Z Axis appraisal please

[JAZZCNC]

Yep pissing on your own leg.!!. . . To me your making things harder than need be for very little gain, just get the one which makes building easiest. Standard ballnut does the job great and easy got.

[EddyCurrent]

Latest.
Spec: machine workable area 900x600x150

Plate: 20mm aluminium
Ally profile: 80x40
16mm ball screws (X-1610 Y-1610 Z-1605)
2 off ball screws for X driven by 1 motor via timing belts
15mm HIWIN bearings
'Standard' ball nuts (possible slightly modified by a mm or 2)

I know, a bit of backtracking, but I've been agonising over the L gantry and I just can't get away with that big plate down the back of the Z axis that joins the top and bottom, plus I don't like the ball screw that far back even though I know Jazz said it would not be problem in the real world. This is a C gantry and I'm planning to fit bellows (from here Machine Bellows - Arc Euro Trade) along the front to keep crap out.
You may notice that the Z rails do not meet the front plate, I'll have to mount the rails to the plate and shim the bearing carriages out by about 4mm, it was the only way to get the ball nut in. I still might get a RSW nut for experimenting with.

You might think the Z axis backplate hangs too low below the gantry and that is probably true, I might have to raise the whole assembly a bit but I'd still like Z travel of 150mm

It's probably best to move this to a build log next.

[JAZZCNC]

plus I don't like the ball screw that far back even though I know Jazz said it would not be problem in the real world.

Ah ah I find this hilarious.!! . . . For some reason you don't like the screw at rear but yet your happy with the rear plate extending down below gantry and flapping about seriously weakening and compromising the quality of finish.?

Think about the machine your building not what some spread sheet that somebody who's never probably built a CNC machine predicts is the optimum location, Or from looking at some poorly built machine who's getting less than expected results then blaming there crappy build quality on design style.

Now I'm not saying that rear is best or optimum but Both ways will work equally same and I'd challenge you to spot the difference in quality of finish in 99.9% of cut materials if I built 2 machines in each style. . . My preference for rear mounted screw is poorly for ease of build and neatness with motors/belts etc and with cable management etc so not messing around with floppy bellows and motors sticking out side of machine.

Your insistence on keeping Z axis slim and low is seriously compromising the machine.!! . . . Front or rear screw no matter but this Z axis is wrong.
Lift the bearings and rear plate so they are supported by the gantry other wise the back plate will flex under cutting forces.!

[Boyan Silyavski]

Why don't you use steel profile gantry? Or at least steel profile and aluminum profile mix if you insist on using aluminum profile? Profile is much stronger than plate. It would be good idea to check your design with the calculator posted in the sticky area of the forum. Personaly i dislike plate sides of gantry if they are not double, imitating profile.

[EddyCurrent]

Why don't you use steel profile gantry? Or at least steel profile and aluminum profile mix if you insist on using aluminum profile? Profile is much stronger than plate. It would be good idea to check your design with the calculator posted in the sticky area of the forum. Personaly i dislike plate sides of gantry if they are not double, imitating profile.

I was thinking I might add a 12mm plate to the flat back side of the gantry to tie it all together. I know some designs have double gantry sides but I'm conscious of the gantry weight and also feel the depth of the gantry cross members will prevent any flexing of the sides. But I may be wrong !

Thanks for your comments

[Boyan Silyavski]

I was thinking I might add a 12mm plate to the flat back side of the gantry to tie it all together. I know some designs have double gantry sides but I'm conscious of the gantry weight and also feel the depth of the gantry cross members will prevent any flexing of the sides. But I may be wrong !

Thanks for your comments

I assume your gantry will be 160mm high, that means for the 600mm travel you will use something like 1m of 160x12mm plate which weights 15kg. For example 1m 160x60x4mm steel profile weights 9 kg and would be stronger than your combined plate+aluminum profile.

[EddyCurrent]

Continues here

http://www.mycncuk.com/forums/router...eady-eddy.html

[EddyCurrent]

Ah ah I find this hilarious.!! . . . For some reason you don't like the screw at rear but yet your happy with the rear plate extending down below gantry and flapping about seriously weakening and compromising the quality of finish.?

I mentioned this in my post because I knew it was crap but I'm changing it now.

Think about the machine your building not what some spread sheet that somebody who's never probably built a CNC machine predicts is the optimum location, Or from looking at some poorly built machine who's getting less than expected results then blaming there crappy build quality on design style.

I agree but I like the look of this design better anyway

Your insistence on keeping Z axis slim and low is seriously compromising the machine.!! . . . Front or rear screw no matter but this Z axis is wrong.
Lift the bearings and rear plate so they are supported by the gantry other wise the back plate will flex under cutting forces.!

Same as point 1

What happened was I was concentrating on the top bit and forgot about the overhang until I posted the pics and saw it.
Also moving it to a Build Log next.

Thanks for comments.