Thread: Purchasing a z-axis?
Ace, how longs it taken so far?
This looks like one of mine?
If so this is the details, and it is mm pitch.
Visit Us: www.zappautomation.com
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!
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.
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...
It's all in an attempt to get rigidity in the head. I wanted to have two Y rails/carriages, separated by a distance in the Z direction, to provide rigidity against the Z rail flexing. I wanted to have just one lower carriage, back to back with one of the Z carriages, but then there's no way to attach them both to the plate, because you can't get to the bolt holes of one when the other is in place.
So I ended up with the slight extravagance of two lower Y carriages, and the lower of the two Z carriages, all in a line as close together as possible at the lower end of the bracket. Since I had a spare carriage, I thought I might as well use it. Then there is the top carriage, providing a brace and incidentally holding the other end of the driving plate.
I'm open to suggestions though. My main intention is to make it as rigid as possible, which hopefully would make a good foundation for any further improvements in future.
I'm also planning on having a fair bit of Z travel, as I want to be able to machine moulds from foam. I've opted to raise the height of the X rails well above the table, so the gantry doesn't have to be so tall, and then fit the Y rails with a wide horizontal spacing. At the moment I'm hoping to be able to run the Z axis down between the two Y rails (see other thread on my small diameter DIY spindle to see how this might be possible).
The idea for this configuration came from looking at this machine: http://www.mechmate.com/, although I'm not building something this big (mine's about 800mm x 600mm x 250mm working volume).
Sounds like your machine is to be similar size to mine - slightly shorter. I was thinking that for most jobs, if I didn't need the full Z travel, I would pack the job up a bit so the cutter isn't operating at full travel all the time.
I'd had the same idea of packing the base up for shallow stuff. I may make up a second tall base from MDF using a torsion box design.
The challenge is to reduce flexure to the minimum, both by designing in stiffness and by restricting the offset distance of the cutter from the centre of each beam.
My Y axis beams are 2" square, 10g, alloy box sections, with 16mm fully supported SBR type rails fitted to the top of each. These beams are on 6" centres horizontally, so I have a gap of 4" between them. The Z axis will (I hope!) fit into this space, so that the cutter can retract back between the rails for maximum usable operating depth. Including the height of the rails, my Y axis beams are about 4.8" tall, so I'm hoping that they should remain stiff enough in the vertical axis to not noticeably deflect under the weight of the carriage and spindle.
I'm mounting the rails on top of 8" long 4" x 2" channel sections, mounted with the short faces at top and bottom, bolted directly to the carriages of the X axis. The X axis rails are also 16mm fully supported SBR ones, bolted to the top of another bit of 4" x 2" channel mounted on edge, with the bed bolted beneath. This gives me a nominal Z axis working height of about 248mm, although I can gain a bit more, if needed, by lowering the table a bit more.
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