# Turning 4140 with HSS



## Glenn Brooks (Mar 17, 2019)

anyone have experience turning 4140 with HSS?  Iam getting a very rough finish cut turning some 2 1/4” OD 4140 on an old 18”x 72” Cincinnati traytop lathe.  The lathe is equipped with an old school lantern tool post.  

My finish surface on the 4140 material exhibits a lot of concentric tool marks, maybe 2-3 thou deeper than the rest of the finish surface. 

Wondering if 4140 is naturally hard to work with HSS, or my technique is off, or maybe some excessive wear with lead screw not holding the compound and tool assembly rigid enuf???

Anyone offer advice?

Thanks
Glenn


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## pdentrem (Mar 17, 2019)

I turned a set of new shafts for our slitter using 4140. Got a reasonable finish but I finished them on the grinder to a fine finish. Ground a HSS tool to cut along the length with minimal chatter. I was removing .100” per pass at the beginning to reduce from 2.5” to 1.125” on most of the length on one side of the thrust collar and 1” on the other side.


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## Bob Korves (Mar 17, 2019)

4140 can certainly be cut with HSS.  The secret to success with higher carbon steels is to avoid problems with work hardening.  Make real cuts, not skim passes, with feeds that get under the hardened surface of the previous cut, both in depth of cut and in longitudinal feed rate.  Do not let the cutter dwell stationary at the beginning or end of the cut, keep the tool off the work unless it is cutting for real.  Use proper planning to make the finish cut a real cut, not a light cut.  If the metal work hardens, you will have problems.  Summary:  Have a plan for the cut before you start.  Get in and get going, keep moving until you get out.


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## machPete99 (Mar 17, 2019)

Using Cobalt based tooling will help...


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## mksj (Mar 17, 2019)

I just turned a spider out of 3" 4140 HR, I use carbide CCMT inserts (32.51), so for HS would decrease the RPM. I was running at around 450 RPM with a SFM of around 350 with a DDOC of 0.080". The key I found to a better surface finish and also breaking the chips was higher feed rates. I started out at around 0.003 IPR and the finish was poor, pushing the feed up to 0.007-0.008 IPR significantly improved the finish and finally broke some of the swarf into smaller pieces as opposed to one continuous strand. You need to cut the steel as opposed to deform it and work hardened which is probably happening at the lower feed rates. Need a sharp small round nose profile on the HSS. You may also have a rigidity issue or play that is causing chatter.

4140 HR 3" with lower feed and then higher feed, significant improvement in surface finish and less chatter with the higher feed rate.



Finished 4140 spider, other than some light sanding the surface finish was very good.


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## Bobby Bailey (Mar 17, 2019)

A lot of our work is on 4140. We get nice finishes using HSS insert tooling with .020 radius.


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## mikey (Mar 17, 2019)

Glenn, I've only done 4140 maybe a half-dozen times so that doesn't qualify me to say much but I did use HSS and got pretty good results. My tool has standard relief angles to handle the cutting loads (about 8 degrees) but I use slightly larger side and back rake angles (12 for both, if I recall correctly) to get the chips out faster. I also use a small nose radius, about 1/64", to reduce deflection because I want the tool to cut and not rub.

4140 work hardens readily so you have to make sure you're cutting continuously, use sharp tools and coolant and get the chips out fast. I don't have a big machine so I cannot take huge cuts. Instead, I take smaller cuts and feed faster and this seemed to work better for me. I tend to do this for all higher carbon steels on my smaller machines - rough slow and finish fast. 4140 has a cutting speed of about 70-200 but I go slower, about 50 sfm, and take decent cuts when roughing. For finish passes I crank it up to 200 sfm and slow the feed. 

I'm doing this from memory and we all know how that goes. Good luck!


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