Follow along by carefully examining this section’s figures, and we’ll explain how this solution will work so the title of this section will become clearer. (Don’t worry about any MDF pieces or other stuff in the pictures; for now, just pay close attention to those pieces we point out in the figures.)
The two items in Figure 4-1 are 4" lengths of aluminum angled rail. The rail consists of two 1/8"-
thick walls that meet at a 90 degree angle (right angle). Rail width is measured on the outside wall from the outside edge to the edge where the two walls meet. You can typically purchase this angled rail in lengths of 4', 6', and 8'. (Rail is easily cut with a hand saw, but feel free to use whatever method you prefer for cutting the rail to the proper lengths needed for your CNC machine. You’ll also be told the proper lengths to cut your rails in later chapters.)
Figure 4-2. The angled rail’s width is measured from the outer wall edge to the inner wall edge.
Riding the Rail
The two items in Figure 4-1 are 4" lengths of aluminum angled rail. The rail consists of two 1/8"-
thick walls that meet at a 90 degree angle (right angle). Rail width is measured on the outside wall from the outside edge to the edge where the two walls meet. You can typically purchase this angled rail in lengths of 4', 6', and 8'. (Rail is easily cut with a hand saw, but feel free to use whatever method you prefer for cutting the rail to the proper lengths needed for your CNC machine. You’ll also be told the proper lengths to cut your rails in later chapters.)
Figure 4-1. Two pieces of aluminum angled rail
Figure 4-2 provides more detail on how to find the proper type of aluminum angled rail; even
though rail is often labeled with the proper measurements, don’t trust the sticker. Measure it yourself to make certain you’re buying the proper thickness and width of rail.
though rail is often labeled with the proper measurements, don’t trust the sticker. Measure it yourself to make certain you’re buying the proper thickness and width of rail.
For your CNC machine, you’ll be purchasing two different widths of rail: 3/4" and 1 1/4". The
thicknesses on both rail widths will be 1/8". The rails shown in Figure 4-1 are 3/4" wide and 1/8" thick.
thicknesses on both rail widths will be 1/8". The rails shown in Figure 4-1 are 3/4" wide and 1/8" thick.
Now take a look at Figure 4-3. This figure shows a few of the pieces of hardware that you’ll be purchasing—bearing, bolt, and nut. It also shows the three items assembled.
Figure 4-3. Bearing, bolt, and nut, and the three items assembled
Just in case you’re wondering, the bearing shown in Figure 4-3 is the same type of bearing you’ll find used in skates!
Before we continue, we want to give you a preview of what a final bearing-rail assembly (BRA) will look like. Figure 4-4 shows a piece of angled rail with four of the little bolt-bearing-nut assemblies mounted to it.
Figure 4-4. A BRA—one rail and four sets of bolt-bearing-nut assemblies
Now let’s take a step back; you need to know how to take four of the bolt-bearing-nut assemblies and attach them to the rail to make a BRA.
After the bearing, bolt, and nut are assembled, they must somehow be attached to the piece of rail you’ve cut. Attaching will involve three steps: drilling a pilot hole, drilling a 17/64" hole, and tapping the hole (creating threads in the hole for a bolt to be screwed into).
First, you’re going to drill pilot holes in the rail. Let’s talk about pilot holes. You’re going to hear that used in many places in the book, so this is a good time to explain what they are and why they’re important.
Drilling a pilot hole could also be considered predrilling because you’re going to use a very small diameter drill bit to drill a starting hole. Take a look at Figure 4-5 and you’ll see a small piece of metal— about 1/16" thick—with a single hole in it.
This little piece of metal serves two purposes. First, we’ll place it over the rail where we wish to drill a hole. This will allow us to drill the hole in the same location on every piece of angled rail that we’ve cut. Figure 4-6 shows how the metal is being used as a template for drilling a hole in our angled rail. (You can purchase a small scrap of 1/16"-thick metal like this at a local hardware store cheaply.)
The small hole will allow you to more accurately drill a final hole in the aluminum. Notice in Figure 4-6 that the metal piece has been pushed completely up against the inner edge of the rail (where the two walls meet) and is flush with the end of the rail as well.
Figure 4-5. A piece of metal with a single pilot hole in it
Figure 4-6. Drilling a pilot hole into an aluminum angled rail piece
Figure 4-7 shows the measurements for where the pilot hole was drilled in the metal template we used.
Figure 4-7. The metal template used to accurately drill pilot holes
We know that the rail will require four sets of the bolt-bearing-nut assembly, so you can simply flip the metal template over and drill a matching pilot hole on the opposite side of the rail’s wall. Then drill two more holes on the rail’s other wall. Figure 4-8 shows a rail piece with all four pilot holes drilled.
Figure 4-8. Four pilot holes drilled into a rail
The small pilot hole will allow you to center a larger drill bit more accurately. Most drill bits have a sharp tip on their end that will fit inside the small pilot hole—take advantage of this fact and your drilled holes will be more accurately placed.
The next thing you’ll need to do is enlarge your pilot holes. For all the rails you will make for your CNC machine, you’ll be drilling 17/64" diameter holes. You can see in Figure 4-8 the larger diameter drill bit that will be used to make these holes. The 17/64" drill bit typically comes with the package that includes the tap and is usually labeled as a drill bit and tap for 5/16" screws.
