Constructing Janus, by Dirk

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Re: Constructing Janus, by Dirk

Postby BenJackson » Mon Sep 12, 2011 4:10 pm

You will get a lot more charring if you are not at the right focal distance. I had lots of trouble burning wood and cuts with obvious beveled edges when I did my first plywood tests at the wrong distance.

The quality of the charring does depend on the glue. I had some thin scrap (packaging from something) which had some seriously nasty glue. Cutting it produced a bunch of gunky residue and the part smelled horrible. Some 5mm plywood from Home Depot cuts fine with just a light smoky smell (full power, F400).

I'm working on a pulse-per-inch style cut for EMC2 which I just tested on paper with good results. Since nowhere is the duration of the pulses mentioned I still have some work to do to get the settings tweaked for wood.

BTW that thin veneer balsa core is what my makerbot is made out of.
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Getting good results now!

Postby dirktheeng » Sat Sep 17, 2011 12:45 am

All,

I ordered plywood from KentCraft Company in Toledo. I got 3 1/8" sheets and 3 1/4" sheet measuring 20x30 (1/6th of a sheet of baltic bearch each) shipped here for a total of $35. I cut one of the 1/8" plywood pieces down into 3, 10x20 pieces and started to work with it some.

I found the focus by moving the z axis up/down and then moving the laser head in the x direciton at a fast jog and manually firing the laser. I kept adjusting the z axis until the line looked to be the narrowest. Eventually I will add a z axis homing switch so I can set the height automatically. Finding the focus is a bit tedious. If anybody has a better solution, I am all ears. I tried searching for "ramp test" but I couldn't find much on the forum. It may be hidden in a document somewhere, but didn't find it.

Anyhow, I set up the 1/8" plywood and started making test cuts. Man you can cut that stuff fast! I cut it at 1000mm/min and the cuts came out beautiful! The plywood made such a difference! The edges are just browned some with no charing and it doesn't leave a bit of black on your hands when pick it up. It also barely smells.

Here's a couple pics:

DSCN4181.JPG
test pattern


DSCN4182.JPG
Edges


I got the axis calibrated... now my cuts are within about 0.002" of what they should be. I have a little work to do to get the gantry perfectly square

I also made a temparary ventilation system out of card board and duct tape. It is about the most getto thing I have ever made, but it works pretty well. Also, my little 200CFM fan really does the job. I can really see the smoke move in the unit and I really can't smell anything when it cuts as long as I don't have any leaks in the duct work past the fan. Yes, the fan is on the inside of the cardboard in the window. In the final design it will definatly be on the outdide so it draws any leaks out.

DSCN4180.JPG
Temp. Ventilation setup
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Using Arduino to set power/PPI on the laser

Postby dirktheeng » Sat Sep 17, 2011 12:47 pm

BenJackson wrote:You will get a lot more charring if you are not at the right focal distance. I had lots of trouble burning wood and cuts with obvious beveled edges when I did my first plywood tests at the wrong distance.

The quality of the charring does depend on the glue. I had some thin scrap (packaging from something) which had some seriously nasty glue. Cutting it produced a bunch of gunky residue and the part smelled horrible. Some 5mm plywood from Home Depot cuts fine with just a light smoky smell (full power, F400).

I'm working on a pulse-per-inch style cut for EMC2 which I just tested on paper with good results. Since nowhere is the duration of the pulses mentioned I still have some work to do to get the settings tweaked for wood.

BTW that thin veneer balsa core is what my makerbot is made out of.


Well, this got me thinking too about how I can do this with Mach3. I started messing around with Brains, Modbus, and the Mach3 screen editor. I'm going to see if I can't make one of the arduino's I have laying around into a PLC that I can use with Modbus. I can use a a Brain to send feedrate information and pulse settings to the Arduino and then use the arduino to set the pulse to send to the on/off signal on the laser PSU.

Optionally, I could directly listen to the pulses sent to the motors and use the brain to send just the pulse settings over ModIO. That way I could get a faster update of the total feed rate and adjust the pulse frequency accordingly.

What do you think?
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Arduino Frequency counter setup

Postby dirktheeng » Sat Sep 17, 2011 4:11 pm

Ok I found a way to set up the arduino to do a hardware based frequency counter.

