Boring tips (lathe!)

[Cenedd]

Can anyone give me some pointers on boring if I'm missing something? I've just finished a piece that was 28mm diameter aluminium with a 24mm bore through most of it and 14mm through the last bit....so it couldn't have been started from pipe. I'm eyeing up some morse taper drills to hog out future pieces but I'd like to understand if there was anything I could have been doing better.
I drilled to 12.8mm - the largest parabolic drill I have - and then bored from there. The piece was 100mm long with 74mm of that being finish piece and the rest being grip stock. The bore was 62mm deep. I had no tail support (because I was boring) and no real room for a steady rest. I was hoping to take 0.5mm DoC roughing passes with a 12mm bar and an aluminium profile CCGT insert but I found that chips seemed to build up either between the insert and the end of the bore or between the insert and the wall and either bend the bar away from the wall or move the part away from the bar. Stock was 6082T6 but just didn't want to break a chip in most conditions - had some success at low DoC finishing passes on the outside at unsettling rpm (1700+) trying to avoid screaming resonance....but wasn't comfortable trying to bore at that pace

In the end, I roughed with a tiny bar until I'd got more clearance and improved things with WD40 and some clearance by blowing air intermittently down the spindle....but it wasn't really what you'd call productive. More obstinately determined to get it done.

Any pointers that I'm going about it all wrong would be quite welcome actually Also testimony that it's supposed to suck that much would make me feel better if that's actually the case!

[David Powell]

HI, You MIGHT do better by starting with a rather larger diameter piece, drilling and boring as needed, then supporting the outer end with a centre and reducing the diameter as needed. If turning the outer diameter gave problems, or the thought worried you then , before starting on the workpiece you could make a slide fit plug which you could use as a gauge for your bore and put it in while turning the outside to help avoid chatter.
I hope this is of help and thought provoking. Tegards David Powell.

[Bented]

When roughing raise the feed rate and DOC until the chip breaks.
Save the light cuts for finishing, molded chip breaker inserts often do not work well at small DOC's. Use flood coolant if possible. Last week I did a blind 2.376- 2.377" bore 11 13/16" deep in 304 SS, the chips were a mess. If there is no thru coolant bar available I often tape nylon tubing to the bar.

Only the last pass counts.

[loose nut]

I was going to but this subject is just so boring! 😂

[BCRider]

While reading your post I was thinking the same thing as David suggested. Namely do the internal work first and hold the stock deeper in the chuck.

To avoid the frequent issue of three jaw chucks having some runout leave a bit of stock for the finishing passes. Shift the part in the chuck so you can reach the OD for work on it and with the new grip make your finishing passes to size which will also make the ID true to the OD work you do on the part.

[Toolguy]

When running a boring bar, set the cutting edge a little above center. When the edge is on center and it bends down (it always does), then it digs in deeper and chatters a lot. When the edge is above center and it bends down, the pressure is relieved, and it cuts smoother.

Also, you can center drill one end and do a clean up cut on that end on a live center, then hold that end in the chuck and dial it in (adjust-tru or 4 jaw) then bore the hole. Then extend the part out, dial in the chuck end, and put a pipe center in the bored hole and finish the outside.

[LKeithR]

...Any pointers that I'm going about it all wrong would be quite welcome actually...

You never mentioned what lathe you were using but I'm going to assume that if you were having problems
with such a small piece that your lathe isn't very rigid. I agree with some of the other comments about doing
the whole job without moving it in the chuck--bore the ID and then turn the OD. Don't be afraid to sacrifice a
little extra material if it makes the job go easier.

I really like the sharp, uncoated inserts designed for aluminum, especially for finishing, but sometimes they're
not the greatest at breaking a chip. Depending on application and material I will often use a steel insert with
a good chipbreaker for roughing cuts and avoid (or at least reduce) chip buildup in the bore. Experiment with
some different inserts till you find one that works reasonably well on your machine...

[Cenedd]

Some things to think of there. I'll clarify a couple of things though:
I held it in a 3-jaw because the stock was larger than my largest collet. Any runout would be turned out.
It didn't really occur to me to use a 4-jaw because I wasn't expecting to have difficulty...but the extra jaw might have given a better hold.
I turned the OD first but only enough to true it up and not to reduce the diameter.
Then I faced, drilled and bored the ID before coming back to the OD and some other details. The only problem I had with the OD was because I'd basically created a bell. Raising the rpm stopped it resonating.
I couldn't have put the part any deeper in the chuck as it's only a 100mm chuck so the through-bore is smaller than the work-piece. I could have lost the grip stock and reduced the length by about 30mm at the cost of complicating things by having to flip the part to get to the full OD.
A pipe centre is something I don't currently have but probably would have helped turning the OD - even just to reduce the ringing.

Lathe: I'm running a Sieg SC4 so it's not bad for the price range but it's no Monarch/Colchester/other decent old iron.

I mostly run aluminium profile inserts for everything except harder steels (and I'm talking above EN8) as they seem to run better for the sorts of DoC I'm going for. A 1mm DoC for an OD turn is about as heavy as I go. I can certainly try a steel profile insert and see if it breaks the chip any better.

