Drake in Modern Machine Shop Magazine:
Realizing the CNC Thread-Grinding Concept

Hardware and software capabilities must be combined for productivity with precision in thread grinding. The critical element in achieving the full potential of CNC automation for thread grinding is the ability of the diamond roll to contour-dress wheel forms, accurately and automatically.

Three factors influence the decision to grind threads rather than turn them: material, accuracy and complexity. In hardened materials, normal thread-cutting or rolling methods are not practical. Thread grinding produces much more precise work, holding to "tenths" or better, compared to turning. And, because of the ability to eliminate tool wear effects by dressing the wheel, grinding is the method of choice for threading, whether for full or interrupted cuts, as on taps.

Often, grinding also displaces cutting in many thread applications because:

• Grinding will usually produce a better finish.
• Grinding typically allows faster set-up.
• When machining, any change in thread variables requires a tool change.
  With CNC grinding, you just redress the wheel to the appropriate form.
  

Machine Design.
In the design, engineering and production of a range of thread grinders, the best of several disciplines must interact to achieve new standards of precision and create machines that are "parts ready," as delivered. By blending traditional machine-building craftsmanship, advanced component design, accessories and subsystems, the latest in CNC technology and software customization techniques, it is possible to produce thread grinding machines that provide exceptional versatility and cost-effectiveness. This is a review of the principles that can be used, first to create a machine configuration that is capable of high precision work, and then to equip it with the "brains" to execute that precision work, repeatedly.

No matter what the base material is (for example, cast iron or steel weldment), the strength and stability of the base are critical to grinding with a minimum of vibration. A stiff base is required to respond to very small increments of movement at a consistent rate. This dimensional stability is vital for producing excellent surface finish and assuring part-to-part and shift-to-shift repeatability.

The slideway system also contributes to the stiffness of the machine. To eliminate unwanted play or side-to-side motion, guideway length must be many times greater than the way width and greater than the way spread. AC servo motors provide excellent positioning capability. The most appropriate slide system and materials should be selected to suit each specific application:

• Nonmetallic liners.
• Linear recirculating ball carriers with guideways.
• Hydrostatic.

For optimum positioning and repeatability, slide-mounted and sealed-glass scales are available, which can provide feed-back resolution of 0.000,004 inch.

Servo Drives.
To assure accuracy, precision ball screws and preloaded ball nuts are used in conjunction with CNC servo drive units. The use of hollow-shaft servo motors carries the thrust loads developed by the servo motor/ball screw assembly. This arrangement eliminates the normal ball screw bearing package and flexible coupling.

All key machine functions must be controlled automatically, precisely and consistently to maintain reliability. The servos allow the machine to be electronically "stiff," as well. The system should recognize and compensate for process variables, such as a hard spot suddenly encountered in the workpiece. High feedback resolution allows the machine to adjust to these changes in force without excess movement of the wheel. The ability to maintain speed and position under varying loads is essential in grinding through an interrupted cut.

Since programs are designed for the specific parts to be produced on a CNC thread grinder, setup is greatly simplified, with setup time often dropping from hours to minutes. Cycle times are minimized by using the most efficient paths and sequencing. The ultimate goal of emphasizing exact control is improved productivity and repeatability. Figures 1 and 2 show examples of where exact control of servo drives is critical.

Another is the capacity to grind ball screws of extreme length-to-diameter ratio. Such long workpieces require the use of steady rests; however, the rests themselves often induce minor variations or bending of the screw. See Figure 3. Through the use of probes and effective software, not only is it possible to position the wheel properly within the machined groove of the ball screw for final grinding, but the machine can also "map" the OD by probing across the screw's length and automatically compensate to maintain precision throughout the length of the track. Menu data tables can be provided to allow input of pitch diameter measurements for finished passes.

Total Abrasive Capabilities.
On a CNC thread grinder, a full range of spindle speeds and power capacities is necessary to allow the use of all modern abrasive media: aluminum oxide, ceramic, cubic boron nitride (CBN), and diamond. For external work, wheel speeds of 1000 to 3500 rpm (up to 16,000 sfpm) can be achieved with an 18-inch wheel diameter. Internal machines typically operate in the 20,000 to 70,000 rpm range. A machine's range of speeds determines what workpiece materials can be successfully and efficiently threaded.

Contour Dressing.
CNC contour diamond-roll dressing is the one way that thread grinding can be fully automated. A precision diamond roll can be used to create the exact thread-form profile on the abrasive wheel, using a continuous path movement and holding tolerances in the tens of millionths. See Figure 4, which shows a stylized schematic of the "mirror" relationship between the negative thread and the positive wheel form.

Automatic dressing assures that the wheel is properly shaped and sharpened at all times. Frequency of dressing depends on such factors as the surface finish needed, wheel type and removal rate.

Another feature of the diamond-roll dressing is that the ratio of the speeds of the diamond roll and wheel can be adjusted to optimize performance. For example, increasing the feed rate will make the wheel more aggressive, while reducing it will allow the wheel to generate a finer finish on the workpiece.

A last option with diamond-roll contour dressing is that in which the roll and wheel rotate. "Counter" dress has roll and wheel moving in opposite directions at point of contact and typically yields a finer finish. "Uni" dress has roll and wheel turning in the same direction. This tends to open up the wheel surface for more aggressive grinding.

Software Issues.
With CNC technology, all of the vital parameters for precise, repeatable wheel dressing can be built into the software program. With the precise positioning described earlier, the diamond roll can create a wheel contour to mirror the desired thread in 60-degree, Worm, Acme, Buttress, Gothic Arch, or Annular Groove, in inch or metric standards.

Data such as wheel and work sfpm, number of passes (rough, semi-finish and finish), depth per pass, frequency of dress, and so on, can all be entered through the menu. Formatted prompts are provided to assist the operator. With these facts available, it is possible to provide proof of cycle time, generating a performance guarantee in parts per hour for a given machine-wheel-and-part combination, setup after setup.

Some machines offer built-in parts generation software. With this customized software, the user is not required to do any part program development or EIA-formatted programming. All part characteristics can be entered into the system via menu entry. These systems precisely generate both wheel form and grinding cycle and may feature diagnostic messages, system status, fault messages and data storage for future recall. See Figure 5.

Certain systems offer an optional "macro-executor" utility, a feature that provides the ability to configure menus and prompts that are tailored to individual needs. Basic menus should include:

• Parts variables
• Set-up variables
• Dress variables
• Wheel and roll variables

Help screens to support these menu functions are a variable feature. Also very valuable is an additional feature, a "handy" screen, that gathers and displays the data that each user chooses to have available to the operator. It duplicates facts that are shown within the basic menu screens, bringing them together into one, handy display.

Programming should begin with the customer's operating personnel working with the builder's engineering specialists who can size and spec the system to the individual application. The engineers can then create specific function modules that plug into the machine's CNC system. Even after a machine is installed and running, it should be a relatively simple matter to have new control modules created to accommodate changes in parts or processes.

Families of Parts.
By partnering with each customer's design and manufacturing engineers throughout the development of a machine, it is possible to simplify setup for a family of parts by identifying and controlling all of the key process parameters involved. Simplified setup allows grinding machines to be used for both high-production volume and lower quantity, job-shop environment work, with equal effectiveness.

The final step is to isolate and analyze both machine and process variables and program the software to make the necessary adjustments to compensate for them.

A New Standard in Grinding.
In concept, CNC thread grinding is clearly an attractive option. More important, by blending traditional craftsmanship, advanced science and a willingness to custom engineer each machine, shops are finding that new levels of reliability, performance and cost effectiveness in thread grinding can be achieved.