Then mount the work between the live and dead centers of the lathe. CENTERING THE WORK.—To center round stock where the ends are to be turned and must be concentric with the unturned body, mount the work on the head spindle in a universal chuck or a draw-in collet chuck If the work is long and too large to pass through the spindle, use a. Feb 18, 2020 - Sometimes you just need to laugh. See more ideas about machine shop, humor, mechanic humor. Customers trust us to provide the critical repairs to their mills, lathes, iron workers, benders, and other machinery that keep production running. When a piece of equipment is down or not functioning properly, an experienced technician can make a visit to the customer’s facility in order to make an initial diagnosis.
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Lathe – centering a workpiece on a four jaw chuck
With a three jaw chuck the chuck determines the centering of the workpiece. It is seldom perfect but is often good enough. When it isn’t good enough the solution is to use a four jaw chuck. But, of course, the user has to align the workpiece in the chuck.
There are three basic situations where this might happen:
the workpiece is round
the workpiece is rectangular or square
a punch mark on the workpiece has to be aligned with a center in the tailstock.
The key tools for centering a workpiece in a four jaw chuck
All centering on the lathe is made much easier with the aid of the right tooling. This is not a DTI on a magnetic stand. It is a DTI fitted to a bar so it can be held in a quick change toolholder. This is because this tool is frequently used and it is worth a quickchange toolholder so that it is always ready for use.
The DTI can be either metric or imperial. But it must not be too sensitive and it must have a sufficinet range. For example it can be seen that a DTI with a sensitivity of 0.01mm per division and a range of 10mm is used here. It is assumed that there is a secondary scale showing whole millimetres.
Notice that the dti is clamped it is not held with a grub screw.
The DTI is fitted with an extension bit so that it can touch the surface of the workpiece not only when has a small diameter but also when the workpiece is short and the dti has to work right up against the chuck.
3301 centering a 4 jaw chuck
It is also necessary that the tip of the DTI is at the center height of the lathe.
The other tool needed is second chuck key. This key is different in that it can be used on a socket when it is farthest away from the user. Because of the distance and because the space is limited this key is shorter and narrower than the main key. It is not suitable for tightening up a screw but is used when adjusting an opposing pair of screws.
Both of these are always needed but there is another tool that is needed in the case of centering a punch mark or a center in the workpiece. This is a piece of round metal rod, steel is fine. This is about 150mm long. on one end it has a male 60º center and at the other it has a female 60º center. Needless to say both centers must be concentric with the rod.
Centering
In all cases when centering the workpiece it is moved by means of the screws on the chuck. However when measuring any deviation this can only be done when all four screws are reasonably tight. For example, a round bar might be centering in the horizontal plane but if the vertical screws are loose it may be tilted in the vertical plane.
In all cases, in general when one screw is loosened the one opposite will have to be tightened to move the workpiece and to hold it securely. But for the last hundreth or so the movement required can often be achieved simply by tightening one screw though very tightly.
Centering a round workpiece in a four jaw chuck
There are several videos on YouTube showing how to do this in an apparently effortless manner. What the viewer might not realise is that the key to this is to align the workpiece quite accurately before using a DTI.
Usually when centering a round part, the DTI would be near the chuck end of of the workpiece.
The difference here is that in real life when a round workpiece is fitted in the chuck it will be off center. Sometimes it might be quite accurate but it is quicker to align it as accurately as possible by sight and then to not assume it is accurate enough. What usually happens is as the workpiece is rotated the needle on the DTI wizzes all over the place.
The method here is to ignore the big needle and to use the small needle on the small dial.
With one pair a screws horizontal, just touch the surface with the DTI. Move the cross slide till the small needle touches the 5 on the small dial.
3302 centering a 4 jaw chuck
Rotate the chuck by 180º. Usually the reading will now be between 0 and 10 on the small dial. The workpiece is moved horizontally to halfway between the new reading and the previous ie 5.
If the workpiece is out in both the vertical and horizontal directions it might not be possible to make a big adjustment in the horizontal plane because the workpiece will catch on the vertical jaws. One of these might need to be loosened to let the workpiece through. However at all time when a reading is being made all four jaws must be tightened though not to their final tightness.
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This will align the workpiece in this direction to within the normal range of the DTI.
The chuck is rotated by 90º and the workpiece is again centered using the small dial.
The whole process is repeated again but this time using the large needle and large dial.
This should get the workpiece centered to within a few hundredths. The final adjustments can be made simply be the tightness of the screws.
Centering a square/rectangular workpiece
In these cases the problem is that the DTI cannot touch the workpiece continuously as it rotates. Each time the surface changes, the DTI has to be moved to the right, the workpiece rotates and then the DTI can be moved back. Very often, as the DTI moves back, the probe as the be pulled back using your fingers. If the DTI is on a magnetic stand this can often alter the readings. This is one reason why the clamp as described above is recommended.
In the case of a square piece unless it is square attempting to center it accurately is probably pointless. Square means alle angles are at 90º and all four sides are the same length across. In the case of a rectangular piece, all four corner are 90º and both opposite pairs of sides are parallel.
There are at least two distinct ways of doing this:
Method 1 – using the DTI
Method 2 – using a height gauge
Centering a workpiece with a punch mark of center in it
The simple way of doing this is by fitting a center in the tailstock and moving the workpiece till the punch mark or center fits the center in the tailstock. In this case the workpiece can be any shape that can be held in a four jaw chuck so long as the mark can be aligned with the tailstock.
