Chip breaking in CNC turning

CNC – Why don’t my chips break properly in CNC turning ?

Why should chips break anyway ?

Improper chip breaking can result in poor surface finish, CNC machine downtime to remove the chips periodically, and higher temperatures at the cutting edge. Continuous chips are therefore not desirable.

Why don’t they break ?

In brittle materials chips will break on their own.

In ductile materials, chips are broken in 3 ways:

1. On their own (like when you bend a piece of wood beyond a certain angle).

2. By pressing against the tool

3. By pressing against the workpiece

Chip breaking methods in turning. Pic courtesy Sandvik.

The key is to bend the chip so that one of these happens. This is done by the chip breaker geometry at the cutting edge. Inserts have different cutting edge and chip breaker geometries for different workpiece materials and depths of cut. If the chip thickness is too low, the chip does not even touch the chip breaker and does not curl, as shown in the fig. below. The chip thickness is equal, or almost equal, to the feed rate in mm/rev – equal if the tool’s approach angle is 90 degrees, less if the approach angle is more. So if the feed rate is too low, the chip does not break. This is usually the problem when you are getting continuous chips.

Chip breaking and feed rate

So how does one decide the feed rate ?

The tool manufacturer’s catalog will have diagrams for various chip breakers, like the one below. ET, RT, etc. are the chip breaker names. The various coloured shapes show the recommended depth of cut and feed rate. If you are using an insert with an ET chip breaker, for example, you must ensure that the depth of cut and the feed rate combination is within the blue shape. It also shows that with ET, you can forget about chip breaking if the feed rate is less than 0.2 mm/rev.

Chip breaker operation areas. Pic courtesy Taegutec

Click here for a short explanation on the question “Why the hell should there be different chip breakers ?”

Action point

Chips not breaking ? Look at the chip breaker geometry diagram in the tool manufacturer’s catalog, change the feed rate.

Pic. and text source: CADEM NCyclopedia multimedia CNC training software.

Etc.

Band at breakfast

I saw this cheerful sight one morning while cycling to work. Members of a band having what definitely looked like a hearty breakfast, adding some colour to the road in the process. I asked if I could get some photos, and the band members cheerfully posed for a whole lot of them.

Must have been getting ready to put in a hard day’s work at a wedding (or a funeral – it’s customary for a band to give a warm sendoff in many communities, with film music).

Speaking of brass bands, they seem to have evolved far beyond just weddings and funerals. See this video of a live performance by the Jaipur Kawa Brass Band. They play all over the world, and have their own web site.

I read somewhere that Louis Armstrong, the jazz great, also used to play in jazz bands at funerals when he was starting off as a musician. Like in this video of a New Orleans jazz band playing ‘When the saints go marching in’ at a fun funeral. The dear departed too must surely have been tapping his/her foot in the casket.

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