Internal turning and the L/D of the boring bar

Internal turning, or boring, turns the inside of a hole with a bar that reaches into it. The bar holds the insert at the end of a cantilever, and that cantilever is the whole difficulty. Mitsubishi Materials says it plainly: “For boring, the deeper the hole to be machined is, the longer the boring bar needs to be”, and compared with an external holder the overhang is much longer and prone to vibration. At the same time the hole limits the bar’s diameter, and the chips have to find their way out of the part. ISCAR puts the threshold low: “where L/D>3, chatter vibrations become apparent”.

L/D is the overhang divided by the bar diameter. Mitsubishi defines L as the length “by which a boring bar hangs out from the tool post to the actual cutting edge”; BIG DAISHOWA adds that the ratio belongs to the tool, calculated from the length of the bar, “not the workpiece bore diameter and depth”.

Why the overhang counts three times

A boring bar bends like a cantilever with a load at its tip. Kennametal’s calculator and Isakov in Cutting Tool Engineering use the same formula:

$$\delta = \frac{F \cdot L^3}{3 \cdot E \cdot I}, \qquad I = \frac{\pi \cdot D^4}{64}$$

with the load F, the overhang L, the modulus of elasticity E of the bar material and its moment of inertia I; for a bar with a coolant bore, I = π (D⁴ − Dᵢ⁴) / 64. The overhang enters cubed and the diameter to the fourth power: twice the overhang bends the bar eight times as much, twice the diameter sixteen times less. Isakov’s rules follow from it: “the overhang should be as short as possible”, and “the bar diameter should be as large as possible to achieve the maximum moment of inertia.”

Bar material

steelheavy metalcarbide

Bar diameter D

25

mmOverhang L/D

4.0

Radial load F

500

N

Deflection at the tip δ

42.0 µm

Overhang L

100 mm

Moment of inertia I

19,175 mm⁴

Guide limit (Sandvik; heavy metal: Kennametal)

4 × D · within

Only the modulus separates the materials, and Kennametal gives these values for its own bars:

Hardness is not in the formula, Isakov notes, so a hardened steel shank bends just as much. Deflection, in Kennametal’s words, “can affect the surface finish or in extreme circumstances the part’s dimensional tolerances”, and its calculator is “only intended for planning purposes”.

The longest overhang, maker by maker

The stiffer the bar, the further it may reach. Every maker states its own limit, and they do not quite agree.

Sandvik CoromantISCARMitsubishiTungaloyKyoceraIngersollSecoKennametal

So a steel bar stops at about four diameters, and Ingersoll is more cautious still. Carbide reaches about six, with makers between five and seven. Beyond that, damped bars: inside, as ISCAR describes its own, “a heavy mass that is supported by a rubber spring element containing oil”; Seco calls it a damping mass inside the bar. ISCAR’s damped blades come pre-calibrated for a 100 mm overhang; depending on the overhang and the clamping rigidity, they sometimes need fine-tuning. Sumitomo even sets a minimum overhang of 3.5 D for its damped steel bars.

Bar size and clamping

The largest bar that leaves room for the chips. Mitsubishi: “choose a shank diameter as large as possible”, but “if the shank diameter is too close to the bore diameter, chip control can be a problem.” ISCAR and Ingersoll give a number: the bar’s minimum bore diameter (D_min) should be 10 to 20 % smaller than the bore being machined, so that the chips can get out (and, ISCAR adds, for deflection). ISCAR’s example: for an 80 mm bore, a 60 mm bar rather than an 80 mm one.

Clamp at least four diameters. ISCAR: “The minimum recommended length is 4 X bar diameter”; Ingersoll: “It’s strongly recommended that the length of the mounted shank be at least 4xD.” ISCAR prefers holders that grip the shank all round with even force, ISCAR suggests a counterweight or mounting a heavy bar upside down, Ingersoll the upside-down mounting; both reduce the moment the bar’s own weight puts on the tool post.

Centre height. “Precise center height positioning is another important factor to achieve the right dimensional tolerances”, writes Tungaloy; its round BoreMeister shanks have no flats and are set with a centre-height device and a dial gauge, and Sandvik sells EasyFix sleeves for the same job. Mitsubishi adds that “Lowering the centre height slightly increases the rake angle and therefore can lead reduced vibrations.”

The insert: keep the radial force down

The force that bends the bar away from the wall matters most. Mitsubishi: of the three force components, “the back force has a far greater influence”, and “A back force value of 0 is ideal”. ISCAR: “The main way the insert can improve the machining stability is by minimizing the cutting forces.” The makers agree on how:

• Entering angle near 90°. “Stay as close to a 90-degree entering angle as possible” (Ingersoll); ISCAR and Mitsubishi say the same. The turning operations page shows why: at 90° the passive force is smallest.
• Small nose radius. “Larger corner radii create higher back forces” (Mitsubishi); ISCAR adds that the depth of cut should be larger than the nose radius.
• Positive and sharp. Mitsubishi: “positive inserts are usually used for boring”; negative geometry “increases cutting resistance and can easily cause vibration.” ISCAR asks for a positive rake and a small edge hone.
• No wipers. Ingersoll: “Not recommended due to more radial engagement that can cause more tool pressure and vibration.”

Two exceptions from Mitsubishi: in very small bores the rake has to be negative to clear the wall, and large bores can take negative inserts for economy.

Speed, coolant and chips

Mitsubishi: “it is important to take into consideration the L/D when setting cutting conditions and selecting the shank material”; above the steel bar’s limit, depth of cut and feed have to come down. Its boring-bar remedies include “Decrease the cutting speed.” A damped bar turns this around: Ingersoll says it allows higher feeds and speeds, and ISCAR asks for constant spindle speed (G97) with its damped bars, because the speed strongly affects the vibration.

Coolant through the bar helps carry the chips out (Ingersoll, Kyocera, Tungaloy). The chips should leave towards the opening: “Clogged chips often occur due to chip flow towards the inside of the workpiece”, Kyocera writes of its boring chipbreaker. Ingersoll adds that “Long chips can cause vibrations”, so the insert should break them.

When the bar chatters

Mitsubishi’s remedies for a vibrating boring bar (it notes that their order is of no importance):

1. “Limit the overhang”
2. “Increase the shank diameter (reduce the L/D)”
3. “Change the shank material (steel to heavy metal, heavy metal to carbide)”
4. “Use a carbide shank”
5. Lower the centre height slightly, for a larger rake angle
6. “Decrease the cutting speed.”
7. “Change to an insert with a smaller corner radius.”
8. “Change to an insert with a sharper cutting edge, or a larger rake angle.”

BIG DAISHOWA’s table for boring heads lists the same: shorten the tool, a larger or carbide or heavy-metal bar, less speed, a smaller nose radius and 90° holders. In one of its examples, shortening a Ø16 mm bar from 10:1 to 6.5:1 raised the feed from about 20 to 200 mm/min. Past the limits above, the answer is a damped bar.