SEPTEMBER 2005 / VOLUME 57 / NUMBER 9 BY SUSAN WOODS, FEATURES EDITOR

Machine builders develop technologies that improve thread grinding.

hread grinding is one of the most piece around its own axis challenging grinding tasks, be- while the linear axis T cause the operator has to be con- moves the workpiece past cerned with precise lead, pitch, and the to cre- flank and helix angles. It is not just ate the lead. The wheel grinding to a diameter, but producing a axis feeds the grinding specific form to a specific diameter. wheel into the workpiece The form may be a 60° V thread, a 29° to control the thread angle Acme thread or a gothic thread depth. for a ballscrew. This is difficult consid- Today’s CNC technol- ering the grinding wheel that is required ogy and the capability of to hold a fine radius in the root of the processors and digital thread also has to remove the rest of the drives make the task of material in a cost-effective manner. combining all basic mo- Because of material hardness and Drake tions relatively easy. This high-precision and fine surface-finish task can be performed by a requirements, some parts must be servomotor connected to a thread-ground. Precision ballscrews, ballscrew and table slide. lead screws, worm gears, thread gages However, thread grinding ma- and automotive steering components chines are available that are equipped are good examples. Precision parts with linear motors that eliminate the with internal threads that are typically need for the ballscrews on the X and Z thread-ground include ball nuts and axes. This is beneficial because linear thread ring gages. The GS:TE-LM linear motor grinder motors allow for greater acceleration, To meet the needs of companies that thread grinds a high-precision part. velocity and contouring capability. produce such parts, grinding machine Thread grinder builders have resis- builders continually develop new tech- Look at Linear ted the move to linear motors in the nologies. This article looks at some re- Basically, a thread grinder consists past due to the “cogging” nature of cent developments. of a rotary axis that spins the work- their movement at slow speeds, which are typical in thread grinding. (Typi- He added that linear motor ma- cally, the workpiece is moving between chines eliminate backlash, windup 1 and 12 sfm.) Cogging refers to and other problems associated with “jerky,” or nonuniform, motion. It is long-ballscrew drivetrains— caused by magnetic forces that cause particularly on the Z-axis. speed changes. Recent advances in lin- Backlash is lost motion in ear motor technology—the way the the interface between the magnets are aligned—have eliminated ballscrew and the nut. When this problem. changing direction, there is a “Another reason linear motors were small amount of rotation of the feared in grinding applications is ballscrew by the servomotor that grinding swarf,” said Jim Vosmik, pres- does not result in movement of the ident of Drake Manufacturing, Warren, nut. Windup means the ballscrew, the ITMF Ohio, which offers the GS:TE-LM, a coupler between the ballscrew and the International Machines of Florida’s thread grinding machine equipped with servomotor, the nut mount, and the Series 2005-THG thread grinder. a linear motor. “However, grinding screw mounts all give slightly while swarf will contaminate and destroy a trying to overcome the inertia of the ballscrew version if the user is mostly ballscrew assembly faster than a linear table and workpiece. It results in posi- doing thread grinding. “The tolerances motor, so there is really no greater risk tional error. of most threads are not so tight that they to a linear motor grinder than one that While acknowledging the advantages would require the extreme positioning utilizes ballscrews.” of linear motors, “there is the fact of accuracy that the linear motor solution machine price, performance and feasi- offers. A linear motor would be more bilities,” said Karl Giebmanns, presi- suited to extremely tight-tolerance dent of International Tool Machines cylindrical grinding.” of (Palm Coast) Florida Inc. “Be- cause customers prefer to pay as little Where There’s a Wheel … as possible for a grinding machine Another option for those who pro- that can achieve quality results, the duce threads is axis-parallel thread servo/ballscrew concept is still dominat- grinding, which can be performed on ing and will not be replaced any time United Grinding’s Studer cylindrical soon on machines requiring only short grinding machines. This capability table strokes. With [most] thread makes it possible to perform thread grinders, you have very short strokes, so grinding, along with regular cylindri- improved acceleration and deceleration cal grinding, in one setup. Normac doesn’t even come into play.” On a cylindrical grinding machine, The MX17 universal thread grinder’s wheel He added that the high temperatures the wheel is at a 90° angle to the work- and dresser operate in the same plane, generated by some linear motors are piece. Most threads have a helix angle, which helps ensure accuracy of the final costly to contain and have an impact on requiring the grinder to tip its wheel thread form and reduce dressing time. the final machine price. “The two major sideways slightly to match that angle. Linear motors can provide fast speeds priorities for the customer regardless of the maximum length the should always