Product category:
Machinery and Production Equipment
News Release from: Leader Chuck Systems | Subject: Collet chucks
Edited by the Engineeringtalk Editorial
Team on 21 November 2000
Chucks improve levels of process
capability
Leader's Hainbuch chucks improve process capability at Delphi Diesel Systems.
A Hainbuch collet chuck system is playing a key role in enabling Delphi Diesel Systems to achieve improved levels of process capability during turning of precision parts at its Cheltenham plant In addition, the system has also reduced changeover times and eliminated the need for Delphi Diesel Systems to bore out the jaws to suit individual workpieces
This article was originally published on Engineeringtalk on 20 Jul 2000 at 8.00am (UK)
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Formerly Lucas Diesel Systems and now part of Delphi Automotive Systems, the company manufactures diesel fuel injection systems for vehicle engines and units for power generation plant.
The capability improvements obtained by changing from a standard three-jaw chuck to a 65mm Hainbuch collet chuck on a CNC lathe were measured during pre-production trial machining of injector nozzles.
The new chuck's contribution to improved process capability when machining the nozzles is attributed to the collet's precise 360 deg clamping, and its positive and powerful pull-down clamping force.
Capable of handling parts up to 1000 mm diameter, the Hainbuch system, available from Leader Chuck Systems, ensures high-accuracy workholding by having a clamping diameter precisely that of the workpiece being held.
It is capable of clamping to provide a total indicator reading (TIR) of below 0.004mm and is reckoned to have a clamping force of 50% cent higher than a conventional jaw or collet chuck with the same drawbar pull.
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The changeover to the new chucking system at Delphi Diesel Systems has also reduced by two-thirds the time it takes to change from one size of workpiece to another.
Compared with the three-jaw chuck, this is typically down from one hour to 20 minutes.
It has also eliminated the need to bore jaws to suit individual workpiece diameters since the quick-change collets are supplied to Delphi Diesel Systems in the exact diameters needed, accurate to 20 microns.
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"The jaws are ground to the specific size we want to grip," explains operations support engineer Paul Phillips.
"We are also interested in quick changeover times because our batches tend to be small, typically 50 to 100s.
We don't want to spend time clocking the chucks.
We want to take one collet out of the chuck and put another one in - and know that it is right.
These days we no longer suffer from concentricity problems." Trials have involved comparing process capability on the machine when fitted with the two types of chuck.
Comparisons have been made during the machining of critical close-tolerance diameters and lengths on the nozzle bodies.
These are held to 50 microns or less.
The improvements achieved are exemplified by the consistent accuracy with which an external shoulder on one 45 mm diameter nozzle is now machined.
As a result of the change to the Hainbuch system, the process capability for this particular shoulder relative to a previously machined internal bore has seen an improvement of over 200 per cent in Cpk.
Even better improvements in process capability have been achieved on longitudinal dimensions, for example between an external shoulder and the end face of an internal bore seating.
Here there has been an improvement of over 500% in Cpk.
This is attributed to the greater accuracy with which the workpiece is pulled down when loaded, with the Hainbuch collet action pulling the part back against a backstop within the chuck.
Following these machining comparison trials, work has continued to improve the overall capability of the machine and the process to achieve the 1.33 Cpk required for the operation to be allowed to go into production.
With a process capability at this level, workpieces can now be machined without the need for either in-process gauging or continuous monitoring using statistical process control.
The decision taken some 12 months ago to change to Hainbuch workholding was the result of experience gained on two larger lathes supplied as turnkey systems with a proven process capability.
The two machines are used to hard turn barrels for fuel injection pumps in a range of diameters from a variety of case-hardened and through-hardened steels in the 60 to 63Rc range.
It is the precision of these machines combined with the accuracy of the collet chucks that enables internal and external features of the barrels to be machined to tolerances of 20 microns.
For initial hard turning, barrels are held on a mandrel that locates in the honed bore of the barrel and is gripped by a Hainbuch collet, with a tailstock giving back-end support.
A range of 10 to 20 mm diameter collets is kept for holding the mandrels.
A second turning cycle, including end facing, is then performed with the part held in a collet on its outside diameter.
The barrels are afterwards ground internally and externally.
One machine was originally supplied with 42mm Hainbuch chucks; the other with a 65mm chuck.
Today, one still has a 65mm chuck but the other has been increased to 100 mm capacity.
The machine used for soft turning of nozzles also has a 65mm chuck.
For workholding on all three machines, Leader Chuck Systems has supplied 50 different collet sizes to give a workpiece holding size ranging from 6 to 100mm diameter.
With their high-precision gripping, the collet chucks have proved to be ideal, as Paul Phillips points out: "The older machine and its Hainbuch chuck have been turning hardened barrels to within 20 microns for the past seven years, and they are still holding those tolerances today.".
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