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goose
01-30-2010, 08:51 PM
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.

Thank you,

Gary

danlb
01-30-2010, 08:55 PM
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.

Dan

JTToner
01-30-2010, 10:23 PM
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.

RKW
01-30-2010, 10:53 PM
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.

Black_Moons
01-30-2010, 10:54 PM
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

hssmike
01-30-2010, 10:57 PM
Gary,
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.

Mike

luthor
01-30-2010, 11:13 PM
What is the condition of the lathe? You can't part off successfully on a worn out machine.

Black_Moons
01-30-2010, 11:16 PM
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.

goose
01-30-2010, 11:18 PM
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.





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!)



Scary....Dam right ! (that's why I had the blade just a hair above centerline.)

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.


Gary

Rustybolt
01-30-2010, 11:20 PM
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.

airsmith282
01-30-2010, 11:29 PM
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.

Thank you,

Gary


first off you need to set the blade a slightest hair below center,
2 make sure its sharp
3 use cutting fluiid and ots of it even on aluim
4 take extreamly slow feed and i do mean slow and the slowest speed you can get out of the lathe
5 not nessasarry for a relief cut but its not a bad idea either,
6 do not oever extend the blade length start at 1 inch if you need more you can adjust later if need be..
7 follow these steps and you will do just great, ignore them and you will be asking this same question again ,,

8 i have parted off 2 inch aluim and steel and have had no problems doing it this way, as soon as ai try and push it i get trouble so i leanred not to push it,
i can now and most often do part at 230 for lauim and steel and brass and bronze i have done at upto 975 rpm cause brass try;s to catch and bit all the time ,, dont push the feeds and speeds untill you can do it slow take your time there is no rush ..
9 does not matter how small or how big of a machine you got ..if you push it you will have problems

Black_Moons
01-30-2010, 11:40 PM
Oh, some other good advice: Lock your carriage, lock your compound, And as an intresting option you can run the blade upside down (and in reverse, or on the back of your cross slide) to provide much better chip clearance (as chips fall outta the cut insted of built up ontop of the blade), continious air blast can work to clear chips just as well though im sure, But this just another 'advanced' option, it should work fine normaly.

Getting a *taller* blade can help.
If you have a BXA toolpost, the standard cutoff holder for it can take 11/16" blades, or 3/4" if you file the top of the blade holder groove like I did (Doesnt even contact the blade in normal operation, so the modification doesnt affect its ability to hold the smaller blade or have any chance of screwing up the holder if you do it wrong)
Note that it does add a little bit of rake that can help, Don't add any (well, as much) rake on your tool if your using these
(For the record, I use the affordmentioned 3/4" tall blades at 3/32" thickness on my 12x36 without problems in both wedge and T style)

Theres also 'cutoff blade' to square shank holder dealies you can buy. these generaly do not add rake.

(mmm at air/mist system for cutoff)

darryl
01-30-2010, 11:41 PM
I can see that play in the crosslide and compound can easily allow the workpiece to drag the cutter inwards by friction from the sides of the cutoff blade. All of the area on the workpiece in the groove that's below the center axis can impart both a downward and an inward force to the cutter. Normally we only support the cutter against downward forces.

My thinking in theory says that the cutting edge should be just a tad above center to impart some small force tending to keep the crosslide positioned against the crosslide lead screw. That may tend to counteract the tendency of the cutter to dig in because of the top rake angle. Just what seems right to me, in theory at least. I have not done a lot of cutoff work on the lathe, preferring to start the mark on the lathe, then cut it off on the bandsaw. I would normally be facing the cutoff ends anyway, so there's always a remount of the workpiece required.

JTToner
01-30-2010, 11:53 PM
One more thing - don't forget to lock the carriage.

websterz
01-30-2010, 11:56 PM
http://www.gadgetbuilder.com/VerticalShearBit.html#Parting

Adding the "V" notch to the top edge of your parting tool will work wonders. I part lots of 4140 and the notch makes the chips flow so much more freely out of the cut. Give it a try...you'll like it! :D

x39
01-31-2010, 12:11 AM
What are you holding the workpiece in?

airsmith282
01-31-2010, 12:16 AM
it dont matter if you lock the carriage or the comppund. the blade he is using will flex either way , even at 1/2 to 3/4 of an inch and locking it all down the blade will still flex, when your using thoes skinny blades you have no choice but to take this as slow as you can or you get into problems also dont take the cut where you want it take it back a bit to allow for hte flex so you can flip or peice and square it off after to the proper size you need,

DaHui
01-31-2010, 03:51 AM
Just get an insert tool and you won't have to worry about it anymore :)

oldtiffie
01-31-2010, 04:21 AM
David.

