Achieving Vibration-Free Boring and Turning
Any machinist who has worked on a boring operation with deep length to diameter ratio understands just how difficult the process can be, and how detrimental vibrations become to the success of the machining operation.
Chatter/vibrations invariably lead to poor surface finish, poor tool life, excessively lower productivity (especially in turning and boring operations), in addition to other indirect disadvantages such as a noisy shop environment, machine down times and frequent requirement for machine tool maintenance.
There are several factors that contribute to the vibrations:
- Transient vibrations caused by machine motions such as rapid feeds, initial engagement impact of cutting tools etc., transferred thru foundation of the machine-tool structure until they are naturally dampened
- Forced vibrations such as those seen from the repeated period engagement of tool (eg. Milling operations or interrupted cut in turning/boring operations, or those transferred from adjacent machines (ill designed foundation or machine tool structure etc.)
- And lastly the self-feeding or self-excited vibrations (chatter) coming from the dynamics of the machining process itself, coming from feeds, depth of cuts, speeds and many other factors related to a particular machining process
Additionally, incorrect tool selection, insert selection, approach angles, cutting data, even the grades can have a severe negative impact on any boring operation.
Let’s take a deeper look at some of the main factors that can severely impact the chatter behavior of internal turning (boring) process.
The length to diameter “L/D” ratio
Vibration occurs when machining long, small-diameter components or during internal machining using boring bars with a long projection length. This is particularly the case if L/D > 4, where L= length of the workpiece and D= diameter of the workpiece
The deflection of a cantilever beam (for example a tool clamped on one end and free on the other end, being forced to bend thru cutting forces), is proportional to the cube of the free-hanging length. Thus the longer the length of the tool, that much higher the deflection, leading to proportionally higher amplitude of vibrations
Boring bar materials
Typically, up to L/D= 4, a standard steel boring bar works just fine. Up to L/D=6, a carbide boring bar gives good performance. Beyond L/D > 6, an anti-vibration boring bar, such as Accureˑtec boring bar, is required for vibration-free operation.
In general, an insert with a positive clearance causes the least vibrations. The higher the clearance, the lesser the “rubbing” and vibration on the finished workpiece surface.
A double-sided negative insert has low (theoretically zero) clearance. The clearance on the flank face is introduced thru a tool pocket (rather than insert shape). This, however, adds a negative rake angle on the rake face thus leading to a more pronounced vibration tendency.
A single-sided negative insert is designed specifically for heavy roughing. Thus it has the disadvantages of high vibrations just like a double-sided insert, but additionally the insert bottom surface has a large and secure contact with the insert pocket, thus adding strong support for roughing applications (and strong tendency towards vibrations especially if one tries to use this insert for boring operations.
Included angle of the insert (in other words the shape of the insert) has a lot of influence on the tendency to vibrate. A full round insert tends to push a lot of cutting forces radially into the workpiece being machined. This usually means a lot stronger work holding and a lot stronger machine tool is required. While this may be a good option for roughing, especially large OD components, this may not be the best suited choice for a boring bar operation.
Approach (lead) angle
Similar to insert clearance and included angle, a feature that is the best for rough turning/boring, in general, leads to a negative outcome on the vibration characteristics of the boring operation. An approach angle of 90O ensures majority of the cutting forces go into the spindle axially, thus the radial forces are nearly zero. This reduces the tendency of a long, thin workpiece to vibrate. The more the radial forces, the more the tendency to radially deflect, the more the tendency to put the forces into the chuck, the more the tendency of the workpiece to correct itself radially, the more the tendency to vibrate. For this reason, a 75O lead angle would be worse than a 90O lead angle, and a 45O would be the worst because of the forces and vibrations induced into the boring operation. This is also why there are a lot more external turning holders with 45O lead angle for roughing, compared to internal boring tools.
