CNC Tool Grinder | CNC Lathe Machines

CNC tool grinders get friendly

Mason, Frederick

The overworked term "user-friendly" really means something when it comes to controlling the motions of five and six-axis CNC tool and cutter grinders. This class of machine tool was introduced in 1977 and became quite capable of producing and regrinding all the basic round-shank fluted cutting tools. Yet, the software and control were definitely not friendly and ranked up there with five-axis mills in the difficulties involved in programming and controlling them.

The major news about CNC tool grinders today is that they are simpler to operate. The software requires only a page or two of data input for all the basic round-shank fluted cutting tools. User interfaces have advanced from writing command lines to graphical fill-in-the-blank formats. Operators can enter operating parameters and tool geometry with less time and trouble. And the Pentium, 486, or other fast computer chips calculate the paths of the tools and grinding wheels in seconds rather than minutes.

Whether machine builders use their own or industry-standard controls, they have all written their own grinding software for a wide variety of tool types. It is still true, however, that some data input formats are easier to work with than others. This is important when there can be more than a dozen questions to answer or a data screen or page. Setup time minimization is particularly critical for regrind shops or in-house cutter grind departments doing small lot of tools.

The other significant development is that the designs of the CNC grinder hardware have incorporated more years of experience. They have moved away from the lightweight machine designs inherited from manual cutter grinders and become more rigid and productive for grinding tools at high stock-removal rates.

On some models, coolant pressure is up to 100 psi (690 kPa) or more, coolant filtration is more sophisticated, the grinder's moving elements are well protected, sometimes with positive air pressure to repel dirt and liquid. Several models now come with polymer-composite or cast-granite bases to minimize vibration. The latest axis drives and encoders provide many of these models with positioning accuracy and repeatability equal to or greater than the needs of most tool manufacturers and resharpening shops.

Most CNC tool grinders are designed to use wheel packs with two, three, or four wheels to grind most tool types. This eliminates individual wheel changes but creates some complexity in getting wheels to the tools without interference. A wheel pack may consist of an OD wheel, a cup wheel, and probably a third wheel.

Popular New Design

Walter Grinders (Fredericksburg, VA) introduced its five-axis Helitronic Power double-ended spindle CNC tool grinder at IMTS a year ago. Aimed at both cutting tool manufacturing and regrinding, the fully enclosed, compact machine is fast and rigid, grinding a 3/4" (19 mm) diam four-flute carbide end mill from a solid blank in 5 min, 15 sec.

The software package has a data-bank with the commonly required tools to provide faster setup. Screens for the operator interface are nearly identical regardless of tool type, reducing the learning curve and simplifying data input. Normally only two screens of data entry are required, although up to eight can be used if additional data are required for special customer applications.

The data--such as primary and secondary clearance angle, rake angle, etc.--for HSS and carbide tools now reside in the database. The same goes for operating parameters such as feeds and surface footage, thus optimizing wheel performance. When the operator enters the wheel diameter, the rpm is automatically selected--according to the surface footage in the file. The machinist may, of course, override the supplied data without affecting the data residing in the database.

The machine's work envelope is an 8" (300 mm) grind diameter and tool length of 11" (280 mm). The double-ended grinding spindle on the cross-slide and the +200' C-axis rotation of the workhead give this circular machine design great flexibility. This was demonstrated by grinding 16 different tools (cylindrical and tapered end mills, step drills, and other step tools) with changeover times ranging from 1 to 3 min.

For automated tool certification, Walter is developing (with a partner) a three-axis and four-axis tool measuring machine employing lasers and cameras to measure rake angles and primary and secondary clearances on the OD and on the end teeth. This could, potentially, be coupled with a loader to transport tools from the grinder to the tool checking machine, for automatic tool verification and feedback to the grinder. This Helicheck device uses some of the same software as the Walter grinder data-bank.

Fast Tool Manufacturing

The Model 4000 five-axis Super Truflute from Unison Corp. (Ferndale, MI) is built for fast cutting-tool production and incorporates all the company has learned in the last forty years. The five-axis machine has hydrostatic round ways on the X, Y, and Z axes and a 22" (560 mm) cross roller bearing for the wheelhead swivel, providing extreme rigidity. This allows it to flute solid carbide at very high rates--perhaps twice what was possible even five years ago. For example, a standard 1" (25 mm) carbide four-flute square end mill is manufactured complete from the solid in 5 min, 15 sec. A 1" four-flute carbide ballnose end mill takes 7 min 30 sec from the solid.

The spindle drive is 10 hp (7.5 kW), speeds are from 2200 to 7500 rpm, and the maximum wheel diam is 10" (25 mm). The grinder takes tools up to 12" (305 mm) diam and up to 14" (355 mm) long.

