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Model Engineering Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Lathe Tools

The tool that is held in a toolpost on the lathe cross-slide and which is feed into the rotating workpiece to machine it to size.

Lathe tools are made from high speed steel (HSS), Tungsten Carbide or other very hard materials such as diamond.

Angles and Clearances

lathe tools The cutting and clearance angles of a lathe tool will be dependent on the type of material they are to be used on and the required quality of finish. The basic angles are shown below:

plan view

The shape of the tool in plan view is as important as all of the other angles and should be set according to the material being worked. The tool shown is suitable for roughing out the shape. The angle of the tool in plan view should be less than 90 if a step is required in the diameter of the workpiece. The nose radius should be kept large for roughing purposes, but may need to be reduced for finishing into the corner of a step.

side view

Increasing the rake angle on the side or back will produce a force that will draw the tool into the workpiece. The tool will tend to run and as such the cut will not be so controllable. It is important that the height of the tool tip is on or just above the centre of the workpiece. If the tool is too low the tool will be drawn into and under the workpiece and for small diameter material may result in them bending or breaking. The front clearance is there to stop the tool rubbing on the workpiece. When machining some materials, of note is aluminium, some of the material may build up on the tip of the tool and so produce a new cutting edge. The result will be a poor finish.

front view

The overhang of any tool should be kept to a minimum as it is important that the tool is as stiff as possible. If the tool starts to chatter check that is held firmly. This may be particularly true if too many shims have been used to adjust the height of the tool. However, there is the odd occasion where a large overhang is required, especially on small lathes, to work into an area where the saddle or tool post will not clear the workpiece. On this occasion the cutting rate should be reduced to reduce the forces on the tool and hence reduce the displacment of it and hence chatter.

MaterialSide clearance Front clearance Side rake Back rake
Aluminium1281635
Brass1085 to -40
Bronze1085 to -40
Cast Iron108125
Copper12102016
Mild Steel10 to 12812 to 188 to 15
Stainless Steel10815 to 208

Bell Mouth

The flaring or tapering of a machined hole, usually made at the entrance end because of misalignment or spring of the cutting tool.


Boring Tool

A boring tool is used to enlarge a cylindrical hole, sometimes the tool bit the boring bar and the tool holder are incorporated in one solid piece.


Centre Height

Set up the lathe tool so that the very top edge of the tool is at centre height, you can do this by aligning the tool with a centre in the tailstock and then packing under the tool with shim.

If the tool is too low then you will leave a "pip" in the centre of the bar when you face it.

A set of feeler gauges makes a good starting set of shims.


This shows the result when the tool is at the correct height.

I use a mixture of brass, steel and aluminium shims. Thin brass shims are great to get the height exactly correct.

With a larger lathe and so larger tool sections it is still useful now and again to use small tools. The Warco WM240B takes 12mm tool steel and this only needs a slight height increase to reach centre. Now and again I still want to use 6mm tools and for these occasions I have a piece of 6mm thick mild steel shim to bulk up the tool height.


This again shows the tool has been set too low.


If you want a varied selection of shims then quite useful to buy a set of feeler gauges and cut them down into a selection of shims.

This will allow you to get the tool at exactly the correct height.


The shim is place under the tool.

Be careful as you want the shim to cover the full length of the tool in the holder, or as a minimum the distance between the bolts that are used to lock the tool in place. If the shim is too short then there is a chance that tightening the tool down will cause the tool to tilt at an angle.


Chatter

Chatter marks on the workpiece are the result of vibration of the lathe, saddle, toolpost, tool or workpiece itself.

Sometimes it can be very difficult to stop this from happening and I often find this is because I′m using the lathe at it′s limit of capacity or capability.

Tool overhang can result in vibration, the upper image shows the tool has been located a long way out from the toolpost, shortening the overhang will increase the stiffness and so increase the frequency of any mode of vibration associated with the tool in cantilever.


HSS Tools

The most common material for lathe tools is High Speed Steel (HSS). This is easily ground using a bench grinder and a final cutting finish can be achieved with a wetstone or a diamond pad.

When starting to use a lathe it is quite useful to start with a pre-ground set of tools.

Review - Soba 12mm HSS Lathe Tools - These are not cheap, but a great way to get a set of tools to start working with.


Parting Tool

A thin blade used to cut parts off. Also used for making grooves such as in pistons for rings to sit.

Parting tools are normally just a couple of mm wide. The blade itself is deep to give it strength.

When parting there is nowhere for the swarf to go and so there is a tendency for parting tools to jam.


A parting tool in use in a lathe.

In this case being used to make grooves for cooling fins.

There are a number of key parameters to remember when using parting tools:

  1. height is exactly on centre
  2. cutting angle = 35 for aluminium
  3. ensure the holder is tight with just enough overhang to make the cut
  4. use plenty of cutting fluid
  5. Lock the saddle
  6. Low speed - 150rpm or less
  7. Use parting blades that are at least 12mm deep
  8. Check alignment, then check again
  9. Hone the tool on a diamond abrasive

Read more on Parting Tool.


Toolpost

A quick brush of the tool post to remove any swarf is always a good idea as it has a habit of getting embeded in the shims otherwise.

....or it raises the tool slightly, moving off centre what was otherwise a good setting.

The worst case is it gives an unstable tool that can break or damage the part you are turning.


Tungsten Carbide Lathe Tools

I use tungsten carbide tools when I machine cast iron and sometimes brass. In the case of brass mainly because the tool angles are correct.

Some tungsten carbide tools consist of a mild steel shank with a piece of tugsten carbide brazed in place and then ground to shape.

Review - Glanze TCT Tools - With the Warco 240B lathe I needed some larger carbide tipped cutting tools

See also: Lathes, Lathe Tools.

Model Engineering Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z