Clamp

A clamp for clamping a workpiece comprises a base member a clamping member mounted on the base member for linear movement relative thereto and a drive member mounted on the base member for rotary movement relative thereto. First and second cam elements comprising a face cam are provided on the drive member and the base member, at least one of the cam elements including at least two portions of different pitch. The arrangement is such that rotation of the drive member relative to the base member causes linear movement of the clamping member relative to the base member.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a clamp. In particular, but not 
exclusively, the invention relates to a clamp for exerting a pushing or 
pulling force on a workpiece. 
2. Discussion of the Known Art 
An example of a clamp for exerting a pushing or pulling force on a 
workpiece is that sold by HMC-Brauer Limited under the model numbers 
CP1005 to CP1008. This clamp has a roller cam comprising a pair of rollers 
mounted on spur axles that engage helical tracks provided in the inner 
cylindrical surface of the clamp body. Rotating a handle causes the 
rollers to move along the helical tracks, which drives a clamping member 
in the axial direction of the clamp. The helical tracks include portions 
of steep and shallow pitch, which allow the clamping member to be brought 
rapidly into engagement with the workpiece before the clamping force is 
applied. The clamp is very quick and simple to apply, with a single 
movement of the handle providing both the initial fast travel of the 
clamping member and also the final clamping force. 
The main problem with the clamp described above is that axles of the roller 
bearings may shear or be distorted if an excessive force is applied. The 
nominal maximum holding force of the clamp is therefore only 500 daN, 
which can easily be applied with a hand impact on the handle. In practice 
in the workshop, the clamps are often mistreated and a mallet may be used 
to increase the clamping force. This frequently leads to damage to the 
clamp. 
The above mentioned problem is compounded by the fact that travel of the 
clamping member is limited by the length of the helical tracks. If the 
rollers reach the ends of the tracks before a sufficient clamping force 
has been applied, there will be a temptation to apply the clamp harder as 
there is no clear indication that the point of maximum travel has been 
reached. This is again likely to cause damage to the clamp. 
A further disadvantage of the clamp is that it is relatively complicated 
and expensive to manufacture. 
SUMMARY OF THE INVENTION 
It is an aim of the present invention to provide a clamp that mitigates at 
least some of the above-mentioned disadvantages. 
According the invention, a clamp for clamping a workpiece includes a base 
member, a drive member mounted on the base member for relative rotational 
movement about an axis of rotation, and a clamping member mounted on the 
base member for linear movement in the direction of the axis of rotation. 
The clamping member has a workpiece engaging portion for engaging a 
workpiece at a point substantially aligned with the axis of rotation and 
for exerting a clamping force on the workpiece substantially in the 
direction of the axis of rotation. The clamp also has a first cam element 
on the drive member and a second cam element on either the base member or 
the clamping member for engaging the first cam element. The first and the 
second cam elements include a face cam, and at least one of the cam 
elements includes at least two portions of different pitch so that 
rotation of the drive member relative to the base member produces linear 
movement of the clamping member relative to the base member in the 
direction of the axis of rotation. 
The use of a face cam rather than roller cams ensures that the cam elements 
are always in compression, which allows far higher clamping forces to be 
applied. All clamping elements of the clamp are, in fact, solely in 
compression during use, and this means that the clamp is extremely robust. 
The use of a mallet to apply the clamping force is therefore permissible. 
Because the cam includes two portions of different pitch, the clamp can 
provide a fast initial travel followed by a high clamping force. As with 
the prior art clamp described above, both the initial travel and the 
clamping force are applied with a single movement of the handle and this 
makes the clamp very quick and simple to use. 
It is possible to form the face cams integrally with the drive member and 
the handle member or the base member, for example by investment casting. 
This makes the clamp very simple and inexpensive to manufacture and 
assemble. 
Advantageously, at least one of said cam elements includes a portion of 
shallow pitch and a portion of steep pitch. Preferably, at least one of 
said cam elements includes upper and lower portions of shallow pitch and 
an intermediate portion of steep pitch. 
The shallow pitch of the cam surfaces is such that the clamp does not 
release itself. 
