# Lathe finish has gouges and surface is very rough



## dansawyer (Jul 24, 2021)

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


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## markba633csi (Jul 24, 2021)

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


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## benmychree (Jul 24, 2021)

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".


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## dansawyer (Jul 24, 2021)

benmychree said:


> 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


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## SLK001 (Jul 25, 2021)

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.


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## higgite (Jul 25, 2021)

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.


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## WobblyHand (Jul 25, 2021)

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


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## mattthemuppet2 (Jul 25, 2021)

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.


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## mksj (Jul 25, 2021)

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.


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## dansawyer (Jul 25, 2021)

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.


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## markba633csi (Jul 25, 2021)

With HSS you can slow it down, your previous speed would be suitable or maybe a bit faster like 750
The tool makes a lot of difference- carbide tools seem to cause the most trouble for people than any other single thing I've seen
-M


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## sdelivery (Jul 25, 2021)

Much of getting a  good surface finish is the back loading that happens to the slides.
.005 doesn't load the tool or machine at all take a .100 at those same numbers and look at the difference.


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## sdelivery (Jul 26, 2021)

Much of getting a  good surface finish is the back loading that happens to the slides.
.005 doesn't load the tool or machine


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## jwmay (Jul 26, 2021)

Is this on your Colchester? Poor finish can result from so many different things. The best a person can do is eliminate any rigidity problems, switch bits, and play with feed speeds and depth of cut until you've figured it out. I think experience plays a big part as well. I don't know your experience level, but I've experienced changing one small variable make all the difference in moments. I think stacking many such experiences up over time is what gets us there in the end.


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## dansawyer (Jul 26, 2021)

jwmay said:


> Is this on your Colchester? Poor finish can result from so many different things. The best a person can do is eliminate any rigidity problems, switch bits, and play with feed speeds and depth of cut until you've figured it out. I think experience plays a big part as well. I don't know your experience level, but I've experienced changing one small variable make all the difference in moments. I think stacking many such experiences up over time is what gets us there in the end.


Yes, this is the Colchester and I am definitly a novice. I have tried various bits, speeds, and feeds. Using sharp HSS seems to produce better results. There is one remaining question relating to your comment on rigitity problems. 

The second picuture shows a pattern in the result that must be related to feed, the photo shows distinct bands. I do not believe these come from the head or spindle, I hope I am not wrong about that. The bands are difinitly visable in the center of the pattern. They are also visable in the cut to the left in this photo. These were all made with a relatively slow feed, .002 to .008 per rev. 

The first photo shows 2 bands just to the left of the tool. The first band was manual feed at a very very slow rate, this is actually a pretty clean cut. The second band was auto feed at .002 per rev. It definitly shows banding and a rougher feed. (now I need to sort out if there is a pattern in the spindle or how the feed mechanism could be causing a pattern in the saddle   .)


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## WobblyHand (Jul 26, 2021)

You might want to consider reading post #104 of the epic lathe tool grinding thread.  (You really should read the whole thing, but my link gets you to where mikey gives instructions on making 3 HSS tools.)  The tool itself can affect the finish.  Check it (under a magnifier) for any chips or defects in the cutting edge.
mikey's epic tool grinding thread post #104
I learned a lot by reading the thread by @mikey


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## sdelivery (Jul 26, 2021)

It is rare for a professional machinist to take cuts of .002-.009
If you have .200 to remove to be on size then based on your machine and tooling it's one pass at .150-.180 mic the part and dial into the finish size.
A huge part of lathe work is pushing hard enough with the tool to take the spring out of the machine.
There are circumstances that make this more difficult and that is why it takes time and experience to become a good machinist.
Some material you increase the feed and the material flows as it is removed and the finish is beautiful. 
Speeds and feeds are  important and one thing that makes the difference between a machinist and an operator.


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## jwmay (Jul 26, 2021)

I'm not really qualified to give advice on it, because I'm still learning myself.  I sure do like the look of those Colchesters. I read they make good stuff. FWIW, I don't think you need to worry about the saddle or feed mechanism being at the heart of this issue. Watch some videos on YouTube about tool grinding, and read the linked post if you're a reader.  I did both. It helped. But there is a moment when you get the idea of what you're after, and the grinding gets quite a bit easier after that. Also your lathe is capable of running carbide inserts. When I had a lathe that could do that, faster and deeper was almost always the answer. YMMV


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## martik777 (Jul 27, 2021)

Try a cut with different material and maybe work on focusing that camera 

That banding makes me think there is a problem elsewhere, like in the spindle or bearings


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## SLK001 (Jul 27, 2021)

The bit you show in your pictures is very poorly done.  Your bit should have a _radius _on the end - not a flat.


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## Papa Charlie (Jul 27, 2021)

Your tool tip looks like it is square. Hard to tell from the image.

I think you need to work on your tooling. The metal is being torn off not cut.


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## homebrewed (Jul 27, 2021)

Buildup of swarf on the cutter could be causing some of the banding you are seeing.  Tool bit geometry and/or choice of cutting fluid might need to be changed.


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