Surface Finish Inquiry

[JordErik]

Hi Everyone

I'm currently working on a 8" diameter piece of hot rolled steel for a backplate for my 4 jaw chuck. I'm having some issues with the surface finish that I'm getting, and i know that low carbon steels are notorious for producing bad surface finishes. I read about the vertical shearing tool and i plan on grinding one to try out, but the finish that i get when facing is much worse than when turning, so that makes me wonder if there is another issue going on.

When running the lathe last night i turned the thread tenon and faced the shoulder behind it and got this rough, inconsistent finish. It made me wonder if the tapered gib in the cross slide needs to be adjusted, but i thought i would ask before trying to chase a gremlin out of the machine.

The lathe is a Voest DA 150, i have the work bolted through the back my faceplate, turning approx 280 rpms, feed was set at 0.005 IPR, i was using a WNMG insert, and the workpiece is a low carbon hot rolled steel.

I dont get a lot of free time after work, otherwise i would have played around with trying HSS instead or some possible solution, but ill have time this weekend. I'm also coming up to my dimension, so i dont want to run out of material. Let me know what would be the first thing you would try out to improve the finish I'm getting, or if theres any info I'm missing.

Thanks in advance!

 

[mmcmdl]

SFM . Gotta get the rpms up as your diameter gets smaller .

On another note . My first lathe back in 1982 was a Voest , you have a full pic of your lathe ? I should've held onto it but had to move .

 

[sdelivery]

To light of a pass can leave you a ruff finish but yours reminds me of having a tool with a smaller cutting radius then the feed rate.
Tools LIKE to be loaded and generally get there best finish that way.
Also lock your saddle during facing operations.

 

[JordErik]

mmcmdl, heres a photo of her right after i finished putting that chip pan and deflector together. Not sure of the year, i have to try to find a serial number and look that up somewhere.

The surface finish wasnt bad closer to the center, but definitely worse closer to the OD. I forgot to lock the saddle, so ill try that and take a light cut. When i originally faced the sawed surface, it came out much nicer than this, and everything was the same. I forget the nose radius, but i believe it is 1/32 on my insert.

 

[DavidR8]

I learned recently that the minimum DoC with an insert is related to the nose radius. A larger radius has a deeper minimum DoC.

 

[mmcmdl]

mmcmdl, heres a photo of her right after i finished putting that chip pan and deflector together. Not sure of the year, i have to try to find a serial number and look that up somewhere.

Mine was identical to yours ! Very nice lathe . Mine did a faceplant when moving it into the basement so my handles were a little different from yours !

 

[benmychree]

I would use a positive rake insert and use coolant or TapMagic while finish turning, that should improve finish.

 

[JordErik]

Ok, we have a result. I changed to a cnmg insert with a 1/64 nose radius, upped the rpms to 320, dropped the feed to 0.003 IPR and locked the carriage and i got a much better finish. This is probably as good as i can ask from low carbon steel without trying that vertical shear type of tool. I can barely feel the texture of the finish, and this is much better starting point to try to sand/polish.

Interestingly, i got nearly a mirror finish right inside of the OD, probably because that area cools faster and quenches harder than the interior of the bar when its rolled? I also got a mirror finish just past the holes that i drilled for the mounting bolts, where there would have been an interrupted cut. I wonder why that would be.

Thanks for the advice guys!