As mentioned in Chapter 11, the y-axis that you build in Chapters 10 to Chapter 12 will consist of six pieces of MDF. Those MDF parts are

This chapter will focus on cutting, drilling, and mounting Part P, the Gantry Bottom Support. The completed CNC machine image shown in Figure 11-1 hides Part P, but we've provided a shaded outline of where the piece will be placed.

Figure 11-1. Part P is mounted underneath the table.

Please read through this entire chapter first before you begin cutting and drilling. Part P is a critical piece to the smooth operation of your final CNC machine, so take your time and proceed carefully.

In Chapter 10, you built two BRAs, each with a length of 7". Those two BRAs need to be attached to the gantry sides at this time. You'll use six 1 1/4" bolts and nuts to secure each BRA to a gantry side. Mount the BRAs so that they ride the 4' rails mounted to the sides of the table. Take a look at Figure 11-2; it shows Part R and the six predrilled 1/4" holes that will hold down the BRA.

Figure 11-2. Part R with the six holes used to bolt down the BRA

Figure 11-3 shows a gantry side with its BRA mounted and able to move along the 4' rail bolted to the table's side.

Figure 11-3. A single gantry side with a BRA rolling along the table's rail

For best results, take off the bolt-bearing-nut assemblies before bolting down the 7" rails. Don't tighten down the bolts until you've measured and confirmed that each end of a rail is the same distance from the bottom edge of the gantry side. Figure 11-4 shows the proper locations to measure to ensure that the BRAs are mounted perfectly parallel to the table surface.

Figure 11-4. Mount the BRA rails so that they are parallel to the table surface.

Now, this is where it can get a little tricky-having a second (or third) set of hands will really help. You're going to need to hold the two gantry sides (with BRAs attached) to the table as perfectly vertical as possible, and measure the distance between them under the table. Figure 11-5 shows the two gantry sides being held to the table with clamps, but feel free to use any method you like as long as you can keep the two sides as vertical as possible and not angling inward our outward.

Once Parts Q and R are in place, carefully measure the distance between them from under the table. Figure 11-6 shows what you need to measure.

Figure 11-5. Mount the gantry sides (Parts Q and R) to the table.

Figure 11-6. Measure the distance between Parts Q and R.

Write down the measurement of the distance between Parts Q and R so you don't forget it; you'll need it to cut Part P to the proper length, as well as another MDF piece you'll cut in Chapter 12. If you like, go ahead and write this critical measurement below, on this very page, so that you'll have it when you need it:

Inside distance between Parts Q and R: ____________

If you consult the CNC machine plans, you'll note that Part P has dimensions of 7"×2' 2 11/16" (2 feet, 2 and 11/16 inches). While the 7" height is correct, you're going to use the measurement you just made for the inside distance between Parts Q and R. Cut that single piece of MDF now. Figure 11-7 shows our Part P cut (our measurement was 26 3/4") and marked for drilling.

Figure 11-7. Part P cut and marked for drilling

You're going to drill six 7/16" holes for the cross dowels that will be inserted into the surface of Part P-three on the left and three on the right. You'll also drill six 1/4" holes for the 3" bolts that will be inserted into the sides of Parts Q and R and screw into the cross dowels-three on the left edge and three on the right edge. Figure 11-8 shows Part P with the drilling completed.

Figure 11-8. Part P with holes drilled for bolts and cross dowels

Next, use the plans to measure and mark the holes for drilling Part E. Part E prior to drilling is shown in Figure 11-9.

Figure 11-9. Part E needs to be drilled and bolted to Part P.

Next, mark and drill Part P where Part E will be bolted to it. Countersink the holes on Part P as shown in Figure 11-10.

Figure 11-10. Parts E and P drilled and ready to be bolted together

Two 3" bolts will be inserted into Part P through Part E, and nuts will secure them, as shown in Figure 11-11.

Figure 11-11. Parts E and P bolted together

It's easier to attach Part P (with Part E bolted to it) to Parts Q and R if you take the gantry sides off the table. Don't tighten down on the bolts yet, though-leave them loose so you'll be able to fit the BRAs back over the rails and then tighten them down.

Lift up the table on one end and slide the partial frame under the table leg. Carefully fit the gantry sides so that the BRAs clamp onto the 4' rails. Once the BRAs are fitted in place, tighten down on the bolts on the sides of Parts Q and R to hold the partial frame snuggly to the table. Figure 11-12 shows the partial frame mounted to the table.

Figure 11-12. Partial frame with gantry sides and bottom installed

Next, slide the gantry sides and bottom along the rails as far forward as possible, toward one of the table legs. Use clamps to keep Parts P, Q, and R from moving back and forth along the tabletop. Using a hand drill with a 1/2" drill bit, insert the bit into the x-axis leg's lead screw bearing, and drill a horizontal hole through Part E, as shown in Figure 11-13.

Remove Part E from Part P and attach a 1/2" square nut, as shown in Figure 11-14. Use two 1 1/2" bolts and matching nuts (see Figure 11-15) to secure the square nut over the 1/2" hole. The heads of the bolts will clamp down on the surface of the square nut and hold it in place.

Figure 11-13. Drill a hole through Part E using a 1/2" drill bit.

Figure 11-14. Use bolts to secure a 1/2" square nut to Part E.

Figure 11-15. Use nuts to tighten down and hold the square nut on Part E.

Reattach Part E to Part P. Next, insert the x-axis lead screw you cut back in Chapter 9 through a lead screw bearing in one of the tabletop legs. Thread the lead screw through the square nut bolted to Part E, and then keep threading the lead screw until the opposite end exits the other tabletop leg's bearing.

At this point, you should have the following items completed:

For the work performed in this chapter, you will use

In Chapter 12, you're going to complete the y-axis frame by cutting and drilling two more pieces of MDF that will be attached above the table's surface and between Parts Q and R. When done, the y-axis frame should be sturdy and allow for smooth movement forward and backward on the table surface (x-axis).