[Anita]

Hi

Has anyone ever considered using a cable drive instead of a ball screw? Roland do it on their smaller machines but why could it not be scaled up to slightly larger ones. If the cable has sufficient tension and it supports the slider in both directions so it can be pulled both ways. The only way I can see there being problematic backlash is if the cable is lose or it stretches (could use steel or kevlar). There could be some positional error from one end to the other as gravity would pull down on the cable but this could be orders of magnitude less than the tension applied to the cable with it still able to run freely. Also if the slider can slip on it's pulleys relative to the cable that could create problems but again if the tension in the cable is high I can't see that being likely.

Of course this does lead to another idea. If you get very large and the cable is just to huge, a toothed belt could work as well so long as it is stretch free and the teeth are spaced with sufficiently low tolerance for the application.

I want to mill up to Aluminium only or maybe steel very slowly and am planning the following build but I just don't want to pay 1000's for ball screws if there is another way that just might work.

I notice there are plenty of American’s on this forum so please excuse my euro units but I’m from New Zealand and we are two simple here to calculate anything other than base 10 in our heads.

The machine needs to be able to machine aluminium reasonably fast
Travel >300mm x >300mm x >100mm

Spindle 200W Maxon 4 Pole EC Spindle 127mNm at 1590RPM is quite impressive for the size (30mm dia) http://www.maxonmotor.com/medias/sys.../12_178_EN.pdf) - already have a very similar one to this data sheet and a set speed maxon controller, I plan to connect it to the spindle shaft with a series of pulleys to adjust speed and torque as with some manual mills so I can maybe slow it down and torque it up enough for slow steel cutting at a pinch.

Total indicated run out <2μm – using a spindle (custom made if I can’t find one - does anyone know where you can buy these?) just with a shaft and two quality Japanese bearings a long way apart

10mm ISO 15448 collet

Mech Res ± 0.01mm

Positional accuracy ± 0.1mm/300mm

Repeat accuracy ± 0.05mm

3.5A steppers (http://www.geckodrive.com/gecko/imag...0Datasheet.pdf)

4 Axis combined driver and brake out board to allow for later upgrade (G540 4-Axis Motor Control) does anyone know if this has low RF interference?

Ball Screws or - and this is primarily why I posted in this forum because this is my big question – cable or belt.

:withstupi

Thanks heaps!

[wildwestpat]

I have been considering this for a wood lathe with a live router head with a 1 meter travel. The problem is IMHO the spring due to elasticity of the cord and errors due to the concentric run-out tolerances of the all pulleys used. Cord with a carbon fibre continuous core would help but there would still be a lot of spring in the system and this would IMHO lead to problems with surface finish with heavy cuts due to the tooling load on the cord system. Toothed belt based designs exist in the larger sizes but again the elasticity of a long length of belt has to be reconciled when cutting tough materials - most of these are wood router based not heavy metal cutting designs.

Just my thoughts on what I have been considering and here is a link for carbon fibre cored string http://www.fibraplex.com/ProductPage...e%20String.asp but the resulting elasticity and springiness leading to resonances need to be proved with a build. A toothed belt has some inbuilt damping due to the moulding compounds used.

Regards - Pat

[Anita]

Hi Pat
Thanks for not shooting down the idea completely and for that great link. I think I can deal with the pulley concentricity but getting them all within tolerance on diameter may be slightly harder since measuring it across a hemispherical cross section would prove tricky, about the best I could do would be to use a ball micrometer. A rubberised carbon cable sounds like a good way to go for sure though. If I have a 3 or 4 pulley set up it should give a fair bit of mechanical advantage to the motor especially if I gear it down with a pinion on the shaft and coil the cable round the shaft of the connected gear. I'm not planning on making deep cuts and I only really need to machine wood, plastic and sometimes Aluminium.

Regards