Is it even possible?

[DPittman]

Is it even possible that steel could vary so much in composition that it would cause such inconsistencies in finish? This is a chunk of unknown 1.5 diameter round. I tried various cutting tools and angles and speeds and although there was some better than others the finish was inconsistent with all of them. The tool seems to cut in some areas better than others. This is a a sanded finish in the picture.

 

[Tom Kitta]

Could be lots of things - like for example your feed was not on automatic, change in speed of the lathe under load, chatter etc. Could be also rubbing of part vs. tool - i.e. cut depth / tool geometry. Your tool may not be cutting as much as tearing the material off.

What was the steel, size, speed, feed, tool type etc?

This is more frequent with construction grade steel that is gummy, like C36.

If you have some scrap in this steel you could:
1. run aggressive pass at close to max of your lathe - take a lot off, like 1mm per side or more and moderate feed - see how it looks - make sure SFM produces blue swarf.

2. light pass, say 0.1mm or less at a very high SFM and moderate feed.

3. change tool type i.e. insert or HSS.

 

[DPittman]

Could be lots of things - like for example your feed was not on automatic, change in speed of the lathe under load, chatter etc. Could be also rubbing of part vs. tool - i.e. cut depth / tool geometry. Your tool may not be cutting as much as tearing the material off.

What was the steel, size, speed, feed, tool type etc?

This is more frequent with construction grade steel that is gummy, like C36.

If you have some scrap in this steel you could:
1. run aggressive pass at close to max of your lathe - take a lot off, like 1mm per side or more and moderate feed - see how it looks - make sure SFM produces blue swarf.

2. light pass, say 0.1mm or less at a very high SFM and moderate feed.

3. change tool type i.e. insert or HSS.

Thanks I did try pretty much everything you touched on other than trying to take an aggressive pass. I will try that next time. Thank you

 

[cuslog]

I get that sometimes, mostly with mild steel and light cuts. Mild steel can be kind of gummy. I think what's happening is you're getting some built up edge (BUE) right at the cutting edge and then every once in awhile, it smears, kind of galling the surface finish. Only way around it (that I've found) is more aggressive cuts ( higher feed rate and more depth of cut). A smaller nose radius helps a lot too.

 

[Susquatch]

When milder gummy steel does that to me, I try the things already mentioned but many times I have better success with a small (1/4" OD) round insert or hss ground to shear the surface with a vertical slicing action. The latter almost always works but is slower than freezing cold mollasses in Alberta. If you are not producing 0000 steel wool swarf, you are not doing it right.

Another advantage of a shear bit is that you don't take a lot off so it doesn't totally ruin a part already close to design diameter.

 

[Susquatch]

Here is a photo of my goto shear tool. The height VS the work isn't all that important as long as the edge cuts (slices) the steel properly. Raising the bit VS the work reduces the slice angle lowering it increases it.

The bit is presented perpendicular to the work and moved right to left SLOWLY along the work. It takes tiny finishing cuts less than a thou deep.

Here is another version that I like to use to deburr an end hole with. It is plunged into the hole sort of like a boring bar, but the cutting edge is at the top so it is inserted in a way that it cuts the steel wool from the top of the hole not the side. Basically, the hole swoops over the bit. The result is a totally clean perfectly square hole with a sharp edge not a bevelled edge.

 

[Susquatch]

The bit might be confusing it does not cut with the point or corner, for this bit, it cuts (slices is a better term) on the vertical edge.

 

[DPittman]

Interesting.

Here is what I use for a shear tool. It usually leaves a beautiful finish on aluminum and brass (I know that's not too hard) but I haven't used it much on steel.
So I tried that and I would say the finish is somewhat better but still inconsistent.

 

[Susquatch]

I can see how that would work but it isn't what I meant at all. I think that would still tend to tear or even gouge if things started vibrating. The cutting edge on a shear bit is tangent to the surface and very slightly skewed to the right above the part and to the left below the part so it peels a very tiny wire just like steel wool off the surface. Properly setup, it will make a mirror finish every time. It will even work on Stainless which is gummy as hell.

Think of it a bit like the blade of a scissor held tangent to the work and then canted a few degrees clockwise.

The bit must be polished razor sharp. I actually use a leather wheel on my tool grinder to burnish the edge. It will not dull easily because the bit is so large around it and that soaks up all the heat.

 

[Susquatch]

If you have time to wait, I'll try to do a video tomorrow.

 

[DPittman]

If you have time to wait, I'll try to do a video tomorrow.

Oh thanks I know what you mean, but others might find the video helpful.
I am able to get off "steel wool" like shavings with this bit also. But as with yours, it has to be razor sharp also.

 

[Susquatch]

I think it's hard to describe the tool and the way it gets presented to the work in words. I tried to draw it just now and failed miserably. I'm not even sure I can do a decent video. The cutting edge is hidden behind the 1/2 tool bar. I'll see tomorrow.

The bit you showed in your photo is more like the round insert tool I described earlier. And yes, it works better than pointed tools. And yes, it will produce steel wool if it is moved slowly.

But I would not describe it as a shear tool (I being the operative word there) the shear tool has a vertical edge not a horizontal one. And the vertical edge extends both above and below the work. This edge is canted clockwise a few degrees and then moved along the spinning work.

I dunno. Maybe I won't try explaining in words any more tonight. Catch you tomorrow.

 

[DPittman]

Maybe I shouldn't have called my round tool bit a shear tool but it works very much similar when I position it as seen in the video at the 4:00 minute mark. I have used the square bits also as seen in the video but I feel that the round bit usually gave a nicer finish in brass and aluminum so I thought I would try it on this steel.

 

[Susquatch]

Ok, the shear tool they describe is getting there, but only half way. It's rotated about 45 degrees from vertical. For the tool to cut steel smoothly it has to be much more straight up and down. Just a few degrees from vertical. I like to let the grinder make a slight curve in the tool bar so I can find the right angle just by moving the bit up or down a bit.

 

[Susquatch]

Take another look at the tool bit photo I attached earlier. It is in a standard BXA tool holder so you can see how vertical it is. It is also shown the same way it sits on the lathe with up facing up. That edge is like a vertical knife.

 

[Susquatch]

If you google "vertical shear tool lathe" you will find a few good and bad examples of the shear tool I described. I was a bit surprised to see how many videos there are as I have never ever seen anyone else use one.

Anyway, here is a video by Tubalcain where he uses one. Maybe not quite like mine because his edge is straight not curved so he cannot fine tune the angle like I do to improve the finish. But if you watch the whole video, you will come to understand why I like to use a vertical shear tool whenever finish is important - especially on soft steels and Stainless.