Here’s a statistic to make you think. Onein 10 of Martin-Baker’s ejector seats

actually saves a life.Visit the website ofthe world’s most experienced ejectionseat and related equipment manufacturerand you’ll see the current tally of livessaved stands at 7046. Six lucky peoplecan owe their lives to these seats in 2004alone.They even get to qualify for lifemembership of the ‘Martin-Baker TieClub’,confined to persons who haveejected from an aircraft in an emergencyusing one of the company’s seats.

A few more numbers.This Britishcompany,located in the quaintly namedvillage of Higher Denham,near Uxbridgein Middlesex,boasts a 50-year pedigreeand 80 per cent of the global market,supplying 90 air forces worldwide.Todate,182 different aircraft types havebeen supplied with over 69,000 seats.

And these systems cannot afford tofail. Reliability of all mechanicalcomponents – even though only used fora few seconds in an emergency – is vital.That means stringent manufacturingstandards for the variety of highlycomplex,multiple-featured componentsthat go into every seat.A demand for highquality puts emphasis on quality control,and the ability to produce componentsaccurately with high repeatability.

Martin-Baker makes most parts in-house and the most widely used materialis aluminium. In machining thecomponents,the company uses 19 CNCmachining centres in its 500,000 ft2

production shop,including 10 Makinos –all having in-process touch probegauging.

In addition to the machining centres,the company boasts CNC punchingmachinery plus 16 CNC turningmachines,including eight mill-turn unitsboasting drilling and milling.

Most recently installed are three 4-axis Makino A55 high-speed machiningcells with a fully integrated 60-pallet railcar system,acquired to deliver greaterefficiency in production of detailedmachined parts,reducing lead times andeliminating work-in-progress processes.Indeed,most parts are now machined onthe A55s,all with identical set-ups forflexibility in case of fluctuations indemand.Vital in the operation is areliance on spindle-mounted probes tocontrol operation.

Makino machining centres were

EJECTION SEAT MANUFACTURER DEMANDS HIGH ACCURACY

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For the one in 10Ejector seat manufacturer Martin-Baker relies on accurate and repeatable manufacture of high-quality,

complex components that simply can’t afford to fail. And it’s using CMM and machine tool probing to

do it. Mike Nash took a closer look

chosen,says Peter Smith,productionengineer because:“They [supplier NCMT]had lots of experience in pallet loadingsystems.”Critical since Martin-Baker wasgoing for unmanned running. So NCMT’sexperience provided a comfort factor.

FINDING DATUMIn operation,a pallet is loaded into amachine with the material located insimple fixtures,without any accuratedatum location made. Probes are used fordatum setting and in-process machining.In each case,the first feature is probedwith a Renishaw MP10 probe so Martin-Baker does not have to rely on theaccuracy of the fixture,as thismeasurement establishes the datum onwhich all subsequent machining is

based.“Essentially there is no fixturing toworry about,”explains Mr Smith.Andbecause of that,when a first-off goes toinspection (see box item),anothercomponent can be up and runningquickly using generic fixturing.

“The probe is also used when neededduring the course of the program,”headds,for probing the machined surfaces

“then machining the next features relativeto those already machined.Thisguarantees the positions of bores andother features that must relate to eachother accurately.”All components arethen probed at the end of machining,which allows the machine toautomatically make ‘intelligent decisions’such as when extra operations areneeded. In-cycle probing can be used todecide whether extra cuts are needed,forexample,or whether there are any otherissues with components as they are beingmachined.

The flexibility of the probing systemallows it to be used only when ameasurement is required,with very shortroutines involved. Programmers needonly add one or two lines of code to a NC

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PROBING

www.machinery.co.uk ● MACHINERY 2 April 2004

The importance of accuracy and

repeatability at Martin-Baker

below. Left – a QC10 ballbar

performs a quick performance test.

Centre – the 60 pallet loading

system feeding the Makino A55s.

Right – MP10 spindle probes set the

component for further machining

operations

On closer inspection

Martin-Baker’s inspection areaboasts four CMMs, two MitutoyoEuro C9166s plus two smallerC9106s, all employing multiplestylus arrangements with stylusracks duplicated on each machinefor flexibility, so parts can beinspected on more than just asingle CMM.

A grid-type workholding systemis used to locate parts, withmachine operators conductingtheir own inspections,“representing a change in the workethic,” reports Jim Kelly, inspectioncell leader. To support this, the partto be inspected is loaded, withprogram then selected from aseries available and a fullyautomatic routine then run. Acomprehensive list of instructions,detailing orientation and fixturing,is also provided.

Each part’s paperwork includesan ‘issue’ number (a codeidentifying the inspectionprograms). “This makes sure thatno job can be subject to the wronginspection program,” says Mr Kelly.“If we put in the wrong issue it willautomatically throw you out, andwill not run the program.”