The first and second cam elements are advantageously matched substantially 
to one another. This maximises the contact area of the cam elements, thus 
reducing friction and minimising wear on the cam surfaces. 
The clamp may include a resilient member arranged to resiliently oppose 
linear movement of said clamping member in a first direction. The 
resilient member, which may be a spring, may be employed to ensure rapid 
return of the clamping element when the clamp is released. The resilient 
member advantageously biases said first and second cam elements towards 
one another. 
The first and second cam elements are advantageously arranged to permit 
continuous rotation of said drive member in the clamping direction. This 
prevents the clamp being damaged in an attempt to force the handle beyond 
a fixed stop. If the handle is rotated beyond the point corresponding to 
maximum travel of the cam elements, which is indicated by alignment of 
marks on both the base member and the drive member, the clamp is released 
and incomplete clamping is indicated audibly and visually. 
The cam elements may include stop surfaces to limit rotation of said drive 
member in a second direction of rotation. This ensures that the handle 
always returns to the same position when the clamp is released. 
The clamp may include means for preventing rotation of the clamping member 
relative to the base member, thereby preventing damage to the surface of 
the workpiece. 
Alternatively, the clamp may include a lost motion mechanism to allow 
limited rotational movement of said clamping member relative to said base 
member. Rotation of the clamping member may be useful for providing 
additional clearance for insertion or removal of a workpiece. Such a 
pending British patent application No. 9508951.2, filed Nov. 13, 1996, the 
contents of which are incorporated herein by reference. 
The drive member may include a handle for manual rotation thereof. 
Advantageously, the clamping member is arranged to engage the workpiece at 
a point substantially aligned with the axis of rotation of the drive 
member and to exert a clamping force on the workpiece substantially in the 
direction of that axis. 
Advantageously, the base member and the drive member comprise metal 
castings. 
An embodiment of the invention will now be described, by way of example, 
with reference to the accompanying drawings and the applied claims.

DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of the clamp, which may be described as a cam ram, is shown 
in FIGS. 1 to 3. The clamp comprises a base 101, a handle 102 and a 
clamping member 103. 
The base 101, which is shown in more detail in FIGS. 4 to 8, comprises a 
substantially triangular base plate 105 and a substantially cylindrical 
body member 106 of outside radius R.sub.0 that extends perpendicularly to 
the plane of the base plate 105. Three holes 108 are provided in the base 
plate 105, one adjacent each corner, which receive bolts for mounting the 
clamp in use on a work bench or machine tool. Three index marks 109 are 
provided around the circumference of the body member 106. 
A cylindrical bore 110 of radius R.sub.1 extends axially through the body 
member 106. Rectangular recesses 112 that extend from the lower surface of 
the base 101 to approximately two-thirds its height are formed in opposite 
sides of the cylindrical wall of the bore 110. 
The upper face of the cylindrical body member 106 is formed as a cam 
surface 114. The profile of the cam surface 114 is shown in FIG. 8 and 
comprises three identical equi-angularly spaced portions, each of which 
includes a shallow pitch lower section 116, a steep pitch middle section 
118 and a shallow pitch upper section 120. In the embodiment shown in the 
drawings, the lower and upper sections 116, 120 each extend through an arc 
of approximately 50.degree. and have a pitch at the outside edge of the 
cam surface producing a rise of 1 mm. The middlesections 118 each extend 
through an arc of approximately 20.degree. and have a pitch edge producing 
a rise of 6 mm. The total rise of the cam surface is approximately 8 mm. 
The ends of adjacent portions of the cam surface 114 are connected by 
substantially vertical walls that form stop surfaces 121. 
The upper sections 120 of the cam surface 114 extend radially inwards, 
forming lugs 122. The inner edges of the lugs 122 are coincident with an 
imaginary concentric circle of radius R.sub.2 where R.sub.2 &lt;R.sub.1. 
The handle 102, which is shown in more detail in FIGS. 9 to 13, comprises a 
substantially cylindrical body member 124 and a handle member 126 that 
extends substantially radially therefrom. The handle member 126 has an 
H-shaped cross-section and tapers towards its remote end. At the remote 
end of the handle member 126 there is provided a hand grip 128 in the form 
of a spherical ball. 