be based on machine is capable of grinding. The the outcome and quality of maximum speed of a ballscrew depends the part produced on the on its diameter and length, which can be grinding machine and the a problem when thread grinding longer price,” Giebmanns said. workpieces. As the mass of the ballscrew “Using a new technology just increases with its length, acceleration is for the sake of using it does limited and stiffness decreases. not necessarily make the With the linear motor, “you elimi- grinding machine better.” nate some of the problems with getting Mike Head, inside sales a master ballscrew in the machine that manager at United Grinding can hold the workpiece in its pro- Technologies Inc., Miamis- grammed position at all times over its burg, Ohio, said the linear United Grinding entire length,” said Vosmik. “This motor on its universal grind- On a cylindrical grinding machine, axis-parallel means you can produce 1m or longer ing machine would not be a thread grinding, right, allows thread and screws that are JIS C1 or better, with- benefit for thread grinding cylindrical grinding in one setup. An inclined out fighting the machine.” and he would recommend the grinding wheel, left, does not. Axis-parallel thread grinding is char- switch to another wheel with a turret The following companies acterized by a wheel not inclined to the wheel system and then grind the thread contributed to this report: helix angle of the actual thread. The on the part in the same clamping.” grinding wheel is perpendicular to the No special equipment is required to Drake Manufacturing Z-axis so that the rotation axis of the dress the threads. The StuderThread (330) 847-7291 wheel remains axis-parallel to the software helps generate the thread www.drakemfg.com Z-axis. dressing forms by allowing users to call During wheel dressing, the resulting up pre-existing examples that can be thread-profile distortion is taken into easily modified to meet their specific International Tool Machines account by StuderThread software. thread. This greatly reduces the pro- of Florida Inc. The software calculates the distortion gramming time required, as the soft- (386) 446-0500 and generates the dressing routine. ware generates the machine code based www.itmfl.com “What is happening is we are de- on simple data that is either directly the thread flank angle in the input or imported using a DXF file. form that we dress into the wheel,” said Drake Manufacturing also offers this Normac Inc. Head. “Because the wheelhead is par- wheel correction feature via its Smart- (828) 209-9000 allel to the part rather than inclined, Form software. It allows high-helix and www.normac.com this deformation that we purposefully axis-parallel internal thread grinding. induce results in a correct thread form “Instead of the wheel cutting at the cen- on the part.” terline, at lead angles above 10°, the United Grinding Technologies Inc. The benefits of this feature are the wheel starts to wash out the thread (937) 859-1975 user can grind the thread as well as form,” said Vosmik. “We have software www.grinding.com grind part ODs and IDs in one setup. onboard that corrects for that washout This results in better part quality be- and puts the ‘wrong’ form on the wheel cause one clamping equals better con- to get the right form on the part.” other, so there are limitations as to how centricity between the ground features; Another feature that not all thread the wheel is formed.” quicker turnaround because there is grinding machines have is a dresser On Normac’s 7-axis MX17 universal only one machine to setup and thread grinder, the 2-axis CNC program instead of two; and dresser is mounted on the same less part handling, which re- housing as the grinding wheel. sults in less part damage. When the wheel is set to a partic- “What would need to be done ular lead angle, the dresser main- on two machines, a dedicated tains position relative to the thread grinding machine and a wheel, so they are both operating cylindrical grinding machine, in the same plane. This helps en- can be accomplished on a single sure accuracy of the final thread machine in a single clamping, form—especially forms with resulting in less initial capital higher lead angles—and reduce expenditure,” said Head. dressing time. As an example, he cited a An additional benefit to this transmission shop making dresser configuration is the abil- bearing journals with a thread ity to dress the grinding wheel on the end. With a standard during roughing, which boosts United Grinding thread grinder or cylindrical With axis-parallel thread grinding, the wheel is productivity. Positioning resolu- grinder, the user could do one perpendicular to the Z-axis. tion of the dressing unit is within or the other. “You could grind all the di- that allows for traverse dressing of the 0.0001mm. ameters, but then you would have to wheel. Grinding machine builders continu- break the setup, send it through a spe- According to Jerry Martin, sales en- ally develop technologies like the ones cial thread grinder and grind the gineer at Normac Inc., Arden, N.C., mentioned, which improve the thread thread,” Head said. “The Studer ma- “many times the dresser is mounted in grinding process. Users just need to be chine can grind the diameters and then one place and the grinding wheel in an- made aware of them. q

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