Hi all

I have to finish the bores of a huge connecting rod which has bore dimensions of 250 and 310 mm with tolerances of about 20 microns and depth of about 140 mm.

Now the problem is everytime we finish the 2 bores with the required tolerances using boring bars, we get the correct tolerances after machining. However, after a day or two, the bore sizes go haywire. The bore becomes oval or has taper or both with tolerances varying over 50 microns. This completely renders our connecting rod to be rejected as the bearings do not fit in well.

We have tried the process of doing the roughing operation on one day and then finishing the bore in the next day so that stresses developed (if any) in roughing could be released. However we have not found better results.

Also it has been found that the bore sizes do not change beyond 5 microns if the bores are initially filled with weld material (welding is done on connecting rods whose sizes become unacceptable). So we assume that weld material being harder, does not cause the bores to change. However this cannot be done for all the connecting rods as it becomes very expensive and impractical.

The material used is Mild Steel with hardness of about 170-180 BHN and the bores are initially gas cut. We are using air instead of coolant for the boring operation as this gives somewhat better results than using coolant.

Your help in this regard would be highly appreciable since we have tried many alternatives but are not getting good results. Please suggest if we are going wrong somewhere.