A benefit of the Renishaw styliis that calibration doesn’t need totake place every time theconfiguration (part/styli) ischanged. The styli is selected aspart of the program and thenmeasures – the resulting reportseven list checks the machinescannot perform, and which arethen done manually. Typicallyinspection time before was about aday but has now been cut to 2hours on a typical component. Andevery component is checked.

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PROBING

program to take these measurements,with the resident probing softwareallowing the probe to be loaded onto thespindle from the tool changer,workingout the necessary moves andautomatically updating program offsets.

Martin-Baker also makes fullproduction use of three 20-year-oldMakino 106 CNC machines. Here MP10probes have been retrofitted,and areused for job set-up and datum setting,such as on forged billets,which areclamped into a vice.The probes haveproved useful in detecting variation inthese forgings. Readings can be used bythe machine’s control to decide wheremachining starts and how many cuts arerequired,for example,eliminating any‘fresh air cuts’ that might be introduced orany scrap – allowing intelligent decisionsto be made on cutting cycles.

In addition,the positional accuracy ofthe indexing of these older machines ishigh enough. So the spindle probe is usedin-cycle to reset the datums after everyindex,taking points on a part’s surface toestablish its actual position.“Without this,the machine couldn’t do all the faces ofthe component,in fact we couldn’t easilymachine some of these components atall,”says Mr Smith. Part position isdetermined with compensation thenapplied automatically to the partprogram.

One typical example of this approachis the slots on ejector seat guide rails(picture,left),fitted inside the aircraft andalong which the seats are propelled whenactivated.A critical feature here is wallthickness.The rail must not be too thick(to keep weight to a minimum) but itmust also be sufficiently strong to take theforces involved.A probe checks multiplepoints along the rail – which is clampedto the machine bed – with the resultingreadings fed into the CNC and themachining program then updated toensure the correct amount is machinedin each place. Prior to retrofitting theprobes,this job took two hours to set upmanually,but now it takes just 30 minutes.

Martin-Baker has subsequentlyinstalled two large Mori Seiki mill-turn MT2000 machines to provide extra capacity

to cope with demand,boost productivityand upgrade capability.

Capable,says Mr Smith,of machiningover 400 different parts,including moreunusual shapes,one of the machines willrun 24 hours unmanned. He knows thisas the machines ran for six months atMori Seiki in Milton Keynes before beingtransferred to the customer. Fredk Pollard,the UK agent for Mori Seiki,has beenonsite for some time training Martin-Baker staff. Mr Smith’s plan is that,overtime,some component work will betransferred from the A55s on to the MT 2000s.

In terms of probing,having seen thebenefits,Martin-Baker insists on probesfor all new machines and the Mori Seikilathes are fitted with Renishaw MP700probes.Typical applications includechecking collet diameter,but,accordingto Mr Smith,the probes also help identifylikely rework and re-machining problemsmuch sooner,alleviating the need toscrap the component and,whilst he’s notspecific,“save a lot of money”.

Of course,using probes on aninaccurate machine would be pointless,so Martin-Baker uses the Renishaw QC10ballbar.The system follows the radius of acircle described by the machine in twoaxes,controlled by a simple programrunning in the machine’s control.Atransducer in the ballbar records anyvariation in the measured radius and theballbar software,running on a linked PC,displays and diagnoses the results.

The test enables the software toidentify and quantify the positioningerrors and their cause. Errors can becorrected through adjustments,saytrimming servos,with the exact figureentered,and another test run afteradjustment to confirm improvedperformance.

ON THE BALLThis ’10 minute test’ is performed onevery CNC machine in the factory atthree-month intervals.“If we have anycrashes,we check them immediately,”saysMr Smith.

Generally,as Robbie Mitchell,machine tool maintenance engineer

points out,machines describing pathswith a circularity error greater than 25microns means that they cannot holdmost specified component tolerances,increasing the likelihood of scrap partsand increased machine maintenance.Not only has the ballbar allowedplanned,predictive maintenance,as MrMitchell can look ahead and see howmachines are progressing,but it has savedtime on the shopfloor.“It takes youstraight to the fault and is much quicker.”

If Mr Mitchell previously thought“something was up with a machine,”heused to spend a morning locating theparticular problem. Diagnosing andputting right a machine crash used totake pretty much half a shift.“Now I knowwhat to do after 10 minutes,”says MrMitchell.

The ballbar is also able to help gaugemachine capability with relevantinformation passed back to theproduction engineering department toallow jobs to be planned more effectively.Such jobs are now put on those machinesthat “we know are capable of doingthem,”says Mr Mitchell simply. M