getting the spring out

[Loose nut]

Yesterday I was boring out a piece of 3/4" thick aluminum plate, 3" ID, grade unknown, and when I was about .050" under size I made a couple of cuts with out moving the cross slide to turn the "spring out" so I could get an accurate measurement before making my final assault on the finish dia. Now after about 5 or 6 cuts with the same settings I'm still getting a little swarf coming off, not much maybe a fraction of a thou. This was with a 3/4" dia boring bar with HSS tool.

At what point is it finished, It seems like it should reach a point where the tool doesn't cut anymore, can anybody explain this to me.

 

[Kermit]

I Know NOTHING about it so let me give you my best guess.

;D

Assuming there is nothing wrong with any of the mechanical parts, could it be the micro changes associated with metal expandng and contracting with changing temp? I know the guys at work sometimes have to put the metal pieces in the fridge before they put them on the measuring table.

Other than that ^ I'm blissfully ignorant of any REAL reasons.



Comedy break over, let's get this man some real help with his problem...
Kermit

 

[Maryak]

Loose nut,

How much overhang was there on the bar. ???

The best I can come up with is too much and your have a vibration line in your cut. Each time you go again at the same setting, the tool hits one of these little bumps, bounces and makes another set of little bumps.

Reduce the amount of bar overhang to the minimum possible, sharpen your tool bit and take a couple very light cuts at the same settings.

Another thought is that being aluminium you may have a small piece welded to the tip of your tool. If so clean it off and try another cut.


Hope this helps. ??? ???

Best Regards
Bob

 

[jack404]

i get some "flex " from lighter boring bars on occasion
this makes a similar story, think your done cutting and its still peeling off a line of swarf

a friend here had a new lathe and did not seat the cross slide mitres real well after getting it

once he had it sorted that stopped his issues

other than those two it could be a few things..

good luck with it loosenut

cheers

jack

 

[rake60]

I think Kermit may have the answer on this one.
Aluminum can warm up very quickly when it is machined and will expand slightly
as a result. It doesn't take a lot of heat to make that size move a little.
It also dissipates heat very quickly and will contract as it cools.

If the stock warmed up a little while roughing the bore, it would be cooling as
lighter cuts were being taken. Until it reaches ambient temperature the tool
will keep fuzzing out what has shrunk back in.

Rick

 

[Kermit]

NO $%&* !! ;D

Wow, Okay. I had no idea but the assumption of no problem with the tools was really the only one I could make as a true greenhorn...Then I was left with nothing to go with but the physics of the item being turned. Aluminium

I do like the other scenarios presented. I will be well armed with information long before I ever get to start cutting.

I think I'll stop at (+1) in the guessing game and just start listening more. :-X

Leaving the casino before he loses his winnings,
Kermit

 

[Loose nut]

I'm thinking it is a combination of flex in the boring bar, I would have thought that a 3/4" bar was heavy enough to stop this in something that cuts as easy as aluminum, maybe not and the metal heating up. I don't have as much problem as this in brass or steel unless I screwed something up. I also sharpened the bit and made sure there wasn't any build up on it but it could have used more rake and relief, I don't know if the tool geometry would make that much difference. I don't have this trouble when turning.

As far as boring deep (bore size relative to the size of the boring bar) in a pipe or casting where you have no choice but to have the boring bar extended, how do you minimize flexing so the the bore comes out chatter free, parallel and not tapered.

 

[joeby]

Long overhangs will be a problem no matter what you do. If it's a through hole you can mount your work on the carriage and run a boring bar between centers; but blind holes can be tough. Best you can do is use the largest diameter bar you can and light finish cuts. Watch your speed so as not to start chatter, if chatter starts you almost have to take a deep enough cut to get under it, otherwise it acts like a record and you keep "recording" what the cutting edge picks up. If you have some disposable income, solid carbide bars help too.

Tool geometry can cause what you describe too. If the clearance is not enough and the tool "heels" in the bore, you effectively have a built in depth of cut limit.

Kevin

 

[rake60]

The boring bar should be extended the length of the bore
+ the length that fits your personal safety comfort zone
for disengaging the feed on the machine.

When I first started machining that safety comfort zone was 1/8"
As I got more experienced and cockier that dropped to .030"
Now I'm getting old and a little slower it's back to 1/8"

How heavy were the rough cuts?
Aluminum is very free cutting. I just can't imagine a 3/4" boring bar
flexing that much when cutting it. I'm not saying it couldn't happen,
but I'd be very interested to see the conditions that made that possible.
It's what I love the most about this career and hobby.
The metal teaches you something new every day!

Rick

 

[Loose nut]

It's what I love the most about this career and hobby.
The metal teaches you something new every day!

It makes me want to start throwing things.

 

[deverett]

I'm thinking it is a combination of flex in the boring bar ...

... As far as boring deep (bore size relative to the size of the boring bar) in a pipe or casting where you have no choice but to have the boring bar extended, how do you minimize flexing so the the bore comes out chatter free, parallel and not tapered.

Tubal Cain used to advocate putting a dollop of Plasticene on the tip of the boring bar during the boring process to change the harmonics and thus the (micro) chatter pattern - if no larger boring bar was available.

Dave
The Emerald Isle

 

[joeby]

A strip cut from an old innertube wrapped tightly around the bar and taped in place works for controlling chatter also.

Kevin

 

[Peter Neill]

Grab hold of the saddle at either end and try and twist it, if it moves even a a bit, then check the gibs on your saddle are tight, and if not, tighten them.
If there is any play there, then the saddle can cock over and twist just a touch in one direction when you drive it forward, and then in the other direction when you drive it backwards.
If this is happening you can get more of a spring cut apparent on the return stroke if you come out of the bore at the same setting.

Peter