Accurate Strong Gantry

[Boyan Silyavski]

When I think of a standard VMC the real heavy consoled spindle house running up and down on a vertical rails with about 400 mm or more offset and the bearings stands it- it is their job.

Yeah, but with sliding counter weight at the back to balance the weight of the head so at the end the bearings have less push when static than a gantry cnc

[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

[Boyan Silyavski]

According to the charts HTD 5M is well in the charts for what you will use it for. I would say 25mm wide and steel corded. I wouldn't go with other braids cause Kevlar and so stretches. Pulleys should be 20 - 22 teeth. Make the ball screws placement and design so that they are only long for the exact necessary travel. Dont overextend them cause its easier to mount or for other reason. Pulleys should have fitted bronze bushing and the bolts/shafts should be proper ones. Shoulder bolts if you use bolts and steel shafts, don't leave them to rotate without bearing in between.

PS, some additional info:

[vargai]

I am a fan of taper locks or similar shaft lock- simple fast and not expensive - is it good without parallel key?

[Boyan Silyavski]

Then you will need dial indicator and mess around just to center it properly on the shaft. Cause they are not self centering to the needed tolerance. Plus are heavy. Best would be tight fit and you or somebody localy to machine you the pulleys from aluminum.

[vargai]

thanks, already I can go further with design
BR
István

[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.

[JAZZCNC]

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.

Yes Boyan it really does boil down to preference and components used not really so much money. My machine doesn't use belts based on money it's soley down to accurecy and repeatabilty/reliabilty within a budget based on using a stepper system.
Any machine using closed loop or some form of error catching setup Ie Servo's/encoders then I wouldn't use single motor and long belts etc.

Regards belt accuracy etc then to any belt/pulley doubters out there then just see this made with a 1300mm wide router with medium weight gantry.!

[vargai]

Silyavski, Dean,

Thanks for the comments.
This is the case when everybody has right.The proof of the pudding is in the eating. Dean likes his pudding and you like yours.
I will do my one and will taste it.
I am going to install two screws anyway and link them with belt-it seems more simple now than two servos-later it can be upgraded