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This page is designed to help you get the most out of hard turning by describing techniques for successful machining.
Scroll down below to get an answer:
What kind of heat is generated by Hard Turning? How is it handled during the Hard Turning Process?
How should I set up my machine for optimum results when Hard turning?
How should I apply coolant?
What kinds of cutting materials are used in hard turning?
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What kind of heat is generated by hard turning? How is it handled during the hard turning process?
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When hard turning, the cutting zone temperature is approximately 1700 degrees (Farenheit). The majority of the heat from the cutting application should be carried away with the chip. Excessive heat increases tool wear and reduces tool life, so proper application of speed and feeds are critical.
The illustration at the left shows how this heat should be dispersed.
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How should I set up my machine for optimum results when hard turning? The key is to maximize rigidity. You should attempt to achieve the smallest possible tool overhang, and spindle rotation should put cutting forces into the machine bed. Stop cutting if chatter occurs. Coolant (spray mist or flood) is appropriate for continuous cutting--you can achieve up to a 20% increase in tool life with high-pressure coolants. Spray mist is often used in Europe, due to the high cost of disposal (since less coolant is used in all applications), and dry machining is being investigated aggressively in Europe. For intermittent cutting, do not use coolant. Dry cutting may benefit from the application of compressed refrigerated air.
Turning:
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Centerline position is important in hard turning. The smaller the diameter, the more critical the position. The tool should range from on-center, to a little below: Never above center (see illustration, right).
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In many cases, the workpiece dictates the insert geometry and rake angle. Negative rake consumes 20% more horsepower. Cutting forces are higher with negative rake (see illustration, left) |
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Cutting forces are at right angles to the insert cutting edge. Lead angle geometry helps reduce edge chipping. Lead angles help to protect the nose radius from surface conditions, and permit the use of the 100-degree corner of the 80-degree insert. ALWAYS cut at a right angle to the cutting plane, and increase the lead angle to protect the nose radius (see illustration, right). |
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Tool overhang should be kept as short as possible. The maximum is 1 to 1-1/2 times the shank height (see illustration, left) Eliminate any shim or spacer. |
Boring:
There are versatile options for boring. Both Ceramic and PCBN inserts are available in ANSI-standard geometries. Most standard combination boring bars can be converted with a simple seat change to use thick ceramic inserts.
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In general machining, you'll experience tool deflection and chatter. In hard turning, the problems resulting from tool pressure are multiplied, and forces are magnified (see illustration, right):
- Torsional forces try to twist he bar
- Tangential forces try to deflect the bar away
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To reduce tool pressure while boring:
- Use positive rake tool geometry; use sharper 55- and 35-degree styles
- Use the smallest possible nose radius
- Reduce the depth of cut (two cuts are better than one)
- Reduce feedrate
- Increase cutting speed
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When boring, cutting deflection will lower the effective centerline. To counter this "centerline effect", set the tool either on center or slightly above--NEVER below (see illustration, right).
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Lead angle cutting forces are at right angles to the insert cutting edge. Lead angle geometry helps to reduce edge chipping, and lead angle help protect the nose radius from surface conditions. The lead angle permits the use of the 100-degree corner of the 80-degree insert. Use 0-degree whenever possible to send pressure back to the spindle (see illustration, left). |
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The illustration (below) shows various clamping styles and their relative merits. Single-point contact is "good", Ericson or Collet Style is "better", and Full-length split sleeve is "best".
