cable drive

[tool25824]

This is my first post on here. Well over 30 years expereiance designing machine tools.

I have much expericance with capstand drive systems. The best method i have used for axis posiitioning is as follows.

The cable is attatched at each end of the none moving element, (bed). The cable goes throu a capstand which is groved (Pitch like thread) with the cut being just larger than cable dia and just under half the diameter deep. ( Referance hoist drums. (Not winch but hoist.. big difference.). Of course this capstand is attatched to the slide. The cable needs to lay in a tray with zipper rubber over it. Keeps the dirt out. I have used this method on large planers to wafer handel equipment works great.

Oh yes... the cable is tied at each end on the bed. Not on the capstand. Anything more than 3 wraps on the capstand is said not to slip.... I use five just to be sure. Biggest accuracy issue is thermal.

The life is long as long as you follow the cable manufactures rules about min radius any drums or idllers.

I know this post is old just some info. Hope this helps with any issues.

[handlewanker]

......and after a few traverses back and forth you get slip/creep around the drum that makes any accurate positioning a non event.

This is no better than a friction drive, and without toothed gearing etc, you will have creep around the drum and so lose position.

All CNC machines need to have position repeatability for back and forth movement

I have seen a drive with wire where the wire is attached both to the drum and the ends of the frame....the drum being grooved to ensure the wire did not cross over itself, but this was just for a drive not a resolution counter.

We had a wire drive on a tool and cutter grinder........Jones and Shipman I think....possibly a Cincinnatti.

When you consider that a resolution of .02mm is about as rough as you'd want to have for a CNC operation, the stretch in a piece of wore makes this impractical.
Ian.

[tool25824]

Thank you for the response. I understand excatlly what you are describing.. .. My experiance with is on several machine tools which the lenght was impractal for Screw or even Gear Rack. I installed one on a ship shaft lathe over 200 Feet... Built by Mesta Machine in Pittsburg PA. i experiaced the same thing you are describing. After many hours of testing measuing the creep was descovered. There is no slipage, just creep. After days of cusing and discussing it was discoved that the creep is caused by diving the slide, say in the minus dricection the cable streatched on the minus side and relaxed on the plus side thus causing the creep in drum drinving. The side with the increased tension, the minus side the cable actually streatches. This giving the cable a smaller diameter. Thus chaging the pitch diameter of the drum. You can see the specs on the this on US steel improved plow wire rope data tables. Of course this diameter change is what is we are callingl the creep, we shall say on the drum.

The cure fo this .... while simple today.30 years it was a challange. and there are several options. With the computing power today my suggestion would be to mesure slide resistance , motor tq., use a lookup table, gathered from imperical data on the machine, and inject the correction in the position loop. Further correction could be made as we did on the large lathe put sensors in the slide way and correct at the grid points along the axis. There are severa other ways this is just one.

Now that that is said. I feel, and i will test this and report back, That with cable tension at 40% of the tensial strenght the added load of the slide moving will cause very little change in a small machine. Remember on a groved capstand drive i have never seen slipage. You must also remember the cable alignment on one side of the capstand is not in line with the other so the cable mount points must be offset the amount of first and last wind on the drum.

Let me thank you again for your response. I hope this helps you. I am retired now and building some small routers, mills, and plasma machine to keep me busy. I found this forum when a guy asked me about cable drives after seeing a photo of a cable dirve I did on a 120 foot sate dish array in Arizona.

I would be gald to discuss this further if you like. I enjoy helping people. By the way my wife of 35 years is from Sidney.... We live in Houston now.