CNC MACHINING NEEDS TO KNOW

Table of Contents

CNC MACHINING NEEDS TO KNOW

CNC is the abbreviation of computer-controlled numerical control, which is an automatic machine tool controlled by a program. The control system can process the program specified by the control code or other symbolic instructions according to the predetermined logic, and decode it through the computer, so that the machine tool can perform the specified action, and the blank can be processed into semi-finished products and finished parts through tool cutting.

custom cnc machining service china manufacturer

To work successfully on a CNC machine, a program instructs how the machine should move. Use CAM software to code with the CAD model provided by the customer, give the CNC machine tool programming instructions, load the CAD model into the CAM software, and create the tool path according to the required geometry of the manufactured part, after determining the tool path, the CAM software will create Machine code, which tells the machine how fast to move, how fast to turn the blank and/or tool, and in the X, Y, Z (3-axis), A, and B (5-axis) coordinate systems.

4 Axis Cnc Machining

CNC machine tools can be roughly divided into 3-axis and 5-axis machine tools. 3-axis machine tools mainly include 3-axis lathes and 3-axis milling machines. 5-axis machine tools are available in various forms such as continuous type or integrated milling and turning type. During use of a 3-axis CNC lathe, the cutting tool remains stationary and the part blank rotates. In contrast, on a 3-axis CNC mill, the cutting tool moves while the blank remains stationary. If a square feature is required on a round part, first create the round geometry on the CNC lathe and then create the square feature on the CNC mill. Parts can be manufactured more economically with a 3-axis CNC machine than with a 5-axis CNC machine, especially parts with cylindrical rotating features, which can be achieved to a high degree of precision with a CNC lathe. However, it is very difficult to manufacture complex parts with 3-axis machine tools. 5-axis machine tools can move simultaneously or successively in 5 directions, and can handle more complex parts.

Aluminum CNC machinery

With computer-controlled machine movement, the X, Y, and Z axes can move simultaneously to create everything from simple straight lines to complex geometric shapes. However, despite advances in machining and CNC control, CNC machining still has some limitations and cannot create all shapes and features.

1. Basic CNC machining

1.1 Tolerance

a. Standard tolerance

If no CAD drawing or dimensional tolerance specification sheet is provided, the model build product conforms to the following specifications:

By default, steep and sharp edges will be blunted (rounded). If there are clear and sharp edges that must be preserved, please inform us in advance when placing an order.

Default dimensional accuracy (length, width, height, diameter) and positional accuracy (position, concentricity, symmetry) is 0.1 mm

For the accuracy of orientation (parallelism and perpendicularity) and shape (cylindricity, flatness, roundness and straightness) features, tolerances are applied as follows:

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b. Part tolerance

Whether the tolerance is acceptable is determined by the designer based on the shape, fit and function of the part. Standard tolerances used for machining are 0.1 mm for metal parts and +/- 0.2 mm for plastic parts, unless specifically requested by the designer. If tighter tolerances are required (say 0.05mm tolerance), information about which dimensions require tighter tolerances must be marked on the drawing or communicated to us in advance.

part tolerance

It is important to remember that tighter tolerances will incur additional costs due to increased scrap, additional fixtures, special measuring tools and/or longer cycle times (machines may need to be slowed down to maintain tighter tolerances) . Depending on the tolerance callout and the geometry associated with it, the part cost can be more than double the standard tolerance.

The overall geometric tolerance can also be applied to the drawing of the part. Different tolerance requirements will cause additional inspection, production time, and thus may also incur additional costs. In order to minimize costs and save money, it is recommended to apply tight tolerances only to critical areas or overall part geometry.

1.2. Size limitation

a. Milling machine: Part size is limited by equipment size and depth of cut required for part features. The processing depth is not equal to the Z-axis stroke of the equipment. The processing depth of each part is determined by the part itself. Generally speaking, the depth of the groove does not exceed four times the width of the groove, and the thickness of the drilled hole does not exceed ten times the diameter of the hole.

b. Lathe: Our lathes can machine parts up to 450mm in diameter, but larger parts can be manufactured in special cases.

