Screw, Worms

Deep groove dry milling machine and a super precision helical grinding machine

An innovative deep groove dry milling machine and a super precision helical grinding machine has transformed the production of precision helical profiles such as rotors and worm gear shafts.
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The production of precision helical profiles such as rotors for fluid and air compressors and worm gear shafts used in rotary positioning tables has been transformed by Holroyd using an innovative deep groove dry milling machine and a super precision helical grinding machine with on-board measurement and auto adjustment of the grinding wheel surface.
The use of these machines represents a revolution in thinking for the industry and it's resultant improvements in production speed, accuracy and consistency are impressive to say the least.
The use of these two machines, a Holroyd 2E high-speed rotor milling machine and aHolroyd TG150 thread grinding machine is best illustrated by a dedicated compressor rotor production cell at the company's manufacturing plant in Rochdale.
Although in this instance they are being used to produce rotors for compressors, both machines are designed to produce complex helical profiled components and are equally at home producing gear parts such as worm shafts.
Both machines were designed for this dual purpose and are used and sold as such.
The cell - the 'Gemini' production cell at Holroyd produces on average 40,000 compressor rotors per annum.
In the last year of production there has not been a single failed or scrap component or a missed production target.
The reason for this consistency is essentially the use of dry milling, on-machine measurement, and on-machine auto adjustment.
Before we look at the details, we should take a brief overview of the process line.
Stress relieved (C1141) grade steel is delivered in bar format, the rotor shafts are turned conventionally at both ends, and the blanks loaded onto a unique Holroyd 2E helical deep-groove milling machine (more about this later).
Five helical grooves are then milled from the blanks, leaving 250 microns of metal for final grinding.
Spindle shafts and rotor ends are then finished on a Danobat CNC lathe.
The rotors are moved between machines on colour-coded pallets to avoid confusion, although they do look significantly different at each stage of the process.
The semi-finished rotors are then placed onto an automated loader for final grinding on a Holroyd TG150 thread-grinding machine.
Every component leaves this machine finished, measured, checked, and approved without any need for external measurement equipment.
It is worth mentioning at this point that Holroyd designs and builds all its machines on site, the machines are sold to other manufacturers and also used in-house to produce finished products for customers who prefer to sub-contract.
This way of working ensures that the development of Holroyd machines remains integral to achieving the finished product and prevents the company from having to rely on machines out in the field for long term feedback and development work, as is the case with so many other machine tool manufacturers.
Dry milling helical profiles from blanks - looking at the important areas of this process in order, we first arrive at the Holroyd 2E machine; this cuts a 25mm deep and 30mm wide helix groove in the rotor blank.
The piece is fed in a reverse action back across the cutting head, which moves vertically, and horizontally in the opposite direction in order to achieve the helix groove profile.
This machine forms a groove in a single pass and takes only seconds to do it.
The cutting head is able to remove so much material in one action because the heat generated is transferred to the swarf.
The swarf is then removed from inside the machine bed on a conveyor.
Since there is little or no heat transfer to the work piece, there is no cutting/cooling fluid required.
The machine works dry and so is also very compact and economical, as it does not require additional liquid filtering, pumping, and cooling apparatus.
Paul Hannah, Engineered Products Director at Holroyd comments that, "the swarf takes the heat here, allowing us to remove a large amount of metal very quickly and without causing a significant temperature change; the piece temperature remains stable at about 6 degrees above room temperature during cutting.
By quickly cutting the blanks to within 250 microns of the finished piece we ensure that the finish grinding does not have to remove too much metal and we achieve the end results in a much shorter cycle time."
Thread grinding procedure - after the rotor ends and shafts have been machine finished, the rotors are then finally ground on a Holroyd TG150 thread grinding machine.
Rotors are transferred to an autoloader feeding the thread grinding machine.
The TG150 auto-loads the part and clamps it hydraulically using a close-tolerance shaft chuck, being supported at the non-drive end by a hydraulically operated tailstock.
The machine checks the part's position against a datum using a precision ruby tippedscanning probe mounted within the machine.
The grinding head then generates a first thread/flute to within a 20microns of the finished accuracy.
The grinding wheel creates the convex or concave curved profile of a male or female rotor by grinding out from the centre crease with two surfaces of the wheel simultaneously, one side shaping the inside and other the outside of the helix.
The probe then scans the profile to confirm the tolerances, if the profile is correct and within tolerance, the next flute is ground to finished accuracy.
The second flute is then checked whilst the work piece is still in the machine, if it passes, then the remaining three flutes are ground and the first one finished, removing the last 20microns left by the first pass grind.
If the part does not meet tolerance requirements after the first flute has been ground, then the TG150 calculates the necessary adjustments to the grinding wheels form or axis positions and then re-dresses the grinding wheel using two diamond-cutting wheels housed inside the grinding wheel arm.
After the grinding wheel has been re-dressed, another flute is ground to 20microns from the finished size and checked.
Providing the tolerances are now met, the next flute is ground to finished dimensions and checked, this confirms the adjustment was correct and the rest of the threads are ground including the first two, which are finished.
Paul Hannah states that, "the re-dressing procedure is carried out automatically by the machine before the first thread is ground and rarely does the machine need to make any further corrections after the first pass.
On the occasion that it does, the correction is effective, and the second thread passes, the grinding cycle then proceeds as normal.
The process is so reliable that if a part did not pass after the machine had had two chances at correction, then we would know there was a problem with either the preparation work or the materials being used, i.e grinding wheels or steel."
