Værktøjsståls bearbejdelighed og skæreydelse

Introduktion

Værktøjsstål refererer til en række legerede ståltyper, der er specielt udviklet til fremstilling af skære-, formnings- og faconværktøjer. Deres høje hårdhed og unikke egenskaber giver fremragende holdbarhed og lang levetid til værktøjsapplikationer.

Men de samme egenskaber, som giver værktøjsstål enestående ydeevne, gør dem også udfordrende at bearbejde og slibe. De hårde karbider, behandlingsbetingelser og legeringstilsætninger, der giver styrke, slidstyrke og andre egenskaber, påvirker bearbejdeligheden negativt.

Denne artikel giver en grundig gennemgang af værktøjsståls bearbejdelighedsfaktorer, skæreværktøjets ydeevne, bedste bearbejdningspraksis, innovationer og metoder til at overvinde vanskelighederne ved bearbejdning af disse vigtige værktøjsmaterialer.

Definition af bearbejdelighed

Bearbejdelighed refererer til, hvor let et materiale kan skæres, bores, slibes eller på anden måde bearbejdes. Nøglefaktorer omfatter:

  • Krævede skærekræfter
  • Opnået værktøjslevetid
  • Mulige skærehastigheder og fremføringer
  • Kvaliteten af den producerede finish
  • Spåndannelse og brudkarakteristika
  • Slibeydelse

Materialer med god bearbejdelighed muliggør nemme, hurtige og billige bearbejdningsprocesser. Værktøjsstål giver udfordringer i de fleste af disse henseender.

Værktøjsstål Egenskaber, der påvirker bearbejdeligheden

De unikke egenskaber ved værktøjsstål, der giver fremragende værktøjsydelse, hæmmer også bearbejdeligheden:

Hårdhed

  • Den høje hårdhed (50-70 HRC) i værktøjsstål kræver betydelige skærekræfter og forårsager hurtigt slid på skæreværktøjer og slibeskiver.

Hårdførhed

  • Den moderate sejhed i værktøjsstål gør præcis skæring vanskeligere og mere energikrævende. Legeringerne modstår deformation og skæring.

Legerede karbider

  • Hårde partikler af vanadium-, krom-, molybdæn- og wolframkarbider forårsager ekstremt slid på skærekanter og slibeskiver.

Varmebehandlingstilstand

  • Den hærdede tilstand af værktøjsstål bidrager til deres hårdhed, styrke og dårlige bearbejdelighed.

Derfor er det 100 gange sværere at bearbejde værktøjsstål end stål med lavt kulstofindhold. Det kræver omhyggelig procesplanlægning.

Større Værktøjsstål Bearbejdningsmetoder

De primære processer, der anvendes til bearbejdning af værktøjsstål, omfatter:

Drejning

  • Udføres på drejebænke
  • Brug af værktøj med hårdmetalindsats
  • Kræver lavere hastigheder og tilførsler end andre materialer

Fræsning

  • Fræseoperationer til at forme komponenter
  • Behov for vendbare hårdmetalfræsere
  • Mere udfordrende på grund af afbrudte nedskæringer

Boring

  • Boring udført med hårdmetalbor
  • Peck-boring nødvendig for at fjerne spåner
  • Oplever langsommere hastigheder og hurtigt slid på hjulene

Slibning

  • Slibning for at opnå endelige dimensioner og overfladefinish
  • Hard bonded wheels mandatory; diamond sometimes used
  • High wheel wear compared to other alloys

Wire EDM

  • Electrical discharge machining to erode material
  • Does not contact the surface so hardness not an issue
  • Provides excellent finish

Additional Processes

  • Planning, broaching, and other processes also utilized when feasible
  • Carbide tooling essential for reasonable life

Værktøjsstål Cutting Tool Performance

The cutting tools utilized must withstand the rigors of machining the tool steels themselves:

Carbide Tools

  • Carbide grades with high hot hardness like C5 and C6 recommended
  • Higher cobalt content provides increased strength and wear resistance

Tool Geometry

  • Positive rake angles around 7-15° work best
  • Refined edge preparations improve cutting performance

Coatings

  • TiAlN, TiCN and other ceramic coatings provide lubricity and edge protection

Tool Life

  • Carbide tool life is still relatively short compared to cutting easy alloys
  • Focus is on optimizing cutting parameters within tool life constraints

Tool Forces

  • Cutting and feed forces are higher than when machining carbon steels
  • Stout toolholders needed to resist deflection under forces

Careful cutting tool selection and application is key to cost-effective tool steel machining.

