Trouble with parting off

[AlanP]

Here is one for you lads over the pond when you wake up in the morning. ;D

While parting off, and no mater how carefull I am, with or without coolant, about halfway through the cut the tool seems to dig in, this results in the tool going under the work piece and bending the tool holder or breaking the blade, the tool is set dead centre to the work piece. This is happening on a piece of 8mm mild steel round bar. I have had it happen on a similar size piece of brass.

Any help would be appreciated, please bear in mind that I have only been using a lathe for about three weeks
This is the tool I am using

 

[randyjaco]

Alan,
This is a very common problem with the smaller, less rigid lathes. You might do a search on this web site because I am sure it has been covered in detail.
The first thing is you have to make your lathe as rigid as possible, This means tightening everything down and locking them in place other than your cross feed. You might even tighten up your cross feed gibs a bit. Next make your tool as short as possible. Make sure that your tool is as sharp as possible. You may have dulled it or chipped it in the last time it grabbed.

Now with lots of coolant start the cut again very slowly. You will have to clean up the bugger you made the last time. The important thing is to keep the tool moving slowly and evenly. Don't get in a hurry.

Back when I had my South Bend 9", I'd just do a guide cut with the cut-off tool and the grab a hack saw and cut it off manually. It was ofter much faster in the end.

Another trick I learned was to start your cut until you get 1/3 -1/2 way through. Then move your carriage over .01" - .02" and start a fresh cut. There are certain materials that will expand some when hot and clamp down on the bit thus helping to pull it down.

Good luck, all of us have had the problem at one time or another. Moving up to a heavier, more rigid lathe is usually the final answer if one plans to do a lot of cut off work.

Randy

 

[Galileu]

This used to happen to me in my 7x12 Real Bull (Einhell), even after I had set the gibs and saddle slide plates. Recently I made a Norman patent toolpost to replace the original 4-way toolpost, resulting in much improved rigidity; I have since used the parting off tool with no apparent digging in. It seems to me that it is really a question of rigidity, even after all the slack and side play is corrected in the sliding parts.

Best regards,
Jose

 

[jgedde]

I have the same problem with my Atlas which is far from rigid but I can part-off without too much hair loss. Tightening everything down works but isn't the whole answer.

Use as little tool overhang as you can.

Use a parting tool with generous side relief. The guys at work gave me one like this and it made a HUGE difference.
http://www.wttool.com/index/page/category/category_id/15869/

Use a sharp parting tool.

If you're using a old lantern toolpost type parting tool holder, throw it away - FAST!

Often times setting the cutter just higher than center wil help a bit since the lack of rigidity in most smaller lathes force it to the centerline anyway.

Use a lot of lubricant (not necessarily coolant).

Don't try to feed too fast, but not too slow either.

Speed should be slow, but not backgear slow.

And, watch this video series:
[video=youtube;82LtUTBmxwQ] http://www.youtube.com/watch?v=82LtUTBmxwQ [/video]
[video=youtube;w0N6K9y2i2M] http://www.youtube.com/watch?v=w0N6K9y2i2M [/video]
[video=youtube;PgS5GFQXSQo] http://www.youtube.com/watch?v=PgS5GFQXSQo [/video]

If I can part on my old "flexible flyer", you can too! Good luck.

John

 

[jgedde]

I took a look at your photo. Don't grind the top rake. Just keep it flat along the top. That helps a lot! The tool is too grabby with such an agressive top rake.

 

[Pacer]

If that is indeed a pic of your set up for parting, then I agree the ground in back rake can very well be your biggest problem - along with the excessive overhang on the blade. Use the blade 'as is', no back rake.

The talk of smaller lathes being notoriously difficult to part on is true, however we dont even know what size your lathe is ... could you say?

 

[AlanP]

Thank you all for taking the time and trouble to reply to me, I can see a lot of things that I have got wrong, I will start by grinding the other end of my blade with no top rake and then go from there.
The video's are really helpful
Thanks again
Alan

 

[Tony Wells]

If someone else has already mentioned this, I apologize. I must have missed it. The tendency for the work to climb, or the blade to flex is controlled by the cutting forces. Enough rigidity can handle more flex force, naturally. Often overlooked is the factor of blade width. It's obvious that a .250 wide part off blade, such as might be used in a large industrial situation would in no way be practical on a home shop on a 7" machine. Simply too much force is exerted both up on the work, and down on the tool. Not to mention the horsepower needed. So, bottom line is this: use the thinnest blade you can and still get the job done. Of course, thin blades tend to flex sideways, so you have to contend with that. Nose grind has a great effect on that. Square grind will pull less than a bevel grind such as used to minimize the burr for instance. If the cut is relatively deep, common practice is to start with the blade protruding from the holder a minimum to ensure a straight groove to begin with, then extend as the cut gets deeper. You also gain side support from not having stepped over as you plunge. There should be a bit of relief in the tool, but not much.

 

[Zigeuner]

I use a standard 5/8" X 3/32" cutoff blade with a small chipbreaker cut in the top. I set it slightly below center and run my 13 X 40 Enco at 115 rpm. If I "feel" the cut through and proceed slowly, I can do it every time.

As mentoined above, I will frequently back out, move over about .020 or so and cut down again to widen the cut, I did this today on a 5/8" SS rod and it worked very nicely.

The most important thing is to move slowly and cautiously and feel the cut through while watching the chips come off.

 

[BRIAN]

Hi Alan

It seems the lads have most of the points covered , But I will add my 2 cents worth. it may help.

If possible wind the compound slide back to eliminate the overhang, try to keep the tool over the cross slide.

A saddle stop will help If you want I will post a photo for you, Simple and quick to make.

The Gib's are allways a problem so look at my Download (top adjustable Gibbs for 7x129 in downloads section.


If I can be useful Just ask. BRIAN