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Spun Glass Blankets for Press Stretching

Spun Glass Blankets for Press Stretching MANUFACTURE Spun Glass Blankets for Disadvantages of Grease However, though this method solved one prob­ Press Stretching lem, it immediately produced several new ones. It was very unsatisfactory because of the messy and dangerous working conditions created by the large quantity of grease deposited on the material and A New Technique Developed at the Glenn Martin the press as well as the dangerously slippery con­ dition of the floor and walkway caused by the Factory at Baltimore to Overcome Difficulties in accumulation of excess lubricant. The handling of the formed parts in a greasy condition was the Hot Forming of Magnesium Sheets very hazardous to the workers and provided an opportunity for hand cuts and bodily injury resulting from slips and falls. Another objection of great importance was the fact that grease re­ some cases adhesion also caused ruptures in the SPU N glass, woven into thin blankets, is being moval proved to be a costly and time-consuming surface of the sheet or damage to the forming die used by the Glenn L. Martin Company, processing operation. if some separating medium were not placed Baltimore, in the stretch forming of mag­ between the part and the form block. nesium sheets as a successful substitute for other, The practice often used in industry for the less efficient materials. solution of this problem is to paint the material The Solution for Aluminium The Problem to be formed and the stretching block as well with The substitution of a sheet of rubber for the a heavy coating of grease. With the presence of Originally, in the stretch forming of sheet grease effected favourable results and continues aluminium, adhesion between the surfaces of the grease between the stretch block and the material, to be a satisfactory solution for the cold-forming sheet and the stretch block caused an uneven co-extensive movement becomes possible in the of aluminium sheets. However, with the advent flow of the metal, producing an unsatisfactory, sheet metal. That is, the desired extension is of magnesium the necessity for hot-forming this irregular, wavy-surfaced finished product. In effected without excessive friction. material made the use of rubber impracticable because it smoked and smudged under the heat. Reversion to grease, oil, graphite or other lubi- cants was equally undesirable in a hot-forming operation. Oil, for example, had a flash point of 425 deg. F. The others brought back the old, wasteful uneconomical and dangerous condi­ tions which the company had sought to avoid from the beginning. After considerable investigation by Martin manufacturing research engineers, R. E. Berger and H. P. Hessler, a spun glass blanket, 0.027 in. thick, was found to be the perfect solution to all problems—far superior in use to all lubricants as well as to rubber. In fact, experiments proved it to be unique in this particular application. For example, in a comparison of two forming opera­ tions, identical except in the substitution of the glass blanket for graphite, it was demonstrated that 70.1 minutes were consumed using the old method and only 4.9 minutes with the glass blanket—a net saving of 65.2 minutes in favour of the new Martin development. Properties of the Blanket Experience with the glass blanket shows that its high effectiveness is due to: (a) Its heat resistance. (b) Its very high tensile strength when drawn TABLE I: COMPARATIVE TIME ANALYSIS OF GLASS CLOTH vs GRAPHITE AS A FRICTION REDUCING AGENT IN tight. STRETCH FORMING MAGNESIUM (c) The hard, smooth surface which the fibres of the glass blanket present to both form block Operations Required using Graphite Time Operations Required using Time Part description Method required Glass Cloth Method required and sheet metal, thus promoting a sliding action between them. Magnesium Skin (1) Spray parts with graphite 50 0 min. (1) Heat form block to 550°F. initially 0∙20 min. (d) The relatively low thermal conductivity 0∙125in.x4g.x72in . (Connect terminals and throw switch)* of the blanket (when heated sheet metal and un- (2) Paint form block with graphite 0∙40 min. (2) Place skin between pre-heat platens 1∙50 min. heated form block is used). for heating to 650° F. (c) An accompanying passivity of the glass (3) Heat form block to 550° F . initially 0∙20 min. (3) Place glass cloth blanket over form 0∙20 min. cloth which permits it to elongate partially with (Connect terminals and throw switch) block the sheet metal during the stretching operation; (4) Place skin between pre-heat platens 1∙50 min. (4) Remove skin from pre-heat platens 20 0 min. this being due to the natural flexibility and loose­ for heating t o 650" F . and clamp in stretch press ness of the weave. (5) Remove skin from pre-heat platens 2∙00 min. (5) Stretch form to proper contour 0∙50 min. (f) Practicability of stretch-forming heated and clamp in stretch press. sheet metal over an unheated form block since (6) Stretch form to proper contour 0∙50 min. (6) Remove from stretch press and stack 0∙50 min. the glass blanket acts as a heat resistant insulator, retarding transfer and loss of heat from material (7) Remove from stretch press and stack 0∙50 min. to form block. (8) Remove graphite lubricant by 60∙00 min. Use of the glass cloth thus far at the Martin scrubbing in chromic acid-calcium nitrate bath and stackt Company indicates that it (1) facilitates produc­ tion, (2) eliminates operations, (3) produces more Total time per part 70∙1 min. Total time per part 4∙9 min. satisfactory parts. With further experience in this practice more data will become available on the •In many cases healing of the form block is unnecessary because of Ihe insulating effect of the glass cloth which permits retention of the heat in the part being formed. life expectancy of the spun glass blankets when †AI1 lubricant must be removed from the part or the graphite particles will form individual galvanic cells o n the magnesium surface and utilized under these conditions. thereby induce corrosion. October 1948 315 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