Unison's own control employs an Intel 90 MHz Pentium processor for very fast calculations--the first Pentium chip in a machine tool control, says Unison President Fred McDonald. For example, the calculation for a tapered ball-nose end mill takes no more than 30 sec, compared to minutes on older-generation controls with slower processors. A large-type fill-in-the blank approach characterizes the new software, making it clearer for operators to use. Standard data pages, with a uniform format from tool to tool, can be filled in to program standard tool types.

A touch probe is standard on the Model 4000 for tool remanufacturing, although for tool resharpening alone, Unison offers a less costly, less powerful Model 2700. Its motor is 3.5 hp (2.6 kW) and revs to 7200 rpm. The 2700 carries the same software system as the 4000.

Bramac (Tucson, AZ) is another builder specializing in machines for the rotary cutting tool manufacturing industry. Its five-axis Model 038 CNC tool grinder can have up to five wheels in its wheel pack and has spindle options of either a standard 7.5 hp (5.6 kW) unit or a 20 hp (15 kW) high-speed unit for four-times faster feed rates cutting with CBN. It is offered by Ameri-Swiss (Scottsdale, AZ). The standard spindle revs from 100 to 7000 rpm.

Windows-Like Software

The five-axis CNC cutter grinding Quinto software package from J. Schneeberger (Elgin, IL) represents a considerable improvement over the software for their first CNC cutter grinders of a decade ago. It is in today's software mainstream of creating grinding programs by entering the tool data and wheel data into screen pages for given tool types. The screens do not confuse the operator with questions or data slots unrelated to the program at hand.

A helpful software tool is the flute simulation program, it displays the cutting tool, and the program may be adjusted interactively on screen if the flute parameters are not what the operator would have expected. This is not unusual, since even an expert can make only a rough guess as to what a flute will look like, given the combination of several tool and wheel parameters.

A standard database is supplied and is further expanded with user-defined tool designs. Database values or sensor-measured dimensions, such as the spiral pitch, are displayed in the editor window for verification. The CAD drawing of profile tools and step tools may be imported into Quinto, and the software calculates the grinding motions required, with the necessary compensations for spiral pitch, slot shape, or axial angles.

New among the line of Schneeberger CNC cutter grinders is the five-axis Gemini model, with Fanuc control, using a moving column design for good operator access. The wheelhead will swivel 360+' to achieve any desired helix angle. The machine is designed for resharpening, with a 5.3 hp (4 kW) water-cooled motor and double-ended spindle, capable of holding six wheels at a time. This reduces setup time if standard wheels are used.

Still another five-axis cutter grinder sporting a new windows-type graphical user interface is the Alfred H. Schuette WU 400, from Grinding Technology Inc. (Shelton, CT). It carries a NUM control, with Schuette's software. The multi-window software system "is pretty intuitive, with the screens showing you what you are changing on the tool or on the grinding wheel," says Rick Huston, GTI applications engineering manager. The probe can automatically determine effective tool length, tool diameter, lead, and radial position of the tool. The design of this double-ended spindle machine rotates the spindles around the cutting tool, rather than vice versa. There are three spindle options, with top speeds of 7000 or 12,000 rpm.

New Graphical Interface

Stan Huffman, founder of Huffman Corp. (Clover, SC), introduced a multiaxis CNC tool and cutter grinder for ball and radius end mill grinding in 1977. His designs used superabrasive grinding wheels, high-pressure flood coolant, and an off-line programming system to produce accurate and repeatable geometries on ball nose and corner radius end mills. Huffman's H150 series is, says David Drechsler, manager for grinding systems, "better, faster, and cheaper." The HS 150 series uses GE/Fanuc controls, drives, and motors.

The entire machine--including its cast iron base--has been optimized with finite element analysis. The rails and ballscrews are twice the size of previous models. Three different single-spindle models, and a dual-spindle model for high production, are available. A 10 hp (7.5 kW) spindle motor is standard. On the HS-155, the maximum wheel diam is 6 (150 mm) and the wheel spindle reaches 6000 rpm.

"Our graphical interface software is mouse driven, displays a graphic of the cutting tool, provides a menu for entering numerical and geometric data, and greatly reduces changeover and setup times," says Drechsler. Windows-style screens lead the: operator through sequential choices: identifying the part or type of part, retrieving a data file, probing the part, making a finished part program, and downloading the program. Also aiding setup, he adds, are the quick-change HSK toolholders and wheel arbors, and the integrated probing. Software optimization and increased axis rapid travel rates have reduced grinding cycle time.

Unusual Six-Spindle Approach

The six-axis Ewamatic 106 from Ewag Corp. (Lincoln, RI) is one of the CNC tool grinders for grinding fluted tools to offer a radically new design. Up to six grinding spindles are arranged in a circle and driven from a central 10 hp (7.5 kW) motor, one at a time. Each spindle is capable of carrying up to three wheels.

However, the real benefit of this design is less the increased number of wheels that may be carried and more that each spindle has unfettered access to the tool with a single wheel, rather than a conventional three-wheel pack, says Sam Lerch, Ewag president. Spindles are shifted in two seconds. The table carrying the grinding spindles may be inclined +25' to -15'. The control is the NUM 1060 Series II.

