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Lathe – Boring


Boring can be used to make holes that are sure to be straight and whose bore diameter is the same from end to end. It is usually necessary that the diameter of the bore is a certain size. A common job that is often done by boring is making the bore of a cylinder in a cylinder block.

Holes like this could be finished by reaming to size but larger reamers are expensive and the size needed is always different even if only by a small amount from the reamer that is available.

Usually the part being bored will be held, by some means or another, on the spindle of the lathe. The tool being used for the boring will usually be mounted on the saddle.

For the bore to be straight it is essential that the axis of the spindle is parallel to the bed of the lathe. It is also essential that the tool moves parallel to the bed of the lathe. The best way of doing this is so that the movement of the tool is obtained by the movement of the saddle – not the top slide.


Boring – holding the workpiece

Often when boring, the workpiece is not necessarily round. This means it is not balanced. In which case it either needs to be balanced by means of some sort of counterweight or the spindle must only turn slowly enough to prevent any vibration.

The workpiece can be held by any of the following means:

Clamping on to a face plate

fig boring with workpiece clamped on the face plate

Face plate with angle plate

fig boring with workpiece held on angle plate mounted on the faceplate

Face plate with Keats block
Lathe - Keats block

Lathe – Keats block

fig boring with workpiece mounted on a Keats block

Where the workpiece is mounted on an angle plate or a Keats block it is possible for more than one hole to be bored and be “perfectly” parallel to the first so long as the setting on the angle plate or Keats block is not altered.

In this case the workpiece is not balanced. Though it might sound easy to balance the workpiece, in practice, it often is not. In this case the space available on the face plate was totally inadequate to take the weight necessary even if it had been available.

However, it was quite practical to machine this workpiece simply by turning it slowly enough.

Three jaw chuck

It is fairly safe to always assume that a tree jaw chuck is not going to hold a round workpiece concentric to the axis of the spindle. But if a hole is center drilled, drilled and bored then its axis will be concentric. If the outside of the workpiece is turned then any part of this turned surface will be concentric with the bored hole.

Four jaw chuck

A four jaw chuck can be used to hold many irregular shaped workpieces, for example, a cylinder block They can also be aligned accurately so any hole bored will be in the right place.

If more than one hole is being bored and the bores need to be parallel then it is necessary to have at least two surfaces on the workpiece that are flat and will ensure cthat when the workpiece is moved its axis will remain parallel to the axis of the spindle.


The boring tool

Because of the nature of boring, the boring tool is often just a round bar and is usually referred to as being a boring bar. Though the part of the tool entering the workpiece might be round, the part being held is often square for ease of clamping.

Boring bars can take any of the following forms:

 A bar made from a single piece of high speed steel

Because HSS is difficult to grind this type of tool is often only suitable for smaller tools. In the example shown it is clear that a lot of HSS has been removed. This means that this sort is often only suitable for tool that are fairly small in length and the bore diameter.

fig boring bar ground from a single piece of HSS


6027 forged one part boring bar in HSS

6027 forged one part boring bar in HSS

fig HSS boring tool

It is possible to making a boring bar from a rod of HSS without removing a lot of material by using a rod at a very slight angle.

Fig boring using a rod of HSS at a slight angle

In this photo the angle has been greatly exaggerated so it is visible

A bar made from a single piece of carbide


The advantage of a carbide boring bar is that a carbide bar is stiffer than a steel bar of the same dimensions.


A bar fitted with a piece of HSS welded  or carbide brazed on

fig boring bar with HSS tip brazed on

fig boring bar with carbide tip brazed on


A bar fitted with a separate piece of HSS or a carbide insert

fig boring bar fitted with a piece of HSS


Very often it is necessary for the piece of HSS not to stick out of the back of the boring bar so collecting very small pieces can be useful.

Because the boring bar has to be long enough to fit a grub screw to hold the HSS bit often the HSS will not be at the very end of the tooling whilst keeping the diameter to a minimum.

fig problem with HSS

fig boring bar fitted with a carbide insert


A solid carbide boring bar fitted with carbide inserts



Geometry of the boring cutter

The principles regarding the geometry of the cutting edge for boring are the same as when turning. But there are two other problems. Firstly it is not possible to see where the cutting is happening. Secondly the tooling must not hit the side of the hole where the cutting edge is other than at the cutting edge also the back of the tooling must not hit the other side of the hole.

