If the title of the chapter seems strange, let us explain. The CNC machine you'll be building is made up of 26 varying-sized pieces of MDF material. Those pieces have to somehow be connected in such a way that the machine doesn't fall apart. Keep in mind that your CNC machine will be moving-the z-axis will be moving up and down and side to side on both the y-axis and the x-axis. Often these movements will start and stop quickly. Your CNC machine must be able to withstand the twisting and bending and other stresses required for its operation. Wood glue simply won't do, and the mechanical way of fastening has the added benefit of disassembly. This is a machine, so we're going to treat it like a machine.
This short chapter is going to introduce you to two methods for connecting one MDF piece to another. Both methods create strong connections that will be difficult to break, and both methods use the same tools. We'll leave it to you to decide which method you'd like to use, but we favor one in particular that we'll discuss shortly.
Keep in mind as you read this chapter that almost all the pieces of MDF that you will cut and drill will connect using one of these methods. Pick one and become familiar with it. Listen to the tips and advice we provide, and your CNC machine will be a rugged and durable device for years to come, with minimal maintenance.
Before we show you the two different methods, we want to explain how the two methods work by using some simple figures and photos. First, take a look at Figure 5-1; it shows two pieces of MDF.
For most of your CNC machine assembly, you will be joining MDF pieces at right angles. Figure 5-2 shows how the two pieces of MDF will connect.
The pieces will be connected using a series of drilled holes; MDF piece A will have a single hole drilled into it, as shown in Figure 5-3. MDF piece B will have two holes drilled into it, also as shown in Figure 5-3. Don't worry right now about where to drill the holes; you'll get that information in specific chapters when you begin to cut, drill, and build your machine.
Caution
Part A and Part B in these figures are not the same Parts A and B for your actual CNC machine; these are waste pieces of MDF and are only used to demonstrate two methods for connecting pieces.
The hole size drilled in Part A matches the hole size drilled on the edge of Part B. The second hole drilled into the surface of Part B will be of a different and larger size.
When you're reading the CNC machine plans, you'll be told what size holes to drill, but in most instances where you're connecting two pieces of MDF at right angles, the matching holes will be of 1/4" size (use a 1/4" drill bit). The surface-drilled hole (on Part B) will be of a larger size and will depend on whether you use method 1 or method 2 for joining MDF pieces at right angles.
Now that you understand how these two pieces are to be connected, let's look in detail at the two methods available. Note that in each method, a different-sized drill bit will be used to drill the surface hole on Part B.
Take a look at the hardware components in Figure 5-4. There are two bolts-the length of the bolt you use to connect MDF may vary, but in this instance, it is 2" in length. The smaller components are often referred to as either cross dowels or barrel nuts (and they're also occasionally called knockdown nuts). When visiting a hardware store, if you choose to use method 1, be sure to mention these names to the salesperson if you can't find them. For purposes of this book, we're going to use the term cross dowel when referring to this part.
Tip
Cross dowels can sometimes be slightly expensive when purchased individually. You'll be using over 50 of these as you build your CNC machine, so buying them over the Internet may provide you a lower cost per piece.
If you look closely at the cross dowel, you'll see that one end contains a slot-you'll be using a slot screwdriver with this component. Notice also that the cross dowel has a hole through its main body and the hole is threaded-meaning a bolt with threads can be screwed into it. Figure 5-5 shows the bolt screwed into the cross dowel-notice that the slot on the cross dowel is parallel to the bolt. (In other words, the slot points in the same direction that the bolt points.) You will use a slot screwdriver to orient the slot on the cross dowel to match up with the insertion of the bolt.
The cross dowel has a diameter of almost 7/16" and will be inserted into the hole drilled on the surface of Part B. This will require drilling a 7/16" hole in the surface of Part B, as shown in Figure 5-6.
Note
The cross dowel is slightly smaller in diameter than the 7/16" hole to provide some wiggle room for fitting it into the hole.
Notice that the holes on Part B are aligned. You can see a pencil line in Figure 5-6 that runs from the bottom of the 7/16" hole on Part B; this line is useful for drilling the 1/4" hole into the edge of Part B. If you stick the bolt into the 1/4" hole on the edge of Part B, you should be able to view it by looking down into the hole drilled on the surface, as shown in Figure 5-7.
After removing the bolt, we next take Part A and place the 1/4" hole drilled on its surface over the 1/4" hole drilled into the side of Part B, as shown in Figure 5-8. We also insert the cross dowel into the surface hole on Part B with the slot facing up so we can see it-this is also shown in Figure 5-8.
Figure 5-7. If drilled properly, the inserted bolt will be visible when you look into the surface hole.
Use a slot screwdriver to twist the cross dowel so the slot runs parallel to the 1/4" drilled holes in Parts A and B. Now we're ready to insert the bolt. This may seem a little tricky at first, but it gets easier as you do more of them-practice makes perfect!
