Need help identifying culprit of bad swarf from parting off on lathe (pics inside)

9t8z28

Registered
Registered
Joined
Aug 16, 2016
Messages
454
I rarely have an issue parting on my 8.5x20 Sieg C4 lathe but when I do it’s when I get less than an 1/8” away from the part falling off or right was I first start the cut. The blade either stops cutting or it doesn’t cut at all. If I apply more pressure to make it start cutting this long flat pile of what looks to be microscopic pieces of metal comes off. It slowly builds up and works its way back on the tip of the tool. I have sharpened the parting tool numerous times (I get it razor sharp) and messed around with the height as well. It seems to cut the best when I have it slightly above center.
I am using a Somma Tools T-15 .094” thick T-type blade in an Aloris AXA parting tool holder (I believe its a #71) and a solid tool post and my carriage locked down. Just in case anyone asks, yes I have checked the height after I locked the carriage. Lastly, I made sure that the blade is perfectly straight and not leaning or tilted. The material I seem to have an issue parting is 1018 CRS, 12L14, and general low carbon steel. I do not have this issue with tool steel or aluminum.
My parting speed is usually around 230-350 rpm’s and after the cut is finished the part is not so hot that I cannot touch it. Don’t get me wrong, it’s hot but not to the point that it will burn me. Also the motor does not bog down at all.
Oh, I almost forgot to add that I use a moderate feed. I make sure that the chip keeps rolling off. If I have to stop to put more oil on my brush I back out so that the tool does not rub and potentially work harden the material, or at least I think that is what’s going to happen if I let the tool rub without feeding.

Here is a picture of the swarf that I occasionally get.
4ACCF9E2-C9FA-4716-98F3-9B7F69B76C5A.jpeg
Here is a picture of the part. Can anyone see what’s going on judging by the face of the material? This is the part that falls off. I should probably add that when I face off the part still left in the chuck I do not notice it being concave or convex. A few thou cleans up the face.
23466E0B-AD24-4DA9-9198-FD2F90F97800.jpeg
 
Could you please post a photo of the blade mounted on the lathe in the holder in such a way that we can see the actual clearance angle from vertical it has while in use? From the photos and your description, it seems like the blade might be rubbing on the work below the cutting edge (not enough relief.)
 
Does that mean the end relief is rubbing? When I parted off this tool it cut very well until the very end. This tool stays very sharp As well.
That 2nd picture shows the tool is rubbing
 
Last edited:
Will this work ? F37B75FF-F0DC-4BAB-90D5-87954E989D8B.jpeg
Could you please post a photo of the blade mounted on the lathe in the holder in such a way that we can see the actual clearance angle from vertical it has while in use? From the photos and your description, it seems like the blade might be rubbing on the work below the cutting edge (not enough relief.)
 
The steel is grabbing the cutoff instead of being cut, which is why you have that chip. (it's not swarf, it's a chip, hence the term "making chips") Your front clearance, and your side clearance is wrong. I always make my front clearance steep and the radius of my grinding wheel. Also put a slight angle on the front, the higher angle on the side that you are cutting off so as not to leave a massive center tit on the finished part. That being said, it wouldn't surprise me also that you're either cutting too slow or your feeding too fast. Contrary to what alot of people here think, crank up your speed. You're only cutting cold roll steel, not magnesium. Look at a youtube video of a B&S or Swiss screw machine running. They sure don't drop down to back gear RPM to do 15 different operations.
 
I just saw your pic. Put more angle on the front
 
Yes, more relief angle on the blade. The slope of how it is mounted to the tool holder reduces the clearance by that angle. I use the same style holder that you are using, and it has 3.5 degrees of upward angle, which needs to be put into the blade so it is vertical, zero clearance angle to the work. From there you probably want perhaps 7 degrees more (depending on what you are cutting), which would require a 10.5 degree total clearance angle on the blade itself. It is difficult to read the protractor in your photo, looks like maybe 8 degrees to the work. Try 2 or 3 degrees more angle and see what happens.

Also make quite sure the cutting edge is not above the center of the work. Match it up with the concentric rings of a facing cut on some stock.
 
Last edited:
Here is a pic of the swarf or chip when it is cutting good. The other pic that I posted earlier of the single chip is what I get when it stops cutting.
I tried to unroll the one roll of chips to open it up but it wasnt happening lol. Do these chips or swarf look normal ? I thought they did. I took the picture with my iphone looking theough a magnifying glass. My iPhone takes ****ty close up pictures by itself
0168722C-7A99-4D73-9D41-8F1EBA4F77DB.jpeg
 
I know it’s kind of hard to see but the Square head protractor is reading 8.5° . I had it at 7° which is what I thought was recommended and it didn’t want to cut so I added a little more (not a lot more) to 8.5° and it started to cut so I left it at that and figured the problem lied somewheres else. So you are recommending an additional 3° so that should put me at about 11.5°- 12°
Yes, more relief angle on the blade. The slope of how it is mounted to the tool holder reduces the clearance by that angle. I use the same style holder that you are using, and it has 3.5 degrees of upward angle, which needs to be put into the blade so it is vertical, zero clearance angle to the work. From there you probably want perhaps 7 degrees more (depending on what you are cutting), which would require a 10.5 degree total clearance angle on the blade itself. It is difficult to read the protractor in your photo, looks like maybe 8 degrees to the work. Try 2 or 3 degrees more angle and see what happens.

Also make quite sure the cutting edge is not above the center of the work. Match it up with the concentric rings of a facing cut on some stock.
 
Back
Top