What is High Speed Machining?

High Speed Machining (HSM) is a machining technology that focuses on high spindle RPM’s with light and low pressure cuts, which results to faster material removal rates. HSM was first incorporated by Lockheed in the eighties and adopted by others in aerospace manufacturing. It’s now an industry standard for almost all machine shops.

Brief history of High-Speed Machining?

Before HSM was evolved, most machine shops utilized conventional machining which differs greatly from high speed machining. Conventional machining has a greater contact time which will result in greater cutting forces between tool and workpiece than HSM. This will result in less accurate work piece and inferior surface finish.

Research on HSM started in late fifties and Lockheed was the first company to incorporate. After research and technology advancements others started following in various industries. It was at this point many machine shops incorporated HSM as a preferable option to conventional machining.

High Speed Machining technology is a state-of-the-art machining technology that has helped machine shops economically by bringing down costs and increase outputs using shorter cycle times with minimum machinist interference.

Where Is High-Speed Machining Used?

High-Speed Machining has now become industry standard for all machine shops. With the advent of CNC machines, HSM has became a cutting-edge technology for all metalworking machine shops. This is an important manufacturing technology that is widely being used in multiple sectors such as aerospace, automotive, medical.

Most precision parts are manufactured from complex alloys which are very hard to machine. Combination of HSM in CNC machines, CAM software has helped all manufacturers in easing the machining process on complex alloys reducing the cycle times and increasing production rate.

High-Speed Machining Technology

Modern world demands throughput and that is a driving factor that can make or break any machine shop. This can be achieved by incorporating HSM technology using CNC machines and CAM systems. Implementation of HSM requires machinists to understand few important concepts :

  • Rigidity of machine tool, workholding and workpiece -
    • This is very important and first step for any machining process.
    • Appropriate tool holding and workholding must be chosen depending on the application.
    • Workpiece clamping needs to be very robust to hold up the high cutting forces. Workholdings like dovetail vises, strong edge clamps, chick vises, strong mitee-bite clamps are preferable as they offer extremely high claiming pressures.
  • Machine tool Selection -
    • Depending on toolpaths, depth of cut, choosing the right cutting tool is very important.
    • If the machine tool is utilizing endmills or drills, appropriate length to diameter ratio has to be chosen.
    • It is also important to have a dedicated roughing tool and finishing tool to reduce tool wear and increase tool life between operations.
    • Short and stub tooling will always enhance accuracy, repeatability and surface finish
    • Cutting Tool balancing is another important factor. When holding tight runout and concentricity it is advised to use shrink fit tooling. This ensures balance at high RPMs.
  • Chip thinning -
    • Controlling chip thickness can significantly improve productivity and reduce costs.
    • Length of chips are the result of many things - depth of cut, feed rates and the proper optimization of machining techniques, etc. all of which will result in less tool wear, more process stability and yields greater control when holding tight tolerances.
    • Machining with a radial depth of cut less than 50% of cutter diameter can cause the chips to be thinner prolonging the tool life and helping reduce cycle time.
    • Proper feed rate adjustment need to be made to control the depth of cut which controls tool engagement - “chatter” can result when not done correctly.
  • Coolant -
    • Coolant / Cutting fluid plays very important role in any machining process.
    • When operating on low speeds, coolants help in lubricating the process of cutting. In HSM processes coolant is extremely important in helping cool the workpiece, adn preventing work hardening.
    • High pressure coolant is used to remove the chips while the tool is performing the cut to prevent chip built up which can cause rapid tool wear, unacceptable surface finishes on the workpiece and can even cause fire when excessive chips are accumulated.

Diamond, Carbide, Cobalt are the most common materials used to make tools.

Challenges in High-Speed Machining

Simply increasing speeds and high feed rates can cause catastrophic problems, so proper counter measures need to be taken to generate a successful machining process. Some of the challenges are-

  • High RPM’s generate lots of heat, and during machining can make some metals extremely hard (work-harden) and will cause tool-failure.. so proper high pressure coolant is needed to remove the chips and lower the cutting temperatures.
  • High cutting speeds and feed rates can also increase the risk of vibration, which can result in chatter resulting in poor surface finishes and visible tool lines / steps. This can drastically reduce tool-life and increase the possibility of scraping the workpiece, and so work-holding, fixturing, tool holders need to be routinely inspected to ensure rigidity.
  • Cutting forces are significantly higher high speed machining than in conventional machining. One of the most common problem is tools slipping inside their holders which can cause tool breakage and/or machine crashes.
  • Many older machines do not have the RPM’s necessary for high speed machining, which is why newer more rigid CNC machines are needed.
  • Toolpaths and programming used in high-speed machining can be challenging. Advanced CAM software, inclusive of functions that calculate speeds, feeds, input and exit cut angles, etc. is additionally necessary to ensure the proper outcome.

Common Processes

Turning - Turning is the process of removing material from the outer diameter on a cylindrical part. Material removal rate is controlled by the feed rates. Some of the common turning tools are called inserts. Cutting inserts are removable cutting tips with multiple cutting edges that can be flipped or rotated without disturbing the overall geometry of the tool. There are different grades and different geometries of inserts for various applications. Inserts are mounted onto an appropriate tool holder and the workpiece is mounted on machine spindle.

Drilling / Reaming - Different types of drills are utilized for CNC machining. Standard drills, Coolant fed drills, drill bits (just like inserts) are some of the common cutting tools that are used for drilling. Different drills have different geometries (2 Flute, 3 Flute, 4 Flute) and depending on the application the correct cutting tool has to be chosen. Reamers are used for finishing holes, which are generally used after drilling to hold tighter tolerance hole sizes.

Boring - Boring is similar to turning, except it is done on the inner diameter on a cylindrical part. This is most commonly done after reaming, when holding very tight tolerances. Boring inserts can also have multiple cutting edges on a boring bar. The common machining practice is that you hone for concentricity and ream for size.

Milling - There are many different types of multiple fluted endmills available in the market today. Endmills are chosen based on milling machines, tool paths, type of high speed milling process. Some of the most common endmills are straight flute endmills, ball-nose endmills and roughing endmills. Endmills cut rotationally in a horizontal or lateral (side to side) direction as opposed to end working tools such as drills that cut straight into material (horizontally or vertically)- depending on machine used. Endmills are generally used for slotting, profiling, contouring and counter-boring.

Optimization of milling can be done by choosing appropriate toolpath to achieve maximum tool life.

  • Peel milling is one approach that uses high feed rates, low radial depth of cut, and high axial depth of cut. This toolpath will maximize uniform tool wear along the entire flute length.
  • Trochoidal milling is another approach that is excellent for machining deep slots with narrow features.

Benefits of High-Speed Machining

  • High Speed Machining allows higher material removal rates which in turn increases the productivity and lowers cost.
  • Proper implementation will result in less cutting forces, lower less heat and less chances of tool or workpiece deflection which is also very important factors in precision machining.
  • The size of chips can be controlled.
  • Using modern CNC machines surface finishes can be controlled by adjusting speeds and feeds.