Thinking about building a second DIY machine

[PhilipTheArab]

The first diy cnc machine is working fine but

Rigidity is not good and I was expecting that anyway. My objective then was to make a cnc that works.

second I need to find a solution for the couplings. As in my case they work for a while, then start slipping in the middle of work.

Any way my question here :

I need 3 meter of 25mm ball screw.

Please give me your thoughts for long travel axis regarding linear motion


good day

Philip


This machine is more than 2 years old

https://www.youtube.com/shorts/Pg7Bt7uamvc

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[joeavaerage]

Hi,
I don't think a 3m ballscrew of 25mm is practical. Firstly I'm doubtful that any manufacturers produce such long ballscrews. Secondly it is highly likely that even a 25mm screw would whip when rotating at even
a few hundred rpm. I think 1.2m, maybe 1.5m in 25mm diameter is practical, but 3m????.

For that length you need to consider rack and pinion.

Another alternative is to have a 3m ballscrew, but not have the screw rotate, but the ball nut. This technique is used on very large machines and the screw is often 50mm in diameter or so....way over the top
for a hobby machine. There have been some good examples of hobbyists whom have made rotating ballnut devices from regular ballscrews, but only at the cost of significant complexity, none-the-less,
some very good machines have been made that way.

From the video it looks like you are using round rails? Is that correct? If so then square rails and matching bearing cars are very VERY much better and way more rigid. That would be a good place to start
if you are wanting to improve your machine.

Craig

[PhilipTheArab]

Thanks Craig for the reply

I agree with what you mentioned.

The linear motion in the machine in the video, yes 4 cm round bar. I only have issues regarding vibration at aggressive rates, but agree about rectangular being more stiff.

For cost reduction, I am also considering fully supported round bar with open bearings.


regarding the ball screw, If there were 3m long screws, I was considering installing several ball nuts to break the waves. But I agree that rack and pinion could be better if not for the alignment of several racks

Philip

[peteeng]

Hello Philip - Helical R&P is the correct answer for that size machine. They cost around the same as a ballscrew have minimal/zero backlash. Recommend not to use straight cut rack. The only reason to go to ballscrew is if your trying to make a very accurate machine over that length then the cost of large precision screws will have to be paid off by you and then your children. Last machine I made I used supported round rails to cut some cost and see how they went. Turned out to be not a big saving and they worked fine but not being preloaded I expect long term they will develop clearance. I'd go square rail medium preloaded cars. Couplings should not slip - what sort are they? I use loctite on everything, no loose bits after that. I'm not sure how you will install several ballnuts to stop vibration? Some long screws have small arms under them that move out of the way as the drive nut goes by, but R&P will be a better solution. Peter

[joeavaerage]

Hi,

yes 4 cm round bar. I only have issues regarding vibration at aggressive rates, but agree about rectangular being more stiff.

Vibration happens when a disturbing force deflects a structure of finite compliance, and it exhibits oscillatory motion. Even if a structure does not vibrate, it
still deflects with an applied force....and ANY deflection means loss of accuracy. The less the compliance, another way of saying the stiffer the machine the more accurate it will be.

The advantage of square rail is that the bearing cars can resist moments that round rails cannot and that the stiffness results from the rail being screwed to an uber stiff frame.
It is the frame that makes the rail stiff....not the rail itself.

Craig

[PhilipTheArab]

Last machine I made I used supported round rails to cut some cost and see how they went. Turned out to be not a big saving and they worked fine but not being preloaded I expect long term they will develop clearance


Glad to get advice from someone walked the path regarding the fully supported round bar for linear motion


I will consider the helical rack and pinion

many thanks


Philip

[PhilipTheArab]

Hi,



Vibration happens when a disturbing force deflects a structure of finite compliance, and it exhibits oscillatory motion. Even if a structure does not vibrate, it
still deflects with an applied force....and ANY deflection means loss of accuracy. The less the compliance, another way of saying the stiffer the machine the more accurate it will be.

The advantage of square rail is that the bearing cars can resist moments that round rails cannot and that the stiffness results from the rail being screwed to an uber stiff frame.
It is the frame that makes the rail stiff....not the rail itself.

Craig

yes vibration and deflections are two different things. I have 4 round bars carrying the carrage. I did not observe any sign of accuracy or deflection, but maybe I was not looking close enough.

I graduated prior to computers. We used to calculate structural analysis, and perform finite element solution by hand and slide rule.

I realy appreciate your comments and help

Good day

Philip