Strange boring prob.... ideas?

[B34M3R]

Ok, first off, I am new to this field, I'm only 22, and most of my learning has come on the job... I'm currently working with aluminum on a mori seiki SH-50 horizontal mill...

Heres my problem... recently this machine has developed a problem with the main bore holding its tolerance... We need to keep a 1.002-1.006... and the bore will jump .002-.003 between one set of parts and the next. This is causing me a freakin ton of stress at work, because I am spending more time keeping that damn bar dialed in then I am working on my parts...

There are some quirks to the problem though... we run 8 parts at a time... and the overall variance in those 8 parts is only about .0002-0003... which is normal. But if I run another set, that set will jump up or down(it alternates, actually) around .002.... And if I run a 1 one-part, the other 7 will be .002 off the single one I ran... even if the tool never left the spindle...

Now, supposedly our setup guy has tried swapping the boring bars out between machines (we also have 3 SH-500s and 2 SH-5000s, all running the same part) and according to him, the problem is related to the machine, not the bar.... he thinks it is related to the spindle, but I fail to see how thats possible. The only major difference between my sh-50, and the sh-500s are the insert drills we use to rough the holes... the 500s are using a shorter(8 in or so) .985 insert drill that comes in from both ends of the part to finish the hole, while my sh-50 using a "prototype" (prototype to my shop anyway) 18" mitsu drill, which ends up only making a .965 hole...

Now, theres 2 obvious differences between the holes that the drills are making.... one is the .020 difference in size, and the other is the finish... in the good machines, the finish looks like standard, shiny, machined alum, even before the boring bar cleans in up. The mitsu drill, however, leaves an uneven blackened finish to the bore...

Oh, and I forgot to mention, the boring bars insert actually moves between sets, even though the adjustment dial does NOT. Lock screws have been replaced, and do not appear to be loosening up at all.



Now, the idea I am tossing around in my head, is maybe some sort of heat expansion issue in the end of the BB... that causes the insert to move, and then as it cools back down, move back the other way? I wouldn't think that .020 more material would cause TOO much excess heat, but if that black finish that the drill is leaving is some sort of glaze, then I guess it could be possible...

Ugh, I'm the new guy running these machines, but I'm the only one who seems to overly care about fixing the issue... and as such, most of the people at work are blowing me off... so please guys, ANY ideas and insight on the matter would be helpful... thanks.

[laka]

Maybe dial the spindle and see if it is perpendicular to the table. Sounds like your tool (drill anyway) isn't perfectly perpendicular and rubbing a bit.

How do you know that the actual insert is moving on the tool? If so, maybe the seat is damaged or worn.

We've had wierd boring problems like this on one of our machines, and occasionally a small chip will get stuck on the taper of the tool and tilt it out a little bit changing the bore sizes on part to part.

Is the boring bar actually hot when it finishes boring a hole? Boring 0.02" shouldnt make your tool very hot at all, not enough to worry about expansion issues. If this was true, then the first hole would be good, and each additional hole would get larger and larger.

If the tool bores consistently during one toolchange cycle, and changes size after cycling through tools again it sounds like there is something funny with your spindle and the tool is not seating in the taper perfectly.

[fpworks]

I've seen this before...nearly identical circumstances. The problem was in tool retention. When the parts go oversize, for one cycle, then go back to size, it IS a runout repeatability issue.

Check the runout, measured on the tool. Might be difficult to check on your bar, so you may have to use a different tool. Execute tool changes, and check again. Also try starting the spindle and stopping, then checking. I bet you'll see the runout jump after one of the tool changes or one of the spindle on/off cycles.

Tools will typically have a tendency to self center if loose in the spindle (like a spinning top) and the spindle is running...BUT cutting action can throw it back off to the side. However, if the tool may also have a tendency to stay off center and not center up while the spindle is on. As you can see, the phenomena is unpredictable, same as your results.

If possible, and if you have a retention force gage, retention force would be the FIRST thing I would check. Also, check condition of the tool holder, retention knob, spindle bore, etc.

Best of luck!

[dertsap]

thats a lot of material to take out for a finished bore

[big_mak]

I'm with FPworks, and would check the draw bar retention. SOmething si not repeatable.

The other thing is that the extra 0.02" of material may be causing some problems. We can't don't know what bar you are using, so we can't even guestimate how the extra material is effecting the bar.

Are you using the same cooling methods? Thru Spindle or Flood, as the other machines.

My Money's on the tool not seating properly!

