Accurate Strong Gantry

**Boyan Silyavski** · 27-11-2014

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** · 27-11-2014

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** · 27-11-2014

understood what you mean, just wanted to reach the perimeter of the machine table with the tool to have access for edge drilling that is advantage at high work piece -but this is not main issue.
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.
I am using 25 mm bearings that is IMO strong enough for normal operation, however I will consider your suggestion-every detail can be important.
Perhaps it will help to support the rail side form above against the constant torque and lowering the console lever according to your suggestion

**Boyan Silyavski** · 27-11-2014

Originally Posted by vargai

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** · 02-12-2014

Hi,
I would like to know where is limit in weight with the moving gantry that can be drive on reasonable way.
My redesigned one is still 100 kg made of steel with a 18 kg motor just for cheking.
I don't think I can go below 80-90 kg.
I don't care if it requires 20-50 W more just pls advice if this is over sized for a general strong gantry
Thanks

**vargai** · 27-11-2014

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** · 27-11-2014

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** · 27-11-2014

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

**Boyan Silyavski** · 27-11-2014

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.