88hs random incorrect spindle speeds

Hello My name is Danny Drinan,

I'm having this randomly occurring problem with my Fadal 88hs, When I set a spindle speed using the M3S command sometimes it will randomly give me a spindle speed way less than what I input such as ( M3S1500 outputting only 200 rpm ) It usually doesn't matter what speed RPM you input , once it starts doing it you will continue to get the same speed no matter what until you return to home position and restart the machine. It does it maybe once every three times or so when you use the machine during the day. My spindle speed adjustment knob is always set to 100% and i only really use it during the machining process. Does anyone have any ideas?

You don't mention which machine you have or the year model. It sure helps determine what system you have.
When you say it's only outputting 200 RPM, what does that mean? The screen says 200 RPM or you measured it with a photo tach and it's 200 RPM or what exactly?
While I'm asking questions, which VFD do you have and does it have a keypad and display? If so, then it might also have some information on it.

Could be belts slipping, not changing gears (belt ratios), bad command signal to the VFD. Hard to tell at this point.

Well I'm not entirely sure on the year, The books I got with the machine when I bought it are dated at 1995 so I assume that's the year? the side of the machine just says Fadal CNC 88hs, I attached some images of the machine to this thread below. It actually says 140 rpm on the screen after I've inputted M3S1500. Additionally I forgot to mention that the exact opposite happens as well, Sometimes I input a slower M3S200 and sometimes I will get 5000 Rpm instead. Its very random.

Most Fadals have air actuated solenoids that change the pulley ratios unless it's the single belt 7500 RPM machine (if I remember correctly). The 10,000 and higher machines have a high ratio and a low ratio pulley set. When speed set it over a certain amount, 2500 RPM in my case on the 4020, it will loosen the low speed belt and tighten the high speed belt. Below 2500 RPM it will do the opposite.

You can also force this by using a .1 or.2 after the speed command in order to tell it which pulley to use. .1 is low and .2 is high.
Example: M3 S500.1 will be 500 RPM in low range. M3 S500.2 will be 500 RPM in high range.

When you do the above and toggle back and forth between the two, you can hear the air solenoid kick in and change the ratio before the spindle comes on.
Removing the 4 screws that hold the spindle head cover on will show you all the mechanism on top of the head.


My machine has a sensor that determines if the tension arm moved and alarms if it doesn't change ratios, however I have had it not alarm on me because it tried to change but never made it to the fully tensioned position. My actuators were worn out and leaking and lot's of other little issues. Mine is a high torque and has the hydraulic amplifier blocks that needed cleaning and resealed.

If you google Fadal Service Manual you will see there are many sources with all the manuals available to download. Your interested in Section 8 - Spindle Drive System.

Sorry, I forgot to mention, once you find the maintenance manual, there is section 16 with a pretty good troubleshooting section. I believe it's chart 3 that might help you.