MAY 2005 / VOLUME 57 / NUMBER 5 BY VAN NISER, ONSRUD CUTTER LP The

Router designed for machining mechanical plastics provide an opportunity to maximize productivity. Way

tics include acrylonitrile butadiene styrene (ABS), Acetal, Delrin, Hydex, nylon, polycarbonate, polyurethane and polyethylene terephtalate (PET).

Cutting Geometry Router bits for cutting mechanical plastics have traditionally been run on CNC routers at high speeds and feed rates. Extensive testing and years of field experience have shown that a tool with a high rake and low clearance performs exceptionally well. It ma- chines mechanical plastics more pro- ductively than tools with other geome- tries and imparts a finer surface finish (Figure 1). This kind of free-cutting geometry is rarely used by shops to machine me- chanical plastics. Most use endmills run- ning on CNC machines. Endmills are robust cutting tools specifically designed for heavy loads, All images: Onsrud Cutter slower spindle speeds and lower feed any wear parts are made from Traditionally, these types of parts rates. These tools, with their minimal mechanical plastics. Common have been fabricated from metal. But flute area, interfere with the ability to M ones include bearings, gears, mechanical plastics are beginning to re- clear the stringy chips generated when material-handling parts and machine place metal because of their increased machining mechanical plastics. Endmills components such as spacers and posi- durability, excellent machinability, and are designed with minimal rake and low tioning mounts where the reduction of exceptional mechanical and electrical clearance, which can aggravate the melt- vibration is essential. properties. Common mechanical plas- ing and rewelding problems common when cutting mechanical Chip Load plastics. Multiple-flute end- Once the correct tool geometry is cho- mills tend to push the chips, sen, the proper chip load is the next con- rather than carve or the sideration. In mechanical-plastics ma- chips like router tools. chining, the recommended chip load Clearly, endmills were de- range is 0.004 to 0.012 ipt, which results signed for ap- in an excellent finish and acceptable pro- plications, but have been ap- Low or no rake, Medium rake, High rake, ductivity rates (Figure 3). This narrow low clearance high clearance low clearance plied to mechanical plastics (metal) () (plastic) range imparts the finest finish through because machinists are com- the continuous generation of properly fortable using them. Figure 1: Tool geometries for metal, wood and plastic. sized or curled chips. Inadequate chip load can lead to knife marks, which ad- Tool Selection chip welding may be a problem. How- versely affect the finish. O-flute tools Mechanical plastics are characterized ever, for part hold-down considerations with a high rake and low clearance help as either soft or hard. By looking at the and through-cuts, left-hand spirals are a eliminate knife marks by slightly rub- chip produced, a machinist can easily standard item. bing the part during machining. determine the flexibility or rigidity of The O-flute spirals are available as the material being cut. Soft plastic pro- single- and double-edge tools in diam- Machining Ways duces a curled chip, while hard plastic eters ranging from 1⁄16" to 3⁄4". When Today’s CNC milling machines are produces a splintered wedge. Generally, machining mechanical plastics, the sin- more than adequate to achieve the proper O-flute tools are applied to soft plastic, gle-edge O-flute spirals impart a finer feeds and speeds for router tools. Spin- while V-flute tools are used with hard finish than multiple-flute endmills. dle speeds of 10,000 rpm and higher, plastic (Figure 2). When small tool diameters are neces- with feed rates in excess of 600 ipm, are sary, the single-edge design, with not uncommon. However, when these its more open flute, accentuates kinds of capabilities are not available or chip evacuation. In terms of bal- feasible, router tools toleranced for ma- ance, a maximum cutting-edge di- chining mechanical plastics can perform ameter of 3⁄8" is recommended for single-edge tools. for mechanical Chip flow (curling Chip flow (violent If balance is an issue plastics action) heavier and action) lighter and or a deeper cut is required, double- easier throw from the more likely to reweld cut area. Best for soft in softer plastic. Best edge O-flute spirals and 3-flute fin- hose machining mechanical plas- and/or tough plastics. for hard brittle plastics. ishing tools