USE CORRECT TOOLS FOR PROBLEM-FREE ACRYLIC SHEET MACHINING
Most machining equipment was originally designed for use on wood and metal substrates. If used properly, however, acrylic sheet can also be machined with the same tools and equipment, including mills, files, engraving equipment, thread cutters, lathes, and reamers. Inadequate equipment or improper cutting tools can cause notching or overheating of acrylic sheet, and may even cause failure of the fabricated item. It is important to ensure that the right equipment and procedures are used for acrylic sheet.
Equipment and Procedures
Milling Machines and Tools
To control vibration and to assure clean cuts, select a milling machine with a heavy base and reliable balance. Equip units with vacuum devices or compressed air to keep chips and cuttings clear from the blade area. This will also assist in controlling heat generation.
Milling cutters vary widely, depending on the application. Use a cutting edge rake angle between 0-5° and a clearance angle of 2-10°. Cutting can be done with high-speed steel, carbide, or diamond-tipped tools.
The initial cost of a carbide tool is higher than steel, but maintains a sharp edge longer, producing a high-quality machined edge. Diamond-tipped tools produce a superior cut and last longer than carbide tools. In many cases, colorants used in colored sheet may shorten tool life.
Use a multi-edged milling tool for machining and set the cutter's rotation speed as fast as possible (minimum 3,300 feet per minute). Modifications to the blade may be required, since most are designed for cutting wood or metal. To reduce chipping, the rake angle should be ground to 0° to 5°. This provides a scraping, rather than cutting action. The clearance angle should be 2° or more to allow for effective removal of chips and cuttings, and to minimize frictional heat buildup. If using a standard milling tool, provide cooling.
Most normal files, hand routing files, and rasps can be used on acrylic sheet. These tools are used for producing a rough surface and are not intended for fine finishing. Desired effect and application dictate tool choice.
To eliminate notching and to smooth saw cut edges, use a scraper made from high-speed steel. Flat-ground triangular files with a maximum edge of 8mm (3/10") are also effective. The file surface must not be hollow ground because its rake angle will produce chatter marks. A flat-ground file should have a 0° rake angle. Use water while filing and sanding to create a smooth finish and lessen stress buildup from frictional heat.
For artistic applications, hand-held tools should be used. For more precise work, use fixed-tracing engravers. Flexible shaft engravers should have a rotation speed of at least 1,000 rpm. Air-operated pencil milling and grinding tools should rotate at approximately 6,000 rpm. To avoid melting (surface whitening) from slow travel, increase the engraving tip's travel speed. If melting persists, use a liquid coolant or air cooling. To avoid chipping, reduce the travel speed of the engraving tip.
To cut internal and external threads in acrylic sheet, use normal taps and dies. Most machine cutters are suitable for this procedure. Since acrylic sheet is notch sensitive, do not machine threads with sharp edges; use rounded threads to reduce stresses.
Note that acrylic threads are not meant to bear loads. If threaded connections are to be unscrewed often, reinforce the internal thread with a metal insert designed for use with acrylic.
A cutting emulsion or oil will produce a polished surface. The oil or emulsion should be tested first for compatibility with acrylic. Acrylic suppliers can often recommend a suitable cutting fluid. Regularly remove chips from internal holes and don't allow the tap to "bottom out."
As a general rule of thumb, cutting speeds for acrylic are about ten times those used for steel. The speed for acrylic should be at least 650 ft./min. The radius of the tool bit should be at least 20/1000th in. High speed steel, carbide-tipped, and diamond tools can all be used. Diamond tools on vibration-free precision lathes give the best results.
Normal reamers can be used on acrylic sheet to deburr drilled holes. All tool edges must be fault-free to prevent grooves or furrows in the drilled surface.
Heat distortion is more likely to occur in plastics, including acrylic sheet, than metals. To minimize the effects of heat distortion and to produce a polished surface, use coolants such as compressed air, water, emulsions, etc. If cooling is not provided, decomposition and irregularities at the machined edges and surfaces may produce high stress areas, leading to crazing (numerous tiny cracks in the material).
If using emulsions, be certain that the emulsion has been tested for compatibility with acrylic sheet. Incompatible emulsions may cause crazing.
Direction of Travel
To achieve a smooth cut, feed the acrylic sheet in the proper direction as dictated by tool rotation.
Inherent stresses and stresses imparted to fabricated items by machining may cause dimensional changes, after machining, when the sheet is heated to its forming temperature (300° F). These stresses can be eliminated through annealing (controlled heating, then cooling of acrylic sheet).
As always, a protective face shield or goggles should be used when operating machining equipment. Hearing protection should be worn during long operating periods. Eye and respiratory protection may also be appropriate, depending on the operation.
Please note: Acrylic sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources. Users should undertake sufficient verification and testing to determine the suitability for their own particular purpose. Be sure to follow manufacturer's safety recommendations for equipment and materials used with acrylic sheet. Written by Grant LaFontaine, Sheet Products Manager, CYRO Industries.
For more information, contact D. Artz, CYRO Industries, P.O. Box 5055, Rockaway, NJ 07866, 800-531-6384, FAX 973-442-6117, Web: www.cyro.com.