Certain machining practices, regardless of the material being machined (plastic or metal), can generate excessive stress. When induced stresses caused by the effects of machining are added to any residual stress, product failure can result.
Causes of excessive stress induced by machining include:
Speed and feed rates will be determined by the finish desired, the amount of stock removed, and the tool used.
The maximum speed that a cutting tool can be operated without generating excessive heat should be determined by actual testing. Machine vibration and way clearances must be kept to a minimum. The following machining recommendations will help minimize the amount of stress induced during the machining process.
1. Heat Generation - should be kept to a minimum during all machining processes.
a. Actual testing should be conducted to determine the maximum speed at which a particular tool can be used without generating excessive heat and/or chipping. All tools must be kept clean and sharp.
b. Cutting solutions of water, soapy water, a suitable lubricant, or cool air jet should be considered for cooling during the machining process.
|CAUTION: Certain oils and lubricants typically used for machining of metallics contain stress-cracking agents that are not compatible with PVC or CPVC materials. Contact the lubricant manufacturer for compatibility prior to use.|
2. Turning and Boring - High-speed steel and carbide tools are typically used for most plastics. A common practice is to follow the feed and speed rates that are typically used for machining brass. Speed rates for high-speed steel tools commonly vary between 250 - 500 ft./min.; speed rates for carbide tools can vary from 500 to 1500 ft./min. Tools used should have less clearance and more rake than those used for steel or other metals.
3. Drilling - Carbide-tipped bits are recommended for high volume production. Extra clearance at the back edges of the flutes is desirable to reduce heat generation caused by friction. Drill speeds can be as high as 12,000 to 15,000 rpm with carbide-tipped bits. Bit points should have an included angle of 55° to 60° for thin sections and 90° for thicker sections; clearance angle should be 15°. Lubrication or an air jet should be provided to avoid excessive heating and aid in chip removal. Commercial high-speed steel drill bits specifically designed for use with plastics are available; they have large flutes for easy chip removal.
4. Tapping and Threading - A high-speed, steel-nitrided, chromium-plated tap with a rake from 0° to -5° is recommended for tapping small holes. Tapping speeds usually vary from 40 to 55 ft./min. The size of the hole should allow for approximately three-quarters of the standard thread depth. When cutting a 60° thread (such as ANSI B1.2.1), the tool used should be ground to cut on one side only, and fed in at an angle of 30° by setting the compound rest at this angle.
NOTE: The machining characteristics of different plastics vary somewhat. The recommendations given are general and may require modification to obtain the best results. The data furnished herein is provided as a courtesy and is based on past experience, limited testing, and other information believed to be reliable. This information may be considered as a basis for recommendation only, and not as a guarantee for its accuracy, suitability for particular applications, or the results to be obtained therefrom. Materials should be machined and tested under actual use conditions to determine suitability for a particular purpose.