Although I left enough space for the epoxy to form a meniscus at the edges, I decided I wanted to get the rails right up to the edges, per the original design. I used a shoulder plane while the epoxy was only partially hardened and it planed down beautifully to remove the meniscus along the outer edges. After the leveling epoxy was completely cured I bonded the aluminum rail mounts (with rails still installed) into place on the gantry, using an adhesive epoxy. I think I forgot to mention earlier during the Z Axis buildup, that all the aluminum bar stock was chemically etched prior to epoxying them to the plywood, in order to get the best bond. On one set of rails, I sanded the bonding surface with 120 grit prior to etching; another recommended method for better bonds. I’ll see how each method holds up and perhaps do some more formal testing of different bonding methods in the future.
To help align the rails, I placed identical length aluminum bar stock in between the rails at each end, where the ball screw supports will be mounted. Then I built part of the Y carriage and installed it on the rails, to hold them in alignment at the center.
Once the epoxy cured I put a dial indicator on one set of guide bearings and measured the error all along the length of the opposite rail. I loosened one to three rail mount screws at a time and made fine adjustments to the rail until I had no more than +/- 0.0005” error throughout the full length.
With the epoxy leveling complete on the gantry face, it was time to prepare the Y Axis profile rails and their aluminum bar stock mounts.
When I had mounted the Z Axis rails, I clamped the rails against a straight edge and then used an automatic center punch to mark all the hole positions on the aluminum bar stock. Then I removed the rail and drilled all the holes on the drill press. That turned out to be less than ideal since the center punch was smaller in diameter than the rail mount holes and I wasn’t perfectly centered with each hole. Luckily there was enough play around the M4 screws to correct these minor errors, but I needed a better method for the Y Axis.
Again, I clamped a straight edge against the registration surface on the side of the profile rail, then aligned that with the aluminum bar stock and clamped the bar stock together with the rail. Rather than using the small diameter spring-loaded center punch to mark each hole, I found a drill bit that exactly matched the diameter of the holes in the profile rail. I inserted this bit in each hole and gave it a good tap with a hammer, leaving a mark at the center of each hole. This allowed me to drill all the holes more accurately at the drill press. I still ended up with some slight errors from my less than perfect alignment of the drill bit with the marks on the bar stock. For the second Y axis rail I tried yet another method…third time is a charm!
Leaving the rail/bar assembly clamped together, I brought the whole assembly to the drill press. I aligned the first hole on one end with the drill bit and could easily see when it was centered. I drilled the first hole, then tapped it and put a screw in. Next I drilled and tapped the hole on the opposite end and put a screw in. Followed up with one in the center, gradually removing clamps as I went. Now I could drill all the holes without having to mark anything. This method was much faster and more accurate as well. With the holes drilled and the rail still attached, the rail acted as a guide for the tap to ensure it was vertical. The tapping operation went much faster this way as well.
I didn’t take a lot of pictures of this whole process, but here are a few shots of the work and progress:
Rails laid in place on the gantry for fit check:
After planing the gantry, I realized that the front face, where I intended to mount the Y axis rails, is no longer perpendicular to the base. Since I was planning on using epoxy for the rail mounts anyway, I figured I could just use epoxy for leveling the front side of the gantry.
With the gantry on its back, I made sure the gantry bottom was perpendicular to the machine base, since that is where the X Axis bearing guides will be mounted. To minimize the use of expensive epoxy, I made a dam out of 1/2″ plywood covered with blue painters tape, which releases easily from epoxy, then sealed at the edges with clay. This dam allows the epoxy to flow only to the areas where the rail mounts and the ball screw bearing support mounts will sit for the Y-axis, with extra room allowed for the meniscus of the epoxy at all edges.
Once the epoxy was mixed with hardener, I poured a small amount into another container and mixed in a little acetone to thin it out. I didn’t want to thin the whole batch because acetone can change the strength properties of the cured epoxy. I poured the non-thinned epoxy throughout the area and let it settle for a few minutes, and then poured the thinned epoxy over the top to achieve a very flat surface.
Remember, all of this work is driven by the fact that I didn’t start with a perfectly flat work surface. If the gantry had been properly square, I would have just attached my rail mounts and bearing supports directly to the wood face of the gantry. However, it is a good learning experience and I’m glad to have been able to test out the epoxy leveling method in case I want to use it for my next machine.
My first major problem with this build. After all the glue was dry on the gantry, I rotated it 90 degrees on my work surface (the machine base) to see what it would look like in its final orientation on the machine. One corner of the gantry sat above the surface by about 3/32”. I first thought it had warped after the glue-up, but back in the position where I constructed it the gantry laid perfectly flat against the base. After doing a lot of checks with a straight edge and feeler gauges I found that my base was not perfectly flat as I initially thought. It was flat along the edges in both length and width, but there was some variation throughout the center. So, between the 40 or 50 thousandths of twist introduced where I constructed the gantry, and another 30 to 40 thousandths in the final orientation, I ended up with the gantry having this 3/32” warp or twist in relation to the base.
The way I decided to fix this was to create a large planing jig for my router. Following some videos online, I mounted two long pieces of plywood (with the factory cut sides up) along the sides of my machine base and used a level and two wires running from corner to opposite corner to get the rails level and without any twist. Then I built a sled for the router that would slide along these two rails, allowing me to run the router over the whole surface with a 1 ¾” surfacing bit. I waxed the router sled and the rails, and used some clamps for stops. My wife helped out by holding the shop vac to collect the dust while I routed the entire surface of the machine base. I clamped one end of the base to my leveled workbench so that the corner with the twist would be up the air and would get milled down. If you want to see more details, this is one of the videos that provided inspiration: youtube.com/watch?v=qtkBZHLJyD0
After routing the top, I flipped the base over and repeated the operation on the other side. In theory, these two sides should now be parallel and without any twist. This will hopefully get me back to square one for having a flat work surface to build the machine on.
