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How to Select a Spindle

When trying to develop efficient processes, many machinists and programmers turn to tool selection first. It is true that tooling can often make a big difference in machining time, and speeds and feeds, but did you know that your machine’s spindle can have an equally impactful effect? The legs of any CNC machine, spindles are comprised of a motor, a taper for holding tools, and a shaft that will hold all of the components together. Often powered by electricity, spindles rotate on an axis which receives its input from the machine’s CNC controller.

Why is Choosing the Right Spindle Important?

Choosing the right spindle to machine your workpiece with is of very high importance to a successful production run. As tooling options continue to grow, it is important to know what tooling your spindle can utilize. Large diameter tools such as large end mills or face mills typically require slower spindle speeds and take deeper cuts to remove vast amounts of material. These applications require supreme machine rigidity and require a spindle with high torque.

Contrastingly, smaller diameter tools will need a higher-speed spindle. Faster speeds and feeds deliver better surface finishes and are used in a variety of applications. A good rule of thumb is that an end mill that is a half inch or smaller will run well with lower torque.

Types of CNC Spindles

After finding out what you should look for in a spindle, it is time to learn about your different options. Spindles typically vary by the type, style of the taper, or its size. The taper is the conical portion of the tool holder that fits inside of the opening of the spindle. Every spindle is designed to mate with a certain taper style and size.

CAT and BT Holders

This is the most widely utilized holder for milling in the United States. Referred to as “V-flange holders,” both of these styles need a retention knob or pull stud to be secured within the machine spindle. The BT (metric style) is popular overseas.

HSK Holders

This type of holder is a German standard known as “hollow shank taper.” The tapered portion of the holder is much shorter than its counterparts. It also engages the spindle in a different way and does not require a pull stud or retention knob. The HSK holder is utilized to create repeatability and longer tool life – particularly in High Efficiency Milling (HEM) applications.

All of these holders have benefits and limitations including price, accuracy, and availability. The proper selection will depend largely on your application requirements.

Torque vs. Horsepower

Torque is defined as force perpendicular to the axis of rotation across a distance. It is important to have high torque capabilities when using an end mill larger than ½ inch, or when machining a difficult material such as Inconel. Torque will help put power behind the cutting action of the tool.

Horsepower refers to the amount of work being done. Horsepower is important for smaller diameter end mills and easy-to-machine materials like aluminum.

You can think of torque as a tractor: It can’t go very fast, but there is a lot of power behind it. Think of horsepower as a racecar: It can go very fast but cannot pull or push.

Torque-Horsepower Chart

Every machine and spindle should come with a torque horsepower chart. These charts will help you understand how to maximize your spindle for torque or horsepower, depending on what you need:

Image Source: HAAS Machine Manual

Proper Spindle Size

The size of the spindle and shank taper corresponds to the weight and length of the tools being used, as well as the material you are planning to machine. CAT40 is the most commonly used spindle in the United States. These spindles are great for utilizing tools that have a ½ inch diameter end mill or smaller in any material. If you are considering using a 1 inch end mill in a material like Inconel or Titanium, a CAT50 would be a more appropriate choice. The higher the taper angle is, the more torque the spindle is capable of.

While choosing the correct tool for your application is important, choosing a tool your spindle can utilize is paramount to machining success. Knowing the amount of torque required will help machinists save a lot of headaches.

8 Ways You’re Killing Your End Mill

1. Running It Too Fast or Too Slow

Determining the right speeds and feeds for your tool and operation can be a complicated process, but understanding the ideal speed (RPM) is necessary before you start running your machine. Running a tool too fast can cause suboptimal chip size or even catastrophic tool failure. Conversely, a low RPM can result in deflection, bad finish, or simply decreased metal removal rates. If you are unsure what the ideal RPM for your job is, contact the tool manufacturer.

2. Feeding It Too Little or Too Much

Another critical aspect of speeds and feeds, the best feed rate for a job varies considerably by tool type and workpiece material. If you run your tool with too slow of a feed rate, you run the risk of recutting chips and accelerating tool wear. If you run your tool with too fast of a feed rate, you can cause tool fracture. This is especially true with miniature tooling.

3. Using Traditional Roughing

high efficiency milling

While traditional roughing is occasionally necessary or optimal, it is generally inferior to High Efficiency Milling (HEM). HEM is a roughing technique that uses a lower Radial Depth of Cut (RDOC) and a higher Axial Depth of Cut (ADOC). This spreads wear evenly across the cutting edge, dissipates heat, and reduces the chance of tool failure. Besides dramatically increasing tool life, HEM can also produce a better finish and higher metal removal rate, making it an all-around efficiency boost for your shop.

4. Using Improper Tool Holding

tool holding

Proper running parameters have less of an impact in suboptimal tool holding situations. A poor machine-to-tool connection can cause tool runout, pullout, and scrapped parts. Generally speaking, the more points of contact a tool holder has with the tool’s shank, the more secure the connection. Hydraulic and shrink fit tool holders offer increased performance over mechanical tightening methods, as do certain shank modifications, like Helical’s ToughGRIP shanks and the Haimer Safe-Lock™.

5. Not Using Variable Helix/Pitch Geometry

variable helix

A feature on a variety of high performance end mills, variable helix, or variable pitch, geometry is a subtle alteration to standard end mill geometry. This geometrical feature ensures that the time intervals between cutting edge contact with the workpiece are varied, rather than simultaneous with each tool rotation. This variation minimizes chatter by reducing harmonics, which increases tool life and produces superior results.

