Educa's BigFoot

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Educa's BigFoot

Postby educa » Wed Dec 28, 2011 10:45 pm

Hi all,

My name is Bart, I'm from Belgium and I intend to make a laser cutter.

This site allready helped me a lot in finding information and now I'll start making my own machine.

The purpose is to be able to accept materials upto 121x61cm (about 4x2 foot, therefore the name of the project BIGFOOT)

I am currently reading a lot of buildlogs and will soon post my ideas and progress.

The machine will not be a 2.x machine, but a lot of idea will be taken from it of course.
educa
 
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Aluminium extrusions

Postby educa » Thu Dec 29, 2011 9:48 pm

Ok here we go with the first real info.

I see on the buildlogs that you guys seem to use misumi 2020 aluminium extrusions.
I did a lookup and found that 2020 extrusions seem to be 20mm by 20mm big. I use the word BIG, but in my humble opinion this is rather SMALL.

Don't you guys have problems with the stiffness of your frame ?

Here in Belgium I didn't even find a source for 20x20 extrusions. The standard is here 35x35 or 45x45

ImageImage

What do you think? Would 35x35 be enough for my frame ?
educa
 
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Linear ball bearing guide rails

Postby educa » Thu Dec 29, 2011 10:32 pm

Now my next material of choice is the slides where the movement is going to be. I see that most people here glue V-rail onto aluminium extrusions and then use grooved wheels. While this might be a good (and cheaper) solution, I do have good experience with ball bearing slides, so I'll go with these since they have only about 5 microns of play between slides and carriages.

The gantry axis will be built upon 35x35 alu extrusion and will use SBC linear rail measuring 1250mm. The carriage is 135mm so I won't be able to cut all the 121cm I have on the material plate. That is not such a big deal however since I will certainly be able to travel 1000+mm and I only want to be able to put quarter E1 plates (121cmx61cm) into the machine so I don't have to use a sawblade and can use the sheets directly from the store.

This linear rail is very strong with width of 34mm and height of 29mm precision hardened steel, so I don't think in combination with 35x35 aluminium extrusion that this will bend in any way in the middle.


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For my smaller rails which carry the gantry, I'll use THK ball bearing precision guides SHS 20 750mm long with each 2 carriages (only 1 will be used). A Carriage is about 10cm, so 75 - 10 = +- 65 cm movement possible.

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This looks like a solution which should be very high precision. It is however not LOW cost .

The rails themselves cost (second hand) about $350 , but then again, I want to be sure that this part is rock solid.
educa
 
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Motors and stepper drivers

Postby educa » Thu Dec 29, 2011 11:04 pm

To be able to move the axes I'll also need stepper motors of course.

So these are the weapons of choice.

Since my machine will be quite big and the rails are also not light stuff, I will use NEMA23 motors with a torque of 3.1Nm . Thats very high torque so I suppose I won't get lost steps when I run the axes.

The power comes from a 400Watt 36V 11A PSU unit

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Then I control the steppers with a CW5045 stepper controller capable of driving upto 4.5 A and doing microstepping in both imperial and metric scale
1 , 1/2 , 1/4 , 1/8 , 1/16 , 1/32 , 1/64 , 1/128 and 1/256
+
1/5 , 1/10 , 1/25 , 1/50 , 1/125 , 1/250
This would allow for very precise control, but tests still have to make clear what microstep settings are acceptable, since higher microstepping makes steppers smoother, but reduce torque

Image

The stepper motors themselves are NEMA23 3.1Nm steppers with dual 8mm shaft
They will be driven at bipolar parallel connection and draw 4.2 amps

Image

The pulleys will be 20 tooth T5 pulleys (5mm tooth spacing) and the belt is T5 Polyurethane Steel reinforced open loop.

Image

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If the pulley is 20 tooth T5, then 1 rotation is 100mm

The motor is 1.8° 200 steps motor, so I should theoretically get following resolutions when microstepping

@ 1 step: 0.5mm / step
@ 1/5 step: 0.1mm / step
@ 1/10 step: 0.05mm / step
@ 1/25 step: 0.02mm / step
@ 1/50 step: 0.01mm / step
@ 1/125 step: 0.004mm / step
@ 1/250 step: 0.002mm / step

I think 1/25 step will be enough since I will mostly design vector cut stuff in coreldraw and PLT is 0.025mm accurate (1/40 mm)
Also, I don't see a point to go higher resolution for engraving since a single dot will be 0.15 tot 0.20mm anyway.
educa
 
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Re: Educa's BigFoot

Postby educa » Thu Dec 29, 2011 11:13 pm

By the way, if anyone has any tips/hints for me while reading these blogposts, the please feel free to post them here. I am willing to learn from anyone!
educa
 
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Re: Educa's BigFoot

Postby dirktheeng » Fri Dec 30, 2011 3:04 am

Bart,

Wow, you got some good stuff so far! I really want to get better pulleys for my system. I'm not a big fan of the plastic ones I have now... not real accurate and I think they are the source of some backlash.

