Quick replacement toolholder for milling machine

A toolholder for securing a tool in the collet of a mill, the collet having a fluted open end wherein the toolholder is a pair of concentrically arranged cylinders, one cylinder inside the other and each having flutes. The inner cylinder has an axial bore for receiving the shank off the tool so that the flared end of the spindle squeezes the fluted ends of the pair of cylinders and the shank of the tool so as to secure the shank of the tool concentric with the collet and spindle of the mill.

BACKGROUND 
1. Field of the Invention 
This invention is related to devices for holding milling cutters (tools) in 
mills and particularly to a toolholder that enables a user to make a quick 
and convenient substitution of tools without removing the collets. 
2. Background and Information Disclosure 
A mill for cutting a metal part including a vise to hold the part and means 
to move the vise and part in three dimensions while machining (cutting) 
the part with a rotating tool has been in widespread use for almost a 
century. One particular manufacturer has been so successful in marketing a 
version of this machine that the machine is universally referred to as the 
"Bridgeport mill" although there are numerous other manufacturers all 
producing an identical machine. 
The toolholder used in the Bridgeport mill is a collet with a bore having a 
thread on one end. The collet slips into the bottom end of the bore of a 
rotatable spindle. A drawbar inserted into the top of the spindle is 
screwed into the end of the collet and draws the collet into the bore of 
the spindle. The shank of a tool is positioned in the bottom end of the 
collet. The collet has three slits along the shank of the tool so that as 
the low end of the drawbar is screwed into the top end of the collet 
thereby drawing the collet into the bore of the spindle, the lower end of 
the collet is squeezed in the flared end of the bore of the spindle so as 
to clamp onto the tool. This construction provides very solid support to 
the shank of the tool This is an important requirement in order to avoid 
vibration which would occur as the tool cuts the metal and cause 
degradation of the finish of the surface of the part. The fluted 
construction of the collet with slots also maintains very accurate 
concentricity of the shank of the tool with the rotational axis of 
spindle. This is necessary for achieving good finish and precision and is 
the arrangement that has been used by millions of machinists over a period 
of fifty years. 
A major inconvenience with this arrangement is that the machinist must 
change his toolbits frequently, inserting larger or smaller tools or he 
may be required to insert a drill bit requiring that an expandable chuck 
with a straight shank be mounted in the spindle. Changing one tool to 
another tool having a different shank diameter also requires that he must 
change one collet to another collet having an appropriate bore to fit the 
diameter of the shank. This involves the time consuming operation of 
separating (by unscrewing) the drawbar from the top end of the collet, 
removing the collet from the bore of the spindle, searching for the collet 
having the appropriate bore diameter to fit the shank of another tool, 
inserting the second collet back into the bore of the spindle, screwing 
the drawbar back onto the end of the newly mounted collet. Because the 
collet universally used for this operation is typically eight inches long, 
the machinist must usually operate the crank handles of the mill to move 
the vise and part down and away from the end of the spindle in order to 
remove one collet and insert another collet. This causes loss of 
registration of the part with the tool bit and consequently loss of 
accuracy in machining the part. Typically, the requirement to change 
collets in machining a pan extends the time required to machine the part 
by about 50%. 
Another problem with this arrangement is that the machinist must have 
available a number of expensive collets, each collet having a bore 
diameter different from the bore diameters of the other collets such that 
there is one collet for every shank size of the machinist's collection of 
toolbits. This requires a considerable cost to the machinists because each 
collet is precision machined and has a taper on an outside surface, a 
thread in one end, a precision bore, axial slots that allow the collet to 
squeeze onto the end of the shank, and the holder must be machined from 
hardened steel in order to extend the life of the threads formed in the 
bore. 
Because of the expense of the collet, the typical machinist generally has 
only one collet per shank diameter and therefore has a limited number of 
collets. Therefore, he typically maintains his tools in a place separate 
from where he stores his collection of collets. The result is further loss 
of time in sorting through his collection of collets and collection of 
cutters in order to match the collet with the cutter. 
THE INVENTION 
Objects 
It is therefore an object of this invention to provide a toolholder which 
secures the cutter to the spindle of and reduces the time consuming 
operation of replacing BOTH the collet and tool when a new size of tool is 
required. 
It is another object that the tool of this invention hold the tool very 
securely such as to avoid vibration generated by the tool rotating against 
the part. 
It is another object to eliminate the cost that is inccured by requiring a 
number of collets having a range of bores to accommodate all sizes of 
tools. 
Summary 
This invention is directed toward a toolholder which has one end adapted to 
receive the shank of a tool and another end adapted for securing in a 
mill. The toolholder is basically a double collet. An inner collet holds 
the shank of the tool and is positioned inside an outer collet which is 
engaged in the spindle of the mill. This construction provides a stable 
support for the tool and maintains precise alignment of the axis of the 
tool with the axis of the spindle while enabling the operator to change 
tools quickly and conveniently by simply disengaging the inner collet from 
the outer collet. The tool end of the inner collet extends from the outer 
collet and a collar slips over the extended end of the inner collet. The 
collar is collapsible by virtue of a slot and set screw in the collar so 
that the shank of the tool is secured in the inner collet when the inner 
collet is not positioned in the outer collet. The collar also functions as 
a stop which provides for inserting each inner collet a repeatable 
distance into the outer collet

