Purchasing a z-axis?

[lateAtNight]

As it turns out its not 400mm, more like 340mm, 230mm usable thread. 16mm diameter at the threaded bit, 10mm shaft, and 6.35 tip. I'll double check that later. Pic attached, with a 300mm ruler next to the screw.

[Gary]

This looks like one of mine?
If so this is the details, and it is mm pitch.

http://www.slidesandballscrews.com/pdf/Machining.pdf

As it turns out its not 400mm, more like 340mm, 230mm usable thread. 16mm diameter at the threaded bit, 10mm shaft, and 6.35 tip. I'll double check that later. Pic attached, with a 300mm ruler next to the screw.

[Tonyel]

Hi Dave

I'm interested to see how you're getting on. I'm also in the process of designing (and hopefully, later, building) a CNC gantry style router.

I was wondering about how much Z travel to allow; seems on the face of it that 250 mm is a good compromise between rigidity and travel. I haven't done any calcs, but just by 'seat of the pants' engineering it looks to me that much more than that and it just looks a bit too far. Most of the time I imagine it will be operating at the far end of its Z travel too, unless I put a riser on the table.

Does your Z axis screw have a 'zero backlash' nut? How much are you looking to get for it?

I attach a pic of my design so far, for what it's worth. Criticism and suggestions are most welcome :D!

[lateAtNight]

I'm sure the guys on here will give you a good estimate on what's a good amount of travel for a z axis (maybe start your own build thread? i'll be doing the same soon enough). I'm just a beginner on these things myself, so I'll leave it too someone more informed :).

The one I got off kip has about 115mm of travel, but I won't be using it all, because I simply will never be milling that far.

I had the ballscrew on ebay for £50, but no one was interested, so I guess I'll take £45 for it. After all a new ballnut alone from Zapp is that much, and it is new. Single nut.

I like the design on your router, but again... I'm just a beginner. I'm using linear rails now myself, and I do like :). My bank balance doesn't though :)

Also, by the looks of it your going to use some square alu tube for the frame? I suppose it depends on what tools you have available to you, but you might want to consider aluminium extrusion frame? I decided to go down that route after making a complete mess of the alu tube. It was alright, but I was never going to be able to make it accurate enough with just a hand drill.

[Tonyel]

Well, I did wonder! Zero backlash sounds like a hard thing to achieve in the real world.

The intention is to build the whole thing, more or less, from steel, on the grounds that it's cheap, can be easily welded by someone I know from work, and is about 3 times more rigid than ally. I appreciate that the associated 3 times higher density might compromise the acceleration of the moving bits and bobs, but I reckoned that it would be fairly easy to upgrade motors and drives in the future, but much harder to rebuild with a rigid frame.

The plan at present is to build the rectangular frame, get it welded as near plane as possible, then get it skimmed or ground to a true plane. A machine shop near me reckoned they could do it for about £160.

I've been thinking about the Z. Although the asymmetric design looks a bit dodgy, I think it should work. After all, the Z slide has to be rigidly constrained anyway, so the slight offset load from the ball screw ought not to bother it. Also, I wanted to minimise the offset of the Z direction load from the shear centre of the gantry box beam, so that it is less prone to bending, and can be lighter. I hacen't got as far as the milling head carrier yet, but intend it to be a channel, for rigidity.

By the way, I'm using the linear rails mainly because I got very lucky and found a load in a scrap yard! Otherwise, I think I'd be using round rails and bearings.

I enclose a close up of the gantry and Z FYI.

Thank you both for your comments and help - much appreciated! Most people just think I've gone mad...

Cheers,

Tony