Selections and Using Carbide Burrs
Burs are sometimes referred to as rotary files and come in many shapes and sizes. Typical applications are weld preparation, weld smoothing, deburring, chamfering, deflashing, and scale removal. Burs are typically chucked in air-driven die grinders and used in hand operations.
Bur Selection: Pick the Best Cut and Shape Style
- Selection of shape and diameter is based on the workpiece and application.
- Burs come in 10 basic shapes: cylindrical, cylindrical ball nose, ball or round shape, egg shape, round nose tree, pointed nose tree, flame shape, included angle, pointed cone, and inverted taper.
- Burs come in fine, standard, coarse, double-cut, and aluminum cut styles.
- Double-cut burs have teeth that provide rapid stock removal in hard materials. They produce granular-type chips, reduce bounce and chatter, and offer excellent operator control.
- Selection of cut should be based on the material and finish required.
- Standard cuts produce smooth finishes on steels, cast irons, and ferrous metals.
- Fine cuts will produce better finish because of the additional flutes and lighter chiploads on harder materials.
- Coarse and aluminum cuts are recommended for soft materials that tend to load and pack in the flutes. These cuts have larger flutes for faster stock removal.
Use of Burs
- Speed varies with the type of die grinder being used, size of the bur, and the material being removed.
- Start at lower speeds and increase speed until the desired result is achieved.
- When placing the bur into the air motor, it should be inserted into the collet as far as possible to minimize overhang.
- Burs should be feathered into the cut with even pressure to avoid digging into the material.
- Do not apply excessive pressure because it can slow the spindle and chip cutting edges. Let the bur do the cutting.
- Never encapsulate the bur in the cut.
- When using in aluminum and magnesium, consider some form of lubricant, wax, or tallow, as it will help prevent the flutes from loading or packing
- Carbide burs, if used properly, will outperform HSS burs by 50:1.
When selecting a vendor for burs, look for suppliers with up-to-date CNC equipment. In the past, burs were hand-ground and very inconsistent in quality and performance. Today. burs from top-tier manufacturers are made on CNC machines designed specifically for grinding burs. They are very high quality and consistent from lot to lot.
Reamer Hand of Cut and Flute Selections
Reamer flute choices vary from straight flutes to flutes with a right-hand spiral (RHS) or left-hand spiral (LHS) orientation. In general, they are all right-hand cut (RHC) regardless of flute spiral direction.
Consider these guidelines for when straight, RHS, or LHS flutes are appropriate.
- Straight-fluted RHC reamers are for general-purpose use in a wide range of materials. They are best used for through holes less than 2x diameter deep.
- RHS/RHC reamers are used for blind holes or deep holes because they carry cut chips out of the hole. They are free cutting and normally produce a better finish. RHS reamers are particularly well suited for cutting aluminum alloys, copper, and other free-cutting non-ferrous material.
- LHS/RHC reamers are an advantage in through holes because they push chips ahead of the tool. They are most often used in steels and irons. They consume more torque to operate, but can reduce chatter in less than ideal setups.
Chatter is one of the most common causes of poor reamer life and hole finish.
Chatter 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 tool’s 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
- 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% to 300% 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.
- 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.
Reamer Wear and Breakage
The following conditions may adversely affect reamer life or, in some cases, damage the reamer through wear or breakage:
- Misalignment of the reamer that creates a bell-mouth condition and premature wear.
- Lack of chip space in the flutes.
- Inconsistent material condition.
- Dirt or burs in the holding collets or spindle.
- Over- or under-feeding or speeding.
- Lack of proper lubrication.
- Bottoming out in blind holes.
- Leaving too little or too much stock to remove.
- Poor hole quality.
- Improper regrinding.
- Careless handling of the reamer.
- Reamer chatter.
- Lack of rigidity of the set-up.
- Wrong reamer design for the applications.
If you experience reamer wear or damage, make sure you have addressed each of these potential causes to eliminate the problem.
Bridge and Car Reamers
“Bridge” and “car” reamers are older terms that refer to a time when misalignment of holes in structural steel components were more common than they are today.
Misalignment of holes for bolts in bridges, early automobiles and railroad box cars required a method to create a usable hole from two or more holes that were not aligned properly.
This method was developed: A rotating tapered reamer is inserted to a depth that creates a usable diameter into which the fastener will fit.
Bridge and car reamers are designed to cut on their sides, as opposed to conventional reamers that cut on the chamfer. Both styles are heavily webbed for strength and are right-hand cut and left-hand helix to push the chips ahead of the tool.
A bridge reamer typically has 3 flutes with a long lead. A car reamer typically has 5 flutes and is slightly shorter, making it more rigid.
Kennametal can recommend a drill size that will leave an appropriate amount of material for the reamer to efficiently size the finished hole.
General guidelines for drill sizes are:
- <1/4” = .010”
- ¼” to ½” = .015”
- ½” to 1-1/2” = .025”
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 and 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.