Racks and Rails

Greetings. One of the critical phases of building this CNC table is mounting the v-rail extrusion and gear racks to the square steel tubing. If this isn’t done with extreme precision, you can’t expect the gantry to move smoothly or squarely.  I didn’t have any guidance on how to go about this so I spent quite a bit of time staring at the table frame and scratching my head.

The first problem to solve was how to ensure that the holes I’d drill in the steel tubing would be in the dead center of the pre-drilled holes in the v-rail extrusion. After my experience drilling holes in the mounting plates for the rails, I knew I couldn’t count on the drill press to be as accurate as I needed. Drill bits like to wander. My friend and trusted adviser, Joe, suggested I build some sort of jig to ensure every hole was drilled in exactly the same place. He’s also a huge fan of his lathe. I never fully appreciated the power of this tool until Joe suggested I use it to make a drill guide. The holes in the extrusion are like a wedding cake- the fat part accommodates the head of the bolt and the shaft fits through the smaller part. Basically I’d take enough material off the outside of a piece of round bar stock so it’d fit snugly into the wide part and then (also using the lathe) drill out the center of the guide with the same drill bit I’d be using to drill the mounting holes. Hopefully the pictures help this make sense. It took me 5 tries to get this right. I’d get the diameter close to fitting into the wide hole and then the next pass on the lathe would make it too small. I didn’t want it to wiggle in there at all.

The second problem was how to ensure the extrusion was held in exactly the right place so all the holes were level.  The width of the base of the rail is .9 inches with the pre-drilled holes centered across this. The plans call for the mounting holes to be drilled 1.25 inches from the bottom of the steel tubing. Some quick math reveals the bottom edge of the rail needs to be .8 inches above the bottom of the tubing. I decided to use my new found skill on the lathe to turn a disc of exactly .8 inches in diameter. I’d then clamp the rail into place with the disc wedged between it and a piece of angle which was clamped to the bottom edge of the tubing.

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Turning a drill guide on the metal lathe.

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Turned this spacer jig to hold the rail exactly 8/10 inches from the bottom of the tubing. I didn’t think I’d be able to get it that precise…
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This shows how I set everything up to consistently drill the mounting holes for the v-rail extrusion. The disc ensures proper spacing from the bottom of the tubing. The center drilled guide fits snugly into the pre-drilled holes in the V-rail.

 

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Another view of the drilling jig.

Once I had the hole drilled I went ahead and tapped it. I drilled out the center of one of my failed attempts at a drill guide to use as a tapping guide, figuring a tiny bit of wiggle room wouldn’t matter once the hole was drilled.  I’d been really concerned about breaking taps during this process because it’s a real pain to try to extract the broken tap from the hole. Using this guide and taking it slowly, I was able to tap all the holes in both rails without breaking a single tap.

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I drilled out one of my failed (not quite snug enough) drill guides to accommodate the tap. Figured once the hole was drilled accurately in the square tubing a little wiggle wouldn’t ruin the threads. Didn’t break a single tap on either rail.

In short, this all seems to have worked perfectly with both rails precisely level with the tubing. I couldn’t be happier or more relieved.

The next step was to mount the gear racks. The plans suggest using industrial strenght double sided sticky tape or welding. I opted to just tack these on. These are also supposed to be mounted 1.25 inches above the bottom of the tubing. My first plan was to mill or cut a piece of angle to the appropriate height to use as a jig. I messed up my first attempt to mill a jig and when I was looking for another piece of scrap angle, I happened on a piece with one edge shorter than the other. Damned if it wasn’t exactly the right size already. I put it right to use with good results.

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Found some angle that rose exactly 1.25″ from the bottom of the square tubing. Used this as a jig to hold the gear racks level while I tacked them into place.

I could have gotten racks of the correct length shipped to me, but it would have been prohibitively expensive. The nice lady at Moore Gear assured me I could match up two shorter pieces by using a third piece.

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Using some scrap to ensure alignment of gear rack seams.

 

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Placing the two rails next to each other on the table, the racks lined up perfectly.

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Checking the alignment of gear racks. Happiness.

 

So again, it’s a huge relieve having this phase of the project behind me! I wouldn’t expect everyone undertaking this to have access to a lathe (as well as someone to tutor them on using one…). However, even If I’d hired the lathe work done it would have been a lot cheaper than having a machinist mount my racks and rails. Now on to the gantry and electronics!

 

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