1.3. Material Selection

Material selection is very important to determine the overall function and cost of the part. Designers determine the material according to the needs of the part, considering the relevant properties of the material, such as hardness, stiffness, chemical resistance, heat treatment, thermal stability, etc.

a. Material Block: Material block refers to the size of the raw material used to create the finished part, generally the size of the blank is larger than the size of the finished part to allow for variation in the raw material. For example, if the final dimensions are 1x 1x 1, the part will have material block of 1.1x 1.1x 1.1.

b. metal: We CNC machine the following metals: Aluminum alloy, stainless steel, carbon steel, brass, copper, etc. Other customized metals

Plastics and softer metals such as aluminum and brass are generally easy to machine and require less machine time, reducing machining costs. Harder materials such as stainless steel and carbon steel must use slower spindle speeds and machine feed rates, which makes machining longer and more expensive than softer materials. Of course, the cost of the material itself is also very different. Even if it is the same stainless steel, the raw material price of 304 stainless steel is twice as high as that of 1018 stainless steel. The following pictures are from top to small: aluminum alloy, stainless steel, carbon steel, brass and red copper.

aluminium
stainlessteel
carbonsteel
brass
copper

 c. Plastics: Our CNC can process the following plastics “ABS, nylon pp polypropylene, PTFE, PC polycarbonate, etc. and other customized plastics”.

Plastic can be a cheaper alternative to metal if the part does not require the rigidity of metal. The following pictures are in order: ABS, POM, PEEK, PC, HDPE

plastic material for cnc machining

Note: Plastics may have difficulty maintaining tight tolerances. Parts may also warp after machining due to the stresses created when material is removed, especially POM material.

2. CNC Machining Design Guide

2.1, 3-axis or 5-axis machining?

CNC machining can efficiently produce simple or complex designs. The more complex the part – i.e. one with curved geometry or multiple faces that need to be cut – the more costly due to the additional setup and machining time required. When the part requires only one set of setup and 3 axes of motion (such as X and Y, and the Z axis for tool motion), the setup and machining can be completed faster, thereby minimizing costs, whereas if the surface is more complex, use 3-axis machining manufacturing can require multiple setups, custom fixtures and other costly machining processes. In this case, 5-axis machining can avoid these problems, but in order to get complex surfaces with a suitable surface finish, small tools are required. These small cuts take much longer than normal cuts, adding a lot of cost. So to help minimize cost and machining time, minimize or avoid the use of curved surfaces.

2.2 Chamfering of inner corners

Since the cross-section of the CNC tool itself is circular, it is impossible to get a perfect right angle or sharp corner when cutting the inner corner. All inner corners should be added with a circular arc. It is recommended to increase the arc by 1-1.5mm, and the minimum acceptable 0.5 mm, the smaller the radian, the smaller the tool required, and the higher the processing cost.

chamfering of inner corners

For example, if a part has a 2.5 in. diameter, a standard end mill would need to tap into a corner, come to a full stop, turn 90 degrees, and start cutting again. Doing so reduces processing speed (additional cost) and also causes vibration (chatter marks). By adding 0.5 to 1mm to the inner radius, the tool can turn slightly without stopping completely. Not only will this reduce the cost of the part, but it will also improve the overall performance of the part.

2.3 Grooving

While small radius tools can be used, sometimes the required depth of cut makes cutting impossible. When the depth of cut is greater than 2 times the diameter of the cutting tool, the feed rate of the tool must be slowed down, which impacts machining time and part cost. For every doubling of cut length, the feed rate is halved and the time more than doubled. Generally, the depth of the groove is not greater than 4 times the diameter of the tool

grooving

2.4 Drilling

Generally speaking, the larger the hole, the deeper the drilling depth can be. It is recommended that the drilling depth should not exceed 10 times the diameter of the drill bit. In addition, from an economic point of view, try to use standard apertures, non-standard ones may require additional tools and knives, resulting in additional costs.

drilling

2.5 undercut

It is difficult for CNC to process undercuts. If there are undercuts, it needs to be carefully designed, and special tools may also need to be customized. The smaller the undercut, the better, and the less the better, it is best not to have it if it is unnecessary. If it must to have, consider the tool Whether it is possible to enter and exit by another path, you can also consider changing to 3D printing to make undercut parts.

undercut

2.6 Minimum wall thickness

If the wall of the part is too thin, it may vibrate and tremble during processing, resulting in a decrease in accuracy, and in more serious cases, it may cause the part to break. It is recommended that the minimum wall thickness of metal parts is 0.8-1mm, and the minimum wall thickness of plastic parts is 1.5mm.

minimum wall thickness

2.7 Slender structures

Slender structures may cause problems similar to thin-walled structures, and slender protrusions will cause unpredictable vibrations, resulting in a decrease in machining accuracy. It is recommended that the aspect ratio should not exceed 4.

slender structures

3. Completion of parts - after treatment

3.1. Thread

There are several ways to create threads in a part: cutting taps, form taps, or thread mills. All of these methods are effective, but designers should keep the following in mind:

  • 1. Always choose the largest thread size the design allows – this makes the manufacturing process easier.
  • 2. The smaller the tap, the more likely it will be broken during production.
  • 3. Thread threads are only processed to the necessary length. Deep threaded holes increase part cost as specialized tooling may be required to meet depth requirements.

