Right hand
Left hand
One of the common lathe operations performed on cylindrical parts is grooving. Grooving is a machining operation that will produce relatively narrow and shallow channel - called a groove - between two walls that are normally parallel to each other. This channel - this new groove - will always reflect the physical shape of the grooving tool used for the machining operation: A square grooving tool with sharp corners ...
... will make a groove that has sharp internal corners
The width of the groove will be determined by the grooving tool insert, and its surface will be very poor. In addition, there will be no comer breaks, and no way to control its size or surface finish.
In spite of all these disadvantages, there is one application of this simplest groove, and that is when it is used to rough-out a groove and prepare it for finishing, generally with another grooving tool.
Typical Groove
It may be a bit of a stretch to define any groove as being typical, but most grooves are not the rough-type grooves but grooves with various degree of precision that share many characteristics. Part-off or cut-off operations resemble grooving, but are never considered precision operations and are not covered in this chapter.
As machining operations go, most lathe grooving operations are not too difficult or complicated in terms of tool setup. A high quality groove always starts with a high quality part program and a suitable grooving tool.
At the time of programming, both grooving tool size and its shape are selected by the programmer. The design of the tool that matters most is generally an interchange able carbide insert. For common square grooves, this is typically an insert that is narrower than the groove width, leaving stock for finishing. This approach produces a groove of a better quality, as the insert follows a groove contour rather than just plunges into the material. For utility type grooves, the tolerances and finish quality are more open than for precision grooves.
When these tools are used on a CNC lathe of the rear type (most common design), they are mounted in such a way that the insert and the clamp face away from the operator (see above illustration).
Although the right hand orientation is far more common than the left hand orientation, there are times, when the left hand tool may be a better choice to make the required groove. When working with a grooving tool, the most important part is the carbide insert.
• A round grooving tool ...
... will make a groove with fully rounded bottom
• A square grooving tool with rounded corners ...
... will make a groove with internal corner radiuses
The list can continue - an angular grooving tool will leave groove walls that match the angle of the grooving tool, etc. There are many types of grooves that can be machined on a CNC lathe. In spite of the many possibilities, the basic principles for both programming and machining remain the same.
Standard Grooves
The two keys to a successful groove cutting is the tool insert, combined with suitable tool path. Always think of a grooving tool as a specialized tool that does normal cutting (grooving), together with forming its own shape into the material. Both cutting and forming will be done at the same time.
Most standard grooves have walls that are perpendicular to one of the machine axes but may have different orientation;
Grooving Insert
Even the most common square grooving inserts come in several designs, particularly in the area of chip breaking. Square grooving inserts also have a very small corner radius, generally ignored in most programs. Another very important aspect of insert selection is its maximum cutting depth. This is also a consideration during the programming process. The CNC programmer not only selects the insert size and shape, but also the command point or reference point of the grooving insert.
I Standard grooves machined on diameter
have walls perpendicular to the X-axis
... these are so called diameter grooves
Grooving Tool
Many designs of grooving tools are offered by many tool companies, but their basic design is the same. The carbide insert is mounted in a holder, either for right hand or left hand orientation. The insert itself has to match the holder and is generally defined by its width and maximum depth of cut. Its corners have a very small radius to add strength and are not considered during programming. The illustration below shows typical tools.
GROOVE MACHINING
As with any other CNC tool path, groove machining on CNC lathes always follows the CNC part program. Part programmers can use many methods of machining a groove, from a very simple one to those where dimensional accuracy and surface quality are of extreme importance. This applies to both manual and computer generated tool path
Standard grooves machined on faces have walls perpendicular to the Z-axis
... these are so called face grooves
Insert Reference Point
All lathe tools require a reference point. This subject was described in the chapter Lathe Offsets, starting on page 161. Like any other tool type, this is a point that is used for exact positioning of the tool relative to the part.
Unlike a single typical turning or boring tools, for grooving tools, the reference point can be in three general locations of the insert (rear lathe example used, viewed from the operator's position):
Lower left corner ... most common Lower right corner
less common Lower middle position ... least common
Standard grooves machined on angles (usually 45°) have walls at an angle to both axes
The Simplest Groove
Regardless of the groove type, the machining process in its absolutely simplest form has only three motions:
Rapid to clearance position XZ Feed in to the groove depth Rapid out of the groove
... these are so called corner grooves
or neck grooves
There are many grooves that will not have their walls parallel to each other but machined at an certain angle. The typical group in this special category is a common O-ring groove.
The following illustration shows the reference point of the insert in relationship to the reference point of the part (part zero = program zero).
While certainly simple, this very primitive groove cut ting will produce a groove that is very seldom practical and it certainly does not belong to the category of precision machining.
Right hand
Left hand
| 195
CNC Control Setup for Milling and Turning
GROOVING ON LATHES
197
Geometry offset measured from the left reference point
Control Settings
The measured amounts in both X-axis and Z-axis will be entered into the GEOMETRY offset number specified in the program. For example, program block GOO 260.0 2-23.1 T0404 M08
Geometry offset 04 is used in the program. During actual setup, the measured X-diameter was -634.570 and the Z-length was -241.760.
The geometry offset entry will be:
GEOMETRY
---
--
Offset
Z-axis
No.
X-axis
01
Geometry offset measured from the middle reference point
02
03
04
|
634.570
-241.760
05
A similar display will be shown for the WEAR offset.
X
Program Evaluation
In order for the CNC lathe operator to cut a precision groove with proper control settings, it is important to study and understand the provided part program. The following five program examples will present different objectives for the final groove:
Tolerances are not specified
Depth tolerances specified | Position tolerances only I Width tolerances only . Both position and width tolerances
Geometry offset measured from the right reference point
As each of the five examples is equally important, they will be discussed separately in detail. In all examples, the focus will be on how to handle each requirement at the control panel. This applies to both initial setup and adjustments during a production run. In all examples, the program will assume all pre-machining (turning or boring) had been completed and the program itself is correct in all aspects. The same basic external groove will be used for all examples, with the necessary changes for each example. The rules that apply to an external groove are similar to internal grooves and even face grooves.
The middle reference point is seldom used.
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