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2.19.2016

Holocraft - The CNC Machining Adventures


 Much has happened with Holocraft since the last blog post. I now have an X-Carve tabletop CNC routing/milling machine now, which I spent the entire holiday season building from the ground up. I needed a place to put the machine on, because I had no space large enough to accommodate it's massive 31" square size (except the kitchen/dining table, but that wouldn't have gone over well with the wife). So I invested in two smaller tables from IKEA that when placed butted up against one-another they create a perfectly sized platform for the machine.


The TARENDO table from IKEA.

The tables are sturdier than they look. I assumed the legs would be wood, just as the top surface of the table is, but they are actually steel, and the whole underside of the table is also braced with similar steel beams. The tables, however, are not completely impervious to wobbling when the machine is dancing about across a hologram, which required some extra fastening and stabilization to be put in place.


The machining setup nearing completion atop the pair of TARENDO tables ordered from IKEA

When I ordered my X-Carve from Inventables.com I went ahead with the option to get the Dewalt 611 router to use as a spindle in the machine. Unfortunately both the DWP611 and the power supply interface board were out of stock and wouldn't be shipped out until January 8th at the latest. I ordered my machine on the 16th of December, which was a Wednesday. Everything arrived (minus the spindle and PSU) the following wednesday in a pair of boxes.

UPS had just left my driveway when this happened.


Since the Dewalt spindle was going to take nearly a month since my X-Carve order was placed to arrive at my homestead, I opted for a backup solution and invested in a high value package deal I found on Amazon for a Konmison 48v DC spindle that comes with it's own PSU, and a set of collets up to 7mm in size (just over a quarter inch). It also comes with its own CNC mounting hardware but it isn't usable on the X-Carve, so I opted for the 'universal spindle mount' that Inventables.com sells on their website, and it has been working out just fine.

Being that both the X-Carve and Konmison spindle have their own PSUs, I opted to stack them so that the fan in the XC PSU would help the Konmison spindle PSU cool down as well, by placing the KSPSU upside down ontop of the XC PSU.

This was the end result:

I call it the 'Frankensupply'.

As you can see, the XC Arduino and GShield are on the left, and the Chinese PSU is strapped ontop of the XC PSU, with the spindle potentiometer strapped down on the corner of the spindle's PSU. I figured that this configuration will best be suited to whatever I come up with insofar as an encasement or enclosure are concerned, cutting holes for the potentiometer, air flow, and power/USB lines. So far I have not been electrocuted, so that's good.

Another issue that arose was how I was going to be mounting my aluminum into the machine as I opted to save 250 bucks by not going for the default wasteboard that comes with the XC machine. Instead, I tried mounting the aluminum directly to the machine frame itself. This *does* work, but it allows for much torque against the X-axis gantry. This is due to the fact that the workpiece sits so far down below the gantry that there is a considerable amount of leverage against the gantry itself, being that the tool is not directly below the gantry but instead jutted out infront of it where the spindle itself is.

The Konmison DC spindle itself has been serving rather appropriately. I have managed to create a few different things with it.. On the plus side it is very light (compared to the DWP611) and runs at a rather decent 12k RPM, according to the seller, but this is something I've yet to determine with something like a tachyometer.


On McMaster Carr's website there is a slew of different metals to choose from. Knowingly, I opted to go for either the 1100 alloy or the 3003 alloy, both of which are extremely pure forms of aluminum. Being that they are pure they are also extremely corrosion resistant, simply because aluminum itself is highly non-corrosive. The other property of aluminum is that it, in its pure state, is very soft. It is on the order of between lead and copper. You can easily scratch it with a pushpin. It's not exactly the softness of lead, but it's definitely not steel.

I had originally opted to go for the softest and purest aluminum available via McMaster, which is the 1100 alloy. The reason for this was simply that it was just the purest they offered, and it was pretty cheap. I started out with five 6x6x0.063" sheets of this (1/16" thick) just to try out. At the time I was still milling the aluminum, using tiny 1/16" and 1/32" ball-nose end mill cutters, and cutting this soft aluminum was wretched at best. It would effectively pile up the removed aluminum along the sides of the grooves. This was disgusting.

I then went ahead and tried out the 3003 alloy, which is actually a bit cheaper than the already-cheap 1100 alloy. It seems to be virtually the same, machining-wise. But sine it's cheaper it's going to be my go-to hologram metal.

The 3003 also comes in a wider variety of thicknesses. Now, provided that I will only be making grooves that are only a few thousandths of an inch deep into the metal, I still need the metal plate itself to not be flimsy and prone to getting bent by slight forces, so thus far I have been opting for sheets that are .050". I actually have on order some more that are only .032" thick, just to see what that's like (plus it's even cheaper).. So hopefully that works out. But thus far, between the 1100 and the 3003 I think I will be sticking with the 3003 simply due to its price.


Makercam.com made life somewhat easy for a while.

Now, being that Holocraft's original means of output was by way of spitting out SVG file paths for the eCraft paper crafting machine, it was a miracle to discover that out of all the free CAM software out there there was only one that could handle the thousands, and even tens of thousands of grooves being output by Holocraft, and that was MakerCam.

MakerCAM made it a snap to convert my hologram toolpaths from an SVG file into a CNC g-code file. This was not without its caveats, of course... For one, the user cannot control how exactly the toolpaths are generated for the paths of any given SVG file. Many times I would find that MC's output would resume cutting a groove from a single side. Over and over it would enter the material to cut a groove, cut through to the other end, then it would raise the tool up over the surface to move back to the beginning side again. To my mind, it would be much quicker to finish a groove, move a little deeper into the material, and then continue back the other way in reverse. No, this wasn't happening with MakerCAM.

