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Old 05-12-2011, 01:05 PM
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Greenbuggy Greenbuggy is offline
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Default n00b machining questions...boring holes & press fit tolerances

Ok so when it gets here (hopefully soon) I will be the proud new owner of a single point indexable boring head with an R8 shank, and a set of carbide boring tools.

I bought this because one of the projects I want to do with the BP is bore out a significantly out-of-round hole in my lathe apron and press in a brass bushing for the pinion shaft to ride in.

My n00b questions regarding this setup are:
- What kind of RPM should I be turning the BP at to bore the cast iron apron? How deep of cuts should I be taking? Currently its slightly larger than 5/8" and out of round.
- How do I center the boring head over an out-of-round hole? My temptation is to just chuck a countersinking bit in the spindle and find center that way, and call it "good enough" but I'm curious what the *right* method would be if I needed to hold tighter tolerance than that.
- What diameter should I bore to if I have a .751" brass bushing I want to press into the hole I bore, and stay put?
- How would I best measure the bore hole without buying a set of plug gauges? I have a starrett caliper and an interapid 312b1 indicator, I'm not opposed to buying another measuring tool(s) but I would rather not buy something I won't ever use again.
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Old 05-12-2011, 05:07 PM
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I have know idea. But I figured I'd say hi and looking to getting the right way to do it to.
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Old 05-12-2011, 05:11 PM
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I'm like Jennifer, don't know but would like to hear the answer.
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Old 05-12-2011, 06:55 PM
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Quote:
What kind of RPM should I be turning the BP at to bore the cast iron apron?
At 5/8", the hole is pretty small & the boring bar (probably a 1/2" bar or a 3/8" bar) will barely be offset from center. Run at whatever speed you want, anything 1000 rpm or under. If you run at 1000 rpm, that's just 163 sfpm, well within the range for a carbide cutter in cast iron.

Quote:
How deep of cuts should I be taking?
Your machine will protest if you try to take too much. Start around .040" on the boring head dial (meaning that the cutter is taking .020" on the radius). If .040" works well try .060, etc. DOC is limited by rigidity of the machine & total length of the boring bar - all R-8 milling machines are none too rigid, and a 3/8 or 1/2 bar is pretty flexible as well.

Quote:
How do I center the boring head over an out-of-round hole?
If there is any metal remaining from the original hole, try to pick that up. If not, the countersink will get you close enough.

Quote:
I'm curious what the *right* method would be if I needed to hold tighter tolerance than that.
Assuming there is a round hole, maybe a drilled hole or a bored hole, clamp an Indicol holder to the quill. Put the head in neutral so the spindle swings without resistance. Mount a dial test indicator on the Indicol & move the table until the DTI stays at zero. See third photo.

Quote:
What diameter should I bore to if I have a .751" brass bushing I want to press into the hole I bore, and stay put?
You want a Class V fit, which would mean .002" (.749" bore) if the bushing were steel. Because it's brass, double that and bore to .747" (.004" interference).

Quote:
How would I best measure the bore hole without buying a set of plug gauges?
Lots of ways to do this.

Use a telescoping bore gage, like the Starrett 579A shown below in first photo.

Use an ID mic, like the Starrett 700 (second photo).

Use a IntraMike or HoleMic but get a bank loan first.
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Last edited by precisionworks; 05-12-2011 at 07:01 PM.
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