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ROLLED VS. GROUND BALLSCREWS, ACCURACY CLASSES AND BALLNUTS
Before we begin, we wanted to point you to this article we wrote on backlash and accuracy. It's important to first understand these concepts since many of us confuse ballscrews with some form of accuracy and precision that tops all other screws. That's a huge misconception!
We're not trying to be picky here, but many times we've heard from many places that ballscrews are the end-all drivetrain component that makes ACME leadscrews and the look like some wallflower at a high school dance - unchosen and left lonely.
What we first need to distinguish are the different types of ballscrews and within those types, there are grades to those ballscrews that "make" it accurate or not.
To keep things simple, we'll just elaborate on the grades 0 to 10, where 0 is the most accurate (this has zero to do with backlash, remember..) and 10 being the least accurate. There are higher grades, but we won't go there since most of us will never see one or be able to afford them if seen. There are the ISO/DIN and JIS standards of accuracy which basically overlap with grades C1, C3, C5, C7, and C10.
As mentioned in the backlash article, accuracy of the screw deals with how much actual travel delta there is for a given distance. Many times we'll see in the imperial world something like +/- 0.003"/12" which means that actual travel may deviate by as much as .003" for each distance of 12" travelled.
Now let's look at another misconception that has its merits and tends to be true, but stands for clarification. Rolled versus ground ballscrews - what are they? Well, rolled ballscrews are the cheaper bretheren of the ground ballscrews. The rolled are the ones that tend to sit in the lower accuracy grades while the ground tend to sit at the higher accuracy grades. While there are ground ballscrews that are grade C5 or even C7, there are rolled ballscrews that are C7 as well.
Before you start buying ground ballscrews it's also important to look at, in the situation of designing your machine for example, how much actual travel there will be. If your converted mini-mill is also goint to get the ballscrew treatment, you could get by with the rolled variety since the longest travel you're going to get is about 9" or so. So to guesstimate, you're probably looking at a 0.001"-0.002" lead accuracy. Not bad at all. What you should be more concerned about is backlash - you know, the thing that turns circles into eggs..
By looking at the picture at the top of the page you see a ground ballscrew that's been preloaded with two ballnuts as well as compared to the one at the bottom which is a rolled ballscrew with a standard nut. This is a major contrast since the ground also happens to be a grade C1 whereas the rolled is a C10 (or maybe that's C7 - we'll need to check).
For many of us starting out in CNC, we're thrown mixed messages and a slew of lingo we have to decipher ourselves. Our goal here is to show you some of the "light" so that you can make your own educated decisions. Stay tuned for more on this topic.