Deep and steady: Drilling Performance

Courtesy of Anderson Dahlen

Anderson Dahlen was able to use a 27 “-long quill to bore a 23 “-deep hole because the spindle had a smaller diameter than the hole.

A long boring tool can cause vibration, so deep holes can be deep trouble. But several strategies can minimize vibration.

Two job shops had different deep holes to bore, but they had the same concern: controlling vibration.

In Rogers, Minn., Tavis Metal and Fabrication was boring a blind-hole 6 ” deep and 0.976 ” in diameter in a structural steel. The hole consisted of a 1 “-deep segment, followed by a 1 “-deep cross-bore gap—which was followed by a 4 “-deep segment.

Complicating the matter, the drill that created the hole hadn’t tracked true, so the hole was “wavy,” according to Jeremy Evans, Tavis Metal’s machine shop manager.

Anderson Dahlen Inc., Ramsey, Minn., didn’t have to worry about waviness. Inside a stainless steel housing, the 7.23 ” through-hole was straight and 23 ” deep.

Both shops had to avoid the bane of deep-hole boring: vibration.

Antivibration Strategies

Thankfully, there are ways to minimize vibration when deep-hole boring. Jack Burley, vice president of sales and engineering for BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill., cited these key strategies:

1. apply the maximum-diameter boring bar,

2. select an appropriate boring-bar material,

3. reduce cutting speed,

4. maximize stock removal before boring,

5. select inserts that reduce vibration, and

6. when needed, use a special tool.

“The largest tool-shank diameter gives the highest rigidity,” said Roland Fleischer, product manager with toolmaker Mapal Inc., Port Huron, Mich., about the first strategy. High rigidity dampens vibration or, at least, keeps the vibration’s amplitude low.

However, the bar design can’t hinder chip evacuation. Tavis Metal ran into that problem with its initial boring bar, which was 0.845 ” in diameter. Everything worked fine in the hole’s first segment, and chips were effectively evacuated.

Unfortunately, some chips fell through the cross-bore into the blind segment and wrapped around the boring head, damaging the finish.

Tavis Metal solved the problem by switching to a 0.845 “-dia. boring head from toolmaker Seco Tools Inc., Troy, Mich., which enables chips to evacuate the hole.

Anderson Dahlen didn’t have to use a boring bar for its 23 “-deep hole.

Corey Bond, an Anderson Dahlen process engineer, picked a Seco 5 “-long, indexable, radial, fine-boring head. The head’s 3.75 ” diameter and bolt-on inserted wing with a 1.641 ” overhang left plenty of room for chip evacuation. The boring head was attached to a 50-taper toolholder and mounted to the horizontal boring mill’s 5 “-dia. spindle. The workpiece was then brought up to the spindle.

Bond then used the mill’s W-axis quill, which can extend the spindle 27 “, to bore the hole. “We were able to use the rigidity of the machine,” Bond said.

Boring-Bar Material

In a deep hole, a boring tool’s resistance to vibration is also affected by the boring bar’s material.

A boring bar is usually made from tool steel, heavy metal or carbide. Each type’s vibration resistance depends on its modulus of elasticity. The greater the modulus of elasticity, the greater the cutting force needed to bend, or deflect, the material. The less susceptible a material is to deflection, the less it is to vibration.

Courtesy of Kyocera

Besides its material’s natural rigidity, a boring bar may be devised with other features for avoiding chatter, like an antivibration mechanism.

BIG Kaiser’s Burley recalled the approximate modulus of elasticity for the three materials: 30 million psi for alloy steel; 50 million psi for heavy metal, a free-machining tungsten alloy; and 90 million psi for carbide. “Steel and heavy metal would tend to bend with less extension,” said Ken King, COO of Kaiser Tool Co. Inc./THINBIT, Fort Wayne, Ind.

However, tool prices significantly escalate from steel to heavy metal to carbide, Burley noted.

He added that heavy metal has a vibration-dampening property and can sometimes work as well as carbide, such as when boring a hole with at least a 6:1 depth-to-diameter ratio.

Sometimes, however, none of the three types is suitable for deep-hole boring.

Courtesy of Seco Tools

In horizontal machine tools, a long boring tool’s weight can be problematic, but may be avoidable by using a boring head and extensions made of a lightweight material, such as aluminum, with their connections made of steel. Some rigidity would be lost, but the loss may be partly offset by making the new bar with a larger diameter than the one it replaced. That solution, though, means losing a bit of clearance.

Burley said each material’s weight can become prohibitive when a boring bar’s diameter exceeds 2 ” and its length exceeds 16 “.

Take a 5 “-dia. hole requiring a 25 “-long boring bar. “There’s no way you could put a solid piece of carbide or steel out that far and still expect it to go through a toolchanger,” Burley said, because it would fall out of the toolchanger’s arm and would exceed the maximum weight limit for the spindle itself.

So, a shop may have to use a multiple-material boring bar, with one material being lightweight. One such combination is steel and aluminum. Burley said those materials can be combined so the boring bar dampens vibration and reduces weight simultaneously.

Reduced Cutting Speed

If a shop encounters vibration, another strategy to reduce it is to reduce cutting speed. Anderson Dahlen had to scale back its insert’s speed from its 425-sfm maximum potential. “I ran it at about 200 to 225 sfm,” Bond said, adding that the spindle speed had to be varied in the cut if the shop sensed vibration starting.

The advantage of lower speeds is obvious: It reduces the cutting force, which helps reduce vibration.

On the other hand, a cutting speed that’s too low can also produce vibration. “I’ve had situations where I kept lowering it, and it got worse,” said Mike Smith, product manager–milling for Seco Tools. “After I increased it a little bit toward where I was before, it hit the correct frequency and worked better.”

“You need a certain cutting pressure to have a stable condition in the cut,” Fleischer said.

The disadvantage of lower cutting speeds, however, is longer cycle time. And, like the cutting speed, the feed rate can be changed, though within its recommended range. If the feed rate is reduced too much, there will be cutting problems, which may cause vibration.

Courtesy of THINBIT