Saturday, 30 October 2010

y-Axis Gantry Design - Draft

I had some time today and am moving along nicely with the y-axis design for the cnc router. Here is a quick update with a couple of design images. Comments are welcome, but beware, it's work in progress. Evidently, some items are virtually floating in free air, so thanks for not pointing that out.

There are a couple of design decisions that might deviate somewhat from the 'standard' router configurations and might be worth discussing:

First, I intend to have the target 'cutting surface' positioned below x-axis shaft level. In a sense, the router will "cut down below". The idea is that this will decrease the required height of the gantry and it works in conjunction of the second point below. In effect, the lower y-axis shaft could be at around the same height as the x-axis shafts - or even below. This will reduce the all-over gantry height and bring everything closer to the x-axis mounting points. It helps, as I am using a fairly tall z-axis (shaft 400mm; z-axis movement range 210mm).

Second, I intend to avoid the "bridge" configuration, where the x-axis drive setup, eg, belt drive or threaded bolt, is centered below the floating cutting surface and connected across to both sides of the gantry. This is probably my greatest fear that it could go wrong (precision issues, or worse, lock-ups). The x-axis will be driven on the side, not the centre as my theory is that side versus centre improves alignment only by a factor of two (you are encouraged to disagree with some maths), and I found the ball bearing setup to resist alignment problems very well.


Cnc y-axis gantry. Work in Progress...
For reference, y-axis shaft length is 600mm.

If you look closely at the design, you will also notice that some of the board positioning has been chosen very carefully to be able to reach screws during assembly, for example, the inner mounting screw for the rear x-axis ball bearings. On that note, have a guess how I assembled the z-axis' nut that sits quite invisibly under the small square mdf board and is held in place by a piece of small mdf board.


Cnc y-axis gantry - from behind.

In other news, I bought a few nuts and bolts today to assemble idlers from my wide ball bearings. I think this will work very well.

6 comments:

legout said...
This comment has been removed by the author.
legout said...

Hi,

I´ve readed your blog. Very interesting and and it´s a joy reading it.
I´m very interested in your belt drive. Did you already have one belt driven axis running? On my cnc machine I´m planing such a belt drive for x and y too. But I wanna do something similar to the "servobelt drive" where two belts are placed on top of each other to minimize bachslash and belt bending. (http://www.youtube.com/watch?v=OdJoVh6DRPA).
One more comment to your x axis. You can install a belt drive on both sides of the gantry and connect both to one motor with a threaded rod similar to this one here (araound 7:00 min in http://www.youtube.com/watch?v=YhQF9SlDUr0&feature=related).

I´m looking forward your next blog entries.

legout

Thomas said...

Hi legout.

Yes, I had previously looked at the servobelt drive with great interest and actually thought of doing it that way. The problem I found with the belts I bought is that there is a small amount of spacing between teeth, so there is a bit of movement.

The belt drive on both sides is an interesting idea. I will first try to build a one-sided drive, and failing that will look at this as an alternative.

Thanks for the suggestions.

I haven't made any progress the last 3 weeks due to other distractions. I hope I'll get back into it in December. Keep up the pressure on me !
Thomas

legout said...

Hi Thomas,

its bad to hear, that there is some space between the teeth. I will contact the dealers before buying.

Are you aware, that the belt drive on one side could destroy your machine? If the side without a belt will cant and the motor is still running on the other side your linear guides could be deformed. I would effort little more money before.

legout

TassieDevil said...

Hi Thomas
I like the robustness of this design. I think you should persist with your current plan. Having one belt drive on one side is not that big a deal. I have seen several machines with one belt, and no reports of problems...your linear bearings and pillow block mounts will go along way to ensuring nothing gets jammed up - so I say press on!!
The only drawback of your design would be perhaps its a little "top heavy" so not sure what might result from this in way of backlash, inertia etc...if you build it strong enough I would imagine this wont be an issue.
Looking at your sketches 9nice, by the way...) you appear to have a bearing of some sorts on the bottom of the z axis lead screw ? If so, will this be able to slide up and down in some sort of special housing ? Would like to hear your design thoughts on that...
So far, I reckon its a ripper of a design..well done.
Regards
Chris

Thomas said...

Hi TassieDevil

Thanks for your encouraging words. I guess I can always build version 2 should version one have any flaws (highly unlikely, of course :-) ).

The bearing at the bottom of the z axis only prevents the thread from bending/vibrating. It does not carry any weight and you can pull the thread right out of it.

The weight-bearing bearing is at the top and is tightened to the thread with two nuts. You can see that better on the photo in the September post called "Up and Down: Z-Axis pictures".

Thomas