3.2 Surface Treatment

__As milled/As Machined

The original finish left on the part by CNC machining is called the “AS MACHINED/AS MILLED”. After simple deburring treatment, small processing lines can be seen with the naked eye, but it still has a smooth touch. Surface roughness is Ra/RMS 125, additional finishing can be done to reduce roughness at an added cost. Increasing the surface finish requirement to 63, 32 or 16 RMS will add cost as the feed rate may need to be reduced or additional post-processing may be required.

AS MACHINED provide the best dimensional accuracy and are the first choice for parts with tight tolerances.

aluminium

__Sand-blasting

Sand blasting is a reductive surface treatment in which a pressurized gun fires a stream of abrasive glass beads at the part to remove thin layers of the surface. This process creates a consistent matte/sand finish on the part and is often used to remove knife marks or imperfections. If your part requires tight tolerance features, then consider that blasting removes a thin layer, which may require masking of critical features, and if masking of surfaces or holes that do not require blasting may result in additional cost.

sand blasting

__Anodizing

Anodizing is an electrochemical finishing process that adds a natural oxide layer to the surface of a part. Anodized coatings protect parts from corrosion and can be dyed in a variety of colors. The coating is non-conductive, so masking can be done in areas that need to remain conductive. Aluminum and titanium are the most common anodized materials. There are two types of anodizing, normal anodizing (type II) and hard anodizing (type III).

Anodized (Type II)

This type of finish produces a corrosion resistant surface and parts can be anodized in different colors – most commonly natural, blue, black, red, green.

anodising

__ painting

This is a process in which powder coating is sprayed onto the part and then baked in an oven. This creates a hard wearing, corrosion resistant layer that is more durable than standard painting methods. Available in a variety of colors to achieve the desired aesthetic.

__powder coating

Powder coating is a surface treatment method that sprays plastic powder on parts. Plastic spraying is also what we often call electrostatic powder spraying coating, and its treatment process is a kind of metal surface treatment decoration technology that has been widely used in the world since the 1980s. Compared with ordinary painting surface treatment, this technology has the advantages of advanced technology, energy saving and high efficiency, safety and reliability, and bright color.

__polishing

Polishing is the mechanical process of grinding with progressively finer abrasives to obtain a shiny mirror finish. The finish is highly reflective, smooth and scratch-free. Polishing is usually done on hard materials such as stainless steel and 7075 aluminum, softer metals such as 6061 aluminum may cause their surface to deform when polished. The highly smooth surface finish also makes it more resistant to corrosion.

4. Summary

4.1 Processing equipment
3-axis/4-axis/5-axis CNC machining center, CNC lathe, turning and milling compound; at the same time, there are gantry milling centers for large parts; precision grinding machines, EDM, wire cutting (fast/medium/slow wire), etc. Processing Equipment.

4.2 Accuracy and size
The positioning tolerance of milling machine processing can generally reach ±0.1mm, and the tolerance of lathe processing can generally reach ±0.05mm; the maximum processing size of milling processing can reach 2100mm x 1600mm x 800mm, and the maximum processing size of turning processing can directly reach 400mm.

4.3 Materials and post-processing
CNC supports the processing of metals (such as aluminum alloy, stainless steel, carbon steel, titanium alloy, magnesium alloy, copper, etc.) and non-metals (ABS, plexiglass, Teflon PTFE, PEEK, PC, Saigang POM, etc.). And sandblasting, anodizing, electrolytic etching, electroplating, silk screen and other post-processing methods.

4.4 Minimum order quantity and lead time
Provide rapid proofing (minimum order of 1 piece), small batch trial production and mass production services; provide samples within 3 days at the fastest, and the on-time delivery rate reaches 95% in the past 7 days for small batch and mass production; the consistency of mass production products is good.

Author: Mose Li

Author: Mose Li

Director of Project Engineering at 3Q Machining

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