are logical selections. T tics have been at the mercy of in- Both of these types of tools can ma- appropriately designed drills for years. Figure 2: O-flute tools (left) vs. V-flute tools. chine materials up to 31⁄8" thick. Jobber drills and similar tools are inad- Most wear plastics are made from Excellent finishes can be achieved when equate in terms of pro- soft plastic. Consequently, O-flute deep cuts of two to four times the cut- ducing clean holes. tools are recommended for machining ting-edge diameter are made at aggres- As with router tools most mechanical plastics. sive feed rates. The double-edge O- designed for machin- O-flute tools are manufactured in flutes are available with a low- or high- ing mechanical plas- straight- or spiral-flute configurations. helix angle to accommodate a range of tics, drills are avail- The choice depends on which direction horsepower requirements. Also, high- able for soft plastics the user wants the chips to flow. helix cutting tools are advantageous in that allow fast plunge Straight tools have a neutral effect, materials over 1" thick. speeds and reduce while spiral tools can influence chip wrap. A 60° point the chips either upward or down- Chip load and edge finish and flat-face rake pro- ward. (For purposes of clarifica- Plastic finish curve vide an ideal plunging tion, a downcut spiral is a left- point. The point re- hand spiral, while an upcut spiral duces the stresses in- is a right-hand spiral.) troduced into the hole For the most part, routers with walls and imparts a upcut, or right-hand, spirals are fine finish without applied because they effectively edge quality Increasing clouding or crazing with Increasing chip load or feed rate evacuate chips. Downcut, or left- (lines or tears in the special point hand, spirals tend to recut chips, Figure 3: Only a narrow range of chip loads, wall of the hole). for mechanical which is not advantageous when or feed rates, can achieve fine surface finish. —V. Niser plastics. cutting mechanical plastics where Typical range is from 0.004 to 0.012 ipt. at spindle speeds of 6,000 rpm and pro- portionately higher feed rates. The key is maintaining proper chip load to enhance productivity and part finish. As previously mentioned, endmills are prevalent in machine shops mostly because of availability, cost and, of course, tradition. The downside of end- mill geometry for mechanical plastics has been addressed, but application of this type of tool is also a major problem. Endmills are being used to machine mechanical plastics in the same man- ner as they have been applied to ma- chine metal for years. Multiple-flute endmills climb-mill when taking a Single-edge, Single-edge, Double-edge, High-helix, 3-flute roughing pass and a finishing pass. straight upcut, spiral spiral O-flute spiral O-flute finishing tool. Many times, taking multiple passes ne- O-flute tool. O-flute tool. tool. tool. cessitates a tool change between roughing and finishing. Also, coolant increase productivity and dissipate heat, ing cycle times and improving part qual- is applied to alleviate problems associ- thus eliminating the need for coolant. ity are integral facets of being successful, ated with heat generation. All these The accuracy of CNC milling ma- it behooves the user to look at ways to factors are time-consuming and expen- chines and hard fixturing with mechan- improve the machining process. Router sive, and they adversely affect the cycle ical or pneumatic clamping devices en- tools designed for machining mechanical time and the cost per part. hance the effectiveness of a router plastics provide such an opportunity. With the use of router tools toleranced tool’s geometry, producing higher- for mechanical plastics, the parts can be quality parts in less time. Machine About the Author machined in a conventional-milling shops that have adopted this type of Van Niser is director of plastic applica- method without multiple passes and the tool report increased output of 40 to 50 tion engineering for Onsrud Cutter LP, aid of coolant. Once again, this is percent and a reduction in secondary Libertyville, Ill. For more information achieved by maintaining a chip load of operations, such as deburring. about the company’s cutting tools, call 0.004 to 0.012 ipt. The increased feed Use of mechanical plastics by shops (800) 234-1560 or visit www.onsrud.com rates associated with heavier chip loads will continue to grow. Because shorten- or www.plasticrouting.com

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