Models for grinding HSS Lathe Tools

There isn't one specifically for this thread i think. You add it to your own little download stashbox.
Hit the 3 bars higher up the page on the left and scroll down for downloads.
You should be able to upload a file under your name somewhere. And it becomes available to others.
 
Karl A said:
I faced in, and then faced out, on a short 1-inch diameter 6061 aluminum rod, using a knife tool. Depth of cut was 0.02 inch. Spindle was turning at 800 rpm. I don't know what the feed rate was. The chips were very different. See below. The chip from facing in is above; the chip from facing out is below.
View attachment 351119
I think I can see why facing out produces a better finish. It's like having a large nose radius.

When I read about facing out with a knife tool, I doubted that facing out means what I means. It seemed wrong -- that to face out is to orient the x-y surface with a negative clearance angle.
Click to expand...

Karl, the reason the chips differ is because the area of contact differs between the two directions. If the tool is oriented properly the side cutting edge just to the rear of the nose radius is doing the cutting when facing out; the nose radius is not involved at all. However, when facing in, the nose radius is definitely involved; depending on how it is ground it can have a significant impact on finishes, chip form and efficacy. Remember, too, that if the cut is deep enough to bury the nose radius in the cut then there is very little cutting edge to the right side of the nose radius due to the shape of the tool so the nose radius is taking the full load. Accordingly, the chip form will definitely differ.

A knife tool is not intended to hog metal. It is an awesome facing tool and is capable of taking microscopic turning cuts but it is not intended for heavy cuts. To me, a 0.020" deep cut is a heavy cut for a knife tool; for heavier cuts a square tool would work better because the geometry is designed to handle it. A knife tool is best used to produced finely finished and accurate facing cuts. With light cuts, it will typically produce a fine finish regardless of feed direction.
 
Mike,

I read the first half of your post several times while having a knife tool in hand as a visual aid. I think that I understand it.

One challenging aspect of understanding what is going on is that the tool moves in two directions relative to the material being cut, due to speed (rotating piece) and feed.

By the way, the old BBC film on cutting tools that you recommended is helpful to me.

I will remember that a knife tool is for light cuts.

Karl
 
The thing to remember is that in order for a tool to cut, there must be an edge to do the cutting. A knife tool has a side cutting edge but because of the acute end cutting edge angle at the tip of the tool, there is no END edge to cut with. Therefore, the nose radius does it all when feeding in with the nose radius buried in a deep cut, resulting in an ugly gnarly chip. The side cutting edge is behind the nose radius and because it is producing a shearing cut on the face the finish is generally fairly good, even with deep cuts.

Now, contrast this to a square tool that DOES have an end cutting edge. Face IN with that tool and the nose radius and end cutting edge are now doing the cutting and the chip will peel off nicely. Of course, cutting forces increase because you have a lot more edge contact so you need to slow down your speed a little or you risk chatter. If you do so, a 0.020" deep cut is nothing for a square tool on a Sherline lathe.

Be aware of your edges, learn how they work and how to use them to your advantage. I suspect most hobby guys can learn a lot by studying how a tool cuts.
 
No, it isn't. Because of the small nose radius it doesn't finish well when turning unless you feed really slow. It excels as a facing tool and finishes well there because the side cutting edge is what does the cutting.

A finishing tool in years past had an exaggerated side cutting edge angle. This was because the shank of the tool was held perpendicular to the work piece and the side angle was ground to produce the desired lead angle. However, today, we have QCTP's so we can use a general purpose tool and just change the lead angle to reproduce that of a finishing tool. This one tool can rough, size, finish and face simply by changing the lead angle of the tool so that the desired part of the edge hits the work.
 
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