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Working With Acrylic Part 2: Drilling & Hole Cutting

Copyright © 2007 by Davis Multimedia, Int'l. All Rights Reserved.
As Printed in October 2007, Volume 33, No. 4 of The Engravers Journal
Figure 1: Using a drill press to drill acrylic.

   If you consider the many attractive qualities of acrylic, you realize why this material is currently among the most popular materials used for awards, gifts and signs, in addition to other types of products such as badges and control panels. Today, acrylic is being used in the manufacture of everything from ad specialty key tags to high-end corporate plaques and trophies.
   Acrylic has several key characteristics which contribute to its popularity in the Recognition and Identification Industry. This versatile material has extreme optical clarity (better than glass) and exceptional weatherability, making it suitable for both interior and exterior use. Of greater importance to engravers is that acrylic can be easily and attractively marked using a variety of methods, including rotary engraving, laser engraving, sandblasting, pad printing and screen printing. In addition, acrylic is easy to work with, i.e. using the proper techniques, it can be cut into a variety of shapes, polished to a high luster, drilled to produce mounting holes, profiled and thermobent.
   What may be more important to customers is the “look” of acrylic. Acrylic is used to create unique and interesting products that make people stop, pick up a piece, feel its smoothness and admire its beauty. Acrylic merchandise closely resembles glass. Indeed, it is difficult to distinguish acrylic from glass merchandise by simply looking at it.
   What’s more, acrylic is a versatile material that can be used for creating a variety of products. For instance, acrylic is popular for both interior and exterior signage, including desk nameplates, wall and desk signs and directories. And many other functional and decorative products can be made from acrylic, including trophies, plaques, desk accessories, key chains, coasters, jewelry boxes, etc.
   For the retailer, offering acrylic merchandise makes sense and dollars. Acrylic products are highly salable and appeal to a broad market, ranging from individual gift purchasers to large businesses seeking corporate awards.
   There are two ways in which you can offer acrylic merchandise to your clientele. One option is to purchase acrylic blanks from industry suppliers and personalize them using your engraving capabilities. Another alternative is to fabricate blanks from sheets of acrylic.
   Many engravers find both of these methods to be useful. For example, suppliers offer a variety of acrylic blanks for items such as plaques, trophies and desk accessories. In many instances, purchasing these types of blanks and keeping them in stock can save you time. On the other hand, other types of products, such as signs and control panels, are inherently custom-made. In these cases, where you need a custom designed product and/or in situations where you need the merchandise immediately, the ability to fabricate the piece(s) in your shop can be very valuable.

Figure 2: Standard twist drill (right) and special-purpose bit with flat cutting edges (left). Figure 3: A quarter-round cutter. Figure 4: A helical end mill.

   As noted, one of the advantages of acrylic is that it can be easily fabricated, provided you use the proper techniques. Part 1 of this article series (Sept. 07) examined various methods for cutting and finishing acrylic blanks. This second installment looks at procedures for drilling, hole cutting and tapping/threading acrylic.
   Before discussing the specific operations, it should be noted that it is important to follow all safety precautions when performing any acrylic fabricating operation. The processes involved in fabricating acrylic can release potentially harmful vapors and gases such as methyl methacrylate (MMA) monomer so it is important to make sure there is adequate ventilation in your work area.
   Acrylic is a combustible material and, therefore, the same fire precautions associated with materials such as wood, paper, etc., should be followed. In addition, always observe common safety rules for all machining operations, i.e. wear safety glasses, keep your work area clean, properly utilize machine safety guards, etc.
Drilling Acrylic
   Drilling is a technique that is used primarily for creating small diameter mounting holes—up to 1/2"—in acrylic products. For instance, you can drill mounting holes in items, such as plaques, certificate covers, control panels and signs, which enable you to attach the product to another surface using screws or other hardware.
   In addition to drilling small diameter holes, you may also need to cut larger holes, e.g. round and/or square holes and slots in control panels. Hole cutting will be discussed in detail later in this article. Following is a look at the drilling process.
   The most commonly used equipment for drilling is a drill press or a hand-held electric drill (cordless hand-held drills are also a popular choice). A drill press is a large, precision-oriented piece of machinery available in both bench top and floor
models (Fig. 1). A drill press usually offers several convenient features, including a built-in work surface, a chuck to hold the drill bit and a drive motor/linkage for drilling straight down and perpendicular to the material. A drill press offers you variable spindle speeds and, in general, more control than a hand-held drill in terms of precise hole positioning and repeatability in hole size and placement.
   A hand-held electric or cordless drill can also be used to create small diameter holes in acrylic. With this type of equipment, however, the position and angle of the hole depends on the operator, so there is invariably less repeatability in hole size, placement and perpendicularity with respect to the surface. Additionally, many hand-held drills only accept drill bits up to 1/4" or 3/8" in diameter.
   Other equipment options for drilling holes in acrylic include flexible shaft-type drills and lathes. A drill press or hand-held drill, however, is usually the more common choice.