Toolguy raises a very interesting point. My bars have flats but the 12mm bar is held in a cylindrical holder so can be adjusted. I normally try to get the flats level by eye but it's certainly something that could be off and that I could try adjusting. Although, that said it didn't seem to be chatter that was a problem, more clearance impaired by chips and more so towards the blind(-ish) end of the bore.
I was hoping to take quicker rough cuts to get closer to size - as Bented says, it's only the last pass that counts - but I feared that pushing on ignoring the warning signs would have torn the workpiece out the chuck - or at least moved it enough to cause 'bad things' to happen. It certainly moved a bit at one point and I had to re-chuck to bring it true and secure.
I'm not set up for flood coolant unfortunately and it would take a fair bit of work and expense to do so.
It could be just a feeds and speeds issue. I was running 1000rpm for the boring and that's perhaps too slow by the book. Generally increasing the rpm means that if things go wrong they go wrong quicker...but perhaps I'm making it more dangerous by being too cautious?

[tom_d]

Any pointers that I'm going about it all wrong would be quite welcome actually Also testimony that it's supposed to suck that much would make me feel better if that's actually the case!

Some thoughts: You are not going about it all wrong. All that needs to be done is to fine tune the procedures. Considering the small lathe you're using some of the options are limited. Putting carbide against aluminum requires speed. Hard to do on that small machine. Max. speeds are best obtained with flood coolant. Again, machine limited. Run the machine at the fastest speeds you are comfortable with that give desired finishes. Ball park figures for aluminum are 200fpm with HSS tool and 600fpm with carbide. Ball park, not carved in stone. Not unusual to need to stop and clear chips when roughing a small bore. WD40 works as lube/coolant. It's not much more than glorified kerosene so use that instead if kero is cheaper, better availability in your area. Water soluble oil in a squirt bottle is good too, if you can.

[Toolguy]

Usually if you're getting chatter/ringing, slowing down the spindle will help. You don't need speed to cut aluminum (or anything else) with carbide. It would cut just fine at 200 RPM. I would try it in the 200 - 400 RPM range. It takes a little longer, but not much. Desired finish has to do with cutter tip radius and feed rate, probably want to be in the .004 to .006 IPR feedrate, and depth of cut.

[BCRider]

...Putting carbide against aluminum requires speed. .....

Not really. Oh sure, if we're after fast production then sure. But the recent experimenting I've been doing with aluminium specific carbide inserts has me using my lathe at well below those supposedly optimum high SFPM rates and the finish is still coming out just lovely. The supposed need for high SFPM with carbide on a variety of metals has also been pretty well torn down by others in previous threads too.

So it's more a case of using carbide permits the OPTION of a higher speed..... mind you.... If higher RPM was used then perhaps a lighter chatter free DOC could have been used and simply make more light passes in the same amount of time? So actual cubic inches per minute of metal removal might have stayed the same as a low speed with heavy DOC?

[Cenedd]

Thanks guys.
Sounds like I need to be doing some testing on something that isn't a part that matters and when I've not got a fairly limited time window to play with
Interestingly/frustratingly I think I have seen evidence that both tom_d and Toolguy are right.....much as BCRider said. I tend to part with an aluminium profile MGMN200 even with steel. It just seems to work better than the blunter steel profile inserts for that. I tend to do that about 300rpm which is far too slow on paper...but works very nicely. I get a pretty good finish at much lower speeds but there does seem to be a drop-off when it gets too slow and I did get an excellent finish with an alu profile WNMG insert (which might have been BCRider's suggestion originally - thanks for that, works very nicely) at nearly 2000 rpm and it was breaking a chip at really tiny DoCs too shaving off about a thou.

I was trying to work out why I've not had trouble previously....and I think I have, but managed it very differently. Before I was taking smaller DoC at lower rpm (about 700) and using the fact that the chip didn't break at all to just pull it out in one long string. Clearly it worked but it was slow and I was pushing for faster progress this time. Clearly a higher DoC breaking small chips and evacuating with flood coolant would be faster - if I could do both of those.

So I can have a play with the angle of the bar and see if going much faster (even HSS calcs are coming out at 2,400 rpm) helps at all. The fact the bar is in the bore should limit the danger should things go awry.
Has anyone got any suggestions of bar diameter to bore diameter sizes? I've previously drilled to 13mm (largest drill) and then I could just about get a 12mm bar down it....but it doesn't leave much/any clearance for chip evacuation. Tiny bar (6mm if I remember) clearly has too much flex at that sort of extension but was useful to get the bore larger before switching. I have some bars in between but haven't used them previously as I was trying to get up to the biggest bar as soon as possible....and I'm thinking now that I'm not trying to pull one long chip, it might be wiser to be using a bar somewhere in the middle - at least at first.

[Cenedd]

Bented I might have to try pinching your taped tube trick and see if it works with just air. I fear it may need a nozzle of some kind though - especially with the amounts my small compressor can keep up with. Worth a play though.

[MattiJ]

I dont like CCGT for boring bars that much, very small clearance between the bar and bore resulting in chips wedging between. DCGT or TCGT works better if bore size allows.

60mm depth with 12mm bar is also pushing your luck. Its doable but you are fighting with chatter at some point for sure.

for ”singing” parts it makes huge difference when you fill or wrap the part with some deadening material.
rubber bands, rubber hose, shop rags, thick layer of tape, solder wire.. almost anything improves the situation and often makes huge difference.

[deltap]

Boring is a slow process compared to drilling. Drill to 22 or 23 mm and then you just have a few finish passes. Even a small lathe should handle drills up to 25 mm.