Corner radius also plays a big role in either leading to high vibrations (with larger corner radius) or reduced vibrations (with smaller corner radius). In many ways, similar to the lead angle or the shape of the insert, the radial forces are much higher with larger corner radii. The larger the radial forces, the higher the tendency to vibrate. Thus, the larger corner radii lead to a higher tendency for vibrations in a machining operation. Additionally, a smaller corner radius (just like smaller hone or edge preparation), leads to a sharper shearing action in the should being machined, thus leading to sharper cut with smaller cutting forces and thus reducing the vibrations in the operation. This is also why button inserts (full round shape), are rarely used for boring operations.
Depth of cut
Not only does the corner radius play a role in reducing or increasing the vibrations in a machining operation, but the correlation of depth of cut to the corner radius also plays a big role in the dynamic. Ideally the depth of cut should be at least 2 times the corner radius, thus ensuring a substantial amount of cutting forces go into the spindle axially rather than into the workpieces radially. This is somewhat counter intuitive, as many machinists are intuitively tempted to reduce the depth of cut to eliminate the cutting forces. However, as shown in the pictures below, the smaller the depth of cut (for the same corner radius), the more pronounced the radial forces. Thus, for a shallow depth of cut [picture showing depth of cut (ap)= 0.25x corner radius (r)], a substantial amount of forces are in radial direction, and the tendency of vibrations is much higher.
It is also important to note that typically higher depth of cut “stabilizes” the cut when using a steel or carbide boring bar because of the reasons stated above. However, for the pre-tuned anti-vibration boring bars, this strategy does not always pay off. So, when using anti-vibration boring bars, it is important to follow the specific cutting tool manufacturers’ recommendations for the depth of cut.
Effective rake angle
Effective rake angle is a well-known factor to influence vibration. The “more positive the rake angle, the sharper the “hook” of the chip breaker, the sharper the shearing action. This is also the reason why aluminums, non-ferrous materials, stainless steels and even gummy low carbon steels respond well to a high positive effective rake angle. The less positive the rake angle, the blunter and more plasticizing the cutting action, the more the radial forces generated. These higher radial forces again go into the workpiece radially, effectively increasing the tendency of vibrations. A typical “flat-top” rake angle (for cast irons) or a “T-land” geometry (for hard turning) tends to introduce a lot more vibrations, hence there is always a recommendation to use a very strong and stable machine tool and work-holding/tool-holding setup for machining these materials.
Just like effective rake angles, coatings play a large role in sharp or dull shearing action while making a cut. PVD (Physical Vapor Deposition) is a physical process of coating the carbide insert, it takes place at a relatively lower temperature in a furnace (roughly 850O F), and it leads to the cutting edge sharpness to be preserved to a much sharper level, with much higher integrity. Typical edge sharpness left after a PVD process is 2-5 µm (80-200 µ-in). In contrast, during the CVD (Chemical Vapor Deposition) process, the coating crystals are chemically grown on the surface in a furnace at a much higher temperature (roughly 1925O F), depositing to a thicker coating. Typical edge sharpness left after a CVD process is 10-20 µm (390-790 µ-in). The thicker the hone/cutting edge, the blunter the cutting action, leading to a higher tendency to induce vibrations. For this reason, an uncoated and polished insert (such as those used for non-ferrous materials, Aluminums etc.) tend to generate the least vibrations, followed by “thin-PVDs” such as Walter’s HIPMS WNN10, followed by conventional PVDs, followed by “thin-CVDs” such as Walter’s WMP20S grade, and lastly the CVD grades.
Some other factors to consider for reducing vibrations in a boring process
Boring operations can be tricky. Proper tool selection, insert selection, approach angles, cutting data, even the grades can have a significant impact towards the success of the boring operation. In general shortest overhang, most positive insert clearance, smallest included angle of the insert, largest lead angle, smallest corner radius of the insert, larger depth of cut, highest effective rake angle, and sharpest cutting edge hone (typically uncoated or PVD coated insert) are the keys to a successful boring operation. Use of antivibration boring bars such as Accureˑtec boring bar can also go a long way towards vibration-free boring operations.