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CNC Mechanical

"Moving Knot" Cable / Pulley System to Reduce/Eliminate Racking on a Moving Gantry CNC Router

After designing and building my latest CNC Router (23" x 36" - see below), I found that I was still seeing a lot of racking of the gantry during cutting that shows up as chatter when the router is far from the middle of the table. I was able to measure about .050" of racking movement by pushing on one side of the gantry. After a lot of research and not wanting to add a second leadscrew (dual x-axis setup), I found a discussion of the "moving knot" on a CNCzone forum. This design has been used for 100's of years on drawers and drafting tables to eliminate the racking action in their mechanisms.

The system works by having two cables that each prevent the gantry from rotating. By having these two forces oppose each other, rotation of the gantry (the racking movement) is prevented. Here is a diagram of what is going on:

Moving Knot Diagram

The installation requires 2 lengths of 3/16" cable, 4 cable crimp sleeves, 4 pulleys, 2 turnbuckes, 4 eye bolts, 2 1/4-20 "t" nuts, 2 1/4-20 fender washers, 2 1/4-20x2" cap screws, and some loctite.

Moving Knot Parts

The original forum discusses using sliding door wheels from the hardware store but these are made from plastic and most don't have ball bearings. I was concerned that these may break or wear out quickly so I found some very good quality alternatives on Amazon.com.

The pulleys are mounted on the 1/4-20 cap screw with a fender washer between the pulley and the wood (reduces stress on the plywood by distributing the lateral force) then screwed into the 1/4-20 "t" nut mounted in the gantry bottom. The pulleys are mounted vertically so that the cables can't contact each other where they cross in the center.

Pulleys Mounted to Gantry

The eye bolts are mounted to the machine to be lined up with the pulleys and are adjusted with the turnbuckles. If your gantry is narrower than mine, you may want to use the eye bolt as a pulley and put the turnbuckle below to adjust the tension so that the gantry pulley doesn't hit it. The cables are terminated on the eye bolts by swaging them with the crimp sleeves. If you don't want to spend the money on a swaging tool, then you can crimp these using a hammer -- just tap them together then rotate 90 degrees and do it again. Don't overdo this or it won't be as strong.

Cable swage

After the turnbuckes are mounted, snug them up by hand (not too tight) and do some 90 degree corner test cuts and accurately measure them. Alternately adjust the turnbuckles and perform test cuts until the machine is cutting perfectly square. When you are done, the cables should be taught but not crazy tight. I get about 1/2" of deflection over a 2 foot span using moderate finger pressure.

After installation, I tried to push on the gantry and was able to measure no deflection on the opposite side. The gantry is now "Rock Solid"!

Here is a video of the system in action:

I have tried several dust collection methods over the past few years including everything from a vacuum hose wire-tied to the router to several different kinds of brush/vacuum systems. The problems with many of the existing systems are that you can't get the vacuum close enough to the bit at high enough velocity to pick up the dust and still leave room to be able to change the bit. I have come up with a design that uses strong neodymium magnets to attach the lower dust brush. This allows quick and easy removal of the brush assembly while allowing the vacuum to be directed where it is needed. The gray strips on the right photo are pieces of weatherstripping to keep the unit from sliding and reduce vacuum leakage. The brush is installed (hot glued) into a 1/8" slot cut into the wood ring. The magnets are glued into 1/8" deep holes and are flush with the wood.

Notice that the fan output of this router (Porter Cable 890 Series) is redirected away from the vacuum while still leaving access to the spindle lock to change bits.

I started building a new CNC machine to allow me to have a larger cutting area. The design has a cutting area of 23" x 36". This has the following features:

This machine has a cutting area that is 9in x 6in x 1.75in and can cut at 40ipm with 60ipm rapid moves.