Why are we drilling using a 17/64" bit and not a 5/16" bit for the 5/16" bolt that will screw into the threads? The hole will be a little smaller than 5/16" because you’re going to cut grooves (threads) into it in the next step. If you drill an actual 5/16" hole, the 5/16" bolt would just fall through. The little extra bit of aluminum left over (3/64") with the 17/64" bit will leave just enough material for the threads to be cut so the bolt can screw into the hole.
Figure 4-8 shows that we’ve clamped down the aluminum rail for drilling using the 17/64" drill bit. Figure 4-9 shows the rail after the holes have been drilled.
Figure 4-9. Larger holes drilled into the aluminum angled rail
Finally, in order to insert the bolt-bearing-nut assemblies, we need to create a thread inside the 17/64" holes for the bolts to be screwed into. To do this, we use a tool called a tap. You can see this tool in Figure 4-10.
Figure 4-10. Cut threads into the aluminum rail using a tap.
The t-shaped tool is used to cut threads into the 17/64" holes. You place the tap bit into the hole, making sure that the tap is as straight and vertical as possible with respect to the hole. You then twist the tap clockwise and the tap bit will cut into the aluminum edges of the hole. Figure 4-11 shows the tap cutting threads into a hole.
Keep twisting—you’ll feel resistance, but it’s shouldn’t be extremely difficult to twist. At all times try to keep the tap bit going straight into the hole, not at an angle. Figure 4-12 shows the tap bit over halfway through the hole.
Keep twisting—you’ll feel resistance, but it’s shouldn’t be extremely difficult to twist. At all times try to keep the tap bit going straight into the hole, not at an angle. Figure 4-12 shows the tap bit over halfway through the hole.
Figure 4-11. Cutting threads into one of the holes in the aluminum rail
Figure 4-12. The tap bit over halfway through the hole
When the tap bit gets about halfway into the hole, it will become extremely easy to twist and very loose. Give it a few more twists and then spin the tap counterclockwise to remove it completely from the hole. Look inside and you should see small threads that will allow a bolt to be screwed in. Do this three more times per rail and you’ll end up with a piece of rail with four threaded holes. Screw in four of the bolt-bearing-nut assemblies and you’ll end up with a rail like the one shown earlier in Figure 4-4. Tighten the bolts down lightly with a wrench—do not apply too much force or the threads can become damaged. We’ll refer to this final piece of rail with four bolt-bearing-nut assemblies as a BRA.
Riding the Rail
Now it’s time to see how your CNC will move smoothly. Figure 4-13 shows an unused piece of rail and a completed BRAs for short.
Figure 4-13. A rail and a BRA
Place the completed BRA so all four bearings rest against the flat wall surfaces of the rail. Slide the BRA back and forth along the rail. Smooth, isn’t it? This is one of two mechanisms you’ll be using to give your CNC machine smooth and accurate movement. (You’ll learn about the second method in Chapter 6.)
Tips and Advice
Do you see the gap between the BRA and the rail it rides on in Figure 4-13? If the gap is too small, the two pieces will rub against one another and can create resistance. This will cause your CNC machine’s movements to be less than smooth. To avoid this problem, it’s important that the spinning bearings be a certain distance “up and away” from the rail it rides.
Fortunately, you won’t have to do any trial-and-error drilling for your BRAs. If you download, print, and use the template we mentioned earlier in the chapter, you can mark and drill all your rails in the proper spot so the bearings will spin smoothly and the gap will be sufficient in size to prevent rubbing.
Fortunately, you won’t have to do any trial-and-error drilling for your BRAs. If you download, print, and use the template we mentioned earlier in the chapter, you can mark and drill all your rails in the proper spot so the bearings will spin smoothly and the gap will be sufficient in size to prevent rubbing.
It’s also very important when tapping the 17/64" holes that you twist the tapping tool straight into the hole. Go slow—twist, stop, and rotate your hands around so you can see all angles. Is the tapping bit going into the hole at any angle other than vertical (90 degrees)? If so, apply some pressure and force the tapping tool back into proper vertical alignment. You might want to cut a scrap piece of rail, drill a few pilot holes and 17/64" holes, and practice tapping—it’s well worth the time so you can create the perfect BRAs to ride the rails of your CNC machine.
You’ll be building a total of six BRAs—two for the x-axis, two for the y-axis, and two for the z-axis. Not all the BRAs will be the same length, but the method for creating them is identical—measure and mark where the four holes will be drilled, drill four pilot holes, drill four 17/64" holes, and tap four holes.
Trust us—creating the BRAs is not hard. Just take your time, double-check your marks, drill those pilot holes, and practice your tapping technique on some scrap. We’ll tell you in later chapters exactly when it will be time to build some BRAs and how many. You’ll do great.
What’s Next?
Next, in Chapter 5, you’re going to learn about two possible methods for connecting pieces of MDF together. You’ll probably be surprised at how easy each method is to use and how strong the connection between the pieces is when it’s done.
Next, in Chapter 5, you’re going to learn about two possible methods for connecting pieces of MDF together. You’ll probably be surprised at how easy each method is to use and how strong the connection between the pieces is when it’s done.
