Patent Application
20110094356
Producing a part on a manual metal working lathe usually involves a sequence of operations such as rough turning and facing followed by finish turning and facing, perhaps boring, and then possibly cutting off the piece, reversing it in the lathe chuck and finish facing the cut-off end to complete the machining of the part. Each of these operations generally requires a different type of cutting tool or a different orientation of the cutting tool and, in the 1950's when most lathes were equipped with a rocker tool post which accommodated a tool holder with an installed tool bit, removing one tool holder and installing and aligning another tool holder required a considerable amount of time and attention and significantly increased the cost of producing the part.
F. Sirola patented in July 1946 (U.S. Pat. No. 2,403,405) what today would be called a quick-change tool post that accommodated a plurality of quick-change tool holders. It was not until the mid 1960's that these devices became commercially available and it was not until the mid 1970's that their price became low enough for the small machine shop to find them cost effective. By this time, however, the quick-change tool post and its associated quick-change tool holder had evolved into a highly practical innovation in metal working technology that significantly reduced the cost of parts produced on a manual lathe: roughing and finishing tool bits can be mounted in tool holders, some oriented for turning and others for facing; cut-off tools and boring bars can be mounted in other tool holders and there are specialized tool holders for processes such a knurling. There is a provision in each tool holder for precisely adjusting the height of the tool bit. By turning a lever, usually at the top of the tool post, any tool holder can be unlocked and removed and another tool holder can then be mounted and locked with precisely the correct height and orientation. Removing and replacing tool holders can be accomplished in seconds and the operator is assured of its proper alignment.
But, at present, there remains the very common lathe operation of threading, where it is necessary to loosen the quick-change tool post from its fixed mounting and precisely align and tighten it in a different orientation. And, upon completion of the threading operation, it is again necessary to loosen the tool post and precisely align and tighten it in its original orientation. Both of these are onerous and time-consuming operations. The present invention is a modified quick-change tool holder that obviates both operations.
In the years since its original invention, the quick-change tool post and its associated tool holder have evolved into essentially three different forms: i.) the fixed tool post with two perpendicular faces that can mate with removable tool holders; ii.) the rotatable tool post with up to four orthogonal faces that can mate with removable tool holders and iii.) a rotatable tool post that incorporates four integrated orthogonal tool holders. (This last form is often referred to as a turret tool post.) The differences between these forms will be discussed in more detail below. The present invention is primarily intended to be used with (a modification of) the quick-change tool post of the first form, which is significantly less expensive and more practical than the other two forms. It will be explained below, however, that the invention can overcome certain significant shortcomings of the other two types in the threading operation. (Please note that the modification of the first type of quick-change tool post will be covered by a sister patent to that of the present invention.)
With reference to the foregoing, the following specifications and the accompanying drawings, in which the preferred embodiment of the invention is incorporated, are now presented. In the drawings,
Referring to
It is well to note that the faces of the quick-change tool post and the mating face of the quick-change tool holder incorporate relatively large flat vertical surfaces, namely the exterior faces of the dovetail tongues and the interior face of the dovetail recess of the tool post and the tool holder, respectively. And this observation (as well as the observation that the metal cutting tool axis is horizontal) applies to all of the commercially available quick-change tool systems as well as to quick-change tool systems covered in the patent literature. It is thus both reasonable and rigorous to specify the orientation of the metal cutting tool axis as bearing a certain horizontal angle with respect to a face of the tool post or to the face of a tool holder. Furthermore, it is noted that the dovetail tongues and recesses are symmetrical about a vertical plane perpendicular to their faces and located at their centerlines. This symmetry also happens to pertain to the single quick-change tool system, of the forms i) & ii) noted above, that does not employ interlocking dovetail mounting faces, to with, that patented by C. Gracchi in March 1950 (U.S. Pat. No. 2,621,396). This last observation pertains to tool holders that can be profitably “turned upside down” as will be mentioned below.
