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Z-Axis rumbling noises: as usual

Martin P

Aug 12, 2004
Germany in the middle towards the left
This is not new, but just another data point for people searching here for this issue.
My FP4AT (D11) developed really bad rumbling noises in the x-axis. It sounds like a bearing has crumbled a ball. On the AT the z-axis servo is very hard to reach, so I ignored it persistently, until no longer possible.
So I figured out how to get all the necessary sheet metal of, which involved moving the table left and right and up and down to fit sheet metall pieces under or over the enclosure. Note to AT owners: when the tool changer door is moved left using the SERVICE menu and the machine is then turned off (for example to work on the servo), the door will CLOSE automatically when turning the machine back on. This can be startling.
So I opened up the top cover of the servo and everything looked fine and clean. Moving the servo by means of digital hand wheel reveals a large wobble of the tachometer winding.
But I was not able to pull or press it off. When comparing the servo to a spare I have, I also noticed that the piece holding the brushes was installed the other way around.
So all I did was to install the holder turned 180 degree. Problem solved, smooth as silk.

TLDR: Z-axis really can be very very noisy just because of a bad tach. The Tachometer brush holder is directional (and is marked accordingly on the plastic).

Warning: Use the Service menu of the D11 with great care. It requires a password for a reason.
The tool changer sub menu MUST only be used at the reference point. Otherwise axis collision WILL happen. I know I am not the only one who had this happen. Be careful!
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For future readers who might be having this issue, also check resistance between the comm bars on the tachogenerator. I cleaned the crap out of mine with IPA and cotton swabs when I had a rumbly/rattly sounding Z and it didn't fix it. Checked resistance and found that while most were ~30 ohms, two pairs of comm bars had an almost dead short. There was a lot of carbon dust built up from brush wear over the decades. I thought some may have filled the grooves between comm bars to the point of shorting them. Made a scraper to clean between the comm bars, and then another scrub with IPA and a toothbrush and I had solved that problem. Z axis was quiet as a mouse after that. It's very easy to mistake that noise for a mechanical problem; very hard to distinguish. I could have sworn it was something mechanical.