Let The Fun Begain, New build on the Table.

[Bush Flyer]

Hi all,
Here is my new drawings for my 1100mm X 700mm X 300mm CNC, the only thing that I kept was the bed, everything is new from there.
I lowered the height of the gantry the Z axis from 125mm to 55mm (that's back to the height that I had my very first drawing) from the bed that should let me put a thin mdf board as a sacrificial board. The travel in the Z axis is 110mm.
The gantry sides are made out of two 20mm thick plate aluminium which will be bonded and bolted together, the first one is solid the second one will be lightened. The Z axis will also use 20mm plate aluminium. The backing on the Y axis is 12mm plate aluminium bolted to Aluminium Profile Extrusion. And the X axis can easily be stiffened buy adding more Extrusion to the Bed if needed.

Ross77,
Thanks for letting me see your drawings, and I have seen this design before and dismissed it as being to difficult to build. The welding part is easy but to make it accurate would mean a lot of shimming, as you know welding one side of the metal joint twists the steel one way and when you weld the other side of the joint it will pull it back again. I am not longer able to Cut and grinding steel due to now being disabled. But I do like the Z axis and I would like to see a photo of your machine.

[Swarfing]

A quick thought!

Flip the 'Y' axis (top to bottom) arrangement over and add another plate to the back to mount the ballnut and you will always have a nice clean screw

[Ross77]

Looks good mate. are you happier with this as well?

Whilst myself and others could carry on making suggestions to tweek if further I think you have got to the point of a build-able machine, without going into calcs you have gone as far as you can.

The only point I would make and it is minor and that is that you might need to add additional support to the top of the X axis bearings to stop them opening up due to the weight of gantry, placing the rails and bearings on their side will improve load capacity but some of the cheaper bearing holders can give and open up the bearing.

I apologise for misleading you with the table distance as I presumed you meant with the z axis at the top of its travel. I see from your drawing you are talking about it a full extension.

I have seen this design before and dismissed it as being to difficult to build

yep thats one of the reasons I abandoned it, cheap but difficult to build. I was mainly showing it to you for the rail orientation.

The welding part is easy but to make it accurate would mean a lot of shimming, as you know welding one side of the metal joint twists the steel one way and when you weld the other side of the joint it will pull it back again

The design was actually for Ali sections and they would have been bonded with epoxy and riveted for good measure. that way no distortion due to welding.

I never got around to building it so no photos I'm afraid.

[Ross77]

Flip the 'Y' axis (top to bottom) arrangement over and add another plate to the back to mount the ballnut and you will always have a nice clean screw

where's the fun in clean? yes you can do that but moves the ball screw away from the Z carriage and reduces rigidity and can lead to racking or binding of the bearings the extra distance can mean the alignment would be a mare as well.

Edit. thought id seen that mentioned before! just realised it was this thread, so same as Jonathan's answer

[Swarfing]

Edit. thought id seen that mentioned before! just realised it was this thread, so same as Jonathan's answer

There you go great minds think alike. The distance is minimal and would not make any noticeable difference. As for binding then you need to make allowances in the bolt holes so that you have adjustment

[Ross77]

Hi Paul
yeah I agree that it probably wont affect the performance to much but without going through all the calcs and then changing/ widening bearing spacing to compensate then it is best left as it is. remember this is being built with a pillar drill so ease of fabrication and also dimensional tolerance is less than parts made on a mil or cnc.

the binding is as a result of the lever arm of the system. the original set up was Bearing-Ballscrew-Load so the ballscrew can control the load directly. moving the ball screw back results in Ballscrew-Bearing-Load so the ball screw has to work through the bearing to contol the load and vice versa, the load is transmitted through the bearings before the ballscrew can control it. Under sudden loads this can cause the system to rotate around the bearing and as open linear bearings have clearance they could bind or cause premature wear.

Again it can be designed out but I think it will be easier to just clean the ball screw

[Swarfing]

Sorry Ross in this instance i would have to disagree. Swapping it over as i said would result in the ballnut being roughly in the same position, as the plate showing on the 'Z' could be set further back still. For this type of build it will work absolutely fine. Have a look around the forum with such a design and ask how they are getting on ;-)

[Bush Flyer]

Its good to know that I'm heading in the right direction, and I am happy with the progress so far.

[Jonathan]

Moving the ballscrew, whilst not disastrous, hardly gains anything. All you're gaining is some protection for the ballscrew from swarf and there are plenty of other ways to do that - for example adding seals or bellows. So even if the difference to the overall machine due to the loss in rigidity is small, you're doing this to correct a problem which is also very small since it's clear that swarf on the ballscrew is also only a small problem. I recall Jazz saying he'd worn out his ballscrews from letting the swarf pile up on them over a few years, but I've not come across anyone else who has had a ballscrew (or nut) fail for that reason.

For this type of build it will work absolutely fine. Have a look around the forum with such a design and ask how they are getting on ;-)

Having the ballscrew on the back on my machine has crippled the stiffness of my Y-axis and although the distance is greater, that shows how the effect can be significant.