The upper and lower edges of the cylindrical handle body member 124 are 
chamfered, as shown in the drawings. An index mark 129 is provided on the 
cylindrical surface of the body member 124, at a point displaced at an 
angle of 180.degree. to the longitudinal axis of the handle member 126. 
A cylindrical bore extends through the handle body member 124, the bore 
comprising a lower portion 130 of radius R.sub.3, a middle portion 132 of 
radius R.sub.4 and an upper portion 134 of radius R.sub.5, where R.sub.4 
=R.sub.2, R.sub.3 is slightly larger than R.sub.0 and R.sub.3 &lt;R.sub.5 
&lt;R.sub.4. The body member 124 therefore includes an inwardly-extending 
circular flange 136, defined by the portion of the bore having a radius 
R.sub.3. 
Provided on the lower surface of the flange 136 is a cam surface 138. The 
profile of the cam surface 138 is shown in FIG. 13 and is matched to that 
of the base body member cam surface 114. The cam surface thus comprises 
three identical equi-angularly spaced portions, each of which includes a 
shallow pitch lower section 116a, a steep pitch middle section 118a and a 
shallow pitch upper section 120a. The ends of adjacent portions of the cam 
surface 138 are connected by substantially vertical walls that form stop 
surfaces 121a. The cam surface 138 extends radially inwards from the 
cylindrical wall of the lower bore section 130 to a radius R.sub.6, where 
R.sub.3 &lt;R.sub.6 &lt;R.sub.4 and R.sub.6 is approximately equal to R.sub.1. 
The handle 102 is mounted for rotation on base body 101, with the upper 
part of the base body member 106 extending into the bore lower portion 
130. The cam surfaces 114, 138 of the body member 106 and the handle 102 
engage one another. 
The clamping member 103, which is shown in more detail in FIGS. 14 to 16, 
comprises a substantially cylindrical body member 140 of radius R.sub.7 
and a circular disc 142 of radius R.sub.5 and R.sub.7 is slightly less 
than R.sub.4. The upper edge of the disc 142 is chamfered and the upper 
face 144 of that disc forms the work engaging face of the clamp. 
A plain cylindrical bore 146 of radius R.sub.9 extends axially through the 
body member 140 and the disc 142. Optionally, the bore 146 may be provided 
with a screw thread 147 to permit a force transmitting rod or an 
alternative work engaging face (neither shown) to be attached to the 
clamp. At the lower end of the body member 140 there are provided two 
rectangular cut outs 148. A peripheral groove 150 is formed in the 
cylindrical wall of the body member 140, adjacent its lower end. 
A compression spring 154 is seated within the base bore lower portion 106 
and abuts at its upper end the lower surfaces of the lugs 122. The lower 
end of the spring engages a pair of washers 156 that surround the lower 
end of the clamping member 103 and are held in position by a circlip 158 
that engages the peripheral groove 150. 
Each washer 156, shown in more detail in FIG. 17, includes two pairs of 
radially inwards- and outwards-extending tabs 160, 162 that engage 
respectively the cut outs 148 of the clamping member 103 and the 
rectangular recesses 112 of the base 101. This prevents rotation of the 
clamping member 103 with respect to the base 101 but allows linear axial 
relative movement thereof. The spring 154 serves to bias the clamping 
member 103 axially downwards (i.e. towards the base plate 105). 
The base 101 and the handle 102 are manufactured from alloy steel, for 
example nickel carbon steel, by investment casting (e.g. by lost was 
casting). Precision casting techniques are employed, providing tolerances 
of .+-.0.005" per inch (.+-.0.12 mm per 25 mm). This precision allows the 
parts to be assembled with almost no machining. The castings are case 
hardened to provide a high surface hardness for the cams and a tough core 
for compressive strength. The castings are finished with manganese 
phosphate, which provides corrosion resistance. The clamping member 103 is 
machined from carbon nickel alloy barstock. 