During a boring job it is possible for the boring cutter to cut either on its end or on its side or both. Either way, it is essential that the cutting edge is at the center height of the lathe.

As with all cutting tools it is essential that the cutting edge has sufficient clearance relative to the workpiece at the point where the cutting is happening.

When cutting an external surface if the clearance is correct at the cutting edge then there is no risk of the cutting edge snagging elsewhere.

Fig clearance angle when external turning

When boring, ie, internal turning, it is essential that, apart from the cutting edge, no part of the face of the cutting tool touches the workpiece at all at any time during the boring operation.

Fig clearance angle when boring

These problems are easily avoided when boring large holes but with small holes the geometry of the cutter becomes critical. In general it becomes almost impossible to bore holes smaller than 6mm diameter. These can often be drilled and reamed instead.

MEW no 53 p60 – some boring information – the geometry of boring tool – Philip Amos


Holding the tool

The main difference between a boring bar and most other turning tools is that whereas most turning tools are mounted so they are at right angles to the bed of the lathe, a boring bar has to be mounted so it is lying parallel to the bed of the lathe.

Fig usual mounting of turning tool

5062 Colchester way of holding tool parallel to the bed

5062 Colchester way of holding tool parallel to the bed

Fig mounting of a boring bar

Secondly, whereas most toolholders have square shanks the boring bar is often round. Often, these round boring bars have flats on them so it is possible to hold them in a conventional toolholder. However it is better to hold them in a sleeve. The sleeve always has a slit in it. When it is used the slit must be on the side as shown.

Fig boring bar in sleeve

At the very least the boring bar or boring bar with sleeve should be clamped at two points which are as far apart as possible. It the bar was infinitely rigid this would be sufficient but since it can flex between these points it is best to clamp it wherever it is possible to do so.

The length of the held part of the boring bar should be at least three times its diameter.

The overhang of the boring bar should be as short as possible consistent with being able to do the job. It is recommended that a steel boring bar should not extend by more than four times its diameter. A carbide tool should not extend by more than six times its diameter. But if one is prepared to remove metal slowly enough higher ratios can be managed.

In general, one should always use a boring bar with the largest possible diameter. However it is essential that any swarf produced can get out. This a particular problem if the hole is blind.


 Use of a boring head in the tailstock

In all of the above examples the boring bar is held on top slide. The diameter of the hole being bored is set by moving the tool using the cross slide.

An alternative method is by using a boring head as used on the milling machine. This is fitted in the tailstock. This works because diameter of the hole being bored can be set by adjusting the cutter in the boring head.

This will, of course, only work if the tapers are the same or can be made to be the same by, for example, using a sleeve.

One snag with this method is that is is not possible to feed the cutter automatically – it has to be done by hand.


Boring holes

If the hole is to be bored in a piece of solid material then it has to be drilled first so that the boring tool can get into it. Usually one would start by drilling the largest possible hole to minimise the amount that needs to be bored out. But, on the other hand, one has to be absolutely sure that the drill will leave something to be bored at every single point on the surface of the bore.

If the cylinder is to be bored in a casting that already has some sort of hole cast in it, then it can be drilled with a drill specially designed for this – a core drill. Otherwise it has to be bored. In this case we start boring using the largest boring bar that will freely fit in this hole.


Making holes the right size

Boring can produce a finish as good as a reamer but getting the size of the hole accurately is much more difficult. Part of the problem is that since the boring bar can bend under pressure, if a large cut is made the size will be different to that if the cut had been very light.

This means that before measuring a hole for a given setting of the cutter the hole must be bored at least twice, preferably thrice, without moving the tool. This is easy to see when having cut just once and then the tool is withdrawn it will cut a spiral on the way out.

For the finishing cut it is worth cutting three times to get the bore the right size and with the best possible finish.


When boring a blind hole it is essential to make sure the tool is not blocked by accumulated swarf.