Figure 5: The correct cutter offset for cutting a 1" diameter hole using a .110" diameter parallel cutter.

   Drill Bits—Choosing the correct drill bit for acrylic will help to ensure simple and successful drilling. Most acrylic sheet manufacturers recommend using highspeed steel drill bits because these bits are inexpensive and readily available, and they work well for drilling fairly soft, non-abrasive materials such as acrylic. However, both solid carbide and Titanium Nitride (TiN) coated drills work exceptionally well.
   For drilling holes in acrylic, a twist drill is recommended. A twist drill is characterized by two spiral flutes running around a straight shank. This configuration provides two cutting edges to efficiently drill through acrylic and allow chips to escape.
   The included angle of the point on a standard drill bit is 118 degrees, which is effective for many drilling purposes. However, when used for drilling acrylic, these bits tend to “grab” the material and also cause it to fracture. The fracturing problem increases with the hole diameter and the proximity of the hole to the edges/corners of the piece.
   For the best results in acrylic, a modified drill bit is recommended. Drilling acrylic generally requires a bit with a smaller included angle than a standard drill bit, i.e. a 60 degree included angle is recommended as opposed to a 118 degree included angle. Also, for the best results, the bit should be further modified by grinding away part of the cutting edges to produce a small flat on each edge (0 degree rake angle) with a clearance angle of about 8-12 degrees behind each cutting edge. Figure 2 shows a drill bit modified for engraving acrylic as compared to a standard drill bit.
   Using a modified drill bit will produce better results overall. This tool configuration provides more of a scraping action vs. a slicing action and helps to minimize chipping and fracturing. It also prevents the bottom surface of the acrylic from “breaking out” and the paper masking from lifting up when holes are drilled.
   Drill bits modified for acrylic are available from acrylic distributors. They can also be purchased from suppliers of engraving materials and accessories. For example, Quality One Engravers, Rancho Cucamonga, California, offers a specially resharpened drill set for acrylic. This set includes 29 bits in 1/16"-1/2" sizes and sells for $125.
Drilling Technique
   With the proper equipment and tools, drilling acrylic is a straightforward operation. When drilling acrylic, it is usually best to use a backup plate, e.g. a scrap piece of engraving stock or other material, although this may not be necessary if you are drilling with a drill press.
   The first step is to clamp/hold the material firmly to the work surface. This prevents any movement of the material during drilling that could result in a distorted hole, and it also serves as a safety precaution.
   It is advisable to leave the paper masking on the acrylic when drilling or when performing any other fabricating operation. The masking protects the acrylic blank from scratches, nicks and other imperfections that may result from the tool, material chips or handling.
   A coolant is usually not required when drilling small holes, provided the drill bit you are using is sharp and properly ground. When drilling large, deep holes, however, you may want to use a coolant to prevent melting and to produce a smoother surface finish. A coolant helps to prevent the acrylic from gumming, sticking, stretching and tearing. Note: If you do use a coolant, be sure to remove the wet paper masking promptly to avoid leaving a sticky residue on the acrylic, which can be difficult to remove.
   Several different coolants are appropriate for use on acrylic. Popular options include a mixture of standard dish detergent and water or a combination of 10 percent water-soluble oil and 90 percent water. WD-40, available from hardware stores, is also an
effective coolant for acrylic. The coolants used for engraving metals are not recommended for use on acrylic. Metal coolants can cause blemishes and crazing on the acrylic.
   Before drilling, it is a good idea to visually check the drill bit to make sure it is running true, i.e. straight. In some instances, the drill bit may be bent, the shank may be worn, or the bit may have been placed in the chuck off-center, all of which can affect the finish and accuracy of the drilled hole.

Figure 6: Tapping and threading is the process of producing threads in drilled holes.