The machine grinds tools from the solid in the size range of 0.5 to 25 mm. The standard spindle speed range is 2000 to 12,000 rpm. For special procedures, one grinding spindle may be a high-speed 75,000 rpm spindle to drive small diamond wheels.

For controlled automatic production of tools, a 3-D measuring system is mounted on the grinding head to take up to nine measurements of the workpiece. The measuring system compensates for wheel wear and temperature-based deviations. For cutting tool production, the machine may be equipped with a robot arm to load/unload tools automatically. The controls for the arm are installed in the standard machine control panel.

Cutters to Tool Grinders

Star Cutter Company started as a cutter manufacturer in 1927 and branched out into building CNC cutter grinders about a decade ago. The Star CNC tool grinders have Allen-Bradley or Fanuc controls and are manufactured by Elk Rapids Engineering (Elk Rapids, MI).

The spindle of the six-axis, 5 hp (3.7 kW) ATG grinder takes a cup wheel, an angle wheel, and two OD wheels. The maximum diam wheel is 6" (150 mm) and wheel speed is 1000 to 5275 rpm.

Star provides dual-probe technology, with one probe for tool location and geometry verification, and a second probe for wheel data verification. Both are interactive with the other software programs, minimizing non-grinding time.

Tool setup is much easier than on earlier models. The operator selects the program for the type of tool to be sharpened and then fills in data for tool geometry, diameter, length, primary clearance angle, secondary angle, etc. Once tool geometry data has been entered, it is stored under a tool ID number for recall later.

Five Star six-axis CNC grinders, as well as a five-axis model dedicated to broach grinding, are used at the Chrysler Trenton Engine Plant (Trenton, MI). Two of the grinder operators in the cutter grind area, Mike Pittiglio and Paul Hacker, report that they generally work on holemaking tools in small lots, from three to a dozen tools being typical, and they can do the setups fairly rapidly with the software system.

In fact, they report, the CNC cutter grinders allow some tools to be completed in a single setup that formerly required three setups on the manual cutter grinders.

If a tool has been sharpened before, says Pittiglio, then all that is needed for proper positioning is to set up the right bushing, if it is a long drill or reamer, and probe a single flute. If the tool had not been run before, then all the flutes must be probed to tell the control the position data. He reports that the present control, with a 486 computer chip, can calculate much faster than two older model Star machines the shop also has.

Some Advantages of CNC Cutter Grinding

CNC tool and cutter grinding produces more consistent tools than manual grinding and is the only reliable way to grind matched sets of identical cutters. Diameters, angles, and lengths can be held more accurately than an expert toolgrinder can produce consistently on a good manual tool grinder. The teeth of a cutter that is CNC-ground are concentric, so that all carry an equal cutting load. The absence of runout yields tool life increases of 1.2 to 4 times that of manually ground tools. CNC-ground tools may yield productivity gains on multispindle CNC machines on the shop floor that outweigh any productivity gains in the toolroom itself.

Further, CNC grinding is much faster than manual tool grinding in part because it operates under flood coolant in an enclosed cabinet, maximizing the speeds employed with the diamond or CBN wheels without burning the tools. The edge quality of CNC-ground tools is superior because the wet grinding eliminates any edge softening from grinding heat Speeds and feeds in end-milling o holemaking operations may be raised 10 to 35% over those used with manually ground tools. It is also possible to achieve more regrinds per tool with CNC grinding because all the teeth were initially ground more uniformly and thus wore evenly.

The CNC tool grinders are expensive, however, ranging from $150,000 to over $400,000, but if they provide quality service to machining departments and minimize downtime and other losses due to tooling problems, they will more than pay for themselves.

A Challenge: Producing a Constant Helix on a Ball Nose End Mill

When milling with a ball end mill, most of the work is done with the end cutting edges, rather than the side edges. The optimum end-cutting tooth geometry is a positive rake angle to the axial center of the tool. The notch rake angle should be positive and blend, without change of direction, with the side cutting edge positive rake at the point of tangency between the ball end and the side cutting edges. In this "perfect" geometry, according to Unison Corp., the primary relief would be the same all around the tool from the straight side cutting edge to and through the ball to the axial center.

Because of the constant helix on the ball, full eccentric relief can be obtained providing such benefits as a longer cutting edge, uniform shearing action, increased chip removal rate, good dimensional accuracy, and good surface finish.

Unfortunately, until now this superior geometry has not been attainable in practice. Gary Vasher of Unison Corp. says the company has developed the machine and software to generate this geometry. "We have three methods of calculating the position of each point on a spherical ball nose," he explains, "and the result is the same with each method. We have also compared each to determine a grinding wheel path that truly produces a constant helix and positive hook around the spherical end of a ball nose end mill."