Here we go again . . . MK4

[Tom J]

As I feared the return to work after Christmas means workshop time has significantly reduced. For various reasons I've only been able to get about an hour in there.

But I have been able to get a few sessions on the CAD and this has meant I was able to go over some of the other designs and have one last go at unlocking some of the compromises. The net result is I've developed some of the other ideas and ended up with what I think is a much better design.

Old one for comparison:


Here is the new version:
iso

iso rear

X axis

Y axis

Z axis

Just X axis showing gantry, ballscrews, epoxy (orange), and custom ballscrew floating end as the ballscrews were not long enough to mount the standard floating end.

X axis drive and belt tensioning arrangement

Side View:


The new features are:
Gantry beams smaller
They are now 80x40x5 RHS steel (down from 100x60x5). I'd put too much emphasis on huge sections, whereas with a double beam gantry I could afford to scale them down and still have plenty of stiffness in reserve over a single gantry.
Because they are smaller I was able to re-configure the X-bearings, bring them closer together, and give more travel in X. They are now only 274mm apart which is getting close to my current machine spacing. I was also able to get the X ballnuts at the centre of stiffness, rather than at the ends of the gantry.

Y rails are much lower
With the smaller gantry sections the Y rails are now lower and closer to the tool, as are the Y axis bearings, which all provides more stiffness

Ballscrews lower
As the gantry sections are narrower I was much happier putting the ballscrews on the front and rear faces as there was much less of a bracket required to join them back to the main Y axis, therefore stiffer.
These are now much lower in Z which also puts them much closer to the tool, which reduces the moments, which lowers the forces on the Y bearings and makes the machine stiffer.
I was also able to put the ballnut in the centre of stiffness, rather than on the outer edge of the Y axis. There might be a marginal gain here I really don't know, but it does look nicer.

Y axis bearings spacing
By re-designing the gantry end plates to free up some space I was able to make full use of the linear rail and spread the Y axis bearings out considerably more than before - without losing travel. This should significantly improve the stiffness due to moment inputs when cutting in the Y direction. When coupled with the lower ballscrews there should be a double win here.
In one of my earlier posts I suggested that double ballscrew on Y eliminates racking - whilst this is true for rotations about Z axis, it is not for rotations about the X axis. To eliminate these you could add another 2 ballscrews lower down, but that is not practical (!) so you do still need to space the Y bearings out even with double Y ballscrews. Rotations about the Y axis are dealt with by having the double gantry beams no worries there.

X drive
I've gone with something a bit different here which does not use tensioning idlers. The steppers are on plates which are slotted, and the steppers are also in slots. Between them I should be able to tension the short belt up to the ballscrew, and the syncronising belt across to the other stepper at the same time by pushing the motor off into one corner. Everything is 5 HTD with 15mm belts.

Cooling
I've decided that there are a couple of options to place the radiator, plus the option just to go for the 'big metal bucket' out of sight. So I'm going to build it and then just see which one takes my fancy. Sometimes you can CAD things too much and get tied up in the last details.


Luckily the new design retains the parts I'd already made so nothing lost. Thank you for the comments made so far, stirring up the doubts I had about some aspects, and making me revisit the previous designs. I think it is all the better for it.

Right, time to start CAMing up some of the parts ready for whenever the next workshop session is . . .

Amazing project - can not wait till is finished
I had similar idea how to drive two ballscrews