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Tool/work deflection

Joined
Jan 25, 2005
Location
New Egypt, NJ
I've been using the boring bar a bit lateley, and
have been astounded at the amount of deflection inherent in it. Or maybe it's the work deflecting. The work is a fairly large socket (finished inside diameter is 2.055"). I'm enlarging the 4 "teeth" on a spanner socket, so the teeth "hit" the cutting bit on their way by it. the bit is as close to the tool post as I can get it to minimize deflection (post is an import knock-off of an Aloris), so there's about 5" of the bar (which is substantial, guessing 5/8" dia) extending ot the other side of the tool post. This end of the bar jumps at LEAST 1/16" each time the teeth of the socket hit the cutteing bit. without changing the cross feed, just making the same cut again, it takes another 3 or so thou! Any way to calculate and compensate for this, or just go slowly and carefully with very light cuts?
 
I've had mixed results w/ boring bars too. By far the biggest factor for me has been a good tool grind. Its amazing how much difference finally getting the edge right will make. I was having problems just like you describe (even down to the teeny cuts) on bronze for heaven's sake, till I finally got a good edge on the tool and it was off to the races and I very nearly cut too much.

Greg
 
Interrupted cuts (especially on hard stuff
like this) are a killer for non-rigid setups.

You wind up exciting the resonance mode in
the bar with the tunk, tunk, tunk.

Almost a job for a toolpost grinder.

Jim
 
As for the cutting edge, I suspect the interrupted cut is hell on the bit. I'm actually surprised how well it's held up; I've now done 3 of these sockets (they only need about .025" taken off, and including setup it takes about 7 minutes). I wonder if my HSS bit is holding up better than a carbide one would? I bet carbide would chip and break, being more brittle. As for a toolpost grinder, I don't see one of those in my immediate future... too bad, I have another project that could use one...
Just amazes me that there's that much deflection in 3/4" of 5/8" dia steel!
 
You might trying tightening the clamp screws on the cross slide. That should eliminate the possibility of the vibration/interrupted cut "pulling" the cross slide farther into the cut.

Proper tool geometry is a necessity when boring, as is having the tip of the tool exactly on centerline.

A good grade of carbide( C5 or C6) might do well for you. I have successfully cut stuff in the 50's HRC at work with carbide, though it didn't take long to put noticeable wear on the insert.

Absolutely stay away from ceramic inserts on interrupted cuts! Ceramic will do a pretty good job on hardened steel as long as the cut is continuous, but normally the first interruption will break the tip of the insert.
 
I would say HSS better than carbide, and if
the socket is too hard and eats the HSS,
stellite or black alloy is another 'impact
resistant' kind of tooling you could try.

I never had much luck with carbide on interrupted
cuts.

Jim
 
Get a bigger diameter boring bar. If I understand your post correctly you're using a 5/8" D bar for a 2" dia hole with an interrupted cut. The bigger the bar, the better, you will still have some deflection, but it will be reduced.
Harry
 
Hi There,

You mentioned that you have an "Aloris Type" tool post. Is it a wedge type or a piston type? The wedge type are much more rigid.

Is your boring bar hardened? A non hardened boring bar will have more spring to it.

Does it have to stick out 5"? If you can shorten it, it will have less spring (but you knew that). If you can increase the diameter of your boring bar, it will be even more rigid (as others have suggested).

Interrupted cuts are hard on tooling. I find HSS the most forgiving in these circumstances. Carbide will usually chip (others have mentioned this too). Take light cuts and time. You will get it done.

Good Luck!
-Blue Chips-
Webb
 
Hi: Are interupted cuts just what the name implies ? You have a good cut going and stop for whatever reason and when you start again it's a terrible cut as if the settings had changed ? Thank you Joe
 
Interrupted cut-

Picture a piece of square stock in the lathe chuck. Picture what happens when you attempt to start turning that piece of square stock. You're only going to touch on the corners of the stock. That is an interrupted cut.

I guess that technically turning the lathe off and coming back in half an hour to finish is an interrupted cut, but it's not what we're referring to here.
 
Webb,
The tool post is a wedge type. Also, the bar isn't sticking out 5" between the post and the work, the other end sticks out the back of the holder 5" because I have the cutter as close to the tool post as possible (about an inch). It's the back end that wiggles and shakes; that's how I know there's a lot of deflection, well, that and the fact that a repeat cut takes so much additional metal. But maybe the tool's getting drawn into the work? I hadn't thought of that. I don't think my machine has cross slide clamp screws, as IMC suggested. It's a '42 SB 10L. Anyone know? Also, I didn't know the tool should be on center; I have it slightly above center so the cutting edge can be well supported and the heel of the bit won't hit the work.
 
beckley,
I am using a 5/8" bar for a 2" hole; for a bigger bar I need a bigger tool post I think? Or a custom made job... the bar I could handle, the piece to hold it is the tough part for me at this point.
 
With that much deflection you should be able to find the source. Try putting a short pipe or even a wrench on the 5" end of the bar. Pull up and down and front and back and look for movement where the bar is clamped in the holder, between the tool holder and the tool post, in the compound ways, in the cross slide ways. You should be able to see or feel the source of the problem. Keep a minimum radius (but not a sharp corner)on your tool bit to reduce the load. That being said, interrupted cuts are not usually smooth sailing, especially on hard or tough materials.
Chaz
 
It is possible that you have a bad boring bar holder and/or toolpost. The 1st wedge type toolpost I had on my 16" Monarch was an import. Whenever I tried to bore a hole in some 52100 that bar(1-1/4" D) chattered like crazy. The problem got so bad that I was in danger of losing a nice production job. I measured the hole in the holder and sure enough it was a little large, in addition the toolpost was not repeating in an acceptable range on any of the toolholders I was using. I bought a domestic(Dorian) wedge type toolpost and a new boring bar holder and never had any further problems. That was 15+ years ago.
Just tossing something out to check.
Harry
 
Hi There,

My mistake! I misread your first post. Try locking the gib screws on the compound and the cross slide and see if that reduces the deflection. If it does, they may have been adjusted too loose.

Another possibility is to check the saddle and see if it "rocks." I have seen a saddle wear more at the front and can "rock" when the cutting load is moved forward of the cross slide.

I hope this helps. Good Luck!
-Blue Chips-
Webb
 
`1/16 sound's like way to much for bar deflection...
not sure about what kind of setup you have on your lathe but it soulds like the brass nut on your cross slide might be loose...i don't mean worn threads but the bolt or what ever holds it to the carrage may have come loose and the cross slide is being sucked back and forth by the intermittent cut...
if you can hang on the top of the tool post with all your weight and the slapping calms a bit look for loose parts somwhere.
 
Hmmm, more stuff to check... I'll try locking the gib screw first, like you suggested, Webb and see what happens (BTW, how do you pronounce "gib"; does the g sound like a "g" or a "j"?). Meanwhile, I've bored another of these sockets (doing them for some of the guys in the shop), this time being very careful with the last few thou, and got it PERFECT! :D
Sounds like I'd better check for loose stuff, and the fit of the bar in the holder... Beckley23, what's a "little large" on the fit of boring bar in holder?
 








 
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