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View Full Version : Z Axis appraisal please



EddyCurrent
11-09-2013, 11:06 PM
I'm reluctant to start a Build Log until some details are ironed out, if you think this is wrong then please let me know.

This a Z axis that tries to keep the overhang to a minimum while still using slightly modified off the shelf ballnut and ballscrew end support.
The ballscrew is supported at the top only but I can't see this being a problem.

All plate is 1/2" - is this enough ?
Ballscrew is 1605
Rails are 15mm Hiwin

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Can't seem to add the sketchup file

njhussey
11-09-2013, 11:38 PM
Two things jump out at me. The first is the carriages are too close together buoy need to space them out more. The second is that the ball screw will whip with no end support so speed will be severely compromised.

EddyCurrent
11-09-2013, 11:44 PM
Okay re. carriages but how can the ball screw whip when the linear bearings are keeping everything firmly in place ? maybe it happens most when the Z axis is at it's highest point ?

JAZZCNC
12-09-2013, 04:36 PM
Ok 12.5mm plate is too thin you need 19-20mm minimum other wise it will flex, you could get away with 12.5 on the rear plate but I wouldn't advise.

The way you have the ballnut on top of front plate means the rear plate is longer than needed really wasting money and adding weight.
The screw not being supported at both ends isn't a problem at short length(<150mm travel) but if having long travel and with you having ballnut right at top then supporting both ends would be better.

Bearings are too close together which has been said. . . . BUT .. . Also you haven't shown how it's fastening to Y axis carriages and I suspect those bottom bearings will be in the way of bolt holes.? It's a common Err and chances are you'll raise them up 20mm or so.?

I wouldn't get too carried away with trying to reduce overhang, the little you'll save won't make a big differance to cutting area but could to performance and quality of cut if taken too far.!!

Boyan Silyavski
12-09-2013, 04:45 PM
The ball screw will not whip. On my small machine its exactly like this. Though if s**t happens and the screw pushes a hard stop bolt/asuming you fix one/ or table bed, everything will bend, instead of just stalling the motor. Or so it seems. At least mine bended a couple of times.

EddyCurrent
12-09-2013, 07:48 PM
Thanks for taking the time to look and advise, now drawing version 2

EddyCurrent
12-09-2013, 10:22 PM
Just a quick one, ball nut choice for Z axis using 1605 ball screw. Trying to keep the back and front plates close together so what would be best type FSI, RSU, RSW ? or am I pissing into the wind and should just opt for a FSUD and have a bigger gap ?
Thanks

JAZZCNC
12-09-2013, 11:56 PM
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
13-09-2013, 09:14 PM
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 (http://www.arceurotrade.co.uk/Catalogue/Machine-Spares/Machine-Bellows)) 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.

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JAZZCNC
13-09-2013, 11:01 PM
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
13-09-2013, 11:17 PM
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
13-09-2013, 11:23 PM
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.

EddyCurrent
13-09-2013, 11:31 PM
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
13-09-2013, 11:44 PM
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
14-09-2013, 12:35 AM
Continues here

http://www.mycncuk.com/forums/router-build-logs/6565-ready-steady-eddy.html