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Re: Plastics question



Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>

Actually, using ANY type of hydrocarbon as a cutting fluid for acrylic or polycarbonate will almost guarantee crazing or cracking. The most readily available "cutting fluid" for these materials is just plain tap water, the "best" cutting fluids are synthetic soluble oils specifically designed for machining solvent-sensitive plastics.

The tendency for acrylic to crack or craze is also highly dependent on the material type (cast or extruded), and the stresses applied to the material during machining (cutting tool speed, feed rate, heating of cutting tool and workpiece, selection of correct tap drill diameter, etc). In reality, tapping acrylic or polycarbonate should be avoided if possible, and the parts through-bolted with a nut and washer on the backside. If parts must be tapped, use long screws to get lots of thread engagement and spread the load over many threads, and control screw installation torque carefully.

The most common machining problem is simply drilling a hole. Using a standard twist drill bit designed for metal is a recipe for disaster, as the rake angle of the cutting edge will "grab" the plastic and pull the bit into the material, leading to tearing-out the material, chipping and cracking. Special drill bits are available for acrylic, but it is easy to modify a standard drill bit for plastic work by grinding the point to a 60 degree included angle (much "pointier" than a standard 120 degree drill bit), and grinding the cutting edge to a zero rake angle. This configuration will "scrape" material rather than dig into it and will never "grab" the workpiece. If you have never tried a drill bit of this type for plastic work, you will be amazed at how easy it is to control the feedrate, and how clean the resulting hole is.

Regards,
Scott Hanson


----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, January 30, 2006 3:23 PM
Subject: Re: Plastics question


Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
(snip)
"When you tap the acrylic for the fasteners be sure to use kerosine if in a lathe. Also gently (very gently) anneal the acyrlic after tapping in your oven to prevent the sharp tap edge points from expanding into cracks that will radiate to the edge of the acrylic and cause fastener failure.

Dr. Resonance
(snip)