[wyldesyde007]

Ok this is old school for cabinet makers but it works for this application quite well!
Take a look at this illustration, it's from pocket door hardware for cabinets...

quite simply what this is is a moving knot, two cables pull against each other to effectively prevent one cable from moving without the other cable moving. I was unhappy with the play I had on one side of my machine caused by my placing the Y screw to the side of the machine, I did this because I wanted open clearance in the bottom of the machine in case I needed to place a large object under the gantry

[jhowelb]

You have my attention, but I don't understand what is going on. What am I looking at? Where does it fit in a machine? What knot? These are drawer slides?

[wyldesyde007]

It cost me about $40 in parts to get my machine rock solid I think it was worth it and this would be a good fix for those of you whose machines work great but are just a bit wiggly saves having to add a belt/chain drive to link 2 screws or adding another stepper.

I picked these up at lowes they are patio door rollers they make awsome pulleys as they are grooved nicely, have an internal bearing and built in standoff.

[wyldesyde007]

After cutting them out of their brackets (i'm kinda interested in these as a v-bearing substitute as they fit a 1/4" steel rod nicely and were designed to hold up a 100lb door!

[wyldesyde007]

See what I mean about v-bearings huh? huh? and if you use the mounting bracket they come in it has a tightening screw that would have been used to raise the door up to tighten it into the track! You could use it to take out the play in your axis'

[wyldesyde007]

Anyways, Back to what I was describing. I used (4) of these rollers,(15') of 3/32" steel cable (plastic coated),(4) 1/8" cable clamps and (4) 5/16" X 3" eye bolts for tightening up the cable.

[wyldesyde007]

I stacked up 2 roller wheels with 1 stack placed on each side of my gantry rails on a crossbar underneath my table, I used a 1/4" bolt to attach them.

[wyldesyde007]

The rollers have a slight hub in the center that acts as a standoff allowing them to turn seperately from each other.
By passing the cable around the pulleys and attatching them to the base as illustrated in the diagram at the beginning of my post you end up literaly tying your gantry to the base! I just set mine up tonite and it is now solid!

[wyldesyde007]

You have my attention, but I don't understand what is going on. What am I looking at? Where does it fit in a machine? What knot? These are drawer slides?

You would use the concept of the "cabling" and your existing rail bearings no drawer slides involved in this fix. there was a 1/4" slop in my setup and I was dreading reworking my machine and adding in another screw/stepper/timing chain. I achieved stiffness by using this cabling system that was designed to keep pocket doors square while they were opened and closed. I've used this system before when building cabinets and happened to remember it.

[jhowelb]

I guess I'm just dense. I don't see what the cable and pulleys do.

Perhaps with a photo of the completed install......................

I can visualize a saw kerf and a 1/4" drill rod on each side of a board fastened with a clamp. Then the little rollers riding the rod. Fixed gantry low budget! Neat!

[andy_ck87028]

This has the feel of something very interesting but I also cannot quite see what it is / how it works.

Can anyone / author open up this concept into its raw terms?

Good thread

Andy

[Big S]

Here is the basic concept, in the picture the red line represents the gantry and the arrow the twisting/racking force. The circles are the pulleys and the white lines are where the cable runs. The pulleys are fixed at a set distance apart on the red line and the ends of the cable are secured.

When the gantry is trying to twist in the direction of the arrow the cable stops the angle changing. When the upper and lower drawings are superimposed it becomes clear that the gantry cannot twist. The gantry can still move along its axis when the cable moves around the pulleys.

I hope this explanation is clear enough?

Shannon.

[andy_ck87028]

That's fantastic.

So, two cables..... one anchored on near end x and the other end anchored on far end x'. The other cable anchored vice versa, i.e. far end x and near end x'

Each cable has two pulleys... one at x end of gantry and the other at the x' end of the gantry. That makes a total of 4 pulleys ... 2 at x end of gantry and 2 at x' end of gantry.

The cables run from their anchor point to the nearest pulley, across the gantry to the other pulley and hence to their final anchor point.

Each cable stops racking in only one direction but collectively both cables stop racking in both directions.

Yes?

Andy

[Big S]

You can also use 1 double width pulley on each end of the gantry since both cables move around the pulley the same way.

Shannon.

[andy_ck87028]

v true ...thx wyldesyde007

[wyldesyde007]

Big S that was actually perfect! I was just about to make a diagram like that but you beat me to it! Thats exactly whats happening, it took me about an hour to rig this up so when I say quick fix; it is. When it was used in a cabinet pocket door assembly it kept 2 seperate drawer slides from moving independantly, by cabling them together they worked in unison keeping the door from sagging.

[jhowelb]

OK, let me see if I've got the concept.
Does this depict the setup?

[wyldesyde007]

OK, let me see if I've got the concept.
Does this depict the setup?

That's it, each cable interacts against the other effectively forming what's called a moving knot, the tension stays constant no matter where the gantry is and as you develop play through rail bearing wear you can tighten the cables and take it out. I chose patio door rollers because they have pressed in bearings whereas rope pulleys just have a bronze if that. A little more pricey but heck! they were designed to hold a 50-100lb door for years! how many of you guys replace the bearings in your patio doors regularly huh? huh? thats what i'm preachin! these things are pretty rugged!

[jhowelb]

When I laid eyes on your rollers, I envisioned two pieces of 1/4" drill rod shown end on in my sketch, clamped to a cross member (black). Rollers (red) riding the rod supporting the trolley of the Y axis (blue). Of course there would need to be at least a pair top and a pair bottom.

You choose the material for the members to suit your own pocket, could be the basis for a low cost, accurate, rugged router!

[Relu]

How about this? You will have equal forces applied to the entire system and you can easy adjust the angle between axes. This setup was used by me on my first hotwire CNC, a few years ago... I intend to use this again, on my next project.

If you have question, I'm glad to answer but please forgive my bad english.