Exploding cutoff blades
All too often I'll have a hss cutoff blade break mid-cut. I just had one explode and get flung 5 feet from the lathe. This usually happens in deep cuts in steel, or even worse, aluminum. What am I doing wrong, if anything?
Situation: hss 3/32nd's cutoff blade (import). Blade is tapered (bevelled.) Lathe in backgear, mid-slow rpms. Manual feed, trying to feel the chip lift off, then advance on the "soft" pocket that results. The blade is positioned slightly above centerline. Using Tap-magic generously.
Going good until about halfway there, then BOOM ! I think tonight's instance (12L14 steel), the chip lifted but didn't peel away, and the cut off blade got hung-up on it.
I've had a cutoff blade jam in a deeeeep cut in AL, and stop the lathe dead in backgear with the V belts squealing. Something tells me this should be avoided.
Is 3/32nd's too thin? Should I be making a relief cut? Certainly I'm not the only one to experience this? I'm beginning to get gun-shy of cutting parts off.
A relief cut is a very good idea. The other is that you want the blade ON center, not above it.
If it's above and it digs in and is forced down, it will dig even deeper. That's probably why it's breaking.
I might be doing it wrong, but I cut off at or near the same speed as when turning, tool centered and exactly perpendicular to workpiece, and side clearance for the full depth of cut, flood coolant on. Right or wrong, I don't know, but it works for me. A few degrees top rake can help. Wearing safety glasses and not standing directly in line with the cut off tool might help protect you from a catastrophic failure.
Make sure the blade is not extended any further than it has to be and on center. As for speed, I have variable speed control and start off slower on the larger OD and than gradually speed up when reaching the smaller diameters.
Try to maintain a continuous chip and occasionally clearing the ones that don't make their way out.
Still have to watch for aluminum and brass since they are so grabby.
what size lathe? (And weight)
I don't think you need a thicker blade, I use 3/32" in my 12x36 1000lb lathe just fine, 1/8" seems too thick for steel on a lathe that small (Yes small)
What would also help is if you checked out how fast you are feeding
Engage automatic cross feed (At the same spindle RPM's) and see what setting corasponds to how fast you normaly feed. Or just time your revolutions and do math, maybe with a indicator for travel on the cross feed.
Getting *dead* on center is very important. Too high and yea it can dig in, too low and the work can bend and ride up ontop of the blade (scary!)
Is the top of your blade got a chipbraker? My blades are just flat toped and it works fine, too much rake can cause it to dig in.
Are you aligning your blade to be *PERFICTLY* aligned to 90 degrees to the work, a degree or two off and the side of the blade will try and cut as its fed in deeper.
You could also grind down the sides of the blade a few mils up to just before the tip of the blade to provide side relief along the length of the blade
Make sure the blade is straight in the holder. It should have clearance on both sides of the blade. If it is not straight in the holder all the relief will be on one side. Don't be afraid to slow rpm's down a little.
What is the condition of the lathe? You can't part off successfully on a worn out machine.
Oh, some other questions that came to mind:
how far is the blade extended out of the holder? It should be no more then is needed (ie just over an inch for parting up to 2" diamiters is what I usally leave it at.. im a little shy about parting over 2")
How far are you away from the chuck while parting? parting is best done within 1" of the chuck
Are you using a tailstock? large tailstock ram forces can cause the work to collapse around the tool causing it to bind. its generaly recommended NOT to use a tailstock while parting.
As recommended, I put the tool on center and also made a relief cut. (Went in a bit, backed out and moved over about .010", repeat)
These seemed to help alot, thanks.
Scary....Dam right ! (that's why I had the blade just a hair above centerline.)
Originally Posted by Black_Moons
Anyways, lathe is about 1000 lbs, 11" by 36" 1.5 HP I was considering getting insert tooling, but I certainly don't want to invest in a holder and inserts and have it damaged by my stupidity.
So far, so good now, thanks all.
RPMs too low. You don't need to cut off in back gears. Even if you only use a brush to drip oil/coolant in the groove, do it. back the tool out to clear chips every once in awhile. Two reasons most cutoff blades break. Dull tool and cut clogged with chips.