27-08-2014, 12:13 PM
About HMT Established in 1953. HMT Machine Tools Ltd was formed as a wholly owned subsidiary of HMT Limited in August 1999. Over the years diversified into Watches, Tractors, Printing Machinery, Metal Forming Presses, Die Casting & Plastic Processing Machinery, CNC Systems & Bearings. Manufactured and supplied over 100,000 machines till date for domestic market. Exported over 18,000 machines.
Commonly Used Pattern Materials
Wood (Teak, Deodar etc.) Metal (Aluminium, Cast Iron, Bronze) Thermocol Rubber Epoxy Resin Foundry Shop Foundry is a shop where metal is melted for casting purpose. The melted metal is poured into the mold and left for sometime for solidification. HMT has 3 furnaces, two of 3MT capacity and one of 1.5MT capacity. All furnaces are induction type & temperature in the furnaces reaches up to 1400-1450˚C. Casting In this process, a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify. Small Parts The section is primarily concerned with the manufacturer of small parts of machine tools. The section is further divided into six subsections. Heavy Parts It is the shop which gives the shape to the largest castings which comes from the foundry shop after being painted rough. This Section is divided into two sub-sections Planning Boring Planning: In this section, the job planning is one of the basic operations done for machining process. Boring: Boring is the operation of enlarging an already drilled hole. The holes are predrilled as in casting only. GPM Assembly These are generally manually worked machines involving no computerized or numerical control. These are semiautomatic in nature. Following machines are manufactured in GPM assembly section. In HMT, Pinjore unit primarily four types of GPM are manufactured- FN2 Milling Machine FN3 Milling Machine Turret Ram Milling Machine Broaching Machine Machines are further classified into Horizontal, Vertical and Universal Milling Machines. CNC Assembly Conventionally, an operator decides and adjusts various machines parameters like feed , depth of cut etc depending on type of job , and controls the slide movements by hand. In a CNC Machine functions and slide movements are controlled by motors using computer programs. Various CNC machines produced in HMT Pinjore unit are: HMC 320M, HMC 400M, HMC 500M, HMC 600M, HMC 800M, VMC 400M, VMC 500M, VMC 800M, VMC 1000M etc PROJECT :CASTING DEFECTS AND REMEDIES DEFINITION OF A DEFECT: The terms "defect" and "discontinuity" refer to two specific and separate things in castings. Defects are defined as conditions in a casting that must be corrected or removed, or the casting must be rejected. Discontinuities, also known as "imperfections", are defined as "interruptions in the physical continuity of the casting". Therefore, if the casting is less than perfect, but still useful and in tolerance, the imperfections should be deemed “discontinuities. TYPES OF DEFECTS Shrinkage defects: Shrinkage defects occur when standard feed metal is not available to compensate for shrinkage as the thick metal solidifies. Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere, therefore as the shrinkage cavity forms air compensates. Gas porosity: Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools . Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength in that vicinity. Pouring metal defects: Pouring metal defects include misruns, cold shuts, and inclusions. A misrun occurs when the liquid metal does not completely fill the mold cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mould cavity, leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or cross-sections that are too narrow. Metallurgical defects: There are two defects in this category: hot tears and hot spots. Hot tears, also known as hot cracking, are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. Proper mold design prevents this type of defect. CLASSIFICATION OF DEFECTS EXTERNAL DEFECTS Blow holes Cracks/hot tear Metal penetration Scabs Fins crushes Defects due to broken moulds Misrun/cold shut Shrinkage cavities INTERNAL DEFECTS Blow holes Pin holes Sand and dirt inclusions Hard spots Open-grains Shrinkage cavities CRUSHES : These defects occur on the surface of casting due to displacement of sand at mould joints or core-prints, leading to irregular shaped cavities or projection on the castings. Causes: Worn pattern or misaligned cope and drag pattern. Low green strength of sand. Badly made mould joints. Inaccurately placed cores. Controls: Proper pattern to be used. Proper clamping of moulds. Use of sand with adequate green strength. Proper placing of the cores. CUTS OR WASHES : Erosion of mould or core by metal over flowing results in ‘cuts’ and ‘washes’ which appear as rough spots. Causes: Excessive metal flow over given portion of the mould surface. Improper gating. Controls: Ensuring the uniform metal flow. Good gating. DROPS: This defect occurs due to a portion of the mould breaking and dropping into the metal. This may occur when the sand mix has a low green strength or it has not been properly rammed or reinforced. Casuses: Improper ramming. Careless hamdling. Worn pattern or patched moulds Controls: Proper ramming. Careful handling FINNING: Joint flash is also called as casting fin, which is a thin projection out of surface of metal castings. Causes: Improper pattern allowing space for fins. High metal pressure induced by gating system Controls: Lower the pouring temp. Use of proper core BLOW HOLES: These appear as rounded cavities either spherical, flattened by accumulation of gases. Causes: Entrapped air or insufficient venting.b Insufficient sand permeability and improper sand mixing. Excessive gas generation from moulding or core sand Controls: Inc. the no. of vent holes. Increase sand permeability. Checked degree of ramming. Avoid turbulent metal flow GAS HOLES Air or other mould gases are sometimes entrapped within the metal due to turbulent flow of metal during pouring. When the metal solidifies this air or gas remains in the metal as a void. Causes: Insufficient and improper venting. Shallow cope. High moisture content in sand. Under baked cores Controls: Proper degassing of metal. Use dry pouring ladles. Dry the core moulds HARD SPOTS Hard spots are caused by chilling action of moulding sands in some metals like gray cast iron with insufficient silicon. These spots are extremely hard and often lead to machining difficulties. Hard spots are avoided by providing uniform cooling and pouring at the right temperature. HOT TEARS AND CRACKS Hot tears are ragged irregular internal or external cracks occurring immediately after the metal have solidified. Causes: Non-uniform section thickness. Insufficient feeders. Improperly placed gates and risers. Use of improper sand Controls: Maintain uniform section size in casting. Allow sufficient cooling time METAL PENETRATION If the sand grains used are very coarse or the metal poured has very high temperature the metal is able to enter the spaces between sand grains to some distance. Causes: Poor moisture control. Soft ramming. Impurities in sand mixture. High pouring temp Controls: Reduce pouring temp. Correct sand composition should be maintained MISRUNS A misrun is caused when the section thickness of a casting is so small or the pouring temperature so low that the entire section is not filled before the metal solidifies. Causes: Insufficient metal fluidity due to low metal temp. Improper gating. Inadequate venting of moulds Controls: High pouring temp. Inc. pouring rate COLD SHUTS Cold shut is caused when two streams of metal which are too cold meet but do not fuse together. Causes: Inadequate venting of moulds. Non uniform casting section. Use of sand with high moisture Controls: Inc the die temp. Alter the metal composition if necessary. OXIDES AND DROSS INCLUSIONS: Inclusions are any foreign materials present in the cast metal. These may be in the form of oxides, slag, dirt, sand or nails. Common sources of these inclusions are impurities with the molten metal, sand and dirt from the mould not properly cleaned, break away sand from mould, etc. Causes: Turbulence due to improper gating pattern. Use of dirty and contaminated ladles. Low pouring temperature. Controls: Avoid turbulence by proper ingate design. Use properly designed running system SHIFTS AND CROSS JOINTS (MISMATCH A mismatch is caused by the cope and drag parts of the mould not remaining in their proper position. This is caused by loose box pins, inaccurate pattern dowel pins or carelessness in placing the cope on the drag