Milling 2.5d balsa wings and purchasing advice

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lsar90
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Joined: Tue May 13, 2014 8:59 am

Milling 2.5d balsa wings and purchasing advice

Post by lsar90 » Fri May 23, 2014 9:41 pm

Hello

My father is an aereo modeller in his free time and is interested in milling balsa-wood to create 2.5d wings.
He made especially little non-motorized airplane models (HLG, hand launch glider) made with balsa wood and carbon fiber like this:
Image

Bigger free-flight gliders always in balsa wood (up to 1 meter per wings) and sometimes (rarely) models such as the one made by criznach (http://www.shapeoko.com/forum/viewtopic.php?f=30&t=2492 )with balsa ribs.

The wings of the models are mostly a single solid whole piece of balsa wood or, for some projects, two or more pieces joined together to form the complete wing.
The bottom plate of the wings-airfoil is flat(obviously for 2.5 dimension) while the upper side is curved and presents various heights in both the x and y axis.
Image
Image

My idea is to buy the eShapeoko (i'm in Europe) to do this kind of milling work (and other like pcb milling), obviously after having done a lot of experience and test with the eShapeoko
(I am new in the cnc world :oops: )
Because of my inexperience, I would ask for advices about the configuration of eShapeoko I have in mind:

-375mm(or 500mm) x 750mm(or 1000mm) with mid supports.
-Dual x and dual y.
-3x Nema 17 motors(2.5A per phase, 47.1N-cm, 200 steps found at a local store at a reasonable price) for both y and z axis.
-1x Nema 23(1.7A per phase, 0.88N-m, 400 steps found on Cvoinescu's store) for the x axis.
-Grbl (Arduino and Gaups shield) with the drv8825 stepper driver with appropriate heatsinks and fan. (maybe later a parallel breakout board with linuxcnc)
-Proxxon rotary tool (I have it already).
-The motor's power supply is a switching 24v 17A. (I don't know if is good for my purpose http://www.aliexpress.com/store/product ... 92132.html )
-Switching power supply 5v 2A for logical supply.
-Switching power supply 12v 3A for cooling fan and some leds for working area illumination (The shapeoko will be instealled in a old cabinet)

For the 2.5d workflow:
-Profili2 (airfoil cad with possibilities to create .stl files of the wings for foam cutting and 3-axis cnc).
-Freecad (for non-airfoil work).
-Heekscnc, Freemil and Openscam.
-G-code sender

The elements that i have already boughts are arduino, Gaups shield and the drv8825 stepper driver.
All of the other elements of the builds are questionable.

So I have many questions and doubts...

Is the trapezoidal screw for the z-axis a good boost in accuracy or in my case it is futile?
Are the motors and the power supply a correct choice?
Suggestions about upgrade?
Any idea about the end-mill can I use?

If anyone has any tips, advice, corrections or additions to make to this build and the relative software's workflow tell me as well :)

Thank you

Cataldo

DanMc
Posts: 257
Joined: Fri Apr 13, 2012 3:34 am

Re: Milling 2.5d balsa wings and purchasing advice

Post by DanMc » Sat May 24, 2014 2:30 am

If I were you I would do the trapezoidal screw, it will make the 2.5D milling a bit faster. Milling from STL files typically involves lots of Z moves and that is the slowest axis, so anything you can do to speed it up is helpful. Accuracy on the Z plane is typically not a problem.

I would also pick up some ball-nose end mills, you'll need those to get a nice smooth finish on 3D surfaces. 1/8" is probably fine, anything smaller is probably overkill for what you want to do.

lsar90
Posts: 4
Joined: Tue May 13, 2014 8:59 am

Re: Milling 2.5d balsa wings and purchasing advice

Post by lsar90 » Sat May 24, 2014 8:44 pm

Thank you for yours answer. The trapezoidal screw upgrade is in my hypothetical cart now ;)

According to the details of the motors I don't know if the only 24V 17A power supply is good.
My calculus are 2.5A per phase for 3 x nema17 = 2.5*2*3
1.7A per phase, 1 x nema23 = 1.7*2
So a total of 18.4A, but I don't know if this is a current that is never reached or the contemporary moviment of the axis, and so of the motors, needs all of the listed current and is good not work near the limit of the power supply.

Is in this case a good choise to use a pair of switching power supply with 36V, 11A or a 24V with higher current??

I have a lot of doubts about the other aspects of the build but this is a good start to find and understand new aspetct of the eShapeoko waiting the return in stock :D

cvoinescu
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Re: Milling 2.5d balsa wings and purchasing advice

Post by cvoinescu » Sat May 24, 2014 9:03 pm

You don't need anything close to 17 A. You won't be able to drive more than 2.2 A per phase anyway (that's the maximum for the DRV8825 drivers); I would be surprised if, with your motors, the total current draw would be more than 6 A. This is because of how chopped drivers work: they are able to turn a relatively high voltage, low current supply into low voltage, high current for the motors.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

lsar90
Posts: 4
Joined: Tue May 13, 2014 8:59 am

Re: Milling 2.5d balsa wings and purchasing advice

Post by lsar90 » Sat May 24, 2014 11:52 pm

Thank you :D

Is the stepper driver (chopped current) basically a h-bridge powered on and off by a pwm signal ?

cvoinescu
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Re: Milling 2.5d balsa wings and purchasing advice

Post by cvoinescu » Sun May 25, 2014 10:40 pm

A driver has two H-bridges, one per motor winding, and they're driven by PWM signals, but it's not that simple. There's current sensing for each H-bridge, so there's feedback, and there are usually three stages per pulse, not two as usual with PWM: first, the H-bridge is "on", then it goes in "fast decay" mode (on, but in opposite direction), then in "slow decay" mode (both legs shorted to ground). The driver controls the proportion of fast and slow decay to get accurate current regulation without causing the motor to heat up too much ("fast decay" causes more high-frequency noise and more heating, but allows the current to vary faster; the driver tries to use slow decay whenever possible).
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

lsar90
Posts: 4
Joined: Tue May 13, 2014 8:59 am

Re: Milling 2.5d balsa wings and purchasing advice

Post by lsar90 » Wed May 28, 2014 8:22 am

Thank you for the comprehensive reply.
I must read more about stepper motor and driver and give a good reading to the datasheets for a better knowledge. There are a lots of useful informations and always not much time.
So I can use a 24V or 36V power supply with less power that wath I had chosen before; something with 7 or 8 ampere should be fine :)

Is the nema 23 (x-axis) an overkill option for my purpose? I have a good proxxon that doesn't weights too much.

Currently isn't entirely clear to me how assemble the larger format eShapeoko and the deluxe frame (there isn't a wiki talking about it); but I think that having all the parts in front of you will clarify many doubts (adding a lots of others doubts :lol: )

cvoinescu
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Re: Milling 2.5d balsa wings and purchasing advice

Post by cvoinescu » Wed May 28, 2014 2:55 pm

The "deluxe frame" is tricky to figure out without drawings (it uses blind joints, which are not obvious if you haven't used them before). I need to ship these orders, and then I'll get right on to the instructions.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

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