Making holes the right length

The only way to be sure that the hole is bored to the same depth on each pass is to use a stop. If the cutter should go too far it is possible for this to cause the slightest movement of the cutting edge that can affect subsequent passes.

The boring process

If the workpiece moves during this process the job will probably be ruined. Make sure the job is held securely.

Face the surface as required.

Center drill

Drill with a small drill.

Drill with the largest drill available but leaving enough metal for boring or reaming.

If the workpiece already has a hole in it the only way of aligning the hole is by boring it. And then, possibly reaming it.


Boring – measuring the hole

The hole being bored can be measured by using a telescopic gauge. This appears to be the most accurate way possible without buying very expensive equipment. The gauge should be put in the hole. It should be tilted and then screwed tight and then pulled straight till it is at right angles to the bore. It is then put at an angle and taken out of the bore. It can then be measured using a micrometer.

Of course it would be nice to be able to turn a piston, bore the cylinder and, hey presto, the piston is a perfect fit.

In practice, It is easier to make the cylinder first  and then make a piston to fit it rather than making a piston and using it to make a cylinder to fit it.

There is always a risk that the cylinder will become oversized and nothing can be done about this.

If the cylinder is made first then the piton is made to fit it. Should the piston become undersized it is easy to make another.

But there is another advantage, to get a really good fit it is easier to adjust the size of the second part made  by using abrasive paper to remove very small amounts of metal. It is far easier to do this on an external surface, ie, the piston, than on an internal one, ie, the cylinder. Also it is difficult to increase the size of a cylinder evenly by a small amount but easier to do so on a piston.


Making holes with flat bottoms

A boring bar will often be used to make accurate holes. These holes will often go all the way through the workpiece. However there are times when a hole made like this or in any other way will need to have a flat bottom. The only way to get a reasonably flat bottom is to use a stop so that each cut when boring is the same depth. However there will be slight variations due to different pressures each time the stop is hit.

If a really flat bottom then the stop needs to be moved right by a few hundredths  so the hole is slightly short and then the boring bar is moved to the right depth. It then cuts from the center of the bottom of the hole outwards to the side of the cylinder.

An example of this would be where a large drill is used to drill a large hole. The bottom is V-shaped. A boring bar could be used to flatten this. But to be able to machine from the bottom of the hole to the side requires a boring bar with a diameter of less than half the diameter of the hole.


Boring when running the lathe in reverse

One problem with boring that does not happen when external turning is that it is often difficult to see  what is going on. One way round this is to run the lathe in reverse. To do this it is essential that if some sort of screw mechanism is being used to hold the workpiece that it will not undo if the lathe is run in reverse.

It is also necessary to use a tool that is the opposite hand of what would be used normally.

fig right handed boring bar

fig right handed boring bar in use

(the use of “right” here might be contentious)


Saddle boring

It is also possible to bore a hole by mounting the workpiece on the cross slide. The boring bar is mounted between centers. This requires a boring bar with a cutter in the middle.

fig boring bar for saddle boring

The diameter of the hole produced is determined by the setting of the cutting edge on the boring bar. The difficulty of adjusting this compared to other methods would make most other methods more attractive.

The size of the hole depends on the position of the piece of hss in the bar. Moving the cross-slide moves the position of the hole in one direction. The position of the hole in the vertical direction depends upon the position the workpiece is held.

MEW no 40 p27 saddle boring – Bob Loader







  1. It is interesting that a boring tool is so basic in it’s shape. This makes sense, but what type of situations would boring be the best option for? I am curious because know much more about drilling, and boring seems to be more difficult. Thank you for all of your helpful details on this type of tool and how to use it.

    • johnf said:

      the problem with drilling is that the hole is not necessarily the right diameter, not necessarily straight and has a poor finish. One way round this is to ream the hole. But it might be that the user does not have the right size of reamer. The general solution is to bore the hole. This can be done with a boring head on the milling machine or with a boring bar on the lathe. On the lathe it can produce a hole of any size accurately and straight and with a good finish so long as the axis of the spindle is parallel to the bed of the lathe (and no play between saddle and bed).
      john f

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