Diameter error resulting from incorrect tool height?

[Winegrower]

I have read several claims that if the lathe tool height is set too high or low there will be a diameter error. It seemed like a small effect, so to understand it I did the math.

It turns out that as an example, a 1” diameter rod turned with a tool offset vertically by, let’s say 0.01”...which seems like a very large mistake to me...gives a diameter error of 50 millionths of an inch.

The error increases as the tool offset increases and as the work nominal diameter decreases.

So a 1/8” diameter rod with the same 0.01 tool height error yields a cut diameter error of 0.0004. Yep, 4 tenths.

Clearly there are more compelling reasons to set a tool on center than induced diameter errors.

 

[Karl_T]

Yep tool height error is far more important on very small stock. In addition to the math you have outlined, on small stock the relief,rake, and cut angles quickly get poor causing much more part deflection.

 

[kb58]

I would think not so much. Typically the cutter is moved in until it makes light contact with the work, the dial is zeroed, and cutting starts. Whether the cutter is a tiny bit high or low may affect cutting efficiency, but I'd expect the resulting diameter to be correct. Part deflection can happen, but how fast the cutter is fed in is part of that, it's never as simple as one thing. For your 1" bar example, I'd expect the cutting process to inherently take into account the small error. An analogy would be like if your car's steering wheel has to be held slightly off-center to drive straight. The car is still perfectly usable as long as the offset is accounted for.

 

[brino]

The error increases as the tool offset increases and as the work nominal diameter decreases.

I suppose that's why tool height is more critical when cutting tapers.
Since with tapers the work diameter is constantly changing, then so too will the errors due to tool height offset.
You will still cut a taper, but it won't be the proper taper.

-brino

 

[pontiac428]

It should not matter, as @kb58 says. It affects an offset, and changes the effective back rake on the tool, but a tool point held at a constant distance from the spindle centerline (that is actually cutting, not rubbing) will cut true if the carriage tracks true. This is a correct statement if the vertical offset is small (not digging or rubbing). With a taper, since the diameter of the work is changing, the offset will magnify error, but it's small, as @Winegrower figured out.

 

[Mitch Alsup]

While tool height error is one component of diametrical error, tool feed rate (pressure at the tip) is another one, and harder to quantify.

 

[higgite]

In addition to what might be an inconsequential diameter error (or not), maybe more important in some circumstances is if your tool is not on center, you’ll leave a nub when you face a work piece, not to mention possible parting woes. Diameter won't equal zero when it's supposed to. That's a diameter error, right?

Tom