In the specification of the invention here, the horizontal angle through which the axis of the tool bit is rotated with respect to the face of the dovetail recess will be taken as positive when the rotation is counter-clockwise when seen from above. Thus the angle of rotation as shown in
The compound rest 22 is attached to the top of the cross feed by an arrangement that allows it to be rotated horizontally full-circle about a vertical axis fixed in the cross feed: a vertical cylindrical hub is located at the center of the conical section 24; the low end of the hub devolves into a short inverted conical section that mates with two bevel-ended dowel pins in the body of the compound rest that are forced against the hub by set screws to lock the compound rest down and in a particular angular position (none of this is shown). The conical section 24 is graduated about its perimeter in degrees and a mark 27 on the bottom of the compound rest adjacent to the degree scale indicates the angular position of the compound rest. (The angular position shown in this figure is designated positive 90° and is the default position of the compound rest; when the axis of the compound rest is perpendicular to the axis of the lathe, its angle is 0° and this position will be designated its home position. Note that, by the conventions adopted here, if it is rotated a certain angle from this home position in the counter-clockwise sense, its angle is deemed positive; conversely, if it is moved in the clockwise sense its angle is deemed negative.) The compound rest includes a feed mechanism that, in this angular position, moves tool bit parallel to the axis of the work piece either left or right by means of a lead screw (not shown) attached to the lead screw knob 23. The precise direction in which the tool bit moves when the compound rest lead screw is turned is denoted the axis of the compound rest. Note that the power feed mechanism of the lathe carriage can be disengaged, the carriage can be locked in place and the manual feed mechanism of the compound rest can be used to machine short cylindrical segments of the work piece.
Also note that the quick-change tool holder can be removed from the tool post face shown in this figure and placed on the face to the right and a second tool bit placed in the other end of the tool bit channel is thus positioned for facing; here a latitudinal power feed can be engaged for successive facing cuts whose depths are determined by the compound rest manual feed.
The position of the quick-change tool post with respect to the compound rest, as shown in
At this point it is convenient to consider a difficulty in the use of a manual metal cutting lathe: since the manual feed of the compound rest is used to machine short cylindrical segments, it is necessary that the axis of the compound rest be precisely parallel to the axis of the work piece (i.e. the longitudinal axis of the lathe), but the 1° graduations of the angular scale are quite small and hard to read; furthermore, it is not uncommon for there to be errors of up to a degree in the positioning of the scale. For this reason the following complicated procedure is required to set the compound rest in its default 90° position: a precision ground round piece of at least ¾ inch diameter must be mounted in a 4-jaw chuck and precisely centered with a dial indicator (of 0.0005 inch or less graduations) and checked for parallelism with the dial indicator over a movement of at least 2 inches of the lathe carnage. The dial indicator is then mounted on the compound rest and, with the carriage locked, the deviation noted on the dial indicator over the approximately 1.5 inch of the compound rest feed range. The angular position of the compound rest should then be adjusted so that (ideally) there is no visible deviation of the dial indicator. This is indeed an onerous procedure.
With the advent of the quick-change tool post and the quick-change tool holder a much simpler procedure became available: without the tool post mounted on the compound rest, the above procedure is performed once and the precision round is left in the 4-jaw chuck; then the quick-change tool post is loosely mounted on the compound rest and a quick-change tool holder of the type shown in
With this operation completed, whenever the compound rest is moved from its default position it can be returned to this position by loosening its two set screws, by installing the tool holder on either the front face or the right face of the tool post and then by moving the compound rest up against a (reliable) face plate mounted on the lathe spindle by moving the lathe carriage or by moving it up against a precision round mounted in a (reliable) 3-jaw chuck by moving the cross feed and then by tightening the set screws to lock the compound rest in place. With practice, this procedure can be executed in seconds. The foregoing issue is of importance here because standard lathe practice calls for setting the compound rest at an angle other than its default position for cutting threads.
A brief description of thread cutting on a lathe is now presented. Screw thread forms can vary widely from square threads to Acme threads to so-called buttress threads, but the focus here will be on the American National Screw Thread Form that is almost universally used for fasteners and machine parts. Here the sides (or faces) of the thread form an angle of 60° with each other; the tops of the threads are slightly truncated, as are the valleys between the threads. (Detailed specifications over the range of threads is given in the reference [c] of the Technical Literature cited.)
