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BACKLASH AND LEAD ACCURACY

What is backlash and why does it matter? What about lead accuracy? How does it affect me and my machine? Or in other words... "In search of 'perfection'"

Also: Ballscrew and ACME Leadscrew Accuracy Charts

1. PREFACE
2. INTRODUCTION
3. BACKLASH IN A MANUAL WORLD
4. CNC TO THE RESCUE?
5. EXPECTATION
6. A PERFECT CIRCLE
7. OTHER DESIGN CONSIDERATIONS AND THE PROS AND CONS
8. CONCLUSION
9. RESOURCES

CNC TO THE RESCUE?

What about a CNC (or computer(ized) numerical(ly) control(led)) machine? Aren't the backlash problems solved by CNC?

Not really, and the end result gets worse! And here's why:

To keep things simple, we won't discuss open and closed loop systems here. Let's just assume that we have a computer that tells the motor how to turn to move the table. There won't be any feedback to the computer to make adjustments for error, etc.. or "backlash compensation" techniques in the controller software of G-Code programming - and this makes up our CNC machine:

1.
COMPUTER > MOTOR > TABLE/MACHINE

Now let's use an example that will better show how backlash affects CNC systems.

2.
Computer says move 1" in the X direction and the > Motor turns the corresponding turns to move the > Table in the X direction by 1"

Now before I throw in the term "lead accuracy" or "lead error" let's do one more thing. Let's also make the table mentioned above to move BACK to its original position (we'll call this X0 - btw, we started at X0).

3.
Computer says move 1" in the X direction and the > Motor turns the corresponding number of rotations to move the > Table in the linear X direction by 1" (this should bring us to X1)

then the

Computer says move -1" in the X direction and the > Motor turns the corresponding number of rotations to move the > Table in the X direction by -1" (this should bring us back to X0)

With examples 2 and 3 we can talk about BOTH backlash and that term "lead accuracy" or "lead error".

So we've converted a machine to CNC with the screws and nuts that came with it. We don't know anything about where it came from, how accurate it is, etc.. and you run the machine. What happens?

Sticking with the backlash example as shown in 3, you would think that the table would go back to its original position of X0 right? Nope. remember, there's backlash! When the screw was reversed in order to go the opposite direction, it had to spin around enough to positively engage the nut before the table would move. Assuming that the screw is 100% accurate (i.e. no lead errors), the machine should return back to some position in between X0 and X1 (hopefully you're closer to X0 than X1 otherwise your machine probably does have serious issues!).

Let's say you fix this by getting an "anti-backlash" nut that delivers 0 backlash and you can get back to X0. Great!

But wait! There's more!

Now let's bring in "lead accuracy" aka "lead error" into the picture.

You noticed something else strange. In example 2 you told the table to move 1" to X1 but your trusty DRO tells you otherwise. Let's say the actual move was to X0.996" (i.e. you're just shy of 1" by four thousandths of an inch - 0.004").

You've just experienced "lead accuracy" aka "lead error". This is the amount of allowable axial error of a given rated screw whether they're ballscrews or leadscrews (we'll talk about this in a bit as well).

Thinking that you can solve your problems, you go out and buy any ballscrew/ballnut out of a catalog because you've heard that "they're more accurate".

You retrofit your machine to accommodate the new ballscrew and ballnut and realize that you still have backlash AND errors in travel. Why? And imagine when to your amazement, you experience positional errors that are even more obscure and unexpected that what you had previously seen. In more complex moves, you basically add up lead errors with backlash and you can get some interesting results at different points..