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The chart (below) shows the relationship to length:diameter ratio and the type of boring bar to be used:
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Length:Diameter Ratio |
Boring Bar |
| Up to 4:1 |
Steel |
| From 4:1 to 6:1 |
Steel with DeVibrator |
| From 4:1 to 6:1 |
Tungsten Carbide |
| From 4:1 to 6:1 |
Heavy-Metal Shank |
| From 6:1 to 8:1 |
Tungsten Carbide & DeVibrator |
| From 8:1 to 10:1 |
Tungsten Carbide & DeVibrator |
| Over 10:1 |
Special Tungsten Carbide composite with DeVibrator |
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How should I apply coolant? The illustration at the right shows the correct application of coolant. The ideal application would be coolant supplied from above and below. The concept is simple: Get coolant to the shear zone, and flood it. |
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Hot pressed Ceramic and PCBN are made in a similar fashion. The material is mixed/formulated, then place into a die cavity. For PCBN material that is to be used for tipping carbide inserts, a carbide backing layer is placed into the die before the raw material.
Under extreme pressure and temperature, the blank is formed and then ground, honed, and lapped.
Different types of binders are used:
- TiC -- TiCn
- Tantalum -- Cobalt -- Tungsten.
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Ceramics Hot- and Cold-Pressed:
Hot Pressed AI2O3 + TiC (Black) Ceramics offer the following features/benefits:
- Can hold +/-.001" (/.0254mm) on diameter
- Best for open tolerance parts
- Up to eight edges per insert
- Economically priced
- Use for roughing and finish with PCBN
- Good Toughness (has the ability to withstand interrupted cutting)
- Good Hardness
- For continuous cutting, tool life can be as good as PCBN:
A specific example of a hot pressed ceramic Insert:
- Kennametal K090 (Ceramic) is a TiC/alumina ceramic (Black) Excellent edge wear provides good size repeatability. Excellent Hot hardness permits higher SFM, and better toughness and thermal shock resistance. It's recommended for high speed/low-moderate feed applications, and semifinishing and finishing of cast iron to 60 HRC, Steels to 65 HRC, and Nickel-base alloys. Recommended SFM is 300 to 600; IPR .004" (.1016mm) to .012" (.3048mm)
Cold Pressed AI2O3 (White) Ceramics offer the following features/benefits:
- Excellent wear resistance
- Relatively low toughness
- Recommended for high speed/low feed finishing of cast iron to 35 HRC, Carbon steel to 35 HRC
Most applications for Cold Pressed Ceramic are soft steels and soft irons, though due to the high AI2O3 content, they offer excellent edge wear resistance, and may apply to light finish machining of hardened steel.
PCBN (CBN) Inserts:
CBN inserts are produced through powdered metal processes using Borazon/ceramic powder. Wafers are cut into slices and brazed to a carbide insert, then edge preparation is done.
There are only a few manufacturers of PCBN blanks; Kennametal works with GE and Deberrs material.
PCBN/CBN Inserts offer the following features/benefits:
- Can hold +/- .0004" (.010159mm) on diameter. Only grinding can hold better
- Up to 8 edges per insert
- Low per-unit production cost
- PCBN holds better surface finishes due to finer micro-structure
- Excellent toughness permits interrupted cutting
- Excellent hardness provides superior edge wear
- Chips take heat away from the part and tool
| The chart (right) shows the comparative performance of finish machining hardened bearing steel (60 HRC) with high and low CBN content inserts.
The level of CBN content directly affects the edge wear resistance and impact. |
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The chart (left) shows the relationship of PCBN Grades to CBN Content.
Low CBN Content provides improved edge wear; high CBN content offers higher strength. |
Should I use Ceramic or PCBN?
There is a tradeoff with accuracy. Generally, if the tolerance is greater than .001" (.0254mm), ceramic would work fine. If the tolerance is less than .001" (.0254mm), PCBN would be best.
When considering the tooling material, it's important to understand the application and critical attributes such as size and finish requirements. The typical brazed tip CBN insert has a cost structure 3-4 times that of carbide. Ceramic, on the other hand, has a cost structure more similar to carbide but would not be used for applications which had a tolerance range smaller then .001”. Parts requiring a greater accuracy would logically use CBN.
Ceramic does not perform well in the presence of high thermal shocks, so it is not generally a good candidate for coolant cutting.
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