Accurate Strong Gantry

[EddyCurrent]

1. 1500 mm long rail position-side or top ?
2. Is there any good carrige pattern to eliminate the double ball screw?
3. At belt synchronizing version1 or version2 is the better? -my logic says the equal belt length in any drive direction give more accuracy .

1. As far as I'm aware the bearings and rail do not care which way you have them but you need to think about how you will mount them so that they are level with each other across the frame, and that they are the same distance apart along the frame.
2. On a 700 wide machine I think you will need 2 ball screws.
3. As you say, the equal belt length method looks best.

[vargai]

Thanks form the prompt answer,
[QUOTE
3. As you say, the equal belt length method looks best.[/QUOTE]

When I am saying equal length I am thinking of the extra synchronizing belt that has the same upper and lower length .
In the lower figure the drive is symmetrical so I am confused which of you mean the best

[EddyCurrent]

Thanks form the prompt answer,

3. As you say, the equal belt length method looks best.
When I am saying equal length I am thinking of the extra synchronizing belt that has the same upper and lower length .
In the lower figure the drive is symmetrical so I am confused which of you mean the best

Version 2 where the motor is central.

[Boyan Silyavski]

The price of the belt and pulleys will be same as the price for one driver more. So if you invest in one more motor you would have it properly made. Not that it would not work like that with one motor.

[vargai]

Thanks your comment, because this is one of the most critical issue here. You are aware of JazzCNC's intention namely two motor can cause big problem at synchronizing failure .
Just example for calculation with overestimated values:On a 1 m long belt 1 mm elongation on a perimeter of a 360 mm pulley gives 1 degree delay- derived to the 20/10 ball screw means 10 mm/360=0,027 mm deviation-looks not bad
But this is just theory again-reported data overrides the best theory.
I am planning to use 200-300 W servo here and master slave drive is quite expensive.
I saw you used two motors on your just made machine-I think steppers that gives torque protection but with potential step loss during operation
You know the target is 1000 N cutting force.
I need to see about this issue in the threads and your opinion is appreciated in the belt versus dual motor question.
Br
István

[Boyan Silyavski]

Hi, calculations are one thing, real life another. I have belt driven machine on all axis. So i believe i know something about belts. The only thing positive in this design is that the belt is perpendicular to the ball screw, means its not driving directly the machine but transferring the movement.

What size type belt and pulleys?

How heavy is the gantry with all motors, rails and so? Not sure if you could drive it with one 300w servo at all.

I know Dean /JazzCNC/ is driving his machine like this, but you have to make sure about the proper belt size, corresponding motor and make sure that the accumulated inertia of the pulleys and ball screws will not limit the machine speed.

And looking at 0.05mm means tthat you must strive at 0.01mm. That means cheap chinese pulleys will NOT work. Have that in mind. And if you need to lower the inertia you must buy expensive aluminum pulleys or custom made that small size. I have 20 crap pulleys laying around cause i thought they were a deal from aliexpress.

[Boyan Silyavski]

PS. It would be better if the spindle center is inside the bearings that move the gantry looked from side. Means you should offset a bit more the gantry to the back. You have to imagine that when all is mounted and weights what it weights the center of the weight must be in the center between the 2 bearing that move the gantry.

[vargai]

belt is common and useful item (coupling) between motors and ball screw reducing the dynamics and revolution with effect of the inertia -I saw it in the Hurco VMC as well that a had worked on earlier.
Short belt is accurate:long synchronizing belt -I do not know . The only thing is sure - producer gives 0,1-0,2 mm accuracy on a belt driven linear unit on 1 m long.(so the elongation is the accuracy)

You are right double ball screw means double inertia and its effect to the required power fourfold.
That is why I am here to overcome this type of doubt , so thank you to share your experience

[JAZZCNC]

So if you invest in one more motor you would have it properly made. Not that it would not work like that with one motor.

It is properly made and it actually works better than twin motors being more accurate and reliable if done correctly.
My machine use this setup and I'm just going to upgrade and replace worn components and seen has this means new custom length belts and new gantry setup etc then it would be perfect opertunity to change to twin motor setup if it was better.? . . I can tell you now the thought never entered my head and I wouldn't entertain swapping to twin motors unless I could use servo's or Hybrid servo system with encoder feedback.

This doesn't mean Twin motor setup is bad or worse because it isn't if setup and run well within the motors speed curve. What it does offer or mean is piece of mind that you never loose sync or have racking issues. No chance of damage to screws if one motor stalls at high feeds etc. Easy to setup and square gantry and never changes after belts have settled down.
Less electronics to deal with etc and while more mechanical items they are very low maintenance and easily available.

There are very few negatives but with some Big positives and if I had any negs it would be belts look messy with more initial work involved and need covering but other than that nothing really and when settled down after a few weeks running they are fit and forget other than yearly maintenance check.

[Boyan Silyavski]

What i meant was, lets say it in other words- that if you don't have a machine shop at home or access to one, it seems more easy to do it with 2 motors, cause no money will be saved buying all that stuff from internet, postage, custom bores and so, plus later the very careful alignment. It could pay off if 1 servo instead of 2, money wise, but like you say Dean, servo is the good way to go with 2 motors. So at the end is just a personal preference. I simply don't like that long belts. In reality my small machine works in a lot of dust and never had any problem with the belts.