Design Matters (Part 3) – Warping

Undoubtedly, part warping is a nightmare for both manufacturers and customers. It can affect the functionality of a part or lead to assembly failure. Generally, part warping is due to residual stress, which can be highly relevant to the choice of materials, dimensions of the part, and manufacturing conditions.


Warped part can be straightened.
Warped part is mainly caused by the residue stress of the material

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Chemical Conversion and Anodizing Processes

Both chemical conversion and anodizing processes are surface treatment to enhance the oxidation process especially for aluminum alloy. Aluminum is usually reactive with water or air to form a stable passive layer of aluminum oxide. Actually, this anti-corrosion protective layer can stop the rest of aluminum metal from oxygenating. In the previous articles, you can learn about:

  1. How to avoid the blotchy and uneven anodizing
  2. Anodizing surface treatment for cosmetic purpose
  3. Anodizing surface can sometimes fail


Aluminum alloy is becoming the most common used raw material for many industrial application. So, apparently, there are more and more CNC machining services projects in which our customer prefer to use aluminum alloy due to its advantages. Generally speaking, properties which make aluminum alloy popular include:

  1. Light weight but still strong enough: The density of aluminum alloy is about 1/3 of steel. High strength to weight ratio make aluminum alloy good material for many application, such as structural, .
  2. Corrosion resistance: Aluminum alloy can naturally generate a protective oxide coating layer on its surface.
  3. Heat and electric conductivity: Aluminum alloy is an excellent conductor and commonly used material in power transmission system.

What is Chemical Conversion?

Sometimes, we use Alodine as an alternative name for chemical conversion coating which applies chromate to the metal substrate. In the past, hexavalent chromium was mostly used in the immersion bath process for coating, but now it has been prohibited by RoHS Directive. Nowadays, trivalent chromium based coating processes are commonly available for commercial application.

Chemical conversion coating:

1.) Creates an anti-corrosive, durable and electrically conductive surface.

2.)Serves a better surface treatment option for aluminum chassis components due to its electrical conductivity, compared with anodizing

3.) Is a pre-treatment to improve paint adhesion prior to powder coating. The paint can mechanically bond to the conversion coating, but not just sit on top of the surface. Otherwise, the paint will be likely to undergo flaking or delamination from the metal surface.

4.) Can also be a primer prior to anodizing treatment.

We offer MIL-DTL-5541(*Ref.1) chemical conversion coatings which form protective films by chemical reaction with aluminum alloy. Generally speaking, these conversion coatings are categorized by the following types and classes:

Type I: Containing hexavalent chromium. Typically, it appears to be gold or yellow in color, which can be optionally specified as “clear” color (as an alternative to no color). However, Type I chemical conversion coating has been prohibited by RoHS Directive.

Type II: Containing no hexavalent chromium, but trivalent chromium. Typically, it appears to be “clear” color (as an alternative to no color).

Class 1A: Providing increased corrosion resistance and adhesive properties for painted or non painted workpieces.

Class 3: Providing increased corrosion resistance and adhesive properties and maintains electrical conductivity. Color ranges from light to dark yellow or gold.

Chemical Conversion Electrical Mount Base Adapter Plate
Parts treated with Chemical Conversion MIL-DTL-5541 Type 1, Class 1A

*Ref.1: See more about MIL-DTL-5541

Reverse Engineering: From Real Component to Print

Reverse engineering, so called back engineering, is to get the design information from the item already made. Afterwards, to reproduce the item total according to the obtained information. Laser scanner, CMM, profile projector, industrial CT scanning are the most powerful measurement tools for reverse engineering. Furthermore, you can refer to Wikipedia for more understanding about reverse engineering.

At APPORO, we provide reverse engineering service for parts made by CNC machining, stamping, plastic injection, die casting. Generally speaking, we use CMM and profile projector to extract the dimensions accurately from the original workpiece. Below showed a CNC machining service project for our USA client who were asking reverse engineering for Rotary Tattoo Machine components.

Original Sample of Rotary Tattoo Machine Components from USA client
Original Sample of Rotary Tattoo Machine Components

Reverse Engineering Step by Step

1.) To extract the sample dimensions by CMM, profile projector and Venier caliper.

