Re: ball screw preferences

>In CCED, "Mark Vaughan" <mark@...> wrote:
>I never really looked at any specs., and really the price is the
>issue here, the cheap ones for routers on ebay I suppose could not
>be that good.

Key point about ballscrews: The MOTION industry is FAR larger than
the CNC industry. And there are MANY application in the MOTION
industry where a ballscrew's low friction/high efficiency is the
desired result, NOT its repeatable positioning capability!

What this means for us pursuing low-cost CNC solutions is that we
are often purchasing ballscrews made for "actuation" rather than
precise positioning. Many of these will have undue cyclical errors
and even pitch variations which get in the way of a DIY anti-
backlash approach. BUT...

As I've written before in this thread and MANY other times:
Fortunately MOST of us do NOT need the kind of positioning precision
we THINK we do to make acceptable parts for our projects and
customers.

>I thought adding oversize balls was more to compensate for wear
>than tighten a loose screw.

Oversize balls is a "high-end" preload technique that some of us
have found can work well in the DIY world using "cheap" ballscrews.
Since it IS a high-end technique, many have not been aware of it.
(it's also one of those I mentioned before which aren't immediately
obvious upon visual inspection.)

>I also thought since small balls have a smaller clearance figure,
>that that meant higher accuracy. Most screw I have stripped in the
>past had ¼ inch balls or thereabouts, the one on the machine I
>stripped are like those you'd find in a bicycle, about 1.5mm, I did
>measure them with a mic to check none were oversized.

See that motion industry vs. CNC comment above... (ballscrews are
used for load-carrying ability AND/OR positioning. Suitability for
the first does NOT imply capability in the other...)

BTW, It's good that you checked the balls carefully. Because another
high-end technique is to use alternating "small" and "big" balls to
ensure their proper ROLLING in the ballscrew and nut channels. The
reduced load carrying of this is offset by multiple ballraces;
making your described ballnuts candidates...

>Do you know any trick for tightening up the end backlash, seems
>that's where all the play is now.

Most high quality end mounts have provision for adjusting play. Or
at least they used to... This can be precision ground spacers
between the bearings(in lower end methods, look for shims); or any
of a number of other things.

In newer machines; there is a trend to reduce costs--even for what
most would consider "expensive" machines--in ways that weren't
formerly used. For instance you see fewer spaced bearing pairs (what
has traditionally been called a "rigid" mount) and more ground face
matched bearing sets (traditionally this is called a "fixed" bearing
mount). In these ground face mounts, you may have to either replace
the bearings (which can be expensive!) or get creative with shims,
etc.
(The high cost of the matched bearings is less overall for the
machine mfr. than the machining and "adjusting" required of top-end
rigid mounting.) Better materials and machining have allowed this
trend towards "simpler". Are they "better"? Fun to think about that.

Hope this helps,

Ballendo