Smooth Haas CNC Tool Changes with an Accumulator Tank

Haas specifications for CNC machines with 40 taper spindles typically state 4.0 SCFM required at 100 psi. Additionally, 3/8” air hose and air couplers are required. Typically, machines with air-oil spindles will require a steady draw of air, while machines with sealed bearing spindles will intermittently draw air. Although the machine air requirement may average to 4.0 SCFM, the tool release piston and blow gun likely can draw significantly more air during intermittent use. Consider that a typical plastic blow gun can draw over 20 SCFM if supplied at 100 psi. We have not collected any data to contest these flowrates, but we intend to discuss how to best plumb new machines for the best performance.
 
We have seen many poorly plumbed machines in the field and heard many diagnoses for poorly operating tool changers that all overlook air supply problems. Tool changers make excessive noise, tools appear to stick in the spindle, the tool changer platter deflects excessively, and diagnoses focus on the spindle taper itself. We by no means intend to solve every instance of these issues with air supply considerations as there are certainly other failure modes, but we know the repeatability and reliability of Haas tool changers can be improved when plumbed properly.
 
A common error occurs when machine owners review their compressor specification, conclude its output exceeds the required 4.0 SCFM, and connect the machine to their air system. There is still no certainty that 4.0 SCFM is actually available at the inlet to the machine air manifold given the plumbing between the compressor and the machine. Using common ¼” air couplings at the compressor, machine, or anywhere in the system can add unnecessary restriction. Excessive hose or piping between the compressor and machine can cause unnecessary restriction leading to less than the required airflow at the machine. Even more commonly, a regulator is placed near the air supply to maintain the recommended 100 psi, but the regulator is often not capable of responding to surges in demand from tool changes and blow gun usage. Often ¼” NPT regulators are used with ¼” couplings directly into the machine, and these certainly should be considered insufficient for any Haas tool changer. The rated pressure, flowrate, or port size of the regulator is almost irrelevant since most regulators simply fail to respond in the time of the tool release piston actuation. 

In this installation, the air compressor cycles between 90 psi and 120 psi, and we choose to regulate the machine air supplies to 90 psi to limit influence from the compressor tank pressure. Using a properly sized 3/8” regulator, hose, and coupling, we have found reasonable success operating umbrella style tool changers. However, significant air consumption during tool changes causes pressure surges when the regulator fails to open quick enough to meet the demand. To ensure 90 psi is available during the entire tool change, we have added an accumulator tank after the regulator. The accumulator tank is fashioned from a portable air tank and plumbed directly into the machine air inlet with 3/8” air couplings and a 3/8” full port safety shutoff valve. The safety prevents discharge if the supply line is removed from the regulator or if other elements are removed from the air system. A T-fitting also allows the 3/8” regulator to backfill the accumulator following large surges in demand. Since large surges in demand are fulfilled by the accumulator, a ¼” coupling on the regulator is found to be more than sufficient for this Haas mill with umbrella tool changer and grease packed spindle. ​

Although even more restrictions could be removed from this installation, the tool changer is found to have minimal impact on the pressure available in the machine air manifold. We have found this configuration to provide consistent smooth tool changing operation. Note that the accumulator tank pressure is maintained below the compressor cycle pressure, so the compressor on-time is not extended by the compressor needing to pressurize the accumulator tanks every cycle. Of course, if the air demand exceeds the compressor output, the compressor will be forced to recover the entire system including the accumulator.
 
Similar performance may be achieved in a facility with large diameter air supply lines regulated to 100 psi. In this case, the large supply lines would act as the accumulator assuming the air consumption was relatively small compared to the capacity available in the lines. Full size 3/8” supply hose and couplings would still be required to avoid unnecessary restrictions. However, significantly large supply lines would be required to offer similar surge capacity to a 5 or 11 gallon accumulator tank. Consider that typical ¾” pipe has ~0.0229 gallons per foot. Neglecting restrictions, 218 feet of ¾” pipe would be required to match the capacity of a 5 gallon accumulator tank, so significantly larger pipe should be considered