The main benefits of making measurements and adjustments on the machine during the normal production cycle are that; not only is every item is fully checked to the same accuracy as it would be on a bespoke co-ordinate measurement machine, but removing the need for off machine measurement cuts set up times to an absolute minimum.
As a result, the speed and repeatability of the overall process is far better than any current conventional approach.
On machine measurement - Paul Hannah states that The Holroyd TG150 machines are "designed and manufactured to the same specifications and tolerances as purpose-built co-ordinate measuring machines (CMMs).
We use the same scanning probes as the best CMMs on the market and also similar style closed-loop servo motion control set-ups featuring high-precision encoders and scales.
Our machines are then calibrated to the same standards as a CMM, giving us the guarantee of accuracy that we required from the TG machines.
We no longer have to physically remove a product to check and validate machining tolerances."
This machine also neatly side steps the thorny question of environmental conditions, crucial to the reliability of external measurement techniques.
The probe is mounted internally, it exists in the same environment as the machine and so temperature fluctuations between machine tool, work item and measurement device do not occur.
If environmental conditions change, the machine adapts.
A simple probe calibration routine at the start of each scan cycle provides a validated reference point to begin machining.
The probing cycle itself takes place in the complete absence of any machining forces and is therefore dynamically and kinematically stable, the weight, precision and stability of the machine bed being comparable to a CMM when the machining elements are static.
The component is held in the same fixture as used for machining and, therefore, corrective rework is possible from an unaltered base point.
Furthermore, the principle of measuring 'on the machine' offers a crucial and distinct advantage: since the part is measured in-situ, the errors that inevitably arise during removal and transport of the part to a remotely sited CMM are eliminated.
The piece does not have to be reloaded into the machine and then re-ground and removed again for validation of the correction.
There is also an obvious (and major) cost advantage to be gained here; by avoiding breaks in production to check accuracy, throughput is maximised and the thread-grinding machine works at its most efficient and productive level.
The Gemini cell at Holroyd works 24hrs on a shift basis.
On-machine dressing of grinding wheels - to complete the work - test - rework cycle on one machine, two on-board diamond-discs dress the grinding wheel.
The photograph shows these components in black on a machine during build-up.
Paul Hannah describes this feature as the main element in removing "the black art" of precision machine set up, "operator skill level is always a variable in complex machining processes, and accurate adjustment often takes years of knowledge and experience to perfect."
Holroyd claim to have distilled this knowledge and experience into the operating intelligence of the machine.
Using the data from the on-board measurement, it is able to accurately predict the minute alterations needed to the profile of the grinding wheel to achieve the desired result and make the changes on-the-fly.
Routine wear in the grinding wheel is also addressed this way since the dressing process is carried out before every new piece is ground.
In addition to the controlled repeatability this system delivers, the time savings again are substantial.
Larger production batches can be achieved and grinding wheel replacement is totally predictable.
The level of on-board adaptation offered by the machine also means that set-up times for new component runs are also drastically reduced as most of the set-up is automated.
Autoloader - bearing in mind how many components are being produced, something that strikes you as soon as you enter the production cell is its compactness.
A contributory factor to this impression is the auto-loader feeding the TG150 grinder.
In order to maximize the machine's operating time, semi-finished parts ready for final grinding are loaded onto a conveyor, the machine then uses a servo driven crane and gantry system to pick each component and load it, the moment the machine is ready.
This is the only aspect of the cell that has been updated since it was first installed.
According to Paul Hannah, the results were beyond their expectations, achieving an increase of three additional completed components per shift.
Operator interface - the front-end software used to control the TG150 is Windows based and so familiar to anyone who uses a PC.
By avoiding the need for complex programming knowledge, Holroyd have made the machine far more accessible for operators other than themselves and ensured that a suitably qualified engineer can achieve target results with basic training.
Controls for routine operation are high durability push buttons on the interface panel, but programming and data viewing is carried out on a touch sensitive screen for speed and maximum ease of interaction.
Holroyd have a policy of installing every new machine in their own premises and training the relevant staff using genuine batches of the customer company's own finished products.
The machine is then transferred to the customer, fully commissioned and the training batches are repeated at the customer's site.
The results - interlocking rotors such as the ones produced by Holroyd's Gemini cell are used in air/gas compressors, refrigeration systems, vacuum pumps, and general precision fluid pumping applications.
The accuracy demanded by these applications can be within 5microns over the complete curved surface and so represent one of the finest mainstream industrial tolerances.
Some gear production undoubtedly does not currently reach these levels of accuracy, but by using Holroyd machinery it becomes consistently achievable.
The profiles may change when producing gear parts, but the accuracy remains.
Overall, Holroyd claim to have reduced the time taken to produce a batch of finished components using the Gemini cell by half when compared to more traditional methods of machining, measurement and adjustment.
They also reckon to have achieved a level of repeatable accuracy a complete order of magnitude higher than any other cost competitive system.
The accuracy levels, they claim, are comparable to those achievable with the use of much slower and vastly more expensive production systems and working environments.
The overall component cost is therefore kept as competitive as possible.
Holroyd are based in England but their market is truly international, exporting 98% of their equipment to companies outside the UK, Europe only represents their third largest market after the USA and Asia.

Holroyd