Velocity of Metal flow in a Sprue - Metal Casting

VELOCITY OF METAL FLOW IN A SPRUE

Green sand casting is one of the simplest and most popular casting processes. The following figure shows the schematic diagram of a simple green sand mold together with all the components of the gating system. 

The gating system of a sand mold consists of pouring basin, sprue, sprue basin, runner, gates, and runner extension. The design of the gating system mainly involves considerations of fluid flow. To avoid formation of casting defects, it is necessary to control the rate of mold filling. Too fast a metal flow causes air entrapment, porosity and dross formation, and erosion of sand, whereas too slow a flow causes the metal to solidify prematurely, yielding defects such as misrun (an incomplete casting) and cold laps (inhomogeneous or layered casting surface). A well-designed gating system evenly distributes the incoming metal to all parts of the mold without causing turbulence or sand erosion.

A pouring basin is either rectangular or square in cross-section, with a fiat base and a fillet at the base near the sprue entrance. Spherical pouring basins with a curved base cause vortex flow and casting defects. A sprue connects the pouring basin to the runner. A sprue is usually tapered to allow for downward laminar flow, and the sprue basin provides space for the metal to dissipate some energy before changing its direction as it flows into the runners.

EXAMPLE

The figure shows a tapered sprue with the metal head H₁ in the pouring basin, and the total metal head, H₂. The cross-sectional areas at the top and the bottom of the sprue are A₁ and A₂. Determine the metal flow velocity at the top and bottom of the sprue. Also derive a relation between heads H₁, H₂ and cross-sectional areas A₁, A₂ of the sprue.

SOLUTION

From the basic fluid mechanics, using Bernoulli’s Principle, we have

The velocity of fluid (v) at a cross-section with head ‘h’ is given by

Metal head at the top of sprue is H₁ and metal head at the bottom of sprue is H₂.

Similarly, considering the metal flow in the gating system,

The metal flow velocity at the top (v_top) of the sprue is given by

The metal flow velocity at the bottom (v_bottom) of the sprue is given by

We know that the fluid mass crossing the areas A₁ and A₂ per unit time is constant i.e. the metal mass flow rate is constant.

So, the mass flow rate (ṁ) at the top of sprue is equal to that at the bottom of the sprue.

Mass flow rate (ṁ) = velocity × density × Area of cross-section

This yields the fundamental relationship A₁√H₁ = A₂√H₂   .

About the Lathe - Introduction of Centre Lathe

About the Lathe

THE METAL LATHE

The lathe is a machine tool in which a piece of metal, can be held and rotated while it is being shaped by a cutting toot. The Lathe is capable of producing cylindrical, conical, helical and spherical shapes. This type of lathe is often referred to as a centre lathe. Small machines are usually mounted on a bench or a stand are sometimes called bench lathes.

THE BED

The diagram above shows a sectioned view of the lathe bed which is well ribbed cast iron. The top of the bed is machined to form 'ways' upon which the saddle and tailstock slide. Most lathes have ' flame hardened' ways to resist ware and maintain accuracy through the life of the machine.

THE HEADSTOCK

The headstock is bolted securely to the 'inner ways' on the left hand end of the bed. The headstock provides a method of holding and rotating the work piece to be machined. It has several distinct parts which perform important operations. These parts include such operations as providing different turning speeds, a set of gears which enable automatic movement of the saddle along the ways and a mechanism for automatic thread cutting.

CARRIAGE

The carriage is the part of the lathe that supports and controls the movement of the cutting tool. The main parts of the carriage are the saddle and the apron. The apron is fastened to the saddle and hangs in front of the bed. The apron contains the hand wheel and lever for hand feed, and a clasp nut for automatic feed and thread cutting. The cross slide is fitted for cross feed of the cutting tool and is operated by hand feed.

THE TAILSTOCK

The tailstock is made up of 2 castings. One rests on the ways and may be clamped to the bed in any position so that work can be performed on jobs of different lengths. The other casting is fitted to the lower one and has a sideways movement that can be used for taper turning. The tailstock spindle is tapered to take large tapered drills and drill chucks etc. The spindle is moved in and out by a hand wheel and screw.

THE COMPOUND SLIDE

The Compound Slide rest is mounted on top of the cross slide and it has swivel base that rotates through 360 degrees. The compound slide holds the tool post which is moved by a feed screw.

The compound slide is also used to control the path of the cutting tool when short tapers are being turned in the lathe. The diagram below shows the compound set to 30 degrees. This means that the cut will be made at 30 degrees to the axis of the lathe.

TOOL POST

The tool post is a device for firmly clamping the cutting tool or tool holder to the lathe. The tool post is fixed to the top of the compound slide. Below is a standard tool post and the square tool post which can hold 4 different cutting tools.

THREE JAW CHUCK

The three Jaw Chuck is 'self-centering’. The jaws all move together and are always the same distance from the centre. This chuck is designed to hold round or hexagonal work. The movement of the jaws is controlled by a scroll plate which is rotated by a chuck key. The drawing below shows a 3 jaw self-centering chuck.

FOUR JAW CHUCK

The 4 jaw chuck has jaws that can be moved independently of each other. This chuck is used to hold irregular or odd shaped work. It requires careful setting up. It holds work firmer and work can be made to run perfectly true with the aid of a dial indicator.

TOOL HOLDERS

There are straight, left, right styles of tool holders. The straight is for general turning, the right hand for facing operations and for turning work close to tailstock and the left hand for turning work close to the headstock.

LATHE TOOLS

The cutting tools or bits commonly used in M.D.T. workshops are made from high speed steel. They are available in 5 mm to 25 mm square section and 50 mm to 150 mm lengths.

The shape of the cutting tool is very important. The tool must be ground so that it is sharp enough to force its way into the work but must retain sufficient material behind the cutting edge so that it can withstand the pressure on it when it is cutting.

The following diagram illustrates the angles and clearances that should be considered when grinding a tool for a specific job.

CENTRES

The headstock and tailstock spindles are bored out to a standard morse taper to receive headstock and tailstock centres whose shanks are also morse tapers. The points are ground to an angle of 60 degrees to fit the countersink section of a centre drill.

The tailstock centre supports the right hand end of the work. The work rotates on the bearing surface of this centre so it must be hardened and well lubricated.

Live centres are used in the tailstock and have built - in roller or ball bearings which enable the point of the centre to rotate with the work.

CENTRE DRILL

Work that is to be supported by a centre must be drilled to fit the centre and to fit the bearing surface. This is usually done with a centre drill which is a combination drill and countersink drill. The cone angle matches the lathe centres.