Re: Ball Screw Quality?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bill Blackburn"
<blckburn@b...> wrote:

> I see that most Ball Screws manufactures are offering different

quality

> of Ball Screws.
> Looking in the Nook Catalog I see three different choices. 0.004
> 0.001 0.0005
> The price jumps considerably when to step up to the next best

screws.

> What are most of you guys using for your Milling machines for CNC

use?

>
> I am planning to use preloaded Ball Nuts.
>
> Regards
> Bill

Hi Bill, there are 2 different criterias to consider when choosing
the quality of the ballscrew.

First the deviation of pitch per foot.
The value given will be a worst case sample, say you have
3 foot of 0.004" quality screw and travel messures 36.012"
over its travel distance.
You could compensate for this amount in the settings of your
controller software and not have any deviation at all.
It is not likely that you will end up with a perfect pitch error
because it will probably vary somewhat inbetween but it should
be clear that you will be able to eliminate a great deal of
error this way.

Second Backlash free operation has 2 basic methods one is the
factory Backlash free single BallNut wich is expensive and
requires the Higher quality ground screws.
The other methode is the use of two Ballnuts wich are spaced
apart by a Antibacklash mechanism.
This mechanism has two functions the first one is to space
the nuts apart in a rigid manner to remove the backlash solidly.
This can only be done to the point of the tightest area along
the travel distance of the screw.
Because we also need to take the pitch error into acount there
will likly be areas within the travel that still have a minor
amount of play.
This is what will be taken up using Preload, it is simply a
spring that is between the nuts and will allow them to expand
even further apart.
The spring used will have a load rating that is a fraction of
the overall load rating of the nut.
This spring type preloading will of course be overcome whenever
there are cutting forces involved that overpower the spring.
This means you would want as little as possible be taken up with
this methode, a screw with less pitch error would ensure this.

With a little consideration I belive the cheaper solutions can
be made to work just fine for a hobby kind of machine.

Good Luck