MACHINING TIPS: Critical Aspects to Grinding – Surface Finishes

The key to any grinding operation is achieving your desired surface finish. The finish of a ground plastic is extremely dependent on the following factors:

  • Wheel Grit

Wheel grit is the most critical factor in obtaining your desired surface finish. A grinding wheel with a rougher grit will remove more material per pass, but the result is a rougher surface finish. A finer grit will remove less material per pass, but give you a smoother finish. The drawback to a finer grit is the potential for clogging of the wheel and over heating of the plastic which results in burnishing or burning of the surface. One needs to balance the choice of wheel grit with desired job speed and resulting surface finish. A popular recommended choice is a seventy-grit wheel. A lower grit number equates to a rougher grit. A higher number equates to a finer grit.

Some grinding wheels have more than a one size of grit per wheel and are helpful in first step material removal and second step surface finishing all in one pass through the grinder. A very commonly used example of a 2-step (duel grit) grinding wheel is a 36/60. This means that the first half of the wheel will be a 36-grit for more material removal and the second half of the wheel will have a 60-grit for finer material removal and a resulting smoother surface finish. There are also 3-step grinding wheels such as a 36/54/120. In this example the 120 grit is very fine and while providing an excellent finish, one will need to feed the material faster to avoid gumming up the wheel with removal debris.

While there are different make-ups of grinding wheel abrasives…an aluminum- oxide wheel is sufficient for most thermoplastic materials and highly recommended.

  • Feed Rate

Another factor in determining an acceptable surface finish is the feed-rate. For a cylindrical or surface grinder, the feed-rate is determined by the speed of the table. For a centerless grinder, the feed-rate is determined by the angle and rpm of the regulating wheel. For the grinding of most plastics, a suggested feed-rate is fourteen to thirty-two ft/min (4.27 to 9.75 m/min). Recommended feed-rates will vary depending on the hardness of the material being ground. Harder materials can run faster, while softer materials will need to be run slower. Keep in mind that the length of rod, plate, or tube to be ground also can affect feed-rate. For longer shapes, adjusting to a slower rate may result in improving the surface finish.

One should expect to adjust feed-rate to optimize surface finish and production time. Too fast a feed-rate will result in a poor surface finish. Too slow a feed-rate will result in increased heat generation at the material surface and burnishing of the plastic. Keep in mind that improper and aggressive feed-rates can also affect the life of the grinding wheel. Too slow and you will experience premature wear of the wheel and loss of tolerance control.

Grinding Feed-rates for Plastic Materials

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  • Coolant

Coolant is required with all grinding methods as this reduces heat generation at the material surface. Grinding dry or without a coolant is not recommended. The use of a flood coolant will flush away the plastic debris and help eliminate burning of the material surface, while also improving the finish and prolonging the life of the grinding tool (Figure 2). A water soluble coolant is recommended to eliminate the potential of any chemical attack on amorphous plastics which can be susceptible to cracking and crazing from aggressive solvents and chemicals. Traditional metal-working coolants contain high levels of “amines” which act as rust inhibitors and can result in chemical attack of some plastics.

Most grinders operate with a closed loop coolant system that requires filtration of the coolant. Otherwise, in a very short time period the coolant becomes contaminated with the plastic debris/powder and looses its effectiveness. The use of a cartridge style micro-filter is recommended to catch the fines. A 20-micron filter should be sufficient for most plastics. Often a second layer of protection to catch larger particles is taken through the use of a simple filter paper set-up. A 55 or 60 micron (55 or 60 µm) filter is typical and this will help to prevent premature clogging of the micro-filter (Figure 3). Regular cleaning of the filter media is critical and timing is dependent on the material being ground.

  • Material Removal Rate

How much material being removed per pass over the grinding wheel is also critical to the desired out come. For typical plastics like nylons (polyamide – PA) and acetals (polyoxymethylene – POM) one can expect to successfully remove approximately 0.030 in. (0.76 mm) of material per pass. Harder plastics like poly-ether-ether-ketone (PEEK) and poly-amide-imide (PAI) require less material removal per pass for optimal finish. For these materials it is recommended to not take more then 0.010 to 0.015 in. (0.25 to 0.38 mm) of material per pass.

For extremely soft materials, like a PTFE (poly-tetra-fluor-ethylene) based product, one should expect some trial and error to optimize performance and finish. A heavier grit (~54 grit) with a lower feed-rate will allow the coolant to clean-out this softer debris and keep the heat generation at the surface reduced for an improved finish.

  • Tolerance and Surface Finish

For most grinders, one can expect to hold an OD tolerance of + 0.0005 inches (0.013 mm). If aggressive conditions are being utilized and clogging of the surface is occurring, then one can expect to experience tolerances in the range of + 0.003 inches (0.076 mm).

It should be noted that when grinding tubular bar one may be concerned about concentricity of the tube wall. Use of a centerless grinder will not affect concentricity. Regarding concentricity, what you start with will be what you end with. However, use of a center grinder could actually improve tube concentricity.

Most grinders will easily provide a surface finish (roughness) of thirty-two RMS. By optimizing operating conditions and following the guidelines noted above, finishes in the range of eight to sixteen RMS are achievable. Finally, for high luster finishing, a three-step grinding wheel (36/60/120 grit) may be utilized. To achieve the highest luster, a buffing wheel could be added as the material leaves the grinder for a final polish. Given the number of factors involved with grinding thermoplastic materials, a little trial and error should be expected in order for one to obtain desired results.

TABLE 2: Grinding Parameters for Plastics

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JIM HEBEL
Application Development and Technical Service Manager

 

MACHINING TIPS: Grinding of Plastics with Refrigeration Assistance
Phone: 610.320.6600
Fax: 610.320.6638
2120 Fairmont Avenue
Reading, PA 19612 – 4235