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cooter
05-19-2005, 08:04 PM
I have a small metal lathe (12x36) which I use frequently. I always seem to have trouble using a cutoff tool in the machine to cut round stock (steel) larger than 1 inch in diameter. After a nice start and cutting into a 2 inch piece approx. 1/2 inch or so the tool seems to want to vibrate and not work properly. I have tried several different angles when sharpening the tool and different speeds, with and without cutting fluid. This is a tight machine and otherwise works well. Any help would be greatly appreciated. Thanks....Gary

kenb
05-19-2005, 08:27 PM
I have a small metal lathe (12x36) which I use frequently. I always seem to have trouble using a cutoff tool in the machine to cut round stock (steel) larger than 1 inch in diameter. After a nice start and cutting into a 2 inch piece approx. 1/2 inch or so the tool seems to want to vibrate and not work properly. I have tried several different angles when sharpening the tool and different speeds, with and without cutting fluid. This is a tight machine and otherwise works well. Any help would be greatly appreciated. Thanks....Gary

I have the same problems with my Myford ML7, which is signifigantly smaller than your machine. I've never been able to part off steel of any size with much success, and I'd pretty much figured that my machine just doesn't have the muscle or rigidity in the toolpost to handle the job properly.
I'm just guessing here, but the fact that it starts off OK and begins to experience problems when the diameter is reduced could point to a cutting speed issue. As the diameter of the cut is reduced, the cutting speed will be reduced proportionally as the circumference of the cut gets smaller.
I'll be watching the future follow-ups to your question carefully, as I think I might benefit from the resulting replies as well.

Cheers,
Ken

metalchomper
05-19-2005, 08:29 PM
Welcome to metalmeet Gary. I'm not a machinist, but did stay at a Holiday Inn last night. Just kidding. In my experience, the top of the parting tool has to be aligned with the centerline of the chuck. I've experienced similar problems when having the cutter too low. I have my cutter set to cut straight in. No angle. For a 2" stock, my speed is around 110rpm.

CCWKen
05-19-2005, 10:21 PM
I'm no machinist either but I've found that low speed and a heavy feed works best. Make sure you're going straight in and the carriage is locked. Your cutting edge should be on center or no more than a couple of thousandths high. It should never be below center. It'll dig in then cut causing chatter.

I use HSS and Cobalt parting blades with a relief (shaped like a V looking at the end). I stone the cutting edges sharp!

Pedalcar
05-19-2005, 10:24 PM
Hi Gary,

this depents on material ,from either the tool or the stock,

I agree with Ken here.

normaly the tool should be on center most of the times but because of maybe a little bending of your tool it will be below center when you start cutting.

So try to set it just above center.

Another thing is that you get worried when the tool vibrates and you kind of back of.That is the wrong thing to do ,try to turn it in a little faster and i don't mean the rpm but the feed so the tool has to move material.

Hope this helps

Ben

jlrussell4
05-20-2005, 04:20 AM
Hi Gary,
I have found that Ken and Ben's advice is what works for me on my 12x48 Craftsman/Atlas. Slow speed and heavy feed. (Yes, you can feed too heavy.)
Jim

bobadame
05-20-2005, 07:18 AM
Keep the blade as short as possible. In other words, extend the blade just enough to pass thru center of the part you're cutting.

Keep the tool holder supported, in other words, crank the compound back so that its ways are supported as much as possible at the start of the cut. The point here is to maximize stiffness of the setup so that the tool point position stays on center through out the cut. Lock the carriage in position, again for stiffness.

Set the cutting edge of the blade exactly on vertical center of the part. If it's set above center it can't cut at all because the point of the blade is not in contact with the stock.

Be sure that the blade is exactly perpendicular to the part it's cutting.

Different materials cut better with different top rakes. Grind the cutter to suit the material. Make sure it's sharp.

Use lots of oil. You're taking a broad cut and a lot of heat will be generated. Oil keeps the metal from galling .

ralph
05-20-2005, 08:29 AM
All good information here. One more, part as close to the chuck as possible.

Unless its absolutely necessary to part off, I'll pull the stock out, cut it on the bandsaw, then put it back in and face the end. Chickens way out. :lol:

cooter
05-21-2005, 03:46 PM
I have a small metal lathe (12x36) which I use frequently. I always seem to have trouble using a cutoff tool in the machine to cut round stock (steel) larger than 1 inch in diameter. After a nice start and cutting into a 2 inch piece approx. 1/2 inch or so the tool seems to want to vibrate and not work properly. I have tried several different angles when sharpening the tool and different speeds, with and without cutting fluid. This is a tight machine and otherwise works well. Any help would be greatly appreciated. Thanks....Gary
Thanks for all the info everyone. I appreciate having found a source and having found people who know what they are talking about!

Kevbo
05-25-2005, 11:24 AM
Another trick that can help is to place a prop between the lower edge of the parting blade and the cross slide. Said prop needs to be narrow enough to fit in the groove only where it is in the work, it can be more substantial down below.

A word of warning though, If you get chatter with the prop installed, the resulting vibration can walk the prop out of position, causing all hell to break loose.

Another thing that can cause chatter late in the cut, like you are seeing is that the weight of the part and the "spring" of the remaining material (to be parted) can resonate. If you can leave the tailstock center just touching the finished part, it can help damp this vibration.

Jacobs once made a free-spinning chuck on a M-2 shank for use turning commutators on armatures with no center holes. I've often wished I had one for parting small stuff. With that you could part between centers, as it doesn't cause axial loading.

Renee n Jerry Conrad
12-17-2005, 08:50 AM
Dad always reminds me to cut some clearance when parting something off. Maybe half the width of the parting tool? Doesn't seem to need a lot. This means make a cut, back out, move over a bit, make another cut, go back and forth. I've not tried parting anything off on our 'mini-lathe' yet but I have a feeling the day will come when I really appreciate the good ideas here. :-D

Renée

wolfthing2000
03-05-2006, 05:58 PM
Rigidity and mass is what it's all about guys! I've been there with this same subject on many an old South Bend belt drive or some wore out loose piece o' ....! then the day comes when you walk up to a Monarch or Lodge and Shipley or other heavy weight lathe and the part off sings through alloy steel like it's butter. We part off to tolerances of .002 on a regular basis .

If you have to make it work on a small machine obey all of the rules about overhang and stock out from chuck. Sometimes I would put a hook on the top aof the blade to make it a super positive edge. NEVER do this in brass as you probably all know anything but a neutral top on your tools or drills will suck into brass!!

Good luck!:wink:

tinkertoy41
03-06-2006, 05:39 PM
The first rule for a lathe C/O tool is to have the correct clearances on all edges, top- sides- front, the top should be left as it come from the manufacture, the sides should be left the same way, the clearance on the front should be seven degrees or less, just enough to keep it from rubbing.
The blade length should be 1/2 the C/O diameter plus1/8 for maximun support.
Only use the fancy top hook clearance after you have gained enough experiece to know why it works.
The tool must be on center or a few thousands below center in order to cut.
Position the C/O blade for cross slide straightness by using a faced off shoulder for reference.
Fluid make a big difference, again use what you got~~
The new insertable C/O tools solve most of these issues.

Thanks Tinkertoy