Constructing Janus, by Dirk

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Replacing the shafts of the idler pulleys

Postby dirktheeng » Sat May 21, 2011 3:37 pm

All,

I got the orders put in to mcmaster and Bart to continue the build. It's been a busy couple weeks with work and my mother in law being in town so I haven't had a lot of time. However, last night I found an hour or so to work and I wanted to give a report on how the shaft replacement went.

On arrival, the shafts did not fit inside the bearings that came with the kit, but I expected that. The shafts did fit very nicely inside the pulleys though (also expected). I could have forced the shafts through the bearings almost by hand. I actually planned on buying a brass spacer with my order at McMaster and using a clamp to press the bearings onto the shafts at the right position. I would actually reccomend this solution because it will help take a little bit of the slop out of the bearings too. Anyhow, I forgot to order that, so I went to plan B. I chucked the shafts into my cordless drill being careful to not tighten the chuck too much. I put the drill in high speed and held a piece of P400 grit sand paper to the shaft. Be careful to move the pinch point between your fingers evenly up and down the shaft and not hold it in one spot too long. This really isn't an ideal solution, but we only need to change the shaft OD by about 3 ten thousandths to make the bearing fit, so we aren't really affecting the accuracy of the shaft by much. It took about 1 minute of sanding or so to make the bearings fit and the shafts still felt and looked polished. I achieved a tight clearance fit. Next, I cut the shaft to length with my dremmel and cleaned up the end. I then positioned the shaft in the right orientation with about 1/8" sticking past the bearing (the opposite side of the bearing to which the pully would be mounted). The fit was tight enough that the shaft would not have moved during operation, but I decided to glue it in place anyhow. I used the trusty super glue to secure the shaft to the bearing. On the end that was left slighly protuding, I carefully used the superglue tip to apply a very narrow bead of glue in the corner where the shaft meets the inner bearing ring. I let the glue set up and afterwards, I could not move the shaft for love or money. After re-mounting the idler assembly in the laser, all of the wobble was gone and the action was clean and smooth like a professionally designed and made machine. The little section of shaft cost me about $0.75 and I think will help with accuracy and also keep wear down. It takes about 20 minutes to do each shaft. This is a solid upgrade for the motion system and I would reccomend it to anybody.

Also, I wanted to mention tha the threads did indeed cut through the tape spacer that I installed earlier so that solution i sno good.
Last edited by dirktheeng on Sat May 28, 2011 2:38 pm, edited 1 time in total.
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Headless Arduino Based CNC Controller

Postby dirktheeng » Sun May 22, 2011 1:38 pm

If you've been following some of my other posts on this forum, You know that I am interested in a USB based, open source controller. While searching around the internet, I found one that was developed based on Arduino and somebody set one up to be completely headless using an SD card, and 2 arduino's set up in a master/slave arrangement (http://forums.adafruit.com/viewtopic.php?f=25&t=13715). It will do 3 axis control and he actually used the same motor controllers as Barts new Mach 3 controller. Here is a picture of the setup:

Arduino Headless controller.jpg
AHC


Here is the basic idea:

One arduino with a 328 chip runs compiled, optimized 3 axis code (GRBL http://dank.bengler.no/-/page/show/5470_grbl?ref=mst). The code reads G-code commands and does the vector planning automatically. It is actually quite powerful, it does lines, circles, and helix as well as spindle control and a few M commands. It also does up to 20 move look ahead and acceleration planning. They get some optimization and speed boost from not running arduino sketches, but flashing the controller manually and loading the compiled code directly. It is open source, documented fairly well within the code, and easy to understand. This arduino reads the G-Code line by line from a serial buffer and exicutes it, creating the correct pulse train. According to the authors, it has been successfully tested up to pulse rates of about 30 kHz. Once generated, the signals go to the motor controllers and on out to the motors. There are videos of this setup working well. It is possible for this setup to run through a text based inteface over USB without a headless controller.

To set up as a headless controller, it takes another arduino which is responsible for reading the G-code from an sd card, and transfering it to the slave via serial interface. There is a buffer write control added to the code on the slave. It looks at the buffer and knows when it is full. When it is full, it throws a pin high so that the master knows not to send anymore gcode until its empty.

I also found a motor control shield for the arduinos running grbl.

https://www.synthetos.com/webstore/inde ... hield.html

So for about $120 + minor wiring we can have a completely functional (all be it basic) headless cnc controller. I plan on using this with my laser and will be modifying it to recognize when to turn on the laser with speed proportional laser power control. I also think we can use the 2 arduinos to set up a decent engraver controller.

More on this later
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Dirks Headless Arduino Laser controller Plan

Postby dirktheeng » Mon May 23, 2011 8:24 pm

Here are the basic parts I plan on ordering. I will be able to load and run gcode and engraving files from an SD card, navagate through a touch screen, control 3 axis motors, control PWM laser power, and have a keypad for jogging around.

The motor controller will be an Arduino Uno (http://www.sparkfun.com/products/9950). Price $30. This will be a dedicated controller and will do nothing but interpret and execute G-code through the GRBL open source controller code. There is plenty of flash ram space in here to add and modify the GRBL code to execute specific laser operations. You can directly connect this board through the USB port to a computer and set it up to stream G-code, but I don't want to do that. I want it to run off of a flash card independantly of my computer so there is no chance of it messing up once I set it up when it is running.

I will run a second controller, probably a Arduino Mega (http://www.sparkfun.com/products/9949). Price $65. This component will be used for a few things:

1) When the laser is cutting, the sole purpose of this microcontroller will be to read G-code from the SD card and stream it to the UNO. If there is enough horse power in the system, it may update the touch screen with laser path progress or something like that. I doubt that this will be a problem given various buffering strategies I can impliment (that being, there shoudl be enough space on the UNO between the serial buffer and operational ram to store several hundred G-code lines ahead of what is being executed at any given time) so that should leave me some slack on the send side to do other things during cutting, but we will have to see.

