New Member and New Build design check

[Sven]

I've been told that the linear bearings and rails are stiffer than anything you could bolt them too.

If that is the case then you'd be better off combining all beams in the gantry in to one structure.
I think you now support the Z on two sides for stiffness, but you actually lose torsional stiffness this way.

Also, it will be extremely hard to line up all parts to run smoothly, with multiple carriages on multiple carriers.
All in all, though it looks very nice, I feel it is overly complex but will under perform.

[Procato]

routercnc - many thanks for your suggestions. It took a while for my brain to get around your notes about the rails on the z-axis but I now I realise what you say and it makes total sense. My original plan was to machine the z box so that when all is put together there is approx 1mm gap between the 3rd rail guides of the z-column and the 'backplate'. I would hold the guides roughly to the 'backplate' with loose screws then inject aluminium epoxy putty into the 1mm gap. I would tighten the screw once the putty had set. This method would I hope would mean I wouldn't need the high tolerance machining that would be otherwise required. Not sure what to do yet so will mull it over.

The order of assembly is also great advice, again some thinking needs to be done.

Re the tensioner - You've caught me being lazy, I will add it in the cad drawing soon but I just wanted feedback before I got into the details. There's quite a big gap between the end plates 80mm.. I plan to have something simple, probably a slot running on the inside end wall of the gantry with a couple of bearings on a screw to tension the belt.



Sven; Many thanks for your input, I have gone to and fro with this thought for a while and I guess what I really need to do is learn the simulation part of the CAD program to get a definitive answer.
I get the idea that unless you build the extra rail perfectly you just end up introducing error and not taking it away but there is this constant niggling at the back of my brain that says when that long Z-column is at full extension, the leverage on two rails will be very high especially in the centre of travel. I can't help thinking 3 rails has to be stronger but that is just a gut feeling and nothing more. It would be very interesting to other peoples thought as well.

[Boyan Silyavski]

Hi,
what are the exact planned distances in mm for the different color lines, see picture below.

Red line bearing spacing distance seems too small for that enormously long Z, doesn't matter that the Z is 3 rail supported. Best would be if is same distance like violet line.
Meaning 250mm is not right here. ~350mm is more sound. Have in mind that mostly and only Lower bearing blocks take the force, so should be up to the task of coping with 900mm extended axis acting as a lever. Proving that with a calculation would be useful. The upper bearing block acts only as lever arm so basically is doing nothing apart from levering, hence the distance here is extremely important.


Why so long Z? Longest reasonably Z IMHO is 500mm. Unless you plan to shape only egg statues . The z assembly will intefere, so i dont see what the benefit will be of so long axis.

[Procato]

Blue 318mm
red 240mm
violet 260mm

The travel on the Z-axis is 350mm which is I think is about correct to mill/sculpt human torso(with separate arms) which is the work envelope that I'm trying to achieve. So I might have exaggerated its length in an earlier post!

So with the 350mm z-axis travel do you still think that this machine would benefit if I increased the distance between the Y-axis rails to 350mm?
Also do you think that the design benefits from the 3rd rails on both the y-axis and z-axis?

[Boyan Silyavski]

Difficult to say.

On my current build both red and violet distances are around 350mm and even without 3rd gantry profile and 3rd rail on z all looks like it will reap aluminum like butter , i would say steel will not be a problem at all with small cutters.

So in your case for me it seems logical to extend the red distance a bit. Though the third rail a kind of compensates for that.

I second the previous advice given to you. Make sure piece by piece its possible to fix all together and align it/ tram it/ . Its a complex assembly so dry test all in software.