I will use a pair of 14-bit counters from Digikey (296-2039-5-ND). They are $0.52 each and are in a 16 pin dip. They are just up counting shift registers and operate at frequencies up to 24MHz, so it is sure not to miss a step sent to the steppers.

I will also look into using the chip on the spark fun i2c 1/0 expantion breakout board (BOB-08130) so I can read the counters via I2C on an arduino. I will then count the steps and use a timer on the arduino to calculate the speed and set the laser on/off based on that. I am going to a friends house to watch the PSU game and then I'll work on this agian some.
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Re: Constructing Janus, by Dirk

Postby BenJackson » Sat Sep 17, 2011 5:54 pm

I was planning to do something along those lines before I really started learning about EMC2 configuration. I am doing the pulsed cuts entirely in software now. The most important "aha" is to quit thinking about it as "frequency in Hz" because it's not about "per second" at all. It's all about "per distance". Pulses per distance with a length (not a duration) of pulse. In EMC2 I can do this by tying the motion.current-vel pin to my module which can convert ppmm to pps and length into duration. This is recomputed continuously to account for things like acceleration.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Sat Sep 17, 2011 6:48 pm

BenJackson wrote:I was planning to do something along those lines before I really started learning about EMC2 configuration. I am doing the pulsed cuts entirely in software now. The most important "aha" is to quit thinking about it as "frequency in Hz" because it's not about "per second" at all. It's all about "per distance". Pulses per distance with a length (not a duration) of pulse. In EMC2 I can do this by tying the motion.current-vel pin to my module which can convert ppmm to pps and length into duration. This is recomputed continuously to account for things like acceleration.


if I put it in a fast enough loop I can just read the pulse count and turn it on and off based on the total distance traveled.
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Finding Dead Nuts square for the gantry

Postby dirktheeng » Sun Sep 18, 2011 12:15 am

After I did my rough square with a caprenters square, I thought I would fine tune it some. I read a few articles on building machinist squars and I thought I would apply a technique for finding true square. If you have a strait edge, you can draw a line with it, flip it over and see if the line lines up with the square edge. Only a true 90 degree square will line up. I use this to check my squares if I drop them or something.

With this in mind, I came up with the following Gcode (made with HEEKS CNC and Inventor):

SqACal.zip
Gcode for square and axis calibration
(583 Bytes) Downloaded 727 times


It is basically a square with a couple notches in it and it cuts 2 of them with the axis lined up in the same direction. Once they are cut, you can take one of them and flip it around. If the gantry is truely square, there will be no gap at the top of the bottom.

Here is a picture of what I started with:

DSCN4183.JPG
Not Square


I would move the table down and move the gantry to the front and position it where I would have access to the set screw on the timing belt pully on the RHS. I could then easily move the pulley forward or back to position it where I wanted it. I would just guess and check. Measuring the true offset was too hard. I also adjusted the second mirror every time I moved it so I would get true square with the beam too.

I ended up with dead nuts square:

DSCN4184.JPG
Square


I used a good strait edge to check the table for flat... it was and then I used the strait edge to keept the 2 right angle pieces striat with eachtother. I just kept going and itterating this way until the gap was gone. I could have done it in about 3 tries, but somewhere in the middle I got confused about which way to move the gantry and messed up. Now I am confident that the gantry is square.

Note: you can see my hands pushing a bit on the square cut... that's becuase after messing with the peices a bit, it developed a bit of a gap, but when I first took them out they were dead square. I also suspect that I may have grabbed one of the wrong peices too after I looked at the pictures because they don't look like the same height. You can also use these cut peices to calibrate the axis steps.
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finding focus

Postby dirktheeng » Sun Sep 18, 2011 12:45 am

I wrote a small gcode scrypt to find the optimal focus. Perhaps this is what was meant by a "ramp test".

Code: Select all
G0X10Y0Z0
M101
G1X210Z50F10000
M102
G0X0Y0Z0


M101 turns the laser on M102 turns it off. The units are in mm and mm/min. The only thing that isn't calibrated is the z axis at this point because it doesn't really need to be. It's definately not moving 50mm, more like 5 or 10.

