Question about using a boring bar versus a reamer for a tight tolerance hole

I had a job last week where I needed a 0.750 inch hole with a plus or minus half a thou tolerance in 4140 steel. The print called for a reamer, but my only good one was worn out. I had to choose between ordering a new reamer and waiting two days or trying to hit it with a carbide boring bar on my lathe. I went with the boring bar, taking really light cuts and checking after each pass with my bore gauge. It took me about an hour to dial it in, but I got the hole to 0.7502, which was right in the middle of the spec. The finish looked great, and the part passed inspection. Has anyone else had to make a call like that on the fly? What's your backup plan when a tool is shot and the clock is ticking?

3 comments

3 Comments

jamie_adams16d ago

Nice save! That's exactly the kind of shop floor thinking that gets jobs out the door. Been there for sure, staring at a worn-out end mill with a tight slot to cut. My backup is usually a fresh, sharp insert on a fly cutter, taking it super slow with lots of coolant. It's not the "right" tool, but it gets the finish and size if you baby it. Honestly, sometimes the workaround feels better than doing it by the book!

king.val16d ago

My buddy Dave at his old shop had a 0.250 slot in 316 stainless and the last good endmill chipped. He took a 0.187 ball nose, ran it at like 30% feed, and stepped over maybe five thou per pass. It took forever but the finish was perfect. The foreman saw it and just shook his head, but he couldn't argue with the part in the box. That kind of hack just sticks with you.

jessej2316d ago

That's just called getting paid by the hour, my friend. Print says reamer, but the lathe doesn't read blueprints. Sometimes the right tool is the one that's actually in your hand and not on a truck somewhere. An hour to baby a boring bar is still faster than two days waiting on a tool truck, plus you get the bragging rights.