How we cut metals or hard substance?

Cutting tools are more ancient than fire.Not only they were the survival weapons of prehistoric man but they were the base of our development too.
The evolution of cutting tools started from bones and stones to now-a-days metals and advanced materials.

What they all have common?

• They all  have to be tougher as well as harder.
• How does both of the terms are different?
• Toughness is the force a body survives before cracking and bulk failure.
• Hardness is surface property.Too much hard means you cannot scratch the surface.

An elephant is tougher with a sensitive skin rather than a crocodile which has harder skin.Just an analogy.

Getting back to cutting tools.Today we use cutting tools to remove material from a work piece giving it a shape accordingly.The material is removed at very high speed by relative motion between tool and workpiece.

And these materials are used for various purposes.
Day by day,our world not only demands quantity of a product but also quality.
Tougher materials are being used in industry so their machining also demands a stronger tool.
This strength of both hardness and toughness which should also be sustained at high temperature and speed is acquired by getting to different materials to make a cutting tool.

The hardest known substance is diamond.So,is it our benchmark for cutting tool?
Yes and No!!
Because during cutting which happens at very high speed a lot of friction and heat can be generated.
So much that carbon atoms can diffuse to some metals which are ferrous.Ferrous means having iron composition.But it is still good for non ferrous.
Diamonds are well known in cutting industry.

So,lets know some revolutionary materials of metal cutting industry at present.
But before that,for an ease  I will tell you the pattern of the list.
The list shows advancement from one point to next.
Advancement in speed and temperature sustained with strength and hardness.Some will be exception that I will specify.

With that tool life is also a parameter.
The working time at a particular speed of tool is called tool life.After that it won't work effectively. The last one is the best available.

With that there are some other blends with these tools which are done due to availability,cost,desired properties and other factors.They are coatings,addition of other substances and treatment by heat.


Carbon steels ;
The basic one.
For wood work and low speed cutting.
The maximum speed can be 8m/min at 250 degree celsius.

High Speed Steel

HSS by exportsindia.com

Here comes the beauty of alloys.The name suggests high speed.
But how the high speed comes?

1. My favorite element : Tungsten
2. With that chromium and vanadium.
3. They are all together in a fixed composition.
4. 18:4:1
5. 18% tungsten,4% chromium and 1% vanadium and then remaining steel.
6. The speed and temperature that can be sustained by tool will be increased comparatively.
7. Molybdenum is also added.Improved tool at low cost than other better tools can be achieved.
8. These are called Molybdenum HSS.

What is special in these that improves the cutting tool?
Tungsten maintains hot hardness and hot strength.

Chromium resists corrosion.
Vanadium increases fatigue strength.
Fatigue strength?
The ability to withstand fluctuation in loads.
Molybdenum resists creep which is prolonged loading at high temperature.
So,here at this point we are getting resistance to fluctuating load and prolonged load due to different elements.
 Used in drilling,shaper,die making,punches,planer etc.Power saw blade are made by this.

If you want to make it super then add cobalt.Cobalt added HSS are called Super HSS.It increases efficiency.Cobalt percentage can varry from 2-15%.

Cemented Carbides

Carbide tools by ucimu.it

When you here term 'cemented' ,then it means bond together.Cemented here refers to bonding but the bonding process of material is done by powder metallurgy.I will brief the powder metallurgy and the whole process of cemented carbides in next post surely.

1. Presence of carbides anywhere shows that it is of brittle nature.
2. Thus here we use tungsten carbide as tools here which are made by powder metallurgy.
3. This has good hot hardness.
4. And the speed can be upto 300rpm.
5. Over that we don't need coolant here.
6. It is brittle and expensive.

Ceramics.

1. Ceramics as tool made of alumina which is material that can sustain a lot of temperature.
2. A lot wear resistant.
3. And 2 times better than carbides.
4. Even it can give mirror finish on cast iron and works on hardened steels.
5. Hardened steels?
6. Steels whose hardness is increased by heat treatment.

But the major disadvantage of ceramic is that they can't resist thermal shock due to poor thermal conductivity.
Mind that Thermal energy and thermal shock are different.
A high thermal energy means high temperature but a thermal shock means high rate of change in temperature.

These can be compensated if ceramics and silver are used together.

Diamond tip glass cutter by ebay.com

CERMETS
Ceramic + Metals

1. More brittle and harder
2. They can sustain wear and are chemically stable
3. They are more suitable for finishing than discontinuous machining.

Diamonds
The rigid structure of carbon atoms in diamond make them hardest but still chemically unstable at high speed to ferrous materials.

Borazon
Coming in the class of diamonds.
And is hardest substance.
Cubic Boron Nitride can cut  super alloys,hardened steels and hard cast iron.
They are of high cost.They are basically made by a layer of  0.5-1mm on a cobalt base carbide substrate at high temperature and pressure.

BORAZON by maxxam.com.pl