Rather than re-constructing the whole gantry on this newly flattened surface, I decided to use the same planing jig to get the top and bottom of the gantry to be flat and parallel. This seemed to work well and now the gantry sits perfectly flat against the base.
While the glue-ups were drying on the gantry, I went back and did some more work on the Z-axis. I finished the router mount for my smaller DeWalt DWP611 router. I glued a few pieces of wood to the carriage with threaded T-nuts driven into the wood. The two router mount pieces get bolted into those T-nuts so that I can install other mounts for my larger DW618 router or anything else I may want to mount in the future.
Next I started drilling and tapping the aluminum bar stock to accept the Z rail mounting screws. I decided to initially install the rails without any machining and see how it works out for this proof-of-concept machine. If necessary, I can always go back and have these machined flat, or have shoulders machined to mount the rails against. The bar stock is still not attached to the carriage in this picture. It is just a test fit prior to epoxying the bars in place.
Having a nice flat surface to work on, I decided to start building up the gantry. I printed out some screenshots from my solid model and wrote in the dimensions by hand, not wanting to take the time to create all the dimensioned drawings in the CAD software. Then I got busy cutting out all the pieces of plywood on the table saw. I used a dado set for cutting the slots in the top and bottom pieces.
I took this image to show how the pieces will fit together. The top is slid back slightly for better visibility:
Next I cut out all of the web pieces from 1/2″ MDF. Here are all the gantry pieces laid in place for a test fit:
Once again, I glued the pieces together in stages to make sure I could keep everything aligned properly. The number of clamps available also limits how much can be done at once.
With the torsion beams completed, I laid them out across two sawhorses and used shims under the legs to get the torsion beams level in both directions. I placed a sheet of MDF on top of these beams and proceeded to build up the base for the machine.
As you can see below, the pattern of webs is not uniform. I was originally planning to have it filled with the square pattern seen at the ends. However, I changed my mind and decided to go with a sparser pattern. It should provide plenty of rigidity for this prototype.
The glue-up was done in two stages, first to attach the sides and all the webs to the base, then to attach the top. Many clamps and weights were used to hold everything together while the glue dried. With the top in place and everything glued up, the assembly is very stiff and doesn’t flex at all when lifting up on a corner.
Here is the Z axis carriage after unclamping from the glue-up and giving it a light sanding. I am using aluminum bar stock to mount the profile rails. I chose this method in case I wanted to mill a perfectly flat surface on these or to mill shoulders (the preferred mounting method for HIWIN rails). At this point the bar stock is just positioned for a test fit. They will be epoxied in place after drilling and tapping the rail mounting holes.
After making this first piece for the Z axis, I noticed it had a very slight warp to it. I don’t think it will be a problem, since I can lay the bar stock on a flat surface, coat the carriage with a thick epoxy, and lay it on top of the bar stock, so that the rail mounts will remain perfectly flat and in the same plane. The epoxy should take up any space due to the slight twist.
I now realize that I will need a perfectly flat work surface to make the remaining parts on. I also will need a perfectly flat base for the machine, so I decided to go ahead and build up the torsion box base at this point. The machine base will serve as my assembly table for the rest of the build.
To build a perfectly flat torsion box, you need to start with a perfectly flat surface. Inspired by this article on popularwoodworking.com, I made two small torsion box beams. This allowed me to practice the techniques for making a torsion box, and then I could use these to create a flat work surface for building the torsion box machine base. You will see later what I mean. Here is a picture of the two torsion box beams being constructed.
I started with the Z axis since the parts are small and if I make any mistakes, it will be easier to redo with less wasted material. Working with the dimensions and images from my solid model, I started by cutting all the pieces out of a sheet of birch plywood to make up the Z axis carriage, laid them out for a test fit, then glued and clamped them together.
This is the Z Axis design:
Gluing up the first pieces:
It is starting to look just like the model :-). It is fun to see something come to life that you have been working on designing for so long!
Lots of clamps:
My table saw left some burn marks on the wood. They will get sanded down later to make it look nicer. After these cuts I spent some time tuning up the table saw to align everything properly. Perhaps a new blade would help as well.
I was not happy with the performance of any of the CNC router plans currently available, nor with the prices of kits and commercial machines. So I decided to come up with my own design. Not to mention, it is more fun to design and build your own, right?
I laid out a set of performance and cost goals and then designed a machine to meet those goals. However, this series of posts is not about that exact machine. I am first building a proof-of-concept machine that is lower cost and easy to build (therefore the wood frame). This will allow me to learn and flush out any issues with the design before building a more expensive and time consuming version of it. I did however attempt to design the wood frame to be as strong as possible so that it will have enough performance to make parts for the originally designed machine.
My rough goals for this wood framed version of the machine:
- Working area of 37″ x 25″ x 6″ (940 mm x 635 mm x 150 mm).
- Accuracy and precision in the ballpark of 0.001″ over 12″.
- Performance – will leave this for later discussion.
- Learn and test the concepts needed to build my originally designed machine.
- Have a machine to start making parts for the next machine.
- Have fun!
I will give some design details in my next post.