Backlash Blues

by Ivan Hamilton 4/16/2008 10:59:00 AM

I've got my mill now, and I've had a little play with it.

One of the things I've noticed is the amount of play in the hand-wheels before they move the table. After turning in one direction, the hand-wheel will move back an indicated 0.2mm before they engage again. This means if you're using the markings on the wheels alone, you may wobble about by 0.2mm.

I took a quick measure of the change in distance between the bearing block and the wheel mount during a left to right transition: 0.1mm. The details of how the screw is mounted to the table is hidden a little, so I disassembled it to get a good look.

Screw (with pin), first thrust bearing, the bearing mount, second thrust bearing and hand-wheel mount

It's pretty easy to see where some slack in the system comes from... there is nothing holding this assembly together tightly. It relies on the placement of the pin and pin bores (it's important to keep in mind here that this is a economy Chinese machine, and not high precision equipment).

With the screw released from the table, I measured the movement of the screw against its "nut" on the saddle: 0.1mm.

0.1mm in the nut + 0.1mm in the bearing mount = 0.2mm total.

At the moment I'm not looking to eliminate this movement. I'm just looking to identify it... I just had a wild realization. In talking fractions of a millimeter (mm), it makes sense to use the next prefix - micrometer (µm). I've never dealt with such little units before. 0.2mm=200µm. Microns baby... microns. We're talking about two and a half times the width of an average human hair.

Maybe I'll just shim it with some average human hair...

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Mechanics | Pragmatism

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Name of author Ivan Hamilton
"My inner nerd can beat up your inner nerd."

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