cnc for marble

[Chaz]

Hii All i am trying to build a big cnc router to use for stone engraving ,i build my first one one month ago but was full of mistakes ,i need advice on solving the problems i am facing right now like loosing steps ,achieving higher speed and higher precision building a rigid machine i will be working on a 3m *1.5 m machine

I suspect before anyone can help you, you need to provide more details. Your question is a bit like 'how do I solve world peace'?

Pics, specs? What speeds can you do, what are the issues etc ...

[yasser]

Dear chaz thank u for your reply and of course u r right here are some photos for my first machine and the major problems are
1- the max speed i can use is 400 mm/sec after that i loose steps dramatically and also this consumes a lot of time as the max step down is from 2-4 mm, i use the steppers directly i think i should be using speed reduction method ,i use nema 34 steppers 11n.m
2- the aluminium sections are very weak for this application i think i will be using steel as the new machine will be 3m in th x axis and 2 m in the y axis so i am searching for a stiff beam design for the length 3m

[Rufe0]

Is this machine to be used only for marble? I would have thought the only way to machine marble would be very slowly. In which case the maximum speed of the machine would be of little importance. So yes it maybe a good idea to use a gear reduction however all gear systems have backlash so you will need to account for that.

[yasser]

would using a belt system be a good idea

[Rufe0]

The problem is at 3m long your going to need quite large screws so that they aren't flopping & wipping around. Large screws generally have a large thread pitch, plus you'd want to use a large thread pitch anyway for rigidity. With a thread pitch of say 20mm your only going to get steps accuracy/resolution of 0.1mm with a stepper motor(with no gear reduction). Which may or may not be OK in terms of accuracy but it means the slow speed movement of the machine would be horrible. So I would suggest you find a way to gear it down or use whatever system to achieve a high resolution. 0.001mm would be good I guess. That would make slow feeds very smooth.

In terms of the machine design I'm sure we can help you design a machine that will be alot more rigid. Do a drawing and we can comment on it. Firstly you need atleast 2 rails per axis, 2 carriages/bearing blocks per rail and 2 screws per axis. I recommend you use thick wall steel box section tube for the construction, however some parts may need to be even heavier duty and so may need to be custom machined for you. I would recommend a dual gantry design. Large seperation between carriages/bearing blocks probably 400 or 500mm. That means you machine will have to be 2000x3500 to get 1500x3000 travel.

I assume that the marble will almost always be a similar thickness and that it won't be very thick. That is a good thing because you won't be able to have much Z travel, in fact I recommend you make it as little as possible.

[yasser]

thnx rufe0 doible girder is a good idea , i thing of using rack and pinion instead of screws and belt driven system for gear reduction i will make the drawing and post it for your feed backs

[Clive S]

1- the max speed i can use is 400 mm/sec after that i loose steps dramatically and also this consumes a lot of time as the max step down is from 2-4 mm, i use the steppers directly i think i should be using speed reduction method ,i use nema 34 steppers 11n.m

Yasser first welcome to the forum. Are you sure about the 400mm/sec as that is 24mtr/min and is very fast. you need to give the specs of the controller and drives also the voltage of the output of the power supply.