Is a center mounted Z-axis Overkill?

I agree this concept would definetly increase the rigidity of the Z-Axis, though I believe it could be overkill dependant on what you are after achieving with your machine!

 

I'd say it would make toolchange and zeroing off a little trickier with the reduced access.

 

Bob,
Great to see someone else thinking along the same lines as me. I've gotten most of the mechanical stuff in house now (from open builds) and have been looking into ironing out the details (in SW). Need to create provisions for belt tension, wire routing, limit switches, etc.
I plan on sharing in my own build thread once I've finalized the design and start machining out the plates.
Really thinking about belt-belt rack and pinion for the X. I'd like to mount with the X motors vertical, which has a couple advantage, but adds more plates, and complexity. Ohh, and the travel as currently drawn is X extrusion minus 240mm, more than some, but not horrible. The Y axis has the belt wrapping around the pair of top wheels and the stepper inside the C.

My setup is intended to house an aircooled 1.5kw spindle using a C-Beam Z. Been thinking a version 2 will likely go to more expensive linear solutions with a ball screw. Would cost more, but offer a stiffer and lower profile. This would fit larger spindles into the same space.

Also need to work out dust collection. This is likely going to be more complex than typical, do to the limited area around the spindle. Might be looking into forming some lexan or something to snake around the spindle motor.

-Sam

 

HI,
mixed feelings about it eventhough in general it is a good idea.

Ok in general it is always good with cnc to stiffen up the machine... till you end up with machines weighting tons -LOL

Looking closely you will find that your spindle-mount should not rotate forwards and back any more (y), but to eliminate sidewards flex one has to space the wheels further apart (like you did) to prevent flex in x. But than one needs longer extrusions (x) to compensate the distance lost - which will flex more the longer they get.
Also these will bend more with the weight than shorter ones - to know if you gained or lost one really needs data and math- ore trial and error.

What happens if one extrusion would ever so gently slope uphills at center while the other downwards? Think this improves the outcome?

The ox works well cause all of its components are about the same in terms of stiffness (or weakness -depends on ones standpoint)
So it is balanced well. All components work in harmony within the design parameters..
NO money and no effort spend on a single apect only marginally improving the product and hardly affect cutting times in the end.

I have posted some pics in a threat here somewhere of a machine using double x...
And as mentioned there too: the z-x setup alone of that machine probably costs more than a complete ox.

greets
and a Happy new year

Flo

 

Hey Flo,

I've gotten my X reworked and I'm very happy with it. So much so that I started looking at the Y (Z carriage) again. I plan on starting a thread this weekend to get some feedback specific to my machine.
Thought I'd mention that at 240mm lost for the X brackets I think it is not too much more than some of the other designs currently accepted. However, if one does not have a well supported X then the added weight will be troublesome.

I've gotten the extrusions in house and they seems to be of good quality. No detectable warping. At any rate, it is just as likely for an extrusion to be twisted as bent. Frankly, with a single beam it will twist more with the carriage in the center vs outboard. Particularly with the large moment realized in most designs currently being shared.
If one beam is curved up and the other down, I'd suggest flipping the one that is down over..

Not sure what the expenses are you are mentioning. Having more plates and extrusions cost more, for me I think they will be offset in ability and size gains. Most of what I'm after is the increase of size. Of course I have the benefit of being able to make the parts on a CNC mill, so I may be underestimating the costs some.

Anyway, I look forward to sharing more with you soon!

Sam

 

What is typical?

Edit:
Got home and pulled up a couple OX designs. Looks like 180mm is typical, so I've given up 60mm more. This is an easily reduced number, but I'm not sure if I'm concerned at this stage.

 

HI,
i was replying generally.
Could have used the phrase: a machine is perfectly constructed once you cannot leave out another part without impairing funktionality..

greets

 

Pretty cool!
I have no idea, how close the being the same length are the extrusions? Can a guy count on +/- on overall C beam length?
My thought is the vertical plates would not be square if one C-Beam was longer than the other. This would put the X? bearings in a misaligned state.
I would suggest putting the cross beams under the X? axis to create the space rather than making the vertical plates taller. Personally I'd rather keep the vertical plate as short as possible and put risers under the entire machine as needed.

I've not been able to find any info (minus one source) on what rapids can be achieved with DIY cnc systems (leadscrew, ball, or belt). Of course loading changes everything, but even non-system numbers are not around. For instance, how fast can a C-Beam (leadscrew), that is 1000mm, just setting on a bench, not in a machine, go?
I plan on doing some testing with me little 250mm C-Beam this weekend (be using a 282oz nema 23 on 24v). However, my experience with a 4ft lead screw(1/2") hot wire(8020 based) was not great. Trying to run at any reasonable speed would cause the screw to whip. I know a larger diameter screw would help, but the C-Beam is 8mm if I remember right. How does the C-beam screw behave at 1000mm or even 1500mm? The last thing I build was actually my first belt system, been really happy with the belts. However, it a very simple down cut hot wire, so no real harmonics to deal with. Really hoping the next router machine with a belt on belt works as good as the hot wire.

Looking at the above again, it is defiantly worth a closer look!