What's a decent cut for a 9x20 import?

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durableoreo

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I've got a bunch of parts to make that require boring out a 2.1" diameter bore to 0.813" in annealed 4140. I've been taking 0.015 per pass. I'm using a 1/2" carbide boring bar with some insert---if it's important, I'll post more details about the tool. No chatter but there's some deflection when I reverse the tool.

Curious what the experienced hands would do in this situation.
 
When boring, always turn the tool off the bore (Id-Est, back the tool of / move it away) before retracting from the bore. Use a method you will easily remember. I have always stuck with either a quarter, half, or one full turn of the cross slide handwheel, though it depends on the bore size and tool size.

As for DOC, that will depend on speed and Feed and whether using coolant (brushed on / sprayed on / flood) as well as material and other factors.

Insert type will also make a difference.
 
For boring bars, I use the thickest boring bar that fits the hole to reduce deflection of the boring bar. I let the boring bar stick out as minimal as possible to reduce deflection further. When there is chatter, I first reduce the cutting depth and if that is not working, the RPM and finally the feed/rev. If nothing works, I change the insert to a CCGT or DCGT type (aluminum) because these are very sharp and will reduce the tool load.
 
They don't last as long as a decent CCMT or DCMT insert ( I guess, 1/3) but they aren't blunted after 1 part. Most of the time, they do the job, even on stainless.
Take an insert that is to blunted for finishing on aluminum and use it on a steel bar. You will be surprised on how it handles.
 
Changed the insert, which reduced the spring. A dull insert was causing the boring bar to deflect, I think.

At 0.015, it is very slow going. And the cuts don't sound good. I bumped up to 1000 RPM, which is great at 2" diameter, close to 550 SFM. At the beginning of the job, where my drills top out, top speed (2000 RPM) is only 327 SFM... a little low for carbide.

I tried one of those speed/feed calculators. A cut of 0.020 at 2" diameter, 0.005 in/rev (520 SFM) is about 0.5 HP at the motor. So I guess that's an answer to the original question. The same cut, but 0.033 deep is 0.75 HP. So it seems that I'm limited by motor power, not rigidity.

The insert seems to last better at higher surface footage.
 
Back in the "good old days" South Bend recommended 1/8" depth of cut for roughing on their 9" lathes, no mention of speed or feed.
 
benmychree said:
Back in the "good old days" South Bend recommended 1/8" depth of cut for roughing on their 9" lathes, no mention of speed or feed.
Click to expand...

Thanks, that's good information.

According to the charts, that's a 3-HP cut!
 
At the adult school where I substitute teach, we tell students to take no more than a 0.025" cut. Less if there is "feedback", like chatter.

When I was working at a valve company, I asked one of the machinists what the deepest cut he could make was, and he said' "I dunno, let's find out!" and took a 0.2" cut on a piece of brass. The machine (a smaller lathe) groaned a little, but it cut it.
 
durableoreo said:
Thanks, that's good information.

According to the charts, that's a 3-HP cut!
Click to expand...
I did qualify that statement; "no mention of speed or feed" or for that matter type and geometry of cutting tool. In my school we ground a chip curling tool, which allowed free cutting of steel "1/32 maximum cut? Ridiculous! Our teacher was an apprenticed machinist and worked as a journeyman at a Navy Yard and also as an apprentice school teacher at the Navy Yard, he knew his stuff and taught it---
 
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