2) During non cutting times, the microcontroller will be tasked with user input via the touch screen (file controll) and a 12 button keypad that will do jogging and such. I haven't decided what kind of data output or screen controll it will have.

3) it will connect to a computer via USB so I can load and unload files to the SD card without having to remove it.

The touch screen shield I plan on using is from nuelectronics (http://www.nuelectronics.com/estore/ind ... ucts_id=31) for 29 GBP's (about $50).

I'm not sure about keypad/keyboard input, but this pad looks promising (http://www.sparkfun.com/products/8653) it's $4.

I may ditch the keyboard and display in favor of a visual C++ program connected to the usb of the MEGA running on my laptop.
Last edited by dirktheeng on Tue May 24, 2011 1:58 am, edited 2 times in total.
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Re: Constructing Janus, by Dirk

Postby BenJackson » Mon May 23, 2011 9:45 pm

For $70 you can get a Mini-ITX (17cm square) PC motherboard with CPU. You still have to add a stick of RAM and something to boot off of (eg USB thumbdrive). I also posted a link to a company that makes tiny DC-DC power supplies you could piggyback off of your exsiting 24V supply.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Tue May 24, 2011 1:27 am

BenJackson wrote:For $70 you can get a Mini-ITX (17cm square) PC motherboard with CPU. You still have to add a stick of RAM and something to boot off of (eg USB thumbdrive). I also posted a link to a company that makes tiny DC-DC power supplies you could piggyback off of your exsiting 24V supply.


I've thought about that and maybe this will be a second generation unit. The reason I like the arduino, at least for the motion control, is that is is really simple and allows direct access to the clock. I can rely on things being calculated and done on a very well defined time frame, which is what we need for a good motion controller. I'm not exactly confident that I can do that easily with a computer. Further, there are advantages to having a buffer separated controller which does nothing but process motion on a very well known and characterized manor.

The controller that feeds the motion controller can be anything that sends G-code on a call. It may make sense to use the Mini ITX for that. Trying to make a motion controller from that is a bit beyond me.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Sat May 28, 2011 2:50 pm

It's been a busy week so I didn't have much time or energy to work on the laser, but I have a long weekend now to make some progress. I wanted to start by showing a couple pictures of the shafts I installed to replace the idler shaft/screws that came in the kit.

DSCN3448.JPG
Shaft Replacement1


There isn't much to this, but it REALLY improves the smoothness of the operation. The belt tension is very nice and even now no matter where the car is on the track. The sound coming from the motor as the car moves is very smooth and even. I beleive this will help some with accuracy as the belt doesn't have to stretch as it moves around the circuit and also help keep things from wearing out.

The second picture shows the end that is glued in. The shaft had to be lightly polished just to fit inside the bearings and it was a hand press fit. For added insurance I put superglue around the bearing shaft joint shown in this picture.

DSCN3449.JPG
Shaft Replacement2


There isn't much too it... sand/polish the shaft to make the fit, cut it the right length, place it, glue it. No need to show the other idlers... same deal, different part.
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Latest updates

Postby dirktheeng » Sat May 28, 2011 3:09 pm

I'm in the middle of waiting for the parts to finish the motion and control system for the laser. I got a few parts in and have been installing them as the come.

Here's a pic that shows the majority of what I have gotten done this week (which isn't much).

DSCN3452.JPG
5/28/2011 update


I got the cable carriers from the local supplier here in WV, the laser mirror/lense mounts and the shafts updated.

I have 2 arduino's on order (should be here 6/2), the keeling 24V power supply (got here yesterday), and barts stepper driver master board (here yesterday as well). My plan for today is to get the stepper driver board mounted and start running the wiring to it. I really wish I had ordered the arduino's earlier so they would be here this weekend! I am very excited to get the motors running! I'm going to go to Radio Shack to pick up the wiring I need and get some standoffs. I really hope they have what I need.
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Faraday cage for driver board

Postby dirktheeng » Sat May 28, 2011 3:14 pm

All,

I was thinking about the EM noise issues and perhapse a good and fairly inexpensive solution is to use some metal screening to make a faraday cage to sorround the driver board. It will still allow air flow for the driver heat removal but will help keep out em noise. I want to see if I can find some brass screen at Lowes and then make a template for the cage. What do you think?
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Re: Constructing Janus, by Dirk

Postby bdring » Sat May 28, 2011 3:32 pm

I think it is probably overkill. Note: A ferrous metal is better for the "M" in EMI.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Sat May 28, 2011 3:44 pm

bdring wrote:I think it is probably overkill. Note: A ferrous metal is better for the "M" in EMI.


Probably, but I can't think of a cheaper solution. a couple square feet of screen from Lowes is only going to be a buck or two. Then all I have to do is print out the pattern, cut it out, bend the pieces and tack ti with a little solder.

I think any conducting metal will work well as the em waves will induce current that will counter the electric feild. There may be some metal which has lower "transmissive" properties than others, but my physics book and wikipedia only say it needs conductive properties. I can test it out by wrapping the screen around my cell phone and seing if the signal dropps.

I think the only requirement for EMI sheilding is that any "holes" be significantly smaller than the wavelengths you want to sheild against.

However, you are right, a ferous metal is needed to sheild against magnetic fields as it will force the field lines around the shield as long as it is not saturated. A conductive shield will only work against an electro-magnetic wave and will not work against static or very slowly varying magnetic fields.
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