You have got it even if indirectly.

Many grind those HSS trapezoidal parting tool bits pretty well at the pointy end but it goes down-hill from there.

The widest part of the tool in the cut should be at the cutting edge. Many grind the front down a little and neglect to grind the top off the tool bit behind the cutting edge.

If the back of the tool or any part of it is wider than the cutting edge it will jamb or break - it has to.

I know it "costs" if you grind it down - but it costs a lot more if you don't. Once any part is ground so that 1/4>1/3 is ground off - snap or grind it of and start again. Parting tool blades are "consumables".

I prefer a chip-curler at and behind the cutting edge as it forms the cuttings into a tight cylinder.

Have a look at any parting tool with a TC insert in it and you will see that the insert is wider than the holder it is held in.

http://i200.photobucket.com/albums/aa294/oldtiffie/Lathe_misc/I-Fanger3.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Lathe_misc/I-Fanger2.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Lathe_misc/Part_off4.jpg

https://www.machineryhouse.com.au/Products?stockCode=L029

https://www.machineryhouse.com.au/Products?stockCode=L028A

https://www.machineryhouse.com.au/Products?stockCode=L011B

https://www.machineryhouse.com.au/Products?stockCode=L073

https://www.machineryhouse.com.au/Products?stockCode=L074

Ian B
01-31-2010, 05:37 AM
Gary,

As Black Moons asked, what kind of lathe do you have? Mention of backgear suggests it could be an old one. You are manual feeding - does your lathe have power crossfeed?

I struggled with parting for years with a small lathe (Myford ML10) - light, no power crossfeed. I know there are 2 camps on this one, but the most success I ended up having was with a rear toolpost, tool upside down. All the slides locked, 3/32" wide HSS tool, sharp, square, on centre height. Still, parting off was never any fun, and tools still broke.

Then, I bought a second lathe - a Harrison M400. 2 tons of it, modern and very rigid. I bought an insert cutoff blade, here's a pic of it in a holder I made:

http://smg.photobucket.com/albums/1003/IanBartlett/?action=view&current=Assembled.jpg&newest=1

The blade's 32mm high, the slot in a T3 toolholder is 38mm high - so it just fits, no side overhang. The 2 left hand screws clamp through the holder onto the blade, the 2 right hand screws clamp the holder.

The tips are 3mm wide, the packet suggests 3 thou - 5 thou per revolution of infeed. I can simply set this on the crossfeed, use flood coolant and part off anything I fancy - totally uneventful.

I set the tool to centreheight, but not within a few microns - and the bigger the diameter, the less it matters. The tips, even when new, do not feel supersharp. Parting off 4" diameter steel barstock goes easily, one pass, no relief cuts. I run the spindle at about 1/2 the RPM's that I'd use if doing normal turning.

The tip has both front and side rake (a sort of hollow top face), resulting in a chip that looks a bit like tiny guttering - rolled inwards rather than being a flat ribbon. This prevents swarf jamming in the groove.

Ian

PixMan
01-31-2010, 09:56 AM
Nice holder you made Ian, very nice. That 151.2 Coromant blade is a good choice. I don't know where you're located, but did you know that in North America you can also use Valenite VSG inserts in it? Their "PG" (Precision Groove) chipbreaker is the one I've been using with incredible results in all materials. It cuts so nice that I am using their 5820 (all-around grade for just about everything) over their tougher 5735 grade and not getting any chipping problems. The inserts are a little less money than the Coromant ones.

I bought a couple of spare blades (VG101) in different widths when they were having a 50% off sale through 10/31. I don't know if it's still going on, but I paid just over $50 each for 2 new blades. I had one from Ebay for less.

I am too unmotivated to make such a nice block. Dad & I have a 16 x 40 Taiwanese Victor lathe. I bought a cheap CA-2 for the Dorian QC tool holder that came with the machine. That cost me $26. I use a Newcomer (made in USA) blade-holder block in the CA-2, that was on sale for $74 from KBC Tools. Really nice setup, and man can I recklessly (or should I say wrecklessly) plow through all kinds of materials. I've cut off up to 2.75" diameters so far without issue. The machine does have coolant though, and that helps. I set it for about .004"ipr on the cross slide feed and let 'er rip.