Cutting threads is generally a multi-pass operation. There are two feeding operations: first, while outside the right-hand range of the thread with the lathe stopped, the threading tool bit is fed toward the axis of the round work piece by the compound rest feed, as explained below, and then the longitudinal power feed is engaged and the lathe is started, so that, with the work piece rotating, the carriage travels to the left at a rate that is determined by the pitch of the thread, i.e. if the pitch is 10 threads per inch, the carnage advances 1/10 inch for every revolution of the work piece. On arrival at the left-hand limit of the thread the lathe is stopped and this completes the first thread-cutting pass. Then the cutting tool is disengaged from the work piece by means of the cross feed; the carnage is moved manually to a point beyond the right-hand range of the thread; the cutting tool is returned to its last position by means of the cross feed and then fed a small amount further by the compound rest in order to begin the next thread-cutting pass.
A threading tool bit, meticulously designed for cutting unified threads, will have a symmetrical truncated triangular tip with an included angle of 60° with chip clearances for a range of thread pitches; the tool bit must be held perpendicular to the axis of the round to be threaded. And accepted lathe practice calls for the tool bit to be fed into the thread with the compound rest set at an angle of 29° counter-clockwise from the direction that is perpendicular to the axis of the lathe in the case of right-hand external threads. In conformity with the convention established above, this angle is deemed positive. (This is rotated clockwise 61° from its default position of 90°.)
A theoretical justification of this feeding practice might be the following: cutting threads on a lathe entails extremely large cutting forces and tool chatter is sometimes observed with consequent roughness on the left-hand face; but the right hand face, subject as it is with only a light chasing cut, can remain smooth and it is the right face of an external thread that bears the force of a tightened nut. Another justification is that putting most of the cutting action on the left-hand face minimizes the play in the gear train and the lead screw of the power feed mechanism.
This feeding practice may have evolved from experience with no theoretical justification, but it seems to be universally prescribed in shop practice manuals: in the large library of a well-known technical institute the practice is called for in all of the manuals published from 1977 to 1984, which are the latest in the collection. But the practice is not universally accepted: a tool manufacturer represented in the large machine tool catalogs specifies that its internal carbide threading tool be fed in a perpendicular direction. The treatise on metal cutting technology [a] cited in the academic references indicates another manufacturer of carbide threading tools allows feeding in the 29° direction, in an alternating +/−29° zigzag manner and in the perpendicular direction. But the two clinchers are included in the technical paper [b] cited in the academic references: first the author relates that the Computer Numerically Controlled (CNC) lathe used in the study had built-in computer software that fed the threading tool in the 29° direction, in the perpendicular direction or in the +/−29° zigzag manner and then the author stated that the 29° feed led to the best results. He also said that some tool chatter was observed in the tests. It is worth pointing out here that the present invention is not needed on CNC lathes since the computer program moves the tool bit arbitrarily.
So finally
At this point it is well to consider the two principal aims of the novel quick-change threading tool holder and to demonstrate that the novel tool holder achieves these aims by means of geometric principles: the first aim is to insure that the axis of the threading tool is perpendicular to the work piece when the compound rest is rotated through a recommended angle from the perpendicular without having to rotate the tool post which should remain fixed on the compound rest; the second aim is to afford a face on the tool holder that can be used to accurately position the compound rest by butting this face up against a face plate or by mounting the tool holder on an orthogonal tool post face and butting the tool holder face up against the round work piece.
As noted above, internal threading is performed with a cutting tool where the cutting tip is at a right angle to the axis of the cutting tool; this axis should therefore be parallel to the lathe axis. Internal threading also requires that the feed angle be the opposite of that used with external threading and thus the compound rest should be rotated clockwise and its angle becomes −a and the required tool holder angle a is easily achieved by turning the same tool holder that was used for the external threading upside down and then mounting it on the tool post face that is perpendicular to the axis of the compound rest.
