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8/10/2019 Basic Lathe Tool Grinding
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25/10/2014 Basic Lathe Tool Grinding
http://www.steves-workshop.co.uk/tips/toolgrinding/toolgrinding.htm 1/4
Grinding HSS lathe tools.
Following a number of requests,
this page details the grinding of
a general purpose lathe tool.
This is an attempt to try and
explain in simple terms the tooldesign, terminology and the
grinding process needed to
create it.
This first image shows a tool blank. There
are 5 faces to consider at the cutting end,
but only 3 of them require grinding. The
back (4) surface and bottom (5) surface can
be left untouched.
The remaining three faces require grinding
along two axis each, however they can still
be created within three grinding operations.
On most tools there is also a fourth grinding
operation which is a radius on the tool tip.
This radius increases tool life and improvessurface finish.
The numbered faces in this image also indicate the order in which the three faces are cut.
Although there are three faces to cut and each of them has two angles to set, the front and
side faces both have two critical angles and two non-critical. The top face has two angles,
both of which are important.
Tool geometry.
The end and side faces both have a clearance/relief angle and another edge cutting angle.
The relief angles are needed to stop the tool rubbing. A tool with a greater relief angleusually has a lower rate of wear, but because there is less material to support the cutting
edge the tool can break more easily, it also cannot conduct heat away so efficiently.
The top face has two rake angles because it can cut both 'into' and 'along' the work piece.
These angles are identified as a 'side rake' angle and a 'back rake' angle. The rake angle sets
the angle of shear for the cut. A greater rake angle reduces cutting forces and gives a better
tool life, but too much rake can make the tool fragile.
The diagram below shows the terminology used to describe cutting tools and the
dimensions shown in redare the critical ones.
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Different materials are best cut using slightly different angles and the table below gives
some suggested angles for the critical faces; but in summary harder materials have a
smaller rake angles and softer ones greater rake angles. The exception is Brass and bronze
which are usually cut with zero or negative rake to prevent 'digging in' of the tool.
* All angles in degrees.
A Tool Grinding Rest
Accurate grinding of the required faces can
more easily be achieved with an adjustable
grinding rest.
The photo on the right shows a simple
grinding rest which is only angle adjustable
(not height) and it has a sliding fence which
can be used to hold the cutter at a set angle
as it travels across the edge of the grinding
wheel.
Some notes on the construction of the
grinding rest can be found here.
Tool Grinding Sequence
Throughout this sequence the tool temperature was kept reasonable by frequent dipping in
a pot of water. Also - always ensure that the safety guards are in place on the grinding
wheel, and always wait for it to stop before adjusting the rest.
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The first face to be cut was the side face.
In this example we are making a normal
right handed tool for the lathe.
Grinding the side face of the tool
The grinding rest was tipped to give the required side clearance relief and the fence
adjusted to give the side cutting edge angle. The work was fed across the edge of the
grinding wheel with a light inward pressure to make the cut.
The angle set by the table is the important angle here as is sets the side relief. The angle of
the fence is non-critical and is simply used so that the side relief doesn't have to be groundall the way along the tool.
Second to be cut was the end face shown in
green below.
Grinding the end face of the tool
The grinding rest was tipped to give the required end relief angle and the fence adjusted to
give the front cutting edge angle. Again the front cutting edge angle was not important,
although it is usually set to make the tip and less than 90 . The relief angle is the critical
one.
If the tool tip is to be radiused, it is cut next.
This can be done as a freehand operation on
the bench grinder, or by hand on a grinding
slip, if only a very small radius is needed.
Grinding tool tip radius by hand
Total Security 2015Page is safe
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Finally the top face was cut. This face had a
side and back rake angle.
The completed tool
To cut the top face, the grinding rest was tipped to set the side rake angle and the fence
angled to cut the back rake angle. Both these angles are important to make an effective
tool.
The tool in action
This photograph shows the tool
cutting a 25mm mild steel bar
and leaving a good surface
finish. Cutting pressure was light
and there was no tool chatter.
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