There are many factors determining the choice between HSS and Carbide cutting tools. Key Market Manager Jeff Boyd discusses the various pros and cons of each.
We’re often asked by industrial engineers: “Should we use High-Speed Steel or Carbide for our solid rotary tools, like endmills, drills & taps?” There’s no quick answer because there are lots of factors involved: tool size, depth of cutting, required material removal rate, tool life, cycle time – and of course cost. Each type of component also presents different challenges, including design, size, batch quantity, material type and hardness.
Therefore, I thought it would good to discuss a little bit about both to help the user understand when to use either.
HSS versus Carbide: General characteristics
In general, the main characteristic of all High-Speed-Steels is a high working hardness with excellent toughness. HSS tools also cost less than Carbide tools and are often a good solution in ‘high-mix, low-volume’ applications.
Carbide is much harder, so it has a longer tool life and faster cutting data than conventional HSS. But the downside of that hardness is brittleness, so the cutting edge on carbide tools can quickly fracture or chip in certain situations. HSS can really excel over Carbide due to its toughness in applications such as where:
• The component to be machined is poorly clamped
• Set-up is not rigid
• The tool is a long-reach type with excessive overhang from the tool holder
• Poor machine-spindle condition
Let’s look at three common machining operations – drilling, tapping and milling – to gain a better understanding of when to use HSS or Carbide tools.
Carbide drills are generally used for high-volume hole production, where the higher tool cost can be justified on a cost-per-part basis. For deep high-volume holes, they are often available with internal coolant ducts, resulting in longer tool life and stable production. Use of through-the-spindle and high-pressure coolant offers excellent chip evacuation, particularly in deeper holes (>3xD), and is the most effective method for cooling the edge in cut.
Carbide drilling is also the fastest way to produce holes in a wide range of metals, due to the higher cutting speeds and feeds possible. However, it’s important to know that in some higher Ni-Cr alloy steels (such as stainless steels) although the hole can be produced with high speeds, the condition of the walls of the hole can quickly work-harden. This can lead to other issues in the machining process, particularly if the hole is to be have an internal thread; the tool life of the tap will be considerably shortened since it will be trying to cut through a hardened skin or surface.
Importantly, Carbide tools can be justified in low-volume production for their higher hardness because they enable harder materials to be machined, potentially upto 70+HRC.
HSS drills have very wide range of uses – from handheld applications to CNC machining in short batch runs – due to their toughness and lower cost. They are ideal for less rigid applications, such as hand-held drilling, stack drilling, and for deep hole drilling where an internal coolant supply is not available.
There are various geometries available for specific material grades, to really cut through the material and leave it in its best annealed condition. Ideal for pre-tap drilling in stainless steel, HSS drills can really benefit the life of the tap when the right geometry is used to produce the hole!
HSS tools are typically the first choice for tapping. They are by far the most common for internal thread production, with many HSS-PM versions available more recently for the various CNC machine tapping applications, different thread types and materials groups.
Given their toughness attributes, HSS tapping tools are also common in the Maintenance-Repair-Operations industries (MRO), with hand taps or straight flute taps the most widely used.
HSS taps are even used in large volume applications as well as the difficult-to-machine material applications, particularly HSS-PM taps. Where they are still the first choice due to the process stability they offer.
Carbide taps are not as popular due to the brittleness of carbide. It tends to chip in most tapping applications, particularly in blind holes. Carbide will fracture in steel applications at full depth, when the tap reverses and breaks the chips that were produced from the down cut in order to back out of the hole. As mentioned earlier HSS has superior toughness over carbide, in the tapping process this really is most important. Due to the nature of tapping being a slow speed-high feed type process, and with the spindle slowing-then reversing at the full thread depth & breaking the chip produced from the down stroke of the machine, it’s this action that the HSS toughness characteristic performs superior than carbide.
That said, Carbide taps can be used for some specific applications, including:
• Tapping hardened steel, with a specific geometry that has negative cutting angles
• Tapping High-Silicon Aluminium (AlSi), as the silicon content is quite abrasive and Carbide offers the best resistance
• Some through-hole tapping applications in steel are possible, but only with specifically suitable geometry
Carbide forming taps can also be justified in high-volume applications. Since there are no cutting edges, you can achieve long tool life without the possibility of chipping and thus justify the higher tool cost. Quite popular in ADC12 (AlSi 8-12%) Automotive Aluminium applications.
Carbide endmills are by far the most popular because they offer the best Metal Removal Rates (MRR). Solid Carbide endmills have become the first choice, given the variable helix designs combined with CAM packages that provide tool paths to suppress chatter from the natural vibration produced in a milling cut. Milling strategies such as trochoidal methods are now quite common.
HSS endmills still have a place, such as for manual milling machines, smaller volumes, less rigid set-ups, and the like. However, their use has declined in recent times due to the many industry advances.
HSS-PM: Best of both worlds?
As noted, conventional grades of HSS have lower cutting speeds applied. But in recent times, HSS-PM (Powder Metallurgy) has been developed to bridge the gap between HSS and Carbide tools.
Simply, HSS-PM is produced from a powder, similar to Carbide. This produces a finer grain structure which allows PM tools to reach a higher hardness than HSS – while still maintaining their excellent toughness. This means you can have a tool that will last longer than standard HSS and which can also be used in high hardness materials – closing the gap between the higher cutting speeds of Carbide tooling.
There are also some needs in rough milling applications for HSS-PM, due to the heavy style of cuts taken per pass. For example, when an aerospace component has a long cycle time, producers like to run their machines ‘lights-out’ overnight to do a lot of the roughing operations. They are not, however, confident to run with Carbide endmills due to their brittleness – so this is where HSS-PM roughing endmills perform best.
Whatever your application and operational considerations, it all comes down to finding the right solution. I do hope this article has given you a clearer understanding of when to use which type of tool. And please do contact us if you have any questions our engineering or R&D teams could help you solve.
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