Setting the zero for a lathe part has been covered ear lier in a separate chapter Most CNC lathe setups benefit from the 20 at the front face of the fin ished part. The XO location is the same for all lathe work – it is always the spindle centerline.
All initial lathe offsets are based on the part zero. The offsets of this kind are called Geometry offsets, and es tablish the distance and direction between the part zero and the tool reference point. This reference point is noth ing more than a fixed point on the cutting tool itself. You may think of it as a tool zero point.
Offsets applied to CNC lathe applications serve the same purpose as offsets applied to CNC milling applica- tions, applications for wire EDM, router, etc. Due to the individual machine designs, there will be some expected differences. The focus of this chapter is on offsets and their applications in CNC turning, applied to the two main axes, X and Z for a rear lathe orientation.
In order to understand various offsets and their appli cations on CNC lathes, you have to understand the basic machine layout and geometry of common cutting tools. Although there are many different lathe designs in in dustry, the principles shown on a standard two-axis lathe apply to all other designs as well, including multi-axis and multi-purpose turning centers, including those with live tooling, sub-spindle, and other features of modern technology of turning centers.
These are factory dimensions and cannot be changed. Note that both the chuck and the tailstock are ignored. A chuck is attached to the headstock face, and different chuck models can be attached to the same headstock. The same applies to different tailstock sizes that can be mounted on the lathe bed.
Both dimensions play a significant role in some very special situations, such as when a complete machine model is used for simulation. In general practice, and in smaller shops in particular, these dimensions are used mainly for reference purposes.
Tool Reference Point
Strictly speaking, the tool reference point can be any where, just as a part zero can be (almost) anywhere. On the other hand, the most practical point location is such that can be easily used during setup. This is no different from milling tools, and using a point of reference at the tool tip makes most sense. There is a difference in a dif ferent area, and that is the shape of the cutting tool. While all milling tools can be represented by a circle, this is not the case for lathe tools. More on lathe tools, see page 151.
Types of Cutting Tools
CNC lathe tools can be divided into three categories:
• Extemal tools
Internal tools I Centerline tools
Now, that the two reference points have been identi fied, it is important to see how they work together. Each on its own is important, but their mutual relationship is the real objective.
In the part program, there is no direct reference to ei ther of these points. As a CNC lathe operator, you have to know where the part zero is and where the tool refer ence point is. A setup sheet or a tooling sheet may be used for this purpose, a standard company policy may be another resource. Interpreting a programmed dimension also helps. For example, if a dimension from the front face to the nearest shoulder is specified as 20 mm in the drawing, and you will see 2-20.0 in the program, you will have a key setup related information.
The word “geometry’, as it relates to any CNC ma- chine tools, is generally used to describe the relationship between two reference points. All relationships between the cutting tool (tool tip reference point) and the part zero (program zero reference point) have to be set at the control before any machining can be done and the CNC program processed. Although each CNC machine has its own unique features in terms of axis definition and ori entation, the machine geometry is always defined in consistent way.
The basic geometry are two dimensions between the face of the headstock and the turret face:
Machine reference point or machine zero or home po sition, they all refer to the same location at the machine – its origin. This origin is defined by the machine manu facturer and for normal purposes, this location is fixed.
In theory, all tool motions programmed should origi nate from this fixed machine zero location. For practical reasons, this is not the case, but the concept behind the subject is important.
On CNC lathes (including both front and rear lathe types with two+ axis models), the machine zero is lo cated at the furthest distance from the lathe centerline in each axis (X-origin of the lathe) and also at the furthest distance from the headstock face (Z-origin of the lathe). These dimensions are factory fixed, are known, and can be verified.
From the illustration at left, the turret position indi cated machine zero for a rear type CNC lathe. It is im portant to understand, that any attempt at a tool motion further away from machine zero position will result in an overtravel condition.
Even the machine zero is not very practical for general tool setup, and programmers do not use it. The only ref erence in the program to machine zero is the G28 com mand – machine zero return; G28 UO WO … incremental specification
Ø38 mm T6061 bar
Apart of their physical orientation, external and inter nal tools cover common machining operations such as turning, boring, grooving, threading, and others. Center line tools are those that do all their work at XO – they in clude drills, taps, reamers, and similar tools. The following three illustrations show the typical tool refer ence point for each cutting tool type (schematic illustra tions only):
This drawing will also be used with tolerances later.
G28 X.. 2..
… absolute specification
CNC Control Setup for Milling and Tuming | 163
Setting up the relationship between part zero and the tool tip (reference point) is done with the GEOMETRY off- set of the control system. The program tool definition is a four-digit number that identifies:
Turret station number
Geometry offset number
• Wear offset number
There is no other reference to the tool setup in the pro gram. Interpretation of the four digits is as follows-tool TO3 is used as an example:
Geometry offset is usually set from machine zero, but this is not mandatory. There are different ways of how to set the X and Y offsets, and CNC operators find many ingenious ways. Many modern lathes have a feature that sets the offset automatically. Manually, each axis is mea sured individually.
The process of measuring the X-geometry offset can be described in a few steps:
Start from machine zero I Make sure machine position shows X0.000 20.000
Place a piece of stock to the chuck and clamp it Move both axes close to the part in the front of it Turn the spindle on to a reasonable spindle speed Move the tool a little below the stock diameter Using handle or jog mode, make a cut in Z- of about 6 mm (0.25 inch), enough to place a micrometer there Do not move the tool but write down the X-position shown on the screen Move the tool to a safe position and stop the spindle Measure the diameter just machined
N28 GOO G42 X40.0 Z2.5 T0303 M08
Turret station number, Geometry offset number – Wear offset number
The first pair of digits applies to both the turret station number and the geometry offset number. The second pair is the number of the wear offset.
Geometry offset is the distance measured from the tool tip reference point to part zero along the X-axis as a di- ameter and along the Z-axis as a distance. In both cases, the direction will be negative for rear type CNC lathes and turning centers.
Wear offset is used for fine adjustments from the orig- inal geometry settings. It is used to maintain sizes, in- cluding tolerances.
Note – Making all adjustments in the geometry offset is not recommended. Mathematically, the results are the same. Practically, this is not the best method. With any input mistake, the original setup amounts will be lost and have to be recreated. Keep the original settings intact.
At the CNC machine, offsets are entered via control panel keyboard, using the INPUT or +INPUT keys:
The Geometry offset is the sum of the previously shown X-axis position and the diameter measured. For example, starting from machine zero with T03, you have cut the stock at a position shown as X-297.560 and mea sured the stock diameter as 136.724. The sum of 297.560 + 136.724 = 434.284
will be input in Geometry offset 3 with the amount of -434.284. The amount must be negative, as the direction of measurement is from tool point to part zero.
Geometry offset for the Z-axis, where the finished front face is the part zero, is often done by touching the front face and adding a face-off amount. For example, the Z-position from machine zero was shown on the screen as Z-685.392. If an earlier tool faced off the part to size, the measured amount is the Geometry offset amount of 685.392, that will be input in Geometry offset 3. The dis play will look like this:
G001 0.000 G 002 0.000 G 003 434.284 G004 0.000
Z RT. 0.000 0.000 0 0.000 0.000 0 -685.392 0.000 0 0.000 0.000 0
INPUT key enters absolute value and replaces the current setting
+INPUT key enters incremental value and updates the current setting
If a face cut is expected, just add the extra amount.