In order to actually control my machine and run my hologram-groove CNC programs to create actual metal surfaces I needed a program that would drive my GRBL-based CNC machine. Among the popular online communities there are a few recommendations that seem to satisfy everybody's needs. Unfortunately, I had no luck with these suggestions because they were not suited for massive g-code programs with tens of thousands of lines of code. These programs could only handle *maybe* a thousand lines of g-code at a time, which was useless for specular holography purposes.

Lo-and-behold, I managed to come across grblControl, which is a relatively newer GRBL controller program that features all of the bells and whistles of the other popular programs with the exception that it runs FAST. It can handle the largest of g-code programs I can throw at it, without breaking a sweat. To anybody using a GRBL based CNC I highly recommend you check it out. It is the only program I have used with my CNC, ever.

It is simple, efficient, and has plenty of features that make machining as painless as possible (which is still rather painful, but at least grblControl doesn't contribute to the pain).

There were a few things that were not exactly desirable about grblControl, but being that it is open source I was able to install and load up QT-creator and dive into the code and start making my own custom version of it. The first thing I opted to change was the fact that it operates strictly in metric, and all of my experience, expertise, and tools are designed for imperial. So, after some hunting and pecking I made my own imperial version of grblControl.

Aside from that I have made a slew of other changes, visually, and also functionally, just to get grblCotrol to be best suited to what I am trying to accomplish with it. Thus far I am really happy with where it's at now.





After a while it became apparent that if I were to have finer control over how the grooves were being cut I would need to implement functionality in Holocraft that would directly output g-code for a CNC machine, and completely obviate the need for any CAM software to generate the toolpaths in the first place.

The way that MakerCAM was interpreting the SVG files was random, at best. I could not rely on the fact that just because a path was defined going from point A to point B that it would be machine in that order. In some cases it seemed to make weak attempts at optimizing the toolpath by alternating directions between successive optical grooves, where when it ended cutting one groove it would then move up to the end of the next groove and cut toward the starting side of the groove definition, but there was no metric by which to reliably cause this to happen, or not happen..

As it stands, my machine has issues with machining holograms (or metal, as it were) due to the mere fact that when it is cutting in the Y+ direction (cutting away from the front of the machine) the leverage on the gantry causes it to lift up, which effectively prevents it from cutting as deep as it is supposed to because it is too friendly to the surface when moving in the Y+ direction.

Conversely, when the machine is cutting in the Y- direction, the gantry leverage just sucks the tool down harder and deeper into the material, gouging it much deeper than would be intended. The end result is that I must cut my holograms with the grooves being formed in one direction along the Y axis. I chose the Y+ direction simply because I'd rather have lighter grooves than inevitably and irreversibly deep gouging grooves that are formed otherwise.

This was discovered when originally cutting holograms where the grooves were traveling from X- to X+, in a left-to-right fashion. What was happening was that the left side of the groove (traveling in the Y+ direction while moving X+ rightward) was lighter than the groove was on the downslope traveling in the Y- direction. This is purely a product of the design of my machine, which was not designed to be used as a sort of drag-engraving machine in the first place, and so ways and means have to be put into place to work around this weakness of the machine.

Another issue that arose was the resolution of the machine itself. Was the X-Carve even capable of distincting grooves into the surface of the aluminum without there being obvious stair-stepping resolution artifacts? Well, we are running with 20-tooth pulleys on 2mm pitch belts means one revolution is 40mm along the belt.. With 200 steps per motor shaft revolution, with 8x microstepping, should be at 40mm / 1600steps =  0.025mm.. Therefore, we should easily have a resolution of at the very most .001" of an inch for our grooves, which seems plenty fine provided that we are malking those grooves fast and smooth and not moving to each exact increment of the motor to scribe the metal's surface.

Now, in practice, what I've found is that running the machine at the highest possible speed (going into GRBL config via '$$' command and playing around with max speeds and accelerations while tweaking the power dials on the gShield going out to the steppers) I've managed to get my CNC to fly like none other. The problem is that when I exert force into the surface of aluminum with a carbide bit at such speeds there is enough leverage at play to allow for what we in the States refer to as 'speed wobble'. This is not a machining term, this is something that happens when you are flying down a hill on a bicycle or a skateboard and your rate of speed just becomes too much.... Too.... Much... The end product being that your handlebars or skateboard begin resonating side-to-side uncontrollably so until the point that a crash of some kind is usually inevitable.

In this case, it results in wobbly little grooves, which are exactly *NOT* what we want, because we are trying to scribe optically accurate/useful grooves into the surface of the aluminum.

So, it has become a balance of slowing the machine down to minimize the wobbles as much as possible, without sacrificing speed, and without introducing a sort of stippling that arises from the actual machine position increments manifesting themselves in the grooves themselves, which are equally as ugly when it comes to optical applications.

Playing with the depth of the groove and the speed at which it is formed has been a bit of a journey, as well as taking other measures to mitigate the 'wobble' by raising the workpiece closer to the gantry to minimize the leverage that the tooling edge has against it. Stiffening up everything on the machine has been another project as well.


Here is a low-fi video of a hologram I have been working on for my younger sister, for her birthday. When I can manage a better camera I will (feel free to donate!).

video



Here is yet another video of another test hologram featuring some random abstract cuboidal shapes merged together in a sort of splatted configuration. Again. feel free to donate better camera ware!

video



Links:

IKEA TARENDO Table - ikea.com
Konmison 300w Spindle Motor with PSU and 13pcs ER11 - Amazon.com
MakerCAM - makercam.com


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