Operation of the clamp will now be described with reference to FIGS. 1 to 
3. In the rest position as shown in those drawings, the cam surfaces 114, 
138 are held in full mating engagement by the biassing force of the spring 
154. In this position, the clamping member 103 is clear of the workpiece, 
which allows the workpiece to be placed in position. 
As the handle is rotated in the direction of the arrow A (clockwise when 
seen from above), the mutual engagement of the steep pitch middle portions 
118, 118a of the cam surfaces 114, 138 causes the clamping member 103 to 
rise rapidly into a position where it is nearly in engagement with the 
workpiece. In the embodiment, a fast rise of 6.0 mm is provided. 
When the handle has rotated through an angle of approximately 20.degree., 
the steep pitch middle portions 118, 118a reach the end of their travel 
and the lower cam sections 116a of the handle 102 come into engagement 
with the upper cam sections 120 of the base 101. These cam sections have a 
shallow pitch and it is therefore possible to apply a large clamping force 
to the workpiece by rotation of the handle 102. In the embodiment, the 
handle may be rotated through a further 100.degree., producing a clamping 
rise of 2.0 mm. Throughout both fast and clamping travel of the clamping 
member, rotation of the clamping member is prevented, so avoiding damage 
to the surface of the workpiece. The limits of the fast and clamping 
travel movements are indicated by alignment of the index marks 109, 129 on 
the base 101 and the handle 102. 
In order to release the clamp, the handle 102 is rotated back in the 
anti-clockwise direction until the cam surfaces 114, 138 are once again in 
full mutual engagement. Further anti-clockwise rotation of the handle 
beyond this point is prevented by the engagement of the substantially 
vertical stop surfaces 121, 121a. The clamping member 103 is driven back 
to its rest position under the biassing force of the spring 154. 
The clamping force exerted on the workpiece is, of course, dependent on the 
force applied to the handle 102. The clamp is designed to be able to 
withstand the use of a mallet to apply and release the clamp. 
If the clamp is over tightened, there is a possibility that the cam 
surfaces may be left at the very limit of their travel. Such a situation 
is potentially dangerous since it is possible that cams could slip over 
onto the next surface, thereby releasing the clamp. 
In order to reduce this danger, the limit of acceptable tightening is 
indicated by alignment of the index marks 109, 129 on the base 101 and the 
handle 102. If the handle is rotated in the clockwise direction through 
more than 120.degree. from the rest position, the lower cam sections 116a 
of the handle 102 will slip off the ends of the upper cam sections 120 of 
the base 101 and the clamping member will be driven back to the rest 
position by the spring 154. This produces an audible clicking noise, which 
warns the operator that the clamp has not been successfully applied. 
As will be apparent from the above description, the clamp can be applied 
and released with only single movements of the handle 102. The clamp is 
therefore very quick and easy to use. The clamp is also capable of 
applying very high clamping forces, since all the force transmitting parts 
of the clamp are in compression. 
Various modifications of the clamp are envisaged. For example, the slope of 
the cam sections may be reversed, so that clamping is produced by 
anti-clockwise rather than clockwise rotation of the handle 102, and the 
mutually engaging cam surfaces 114, 138 may be provided on the handle 102 
and the clamping member 103, instead of the base 101 and the handle 102. 
Further, by providing a connecting rod that is attached to the clamping 
member 103 and extends downwards through the bore 146 and out through the 
base plate 105, the clamp may be used to provide a pulling force rather 
than a pushing force. Alternatively, a pulling force may be provided by 
switching the relative positions of the handle 102 and the clamping member 
103. If the clamping member is provided with a radially-extending arm, the 
clamp may be employed as a down-thrust clamp. 
Whilst the profile of the cam surfaces 140, 172 that is described above is 
preferred, it is envisaged that the cam surfaces may take other forms: for 
example, the pitches and lengths of the steep pitch and shallow pitch 
sections may be varied. It is, however, important that the pitch of the 
shallow pitch sections is large enough to produce sufficient axial travel, 
whilst being small enough to product the required clamping force and to 
prevent the clamp releasing itself. 
Although it is preferred that the two cam surfaces should be matched to one 
another, this is not essential. Instead, a cam and cam follower 
arrangement could be provided.