advice needed for track material

[gary-in-canada]

I'm building a light duty cutter/mill mostly vinyl cutting but some 1/8" plastic cutting also so I do mean light duty. I want to use 3/4 (od) inch "v" grove bearings for xy axis with the gantry weight not more than 10lbs with 4 bearings supporting the gantry weight. Can I get away using some kind of hardened aluminum ground to 90 degree for the "v" groove for the track for the steel v bearings? Should I go with steel tracks and forget aluminum?
Also I need some small pulleys for an "O" ring belt for a small spindle motor 4mm shaft. It seems all I can find are toothed pulleys.

[joesz]

Well there is no "hardened aluminum" you can get it hardcoat anodized but it will not last very long with steel v groove followers which I assume is what you mean running over it. One small nick and the hardcoat will start to flake off over time. If I am understanding you right you just want to use the weight of the spindle to hold this in it's tracks? I don't think that is a good idea even for only light duty cutting.You should design this such that the moving gantry is trapped in some sort of way system or something so it can not tip. Just the acceleration depending on the drive motors you use could cause it to tip and if it tips it will bind the screws and that will cause all kinds of issues. I think that is going to cause you problems of tipping and jumping the track. As for the pulley you can get those at a place called Stock drive components.

[gary-in-canada]

This is for the x and y axis and there will be a double edged track with 4 bearings per side (8 per axis) I didn't mention the other set of bearings as they only come under load when z axis is penetrating material and pushes up the gantry.
So are you saying steel rails for sure? Is there a ratio of accuracy on the wider the track (further apart the top and bottom sets of bearings on a side) the more accuracy and less play or a 2" track width will have the same effect as a 3" track width?

Thanks for the reply and tip on the pulleys...

[joesz]

The wider the spacing the more rigid the setup will be, there is no set ratio as it depends on how accurate you can get the fit to be. Too tight it will bind up too loose and you will have an unstable platform for cutting even if it is very light duty. But spreading the forces over a wider area will in itself create a much more stable platform, think of it as a lever the longer the lever the more force you can create so in reverse the more force it can withstand. Kind of a crude example but it kind of gets the point across. If you expect to get any significant life out of it I would go with steel for sure. Now they make hardcoat anodized track systems but they are not designed for the kind of use you will see out of your system.
If you have access to a Mcmaster Carr catalog they have a lot of devices that can be used to create an x-y slide system.If you do not have access then you can go online and browse the catalog. The most important part of any CNC machine is a stable slide system with no play and then of course a rigid spindle with good bearings and a solid mount. Without these 2 key features your accuracy and finish will suffer and as your tool diameter increases it will create more forces which will in turn cause even more problems. Keep in mind that a major thing you need to over come is what is called stick/slip conditions, this is caused mostly by the force of the screw being offset to the travel of the guide rails it tends to apply a twisting force that under certain conditions will cause binding. This is much more common is a system that say uses ground shafts and bronze or teflon bushings. Replace the bronze bushings with ball bearing bushings and it would eliminate that problem. I have a whole lot of experience in slide set ups like this and I have tried dozens of different methods. The least expensive and easiest to build is hardened ground shafts with ball bearing bushings. It is very rigid it has near zero slop and it also has long life. Mcmaster carr has all the components needed to build such a system. If the travel distance is large then you need to go with the spilt type bushing and the supported rail, also in Mcmaster carr. See the link below.


McMaster-Carr

I am sure cost is an issue so it is going to be a call on your part as to how much cost versus how accurate and rigid the setup will be.

[ozzb]

check MISUMI USA

[gary-in-canada]

Thanks folks for the two catalog links they have all the pulleys, tracks etc I'll need.
Also thanks for the good advice.

Gary