Original samples sent from USA client. Firstly, sent for extracting all the dimensions by CMM, profile projector and Venier caliper. After that, all the dimension information are to create 3D CAD drawing for further CNC machining process. See more about our quality inspection instruments.

Reverse Engineering: Rotary Tattoo Machine - Frame dimensions extracted by CMM
Rotary Tattoo Machine – Frame

Material: Aluminum Alloy 6061-T6

Surface Finish: Red, Black and Silver Anodizing

Type: CNC Milling

Reverse Engineering: Rotary Tattoo Machine - Motor Housing Cap from USA Client
Rotary Tattoo Machine – Motor Housing Cap

Material: Aluminum Alloy 6061-T6

Surface Finish: Red, Black and Silver Anodizing

Type: CNC Milling

2.) From 3D to 2D prints for CNC machine programming

Generally speaking, to convert 3D CAD drawing into 2D CAD print is a must for CNC machine programming. A CAM software can both read and output CNC programming code for production from 2D prints. After that, we will confirm with customer by using the 2D CAD prints marked with dimensions and detailed descriptions. How to make a good 2D CAD print? For instance, various but less view sides of workpiece, detailed but simplified dimensions, marked with important notes if any. After intensive discussions with client, we have reached a consensus to integrate frame components into one part as below.

CNC Machining Services: Rotary Tattoo Machine - Frame for CNC machine programming
Reverse Engineering: Frame


CNC Machining Services: Rotary Tattoo Machine - Motor Housing Cap for CNC machine programming
Reverse Engineering: Motor Housing Cap


3.) Move to production

At last, the customized jigs, cutting tools and CNC programming were ready for production arrangement. You can visit our website for more understanding about our core service. After CNC machining, the items were sent for with high quality anodizing.

CNC Machining Services: Rotary Tattoo Machine Frame
Frame with Glossy Red Anodizing


CNC Machining Service: Rotary Tattoo Machine Motor Cap
Motor Cap with Glossy Black Anodizing


APPORO are still working closely with our USA client for cases with innovative new design, whilst improving the cosmetic surface with clear machining action. Do not hesitate to contact us should you have any interesting cases. We look forward to cooperate with you.

Cross Knurling Profile DIN 82-RGV

Standard specification of DIN 82 (link)


After CNC fabrication processing, the workpiece is usually with smooth metal surface. Knurling is a manufacturing process to feature straight, crossed, angled, diamond-like lines or pattern onto the CNC components. Generally speaking, knurling can perform better grip for finger/hand operation, plastic injection insert or decoration purpose. Sometimes, we also machined multiple shallow slots or polygonal for same above purpose. DIN 82 is most commonly used knurling spec in CNC turning machining field. For example, DIN 82-RGV is with cross knurling pattern. DIN 82-RBR/RBL is with right/left hand spiral. DIN 82-RGE is with diamond-like 30° cross male knurling.


Failure Cross Knurling

Our Belgium textile industrial customer had sent us the original sample made of stainless steel 304. The sample made by Belgium local prototyping manufacturer was with failure knurling surface. See the shorter item in below photo. The measurement of sample was precise and within tolerance, but with bad knurling which was with improper length, too light pattern, and also lack of lead in chamfer at the threaded hole.

Cross Knurling DIN 82-RGV
Cross Knurling DIN 82-RGV Compare Zoom-in


The improper length of knurling do bad for its appearance, a customized full length cross knurling tool can solve this issue. Too light patter may result from the CNC programming or limitation of CNC lathe. Moreover, our CNC production lines can easily machine the lead in chamfer on the edge of threaded hole.

Improved knurling

Therefore, we ordered a customized full length knurling tool to meet the required pattern spec of DIN 82-RGV 0.8. An automatic Japaneses CNC turn-mill machine also helps improving the quality of surface and dimensions. Above photo shows the longer workpiece machined by APPORO is with improved knurling pattern.


We can offer CNC precision parts with several types of knurling surface, such like straight knurling, cross knurling, diamond-like knurling. Our production lines are also capable of knurling on precision plastics parts. Should you have any inquiry for above knuring surface on your CNC workpieces, kindly send us RFQ for free project reviewing without hesitation.