Renishaw's Ballbar quantifies machine capabilities for Gretone

Citation

(1998), "Renishaw's Ballbar quantifies machine capabilities for Gretone", Aircraft Engineering and Aerospace Technology, Vol. 70 No. 5. https://doi.org/10.1108/aeat.1998.12770eab.004

Renishaw's Ballbar quantifies machine capabilities for Gretone

Renishaw's Ballbar quantifies machine capabilities for Gretone

Keywords Machining, Quality, Renishaw

Located in Lytham St Annes, Lancashire, subcontract engineer Gretone operates 16 CNC machining centres to produce airframe components. About 80 per cent of work is for airframe manufacturers and the remainder, precision commercial engineering components. Machining capacity is up to a pallet size of 4,500 × 1,400mm. Batch sizes are six to 250 off, though aerospace companies generally order in tens. The general volumetric machining tolerances on most airframe jobs are ± 0.06mm, but obviously are tighter on pitch-circle-diameter (PCD) and bore specifications. "Before any contract is undertaken", says Gretone's quality manager, David Banks, "we ask ourselves, 'can we make it?' and 'can we check it?'"

When a new job is being considered, the two questions are put to a review committee consisting of Gretone's quality manager; IT (information technology) manager, Mark Wignall; commercial manager, Gary Baxindale and production manager, Ian Hawkins. "Every member of the review team has a input", says David Banks. "For example, if the quality control department know that manufacturing tolerances will be too tight for us, or that a machine would not hold required machining tolerances, then I use my input. To exercise an opinion or input, I have to know what our machines' capabilities are. The Ballbar, manufactured by Renishaw plc, is the quickest way to check and we have a history file on every machine tool."

Gretone uses the Ballbar to carry out a programme of regular checks on all its machine tools (Plate 3). Sometimes it can be shown that a machine can run machining spindle motors for longer periods than originally specified. For example, a Matsuura FX10 2PCS vertical spindle, twin-pallet machining centre had a guaranteed spindle motor change of 2,500 hours. A Ballbar checking programme revealed that the quality could be maintained for longer periods between motor changes. Spot checks can be performed quickly. The reasons why are varied ­ perhaps to verify that certain dimensional tolerances can be machined ­ to check whether any adjustment is necessary after a collision or repair, or to quantify a perceived accuracy problem.

"We always know what a machine tool is capable of doing", said David Banks. "If a machine is giving problems, we can quickly put the Ballbar on and determine if the machine is within the skill level." Apparently, on one machine which had accuracy problems, the Ballbar revealed that the backlash compensation system was over-compensating! The problem was solved by turning it off.

Plate 3 Gretone uses the Ballbar to carry out a programme of regular checks on all its machine tools

Another advantage of using the Ballbar is that the point can be determined at which the machine tool is no longer viable in terms of maintaining accuracy skill levels. The machine can then be sold off before machine operation becomes uneconomic.

It takes about one hour to set up the Ballbar system to check a dimensional relationship between two machine tool axes and receive a ten-point report. The report, outputted on a PC, not only provides a dimensional report but also highlights probable causes of error. Causes could be loose slideway gibs, worn slideways, malfunctioning servo motors, excessive backlash, etc. On the shop floor, inspector Kevin Shurey, is checking the X and Y axes of a Matsuura MC 760 V3 vertical machining centre. Gretone operates four of these "workhorses" of varying ages.

The Ballbar system consists of a ball anvil to position on the worktable or slideway, a bar coupling mounted in the spindle nose (for this test a drill chuck mounted in the spindle nose) and the measuring bar, in this case 150mm long. The Renishaw Ballbar electronic interface device, is mounted on the machine's spindle head and is connected up to the hardware and the PC. First, the spindle nose is registered centre to the ball anvil to give a zero-zero datum. The dial gauge is then removed, the spindle head offset from the anvil by 150mm and then the Ballbar measuring bar components are set up to link the spindle with the fixed ball anvil.

To run the test, the X and Y axes are activated to track the spindle head through a true circular path. The machine first tracks through 180° of arc to allow cycle speed to build and stabilise, then on through a test circle of 360°. The set-up is allowed to decelerate and stop through a further 180°. The cycle is repeated in reverse direction. The software then interprets the measurement data and the data are screened on the PC within a few minutes.

"The readout is invaluable", says David Banks. "It displays the total error with respect to a true circle, then outputs a ten-point breakdown of machine condition". The test revealed a 65µm maximum circularity error. Highlights of the ten-point report included 42 per cent of the error owed to 17 per cent backlash error in X (26.9µm) and 25 per cent backlash error in Y (25.6µm). It was found that 11 per cent was owing to scale mismatch and 10 per cent to lateral play in X and Y. Also, 8 per cent arose from a cyclic error in Y. "The Ballbar is really excellent", says Kevin Shurey, "You can't fault it". On larger machines, such as Gretone's HommaFA1 5/40 5-axes machining centre, different parts of the bed would be checked for a fuller analysis.

The Ballbar was purchased for around £7,000 in December 1996. "We now use it to check new machines before we sign the installation agreement", said David Banks. In general the company checks three machines per month using the Renishaw Ballbar. A history file is maintained of all machines in the plant. Machines include Niigata HN series, Mitsubishi MPAs, Mazak H630, Matsuura FX5s, FX10s, MC-900Hs and MC-760s, Matchmaker 800, Homma FA15/40, Hitachi Seiki HC800A and a Hamai 5VA.

Customer approvals include BAe, Shorts Brothers, Aerostructures Hamble and Lucas Aerospace. Gretone is also approved to ISO 9002 and is currently entering an expansionist phase. The company's group parent, Ariel Industries plc, has funded an 18-month, £2m expansion programme at Gretone for extended premises and eight more machining centres.

For further details contact Renishaw plc, New Mills, Wotton-under-Edge, Gloucestershire GL12 8JR. Tel: +44 (0) 1453 524524; Fax: +44 (0) 1453 524102.