Anyhow, It moves the laser fast so it creates a narrow cut. At the same time, the z axis moves up. Then I look for the narrowest cut witdh and the darkest part and mark it with a pencil. Then I measure from the starting point to the pencil mark and use the following equation:

OF = X*dz/dx

Where OF stands for optimal focus z height, X is the measured distance to the narrowest spot, dz is the z movement in the 3rd line and dx is the total x movement. I then set the z axis to OF, and zero it. It's very handy and quick.
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Re: finding focus

Postby twehr » Sun Sep 18, 2011 1:59 pm

dirktheeng wrote:Anyhow, It moves the laser fast so it creates a narrow cut. At the same time, the z axis moves up. Then I look for the narrowest cut witdh and the darkest part and mark it with a pencil. Then I measure from the starting point to the pencil mark and use the following equation:


This is an automated form of the ramp test. Most of us just program a straight line and run it over a workpiece that is ramped up on one end by a few millimeters. Run the test, find the narrowest point, measure from there to some reliable point on the carriage and you have it. I then designed and cut a piece to 1/8" plywood that fits over the upper plate of the carriage and extends down to just touch the material surface at focus. Very fast, simple, and reliable.
tim
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Using the douchman method to assemble pannels

Postby dirktheeng » Sun Sep 18, 2011 9:38 pm

All,

If you've been following my buildlog at all, you know I am interested in using this laser to cut all the parts I need to make a cart to set my laser on and eventually even make a 4x8 router. One of the methods that I am working on to be able to make parts that are larger than the table is the douchman method. Douchmans are bow tie looking pieces of wood that can be used for decorations, patching dents and holes, or clamping joints together. I am using them in the later sense to keep joints tight during glueup.

I first tried just cutting pannel pieces in a straitforward way aligned during the cut as they would be when they were assembled. However, I found out that though my edges were fairly square, they were definately not perfect... far enough off that they wouldn't joint properly during a glue up. The main problem isn't alignment of the laser, but the fact that the laser cuts in a V shape... that is wider at the top than at the bottom. It may be about 1/64 of an inch wider at the top than at the bottom. While this is fine for most applications, jointing is not one of them. The strongest joints will come from a joint that starts with a joint that is basically seemless (can't even fit a piece of paper in it). Thats why we use a jointer in the wood shop to put a flat, perfectly 90 degree edge on before we glue up.

After thinking about the problem a little bit, I thought I would try flipping 2 of the 4 pannels upside down during the cut so that when they were assembled, one would be cut with the "V" up and the other down. I was banking on the idea that the "v" angle would be pretty consistant and give me a better joint

Here's a pic of what I cut out:

DSCN4185.JPG
Pannels cut


Notice that pannels 1 and 4 are upside down. When I assemble them they look like:

DSCN4186.JPG
Assembled pannels


The douchmans are also cut and in place. When I cut the douchmans, I reduced the height by about 0.002" and increased the width by about 0.007-0.008". I wanted them to be fairly tight in fit and have to somewhat pound them in. I found that you will have to cut several of them at various width increments and fit them to each cut to really draw them in good and tight. That's probably no big deal as we can cut several and put them in a bin to have for assembly.

I also found that the wood does affect the straitness of the joint and kerf width, so these joints aren't perfect. There is a variety of reasons for it. Most notedly, the wood isn't flat, my table is not perfect (yet) and the wood is not really a homogenious material so it cuts differently along the lenght.

That said, over all, I think the cut may work for many applications. In some cases, there may be some gaps that you have to fill with a sanding compound, especially around the top/bottom of the douchmans, but for the most part, the joints seemed to be pretty good. It may be best to use a glue which expands to fill gaps like a polyester foam glue (gorrila glue). I think that may work better than plain yellow wood glue, but we'll see. It's in glueup right now. I put wax paper on the table and glued up the joints and then put wax paper on the top and stacked heavy books on them to keep it flat. Hopefully, that will be enough to get it flat.

Edit: It is also worth noting that the gantry has to be very square to the y axis to get this to work right, espeically if you are flipping the pieces over. If it isn't it will create gaps no matter how square the edge is or how perfect the material is. If you don't flip it over, it doesn't matter as much. The fact that I got gluable joints out of this means that I must have done a decent job squaring up the gantry. I am not 100% convinced that it is perfect yet, but it is darn close. I will know when I start to do larger glue ups.
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