Lathe finish has gouges and surface is very rough

[dansawyer]

The picture below shows two significant gouges in the left and smaller ones remaining on the right. What are the typical sources of the gouges?
The stock is 1 inch mild steel. The lathe is set 500 rpm. The tool is a carbide insert 60 degree triangle. The holder is a half inch. The feed is slow and automated. Similar patterns occur from very shallow cuts up to 20 thou cuts. This surface has been sanded with 600 grit paper.
The surface is typically rough. The chips are predominately 3/4 curls. If I were to describe appearance I would describe the process more as tearing rather than cutting.
(the photo is shightly out of focus, the actual part is noticable worse then the image shows. )
Thanks in advance. Dan

 

[markba633csi]

I would wager your cutting tool is to blame.
Have you tried a HSS tool? Preferably with a broad round tip, razor sharp and with a good amount of positive rake
Low carbon steel like 1018 is prone to giving rough finishes. 1045 is a bit better, in my experience, but they both do it
-Mark

 

[benmychree]

Is this insert tool positive or negative rake? Negative rake tools are not known for nice finishes on mild steel. 600 grit paper, that gives what was called the apprentice boy finish; "high polish on deep scratches".

 

[dansawyer]

Is this insert tool positive or negative rake? Negative rake tools are not known for nice finishes on mild steel. 600 grit paper, that gives what was called the apprentice boy finish; "high polish on deep scratches".

The insert itself if zero rake. If I look critically at the angle at point of contact there may be a very small negative rake at point of contact.
(I have tried to experiment by raising an lowering the holder in the quick changer, this does not have an appreciable effect.)
(I will try both a HSS tool on this rod and cutting higher carbon steel with this cutter)
Thanks, Dan

 

[SLK001]

Mild steel is also like turning soft butter (and not in a good way). To get a decent surface finish on mild steel, grind yourself a shear tool and take 0.0005" cuts.

 

[higgite]

Can’t tell the diameter of the work piece from the picture, but is it possible that your SFM is too low for carbide at 500 RPM?

Tom

Edit: I missed the 1” diameter first time through. Yeah, I’d say 500 RPM is definitely too slow for carbide.

 

[WobblyHand]

1" stock @ 500 rpm = 131 sfm. On the slow side for carbide.

 

[mattthemuppet2]

ground tools (eg TCGT) for aluminium are an excellent option for a good finish on steel. Much lower cutting force = less tearing and flexing of the work/ tool. Still won't give you a leaded steel finish from generic mild steel, but it'll look a lot better.

Also for that diameter 800-1000rpm would work well with carbide.

 

[mksj]

More of a positive rake/sharper cutting edge insert, use an insert with a larger nose radius, increase the feed rate and increased SFM. Often see this issue with too slow a feed and too shallow a cut for the insert nose radius. Too some degree also clearing the chips with air or a FogBuster type coolant system can help. Below was some 2.75" HR 4140 running at around 500 RPM with a CCMT 32.51 insert, I started out with a feed of 0.004 IPR and then doubled it midway. The 4140 needs speed and feed to get a decent surface finish and try to break the chips, softer steels like CR 1018 often have poor surface finish w/o taking a big enough chip and the proper insert geometry. Some types of mild steel, almost impossible to get a decent surface finish as they tear and regrind in the cutting path.

 

[dansawyer]

I switched from carbide to a freshly sharpened HSS tool and redid the test at multiple RPMs and multiple feeds. The best one is attached, the area cut was the section just left of the tool. It cut was 1500 plus rpm and .005 inch per rev, the fasted rpm and the showest feed on the lathe. This was a very light cut.
(this is the fasted rpm on the lathe, it is an older Clasing gear head and is really noisy at this speed, but it works. I am worried this may break a tooth and take the head with it. .
Thanks all.