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11 ways to improve a machined finish

Posted by Alan Hillier on Feb 7, 2018 5:07:07 PM


    

 

 

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Using a variety of sources, we’ve put together 11 simple actions which can help you get better results from your portable machine tools. If you have any more to add let us know!

1. Check the condition of inserts 

A damaged insert will not produce good results, so taking the time to examine their condition will provide some clues into what is going wrong. Some common problems you may encounter are as follows:

  • Rapid Flank wear: Caused by too high cutting speed, or the insert having insufficient wear resistance. The solution is obvious; reduce cutting speed or select more resistant grade of insert.
  • Notch wear: Caused by the feed being too low and/or the material work hardening. Solution: reduce cutting speed or select more resistant grade of insert.
  • Frittering: These are small fractures on the cutting edge which can cause excessive flank wear. The grade of insert is too brittle so select a tougher grade. The problem may also be caused by the geometry of the insert being not appropriate with ‘built up edge’ (BUE) being an issue. You may want to try reducing the feed at the beginning of the cut.
  • Built up edge (BUE): A built up edge causes poor surface finish and frittering when it is torn away. The work piece material is ‘welded’ to the insert due to the low cutting speed. Built Up Edge (BUE) is influenced by three main factors: the properties of your work piece, the surface footage and the tool geometry.
  • Thermal cracks: These are perpendicular to the cutting edge and caused by temperature variations when machining intermittently.

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2. Increase the surface feet per minute (SFM)

Do this especially on carbide steels. This reduces BUE and prolongs the tool life.

3. Reduce feed per revolution

A feed rate that is too high reduces flank wear and also prolongs tool life.  When roughing it is usually ok to use a tool capable of a high feed rate, but when finishing you will need to have a slower feed rate used with a light depth of cut.

4. Use the correct rake angle

Where possible, keeping the rake angle as close to 90 degrees creates cutting forces parallel to the work piece. This reduces flex and produces a better finish.

5. Use a chip breaker

Your goal is to create short chips which get out of the way quickly without scratching the work piece. An insert with a chip breaker will help you achieve this.

6. Increase the tool nose radius 

There is a direct relationship between the insert’s nose radius and the surface finish produced. A smaller nose radius requires less pressure on the tool but limits the feed rate used.  Another disadvantage of a small radius is if the feed rate is more than half of the tool radius which will produce a rough finish( similar to a thread). Remember that when  you use a larger nosed radius, you will need to leave more material on the work piece to allow for a clean finishing cut.

7. Use a ‘wiper’

 A wiper is a type of tool geometry where a larger ‘flat’ section of the insert is in contact with the work piece that follows the main cutting edge and helps to ‘flatten’ the surface.

8Arc-in and arc-out

This technique used in milling reduces the sudden impact as the cutter engages with the edge of the work piece.

9. Different tools for roughing and finishing 

Experts may disagree on this one. Some state that roughing and finishing can be carried out using the same design of inserts used for both, sometimes with a used insert for roughing and a new one used for the finishing cut. 

The other school of thought recommends using 2 different inserts: Roughing using a large nose radius, large rake angle and quick feed rate, followed by a finishing pass using an insert with a smaller nose radius that incorporating a wiper.

10. Check tool holding and work holding:

Often overlooked is condition of the toolholder, which may be old and worn in the pocket where the insert is held. Any movement here may produce chatter.

11. Time to buy a new machine? 

All of the points made above won’t help you of if your milling machine, portable drill, line borer or flange facing machine is not robust and stable. Investing in a well-built machine from a reputable manufacturer will certainly pay off in the long run helping you produce great results for you and your customers for many years.

To see the Mirage range of portable machine tools, visit the products section of our website here

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Sources: canadianmetalworking.com    pmpaspeakingofprecision.com

Topics: On-site machining

   

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