What Are the 5 Most Criticl Parts of the CNC Milling Machine?
What is a CNC milling machine?
CNC milling machines move a rotating cutter and the workpiece under programmed control, to remove material from an uncut billet, to extract a finished part from raw materials.
A 3-axis CNC milling machine operation is a computer controlled milling tool that uses a wide variety of cutters on a machined work-piece, to extract the finished shape of a required component in metals like Aluminum, stainless steel etc. and even glass, copper and wood with great surface finish.3 axis CNC mills are essentially no different from a manual milling machine, but automated to remove the operator from the actual cutting process.
Simpler equipment in the 3 and 4 axis class is also widely used for preparation of precision wooden parts, for furniture and decorative purposes. This can enable high volume repeatability that is impossible to achieve by any other widely available techniques.
3-axis CNC milling machines move the milling cutter in the Z axis and the machined component in the X,Y and Z axes while rotating the cutter to remove material.
This is the logical opposite of additive manufacturing, and a field that is not hard to learn expertise in.
Additional axes of motion provide greater flexibility in the types of components that can be machined. The additional axes can be; tilting the axis of the cutter from the ‘normal’ vertical in one plane; tilting the cutter in multiple planes; rotating the workpiece; and tilting the work piece in two axes.
Description of machines as 5 axis refers to the additional axes of movement of cutters and the movement of the part. This technology allows the CNC milling machine to cut on multiple axes, using a minimum of movements/cutting to realize design and generate high throughput.
CNC machines milling paths are also influenced by material and required quality of outcome such as surface finish and precision.
Milling machines in the 5-axis CNC mill category allow the workpiece or cutter to rotate on the X,Y and Z axes. Despite this suggesting 6 axes, they’re termed 5-axis CNC machines.
By inclusion of the extra rotational axes, the mill can address the machined work-piece from any direction in manufacturing, allowing otherwise impossible features without re-jigging the work-piece and loss of manufacturing precision.
This repositioning is time consuming when CNC milling parts with complex features results in expensive errors and material waste result from loss of precision in referencing of the part to the CNC milling machine ‘origin’.
How do they work?
Precise, extractive manufacture
A CNC machining center is a computer programmable milling machine using rotating metal cutters to ‘extract’ machined components from a billet of material, such as any Aluminum grade. This manufacturing process uses a translated version of a CAD design, written in G-Code, to automate the machine’s functions, delivering rapid high precision and repeatability in machined parts, irrespective of material.
The first workpiece will be checked for accuracy and tolerances, before multiple parts are made.
Logical programing structure
G-Code is a complex sequence of position, spindle speed, cutter type, feed rate, coolant supply and specialist CNC milling software commands, which are easy for engineers to learn, allowing them to create complex machine codes.
Additional instructions used to control non-standard functions are referred to as M-Code, and they are integrated into the program to interpret the CAD data
The CNC machining bit is driven by the spindle, often an asynchronous motor, using variable frequency drive. Some milling machines drive the bit using a DC drive spindle, with an encoder, allowing tool rotational position to be controlled.
The X-Y-Z, axis orientation and workpiece orientation axes of CNC milling machines are often driven in open loop control by stepper motors, providing position control without the complexity of closed loop systems.
The CNC milling machine class is flexible in use on high value and repetitive tasks, following highly adaptive programming design. Multiple components cut from the same G-Code should be indistinguishable.
5 components of CNC milling machines
Spindle
The spindle is an important part of the CNC machining center. The spindle is rotated by a powerful, speed controlled motor and carrying one of the types of steel chuck. In quality and multi-axis milling machines, a separate spindle bearing system carries the cutter chuck, providing improved precision and durability.
The CNC milling spindle’s task is to precisely spin the bit, carried in a steel chuck.
Chucks
Various chucks serve a common purpose in CNC milling machining parts; to grip the smooth or tapered metal shank of milling cutting tools for zero no run-out (eccentricity) or slippage.
An important consideration in chuck type can be cost, as each CNC milling machine bit requires its own metal collet chuck in a CNC machines tool rack. The chucks are designed for quick change, to reduce machining times. Any CNC tool may be re-used many times in machining a single component.
The operation of quick change CNC clamping in milling machines is generally achieved by using one of the forms of collet chuck. A collet is a one piece steel jaw whose center diameter precisely fits the tool shank; whose walls are partially cut through, to allow slight flexibility in the inner/outer diameter under large clamping loads; whose outer taper fits the collet chuck. The collet is squeezed in diameter by the collet chuck clamping force, holding the tool shank in a firm, circumferential grip that minimizes slipping under CNC milling loads.
Feeders
Axis motion construction
The part material to be cut by CNC milling must be presented to the bit in a backlash free motion on all axes. This requires a moving spindle, or a moving table that can be positioned in a highly controlled and repeatable way.
This is usually achieved by positioning the part to be cut onto a 3 axis moving platform on the CNC milling machine that has independent drive motors for each of the axes. This supports the motion on V grooved bearings that provide hysteresis free movement.
CNC table parts are often machined from nodular cast iron, which combines high strength with dimensional stability and resilience. The most critical property of these axis slides is the ability to cover the motion of the CNC milling machine without error.
Axis-motion-drives
The CNC axis drives are applied by recirculating ball screws that allow reversibility without slack, and very precise motion. Important properties of such drives that CNC machining centers benefit from are; very low friction; resistance to wear in contaminated environments; and low inertia, to allow fast changes of feed rate and direction.
The three axes are driven by two methods;
The more primitive method is open loop position control, using stepper motors.
