CNC, Pt. 4: Frame Construction

Construction begins! It's pretty straight forward, just start building in reverse of how it was designed- steel frame first. It is very important to be as precise as absolutely possible when constructing the frame since the rails are bolted directly to it, and you only get about 1/8" of adjustment or less when it's all said and done. Those rails, when locked down, need to be withing about 1/100th of an inch of perfectly parallel or they will bind up and bring the machine to a disappointing halt.



Once the frame was done we mounted the rails (loosely) by drilling and tapping directly into the steel beams.

CNC, Pt. 3: Design

About the frame, and about doing this kind of design work: When working on designs for things that are very mechanical, and not very aerospacy, I tend to shoot from the hip. I have a pretty healthy aversion to math, and I find solids modelers to be cumbersome. I've grown up building things, so my version of shooting from the hip, in general, is still reasonably accurate with error on the side of overbuilt and heavy.

My approach then is to map out as many known pieces and principles as I can, then try to finagle them around in a basic 3D model till everything is as close to ideal as I can manage. I knew my rails (24.5", 19", and 12.5"), and I already had my motors (3 x 280 oz NEMA23 stepper motors). I made quick models of the rails, screws, ball nuts, and motors so that I could align them the way they would be in the machine. From there I modeled rail blocks and assumed that I would use 1/4" aluminum plate to tie them together.

I went with a "split gantry" layout to keep a high stiffness, meaning only one axis has to be mounted on top of another axis.

This whole process leaves everything floating in space, but with all the parts just where they need to end up in the final machine. Therefore all that needed to be done was to model a steel frame around it for everything to attach to, and take measurements...





Strange as it may be, I do all my non-FEA engineering in Maya3d. It's just quicker to mock things up. It also makes it very easy to rig up motion and limits on things so that I can check for interferences and spacing. The machine is designed for stiffness, not efficient use of the rails. As a result the total motion area is 12x12x6 inches.

CNC, Pt. 2: Rotary -> Linear

Ok, Ballscrews and ballnuts. The basic idea is that you need to convert rotary motion (a motor turning) into linear motion (a car rolling down a rail). There are a few ways to do this: You can use a belt, or put little wheels on a track, or use a threaded rod and nut. In the case of a hardened CNC machine like this you need it to be able to provide a large amount of force without getting nudged around or flexing, so that last option, the threaded rod, is a pretty good candidate.

A regular threaded rod will work sometimes, but in general they are very fragile because the edges of the threads are pointy and easy to get chips or bends in them. The simple solution to this is make threads that are flat on the outside so that they are less fragile. This is called an "Acme thread."

There's one step better though, which is to get a very special threading that uses ball bearings to roll on. The advantage of this is a lower friction (meaning more power makes it from rotary force to linear force). In some cases ballscrews can be twice as efficient as an Acme threaded rod. They are, however, generally between 4 and 20 times more expensive depending on just what you get. You do save money on motors, controllers, and power supplies, but it's not always worth it. Do the accounting for your application, and feel free to ask me questions. I'll be happy to elaborate if you're looking to build one of these.



Here's a neat animation of how a ballnut works; notice that the ball bearings have to be recirculated through the nut in order to keep rolling on the screw:



On to the design...

The Saga of the CNC, Pt. 1

This is a kind of post-facto blogging, although the project isn't completely done yet. I realize I should have shared it as it happened so I'm going to try to catch-me-up by going through the whole process bit by bit till it meets up with the current state.

I guess the first place to start is "what the heck is a CNC?" Literally it means "computer numerical control," which is not terribly helpful unless you already know exactly what it is anyway. Engineers are jerks like that. What it is, really, is a semi-autonomous robot that serves the very specific purpose of cutting out parts from wood, plastic, or metal. It's given a list of instructions and then goes to town on making something. Or many things. Or the same thing many times. Sounds pretty useful, right? Well it is. And by that I mean the molds to just about every product you own are probably cut on a CNC of some sort, and in the case of some higher end stuff the product itself is often cut on a CNC.

I've made a couple smaller ones in the past for cutting foam and wax, but I've always wanted to make a serious-business CNC machine that was capable of cutting aluminum and hardwoods. The first step was to determine machine size, and to do that I needed to know what sorts of components I would be using. After an extensive search I settled on a set of rails sold by an ebay dealer named "linearmotionbearings2008." Here they be:


Those are both rails and ballscrews. I'll explain the whole ballscrew thing in the next post, along with the basic design of the machine.

Memorial Day Weekend Fun

Started out the weekend right- by pulling the engine out of the Supra again:

Hybrid Rocket Engine

This is the rocket engine I've been designing. I hope to build it not too far in the future. (Hobby thing, not for work). For scale the outside diameter of the housing is three inches.

Hubble Mission Update

Just started hour 5 of the first Hubble EVA: Everything is going smoothly and quickly, already doing 'get ahead' type work. Nice job, fellas.

Today's Launch

Astronaut Day!