Work Holding Methods By Tom Brown, Matt Charlton and Andy Faulkes
Work Holding Methods The three most commonly used work holding devices are: • Three-jaw chuck • Four-jaw chuck • Face plate • Between centres
What type of work can be held? • 3 jaw chuck used for easy fitting of circular components • 4 jaw chuck can be used for non-round components, or for turning on an eccentric centre • Long jobs can be suspended and turned between centres • Large jobs can be held with jaw attachments on a faceplate
3 Jaw Chuck A chuck with three jaws has a simultaneous movement to centre the job as the chuck jaws tighten.
The three-jaw chuck is used to hold round work pieces of various diameters as the component is centered in the chuck before machining. Very common. Quick and easy to use. Self centering. Advantages of using 3 jaw chucks
Disadvantages of using 3 jaw chucks Although the operator can set the machine up quickly, the job will never be perfectly concentric. If re-work on a finished component is needed, it is hard to align the work back to its original axes in the jaws. Sometimes can damage job if tightened without soft jaws.
4 Jaw Chuck • The four-jaw chuck is used to hold irregular shapes because of its independent action on each jaw. • Heavy to transport. • Accurate with concentricity when using DTI and magnetic base. • Timely set up.
Face Plate • The work piece is clamped to the faceplate, typically using t-nuts in slots in the faceplate, or less commonly threaded holes in the faceplate itself.
Face plate • The face plate is also used for irregular shapes that need to be through-bored. • Used on vertical lathes for holding large jobs. • The job can be clamped concentric when using a DTI. • Timely set up.
Turning Between Centres • Initially requires the use of a centre drill, used in both ends of (a faced) component • Chuck is removed and a morse taper sleeve with dead centre is fitted to the head of the lathe • Faceplate with threaded peg fitted to lathe head • Dead centre fitted to the tail stock (with lubrication) • Lathe carrier attached to the job, which is then supported between the centres • As the lathe rotates, the threaded peg pushes against the carrier and rotates the job at normal lathe speed
Turning Between Centres Faceplate with threaded peg Job Dead centre /w morse sleeve Lathe carrier
Turning Between Centres • Positives: • Can machine much more of the job in one process – no real minimum protruding length as with a chuck • Allows the full length of the job to be machined without difficulty (although flipping the job and moving the carrier will be neccesary) • Job will always be centred when removed and refitted
Turning Between Centres • Negatives: • Lubricant important – can be easy to overheat the dead centre in tail stock end • Sometimes hard to achieve good finish due to vibrations from imbalanced rotating parts • Heavy duty processes not possible – such as knurling – normal chuck required • Precise tail stock alignment needed, or job will end up tapered