DESCRIPTION OF A PREFERRED EMBODIMENT 
Turning now to a discussion of the drawings, FIG. 1 shows an exploded view 
of the toolholder of this invention including an outer cylinder 12, an 
inner cylinder 10 and a retaining collar 14 FIG. 2 is a sectional view of 
the assembled toolholder positioned in the bore 26 of the spindle 28 of 
the mill, (mill not shown). 
The inner cylinder 10 has flutes formed by slots 11 (four slots are shown) 
extending from the lower end of the inner cylinder 10. The outer cylinder 
12 also has flutes formed by slots 13. The lower end of the outer cylinder 
12 has a taper 15. 
The shank 16 of a tool 18 fits into the bore 20 of inner cylinder 10 and is 
held securely by collar 14 slipping onto the end of inner cylinder 10. The 
upper end 21 of the inner cylinder 10 slips into the bore 22 of the outer 
cylinder 12. The outer cylinder 12 slips into the bore 26 of the mill 
spindle 28. As shown in FIG. 2, a drawbar 30 extends down through the 
spindle 28 and screws into the threaded end of bore 22 of the outer 
cylinder 12, such as to draw the toolholder into the bore 26 of spindle 28 
causing the outer cylinder to be squeezed onto the inner cylinder 10 and 
shank 16 of the tool by virtue of the taper 15 and the fluted construction 
of the inner cylinder 10 and outer cylinder 12. 
FIG. 4 shows in greater detail the construction of the upper end of the 
inner cylinder 10. The inner cylinder 10 has a shoulder 21 on the 
circumferential corner of the inner cylinder 10 opposite the tool and the 
slots 11 of the inner cylinder 10 extend from the tool end of the inner 
cylinder 10 to the shoulder 21. This construction improves the purchase of 
the inner cylinder 10 by reducing the resistance to squeezing the flutes 
13 of the inner cylinder 10 against the shank 16 of the tool. 
FIG. 3 shows a nest of toolholders 19 in block 32 and illustrates the 
convenience of using the invention. For many machine shops, a useful 
dimension of bore 22 of outer cylinder 12 is 3/4 inch FIG. 3 illustrates 
toolholders 19 with tools having shanks ranging from 1/16 inch to 11/16 
inch in steps of 1/16 inch. A useful dimension of bore 22 of outer 
cylinder 12 is 3/4 inch so that each toolholder in the nest has an outer 
diameter of 3/4 inch. Tools shown in FIG. 3 may have shanks ranging from 
1/16 inch to 11/16 inch in steps of 1/16 inch. The machinist may change 
each tool to run his job simply by loosening the drawbar 30 and inserting 
the inner cylinder 10 with required tool 18 selected from the nest of 
tools shown in FIG. 3. 
A major feature of this invention is the combination of inner and outer 
cylinders which provides very accurate alignment of the tool 18 with the 
spindle 28 while providing quick change of tools by simply slipping one 
inner cylinder 10 with tool 18 out of the outer cylinder 12 and inserting 
another inner cylinder 10. The inner cylinders may be much shorter than 
the outer collet. A length, L' (FIG. 1) in the range from 1.0 inch to 3.5 
inches is a very practical length for the inner cylinder 10 and a length L 
in the range from 7 to 9 inches is a useful length L for the outer 
cylinder 12 corresponding to a maximum ratio, L/L' of 9 and a minimum 
ratio L/L' of 2. so that there is no necessity to drop the mill table or 
crank the part away from the tool in order to replace the short inner 
cylinder 10 as required when replacing an eight inch collet of the prior 
art. By virtue of this arrangement, the operator can maintain a store of 
inner cylinders 10 holding tools having a range of sizes, enabling him to 
quickly select the required tool for installment in the mill. The inner 
cylinder 10 is much less expensive than the collet of the prior art 
because it requires less material and because there is not a requirement 
to machine a precision taper on the side of the inner cylinder. According 
to an object of the present invention, the machinist need invest in only 
one outer cylinder 12 and a nest of inexpensive inner cylinders 10. 
A steel alloy is the preferred material for fabrication of the toolholder. 
However, other alloys may be useful depending on the application. 
Variations to the toolholder of this invention may occur after reading the 
specification and studying the drawings. I therefore wish to define the 
scope of my invention by the appended claims.