Flexible 5-axis manufacturing cell for close-tolerance unattended machining
In November 2014, Huddersfield engineering firm Reliance Precision (www.reliance.co.uk) strengthened its 5-axis machining capability by purchasing a German-built Hermle C 22 U machining centre. It was fitted with the manufacturer's IH60 integrated handling system and a 24-position magazine for housing workpiece pallets from System 3R.
The flexible manufacturing cell (FMC), which was installed by Hermle's UK agent Geo Kingsbury (www.geokingsbury.com), is the first automated machining centre with a vertical spindle in the Huddersfield factory. All other machines on the shop floor served by pallet pools or twin pallet changers are horizontal-spindle models.
Describing itself as a specialist engineering company providing custom-built, high precision components and sub-systems for diverse applications, Reliance Precision has extensive in-house design, development, prototyping, manufacturing, assembly and test facilities. Half of turnover is derived from the scientific and medical sectors, while aerospace and defence accounts for another 30 per cent.
The company has operated three 5-axis vertical machining centres from a different vendor for several years but wanted to upgrade this area of production to meet customers' demands for ever higher precision.
Alex Greenhalgh, Estimating Manager, commented, "The flexibility and high level of spindle utilisation possible on the Hermle C 22 U FMC were prime reasons for selecting it.
“We wanted to increase efficiency while reducing manufacturing costs, which the automation equipment enables by allowing extended periods of minimally-manned machining.
“The workpiece magazine is not fixed in front of the machine, but moves to the left at the touch of a button. The configuration provides free access to the working area for setting the machine as well as running off small batches or items requiring a high level of operator intervention. This level of versatility is important to us in order to future proof our operations.”
He went on to say that they have been impressed with the accuracy and reliability of a 4-axis Hermle machine installed in 2013 to manufacture a tightly toleranced part for an aerospace customer. Moreover, the service provided by Geo Kingsbury in terms of installation, commissioning, applications support and training has been exemplary and a good relationship has developed between the two companies. These factors had a significant bearing on the choice of the latest machines.
Kenny Marsden, Reliance Precision's Milling Department Team Leader said, “Even though the C 22 U has been producing for only a few months, its accuracy is already evident. It has been proved by the on-machine probing, which verifies that tolerances have been achieved.
“The system is so precise that it prompted us to question results from our existing shop-floor co-ordinate measuring machines. It led to our purchasing two new Zeiss Duramax CMMs, one for milled components and another for turned parts.”
He confirmed that manufacturing processes on the Hermle are right first time and remain within tolerance, from first- to last-off. The machine can therefore be left to run largely unsupervised. In contrast, to ensure that some of the more difficult jobs are in control on other machines, it is difficult for the manufacturing engineer to step away.
A majority of 5-axis machining on the FMC at the Huddersfield factory concerns prismatic components, where access to multiple sides of the part with a high degree of accuracy is the primary requirement. It ensures that the tool can machine the component more efficiently in fewer operations, without the need for costly fixturing. Dimensional accuracies and positional relationships on components are often demanding and minimising reclamping helps to achieve them.
Feature-rich aerospace parts in titanium and stainless steel, for example, frequently require bores accurate to 5 microns, true dimensional positions to within 10 microns, and have 5-micron tolerances for squareness and parallelism. An instrument for a down-hole oil and gas application has two steel dowels hardened to 60 Rc on which two machined flats have to be within ±2 microns of the centreline.
These levels of precision are achieved routinely on the FMC with minimal oversight, as well as on the original 4-axis Hermle, whereas a significant amount of manual intervention was previously needed on other types of machine at Huddersfield to achieve similar quality. Much production work has already been transferred to the 5-axis machine, especially the more complex and exacting aerospace jobs.
Time savings are considerable. In one instance, an aluminium housing that used to take up to an hour to clock into position on another machining centre takes just 5 minutes to set up on the C 22 U. It is largely due to in-cycle probing using a Blum TC52 in the spindle, which allows initial part location to be precise, rather than having to define its position accurately in a fixture. Consistency and repeatability are just as good, if not better, and cost savings on workholding are another benefit.
The success of the Hermle installations has already prompted Reliance Precision to order a third machine - another 5-axis C 22 U but without the pallet handling system. It will be used for development, freeing the FMC for production. However, the table of the new machine will be automation-ready and could be interfaced with a System 3R pallet magazine at any time. Installation of a fourth Hermle, another automated 5-axis cell, is already envisaged.
Machine specification includes very high accuracy
The C 22 U was supplied with an enhanced specification including full consideration for thermal stability of the cutting fluid and enhanced accuracy for all machine axes throughout the entire 450 x 600 x 330 mm working volume.
To assist in achieving such high precision, the shop floor is maintained to within ± 1°C. Heidenhain linear scales are fitted as standard to ensure high resolution feedback. The thermally stable polymer-concrete bed is vibration-resistant, contributing further to high accuracy as well as extending tool life. The table does not move, so the machine is thermally symmetrical around the X-axis and compensation needs only to be applied in the Y-axis. The three linear guideways are above the working zone, keeping them free from swarf. Probing compensates for any thermal movement in the Y and Z axes.
Reliance Precision has standardised on the HSK-A63 spindle interface for its Hermle machines, providing stable face-and-taper contact with the toolholder. The HSK tool system provides better accuracy in the Z-axis, especially at higher spindle speeds. The toolholder is also smaller, lighter and stiffer than conventional 'BT' type spindle tooling. Blum laser tool length measuring and breakage detection is included, commensurate with unattended machining duties.
To help maintain the high level of precision, Hermle condition monitoring was performed after the machine was commissioned at Huddersfield to benchmark its performance over a standard range of tests. They are repeated periodically to determine accuracy drift and predict when the next routine maintenance will be required.
Mr Marsden continued, “There are some niceties on the Hermles that you only get on top-end machines that are very useful.
“For example, the high-pressure coolant is fully programmable anywhere from zero to 80 bar, enabling us to match coolant delivery to the type of machining operation for best results and minimum energy consumption.
“Another good feature is the protection afforded to the spindle provided by six collapsible sleeves. Geo Kingsbury is able to replace the crushed sleeves and get the machine running again quickly, saving a lot of time and potentially the cost of a new spindle.”
A System 3R GPS 240 chuck has been integrated into, rather than bolted onto, the rotary table of the FMC at Reliance Precision to ensure high precision and avoid losing Z-axis travel.
So that the manufacturing engineer is able to view the machining area when the pallet magazine has been pulled in front of the machine for an automated production run, Reliance Precision has installed a camera pointing at the table and an external monitor positioned above the machine's Heidenhain iTNC 530 control system. The images could be monitored remotely, but this is not done at present.
Mr Greenhalgh concluded, “The performance of the Hermle machines has really opened our eyes as to what is possible with milling and is helping us to build on our reputation for top quality and timely delivery.
“Right from the start in 2013, the 4-axis machine resulted in a quantum step forward in our technical ability, placing us firmly in control of all our processes and avoiding manufacturing engineers having to constantly attend to the more difficult jobs.
“Now, with two 5-axis Hermle machines and a third planned, and with the added benefit of automation, we are able to achieve high levels of process control and efficiency on demanding components with micron level tolerances.”
Press release issued by Reliance Precision Ltd on May 15, 2015
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