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FP4NC Graphics Repair


Dec 13, 2002
Benicia California USA
So perhaps "repair" is a bit misleading as applied to the control graphics.
So one of the nice features of the Dialog4 control is its verification graphics. By current standards its pretty crude.
Can view the cutter path in any single plane. (product of any two axis)...Its pretty nice to have when you are about to push the cycle start button on a fresh program on an unobtainable part.....

At any rate about a year back (might be more, time has a way of compressing for me) i rebuilt the "Z" axis servo including new bearings throughout....
The servos on the FP-NC's (Siemens) are fitted with an internal brake. This is a permanent magnet setup that has a circular magnet that holds against a disc. The magnet is fixed to the case of the servo and the disc is fixed (sort of, has springy coupling to allow the disc some axial compliance). There is also a coil with the magnet.
Under normal power off state the magnet grips the disc and prevents rotation of the servo.
When powered, the magnet coil gets 24vdc applied and that cancels the magnet and releases the brake...This happens when the machine is under servo control, or when the handwheels are in play.

For what ever reason the magnet on my "Z" servo has become weak...In fact the "Z" axis drops under its own weight (ball screws will do this) when the control is powered down...Generally this is not an issue.

Well on the Dialog 4 control, when you switch to graphics mode to plot a program, the servos are powered down, and the electronics used to normally run the servos, are used to drive the graphics display.
Enter my problem......
When my machine went to graphics, the weak servo break (Z) allowed the Z axis to droop...This motion is detected by the control through the direct reading Heidenhain scales...Causing an immediate
"E" stop....Frustrating!

I did devise a "work around" by blocking the "Z" slide before going to the graphics mode, but it was a real pain and not a proper solution....

Enter E-Bay. Some time back there were some FP4NC parts that came up on e-bay, and to my good fortune there was a complete ":Z" axis servo.
This was a shot in the dark. No way to evaluate the condition of the servo, and i took the leap.

Well turns out that after a full inspection, the (new to me) servo was in excellent condition....Brake seems much stronger than on the original servo.
Made the swap yesterday, and i am glad to report that the new unit works perfectly.....Axis held without any sag when power is turned off.
A little time spent tuning the Tacho and drift and i am back running with graphics running restored (no E-stop) and no slide sag when powered off...
Cheers Ross
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Hot Rolled
Jun 22, 2015
Denver, CO
Nice work and thanks for sharing. Had the same Z axis slip on a Siemens DC servo during startup on my Hermle. Had to rebuild the Z axis card to get it to function correctly (just recapped the entire board). Had to block it from underneath for about 2 months. That got really old really quick.