Considering the varied requirements of end users, the selection of an appropriate machining centre demands certain clarity. Starting from a simple 3-axis and a spindle configuration to multi-axis & multi-spindle configuration, the selection range is really wide. Different user segments necessitate certain features and configurations.
Due to continuous demand for enhancing the productivity and performance of the machine, there have been developments in the sub-systems used on the machine tools. For those who are looking out for consistent output and quick set-up changeover, the entry level of VMCs or HMCs with 3 linear axis and a spindle makes the job shop versatile for undertaking of diverse machining jobs.
Criteria for selecting the right machine
Some of the factors that would need some consideration in order to select the machine and its configuration include size of the work piece, nature of operations, number of set-ups required to complete the operations, special features required to support intended process, requirement to camouflage loading and unloading time of the components when the machining cycle is in progress, methodology of work piece holding and clamping, cost of manufacturing, precision level of the component (demand on the process), lifetime cost of ownership, technical abilities of the shopfloor manager, analysing if the customer is a job shop or end user and if the requirement is a replacement machine on an existing line of machines.
The four salient features which ought to get the main focus while deciding an appropriate model or version of a machining centre would be the axes strokes, spindle speeds, floor space requirement to install the machine and spindle power and torque. Based on the size of work piece and the work holding fixture, table size will get decided and this, along with the work envelop to be covered, will decide the axis strokes. Type of operations and speed of operations will directly influence the spindle taper size and the spindle power required.
Yielding a faster return on investment
According to HS Ramprakash, Vice President – Engineering, Ace Manufacturing Systems, the usual concern that they get when they suggest a horizontal machining centre to a customer is that it is too expensive. Machine tool users look at machine cost but merely miss out on return on investment.
By design and construction, VMC is much lower in cost compared to the HMC. Normally, VMC is less than 50% of the cost of HMC for its equivalent features. But the return on investment is much faster in the case of HMC. Typically, a HMC comes with an index table or a rotary table.
Replacing three machines with one not only reduces the investment cost, but also the floor space, elimination of redundant tools and the fixtures. The productivity that one can achieve with an HMC can yield a faster return on investment. Some of the advantages of HMC over VMC are reduced manpower and operator intervention, more effective spindle utilisation, shorter cycle time, reduced inventory (work in process) and easy chip evacuation. Although HMC might not be the choice of machine for all the components, there are factors like quality, application, return on investment, cost, etc. which can make one consider an HMC.
Irrespective of the geographical location, where the machine would be installed, smaller foot print machines are popular with users. A machine designed for smaller foot print needs to be highly reliable, as any break down (even planned preventive maintenance) will be difficult to attend in a line, due to reduced accessibility.
Salient features of the machine
Spindle speed and power/torque: This is a very important feature, which has to be understood and selected depending upon application. Generally, if the machine is meant for machining an aluminium component, the spindle speed should be minimum 80-8000 rpm. It is advisable to opt for direct drive for precise application. For precision machining on cast iron/steel, direct drive with higher power spindle motor is recommended. The spindle run out will be very good since there is no side load on the spindle due to belt drive.
The power up series motors are used when wider constant power range is needed. However, these are expensive compared to normal motors of the same power. Generally, these motors are selected for application where bigger dia-cutters like, 100/125/160 mm are used extensively for machining cast iron/steel components.
Spindle bearing and spindle speeds: Spindle rigidity is inversely proportional to the maximum speed of the spindle for a given size of spindle bearing. Higher the spindle speed, lower is the spindle rigidity. Lower the maximum spindle speed, higher is the spindle rigidity.
Higher preloaded spindle bearing of ø70 with a spindle speed range of 45-4500 rpm is very good to machine steel & cast iron components. Ø 80 mm spindles with medium preload are still better to machine steel & cast iron components. The tool life on higher preloaded bigger dia-spindles is better. Often, an option of spindle cooling can be offered to ensure retention of running accuracies on prolonged/continuous use.
Axes rapid traverse & chip to chip time (CTC): The chip-tochip time (CTC), slide acceleration, rapid traverse & spindle acceleration decides the idle time during the component machining cycle. When the acceleration/deceleration time of the slides and spindle are lower along with higher rapid traverse of the slides, the idle time and hence, the chip-to-chip time is lower. In the long run, high speed machines will earn better return on the investment done.
Size of the machine: Generally, the VMC’s sizes are referred and identified by the Y-axis stroke and X-axis stroke and HMC’s sizes are identified by the pallet size, which decides the size of the component that can be machined. The size of the table of VMC will decide the size of the component that can be mounted and the axes strokes will decide the area that can be machined.
Daylight area: In VMCs, it is the gap between the spindle face and the top of the table. Lower the daylight, shorter is the height of the component that can be machined. Whenever index tables or rotary tables are used with cradle type fixture, we need to have higher daylight area to ensure that the component with the fixture will not interfere with the cradle base. The component height and width will decide the additional daylight area required on the machine.
Coolant through spindle (CTS): This feature has become more prominent these days due to its ability to reduce the running cost of the machine by reducing the cycle time and increasing the tool life. The main application is for drilling of holes of L/D ratio up to 30 and insert type-drills of L/D ratio up to 10. They are also used for other machining application like reaming, boring, etc where accuracies and depth of cut play a critical role.
As a thumb rule, we need to supply 0.75 to 1.25 times ‘D’ (size of the hole) lpm oil coming out of the tool for effective chip evacuation. We need high pressure and low flow for smaller dia components. For bigger dia, we need low pressure & high flow.
It is very important to ask the tool supplier to provide the size of the oil holes for critical applications and long L/D ratios. Another important aspect for CTS is to supply coolant with 20 micron filtration for better tool life and high machining tolerances.
Automatic pallet changer (APC): Essentially ‘APC’ is a productivity enhancer offered on majority of the VMC’s as an optional feature and in some of the HMC’s, APC is a built-in standard feature. APC option is selected basically to reduce the idle time. While the auto cycle is in progress on the spindle side, operator can load/unload at the station side in the back ground.
While selecting this option of ‘APC’, one should keep in mind the operator’s fatigue if the automatic cycles are of shorter duration (<60 seconds).
Number of tools in the magazine: Number of setups and number of tools required by the application decides the tool storage capacity. On the VMC machine models with single spindles magazine with capacity of 6/12/16/20/24/30 and 40 are offered.
On VMC with two spindle variants, magazine to support each of the spindles has a storage capacity of 20 tools. On the HMC machine models, the tool storage capacity ranges from 12 to over 100.
Chip conveyors: A conveyor is determined by the material being machined and the nature of chip produced by the process. Normally for aluminium, cast iron, nylon, nonferrous material like brass, bronze, copper, etc, scraper type chip conveyor with rotary drum filtration is recommended. On the other hand, steel material machining application, which will result in longer chip generation, needs slat cum scrapper type chip conveyor. In certain applications, magnetic chip conveyor can be effectively adopted. A coil/screw conveyor can also assist in evacuating the chips to rear end.
These are some of the important considerations that will help manufacturers to choose the right machining centre with the essential add-on features for the application intended.
The article is authored by HS Ramprakash, VP – Engineering, Ace Manufacturing Systems