6. Choosing the Wrong Coating

end mill coatings

Despite being marginally more expensive, a tool with a coating optimized for your workpiece material can make all the difference. Many coatings increase lubricity, slowing natural tool wear, while others increase hardness and abrasion resistance. However, not all coatings are suitable to all materials, and the difference is most apparent in ferrous and non-ferrous materials. For example, an Aluminum Titanium Nitride (AlTiN) coating increases hardness and temperature resistance in ferrous materials, but has a high affinity to aluminum, causing workpiece adhesion to the cutting tool. A Titanium Diboride (TiB2) coating, on the other hand, has an extremely low affinity to aluminum, and prevents cutting edge build-up and chip packing, and extends tool life.

7. Using a Long Length of Cut

optimal length of cut

While a long length of cut (LOC) is absolutely necessary for some jobs, especially in finishing operations, it reduces the rigidity and strength of the cutting tool. As a general rule, a tool’s LOC should be only as long as needed to ensure that the tool retains as much of its original substrate as possible. The longer a tool’s LOC the more susceptible to deflection it becomes, in turn decreasing its effective tool life and increasing the chance of fracture.

8. Choosing the Wrong Flute Count

flute count

As simple as it seems, a tool’s flute count has a direct and notable impact on its performance and running parameters. A tool with a low flute count (2 to 3) has larger flute valleys and a smaller core. As with LOC, the less substrate remaining on a cutting tool, the weaker and less rigid it is. A tool with a high flute count (5 or higher) naturally has a larger core. However, high flute counts are not always better. Lower flute counts are typically used in aluminum and non-ferrous materials, partly because the softness of these materials allows more flexibility for increased metal removal rates, but also because of the properties of their chips. Non-ferrous materials usually produce longer, stringier chips and a lower flute count helps reduce chip recutting. Higher flute count tools are usually necessary for harder ferrous materials, both for their increased strength and because chip recutting is less of a concern since these materials often produce much smaller chips.

Key Tool Holding Considerations

Each tool holder style has its own unique properties that must be considered prior to beginning a machining operation. A secure machine-to-tool connection will result in a more profitable shop, as a poor connection can cause tool runout, pull-out, scrapped parts, damaged tools, and exhausted shop resources. An understanding of tool holders, shank features, and best practices is therefore pivotal for every machinist to know to ensure reliable tool holding.

Types of Tool Holding

The basic concept of any tool holder is to create a compression force around the cutting tool’s shank that is strong, secure, and rigid. Tool holders come in a variety of styles, each with its own spindle interface, taper for clearance, and compression force methods.

Mechanical Spindle Tightening

The most basic way in which spindle compression is generated is by simple mechanical tightening of the tool holder itself, or a collet within the holder. The downside of this mechanical tightening method of the spindle is its limited number of pressure points. With this style, segments of a collet collapse around the shank, and there is no uniform, concentric force holding the tool around its full circumference.

tool holding

Hydraulic Tool Holders

Other methods create a more concentric pressure, gripping the tool’s shank over a larger surface area. Hydraulic tool holders create this scenario. They are tightened via a pressurized fluid inside the bore of the holder, creating a more powerful clamping force on the shank.

Shrink Fit Tool Holders

Shrink fit tool holders are another high quality tool holding mechanism. This method works by using the thermal properties of the holder to expand its opening slightly larger than the shank of the tool. The tool is placed inside the holder, after which the holder is allowed to cool, contracting down close to its original size and creating a tremendous compressive force around the shank. Since the expansion of the bore in the tool holder is minuscule, a tight tolerance is needed on the shank to ensure it can fit every time. Shank diameters with h6 tolerances ensure the tool will always work properly and reliably with a shrink fit holder.

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Types of Shank Modifications

Along with choosing correctly when it comes to tool holding options, tool shanks can be modified to promote a more secure machine-to-tool connection. These modifications can include added grooves on the shank, flats, or even an altered shank surface to aid in gripping strength.

Weldon Flats

A Weldon flat can be used to create additional strength within the tool holder. The tool holder locks a tool in place with a set screw pushing on a flat area on the tool shank. Weldon flats offer a good amount of pull-out prevention due to the set screw sitting in the recessed shank flat. Often seen as an outdated method of tool holding, this method is most effective for larger, stronger tools where runout is less of a concern.

ToughGRIP Shanks

Helical Solutions offers a ToughGRIP shank modification to its customers, which works by increasing the friction of the shank – making it easier to grip for the tool holder. This modification roughs the shank’s surface while maintaining h6 shrink fit tolerance.

Haimer Safe-Lock™

In the Haimer Safe-Lock system, special drive keys in the chuck interface with grooves in the shank of the tool to prevent pull-out. The end mill effectively screws into the tool holder, which causes a connection that only becomes more secure as the tool is running. Haimer Safe-Lock™ maintains h6 shank tolerances, ensuring an even tighter connection with shrink fit holders.

haimer safe-lock

Key Takeaways

While choosing a proper cutter and running it at appropriate running parameters are key factors to a machining operation, so too is the tool holding method used. If opting for an improper tool holding method, one can experience tool pull-out, tool runout, and scrapped jobs. Effective tool holding will prevent premature tool failure and allow machinists to feel confident while pushing the tool to its full potential.