Does C7 Ballscrew + Linear Scales = "C3" Ballscrew?

[floridamachineshop]

I'm rebuilding an entire VF2 from scratch and upgrading it to 5-axis.
I want to put money where it will do the most good. (Every area where I spend less, is an opportunity to spend more elsewhere.)
Of course 3 brand new C5 or even C3 ground ballscrews would be best.
I'm running Aikron linear encoders on every axis (around 0.0002 accuracy if I remember correctly).
Provided that a rolled C7 ballscrew has sufficient preload... how bad would it be to rely upon the feedback going directly from the Aikron linear encoders to the Hickory control to get the kind of linear accuracy I'd get from a C5 or C3 ground ballscrew?

I suppose this means that the Hickory control would have to "work harder" to position X, Y, and Z, because of the greater discrepancy between motor position vs table/spindle head position, resulting from a rolled C7 ballscrew's lower pitch accuracy instead of a C5 or C3 that's ground.

If I throw a budget ballscrew at a Hickory control, but give the Hickory control direct linear encoder feedback thinking it'll "make up for the ballscrew being a rolled C7", is this a bad idea? I've already purchased all the scales and the Hickory encoder expansion board some time ago, so the scales are going in regardless.

[spikee]

Somebody once told me that kern just runs c7 ballscrews and fixes the issues with comp/scales.

[suntravel]

Somebody once told me that kern just runs c7 ballscrews and fixes the issues with comp/scales.

nope, they are using very high quality ground ball screws with a defined preload.

Uwe

[spikee]

The guy worked at Kern . When it comes to the screw itself, it's either C7 or C5. Still, they're high-quality screws. The C rating mainly refers to the non-linearity error, which can be mapped and calibrated for accuracy. Of course, you still need proper preload in your screw.

In lithography, the actuator of the stage itself not not particularly high in mechanical accuracy, The accuracy all comes from good feedback using interferometers on the high end and perhaps still encoders for low-end applications.

Laser compensation of a machine is the same as doing it with a fixed encoder in your system. You map out the nonlinearity errors and compensate them in software. This is a very common thing to do as a performance optimization aspect of a CNC machine.

[suntravel]

Actually the Pyramid is with a hydrostatic screw drive...

https://cdn.prod.website-files.com/60db ... nallow.pdf

Uwe

[spikee]

Actually the Pyramid is with a hydrostatic screw drive...

https://cdn.prod.website-files.com/60db ... nallow.pdf

Uwe

The micro does not

[suntravel]

Actually the Pyramid is with a hydrostatic screw drive...

https://cdn.prod.website-files.com/60db ... nallow.pdf

Uwe

The micro does not

Because it is with linear direct drives...

https://cdn.prod.website-files.com/60db ... imiert.pdf

Uwe

[suntravel]

Micro Vario is with ball screws

https://cdn.prod.website-files.com/60db ... rio_EN.pdf

With rolled C7 screws it is not as good keeping the same preload over the travel than with precision ground screws...

Uwe

[suntravel]

Back to topic, yes you can improve C7 screws with a fine scale for better precision. That is what I do on my hobby machines on a budget.

But a precision ground C3 screw with better scales will outperform this cheap setup in surface finish, precision and long lifetime.

Uwe

[spikee]

Preload is the big thing here. You want repeatability in this sense, not accuracy. he accuracy you can solve with scales etc.