Trying to bore some 6061 T6 with a solid carbide boring tool and can't seem to get a decent finish. The part is 1.5" diameter turning at 1100 rpm on my Compact 5 which seems to work well for most other work. I'm using a little "Tap Magic for Aluminum" as a cutting fluid and the tool is sharp. While turning the part makes a "singing" sound (part has a thin wall) and the finish is sort of "serrated". Any ideas???
Chatter. You've got to damp that singing sound, which is causing vibration and chatter.
Doubtless some others here will recommend changes in tool geometry, lubricant, feeds, or speeds. Some of them may work. Having had to machine a lot of thin-walled pieces of lab equipment (many years ago), I prefer heavy-handed approaches. Some are:
1) Snap a big rubber band around the work -- multiple wraps. I've used slices of inner tubes for big work, such as brake drums.
2) If you have a lot of parts to make, turn and bore a piece of metal or wood that just about fits around the work, leaving room for a rubber band or (better) a couple of O-rings that will fill the gap. Slide it over each part before you machine it.
3) You can try wrapping layers of duct tape around the part, gently, so the accumulated pressure doesn't distort it. This is a onesie-twosie deal only. It worked once for me, and then it didn't work the next time I tried it. You never know...
Basically, you want to get the resonant ringing to stop. You can trick it out by countering the resonance or damp it out with mass or rigidity. One way or another, there aren't many of these that can't be solved one way or another.
The serrated finish is the result of tool chatter..which is the singing sound.
Make sure your tool is overhanging the minimum amount. Too much boring bar sticking out becomes a spring. Thicker is better.
Make sure your tool is dead nuts at center
Make sure your tool has sufficent relief. If its dragging in the slightest..it will constantly be pulling the tool away from th work and then springing back, which is usually indicated by :serrations".. think of it as thousands of little divits as the tool springs back and digs in, then loads up and is pushed away, then springing back and digging in again.
Increase your rpm by 50%, with around .005-.007 per rev feed rate... and see what happens, go up and down with your RPMs. Aluminum can be cut really fast...most of the time..Your mileage may vary..
"What do you call someone in possesion of all the facts? Paranoid.-William Burroughs
I'm not sure what the shape is that you're boring but if you can turn the OD last I'd bore the insides first then turn the OD to get your thin wall. Stuffing the ID with paper towels, dry or wet, or even using a low melt fixturing alloy will dampen the ring. It's easier to control it if you bore the ID first I think.
I use this method when turning rocket nozzles.
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Thanks for the suggestions guys. I wasn't completely sure that it was chatter but you've confirmed for me. I think the easiest solution is to bore the inside first, the I/D is about 1.5" turned into 2" stock, then the O/D gets turned to 1.650. It's a one off...
Is this the Compact-5/CNC (which I have), or the plain vanilla one? With the CNC version, you have a pot to vary the speed through a fairly wide range (as you certainly know if you have that machine), and your problem is that the your current speed is just right to resonate with the length of extended boring bar. Try turning the speed both up and down to see if either improves matters.
Is your boring bar a *solid* carbide bar (stiffer than steel with carbide tips). Whether solid carbide or steel with carbide inserts or brazed, you can also tune the resonance by changing the amount of unsupported bar. Normally, you want the minimum extending to reach to where you want to cut, but sometimes you *might* improve things at a given speed by extending the bar a bit more (which will shift its resonant frequency down the scale, and perhaps tune it out of the range which your motor speed is trying to drive things.)
Of course -- the brazed carbide insert tools for boring heads on milling machines are necked down so shifting the extension won't have much affect on the resonance -- it is defined by the necked down section.
Obviously, the larger the diameter of the boring bar, the more rigid. I commonly use a 1/2" solid carbide with insert on my 12" Clausing for smaller holes, and up to a 1" HSS one for larger ones.
If it is an insert tool, (as are the ones for the Compact-5/CNC), and you have a choice between TiN coated and uncoated, the uncoated (with chipbreaker grooves) are sharper than the coated ones, and will do better in aluminum.
What is the toolholder system? The Compact-5/CNC has a nice solid quick-change toolpost which should do the job with little problem with proper tool extension. Or it has a turret assembly which gives you little control of boring bar extension. I have no real idea what the manual Compact-5 uses for a toolpost, but it might be less rigid than the quick-change toolpost for the CNC version.
That is a fairly thin one, which would tend to be slung off of rotating workpieces, and might be better as a cutting fluid for the stated purpose -- tapping holes. Try kerosene, or for quick-and-dirty, try WD-40 for aluminum cutting.
All above -- plus lots of other good views already posted.
In my experiences with the stuff, once the chatter has left marks, it it difficult to be rid of. The variations in the force acting on he cutter start more chatter. Taking a heavy cut is only useful if you are not nearly at finished size.
I have saved chattered parts when boring, by taking a few passes very light depth of cut at slow rpm, slow feed rate, with a very sharp tool. A couple passes to get past the marks or to sneak up on the final size. Basically, I ran at too slow a speed to set the boring bar chattering. A sharp cutter with geometry for the material being cut is a definite help. Solid carbide is as good as anything and better than some things, but if the egde geometry is not right for the aluminum, you will have to work harder than you should to get any degree of finish.
As always Don, thanks for your valued input. I asked about the C5 on this group some six years ago and you directed me to the right decision to find a manual version. I've made many parts (some to "tenths") on it since and have collected almost all the accessories. I've even turned another one into a precision grinder, but that's another story :)
I'm using a solid carbide bar with a 1/4" shank held in a home made split clamp type holder. I then clamp this into the lathe holder on my C5. I've never had this problem boring with it before, but then I haven't turned anything this big with such a thin wall. I think you may have something there with the resonant frequency and will try different speeds. I do think though that boring the I/D first, then turning the outside, will help a lot.
Funny you mention WD40 for an aluminum cutting fluid as my 30 year plus tool and die maker buddy has said this also, think I'll have to give it a try!
-- Terry Keeley
Nothing improves until someone stops and questions an accepted assumption...