Finally, before stating the claims, a number of matters are discussed: there are at least five manufacturers of quick-change tool posts and tool holders and each manufacturer can have four or more sizes. There is some interchangeability between the posts and the holders of the various manufacturers, but this is not guaranteed. Most of the dovetails on the holders have the 45° angle that is shown in the drawings in this patent application, but some dovetails may have other angles. Furthermore there are at least two types of locking mechanisms: piston and wedge. It will be claimed below that this patent covers all tool holder sizes without regard to the angle of the dovetail or, for that matter, without regard to the details of the interface between the post and the holder including the locking mechanism.
It was noted above that there are three different forms of quick-change tool posts and that the invention covered by this patent application is primarily intended for the first form, which is fixed on the compound rest. The second form can be rotated through an arbitrary but fixed number of positions (which always includes 0°, 90°, 180° and 270°). It may be that some tool posts rotate clock-wise and counter-clockwise by an angle of 30°, which would be an acceptable threading angle, but it is very unlikely that rotation of 29° would be achievable, which is the recommended angle. So the invention presented here allows the recommended angle on these tool posts. It is also quite unlikely that a rotation of 14.5°, which is recommended for cutting Acme threads, would be achievable or anything close to this angle. So the present invention is needed to cut Acme threads if the tool post is not to be removed from the compound rest. Similar comments apply to buttress thread.
The third form of quick-change tool post is often called a turret tool post and one, in particular, has 12 positions which would include the 30° rotation noted as acceptable above. But the tools that can be mounted on these turret posts include a dovetail tongue on which the threading tool holder presented here can be mounted; thus the recommended angle of 29° can be achieved. And, clearly Acme threading tool holders can also be mounted on the turret tool holder.
The quick-change threading tool holders that are the subject of this patent application can be used with an un-modified quick-change tool post of the first form, i.e. a tool post with only two mounting faces. In the case of external threads the tool holder is mounted on the right-hand face of the tool post so that the threading tool bit is perpendicular to the work piece with the compound rest rotated 29° counter-clockwise. With this arrangement the tool post itself is to the left of the tool bit tip and its width of roughly 3 inches may interfere with the chuck that is holding the work piece if threading within 3 inches of the chuck is desired.
There is an existing, commercially available, product called a swivel cartridge tool holder that incorporates a carbide insert swivel head or a high-speed steel swivel head, both of which can be rotated horizontally left or right by 15° increments from the axis of the tool shank. The manufacturer recommends the device for left- or right-hand turning, boring, facing or chamfering, but makes no mention of its use for threading when, in fact, it can be used for external threading with the compound rest rotated 30° off the perpendicular and with the device mounted in the standard quick-change tool holder which is, in turn, mounted on the (modified or standard)) quick-change tool post without moving the tool post from its fixed position. So this existing device accomplishes, in a different way, one of the principal aims of the present invention but only in the case of standard, unified external threads, and this only approximately. It cannot handle Acme or buttress threads and, perhaps more importantly, it does not enable the lathe operator to set the compound rest angle precisely without reference to the possibly inaccurate angle scale, which is another principal aim of the present invention. And (since the head cannot swivel more than 90°) the device cannot cut standard internal unified threads with any rotation of the compound rest, or any internal thread, for that matter.
A final comment: the particular shapes of the quick-change threading tool holders shown in the drawings in this patent application are reasonable and probably feasible, but the only crucial feature of the shapes is the horizontal angle between the axis of the tool bit channel or the boring bar receptacle and the mounting face, which shall be 29° in the case of unified threads (or other particular angle that is recommended on the basis of experience or of theoretical considerations to best perform the threading operation). Other aspects of the shapes will be determined by mechanical design considerations such as increasing rigidity or minimizing manufacturing costs.
This application claims the benefit of the U.S. Provisional Patent Application entitled “QUICK-CHANGE THREADING TOOL HOLDER”, Ser. No. 61/169,958, filed Apr. 16, 2009, which is incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61169958 | Apr 2009 | US |
