Lathe Tool Offset Memory Basics

Chances are your lathe already has a quick change tool post with a handful of holders on it, and you reach for the same few cutters on most jobs. The obvious benefit is the tool change itself. The cutter comes back at the right height and the right angle, with no shimming and no checking against the tailstock center.

The second benefit is that the cutting edge also comes back to the same position, and that is the one most machinists leave on the table. Tool Offset Memory (also called the Tool Library) is built on it: TouchDRO stores where each cutter's edge sits and shifts the readout when you swap tools, so the coordinate system survives the tool change instead of being re-established after it.

Indexable insert tooling makes this easiest, since a fresh edge is restored by indexing the insert. Hand-ground HSS works too, as long as the edge stays put between the day you measure it and the day you use it. If you know tool geometry offsets from CNC turning, or the tool table on a conventional DRO console, this is the digital readout's version of the same idea.

Basic Concepts

Tool Offset Memory is often one of the least understood and underutilized features of a Digital Readout. Unfortunately, the clunky and un-intuitive user interface of a traditional Digital Readout often discourages the machinists from using the Tool Offset Memory. In contrast, TouchDRO offers a simple and convenient user interface that makes programming and using tool offsets very straightforward.

While turning a part on a manual lathe, you often need to use a few different cutters. For instance, the process might look as follows:

Use a roughing cutter to quickly hog off material and get the part close to the final dimension
Switch to a finishing tool for the last few lighter passes to get major OD features to final dimension
Switch to a grooving tool to cut OD grooves, etc.
Use a boring bar to machine ID features
Finally, use a cutoff tool to cut the part off

Each time you switch the tool, you need to re-establish the coordinate system, either by taking a test cut and measuring the diameter, or touching off the tool to the surface and re-entering the previous dimension into the DRO. The obvious reason for that is that these cutters have different stick out and left-to-right offset, so their cutting edges are not in the same position. In other words, the cutting edges are offset in relation to each other.

If your lathe is equipped with a quick change tool post, those offsets are constant as long as the tool stays mounted in its dedicated holder, and the angle of the tool post doesn't change. The idea behind the Tool Offset Memory is to store these offsets into TouchDRO's memory, so you can simply re-call them when swapping the tool.

How Does it Work

When you recall a tool from TouchDRO's Tool Offset Memory, the application does some basic arithmetic under the hood. The process is similar to this:

When you start machining the part, you select the roughing cutter from the list. In other words, you are telling TouchDRO that you are about to use a cutter (that has some arbitrary offsets) to take the first cut
You then take the first facing cut. Without changing the cross slide position, you can now measure the resulting diameter and enter it into TouchDRO as the X axis readout.
The application now knows that, taking into account some tool offset, the diameter of the part is N
When you swap the tool, you can select the new tool from the list.
TouchDRO will load the offset for the new tool, subtract the old tool offset and adjust the dimension.

Requirements

In order to use the Tool Offset Memory, you need to have a way to reliably re-insert the cutter so that its cutting edge position stays unchanged. In practice, this likely means the following:

The lathe should be equipped with a "quick change" tool post
Each stored tool should have a dedicated tool holder. The tool should either not be removed from the holder, or there should be a way to re-mount it in the same exact position
The angle and the height of the tool holder in relation to the lathe's axis of rotation should remain the same as it was when the tool was entered into memory

If you routinely use hand shaped and sharpened HSS cutters, you can either re-program the offset each time you alter the edge, use a jig to re-mount the tool with the edge in the same position, or simply omit storing offsets for the tool into memory.

The Compound Slide

A common piece of advice is to replace the compound slide with a solid mounting block before using tool offsets. That is not necessary. What the stored offsets depend on is the angle of the tool post; the position of the compound underneath it is not part of the arithmetic.

The reason is in how the offsets are defined. Each tool is measured against another tool rather than against the machine, so what TouchDRO stores is the distance from one cutting edge to the next. Moving the compound displaces every cutter by the same amount and leaves those distances untouched.

What moving the compound does change is the workpiece coordinate system, and re-establishing that takes two light cuts. Face the end of the part and set Z, then turn a short length, measure it, and enter the diameter for X. Every offset in the library is still good.

Swiveling the compound is the case that needs care, since the tool post turns with it. A compound that has been swung over to cut a taper needs its post re-squared before the stored offsets mean anything again. A common approach is to set the post to a known angle by indicating the flat side of a tool holder before programming or using a given cutter. The majority of your cutters are very likely held either perpendicular or parallel to the axis of rotation; any that are held at an angle can be programmed and used at a predefined angle set with an angle block or a sine bar.

Fig. 2: Checking Tool Holder Angle with a Dial Indicator

Replacing the compound with a solid block is worth doing for rigidity. It has no bearing on Tool Offset Memory. If the compound is not tracked by TouchDRO, it should not be moved during machining for the obvious reason that the readout can't see it.