# Thread: I-Channel CNC Egyptian plasma

1. Interesting approach using wire rope. What is your strategy for aligning and maintaining the alignment (e.g. set/stretch compensation) over time?

2. Originally Posted by WTF-J
Interesting approach using wire rope. What is your strategy for aligning and maintaining the alignment (e.g. set/stretch compensation) over time?
Thanks for the question,
The strategy depends on the feedback motion from the moving gantry which will be used to move both of the two wire tensiors (at the ends) front and back parallel to the threaded pulley axis.

This will be achieved by converting the linear motion of the gantry to a rotation motion via another separated wire rope and pulleys at both of the ends of the wire rope, and then converting the rotary motion to a linear motion for the wire tensioners which will be vertical to the gantry motion and parallel to the threaded pulley axis via a threaded rod by the same pitch of the the threaded pulley to ensure that the wire ends will be aligned with the pulley every where.

But I want to inform that I have tested the motion without feed back motion and found that the pulley threads were able to move the wire rope and the wire tensioners without the feedback system depending on the rolling bearings installed in the wire tensioners.
But even it is enough, I will use the feed back motion to ensure a smooth movement and prevent the horisontal stress component on the thread that will be used to move the wire tensioners.

pls see the attached photo that may clarify better.

3. Herewith a video of the pre-test of the idea which shows the smooth movement and the very tight attaching between the wire rope and the pulley even before installing the feedback module which has not been finished yet.

I am using a 3.0 mm diameter wire rope
pulley outer diameter = 80.0mm
thread pitch = 5.0mm (this means that the gap between two winds is 2.0mm)
One revolution of the threaded pulley gives us 3.14*80.0 = approx. 250.0 mm displacemend.
2.5 meter length needs 10 revolutions of the threaded pulley.
So the total height of the thread must be 5.0 mm x 10 = 50.0mm
By adding two revolutions because the wire rope is winded arround the pulley by 3 revolutions, So the total height needed = 5.0 mm x 12 = 60.0mm.

The actual height of the threaded pulley is 70.0mm.

4. A quick tip on the *.rar videos. You are saving ~100K or so on each of these files, so it really isn't worth the effort and not everyone has WinRar.

5. Originally Posted by Islam
Thanks for the question,
The strategy depends on ...
I believe I get the concept. (Most residential garage door openers in the US have a similar set of pulleys connected by a torsion bar.) The question is how you will compensate the gantry alignment as the wire rope on each side takes a set and stretches. (Assuming you will be holding reasonable precision in your XY tolerances. (The precision in the Y axis depends on the ability to tune out variation between the two wires and their tensioning.)

7. Originally Posted by WTF-J
I believe I get the concept. (Most residential garage door openers in the US have a similar set of pulleys connected by a torsion bar.) The question is how you will compensate the gantry alignment as the wire rope on each side takes a set and stretches. (Assuming you will be holding reasonable precision in your XY tolerances. (The precision in the Y axis depends on the ability to tune out variation between the two wires and their tensioning.)
Firstly I would like to notify that a little bit of stretching the wire rope results in a very strong adhesion between it and the threaded pulley without need to stretch it very strong.
This is the reason of why I used the threaded pulley, three windings of the wire rope arround the pulley inside the thread ensures a very high friction without the need of strong tension of the wire rope.

About the gantry alignment, let us talk by calculations.
The displacment of each side of the gantry depends on three factors:
wire rope diameter
Tensioning the rope wires by the min. tension which just make the wire rope free of relaxation and laxity.

The two threaded pulleys in each side of the gantry are simillar to each others with the same outer diameter and the same inner thread diameter, and made of cast iron.

Althoug 2.0 mm wire is enough, I used 3.0 mm diameter wire to be more rsistant to stretching.
Even stretching and relaxation after the wire took its set inside the thread, diameter change of the wire will be negligible, also diameter of the cast iron pulleys will be almost as it is.

So by stretching the wire from the screws of the wire tensioners will restore the states without concern of changing the pitch like in the sprocket chain or the timing belt.
Also we can insert strong springs at the end of each wire tensioners to ensure that the wire will be tensioned for the longest period.

Anyway, It is the first time for me to use this approach of threaded pulleys that I havent found any body used before in the CNC driving mechanisms.
Even in the elevators or garagues doors flat pulleys are used with only .5 of the winding arround it.
So I could not estimate accurately the accuracy of this designe even after what I mentioned before running the machine at least 60 hours.

8. Gantry with its wire tensioners module has been installed today.
The wire rope has not been installed yet.

9. Originally Posted by Islam
Firstly...
Thanks. I appreciate the thought and the innovation in your approach. Will be interested to see how this works out.

10. ## cool

Looks very interesting. Cant wait to see it work.
Dave

11. Photos of the wire tensioner of the gantry after installation

12. Vid of the wire tensioner and gantry movement.

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