I’ve just finished making the main bearings for my model engine and thought others might find some of this interesting.
There will be a lot of photos in this thread.

The main bearings of this engine are of the cast white metal variety so the bearing housings were bored out to 1.125” which allows for the bearing to be bored to 7/8” leaving 1/8” of babbitt material all round for the bearing itself.

The bearing caps were secured onto the crankcase casting and the centre location of the main bearings marked out. The crankcase was mounted on the mill table then aligned and centred with the horizontal spindle ready for boring operation. A slot drill was used to produce a small flat which would prevent a drill from wandering when starting. The pilot hole was spotted with a centre drill then a 6mm stub drill was used to go right through. The pilot hole was then opened up using a 1/2” stub drill then a 5/8” MT2 drill bit.

Main Bearings 01.jpg
Ready to drill the first pilot hole through the casting with a 6mm stub drill.
Main Bearings 02.jpg
The pilot hole was increased first using a 1/2” stub drill then with a 5/8” MT2 drill.

With the first bearing pilot hole opened up to 5/8” the pilot hole for the second bearing was drilled. A 100mm ER11 chuck was mounted in the spindle to reach through the casting and hold the slot drill, centre drill and 6mm stub drill before going straight through with the 5/8” MT2 drill to finish the pilot hole.

Main Bearings 03.jpg
Centre drill mounted in an ER11 chuck to spot the pilot hole.
Main Bearings 04.jpg
The 5/8” MT2 drill ready to finish the second pilot hole.

It was at this point I discovered two flaws in my setup of mounting the crankcase flat on the mill table and using the horizontal spindle for the boring.

The first was that the casting had moved slightly under the load of drilling the second pilot hole meaning it was no longer correctly aligned with the first bearing or the cylinder mounting face. This was not a huge problem as there was still plenty of material left and this could be easily rectified during the final boring operation. I should have mounted a positive stop behind the casting to prevent this from happening, but the thought did not occur to me at the time.

The second problem was a bit of a show stopper for this setup though as there was not quite enough room to mount the boring head in the horizontal spindle and still have clearance for the extended boring bar to machine the first bearing. I could have used a shorter bar for the first bearing then used the longer bar for the second one, but the long bar is solid carbide so it’s very rigid and I preferred to do both bearings in the same pass.

It’s at times like this that I really love my Thiel mill because I can remove the universal table and mount work directly to the X axis slide. After a bit of juggling, the crankcase was mounted to the X axis vertical surface and aligned to the vertical spindle making sure to include a positive stop under the case this time. I also left the parallel used to align the casting in the correct plane in place and used a DTI to monitor if any movement occurred during the boring operation.

Main Bearings 05.jpg
Crankcase casting mounted directly to the vertical X axis slide.
Main Bearings 06.jpg
Another view of the crankcase mounted on the mill showing the positive stops, the parallel and DTI.
Main Bearings 07.jpg
Action shot of the boring operation, this is about three quarters through the second bearing housing.
Attached Images




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