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End Mills: 10 reasons why they can fail


No matter how accurate and robust your milling machine is, if you fail to make the right decisions before you start machining you'll run the risk of creating poor results, tool breakage and injury.  Remember that each type of metal has very different properties and varying on-site machining tasks can vary considerably so a single method does not apply to all.

On-site machining projects using portable milling machines are usually straightforward as compared to the type of work carried out by CNC machines in a factory setting.

Despite the different working environment, there’s plenty of relevant tips good advice an on-site machinist can learn from companies who make the cutting tools - and those who use them every day for repeatable manufacturing operations.

Below we’ve listed some of the major reasons why end mills  can fail.

#1. The end mill is too long

Choose the shortest end mill possible to ensure a robust and stable cutting process. An unnecessarily long tool is more likely to deflect when under pressure,  causing spindle run-out, bad results and tool failure.

#2. Incorrect feed rates

Using a feed rate that moves the cutter into the work-piece faster than it can remove the metal will cause major problems. In general, use a slower feed rate for harder materials but if in doubt refer to manufacturer feeds and speeds tables.

Alternatively try the calculation below.

FR = RPM x F x CL

(Feed Rate = Spindle RPM x number of Flutes x max ChipLoad*)

*Chipload: typically 1% of cutter diameter) For example, if you use a 10mm end mill…

500 Rpm x 2 flutes x 0.1mm chipload = 100mm feed rate per minute

#3. Too much depth of cut

As with feed rates, going in too deep can cause the end mill to deflect when moving in a linear motion.

#4. Not enough coolant

When working out in the field you don’t have the benefit of a constant supply of coolant ‘on tap’. Fortunately, it’s not always required, but nevertheless, variations in material types, metal hardness and faster cutting speeds can lead to conditions where coolant is needed. Fail to duse it and excessive heat can lead to ‘built up edge’, where deposits of metal weld themselves to the cutting tool bit. When this happens you will see rough results.

#5. The wrong number of flutes

End mills are available with different numbers of flutes - typically from 2 to 8. Those with 2 flutes allow for maximum chip clearance but produce a rougher finish and aren’t as strong as those with more flutes. A common milling practice is to remove a lot of material as a rough cut with a 2 flute end mill, and then follow this up with a 4 flute end mill to create a smoother surface finish.

For aluminium (which in machining terms is a soft and ‘sticky’ metal) if there’s too many small flutes on the end mill  the chips won’t evacuate properly - causing a build up inside the flutes. For aluminium, using a flute with 3 flutes offers a good compromise.

Portable Milling Machines Range

#6. Wrong spindle RPM

Understanding the ideal speed is necessary before you start running your machine. Running a tool too fast can cause the wrong chip size to be created - or even complete tool failure!
A low RPM can result in tool deflection, a poor finish, or simply just take too long due to low metal removal rates. If you’re unsure what the ideal RPM is for your project get in touch with the tool manufacturer.

#7. Choosing the wrong type of tool

Do you need to plunge? If yes, then you’ll need a ‘centre cutting end mill’. Not a ‘non-centre cutting end mill’.

#8. Tool holding failure

The more points of contact a tool holder has with the tool’s shank, the more secure the connection will be. A poor machine-to-tool connection may cause tool runout and pull-out’ With frequent use over long periods of time collars on the machine can wear so check periodically.

#9. The wrong coating

Coatings on end mills and drills can do different things; some increase lubricity and others increase hardness and abrasion resistance. 

Be aware of the coating on your end mill. For example; a coating designed to increase hardness and temperature resistance for ferrous metals can have a high affinity for aluminium which causes unwanted adhesion.

For aluminium, more suitable alternatives exist which will prevent cutting edge build up, chip packing - and help towards a longer tool life. See our previous article covering built up edge 

#10. Tool Wear

Over time, tool wear is inevitable, but it can be minimised by paying close attention to the correct feeds and speeds and depth of cut. Replacing worn tools will help to avoid complete tool failure.

Interested in portable milling machines? Download our buyers guide today.

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