my watch list for now, and if I still can't get one I will send for that one in California,( I just hate paying import duty ) I'm not going to buy that nema 23 with a 174 oz.in holding torque, it's just to large.
Just to clarify would running a nema 17 and a nema 23 have in the unit have problems with interference or anything else?
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Re: New CNC laser Build

Postby cvoinescu » Sun Jan 18, 2015 11:27 pm

bush flyer wrote:Just to clarify would running a nema 17 and a nema 23 have in the unit have problems with interference or anything else?

Nope, they'd work fine together. (People mix them all the time in CNC milling machines.) It's just that the NEMA 23 is, as you say, too big for this.
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Re: New CNC laser Build

Postby bush flyer » Thu Jan 29, 2015 7:04 pm

All the aluminium extrusion arrived today along with corner brackets and T-slot nuts etc. I have also ordered stepper motors, stepper motor drivers and 24v and 12 volt power supply. The co2 laser will be a 40 watt size and will be the last thing that I order, I will build the unit before ordering the laser tube, but what I now need is advice, what size air pump and water pump will I need for a 40 watt tube, reading this and other forum I think I need about a 100 gallon per hour of coolant but I could not find out the lift height, and 85watt 90L/min air pump. but once again there is a lot of different information, I would be interested in what you are using?
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Re: New CNC laser Build

Postby TLHarrell » Fri Jan 30, 2015 5:27 pm

My 40 watt laser I'm using a small submersible fish tank pump. My water bucket is a 5 gallon bucket sitting on top of another 5 gallon bucket, bringing it up to near the underside of the table. Overall, there's less than 2' of lift on my system. I recently added some PC water cooling radiators inline with the pump which now keeps the water pretty much right at ambient when making long cutting runs.

For air, I'm using an airbrush compressor. Doesn't take much, you're just keeping the gases away from the lens.
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Re: New CNC laser Build

Postby cvoinescu » Fri Jan 30, 2015 7:14 pm

TLHarrell wrote:My 40 watt laser I'm using a small submersible fish tank pump. My water bucket is a 5 gallon bucket sitting on top of another 5 gallon bucket, bringing it up to near the underside of the table. Overall, there's less than 2' of lift on my system.

If the ends of both the flow and the return hoses are submerged and the hoses have no air in them, you need no lift at all, no matter how high or low the bucket is with respect to the tube. The water is in hydrostatic equilibrium: all the pump needs to do is push it around, not lift it. The flow hose would be connected to the pump, so it would be submerged; and in all systems I've seen, the end of the return hose is below water surface too, or you risk getting air in when the pump is off.
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Re: New CNC laser Build

Postby bush flyer » Mon Feb 02, 2015 3:17 pm

Thanks for the reply guys, I have ordered a 30 litre bucket with a nice strong lid. I found this pond pump that was sold as a replacement for a laser coolant pump but that's delivering 75 litres a minute that would be 989 gallons an hour, I read somewhere if the water is pumped to fast round any system the heat transfer will not be as efficient as The correct flowing pump. But this add did not say how big the laser tube was for this pump? So I'm still unsure what size water pump to order? But I think I'm going to get this one, that pumps 650 gallon/ hour just to be on the safe side I will also get a small computer radiator and two 120mm fans so that I can keep an eye on the temperature with a gauge which I will fit to the top panel. But I will need to know what the operation temperature the laser should be when operating. The same as the air pump I tried to find out which size pump was sold with a 40 or 60 watt laser I found one 90 litres / minute and one with just 45 litres / minute which is correct? I understand that you sometimes get the coolant requirement in the instructions that you get with the Co2 Laser tube, but that is the very last thing I will buy. So if someone has the instructions for a 40 or 60 watt tube would it be possible to have a look and let me know the coolant requirements, Just before I order this pump at the weekend thanks.
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