I want to DIY.. some questions please!

[JAZZCNC]

The unsupported rails are for the 25cm Z axis.

CAN NOT STRESS ENOUGH. . . . . .DONT DO IT. . . . ABSOLUTLY DONT DO IT.

You will regret using unsupported rail and even more so on the Z Axis.!!

[Shinobiwan]

CAN NOT STRESS ENOUGH. . . . . .DONT DO IT. . . . ABSOLUTLY DONT DO IT.

You will regret using unsupported rail and even more so on the Z Axis.!!

Simon, I'd really listen to this advice. I know you think you know what your letting yourself into here but honestly, you have no idea. I mean that in the nicest way possible.

I had to have my Z-axis rebuilt because it was using unsupported rails and was shit. About +/-1mm of flex at the bit and that means you'll get a crap finish even in MDF with anything like a decent DOC and feedrate. Your parts will be undersized, oversized and generally a mess. And you can forget it if your thinking of aluminium. Plus your using a heavy spindle with heavy 16mm alu plate. That's a bunch of weight right there before you've even got to the cutting forces.

Do your self a massive favour, ditch it and go with supported round rail. It'll add nothing to build complexity and hardly anything to the price. Cannot stress enough how shit round rail is, it should be banned on anything calling itself a decent CNC router.

Listen to the knowledgeable folks on here and save yourself from a world of pain, you'll get you a much better finish, more accurate parts and will let you move up to harder materials like alu, Corian and so on.

I'm trying to stress this because I fell in to the same trap your about to and its costs me a lot of money to put it right. Do it right the first time.

[Tenson]

I had to have my Z-axis rebuilt because it was using unsupported rails and was shit. About +/-1mm of flex at the bit

I find that quite surprising. I rather thought with the very short length and least weight of any axis, the Z would be okay running unsupported. If it really is that bad I'll get some supported rails. Building a Z-axis assembly to have opposing supported rails seem quite tricky though at first thought. Any tips / pictures?

Anyone want to buy 2x 30cm unsupported rails? ;)

[Shinobiwan]

I find that quite surprising. I rather thought with the very short length and least weight of any axis, the Z would be okay running unsupported. If it really is that bad I'll get some supported rails.

Anyone want to buy 2x 30cm unsupported rails? ;)

Ask Jazz about it, he came around to help me out with the machine and saw it. Its the cutting forces that will really show this up but even pulling back and forth on the z axis by hand will tell you how sturdy it is. The more rigid the design, the better the finish and accuracy you'll get.

If you do it right you'll be cutting MDF with very tight tolerances and even aluminium won't be an issue. Just keep asking loads of questions on here and tell folks about every design decision your making as they'll be able to advise if its a good one or a bad one. Building a good machine doesn't mean its more expensive but building a bad one will cost you to fix it.

[Tenson]

Building a good machine doesn't mean its more expensive.

Bloody seems like it!

[Shinobiwan]

Bloody seems like it!

lol, ok well not that much more expensive. You'll have a really nice machine if you do it right and it'll last you years.

[Jonathan]

The main advantage with profile rails is that they are preloaded. This means there is negative clearance between the bearings and the rail, so there will not be any play. It also makes any deflection linear and crucially, with rails with 4 rows of balls which are the type you should get, the load ratings are equal in all directions. Conversely supported round rails have little if any preload and the load rating varies significantly depending on the direction the force is applied since the open side of the bearing clearly offers less support. Ideally we would mount supported rails so they 'oppose each other' on Z, however as you say it's quite difficult to accomplish rigidly without a somewhat radical gantry design. Hence most people mount them both the same way round. If I measure the deflection when a force is applied to my Z-axis parallel to X the reading varies quite a bit, due to the uneven load rating.
However this is of course all relative - compared to unsupported rails the deflection is tiny. You wont regret using supported or profile rails on the Z-axis - ultimately depends on what you can afford.