Five machine design features that matter and how they affect a part’s surface finish, accuracy, repeatability and cycle time.
Double Column Design- Provides increased rigidity for accuracy and repeatability during milling, enabling higher speeds, better finish, and a larger work area.
Single column characteristics- Column is affected by leveling. When the column moves, the Z axis geometry changes causing accuracy and straightness problems with machined parts.
The table stacked on the y axis extends its travel past the areas of support resulting in pitch. The column can also move due to axis inertia and tool pressure. The table weight changes based on the part size and may need to be tuned or slowed down.
The double column is a robust, single piece casting with no weldments providing excellent support for a fast moving, cross-rail mounted headstock. The mounting of the axis to this column design eliminates pitch. The column geometry cannot easily be changed by leveling, and the increased mass dampens vibration of cutting.
Cross rails installed in a “ladder design” for greater rigidity, to eliminate head deflection during milling. The cross rail carries a constant weight, for consistent operation. The Z axis geometry is tighter and straightness will not wander.
The use of the double column design increases the available work envelope for larger part sizes. It also decreases the overall footprint because the table can travel under the cross rail. 35% increase for 40”/45% increase for 50”/60% increase for 60”
It also allows the mounting of the ATC outside of the work envelope saving valuable space in the work area. A door that protects the tools changer magazine from coolant and chips is good for this design, since the ATC hangs lower. 30 Tool ATC, Random pocket, Big tool function standard.
Thermal Stability- This low mass spindle head mounted to the solid base and column is best to maximize the latest in material removal strategies (Dynamic, Vortex, etc.) with speed, precision and thermal stability for ultimate part finish.
A ladder design mounting reduces the distance of the spindle centerline from the supporting structure (overhang) for increased rigidity. A direct coupled style motor can reduced the mass of the head which limits the thermal growth potential and increases the speed capability of the axis.
Machines should have an oil type chiller as a standard feature, circulating fluid around the spindle, isolating any heat from the casting. Fluid circulating around the motor or through the motor mounting plate will isolate that heat source from the casting. The motor is the largest source of heat in the head. Direct drive spindle motor will add the benefit of reduced vibration and noise. 40 taper tooling works better with a Big Plus spindle for better tool rigidity, reduced chatter.
Thermal growth can be eliminated though testing. For example, at Takumi every spindle installed in a machine is run through a long term thermal test cycle, measuring the growth. If it falls outside the specification, the spindle is removed and returned to the manufacturer as defective and another spindle is installed in the machine. The spindles need to be specially constructed based upon the torque of the motor for a modern cutting style of high speed vs high chip load. Other methods of compensation include control based features, or in cycle tool probing and compensation.
Multiple coolant systems are needed in the mold industry. Coolant thru the spindle is needed while working in cavities or deep holes. Coolant around the spindle or with flexible lines helps deliver coolant right where it is needed. Cutting air blow is also needed for hard milling, and air through the spindle can be useful.
Many machines are mass produced and lack the attention to detail required for the best accuracies and part finish. Hand scraping for fit and alignment is as old as the first milling machine. In this type of design there are up to 35 components that require precise fit and alignment; many are secondary pieces that relay on the first part alignment and second level alignment to meet specifications of a quality mold producing CNC. Hand scraping remains the best method for working with the types of iron used for machine tools.
Ballscrews should large diameter for strength and fine pitch for accuracy. The pitch refers to the distance traveled by the ball nut, which moves the axis, with one revolution. A fine pitch is 10mm and an example of a coarse pitch would be 16mm. The finer the pitch, the more accurate the axis is. The pitch also factors into the rapids of a machine. To reach higher rapids, the pitch can be increased, or the motors increased in RPM. The ball screw pitch is cheaper to change than the motor size. Ballscrew diameter/Pitch 45mm = 1.77” 10mm=.393” 50mm=1.96” 55mm=2.16”
Linear guideways of the roller type, over the ball type, are preferred, for increased load capability, rigidity and faster motion. They have a larger contact area between the trucks and the rail. Lost motion is a term that covers a lot: Backlash, thermal growth, mechanical flex or slop. Mechanical methods to eliminate this start with ballscrews that are double anchored and stretched. This eliminates backlash in the bearing assemblies and thermal growth. The ball screw is stretched by the amount provided by the manufacturer to amount slightly greater than the expansion coefficient of the ballscrew. Axis motors direct coupled to the ballscrews eliminate lost motion found in belt driven systems. Axis motors with absolute encoders provide increased accuracy of the drive systems at high feed rates, or RPMS. This is important when using fine pitch ball screws. Linear scales provide the best positioning on all axis’. Scales take a direct reading of the axis position to further eliminate lost motion. If there is mechanical slop or thermal growth the scale compensates for that. Passing chiller fluid through the center of the ballscrew is sometimes used. This only compensates for thermal growth of the screw and does not help with other mechanical lost motion. The encoders in a scale system provide speed and position data for the drives and control to operate with better precision.
Once the machine build is complete, it needs to tested, checked and adjusted for accuracy. Full geometries checks are performed and recorded. These are the documented values that will be the guide for installing the machine in the field. Machine assembly starts with a level base, and the level is checked and maintained throughout the build process. With this method, a machine will only need accurate leveling at install and all the geometries will match the original documented values.
All axis are then laser checked, compensated and verified for accuracy and repeatability. All machines are usually ball bar tested. A ball bar test is an all in one test. It can detect lost motion by mechanical and electrical systems, and displayed as the circularity of simultaneous axis motion.
Advanced CNC Control Features- Processing power and advanced smoothing features are needed to reach maximum cutting speed and maintain part finish.
Each machine tool maker works closely with their chosen CNC control maker to deliver the best features and options for the intended market. For production those options might be multiple offsets, interface to central monitoring systems, etc. For mold making, speed and finish are the focus.
Mold making programs have a lot of surfacing data and are very large, so memory size of the control is important. The ability to process this data requires the control to see the data before the tool gets to the feature. Controlling the speed vs finish is needed to rough and finish parts on the same machine. Example -FANUC 31i-MB control is standard with the latest in drive technology, power saving features and motion control. All the features listed here are included in our standard configuration.
1 Gig DATA Server provided the memory needed for large mold programs.
AICC looks ahead in the program to eliminate the accel/decal delays that limit feed rates. Nano smoothing provides the capability to take rough surface data and add data to it for a smooth finish. 600 blocks look ahead provides more look ahead that is needed for complex, flowing surfaces. High speed processing makes it all happen faster and machine condition selection lets the programmer or operator change the feature based upon rough or finish cutting is needed. Included options are specifically designed to augment the mechanical design for greater speed (throughput) and superior part finish.
The control, drives and motors can operate together with nanometer precision. The high-speed options are tuned to the machine by FANUC trained engineers. Each machine tuned individually will give the best results, not with a standard parameter set. These parameters are saved and that backup is made available throughout the life of the machine.