Avoiding Crashes with the Right CNC Spindle Direction
Spindle direction on lathes is often referred to as M03 for clockwise (CW) and M04 for counterclockwise (CCW). Of course, this depends on the location from which you are viewing the spindle. Standing at the wrong end of the machine could cause you to inaccurately label the direction. A simpler, more accurate method is to think in terms of forward and reverse. When standing in front of the machine, this refers to whether the part is turning towards or away from you. M03 can be called forward rotation and M04 can be called reverse rotation.
Most machines today are tooled for operating exclusively in the M03 spindle direction. Let’s look at why it may be more useful to machine parts using the M04 direction. Selecting the M04 command allows you to utilize left-hand turret tooling. Left-hand tooling is an optimum method for two reasons:
1. In left-hand turret tooling, the insert faces the operator. Not only is it easier for the operator to watch the progress of the cutting process, but exchanging the insert becomes much quicker.
2. When machining in the M04 direction, the cutting forces (compression) are pushed into the machine bed. With the M03 direction, the cutting forces (tension) are being pulled away from the machine bed. A machine’s structural design is stronger in compression than in tension because the machine forces are directed into the structure of the machine and down towards the ground. If there is a loss of control of the cutting forces, the energy will be directed into the machine and ground instead of into the tooling.
So what kind of situations could have been prevented if you adjusted for the M04 spindle direction? We’ve pulled some examples from our customers to illustrate the conditions that would lead to a crash. Have any of these issues happened to your team?
Crash #1: The operator had not bored soft jaws deep enough to withstand the cutting forces exerted by the part program. The part pulled out of the jaws and became trapped between the turret face and the chuck jaws. The resulting forces lifted the complete turret casting against the way keeper plates. The turret casting cracked under the 40hp forces. The repair bill was approximately $35,000 and the machine was out of service for nine weeks.
Crash #2: The CNC machine, equipped with a Fanuc control, set the machine datum point by using the G54 command. The operator simply forgot to set the correct work-shift value. To compound this mistake, the first part ran in full rapid traverse mode of 800ipm. The machine spindle was running at 1550rpm in M04 spindle direction. The machine registered that the front of the chuck was the work zero datum point. The turret crashed into the chuck jaws, knocking them completely from the machine. No injuries occurred, but this was a severe collision. As a result, the dowel pins in the turret broke and needed to be replaced. The main spindle cartridge needed realignment as it was knocked from true centerline. These damages, repaired for approximately $1,200, caused the machine to be out of service for five days.
There are many variables in these two crashes, but the simple choice of operating in M04 saved Crash #2 from being a very expensive and possibly dangerous accident.