One thing you will want to do is keep the mass of your gantry cart to a minimum because that is what you want to move fast to do engraving... make sure the mirror assembly is good and stiff too.

Also, I think there is good reason why the printer manufacturers use an encoder strip with the print heads with the belt systems. I think that they can't rely on steppers to get to the sub 5thousanths of an inch they need to do really high resolution printing. I am looking into putting in an encoder strip for engraving. so I don't have to worry about the backlash issues. It seem that most people can't hold much more than 20thousanths with this system and some have had some issues with engraving without compensating for backlash.
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Re: Educa's BigFoot

Postby BenJackson » Fri Dec 30, 2011 4:18 am

dirktheeng wrote:Wow, you got some good stuff so far! I really want to get better pulleys for my system. I'm not a big fan of the plastic ones I have now... not real accurate and I think they are the source of some backlash.


I blamed the wobbly idlers until I had the smooth idler. Finally I cranked up the tension on my X much higher than I had previously and it helped quite a bit.
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Re: Educa's BigFoot

Postby educa » Fri Dec 30, 2011 11:39 am

educa
 
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Re: Educa's BigFoot

Postby mikegrundvig » Fri Dec 30, 2011 1:07 pm

Wow, you have an almost identical plan to me with the rails, higher voltage/larger steppers, and even the drivers/power supply - we are both using the same for the most part. I LOVE your pulleys - I was not that impressed with the nylon pulleys from SDP-SI and I feel they are the "cheapest" grade mechanical part in my design thus far.You have chosen MONSTER rails, large enough to run a good sized CNC mill. Is there a reason you went so large? I'm using 15mm width rails at the recommendation of a friend of mine and even they are rated in the hundreds of lbs. Rails also have a pre-loading that's not inconsequential if you don't have much weight on them. The larger the rail, the heavier it gets. I'm a little concerned that the pre-load will be problematic on my gantry head (x axis) as it's going to take more power to overcome than friction.

As for extrusion rigidity, I'm VERY concerned about this and have spoken with the engineers at 8020 (I've got a great local distributor for em so I'm going this way rather than Mitsumi) and they directed me to a load calculator they have produced on their website. It was/is incredibly useful at determining how much flex and torsion you will get out of the extrusion for a given weight/length/load profile. Long story short, the rigidity of the extrusion isn't the problem if you go reasonable. The problem is how you mate various pieces of extrusion to each other. The "core screw" approach combined with mounting plates is what they suggested as being much stronger all around. Also, the "inside corner" joiners get progressively stronger the more of the corner they consume. Specifically, you start with the 90 degree simple ones they are quite flexible. Then they have the ones that form a small triangle and are pretty good. The strongest they offer actually slide into the t-slot itself with a leg between the two pieces of extrusion. You can just see them in this video. I'm not positive these are stronger than the basic triangle design I linked before but the guy in that video swears his frame is bulletproof and as he needs far more consistent accuracy at a higher speed than us, I suspect he is right. For reference, steppers are not fast enough for his application so he is using large DC servos. Supposedly these are the strongest for the outside corners and look nice too. You can still support these with inside corner brackets as well to ensure an even stronger joint.

Whew, lots of stuff. I'll be watching your thread like a hawk! Thanks!

-Mike
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Re: Educa's BigFoot

Postby educa » Fri Dec 30, 2011 1:53 pm

Mike, the reason I took such large rail for X is because thats the one I could buy on ebay. These rails are extremely expensive so you take what you can get :-)

The steppers should have no problem at all at 3.1Nm to pull this 1 carriage.


For alu extrusions, this is what I can get here locally. They have also very strong connectors.

http://alumes.be/pdf/catalogus_nl.pdf

PDF is in Dutch, but drawings are drawings :D


I'm still not sure if I'll take 35x35mm or 45x45mm rails.

45x45 is double the price of 35x35
educa
 
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