Lathe setup help to bore a casting

[Paul Alciatore]

I did a line bore with my SB-9 using a boring bar that I fabricated from a 1" diameter, 1018 steel round about a foot long. As you can see in the photo, I supported the boring bar at both ends, but I understand that is not possible with a blind hole. In that situation I would try to keep the amount of projection from the lathe chuck as small as possible while still reaching the bottom of the hole.



The cutting bit which is visible sticking out of the top of the boring bar, is a broken end mill which I reground. It is held in place in a matching hole in the boring bar with a set screw. IIRC, I also ground a flat on the bit so it would keep it's orientation when the set screw is tightened. The bottom of the hole for the bit is threaded and there is another set screw which allows easy adjustment of the protrusion of the bit. This was not a difficult thing to do; I simply drilled with a tap bit through the boring bar and then enlarged the top 3/4s of it with a drill bit the same size as the end mill's diameter.

A feature of this boring bar which made the job a lot easier is the flat that you can see on the top of the boring bar. The cutting bit is in the middle of that flat and it allowed me to use a depth micrometer with a spacer that fit over the cutting bit to measure the protrusion of the bit. I took a reading when I first started boring and wrote it down. Then I was able to adjust the bit for additional increments for succeeding passes with micrometer accuracy. Again, IIRC, I hit my desired diameter (25mm) within 0.025mm or 0.001". The bore was also parallel to the sides of the block with similar accuracy.

The hardest part of the job was measuring the diameter of the hole without removing the block from the lathe. You can see a turned area on the right end of the boring bar: this has a known diameter. I subtracted that diameter from the desired 25mm and divided by two. That gave me the size of a drill bit that I could use for a rough measure. This needed a gentle touch to avoid deflecting the boring bar. I checked at four points at 90 degree intervals.



For the final diameter measurement I purchased an inexpensive pair of internal bow calipers and ground the jaws down to fit between that turned area and the final bore size. These were used to transfer the diameter to my micrometer so, with a good technique, I could measure down to several tenths.

With these measuring techniques and the flat on the boring bar I was able to easily dial in the needed changes in tool protrusion.

[mattthemuppet]

Can't see any reason why the 4 jaw won't work. Just keep track of which jaws you're moving in which direction and use a dial indicator to set the depth of cut.

[Mr Fixit]

HI BC,

I think I understand your post. The larger bar is held in the chuck with a even larger holding piece or "stub". Then work the bore depth of cut in increments with a shorter bar to the final depth which is 4.5" so that it is as solid as can be for the full depth. If I take light cuts since it's cast iron and I do multiple passes per depth of cut changes, will it matter if it's done stepped vs all at once? ultimately it has to stick out a whole bunch to get the final depth of cut.

What about a light press fit plug in the bore of the casting that guides the boring bar for the last half of the cut depth. Make it out of say brass or aluminum and oil the bearing point of the bar?

Another idea a coworker suggested, was to set it up on a BP clone mill, would the smaller boring bar perform OK for the depth of cut? Again we would need to use a boring head to get the cut correct, I have a Criterion DBL-50MM with a straight shank but this is only able to use a 12mm bar is that a possibility?

Appreciate the help in figuring this out.

TX
Mr fixit for the family
Chris

[David Powell]

If you want to use a Bridgeport or similar mill to bore a hole you may get a better job by locking the quill and feeding by raising the knee, Many quills are rather loose, and few are capable of tightening by gib or pinch screws.
Regards David Powell.