This is the second article on a small CNC router build – go here for the first article to see where it all starts.
In this article, we will describe a small departure from the original plans in the use of a different strategy for the ball-bearing trucks that are used to roll the axes back and forth along their linear rails. These trucks, like many other homebrew CNC trucks, are made of skateboard bearings bolted to the outside of aluminum right angle stock, which is available in any local big-box hardware store and can be cut and drilled with simple tools. It’s a sturdy setup that gives a nice smooth motion.
Our beef with the original trucks is that they used undersized screws and oversized holes to hold the skateboard bearings. This was done, presumably, to offer some adjustability as well as to allow the bearings to be mounted directly in line with each other. That’s cool and all, but since there will always be some dead space between the inner bearing race (which is 5/16″) and the screw (which isn’t), it seems like one good whack to any of the axes could push the bearings off their alignment and leave the truck rattling a bit loose. Even in the case of no impact, the axes will be loaded with the cutting force and seem like they could drift over time if the friction of the washer were the only thing holding the bearing race in place.
So, we decided to use JRGO-style trucks with offset bearings and full 5/16″ hardware. This gives a sturdy setup with good tolerances with a minimal amount of work. The biggest challenge was to determine the proper spacing to be used. Since the 5/16″ bolt heads take up over half of the width of the inside of the aluminum angle, they cannot be mounted directly in line with each other and must be offset a bit to get the bolt heads to fit. This seems like it should be no big deal, especially since the offset between the bearings is a fraction of the distance between the two bearings on one side. For all intents and purposes, the bearings should be very resistant to twisting and torquing even with their slight offset.
We won’t say the actual lengths of aluminum angle used in the project, but for each of them the drill template is the same. For the outsides (bearing closest to end), we drilled 5/8″ from the end and 9/16″ from the outer edge of the aluminum stock. For the insides, it was 1 1/8″ from end and again 9/16″ from the outer edge. This may seem confusing, but is really quite easy – measure one side (say the outside bearing), flip and use the same measurement on the opposite end. Then turn the angle stock around and do the “inside” measurement. Flip around again and mark the other inside point and you are done. All our measurements and marking was done by hand using an adjustable combination square – an invaluable tool for this project. These distances can be modified a bit, but these are the distances that will protect the bearing in case one of the axes crashes against the end of travel – the aluminum angle will hit before the bearing does.
The holes we will drill are 5/16″, and will need a small pilot hole first. As with anytime you’re drilling aluminum, first punch a divot at your intended drill point with a nail or something to keep the drill bit from wandering. Drill a small pilot hole, and then swap to the 5/16″ drill bit and drill the final hole. You should be able to fit the 5/16″ x 1″ hex bolts with relative ease. But we’re not there yet – remove those bolts and get your router out, it’s time for some danger!
Figure 1 – Drill Template for our Variety of JRGO-Style Trucks
Bolt Mounting Slots
We’ll still need to mount these babies flush to a piece of wood, and the Solsylva method of filing and drilling right at the peak of the angle seems tricky. Additionally, we’re going to use 5/16″ x 2″ hex bolts for a heavy duty mount so we will want to countersink the entire 1/2″ hex bolt head right into the angle aluminum itself. And, we’re lazy… So we took the easy way out and used a router with a 1/2″ bit (and a simple homemade jig) to mill a small channel right in the peak of the alumimum angle. You do have a router, right? You’re building a CNC router, aren’t you? So go buy one if you don’t already have one!
The routered notches for the hex bolt heads are visible in the pic below. This may look like a tricky cut, but is actually dead simple if you have some spare bits of wood that you can make a jig out of. What you want to do is make a wooden channel about 1 3/8″ wide that the aluminum angle slides into, peak up. Don’t use measurements, press fit it by hand to make sure the aluminum is held TIGHTLY! Two strips of 1×2 nailed to a 2×4 worked great, and in our case came out perfectly flush with the peak of the aluminum angle when shimmed with a couple of washers. At the end of the wooden channel, you’ll nail another piece of wood to act as a backstop to keep the aluminum from sliding any further. One last chunk of 2×4 right outside of that to act as an alignment stop for the router, and you’re ready to go.
The idea is that you stuff a piece of aluminum angle down your wooden channel until it hits the 1×2 endstop. Then, the router can slide laterally across the aluminum angle to mill out the groove, while being held tightly to the taller 2×4 chunk endstop. This will cut a groove that is always the same distance (1 3/8″ to beginning of slot) from the end of the aluminum stock. Stick in a piece of aluminum, mill a slot. Pull out and stick in other end, mill other slot. Easy as pie! Chock up your router so the bit is sticking down about 3/16″ and with a slow sideways swipe your 1/2″ bit will route the perfect groove to hold the head of the hex bolt. The points of the hex bolt will stick out sideways over the sides and give a nice strong surface to hold onto.
The router will mill mostly through solid aluminum, but will break through a small (1/16″) slot to the inside. We’ll still need to use a drill to zip a 5/16″ hole in the slots for the bolt to fit down into. Then you’re good to go. For the first one, start shallow on the router depth and increase gradually while using a hex bolt head to see how deep flush is for your particular bolt.
We apologize that there are no pictures of the jig that was used to cut these slots, but rest assured it’s very simple. Our target was to place the outer edge of the slot at about 1 3/8″ from the end so it would not interfere with the bolts holding. Ours came out a little closer to the end than that, but still works fine.
A few assembled units.
Figure 2 – Completed Modified JRGO Trucks. Note the Router-Milled Mounting Slots in the Angle’s Peak
The assembly is quite easy as well. Stick a 5/16″ x 1″ hex bolt through each of the four mounting holes and put a nut on. Then a washer, a 608ZZ skate bearing, a second washer, and a final nut. Look to the picture above for a visual assembly example. Since there is not enough clearance for locknuts, use threadlocker, superglue, or Derpina’s nail polish on both the first and second nuts to make sure they will not rotate loose. And please, make sure the threadlocker on that first nut is dry before continuing assembly – Mike!@ In total, the hardware that you will need for these trucks is
- 6 pieces of Aluminum Angle Stock, cut to the proper sizes – About $5 for a 3′ length
- 24x 608ZZ Skate Bearings – About $20 for 50 or so from eBay, with shipping
- 24x 5/16″ x 1″ Hex Head Bolts – About $5
- 48x 5/16″ Hex Nuts – Another $6 for 100x
- 48x 5/16 Flat Washers – Another $6 for 50x
- 12x 5/16″ x 2″ Hex Bolts (for mounting the trucks to their respective wooden frames) – ?? Another couple bucks, probably.
Here’s a pic of the trucks sturdily mounted to the Y-axis table. The flush-mounted hex bolt is clearly visible residing in the slots milled in the peak. Perfect fit!
Trucks in place.
Figure 3 – Bottom of the Y-Table Showing the Trucks and Mounting Bolts
Please note that since you are messing around with the mounting hole spacing, you’ll need to measure and modify the hole spacing for the truck mounting holes. And the mounting holes you drill will need to be 5/16″, of course.
The mounting is very sturdy – for the Y table shown above, a single washer and a single locknut are used to hold the trucks in place. When mounting, don’t tighten too tightly initially. FIRST make sure that the trucks are actually parallel to the edge of their mounting block, THEN tighten – the aluminum angle will dig grooves into the soft pine and lock their position in place for all eternity. You want it to be right.
Until next time – good luck!