High Precision Horizontal Boring Machine
Fives Landis Ltd have recently increased their product range to include the supply of very large grinding machines capable of finishing the main bearing journals and pin diameters of crankshafts for the marine diesel and static engine businesses. The crankshafts, which typically weigh as much as 6.5 tonnes, range from four to eight metres in length and have both pins and journals up to 350 mm in diameter.
As well as assisting with bed design and FEA analysis, three major elements of the machine, the wheelspindle, the headstock and the tailstock have been custom designed by Cranfield Precision. All three axes contain hydrostatic bearings capable of supporting the heavy crankshafts and grinding process loads needed to produce high quality finished workpieces. A large proportion of residual geometric errors in the finished part can be related back to the error motion of the spindles. It is therefore important to design and manufacture the bearings to minimise such problems before they arise. Once built, there is nothing that can be done short of starting again to remove any geometry related errors that appear.
The synchronous radial error motion of the spindles is principally determined by the roundness error in the manufacture of the journal diameters of the spindle. Axial error motion is caused by lack of perpendicularity of the thrust faces. Both errors contribute directly to the systematic errors in the part. Asynchronous error is less likely to cause surface finish defects in a grinding process that uses a large contact area between the wheel and the part, but its effect cannot be completely ignored. The accuracy in manufacture of the matched size and roundness of the housings control the fluid flow and bearing stability.
The design process confirmed that Cranfield Precisionís five pocket hydrostatic bearing design could be scaled up from its current application in high precision diamond turning machines. The final design needed to be more than three times the size of anything that had previously been assembled by Fives Landis Ltd. The machine shop at Fives Landis has been able to manufacture every part of all 15 spindles built in the last 18 months with the exception of the large bores in the bearing housings.
This critical part has had to be machined using large horizontal jig boring machines at subcontractors remote from the works. Size control and roundness has been good, but it has also been costly and there are considerable risks with matching sizes and consequently controlling bearing performance. It is to mitigate this risk that Cranfield Precision are in the process of building a large custom designed high quality horizontal fine boring machine to bring the manufacture of the critical bearing housings into the Fives Landis machine shop.