Alternatively, it can also be aligned by using the special rod and the DTI. The workpiece is fitted in the chuck. It is align as well as possible by eye. The rod is then fitted into the mark or center in the workpiece at the workpiece end and onto a center mounted in the tailstock.
The DTI is set so it touches the rod near the workpiece.
fig aligning a punch mark or center
Any error in alignment shows up on the DTI as the workpiece is rotated. Alignment is achieved by adjusting the chuck until the error on the DTI is zero.
A lathe center, often shortened to center, is a tool that has been ground to a point to accurately position a workpiece on an axis. They usually have an included angle of 60°, but in heavy machining situations an angle of 75° is used.[1]
The primary use of a center is to ensure concentric work is produced; this allows the workpiece to be transferred between machining (or inspection) operations without any loss of accuracy. A part may be turned in a lathe, sent off for hardening and tempering and then ground between centers in a cylindrical grinder. The preservation of concentricity between the turning and grinding operations is crucial for quality work.
A center is also used to support longer workpieces where the cutting forces would deflect the work excessively, reducing the finish and accuracy of the workpiece, or creating a hazardous situation.
A center lathe has applications anywhere that a centered workpiece may be used; this is not limited to lathe usage but may include setups in dividing heads, cylindrical grinders, tool and cutter grinders or other related equipment. The term between centers refers to any machining operation where the job needs to be performed using centers.
Lathe: Work Centering The Mechanic Resume
A center is inserted into a matching hole drilled by a center drill. The hole is conical near and at the surface, and cylindrical, deeper.
Dead center (and live center)[edit]
A dead center (one that does not turn freely, i.e., dead) may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece. Lubrication is therefore required between the center and workpiece to prevent friction welding from occurring. Additionally the tip of the center may have an insert of cemented carbide which will reduce the friction slightly and allow for faster speeds. Dead centers are typically fully hardened to prevent damage to the important mating surfaces of the taper and to preserve the 60° angle of the nose. As tungsten carbide is much harder than steel, a carbide-tipped center has greater wear resistance than a steel center.
When turning between centres, a 'dead centre' is used in the headstock as well as the tailstock. As the one in the headstock revolves with the work, this centre is known as a live centre.
Soft center[edit]
Soft centers are a special version of the dead center in which the nose is deliberately left soft (unhardened) so that it may be readily machined to the correct angle prior to usage. This operation is performed on the headstock center to ensure that the center's axis is aligned with the spindle's axis.
Lathe: Work Centeringthe Mechanic
Running or revolving center[edit]
A revolving center, also known as a rotating center or running center in some countries, is constructed so that the 60° center runs in its own bearings and is used at the non-driven or tailstock end of a machine.[2] It allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures. CNC lathes use this type of center almost exclusively and they may be used for general machining operations as well. Spring-loaded centers are designed to compensate for center variations, without damage to the work piece or center tip. This assures the operator of uniform constant tension while machining. Some live centers also have interchangeable shafts. This is valuable when situations require a design other than a 60° male tip. A live center, which may be hard or soft, is a plain center placed in the revolving mandrel; it moves and is therefore live.
Pipe center[edit]
A pipe center, also known as a bull nose center is a type of live center which has a large diameter conical nose rather than a sharp point. This allows the center to be used in the bore of a pipe or other workpiece with a large interior diameter. While a pipe center ensures the workpiece remains concentric, its main advantage is that it supports the workpiece securely, and can be used for parts whose larger inner diameter prevents the use of a normal pointed center. Thin-walled material such as pipes easily collapses if excessive force is used at the chuck end.
Cup center[edit]
There are two types of cup centers. The woodworking variety is a variation of the traditional live center. This type of cup center has a central point like a normal live center and also has a ring surrounding it. The ring supports the softer material around the center point and prevents the wood from splitting under pressure from the central point. A different variety of cup center is used for metalworking. The metalworking variety of cup center has a tapered hole rather than a conical point. It supports the part by making contact with the outside diameter of the end of the part, rather than using a center hole.
Drive center[edit]
Lathe: Work Centering The Mechanic Job
A drive center, also known as a grip center, is used in the driving end of a machine (headstock). It is often used in woodworking or where softer materials are machined.
Lathe: Work Centering The Mechanics
It consists of a dead center surrounded by hardened teeth, which bite into a softer workpiece allowing the workpiece to be driven directly by the center. This allows the full diameter of the workpiece to be machined in a single operation, in contrast with the usual requirement where a carrier is attached to the workpiece at the driven end. The use of modified shell end mills in a drive center, instead of hardened pins, enables better gripping and prevents breakdown time due to pin stop.[citation needed]
Spring center[edit]
A spring center is a metalworking lathe center for maintaining a cutting tool like a reamer or a tap, in axial alignment with a hole being worked on. It consists of a point backed by a spring to push the cutting tool into the workpiece.
References[edit]
- ^International library of technology: Drilling; Lathe work; Boring-mill work; Working chilled iron; Bench, vise, and floor work; Erecting, 138, International Textbook, 1914, p. 21.
- ^Culley, Ron (1997) [1988]. Fitting and machining. PO Box 12477, Melbourne, Victoria: RMIT Publications. ISBN0-7241-3819-6.CS1 maint: location (link)
Lathe: Work Centering The Mechanic Machine