Slowly insert the 1/4"-diameter bolt into the hole on Part A. Push it through into Part B. Your goal is to have the bolt find the hole in the cross dowel and finger thread the bolt into the cross dowel. While doing this, you'll be using a slot screwdriver to make sure that the slot on the cross dowel remains parallel to the bolt so it can be threaded properly.
The typical mistakes made here are pushing the cross dowel too deep into the surface hole or not deep enough. Either way will keep the bolt from being screwed in properly to the cross dowel. Keep at it-you'll eventually find that finger tightening the bolt becomes more difficult and requires a Philips head screwdriver (manual or electric) to tighten the bolt completely.
Notice in Figure 5-9 that the edge of Part A is flush (or level) with the surface of Part B; this is done by lining up the edges just before completely tightening down the bolt. If you've already tightened the bolt down, loosen it just a bit, align the edges properly, and retighten.
Congratulations. You now know how to connect two pieces of MDF using the cross dowel method. Now let's examine method 2.
You'll be happy to know that method 2 is almost identical to method 1 except that you won't be using a cross dowel. Instead, you'll drill a slightly bigger hole on the surface of Part B (at the same location where you drilled for the cross dowel) and insert a standard nut that the bolt will screw into and tighten to secure the MDF.
Take a look at Figure 5-10 and you'll see the same setup as before; we have two pieces of MDF, Part A and Part B, that need to be secured at a right angle.
You'll drill the same 1/4" holes in Part A and Part B for insertion of the bolt; in Figure 5-10, this is a basic 2" bolt (1/4" diameter) with a matching nut. The larger drilled hole on the surface of Part B will be 3/4" diameter (use a 3/4" drill bit). You will insert the nut sideways into the 3/4" hole.
Place Part A and Part B so the edges are flush (or the parts are matched up at the edges), as shown in Figure 5-11. Insert the bolt into the drilled hole in Part A and then into Part B. Hold the nut sideways, as shown in Figure 5-11, and hand-thread the bolt into the nut. Use a Phillips-head screwdriver to complete the tightening of the bolt.
It's an alternative to using cross dowels that works well. One potential drawback to this method, however, is that over time the bolts will need to be tightened more often; they will tend to loosen as the CNC machine vibrates and moves.
We favor the cross dowel method, but cross dowels do cost a little more per piece. Both methods will allow you to tighten down the connected MDF pieces securely, but cross dowels seem to be less likely to loosen over time and require only tightening of the bolt.
Tip
You might also consider using lock washers or lock nuts. Lock washers are used to prevent bolts and nuts from loosening. You put a lock washer on just before screwing on the nut. Lock nuts have a nylon insert inside them that also helps prevent loosening of its bolt.
Pick a method that works best for you-base it on the skills and tools you have available. You'll get plenty of practice in later chapters as you begin building your own CNC machine, but feel free to try both methods on some scrap MDF pieces and see which method you like best. You may find that method 2 is easier for assembly because you don't have to fumble with aligning the cross dowels-that's fine. You may like method 1 because it doesn't require you to find a wrench to hold the nut for tightening (or hurt your fingers).
Now that you've seen the two methods used to connect two pieces of MDF, we'd like to offer you one additional tip that may make drilling easier. In our experience, we have found that a simple jig (guide) may help in aligning the holes drilled to connect pieces of MDF. With the large amount of right angle joining necessary, a jig can be extremely useful. Once you drill the first hole, finding the intersecting hole location with the proper alignment to the first hole can be trying without some help. A jig will help in two main ways. First, a jig can help with drilling in the proper orientation (drilling a straight hole). Second, the jig will insure the proper location for the intersecting hole and for the hole into the edge of the piece with consistency.
You can build your jig from a small piece of scrap MDF; make certain it's constructed from the same thickness of MDF as you're using for your CNC machine-3/4". Draw a straight line down the center of the piece and then cut the piece in two, as shown in Figure 5-12.
In Figure 5-13, the jig is constructed by gluing the two scrap pieces of wood joined at a right angle and the measurements where the holes need to be positioned. Don't drill the holes until after you've cut the two pieces; measure the location of the holes on the line that was drawn down the center of the pieces.
In Figure 5-14 you can see how the two pieces are joined. After you've selected the method you prefer to use (method 1 or method 2), you can place your jig over the MDF you'll be drilling to drill holes exactly where you want them.
Figure 5-14. Clamp the jig over the piece to be drilled and drill precisely placed holes every time.
If you place the jig over the piece of MDF to be drilled, the two pilot holes shown in Figure 5-14 will place those holes at the standard distances from the edges of the MDF indicated in the CNC plans.
In Chapter 4 you learned how the BRAs (Bearing-Rail Assemblies) allow your CNC to move smoothly along pieces of rail. In Chapter 6 you're going to learn the second half of what's required for CNC movement. The BRAs won't roll themselves, so something has to provide them with the proper force (and the proper direction) for movement. Luckily, it's an extremely simple and elegant solution that you'll find easy to implement.