Steel Framed Router

[Jonathan]

The plate on the gantry to stiffen it is an excellent plan. I think you need to consider more how you are joining the box section, and gantry sides to bearings. Currently you have drawn the box section attached on just one face which is a bit weak.

I have done a drawing for you to illustrate how I think it can be made stronger:




That way the sheet reinforces the joints in addition to the general structure. I would be inclined to weld it for good measure. If not put plates (say 20mm thick) in the ends of the box section and bolt into them.

[JAZZCNC]

Jonathans suggestion is a good one not just for strength but also it protects the ballscrew from flying debrie.
If you must go with 1 screw done the middle then I suggest you make the cross piece that the ballnut fastens to wide as possible. you need strong bracing and width to counteract racking with a single screw.
Something like the pics below will handle just about most jobs at this width with a single screw. A 16mm 10mm pitch would work nicely.

Edit: The sketchs are rough quick knock up's and I would add some more corner braces to the cross piece but it should give you an idea of what I mean.

It probably looks a bit extreme but honestly it's not and the extra work and slightly more expense would be worth every minute and penny spent belive me.
Dont under estimate the forces's even with steppers.!! . . The forces can be extreme when cutting Ali and hard materials and the torque produced by steppers and ballscrews can twist and mangle a gantry built like this in the blink of an eye when the stick stuff it's the fan. :surprised:

[Jonathan]

torque produced by steppers and ballscrews can twist and mangle a gantry built like this in the blink of an eye when the stick stuff it's the fan. :surprised:

Yep, mine has done that once or twice. It's quite surprising. Once nice test I can do is if I disengage the motors and rotate one ballnut by hand the gantry is stiff enough to make the other one backdrive and rotate too... but not much.

I wonder how much difference it would make if you have two ballnuts on the single screw? One on each of the two cross pieces on the gantry.

[JAZZCNC]

I wonder how much difference it would make if you have two ballnuts on the single screw? One on each of the two cross pieces on the gantry.

Dont think it would add much to extra torque but it would certainly add extra precision due to removing backlash and compensating for wear also help with stablizing whip on longer screws.
I have some lovely 25mm Comtop ground Class 3 preload 2 double nut ballscrews for my next project and the difference between the cheap chinese screws and these is night and day. . . . Cant wait to get on with it.

[GTJim]

Thank you all for your very good comments.

Rick - I will redesign the frame and go with 2 ball screws as suggested, possibly 16mm 10pitch.

Jonathan – In my initial sketches the axis was constructed in a similar manor to your drawings but I found the spindle did not cover the whole of the table. So I either extended the frame of the base or took the rails to the extreme of the axis allowing the spindle to go past the work surface while keeping the foot print of the machine as small as possible. But as you have pointed out it has created a weakness and I will take this on board.

JAZZCNC – I also agree Jonathans suggestion is a good one, but as for the protection of the ball screw I was hoping to use brush seals fixed to the gantry to prevent wood chips causing damage. I’m sure I’ve seen this being used on a machine on this site.
I also like your design for the gantry, so I’m back off to the drawing board. I’ll keep you posted.

What are your thoughts on these Chinese screws as I don’t want to work to microns?

As you are all well aware of, everything is a compromise.
Jim

[Jonathan]

What are your thoughts on these Chinese screws as I don’t want to work to microns?

The ones I have (C7) are all smooth with next to no backlash. The backlash is less than the resolution I get so it's not significant.

Apparently they do vary...

[JAZZCNC]

What are your thoughts on these Chinese screws as I don’t want to work to microns?

I dont know about other chinese ballscrews but I have bought and fitted dozens from Chai at linearmotionbearings and they are very good considering the money. . . More than good enough for Diy.
I have some of chai's screws on my big machine and they are just starting to show signs of wear after 3 years of very very hard use cutting mostly Ali plate and in the worst possible conditions due to the fact they are uncovered and partly in the firing line.!! . . It's one of them " I'll do it later" jobs that after 3yrs still aint got done.!!! . . . .

BIG TIP . .DONT Start the bloody thing up untill you've done every thing you planned else they'll never get done.!! . . . So much fun you'll never want the down time.

[Jonathan]

BIG TIP . .DONT Start the bloody thing up untill you've done every thing you planned else they'll never get done.!! . . . So much fun you'll never want the down time.

:heehee: Yep I agree entirely. That's why I don't have any switches on it, or covers and only crude dust extraction ...

My ballscrews are also from Chai. Only just got round to putting better computer on it that I've had for a while. What a difference...RM1610 screw on Y-axis is now going at 36m/min which is the critical speed and it seems fine. I wonder what their limit is :naughty: