What is CNC Machining? | Understand CNC Machining Processes
What is CNC Machining?
CNC machining is a manufacturing process where all the conventional forms of machining techniques such as milling, drilling and turning is done using a computer controlled software. In more technical terms you can call CNC machining as “subtractive manufacturing” process.
As in this, material is removed from a solid piece of block called as “workpiece”. The removal of material is done by different cutting tools.
CNC machining is a automated manufacturing process wherein the computer reads your CAD file and generates a step by step process of making the finished product from a raw material. Few people in the industry also call it CAD/CAM machining, as the drawing and tool path generation is done with the help of a computer.
Earlier, in around 1950s people use to generate the machining operations on NC machines and data transfer relied on punched codes to perform specific operation.
Later the invention of computer enabled NC machine added C to it, changing it to CNC machining.
CNC machining is again classified into various categories depending on type of operations ie Milling or Turning.
What is CNC Milling?
When various Milling tools or milling cutters are used to remove material from the side face or from the upper face of the workpiece, the operation is called CNC Milling.These are considered as the basic machining to be performed to start off with making of a finished product.
CNC milling can further be classified depending on its freedom of movement. In what directions the workpiece can move and in what directions the respective cutting tool can cut.
CNC Machining: 3 Axis, 4 Axis, 5 Axis and Multi axis machine.
What are these Axes?
In CNC milling process as the workpiece or the raw material is held firmly on the bed using fixtures, vices and the cutting tool is attached in spindle holding systems.
Now, the movement of workpiece in X, Y direction and movement of the cutting tool in Z directions allows the machining operations in 3 directions. Hence, such machines are called 3 axis machines. Simple machining operations can be achieved through them so the designer has to keep various restrictions in mind while designing a product.
These are basic form of the VMC machines available in the industry to produce simple mechanical, electronic parts.
Although, few rotational movements can be achieved in here by adjusting fixtures but that would lead to increase in lead time and overall cost.
Similarly for 4 axis and 5 axis there is an additional rotational movement given to X and Y axis respectively. This allows an operator to create integrated holes in unparalleled direction thus decreasing the production time of the job. The multidirectional tool movement in 5 Axis machines allows the tool to machine workpiece in many ways. Highly complicated aerospace profiles, propeller turbine geometries can be created using these machines.
Now, in the category of Multi Axis CNC machines the number of axes ranges from 4 and above. Each axis movement is achieved either by rotating the bed or the spindle tool axis.Continuous movement of all the 5 axis at the time can also be achieved and turbine blade like geometries can be easily manufactured. As these are highly advanced machines the operator needs to be highly qualified and overall the services come at a high cost.
(Inside view of the trunnion table of an Okuma MU-4000V 5-axis vertical machining center. (Image courtesy of Okuma.)
What is CNC Turning?
Turning operation is basically done on cylindrical workpieces to either decrease the diameter or the length.
Conventional way of doing it is on a “Lathe” machine with two revolving centres and cutting feed is given manually by the operator.
The job is holded in a Chuck and it rotates while the cutting tool is stationary. Exactly opposite is the case for CNC milling machines wherein the workpiece is stationary and cutting tool rotates.
To understand CNC Turning, imagine a computer controlled lathe machine, wherein multi tools can be attached to automatic tool changer and multiple operations can be performed without physically changing the cutting tool. The production outcome changes drastically when compare to lathe machine as spindle rotates at a high rpm and there’s more rigidity. Only disadvantage is, it can only machine cylindrical parts, rotating shafts and not sharp geometries.
How can we overcome the above restriction? By combining Milling and Turning operations, Absolutely right. We have a CNC Turn-Mill machine wherein either the workpiece rotates or the cutting tool. It has advantage of both ways of working and complex geometries can be achieved at a lower cost.
DATA ON WHAT IS AVAILABLE IN CHIZEL HUB
Now here are few things a designer needs to know regarding machinability.
Types of Cutting Tools for Different Cutting operations:
In Milling the basic tools are Solid Carbide End-mills, Ballnose cutter, Milling Cutters along with its inserts.
Inserts of the series XPMT, RPMT, special high grade XOMX inserts with multiple cutting edges. These enhance the machining output but at the same time the cost increases.
There are always some standard End-mills and milling cutters available at the machining end.
In holemaking comes Drilling and hole finishing operations
Drilling consist of Solid carbide drills, Drill body with inserts, Boring Bars. These are used according to the material hardness, machining requirement and length to diameter ratio.
Finishing consist of Reamers, Insert type reamers and Finishing boring bars. Mostly 0.3mm material removal happens through finishing operations.
As you know turning is mostly done for cylindrical bars, mostly there are turning tool holders along with its inserts for general turning applications.
For grooving and parting there are special turning cutter bodies with special cutting angle inserts.
Few of the ISO inserts series are CNMG, TNMG, CCMT, DNMG.
Type of Tool Holding systems:
In any VMC machine the first most important feature is its Tool Holding System and then the cutting tool. The former is determined by the machine taper, ie whether its BT40 or BT50 or HSK 50. You might be thinking what are these terms? Basically, every VMC machine has its own capacity standards and are designated accordingly. BT40 is the most common tool holder type and all the everyday machining operations can be performed using these. Moreover these are rigid, tested under high rpm of around 25000 and have almost zero run out.
Also, BT type tool holders assist in Automatic Tool Changer technology to minimize lead time and increase productivity.Sometimes for some high power requirement and some high end machines the back taper is different. They are designed for high performance outputs, like HSK 50, HSK 100 are examples of the same.
Types of Fixtures and Clamping Devices:
The most important feature to control in machining is holding the workpiece, maintain its rigidity during machining and increase accuracy. Many critical factors like hole tolerances, distance between centers are dependent on its fixturing. Using a fixture makes machining easy as multiple operations can be performed.Basically fixturing partially automates the process and production capacity increases.
Types of Fixtures
Mechanical or Hydraulic Vices: These are the simplest type of Holding devices as its self adjusting and clamping pressure is also to the par.Moreover one can adjust the distance between the jaws by adding a special jaw or by add a bushing or add a stop pin to achieve some specific centre distance. Various “hacks” can be done to achieve the direction of cut along with maintaining its accuracy.
Standard Types of Fixturing includes: Clamps, Base, Locating Points and Rest Blocks.
These are the basic ingredients required at any Milling Workshop to perform machining operations. Using Standard Clamps and mounts makes machining cost effective and easy for the operator.
Few other Points :
- Compatibility with the machine bed surface
- As clamps are made of Cast Iron the vibration damping is good.
- Fixtures have modular design
- Universal Chucks can also be mounted on the bed. There are self centering types of chuck, Soft jaws, magnetic chucks, four jaw or sometimes even six jaw chucks.
Design for Machining :
1) Select widest tolerances and roughness surface finish fulfilling functional requirement:
Select the widest tolerances that will give the acceptable performance for operating surfaces. This will reduce the machine cost.
2) Avoid very large or very small L/D ratios:
In Deeper holes the material removal is difficult and also the vibration of tool increases. Try using drills of L/D < 5.
Small diameter holes are difficult to machine as centering is difficult and chances of breakage is more.
3) Avoid sharp internal corners and internal features in long parts:
90 degree sharp corners are difficult to achieve through standard milling processes, so either EDM is required or Wire cut. This increases the cost and time so it is better to provide internal radius equal to the radius of the tool tip.
Again deep narrow holes, deep radius corners should be avoided for better tool accessibility and minimum vibrations.
4) Use of minimum number of machining facilities:
Designing a part so that it is machined in minimum possible steps and using inhouse facilities available. If some operations are to be outsourced then it increases the lead time and cost of the product.
5) Use holes parallel or perpendicular to axis of part: Drills should be either parallel to axis or perpendicular because generating a tool path is easier and holes can be generated wiith minimum settings. Also, using standard size holes is always better as drills are readily available and overall its cost effective.
What are Engineering Tolerances?
One of the key factor to maintain while machining a product is its dimensional accuracy. Now, the maximum upper and lower limit of dimensions which is allowable is called engineering tolerance or machining tolerance.
In CNC machining upto 10 micron tolerance can be achieved. Again this depends on the machine rigidity, tools used and the overall process of manufacturing.
Like the machines use by us at Chizel have under 10 micron tolerance, so we can achieve a best tolerance limit of 50 microns.
Quicks tips regarding points to be provided in component drawings:
- All the section views and an overall 3d isometric view
- Surface roughness values at specific required points.
- Tolerances with their lower and upper limits
- Tapping if any, then internal or external along with its specifications.( LH or RH )
- Any special note for the manufacturer.
Below is the example of one such drawing.
CNC Machining Materials :
Cnc machining can be done on a wide range of engineering materials covering Metals and Plastics both.
In Metals machining is possible on following types:
- Aluminium ( 6061, 6063, 7075 )
Easily available, high conductivity, lightweight and corrosion resistant
- Stainless Steel ( 316, 304, 416 )
High strength, stainless ie corrosion free and weldable.
Resistant to most chemicals, good aesthetic appearance
High electrical conductivity, weak against acids.
- Alloy Steel
Good toughness and strength, heat treatable.
- Mild Steel ( EN8, EN9, EN24 )
Low carbon steel, excellent machinability, direct hardening.
Light weight and above par strength
Designer should choose the material smartly keeping all the factors such as hardness, availability of the raw material, machinability and capital investment behind the cutting tools.
In Plastics, the following categories are machinable.
Cost effective parts, possess fairly good mechanical properties
High tensile and compressive strength, chemically stable as well.
Moisture and wear resistance.
Mostly suited for high temperature applications.
Transparent finish parts , mostly made for outdoor applications.
Few things to keep in mind is the minimum wall thickness for optimum strength, cavity depth, whether it’s machinable or not and maximum part size possible to manufacture.
Post Processing : Often, the parts manufactured through cnc machining or be it additive manufacturing require some kind of surface finishing or post processing to possess some aesthetic qualities. In prototyping, mostly the “FORM” is tested and there surface finishing plays an important role.
Types of Post processing :
- Anodizing: A corrosion resistant film of around 100-200 microns helps against corrosion. Aluminium anodizing is commonly done in black color, although available in gold and red color as well.
- Powder Coating : In this type, the powder is sprayed on either metal or plastic part under warm conditions. Its high finish type of processing and highly durable. Also, works great under different working conditions like chemical involvement, moisture, UV light and wear tear issues.
- Finished Through Machining : According to the requirement of the customer, highly finished parts can be achieved by machining as well. Operator needs to generate roughing and finishing pass cycles and also there are deburring tools to remove the burr around the corner and sharp edges.
Inspection at Chizel :
The next step after the machining is its validation. Whether the product is under engineering tolerances or not? Under this step, the product is either inspected using basic measuring instruments such as Vernier Caliper, Height gauge, Micrometer and other standard hand held methodologies. However, the methods can change depending upon customer requirement and accuracy of measurement. CMMs is one such example where in the entire inspection is done through programmed computer software and accuracy is under 4 microns. Other examples of such types are laser projection machines, profile projectors, UTMs and many others.
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