How do you determine and measure when you have backlash?

[HMF]

Is this correct or an oversimplification?:


"You turn the handle in one direction causing the carriage or table to move, zero the dial on the shaft, turn it in the other direction until the table just starts to move, read the dial, thats the backlash."


:tiphat: Nelson

 

[Tony Wells]

That is correct, and unavoidable completely. You can minimize it in some cases, to a degree, but it represents the clearance in the screw/nut interface. It has to be there or there would be no moving the table. The problem usually lies in the fact that the screw wears mostly in the area of most common use, so any adjustments that are made (provided there are any) make it too tight in less used areas, like near the ends of travel. In that case, it points to a worn leadscrew. Most of us just learn to live with it, until a new screw can be made or acquired. Ball screws can and usually do have less backlash, and last longer. CNC machines will use them, and also within the software have compensation ability to accommodate it.

 

[Tom Griffin]

Backlash is simply the gap between the threads in the nut and the threads on the leadscrew, generally .010"-.015". Unless you are using ball leadscrews in your machines, ALL of them have backlash and this needs to accounted for. When using any machine with a standard leadscrew, make sure to always remove the backlash by feeding in the same direction. On the mill this means feeding against the force of the cutter so it doesn't pull itself into the part and cut too deep, or when moving to a specific position as in drilling, always feeding in the same direction. It also helps to lock the axis of the table not being used just to be safe.

On the lathe, rotate the dial clockwise when turning and external diameter and counter-clockwise when boring an internal diameter, assuming the spindle is not running in reverse.

Those of us lucky enough to have digital readouts on our mills don't need to worry about backlash when moving to a position but we still need to remember to feed against the force of the cutter and/or lock the table to keep bad things from happening, i.e. the part being cut undersize, the cutter grabbing and breaking, or the part being ripped out of the vise and tossed at you (generally a bad thing).

Tom

 

[HMF]

So when feeding, do you move the dial until the carriage starts to engage for movement, and start your measurements there,
to avoid the inaccuracies of backlash? How do you know when to zero out your dial? Do you watch for movement, or do it
just before any movement occurs? Does it really matter, since everything is relative?


:tiphat:Nelson

 

[Tony Wells]

Well, the thing is to never get where you have to know exactly what the backlash is. Say you have a couple of holes on a piece, one 0.0500 from the edge, and the other 1.0000". Once you establish your edge as zero, with an edge finder, zero the DRO or indicator you are using. Now hopefully you were moving into the piece when you found the edge. So now all the backlash is gone, as you were moving in that direction. Simply keep moving to the first hole. Should you over-shoot it, just back up until you know the table moved, and approach the hole location again. Since you are moving in the same direction you were when you found the edge, backlash is not an issue. Now when you move to the second hole, you still should be moving in the same direction. As long as you don't over-shoot the location, you're good. If you do, same deal, just back up and come at it again. NEVER just dial back in the opposite direction and count on knowing how much backlash you have. It will vary all along the lead screw, and you can't keep up with it. Always work in such a direction so it doesn't affect your positioning.

 

[Tom Griffin]

A trial cut also qualifies as finding the edge or surface in milling or lathe work. Simply take a cut, zero the dial and measure to determine how much more material needs to be removed. As Tony pointed out, if you overshoot, back up far enough to remove the backlash and then move to your target position.

Tom