Recommended Cutting Parameters for Tool Steels

Some general guidelines for effective tool steel machining include:

Cutting Speeds

  • Turning/boring speed range of 100-300 SFM
  • Milling speeds from 100-200 SFM
  • Drilling speeds of 10-30 SFM

Feeds

  • Light to medium feeds around 0.003-0.012 IPR turning/boring
  • 4-12 IPM milling feed rates
  • Slow drill feeds of 0.001-0.003 IPR

Depths of Cut

  • Light to medium depths of cut, 0.020-0.100″ typical
  • Deeper cuts increase forces, limiting achievable tool life

Coolants

  • Soluble oil or synthetic coolants provide lubricity and chip flushing
  • High pressure flow directed at cutting zone
  • Through tool delivery ideal for drilling and boring

Following recommended cutting parameters maximizes productivity when machining tool steels.

Yderligere retningslinjer for bearbejdning af værktøjsstål

Some other helpful tips for machining tool steels include:

  • Take light finishing cuts to minimize distortion
  • Allow time for carbide tool heating and gradual wear in
  • Employ slow jog feeds to protect the edges
  • Use peck drilling and chip breaking in deep drilling
  • Monitor tool wear and replace inserts regularly to avoid big crashes
  • Consider rough machining prior to heat treatment when feasible
  • Grind to final dimensions when the highest precision is needed
  • Utilize EDM for difficult features or hard-to-machine grades
  • Vacuum clamping improves hold down for precision grinding

Applying these kinds of best practices results in improved machining productivity.

Advanced Machining Methods for Tool Steels

In addition to conventional machining, some advanced methods are especially helpful for tool steels:

Abrasive Waterjet Cutting

  • High pressure abrasive waterjet cutting avoids tooling challenges
  • Provides good edge quality and geometric cutting capability

Wire EDM

  • CNC wire erosion cuts with electrical discharges
  • Not affected by hardness and leaves excellent finish

Laser Cutting

  • High power laser cutting melts and vaporizes material
  • Allows intricate contours unachievable otherwise

Precision Grinding

  • Utilizes superabrasive CBN and diamond wheels
  • Achieves fine surface finishes and tight tolerances

Cryogenic Cooling

  • Uses liquid nitrogen to freeze and embrittle material
  • Reduces cutting forces and tool wear for easier machining

Applying advanced techniques expands the possibilities for tool steel machining.

Improving Værktøjsstål Machinability

Some strategies can improve machinability of tool steels:

Udglødning

  • Soften the tool steel by heating and slow cooling
  • Allows easier machining but properties must be redeveloped via heat treatment

Hot Machining

  • Cutting at elevated temperatures softens the steel
  • Requires special setups and temperature control

Alloy Modification

  • Adjusting composition can aid machinability but may sacrifice other properties

Non-Metallic Inclusions

  • Adding controlled amounts of sulfides improves chip breaking

Machinability Enhancing Coatings

  • Special PVD coatings like Tecrex can improve tool performance

In highly demanding applications, these kinds of approaches may provide benefits.

Overvejelser om omkostninger ved bearbejdning af værktøjsstål

The difficulties of tool steel machining result in increased costs:

  • Slow metal removal rates require more machine time
  • Frequent tool replacement and dressing leads to higher tooling costs
  • Increased wear on machine components from higher cutting forces
  • Extensive benchwork needed for precision grinding and fitting
  • Additional expenses associated with wire EDM and other processes

These factors result in tool steel machining costs typically being 3-5 times higher than easier to cut alloys like aluminum or mild steel.

Summary of Værktøjsstål Machining Characteristics

In summary, the key points regarding tool steel machinability:

  • The high hardness, strength, and alloy carbides impede cutting and grinding
  • Carbide tools with specialized coatings and geometries are mandatory
  • Much lower speeds and feeds are required compared to other materials
  • Tool life remains short, necessitating frequent insert replacement
  • Advanced processes like EDM and precision grinding are widely utilized
  • Machining costs are considerably higher due to intensity of tooling wear

Careful process planning and tool selection allows efficient machining to achieve final tool steel component dimensions and surface finish.

Ofte stillede spørgsmål

What are the main differences in machinability between tool steels and plain carbon steels?

The hardness, strength and carbides in tool ståls necessitate lower speeds and feeds, heavier cutting forces, and shorter tool life compared to plain carbon steels which can be machined quickly with simple high speed steel tooling.

What cutting tool grades provide the best performance in tool steels?

Carbide grades with high hot hardness like C5 and C6 with specialized ceramic coatings like TiAlN or TiCN generally provide the best combination of tool life and cutting efficiency.

What cutting speed is typically used for turning hardened tool steels?

Turning speeds for tool steels generally range from 100-200 SFM for finishing and somewhat higher for roughing. Much lower than the 500+ SFM used for aluminum or mild steel.

What cooling strategies help overcome the challenges of tool steel machining?

High pressure soluble oil or synthetic coolants provide critical lubricity at the cutting interface. Through tool and high volume flood cooling improves heat transfer.

How much more expensive is it to machine tool steels compared to easy to cut materials?

The intense tool wear and slow speeds needed result in tool steel machining costs typically being 3-5 times higher than machining free machining steels or aluminum alloys.

I hope this guide provides helpful knowledge on effectively and efficiently machining these challenging but extremely useful materials! Please let me know if you have any other questions.