More advanced CNC milling machines use a combination of DC drives and linear/shaft encoders, to achieve closed loop control. This enables the CNC machine controller to discern actual position, rather than step count position. This allows better position control and faster drive of axes.
Rotary Tables
Basic rotary table
Rotary tables are commonplace tools in manual, non CNC machining applications, and their place in a CNC machine shop is as the important 4th axis, either built in or as an upgrade of a 3 axis CNC milling machine. The rotary table allows the machining center to rotate a milled part while machining.
The ability to index for PCD placement of holes, without reliance on X-Y coordinate instructions in CNC milling programming simplifies elements of the G-Code for milled parts by allowing a single command on the 4th axis to deliver perfectly centered positioning.
Tilt-table
An additional capability offered by many rotary tables is to be able to tilt the table. This is considered an additional axis capability, making one type of 5 axis machining center. This is often an aftermarket addition to a 3 axis machine to increase its functionality. It is not as flexible or capable as a fully capable tilt arbor mill CNC milling machine.
Work holding in a rotary table can be achieved by any of the common means used in the CNC machine shop – 3 jaw and 4 jaw chucks, collet chucks, T-bar clamping etc.
Finally, a rotary table can allow a CNC machining center to achieve some of the capabilities of a CNC lathe.
Workholding
Retaining parts on the work surface of a CNC milling machine is a critical function that should not be underestimated. Work imperfectly held is liable to move and can have disastrous consequences. Equally, Aluminum parts clamped too hard can have their value destroyed.
As a rule, the worktable of a CNC milling machine is equipped with slots for T-bar retaining. This allows a wide spectrum of work holding devices – angle blocks, V blocks, vises, indexing tables, swing clamps, magnetic clamps and more to be attached to the tables of milling machines at need.
A common method of retaining workpieces while CNC milling is to use a vise, or a swivel vise. It is also common to use steel swing clamps and step blocks, all held down with steel T-bolts on the CNC milling machine table.
For series production of multiple CNC milling parts, and particularly for mass production of CNC parts, it is common to create custom metal or plastic fixtures for a project. These custom fixtures allow process benefits such as multiple parts on a single build program, faster changeovers to reset the CNC milling machine and increased productivity.
Magnetic tables are used on CNC milling machines, but clearly don’t work well with all metals, so they’re not applicable to many of the highest value/largest volume applications, i.e. CNC machining custom milled parts in Aluminum, Titanium, martensitic grade stainless steel materials and other non magnetic metals.
Benefits of CNC milling machines
CNC milling machines of all types offer a wide range of benefits over competing manufacturing technologies (and manual machining). The most important of these benefits in relation to alternative methods are;
- Low cost repeatability. CNC machining can provide excellent tolerancing of parts and higher repeatability than most manufacturing methods available in wood, plastics, metals and more.
- In Aluminum material milled components, a CNC machining operation competes with diecasting, with better surface finish possible. Unlike CNC parts, diecast metal parts require high cost tooling and often need post cast cutting on critical and high tolerances areas. Where required volumes are relatively low, machining metal parts can be very cost effective.
- Similarly, in machining plastics, rapid CNC services can be invaluable. Plastics have high tooling costs and deferral of these costs can save money. CNC parts are great for prototypes, allowing design iteration without delays and tooling modification costs.
- CNC machining metal parts can be competitive with a sand cast or investment cast form, despite those processes requiring limited tooling. Again, cast parts often require some post machining which can alter the economics significantly.
- Where the highest strength/weight ratio is needed, custom forging for grain structure, followed by CNC machining for precision of fits can result in significant weight reduction, compared with sand cast or die cast parts.
- CNC machining delivers high tolerance prototypes and parts that are consistent and require low levels of process management, once production is established. Extreme tolerances in milled components are among the best available in design for manufactured parts, particularly in the range of metals commonly used in aerospace and in engineering plastics.
- Design for simplified machining is well understood and easy to learn, so where custom parts are developed for CNC machining long term, planning of cutting processes can allow them to be made on simpler machines of 3 or 4 axes, for lower pricing.
- CNC machining service enables fast turnaround and fast design iteration, making it a critical R&D and product development tool that designers learn to love. The material restrictions in CNC machining are minimal, so change of materials can result in no additional production or setup costs.
- CNC milled parts can be made in the widest variety of materials – all metals and plastic, even brass and Copper, reducing design constraints. CNC plastic parts can be a practical method to defer the plastic mold tooling process and allow trials with alternative plastic materials. Aluminum parts can often be milled at competitive prices, even in high volumes that are candidates for Aluminum die casting.
Conclusion
This article aims to give the reader an introduction to the fundamentals of CNC machining and to illustrate the significant benefits theses services can offer. We have introduced the formats of machine, the components that make up these machines and the technical and commercial advantages the process can provide to the designer and the manufacturer. These should help in specifying supplier type and machine preferences, as well as some design issues that should make parts more suited to machining at lower cost.
CNC machining gives manufacturers and product design process engineers capabilities that supply quality parts, fast and at moderate price. While not always competitive at high volumes, CNC machining can serve as mass production processes. And its capabilities can allow mass production CAPEX to be deferred, until the product begins to see sales.
A well run OEM CNC machine shop is a great design and prototypes resource, and well worth some relationship building to allow access to high quality and good service, on demand for your project.
Worthy Hardware is a CNC manufacturing and sheet metal fabrication company,including CNC machining services,CNC milling services, CNC turning services, laser cutting services and stamping services.Call us +86-76989919645 or email us [email protected] for more discounts for your projects.