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Question of finish on manual lathe

Pick your depth of cut. Turn the speed up one notch at a time until it's producing a shiny surface. You might not get there even at top speed, depending on the machine, and the RPM limitations if you're using a very large chuck. If not, increase your depth of cut and try again.

On 1018 with a .010" DOC I'd be somewhere around 1,000 SFM to start.

Formula I use for RPM is (CS×3.82) ÷ Diameter. CS is the cutting speed in SFM (surface feet per minute). That will be different depending on the cutting tool, the material and whether you're roughing or finishing.
 
I've had really good results running sharp cermet inserts when limited to lower SFMs in soft steels and aluminum. They're fragile so don't use them for roughing, though they seem to give good finishes even when chipped, but your size will be off.
 

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You are going to get a lot of suggestions here and many of them may be good. Here is one that I found to be quite significant.

The best improvement that I have found in surface finish with a manual lathe is to have a method of keeping the feed rate of the tool constant. There are two ways I do this. One is to use the power feed through the lead screw. And the other is to replace or supplement the small cranks on the manual feeds with larger ones. This make-shift crank that I made to fit over the cross slide handle produced a remarkable improvement in surface finishes. The photos show it being used while milling with the lathe, but it works equally well when facing a part.

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The TV remote in this shot was just for supporting it at a better angle for viewing. You can see how it fits over the original, ball style crank handle.

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And in this final photo you can see the finish first with the larger crank handle on the left and without it on the right.

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The larger lever arm allows you to move the carriage in this case very evenly while the original small crank almost forces you to take irregular motions which show up in the finish.

The same idea works equally well with the movement on the long, Z axis.

I intend to replace my make-shift wood arms with better ones some day, but even the wood ones work very well. I have a design in mind that would allow both small and large arms in one handle.

These larger handles make me wonder why the makers of lathes always use such small cranks.
 
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You need to learn the best conventional techniques for 1018 type stuff, but at some point you'll wonder how to do better, short of grinding. At that point, learn about shear tools. They only make finish cuts and they can only cut in one direction, but the finish will be 10X better. I show the concept about halfway down this page- https://www.conradhoffman.com/advancedsharp.htm
 
You need to learn the best conventional techniques for 1018 type stuff, but at some point you'll wonder how to do better, short of grinding. At that point, learn about shear tools. They only make finish cuts and they can only cut in one direction, but the finish will be 10X better. I show the concept about halfway down this page- https://www.conradhoffman.com/advancedsharp.htm
Wow Conrad, that is so well done.

Most speeds and feeds are dependant on the quality of the machine, set up and tooling choice.
Usually one goes as fast a one can go intill the the machine or tool reaches some limit like vibration, tool failure, too much heat or what.
 
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I'm surprised nobody saw that the feed rate is way too fast. It looks more like threading.

Drop back to around .004 to.005/rev, and it'll look better. Then go to your slowest feed rate for the finish cuts.

IMHO, a better insert for a smaller manual machine is the CCMT insert. They give a decent finish at lower rpm. I've been using the "medium to finish" style from Sandvik. Seems to give acceptable results. Not 50 Billion RPM, half million dollar machine, acceptable..........but does the job.

1018 is some gummy stuff, but it's the go-to when you need to be able to weld the part. People will tell you that other steels are "weldable", but they're only weldable with pre, and post, heat treatment. So, don't believe everything you hear. You can slop a bead on many things, but it doesn't mean it's a good weld. Things like carbon content, and sulfur content, can wreak havoc with proper welds.

IMHO
 
I'm surprised nobody saw that the feed rate is way too fast. It looks more like threading.

Drop back to around .004 to.005/rev, and it'll look better. Then go to your slowest feed rate for the finish cuts.

IMHO, a better insert for a smaller manual machine is the CCMT insert.

I'm surprised that you think nobody saw that... I saw it right away, but having had this exact same thing happen about a million times, I am pretty sure his feed rate is probably fine. The irregular tearing and chunking that inevitably happens with 1018 when it's cut at too slow a speed often makes it look like a higher feed rate was used, even when it wasn't. I would be interested in hearing what his feed rate was, though. And generally in my experience, running too low a feed rate with too low a cutting speed and too light a DOC in 1018 only magnifies this problem, because it gets even less heat into the cutting zone. And that lathe doesn't look all that small to me. It appears to be holding a tool with a 1" shank, so I'd guess it's at the very least 14" swing.
 
And in this final photo you can see the finish first with the larger crank handle on the left and without it on the right.

View attachment 415033

The larger lever arm allows you to move the carriage in this case very evenly while the original small crank almost forces you to take irregular motions which show up in the finish.

The same idea works equally well with the movement on the long, Z axis.

I intend to replace my make-shift wood arms with better ones some day, but even the wood ones work very well. I have a design in mind that would allow both small and large arms in one handle.

These larger handles make me wonder why the makers of lathes always use such small cranks.
In a strict digital world the movement of a hand is in small increments like a digital value. In some cases with the shakes the values vary more.
Your lever arm reduces the average amplitude of those values in the same distance. :scratchchin:
 
In the ops post #1 it looks like the feed lines at the OD are very irregular, perhaps the insert edge is so dull, or just not-sharp as designed may be compacting the part material, so adding high enough pressure to make the feed not constant. forcing the tool and holder to wander.

Is the op feeding with the lead screw or just feeding by hand?
 
You may actually be taking off too little finish depth with that carbide insert and radius. For a manual lathe, very sharp tools work best, sharp HSS or polished positive rake carbide. I also occasionally turn/modify small titanium parts on a hardinge lathe--honed and polished HSS tools work best, and on soft stainless if a good surface finish is a priority.

I've been finishing 12L14 parts on a HLV with a TGPT insert with .015rad and taking off .001-.002" with no issues. Getting as good a finish as you can get with 12L14.

Also was facing from 2" dia 12L14 from 2" to 0.18" dia, constant feed, constant rpm, same finish across the face, right tool, right material always helps
 
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