   Selecting an appropriate drill speed depends on the hole diameter. The drilling speed should be decreased as the hole size is increased. For example, drilling a .060" diameter hole generally requires drill speeds of 3,000-4,500 rpm; a .375" diameter hole would be drilled at 1,500-2,000 rpm; and a 1" diameter hole would be drilled at approximately 500 rpm.
   In general, use a slow feed rate when drilling acrylic, e.g. .015"-.020" per revolution. Do not force the drill bit through the material as this may cause the acrylic to break and fracture. Also, as the drill bit breaks through the bottom of the acrylic and into the backup material, slow down the feed rate to avoid chipping the material.
   Under normal drilling conditions, the surface of the drilled hole will have a frosted appearance. To achieve clear, smooth holes, try the following technique: First, drill a pilot hole (an undersized hole) into the material. Then, fill the hole with a wax stick. Finally, redrill the hole to the desired diameter. The wax serves to lubricate the cutting action and remove material chips, resulting in an exceptionally clear and smooth surface finish.
   Another way to achieve clear holes is to polish them after drilling. To do this, try using a wood or metal rod approximately 1/8" smaller in diameter than the drilled hole. The length of the rod should be equal to the hole depth plus about 1/2". Mount the rod in a drill press. For polishing hole surfaces which are rough, wind a piece of steel wool, emery cloth or fine grit sandpaper around the rod. If the inner hole surface is smooth and just needs polishing, try using a good grade of flannel and buffing compound. Then, simply turn on the drill press and lower the rod into the drilled hole to polish it.
   Drilling holes in acrylic can be a quick and easy process. A few key points to remember are: 1) Use a modified drill bit to avoid fracturing/breaking the acrylic, 2) Use an appropriate drilling speed based on the hole diameter, and 3) Use a slow feed rate.
   Following is a look at the procedures involved in cutting larger diameter holes in acrylic.
Cutting/Machining Holes
   Drilling is the preferred technique for creating small diameter holes up to 1/2". However, there are occasions when you may need to cut larger holes ranging from 1/2"-6" in diameter, e.g. to create hole cutouts in control panels. The following discussion addresses this type of hole cutting.
   Equipment—Engravers have an advantage when it comes to cutting holes in acrylic. That is, the best way to cut large diameter holes in acrylic is by using a pantograph or a computerized engraving machine.
   An engraving machine can be used for cutting large diameter holes in up to 1/2" thick acrylic. One advantage of using an engraving machine is that you are not limited to circular holes. Depending on the pantograph templates or computer fonts you have, you can cut ovals, rectangles, squares, etc., with most engraving machines.
   Cutters—Selecting an appropriate cutter is important to achieving good quality hole cutouts in acrylic. One of the first characteristics to consider is the cutter material. Either high-speed steel or carbide cutters can be used to cut holes in acrylic. High-speed steel generally works well for cutting holes in acrylic. This material is tough and, therefore, resists breakage.
   However, as EJ has stated in numerous articles, the newer micrograin carbide cutters are usually a better choice for most engraving applications, including hole cutting in acrylic. These cutters are harder and more wear-resistant, and they have better overall cutting characteristics.
   When selecting a cutter, you also need to consider the cutter shape and configuration. A tapered cutter produces a beveled edge which may be desirable for some profiling applications, such as creating badges or key chains. A parallel cutter, on the other hand, creates straight, parallel walls. For most hole cutting operations, e.g. on control panels, straight walls are preferred.
   If you will be using a parallel cutter, it should be at least as wide in diameter as the cutting edge is long. This provides the necessary strength for making deep cuts in acrylic.
   In the past, engravers have primarily used half-round cutters. Today, however, there are additional options, many of which are better suited for hole cutting applications.
   One option is a quarter-round cutter (Fig. 3). Tests with this cutter configuration have shown reduced cutter breakage and less tool chatter. In general, quarter-round cutters also tend to produce better cuts in acrylic as opposed to half-round cutters. (For more information read “Engraving With Quarter-Round Cutters, Jul/Aug 91.)

Figure 7: This chart indicates the correct tap-drill size based on the thread size. Drills are made in fractional, number and letter sizes.

