
Gee Five Zero: Spindle Speed Limiter
G50 is used on most* Fanuc controls to provide a maximum permissible spindle speed when you have set the machine to run by “Fixed surface speed” rather than by RPM (Revolutions Per Minute)
Syntax:
G50 ALWAYS followed by an S value that specifies the max speed (RPM)
G50 Is A Modal Command
Description:
If you are running a job with a surface speed control rather than an RPM control the spindle speed is calculated by the amount of the circumference that passes under the cutting tip at any given diameter. Using surface speed means calculating the surface *passing the tool based on it’s diameter.
This is achieved by multiplying the diameter of the work piece by an average value of 3.15 to get the circumference, then multiplying that figure by the RPM to get the surface speed in meters per minute (the common denomination for metal turning)
Example. A 25mm diameter bar doing 750 RPM is:
((25 x 3.15) X 750) = 59.06 meters per minute surface speed. That’s fine if you are working at one diameter and know you want 59 MPM but if you are facing across the face of the work in the X axis that working diameter is going to get smaller and smaller as the tool moves. So you start at 25mm diameter and at 59 MPM the spindle speed will be 750RPM, as the tool moves towards the centre that’s going to increase. At 15mm working diameter at 59MPM the spindle speed is going to be 1,248 RPM. Ok so 1,248 RPM isn’t too scary but as that tool moves towards the centre that’s going to get silly. So we get to 4mm diameter as we cut across the face and that spindle speed at 59 MPM is going to be 4,682 RPM, that’s getting a tad arse twitchy. As the tool moves onwards (with all the commitment of my cat looking for the extra dreamies he knows I have) you end up at 0.1mm diameter with the control telling the spindle it needs to do 18,585 RPM. While there are some CNC machines that will do that easily the same principle is going to apply if you start at a 300mm diameter 400mm long billet that’s going to be a 220KG chuck of trouble that tries to do whatever it can to reach the 18 thousand or so RPM the constant surface speed code is telling it to do. in the real world, without a limit when the axis gets to a near zero diameter the machine is going to be running at the maximum the motor system can do.
No matter how hard you have clamped that 220kg billet in your new shiny £10,000 Kitagawa chuck that’s going to come out, it’s going to go through the machine door like a momentary inconvenience and then proceed to turn YOU into a paint stain on the floor before heading off into the shop like a deranged serial killer to take out anybody or anything in it’s way. The last thought through what’s left of your head before you die will be G50…
So this may sound a little flippant, maybe it is, however I’ve seen somebody killed by a machine first hand and it’s not something you forget in a big hurry if ever.
So G50..if you run surface speed then ALWAYS ALWAYS use an S value afterwards, if 2,000 RPM will be safe then set you S value to 1,800 as below. With time and experience you can tweak this but NEVER exceed a safe value for S.
G50 S1800