| All about reaming |
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Reaming is an operation that involves finishing a hole that has already been roughly drilled. This is an operation from which one expects to obtain a certain level of dimensional and geometric quality, as well as a smooth surface condition. This is why the emphasis is placed less on productivity, as in the case for other operations, and more on quality, process reliability and result repeatability.
Numerous tools can be used for this type of operation, one example is the machine reamer, which is usually a low-productivity tool, but with under certain lubrication conditions , such as carbide, ceramic or diamond tools with pressurized lubrication through the centre of the tool, allows greater performance to be attained. The reamer follows the rough hole.
Guidelines
Kennametal offers general guidelines for drill sizes that will leave an appropriate amount of material for the reamer to efficiently size the finished hole: <1/4-inch = .010-inch, 1/4-inch to 1/2-inch = .015-inch, 1/2-inch to 1-1/2-inch = .025-inch.
A common misconception is to leave too little material for the reamer to remove. The reamer needs enough material to make its cut. Too little material will cause the reamer to rub or burnish which results in accelerated wear and poor surface finish.
Cutting speeds should be about two-thirds that of drilling SFM for similar material, and feeds should be two to three times higher.
 | Photo from Model Engineering and Model
IC Engine Projects |
Chatter in reaming operations
Chatter is one of the most common causes of poor reamer life and hole finish. It is characterized as synchronized vibrations that are set up in the cutting tool, workpiece, and machine, or a combination of vibrations in all of these elements.
This vibration causes the tool to deflect against the workpiece at a continuous, rapid, and often irregular pace. The toolÕs attempt to restore balance and resume its natural position against the vibration creates chatter. Consequently, chatter leaves poor or torn finishes and can lead to tool failure.
Chatter can be caused by several reasons and can have devastating effects on the quality of the application. Some of the more common reasons include: excessive speed, lack of rigidity in the bushing or machine, insecure holding of the workpiece, excessive overhang of the reamer or spindle, too light of a feed, and insufficient rake or clearance.
Reamers usually perform better at higher feed rates and lower speeds because of the small amount of metal they remove during an application.
A general rule is to run the reaming tool at feed rates from 200 percent to 300 percent higher than those for drilling. Feed rates will vary depending on the material being reamed. Speed rates should be two-thirds of typical drill requirements. This will enable the tool to cut, rather than burnish or rub, the material. Increasing the feed rate will promote tool stability in the workpiece and reduce deflection between the tool and workpiece.
If chatter persists, check the rigidity of the tool in the bushing or machine. Ensure that the holder is secure in the spindle. Often chatter is caused by some component of the machine set-up, and not by the tool.
Be sure to check the holding mechanism for worn or loose bushings or holders. Replace any worn parts that cannot be adjusted and eliminate any movement through adjustments. It is also helpful to check the spindle and other driving parts for adequate strength. Weakened driving parts may cause deflection under the cut.
Eliminate any unnecessary overhang of the reamer or holder. Remember that using the shortest possible tool significantly increases rigidity within any tool. Shorter shanks create less vibration and reduce the danger of deflection and chatter.
It is important to select the correct style or design of the tool. Also consider the type of material being reamed. Choose a style that provides sufficient rake or clearance.
A straight-flute design is common in general-purpose applications. This style is best used in a horizontal position for through holes due to its inability to lift chips from the hole.
Right-hand helical flutes provide a more positive cutting face, which helps lift chips that are free cutting in non-ferrous materials such as aluminum alloys and copper.
Left-hand helical flutes push the material forward and require more thrust. This action takes up the slack in the machine setup and aids in containing chatter. Using the proper tool type will reduce chatter and produce better surface finish.
 | | Photo from Avocet Consulting Pty Ltd |
Optimizing reaming cycle
When optimizing a reaming cycle, increase feedrates results in quicker cycles than running at higher surface speeds and lower feeds. To optimize speed and feed rates, start with the surface feet per minute at the manufacturer's suggested low range and increase the feedrate in increments of 0.001 in. per revolution (ipr) to 0.0015 ipr. Continue increasing the feedrate until an undesirable condition develops, such as an unacceptable finish; a bell, tapered or egg-shaped hole; or poor size. At this point, return to the feedrate prior to development of the unwanted condition. This is, or is close to, the optimum feedrate.
To optimize reaming speed, increase in increments of 10 surface-feet-per-minute (sfm) to 20 sfm. Just as with the feedrate adjustment, increase speed until undesirable conditions develop and then return to the previous sfm resulting in acceptable conditions. It may be necessary to fine-tune speed and feed adjustments to optimize tool life.
Look and listen for signs or sounds that indicate performance. A reamer that squeals upon entry means speed or feed is too high or alignment is poor. Chips should be the right size and color and the finish should have no signs of chatter. Introducing coolant through the tool produces better finishes, superior tool life and higher speeds and feeds.
Depending on the hardness and condition of workpiece material, standard reamer geometries may have to be altered for optimum performance and tool life. Features that are most often modified include circular margins, radial rake and the primary chamfer clearance.
Optimum reamer performance depends on stock removed, the choice of flutes, the runout and coolant. While HSS reamers remove less stock than SC or CT reamers, a good amount of stock to leave for reaming is 2 percent of the reamer diameter for steels and tough alloys and 3 percent for nonferrous materials and cast iron. Proper choice of flute design is imperative for maximizing speeds and feeds.
Several factors contribute to the ability of a reamer to run concentric with the machine tool spindle. They are toolholders, tool overhang, checking total indicated runout (TIR) and workpiece fixturing.
Precision collets are best for straight-shank reamers. When using tapered-shank reamers, make sure toolholders are free of dirt, grit and burrs that could cause improper seating of the shank. Floating holders or bushings, in some situations, compensate for runout, but the best solution is to eliminate excessive runout.
To reduce tool overhang, chuck reamers with no more exposed length than necessary to reach the desired depth. Excessive length leads to additional runout.
Checking TIR indicates the condition of the machine spindle and toolholder. To check, bring the tool close to the top of the workpiece, set up an indicator to pick up the highest point on the land (the circular margin) and turn the spindle by hand. Ideally, reamer TIR should be within 0.001 in.
Workpieces should be securely held in a milling vise or complex jig or fixture. Its movement causes tool breakage, oversize holes, poor finish and shortened tool life.
Coolant-fed reamers introduce coolant through the tool to flush chips away from the cutting edge so that chips are not re-cut. For blind-hole applications, a tool with center coolant delivery is recommended for flushing chips out of the hole. For through holes, flute coolant outlets enable the coolant to flush the chips ahead of the tool.
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