Finish for 4130 on the lathe

[Aukai]

I have tried different feeds, and speeds, CCMT for roughing, and CCGT for finishing, and it still has a grainy finish. Suggestions please.

 

[compact8]

How about trying different depth of cut ? https://www.hobby-machinist.com/threads/finally-able-to-get-good-finish-on-mild-steel.100176/

 

[graham-xrf]

Is that "grainy" as in little bits being torn out of the surface? Get a magnifier, and have a hard look at the cutter tip to see if there is build-up. Carbide tools, with the trick chip-breakers, are built to serve in hard industrial usage. There is actually a (tiny) radius at the cutting edge, making it strong. It needs some force to push into the metal. The finish can be great if the cut is deep enough to let the tool work as designed. It's hot, and hard, and sometimes blue chips.

Although a very sharp edge on carbide is somewhat fragile, Stefan Gotteswinter chooses to "modify" some of his carbide tools by sharpening much of the top radius away, so he can take fine finishing cuts. He is careful to have the back edge radius retained. One has to take care to keep the finish cut gentle enough so as not to damage the now sharper carbide edge. Search out Stefan's video on what he does to carbide tools, and the one about his homebrew slow-speed carbide grinder.

 

[SLK001]

I would use a shear tool (HSS) to get a good finish.

 

[mksj]

Might check a rigidity issue, I would check the gibbs and also the rear carriage guide that is under the ways. Use a cross slide lock. I use CCMT 32.51 IC907 with a feed in the 0.008-0.010 range, SFM in the 300 range with lube/coolant. You also need a sufficient DOC. A CCGT may loose its edge quickly with any significant DOC. I get good finishes in 4130/4140 once I increased the feed, swarf is hard to break unless you are pushing hard with feed and DOC, hard to do with lighter lathes.

 

[DavidR8]

Is it HT or annealed @Aukai?
I have both and the HT leaves a much nicer finish all else being equal.

 

[Aukai]

I don't know about that David, it was an Ebay random purchase awhile ago, it has a slightly dark greenish tinge on the surface. My final pass with CCGT is .010. In good lighting I can see tooling marks like I may need to lay the tool angle back. It looks like I may be cutting with to much nose radius. I'm going to make another one today and see. Thank you for the responses.

 

[Aukai]

Oh, and I was also cutting dry, I'll try coolant this time too.

 

[graham-xrf]

Oh, and I was also cutting dry, I'll try coolant this time too.

There is something other than tooling marks messing up the surface. That surface looks like many of the ones I manage myself, so not great!

You can expect better - like this ..


In Stefan's video, near the end at 12:16, you get to see what he gets. That would be with his "sharpened" inserts. You get even better with standard new inserts used just right, if you can push hard enough, and feed fast enough to get to the recommended rate.
--> Stefan's see 12:16

This Old Tony did a big explain about all sorts of inserts, in "A Brief Chat about Carbide Tooling", and you get to see the finishes from about 24:28.
--> This Old Tony's see 24:28

This is the sort of stuff I aspire to. The reality is still a bit yuk, but at least I know what is possible.

 

[mksj]

Nice video, I think the important takeaway is DOC and higher feed rate, when he increased the feed rate at the very end the surface finish improved.