PixMan
02-01-2010, 01:06 AM
BTW, the Valenite VG101 blades are available in 19mm (3/4"), 26mm (1") and 32mm (1-1/4") variations to allow them to be used even in some fairly small lathes. I don't know for certain, but I believe the same would be true of the Coromant 151.2 T-Max Q-Cut blades.

madokie
02-01-2010, 03:12 AM
some one mentioned running lathe backwards and setting up cut-off blade up-side down ,so chips fall down and out of cut.personally i want to see whats going on when i am cutting & i bet that most folks reading this do to, so thats out for me.ANY ONE DONE IT BACKWARDS?? well running it backwards can work on larger machines, but small lathes run the risk of breaking off dovetails on the cross slide.all the force is pushing up and only dove tails take the force. going forwards the flat & wide areas, on cross slide take the force and preasure.we had problem at work using 1/8carbide inserts with steel holder that fit in quick change holder,steel holder was thinner than insert,about .09 i think and 1 3/4 tall by 6 inches long.lead man broke 2 steel holders @100$ a piece,with in 2 weeks, well we didnt buy any more!!we looked in catalog and found 20$ HSS blade 5/16 thick by 1 3/4 by 5 inches long that fit in same quick-change tool holder just fine,and did plenty of cut-offs in first week of use , we bought 3 more for spares and forgot all about carbide inserts.:)

Black_Moons
02-01-2010, 08:15 AM
I do a lot of my threading upside down and backwards now. (Mainly to thread away from the chuck/shoulder, but anyway) Still havent tryed parting upside down however I will admit.

'seeing' what your doing? What is there to see? nasty chips flying? you still get to see those they just go in a slightly diffrent direction. You still see the tip contacting the work because the work is round and your tool has relief so it does not block the line of sight, if anything you see MORE because the chips are not in the way of you seeing the contact point. the only downside is you won't easily see material welded onto the tip of your cutting tool.

Iv never heard of dovetails being broken off (Unless crashed into the chuck), I would assume some 3/32" by 1" chunk (overhanging about 1") of even hardened tool steel will snap long before several inchs of 1/2"+ thick cast iron dovetail, dispite the 45 (or whatever) degree angle.

Parting backwards/upside down is IMO slightly easyer for small lathes then threading, because you can lock the carriage to prevent it lifting.
(Threading requires the weight of the carriage to hold it in place)

5/16" parting blade? wow that must be a giant lathe. the parting blade I use is 3/32"! less then 1/3 as wide. and my lathe is a 1000lb 12x36"

JTToner
02-01-2010, 10:30 PM
Yesterday I had occasion to part off a 3/4" 4140 workpiece. I used an HSS parting tool, nothing fancy, just a generic Chicom blade from, I believe, Grizzly. I didn't check the thickness, but I ran the spindle at 700 rpm. If I did the math right, that's just south of 140 sfm at the beginning of the cut. Sharp tool, side relief, and square to the workpiece. I hand fed it and no problems. After the parting there remained the tiniest nip. Seems my tool was just a half a tad low. I raised it and ran it across the nip and bingo, no nip! Spot on, set for the next job. Parting is easy, just be sure all the parameters are on the mark. Hope this helps anybody having problems with this operation.
Johnny

airsmith282
02-02-2010, 01:18 AM
i belive that parting off upside down is asking to have an eye put out, if you do it the normal way and you shaatter the blade it tends to not try and take out your eye or logged it self in your heart, i broke a few and all of them missed me by miles, now to do this upside down is very stupied at best, as you are havign to look at what your doing and if it catches and snaps that blade is heading right for your face or chest ,, try explaing that one to the wife if you survive it,

threading upside down has been done for years and no real danger there at all.

but parting thats another story

madokie
02-02-2010, 01:43 AM
yes the lathe we use 5/16 part-off blade is big ,24 swing 15 feet long between centers, russian Ryazan ,company bought it because they needed power feed on the compund to cut carbide centers in oil field shafts with diamond insert.

RKW
02-02-2010, 09:23 AM
I thought the whole idea of parting upside down was more for the smaller lathes (read underpowered) and that speed and HP really would not be much of a danger with the little ones, right?

I part in the normal position but occasionally run into a problem or two when it grabs from chips not getting cleared or sticky material. I also keep the blade as short as possible.


i belive that parting off upside down is asking to have an eye put out, if you do it the normal way and you shaatter the blade it tends to not try and take out your eye or logged it self in your heart, i broke a few and all of them missed me by miles, now to do this upside down is very stupied at best, as you are havign to look at what your doing and if it catches and snaps that blade is heading right for your face or chest ,, try explaing that one to the wife if you survive it,

threading upside down has been done for years and no real danger there at all.

but parting thats another story

benniet
02-03-2010, 09:12 AM
I had the same problem with cutting off and breaking bits. A old guy at work the same guy that is teaching me showed me a way that is a little on the harder side to use a cutoff tool. As you are runing in the bit in move the carrage side to side a little bit so that it cuts a relif all the way through your cut. I warn you that it takes some practice to keep your cut strait. That method is what works best for me. also to keep your bit on center or with a little bit of an up angle about 1 to 2 degrees.