brass boring

trying to figure out where I'm going wrong...

1 3/8" brass hex stock chucked up nicely in the lathe, need to bore a 7/8" x 3" deep hole. i center drill then progressively drill up to 1/2" and slightly shallow on the depth so i can finish later. everything good so far. i switch over to a small boring bar to open it up another 1/8" so i can switch to the 1/2" bar for the finish. The small bar leaves some ripples which i attribute to tool flex, but i figure the bigger bar will be stiffer and there shouldn't be a problem. However, with the bigger bar, the problem is remaining the same. i have lots of ripples, some as big as 3-4thou down the length of the bore. all combinations of fast/slow feed rate, fast/slow RPM, large/small cut seem to make no difference. the larger boring bar is a 1/2 kennametal with clean, sharp insert. Turning the outside diameter down results in clean, smooth finish.

I'm ripping my hair out on this. aluminum, steel, etc. seems to have the same effect.



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I ain't the expert, but here goes: If you are using an "insert" I assume it is a carbide insert. If so, there probably isn't enough relief on the nose. You know, carbide is brittle so the inserts need a lot of support under the cutting edge. I have caused the same thing with boring bearing bronze. I had to switch to HSS cutters, sharpened with lots of front relief so the heel doesn't drag in the cut. Also, you need to make a pretty sharp point so it doesn't take much force to get the tool into the work. If you don't, then springiness will always be a problem. You probably already know that tools want to dig in to brass/bronze, so sharpen accordingly.

You would not have this problem on the od because the relief is working in your favor, whereas in a bore, it's got to be handled carefully. Maybe you can get down there with a magnifier to see the problem to confirm it. I hope others jump in here so I can learn more about boring smooth holes, too. I have just about totally abandoned inserts for boring for the reasons mentioned above. As it is, I have a bunch of 3/16" square cobalt lathe bits that I cut off to fit in my home made boring bars. One nice guy on this list talked about the "spring constant" of steels for boring bars and that helped me with stiffness problems. You are probably in pretty good shape since you are using a Kennametal boring bar. You may be creating a big part of the problem with the small boring bar, since the heel of its insert is probably scraping even worse than the bigger one because of the smaller diameter. And then, because of the springiness created by the larger insert's geometry, those ripples are amplified or at least not easily removed by the bigger bar. I would at least try taking several dead cuts to try to get rid of the ripples before proceeding.

Pete Stanaitis


joel wrote:

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None of those words are numbers. Machining at this level is a numbers game for the most part. A "heavy" cut is different for different people.

We'll assume you're using an actual boring insert and boring bar.

What feed and speed combinations have you actually been using (numbers please!)? Did you look up the recommended feeds and speeds for the tool you're using? That info should be available in the same catalogue from which you selected the boring tool, and it does make a difference so make sure!

Your boring tool sits no more than 3-1/8" out from the tool post.... RIGHT? Overhang is *really* important when you're boring.

With brass, you can always go slower. If I was having problems, I'd probably run at 400 or less RPM to start. Feed rate is *heavily* dependant on the insert so look it up! The cut should not be squeeling ideally. Work to get rid of that or you'll see it in the surface finish.

Good luck. Learning is difficult but worthwhile.



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Robin S.

I do a lot of boring on all sorts of holes, pulleys bushes and the like. I use just one tool its a brazed on carbide onto the steel 3/4in high by 1/2n wide steel. BUT the carbide isnt pointed, its got a 1/8th in radius that doesthecutting. with a 1/4in flat back from the radius. This supports the tool against the work as you feed in the cut. Usual angles on top face and rake. Never cuts grooves or ridges. I can take a heavy cut on ali or a finishing cut on steel/cast ciron. Its a fault in my view of the cutting tip shape your using. Ive a pointed carbide insert on a coromant boring tool and that doesnt work either!! I do harpen my carbide tips with a diamond wheel tho lubed with kerosene. Do try that type of cutting tool shape, I dont think you can buy it my shape, you have grind it yourself. For brass you dont need carbide. Use 01 hardened or any tool steel will do. Yes I use a Le Blond royal from 1942. Ive mentioned it before it was a lease lend from you all . Much used , lovedand appreciated. Ted Dorset UK. .

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Ted Frater

Lots of good advice from others, but no one mentioned that the tool needs to be slightly above center when boring as opposed to slightly below center when turning on the outside of a bar. I expect you know this, but maybe someone new to machining will read this some day and benefit.


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Hi- raise the bar above the center line of the lathe by 1/8 "or more. This will prevent the part of the tool below the side cutting edge from rubbing. Jim.

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Hi- Raise the boring bar 1/16 " or more above the centerline of the lathe. This will help prevent the part of the remer below the side cutting edge from rubbing. I 'never' bore on center for that reason. Jim.

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Lotsa good advice.. but..

When I do small hole boring like that (and yeah I consider 7/8 small) I don't bore.. I ream... 'specially when I need it *exactly* on size... It's just too easy..

--.- Dave (who hasn't posted here in months.. work ya know.. it's a 4 leter word... that leads to that other 4 letter word.. cash)

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Dave August

Yes, but when you have a lathe you don't *have* to spend money on things like taps, dies, and reamers. Randy

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Randy Replogle

But When ya own a 1964 Cessna 182 that Cash helps a lot...

Last time I tried to build a 0-530 from scratch the FAA wasn't to happy..

--.- Dave

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Dave August

Sorry for the long delay in replying...

After some experimenting, the answer is...

Raising the boring bar ~1/16" above the centerline reduced the rippling but did not eliminate it. i then switched over to a brazed on tool as opposed to a insert tool and tried again. MUCH better finish. not polished bright, but that nice sort of "honed"finish. and i couldn't feel or measure any effective rippling.

The difference? the relief on the carbide. the insert had very little, so when the brass was cutting in a tight bore, it would "pack" slightly between the wall of the brass and the insert, causing it to push out from the wall till it "unpacked" and so on and so forth causing the rippling. raising the bar 1/16" gave it a bit more clearance and reduced but did not eliminate it. the brazed on bar easily has 3-4deg more relief, and didn't pack nearly as much.

Thanks for all the help everyone!

--joel > I ain't the expert, but here goes: If you are using an "insert" I

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