[dertsap]

what happens if you do a spring pass ?
i think removing that much material you've probably got deflection , .02 can make a huge difference when boring

[ctate2000]

Could be run out as stated. Could be loose retention stud.

Have you checked the roundness of the hole? How are you checking the hole?
Are you using multiple fixture offsets to position?

Blue the part after roughing and verify you are actually removing material. You may have an oversize hole before you ever begin the finishing pass.

Why are you boring a hole with .004 tolerance? You could drill a hole to that tolerance. Buy a good indexable drill and have the shank ground off center to the cutting diameter by .002 and drill it.

[B34M3R]

Ok, first off, thanks for the replies guys...

Anyways...

Ok, I work in a fairly large (80 machines or so) shop, that has its own engineering dept, setup guys, maint guys, and so on. I am incredibly limited in what I can do myself, since Im "just" an operator. But here we go...

As far as the oversize hole is concerned, that is not the case. The drill actually runs U/S for what our prints call for. I've measured it out to a .965 hole... that is bored to 1.002-1.006. So theres actually .040 being bored out, twice as much as our prints call for. I'm actually fairly concerned about the finish that the drill leaves inside the hole... it is blackened, and not uniform. Unlike the other drills, which leave a nice gleaming surface, which is mostly uniform(some minor vibration rubbing does show up...)

Another thing to consider is, being that my company likes making its money, they have these things pushed as fast as they can go. That drill is going through there at 18 IPM... (i am unsure of the RPM, ill check tonight) and I do not think the TSC can keep up... I mean, there is a ton of coolant being dumped onto the part as well (I assume this is what you mean by flood?), but none of the coolant can get into the hole as it is drilling... its horizontal mill, so once that thing gets over an inch in, its losing the extra coolant. And I think it has something to do with it, because that first inch or so is less/not black like the rest of the hole... Can overheating alum cause actual hardening to the surface if this was occuring?

One of my boss's suggested checking the tool retention, but I do not believe anybody has tested that idea further, I'll check on that tonight. We have also had problems with this machine dropping tools during changes, and tools sticking in the spindle... related perhaps?

As for what I measure with, we have a go pin, a no-go pin, a drop pin(checking for flex, which we always have to some degree or another) and a digi bore mic... the bore mic is what I use personally... although our inspection guys dont... they use the pins (lazy ********)

And we bore because we need to keep the finish below 65.... I know the drills we use now aren't capable of even close to that.

Also, I know the insert is moving because we have a computer-machine-grapher-thingy(i have NO idea what its called) that measures all of our tools digitally... I suppose the retention stud could affect that also couldn't it? Because it clamps into the measuring-machine and measures it while rotating...

Man, we have all these high-tech gadgets at our shop to help us out, and they never get utilized.... =\

Anyway, I've got some pictures of the parts loaded on the pallet, with the bar in the spindle, if that would help you guys understand anything I'm not explaining well....

[ctate2000]

Sounds like you need a new spindle. Dropping tools and sticking tapers are symptoms of inproper taper alignement in the spindle nose. It is possible you may only need a new tool holder. However if you have been using damaged holders you have probably damaged the spindle nose. Retention problems could still be the problem but they can cause spindle damage by allowing the taper to move in the spindle. Check the geometry of the spindle by indicating. You should get only .0001-.0002 of indicator movement in the spindle taper. Any more may indicative of more serious problems.

You are correct, if the drill is leaving a black finish in aluminum then you are rubbing. Drill is not cutting correctly.

[dertsap]

if the insert is moving then the seat on the bar must be cooked , i still think expecting consistency when removing .04 from a bore is a long shot depending upon the setup
many times sticky tapers are due to coolant getting into the spindle and drying wd40 and a rag will fix that if it is the problem
most times it is the simplest things that create the problems
has the job ran ok on that machine before ?

[big_mak]

It all depends on the bar he's using. If this place has all this money, and are getting good results on other machines with another process, they should be using the same process on all like machines.

With those machines and those tolerances, I'd helically interpolate those holes, and ditch the boring bar!

[dertsap]

Originally Posted by big_mak With those machines and those tolerances, I'd helically interpolate those holes, and ditch the boring bar!

good point
i was under the assumption that the bores are going deep concidering the drill is 18" long

i just dont buy into the spindle throwing the tool out that much ,if that is the case then most other measurements from the other tools would be out and inconsistent as well
he says the insert moves , my question is why ?

a sloppy seat may accept .02 material removal without moving , an additional .02 may be enough pressure to make it move