   Another increasingly popular option for hole cutting acrylic is the use of a helical end mill (Fig. 4) or router bit. These tools are similar to drill bits in that they feature multiple cutting edges. This type of cutter helps to minimize chatter marks and produce an overall smooth edge finish. (For more information read “A Look At End Mills,” May/Jun 90.)
   Regardless of the cutter type you choose to use, make sure the cutter is sharp and highly polished. This will ensure easier cutting and smoother edge finishes.
   Cutter Offsets—In order to achieve a hole of the correct diameter, you need to work with cutter offsets. In other words, you need to take note of the cutter width and then offset the cutter path from the shape (hole) to be cut out. The reason for this is that a hole is cut with the outside edge of the cutter and the cutter’s path of travel is determined by the cutter’s axis. If you do not offset the cutter’s path of travel, you will create a hole cutout that is larger than originally intended.
   Determining the correct cutter offset for circles, squares and other symmetrical shapes is simple. To figure out the cutter path, subtract the cutter diameter from the shape’s dimensions. For example, say you wish to create a 1" diameter circular hole using a .110" diameter parallel cutter. Simply use this formula: 1" (hole diameter) - .110" (cutter diameter) = .890". Thus, the cutter’s path of travel equals .890" in this example (Fig. 5).
   Note that asymmetrical shapes require a different approach because the diameter or length and width of the shape is difficult to determine. In such a case, subtract the cutter radius (1/2 the cutter diameter) from the nearest edge of the shape to determine the cutter’s path of travel.
Hole Cutting Technique
   Once you have selected a cutter and determined the appropriate cutter offset, you are ready to cut the hole(s). When hole cutting in acrylic on your engraving machine, it is a good idea to use a backup plate underneath the acrylic to avoid damaging your cutter and/or the machine. Also, you may want to use a coolant to achieve a smooth edge finish.
   In general, use a fast spindle speed and a slow feed rate. Also, if possible, feed the cutter clockwise so that the cutter is turning into the material. This technique usually results in a better cut. Note that you may need to make 2-3 cutter passes, depending on the material thickness and the rigidity of your machine.
   For an exceptional edge finish, you may want to try a roughing cut followed by a finishing cut. To do this, first cut a hole using a slightly undersized cutter and a slow spindle speed. Then, perform the finishing cut with the appropriate cutter size using a high spindle speed. Done properly, this technique should yield a smooth, clear edge finish.

Figure 8: Tapping and threading acrylic.

Tapping and Threading
   Tapping and threading is the process of producing threads in drilled/cut holes, i.e. for screws. There are machines available for tapping and threading, although these are primarily used for production purposes. The procedure is normally done by hand when low quantities are involved. In general, the same procedures used for tapping and threading metal apply to acrylic.
   A tap is a cutting tool having “threaded” flutes along the length and a square gripping surface on the upper shank, which facilitates holding the tap with a tap-wrench (Fig. 6). For the best results, follow the American National Coarse system for screw threads when selecting a tap.
   The procedures for tapping holes are relatively straightforward. Before drilling the hole, you first need to determine the thread size and select the tap. Once you have selected a tap, you can determine the correct drill bit size needed to drill the hole (commonly referred to as the tap-drill size). There is a correct tap-drill size for each tap size.
   Tap and die sets and machining handbooks have tap-drill size charts, such as the one shown in Figure 7, which provide guidelines for selecting an appropriate size drill. For example, if a 3/8–16 tap is to be used (3/8" diameter, 16 threads per inch) the chart indicates that the appropriate drill size is 5/16". Note that smaller holes usually require number-sized drills as opposed to fractional drill sizes, as shown in the chart in Figure 7. Sets of number-sized drills are available from machinery distributors.
   After drilling the hole using the appropriate tap-drill, you can proceed to the threading operation. First, hold the acrylic in a vise or other clamping mechanism to prevent it from moving during threading. You may wish to use a lubricant in order to achieve the best results. If you want to produce threads that are transparent, fill the drilled hole with a wax stick prior to threading. As with drilling, the wax lubricates the cuts and helps to expel material chips.
   Next, select the appropriate tap and attach the tap-wrench. Cup one hand over the wrench and position the tap in the drilled hole (Fig. 8). As you apply downward pressure on the tool, turn the tap to the right to start threading. Note: It is very important to hold the tap perpendicular to the work surface in order to achieve an accurately threaded hole.
   It is usually a good idea to reverse the tap direction as you’re threading, e.g. 1/2 turn forward followed by 1/4 turn backward. This technique breaks the material chips and prevents them from clogging in the flutes of the tap. In addition, it reduces the tendency to cut more material from one side of the hole than the other. After 2-3 turns, you may want to check the straightness of the hole with a square.
   To achieve consistency, some engravers modify this technique by using a drill press. To do this, load the tap-drill in the drill press and turn the chuck by hand to thread the hole. A drill press provides more control over the threading operation by holding the tap perfectly perpendicular to the material, which ensures the correct angle. In addition, you can more easily apply light pressure with the drill press arm. Many engravers find this method of threading to be faster and easier. If you do choose to use this technique, remember that it should be done by hand, not turned on the drill press.
   Acrylic is a versatile material with many diversified applications, ranging from functional control panels to decorative award plaques. Once you know the proper procedures, you will find that acrylic is quite easy to work with.
   This multi-part article series on working with acrylic is designed to familiarize you with the various techniques used to fabricate acrylic. So far, we have cut acrylic and finished the edges (Part 1), and drilled holes, cut holes and tapped and threaded (Part 2). There’s more to come, so watch for the next installment!