I have started a new build of a 4 x 4 CNC router. First a little history.

Technically, this is my third CNC, but my second build. My first, a 4 x 4, was an 80/20 extrusion kit from Fineline Automation. Not a bad machine, but it was a nightmare to assemble, and did not have the features I later learned I wanted. The assembly part was mainly because of tolerances that were too loose in the extrusions (more on that later). I ran it for about two years, and decided to buy my second machine, a Saturn 2.

The Saturn 2 is a welded steel machine. It looked great and held a lot of promise in terms of accuracy, repeatability and rigidity. It was advertised as jig welded and stress relieved. After ordering it, I discovered other owners with some fairly dramatic negative issues with their machines. When mine arrived, shims between the gantry mounting and interface plates were a dead giveaway that something was very wrong. I won’t go into the ugly details, but suffice it to say that pretty much every thing that could be off was off by a lot and it would take a good deal of time, effort and expense to accommodate its defective construction. For anyone who is interested, you can find my thread about it. https://www.cnczone.com/forums/finel...cnc-posts.html I elected to return it. To Fineline’s credit, the owner took it back without resistance. So, the Saturn 2 was my 2nd machine, but I didn’t get it set up for operation.

Now, I’m building my 3rd machine, which merges what I consider the best of several different designs. Although I have MIG and TIG welders and welding skills, I decided not to go with steel tubing. Many will criticize my choice, but cost and logistics were a factor. Welded steel would require stress relieving and milling on a large format CNC mill. In my neck of the woods, the machine shop costs are considerable. As far as logistics, my shop is in my home in a daylight basement. Ingress and egress for large equipment, while doable (I have a cabinet and welding shop worth of heavy tools), is very difficult. I did it with the Saturn 2, but it wasn’t something I would want to repeat often. The problem is more about the physical dimensions than weight.

I will not be covering the electronics in this build. I sourced and built my own for my first machine, and it has worked well. For anyone who might the interested, here’s a rundown on the control side:

PDMX 126
PDMX 107 (Spindle control sister board)
ESS SmoothStepper
Gecko 203V stepper drives
Mach4

I’m using NEMA 34s for X Y Z and A (Y slave).


A note about 80/20 extrusions. While not widely known, 80/20 has cut and length tolerances for its extrusions. The cut tolerance is +/- .002” per inch and the length tolerance is +/- .015. Surprisingly, I’ve not seen any mention of the tolerances in the kit machine literature or any discussion about compensating. I’ve purchased my share of 80/20 over the years and have never found an end that was cut square. I’ve also found that there is variation from piece to piece in the same order, and pieces cut outside of the published tolerance. Some pieces that came with my first CNC kit were off as much as .006” in 1.5” (1530 profiles). Can’t find the tolerance on the 80/20 site? Look at the online catalog, page 850 in Machining Services. Not exactly easy to find.

Tolerances matter. A .002” per inch deviation translates to .136” (more that 1/8”) at the other side of a 68” run. 68” is the length of my cross pieces. Not good enough for my taste. You can’t force the extrusions to square across the width of the side frame, and the lengths varied some, which cause issues when aligning linear rails. I went with shims on my first machine and decided to not go that route again.

Oh, one other thing. Trimming the ends square on my chop saw with a non-ferrous blade was not an option. Unless you have full blade engagement, you get blade deflection. Been there and done that. I’ve even tried blade stiffeners. Helped a little, but still worse than .002”/inch. Makes sense, if you consider that at 5/8” diameter X 4” cut length end mill will deflect when cutting aluminum. If something that thick will deflect, it’s a no brainer that a 3/32” thick circular saw blade will also deflect.


The build:

I wanted a heavier and stiffer machine than the typical 80/20 kit. So, nearly the whole machine is constructed of 3030 (3”x3”) profiles. They are the thickest/heaviest version. The exceptions are the angle bracing for the legs, and the gantry, which is 40-8016 (80mm x 160mm) the metric equivalent of 3060 (3” x 6”). I used metric for the gantry to accommodate the use of a CNC Router Parts’ Pro Ballscrew Z Axis. Although I’m building some of the parts, I decide to buy the Z axis.

Here is a photo of some, but not all, of the 80/20 for this build. Note the blue tape. I took measurements and recorded them on the tape. More on this later.



Dimensions: The Y axis rails are 60” long. The cross pieces (what the spoilboard mounts to) are 68”. There are actually 2 lengths of Y axis rail, which are separated by 5” uprights. The second rail and uprights give the gantry height. This arrangement is different than the side plates commonly used on a variety of machines, including CNCRP Pro model.

My first machine had skate bearing carriages that rode on a steel plate on all three axes. The arrangement is similar to the CNCRP Standard kits. They were just okay for a hobby machine. They worked, but I wanted better. So, my Y axis will have Hiwin 25mm linear rails and the X axis will travel on Hiwin 20mm linear rails.

Okay, what to do about the 80/20 tolerances? My trusty PM 25MV benchtop mill and lots of time. (I didn’t have the mill for my first build). I installed glass scales and DRO on my mill. For anyone contemplating milling extrusion ends and looking for precision, I highly recommend a DRO setup or CNC mill. I only do hobby machinist work and have no formal training. I can say without reservation that I could not have pulled off this build without the DRO and tuning the mill (e.g., tramming side-to-side and front-to-back, adjusting gibs in all 3 axes, oiling and greasing). A mister also helped.

Here is a picture of the 80/20 support stand I built to facilitate milling the ends of the long extrusions. On top of the horizontal is a strip of UHMW, which is very slick. Since cutting the extrusion ends involves moving the extrusions, I needed something minimal resistance. I had some UHMW laying around; it worked great. Really slick stuff.