I’ve never assembled a machine tool before, so I was put off by the rust-preventive coating that needs to be cleaned off, through mysterious and inadequately-explained means.
The manual says to remove the gunk with kerosene or some other solvent, and assumes that the reader is a seasoned machinist who knows what in the world they’re talking about.
After much searching, I found that they’re saying basically “it’s covered in grease; use a grease-cutting goo to clean it, then protect it with something”. The most benign thing I found recommended online was oven cleaner or 409. Many folks swore by kerosene (which is highly flammable) or mineral spirits.
So my plan is to clean the parts with 409 then, for the parts that need protection, use the (non-silicone!) wax I bought for the scrollsaw table.
Because the Y axis gantry is designed to glide on the long (4 foot) sides of the X axis bed, it’s very important that the long sides of the X-axis be as parallel as possible. If they vary, the Y accuracy of the CNC machine will vary from one end of the machine to the other.
(November 23: Now that I think about it, because of the way the rail assemblies sit I think any non-parallel error in the sides of the X table will translate into Z axis error rather than Y error. That’s because as the Y gantry loosens its grip on the X axis table, the gantry will tend to drop rather than slip side-to-side. This is a good argument for designs that have only one rail holding the Y gantry, and the other side rolling free side to side. …but the issue with those designs is racking of the Y gantry. So it’s a tossup.)
No matter how careful the cutter at the lumberyard is, the X axis 2’x4′ sheets will likely be a little out of parallel, and you’ll need to recut them. In my case, the width of the X sheet varied by over 1/4″ in 4 feet.
Today I tried assembling a test of my rail assembly design: 2 bolts, 4 nuts, 2 bearings, and a short bit of 3/4″ aluminum rail, 1/8″ thickness.
I learned why the book design describes threading the bolt into the rail, rather than having a bolt head or nut inside the rail. I found two reasons:
not having a nut or bolt head on the inside of the rail lets you put the bolt closer to the center of the rail, which in turn lets the rail assembly ride higher – making it easier to clear the screws that hold the rails down.
not having a nut or bolt head on the inside of the rail means that nothing gets in the way of the rail assembly lying flat on the board it’s fastened to. Once I tried my assembly design, I found the bolt head on the inside of the rail got in the way – bummer.
So I’m back to using the design in the book: a 3/4″ bolt, threaded into the rail, with a nut and bearing on the outside.
Today my Mcmaster-Carr shipment arrived, containing the first bolts, nuts, and rails for the CNC machine. I couldn’t wait to open it up, but then experienced a good news, bad news, good news story.
By the way, the parts from McMaster-Carr are beautiful, and have well-controlled tolerances and materials.
First the first good news about the rail assemblies: my guess that the book actually meant 5/16″ bolts instead of the printed 1/4″ bolts was correct! 5/16″ bolts fit beautifully into the 5/16″ inside-diameter bearings that will ride on the aluminum rails. Score! One typo detected and fixed.
Then the initially bad news: When I assembled the bearing, bolt, and nut, I noticed that the 1″ bolt is too long for the design: bolts on opposite sides of the rail would interfere with each other. It looks like it should be a 3/4″ bolt instead. No problem; I can order 3/4″ bolts, that would be flush with the interior of the aluminum rail.
But the more I looked at the assembly, the stranger it looked. If I create a threaded hole in the 1/8″ aluminum rail (as the book describes), the bolt would be grabbing that rail with only about 2 threads. That seems to me too little to hold the weight that the X-axis rail assembly will hold.
Now the good news: I started thinking about using 2 nuts per bolt (one on each side of the rail) rather than one. Doing that would distribute the rail assembly weight over the full surface of the nut, which would result in a strong mechanical connection. It would also mean I would drill 5/16″ holes in the rail, and wouldn’t have to tap the holes. Incidentally, there are a two pages in the book that say the rails take 16 nuts instead of 8, which suggests that some version of the CNC machine used this design.
The only down side is that the upper and lower bearings won’t be in line with each other, because the interior nuts would be in each others’ way. I’m not too worried about that because I’ve seen CNC machine designs that use offset bearings with 2 nuts per bolt/bearing assembly (see Homo Faciens’ CNC machine video).
Thinking through the rail assemblies, I now see why the book recommends using a template for the rail holes: for each side of the rail, the two bearings on one side of the rail assembly must be as closely aligned as possible, to ensure a smoothly-gliding rail.
So now my plan for the rail bearing assemblies is to use the following parts:
5/16″ bolt 1″ length (instead of the book’s 1/4″ bolt – a typo)
Bearing (unchanged from what the book lists)
2 5/16″ nuts (instead of the one nut per bolt in the book)
Probably a locknut to keep the interior nut tight.
And instead of drilling and tapping a 17/64″ hole for the 5/16″ bolt, I’ll simply drill a 5/16″ hole that the bolt will slide through.
Next step: I’ll create a rail assembly and see whether it seems to makes sense.
I’m been reading the inspiring book “Build Your Own CNC Machine” by Patrick Hood-Daniel and James Floyd Kelly. The book has generated positive and sometimes negative comments: positive because it’s very detailed, and negative because it does contain occasional errors, is not a complete recipe, and doesn’t represent designs the authors have developed since the book was published (duh) As for me, I like the book because it’s shown me that I can build such a thing, and I think of the book as a guide rather than a recipe.
Lately I’ve gotten serious about building the CNC described in the book: I’ve assembled a parts list (the book lacks one, and contains a few errors) and ordered a few tools and parts to practice the tricky parts of the assembly.
I’ve been doing a bit of mechanical work on the lunar clock, that I started in my previous post.
As a prototype to help me design the laser-cut parts, I cut out a strip of 1/8″ MDF, cut a square for the motor’s shaft and two holes for the motor mounting holes, then mounted the motor to that strip of wood.
A short time ago I excitedly received my very first shipment from the laser cutting service Ponoko. Here’s a narrative of the unboxing, to let you know all the lovely details.
When the box arrived, I plopped it on the floor to cut it open. Our dog Pippa couldn’t help checking it out.
Inside was the 1/8″ MDF I ordered, carefully protected by a sheet of cardboard.
I was surprised to find that the board was covered, front and back, by a protective adhesive, to keep the parts from breaking loose in shipping – nicely done!
The front piece is cut; the back piece is solid. The paper easily peeled from the pieces, revealing a reminder that this is laser-cut: a strong smell of woodburning, which to me seemed to smell like codfish oil. Fortunately the smell dissipated in a few hours, leaving nothing but a nicely-cut piece of wood.
Once the protective papers were off, I could see how the engraving on the Lunar Wheel came out: nicely wood-burned lines, with no noticeable flash-burn. The dog cutout has a little burn, but I’m sure that will come out with a light sanding.
So, there it is: a wonderful first experience ordering my own design from Ponoko – cheers!
Et proiectus est talpa – "and the mole was cast out"