We had a good project come through the shop last month — a pair of large gearbox bearing housings made from ductile iron. Each housing needed a 6.000″ diameter bore finished to H7 fit tolerance, which puts you right around +0.0004″ to +0.0008″ over nominal. With a bore that size, you can’t just drill and ream it — you need a proper boring operation.
We set up on our Wotan horizontal boring mill using a single-point boring bar with a fine-adjust head. Pre-bored with a roughing bar to within 0.030″ of final size, then switched to the finishing bar and crept up to size in two passes. We were measuring with a 3-point inside micrometer after each pass.
Getting a 6″ bore to H7 on cast iron is about patience more than anything. The material machines well, but the big parts can move on you if the fixture isn’t clamped right. Both housings came out within tolerance with a nice 32 Ra finish — ready for bearing press fit. Customer had the gearboxes back in production within a week.