Why is Y axis "squaring" necessary only with twin motors?

[MartinS]

Hi All

In another post regarding a new build, one of the contributors commented:

If you are using two screws and two motors you will have to use two home switches to make sure the gantry is put square every time you home.

You can of course use two screws with one motor and a belt driving both screws then one home switch will be fine.

This might be another example of "the blindingly obvious", but to me it's not......yet!


This was a moving Y gantry machine.


If the machine was well designed for maximum rigidity, and accurately built with good quality components (as best this can be achieved in "home" circumstances), why does it go "out of square" such that "squaring" is necessary?

I can only think that it might be caused by one motor missing a step.......but then why doesn't the gantry resume "squareness" naturally when the system is powered down?


But this suggests that in a single motor, twin belt drive moving gantry machine, the gantry always remains square?..........does it?


Then I wonder what is regarded as "square" or unacceptably "out of square"?

5um/m, 50um/m?


Can someone explain and put this into perspective for me please.


Many thanks

Martin

[magicniner]

If the machine is rigid enough that it can't go out of square you could drive just one side, if it's not rigid enough to drive just one side then it can go out of square and zeroing on one side may not set the machine square.
The exact figures will depend on the individual build and requirements but why wouldn't you want to make a machine be the best it can be? ;-)

- Nick

[MartinS]

-Nick

I WOULD want a machine "the best that it can be". My 40+ years of Engineering has instilled in me the importance, no, necessity of (appropriate) machine precision, accuracy and rigidity.

I am currently trying to get my head around ALL of the elements of what many you guys have been doing for that long that it is probably second nature.

So, expanding on what you have said:

If a machine is not rigid enough such that it needs twin homing squaring, do the belts in the single motor approach keep an adequate squareness?


Can anybody put um numbers on this?


- Martin

[magicniner]

Look up belt manufacturers specifications on length change versus temperature and tension and combine that with your pulley diameters and screw pitches to get a number.
Using a tensioning system which applies equally to two both upper and lower belt runs will almost eliminate errors from temperature and tension, good grade ground ball screws with pre-tensioned ball nuts will help too.

- Nick

[MartinS]

Thanks Nick

..............yes I'll do just that.

[magicniner]

You can get belts with various reinforcements, fabric, more exotic fibres and steel wires which will all perform differently with changes in temperature and tension.