Low cost centre grinds gears to better than DIN 2

Although easy to set and operate, a grinding centre uses a novel machining system to achieve better than DIN 2 accuracy for the form grinding of high precision helical, spur and worm gears.
Holroyd's new Smart GTG2 grinding centre employs a unique (patent applied for) machining system to achieve DIN 2 levels of accuracy - and beyond - in the form grinding of high precision helical, spur and worm gears. Designed with high levels of onboard intelligence, the GTG2 is extremely easy to set-up and operate. It also benefits users with onboard CMM and an automatic, in cycle wheel dressing feature that enables closed loop profile control to be achieved.

'Our design aim was to produce the lowest cost, highest accuracy grinding machine for helical and worm gears in the world, and I am sure that we have succeeded,' says Dr Tony Bannan, Holroyd's Engineering Director, who led the design and manufacturing team that produced the machine.

'Although the GTG2 is designed to provide DIN 2 levels of accuracy, generally, it has exceeded this level under optimum operating conditions, producing its 'best ground' performance with a tooth profile of DIN 1, considered by many to be pinnacle of grinding performance.' The GTG2 is an extremely versatile gear grinding centre, developed for one-off or batch production of high precision helical, spur and worm gears in sizes up to 350mm diameter and 160mm gear face width.

The machine's exceptional performance is the result of a design synergy that combines a 'patent- pending' machining method, with high levels of machine intelligence and onboard features such as automatic co-ordinate adjustment, in-cycle wheel dressing, integrated profile management and CMM.

Holroyd's patent-pending machining method has been developed to compensate for helical twist, an unwelcome condition that occurs when helical gears are 'lead crowned' to improve meshing and reduce noise and wear.

Lead crowning varies the amount of material removed from the flank of a tooth across the face width, by causing the tool motion to deviate from a true helix.

However, in form grinding this has the undesirable effect of causing the profile of the tooth flank to vary across the face.

In many applications this 'error' is not of concern, but in high precision and low noise applications it affects gear wheel performance by concentrating loads on particular areas of the teeth during meshing.

Correction of this problem is achieved on the CNC controlled GTG2 machine by calculating additional motions of the grinding wheel and by then controlling them during the grinding operation using bespoke software written by Holroyd engineers.

In the machining process, the workpiece is rotated about its axis and translated parallel to its axis relative to the tool, and the tool is also moved so as to vary continuously the angle of inclination of its axis relative to the axis of the workpiece.

In this way the generating errors (i.e the deviation of the groove surfaces actually produced from those desired and specified) are reduced along each line of instantaneous contact between the tool envelope and the groove surface being machined.

The resultant benefits are better tooth contact during meshing and improvements in torque transfer efficiency.

Although it performs a highly complex operation, the technology that underpins Holroyd's new machining method is not unwieldy for the user.

Neither does it mean that the machine is difficult to program and use.

On the contrary: the GTG2 is referred to as a 'Smart' machine because of its high levels of onboard intelligence, which make it easy to set-up and operate.

The machine is equipped with Holroyd's own HTG8, 8 axis CNC with facilities such as an Advanced Touch Screen Interface and Integrated Profile Management System.

The touch screen interface enables an operator to take a typical design drawing and enter the specification directly into the machine; leaving high levels of inbuilt intelligence do the rest.

What this means in practical terms is that the machine, by integrating facilities such as automatic co-ordinate adjustment, in-cycle wheel dressing, profile management and automatic programmable cycles, replaces the variable of operator skill in precision machine set-ups and operations with machine intelligence.

By distilling operator knowledge and experience into the GTG2's CNC controller, the machine is able to take data from its (optional) on-board probing system and predict the minute alterations to the profile of the grinding wheel required to achieve any desired profile and make these alterations 'on the fly'.

An example highlighting the proficiency of this system is a workpiece that does not meet tolerance requirements after a first grinding pass.

Following on-machine measurement, the GTG2 calculates the necessary adjustments to grinding wheel form or axis position and then passes the data to a 2-axis CNC wheel dressing system to achieve on machine dressing of the grinding wheel.

All of these operations are achieved automatically and rarely does the machine need to make further corrections after the first pass.

Holroyd's integration of the in-cycle wheel dressing system into the GTG2 is not a one off, rather it is part of an overall design strategy that takes a number of functions crucial to precise and effective gear grinding and integrates them into the machine as Automatic Programmable Cycles.

The GTG2 offers users eight of these cycles to simplify and improve gear production.

In addition to automatic wheel dressing they include: * Gear Tooth Grinding, with Optional Probing and Form Measurement.

* Repeat Cycles with Nesting up to 99 Times.

* Fillet Radius or Trochoid Root Form.

* Gear Crowning Cycle for Modified Tooth Forms.

* Lead Crowning by Bob Crowning, or by Adjusting the Lead or Combination of Both.

* Programmable Constant Peripheral Grinding Wheel Speed, Based on Actual Wheel Diameter.

* Optional Programmable Component Taper Correction.

Optimising the performance of the GTG2 in all of its bespoke and automatic operating cycles is a highly effective measurement system, provided by an inbuilt CMM.

This is designed, manufactured and calibrated to the same specifications and tolerances as standalone CMM, and also uses the same high level 3D scanning probes.

The CMM function is used at the start of each scan cycle to provide a validated reference point for commencement of the grinding operation.

Importantly, the probing operation, which is integral to this process, takes place without any machining forces and is therefore dynamically and kinematically stable.

In addition, as the workpiece is held in the same fixture used for machining, there is no requirement for off- machine measurement, and no consequent loss of position for corrective rework.

As a result the measuring process, overall, is that much more accurate and reliable.

* GTG2 Technology recognised for Queens's Award for Innovation - even before its major market launch, the technology that underpins the GTG2 grinding centre has been recognised by the prestigious Queen's Award For Innovation 2004.

'This award is an early recognition and a validation of all that we were trying to achieve with our design,' said Bannan.

'We have produced a highly cost competitive machine with grinding technology that literally leads the world, demonstrating our continuing ability to produce winners in the increasingly competitive global market for high precision machine tools.