Spun Glass Blankets for Press Stretching

Aircraft Engineering and Aerospace Technology , Volume 20 (10): 1 – Oct 1, 1948

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb031682
Publisher site
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Abstract

MANUFACTURE Spun Glass Blankets for Disadvantages of Grease However, though this method solved one prob­ Press Stretching lem, it immediately produced several new ones. It was very unsatisfactory because of the messy and dangerous working conditions created by the large quantity of grease deposited on the material and A New Technique Developed at the Glenn Martin the press as well as the dangerously slippery con­ dition of the floor and walkway caused by the Factory at Baltimore to Overcome Difficulties in accumulation of excess lubricant. The handling of the formed parts in a greasy condition was the Hot Forming of Magnesium Sheets very hazardous to the workers and provided an opportunity for hand cuts and bodily injury resulting from slips and falls. Another objection of great importance was the fact that grease re­ some cases adhesion also caused ruptures in the SPU N glass, woven into thin blankets, is being moval proved to be a costly and time-consuming surface of the sheet or damage to the forming die used by the Glenn L. Martin Company, processing operation. if some separating medium were not placed Baltimore, in the stretch forming of mag­ between the part and the form block. nesium sheets as a successful substitute for other, The practice often used in industry for the less efficient materials. solution of this problem is to paint the material The Solution for Aluminium The Problem to be formed and the stretching block as well with The substitution of a sheet of rubber for the a heavy coating of grease. With the presence of Originally, in the stretch forming of sheet grease effected favourable results and continues aluminium, adhesion between the surfaces of the grease between the stretch block and the material, to be a satisfactory solution for the cold-forming sheet and the stretch block caused an uneven co-extensive movement becomes possible in the of aluminium sheets. However, with the advent flow of the metal, producing an unsatisfactory, sheet metal. That is, the desired extension is of magnesium the necessity for hot-forming this irregular, wavy-surfaced finished product. In effected without excessive friction. material made the use of rubber impracticable because it smoked and smudged under the heat. Reversion to grease, oil, graphite or other lubi- cants was equally undesirable in a hot-forming operation. Oil, for example, had a flash point of 425 deg. F. The others brought back the old, wasteful uneconomical and dangerous condi­ tions which the company had sought to avoid from the beginning. After considerable investigation by Martin manufacturing research engineers, R. E. Berger and H. P. Hessler, a spun glass blanket, 0.027 in. thick, was found to be the perfect solution to all problems—far superior in use to all lubricants as well as to rubber. In fact, experiments proved it to be unique in this particular application. For example, in a comparison of two forming opera­ tions, identical except in the substitution of the glass blanket for graphite, it was demonstrated that 70.1 minutes were consumed using the old method and only 4.9 minutes with the glass blanket—a net saving of 65.2 minutes in favour of the new Martin development. Properties of the Blanket Experience with the glass blanket shows that its high effectiveness is due to: (a) Its heat resistance. (b) Its very high tensile strength when drawn TABLE I: COMPARATIVE TIME ANALYSIS OF GLASS CLOTH vs GRAPHITE AS A FRICTION REDUCING AGENT IN tight. STRETCH FORMING MAGNESIUM (c) The hard, smooth surface which the fibres of the glass blanket present to both form block Operations Required using Graphite Time Operations Required using Time Part description Method required Glass Cloth Method required and sheet metal, thus promoting a sliding action between them. Magnesium Skin (1) Spray parts with graphite 50 0 min. (1) Heat form block to 550°F. initially 0∙20 min. (d) The relatively low thermal conductivity 0∙125in.x4g.x72in . (Connect terminals and throw switch)* of the blanket (when heated sheet metal and un- (2) Paint form block with graphite 0∙40 min. (2) Place skin between pre-heat platens 1∙50 min. heated form block is used). for heating to 650° F. (c) An accompanying passivity of the glass (3) Heat form block to 550° F . initially 0∙20 min. (3) Place glass cloth blanket over form 0∙20 min. cloth which permits it to elongate partially with (Connect terminals and throw switch) block the sheet metal during the stretching operation; (4) Place skin between pre-heat platens 1∙50 min. (4) Remove skin from pre-heat platens 20 0 min. this being due to the natural flexibility and loose­ for heating t o 650" F . and clamp in stretch press ness of the weave. (5) Remove skin from pre-heat platens 2∙00 min. (5) Stretch form to proper contour 0∙50 min. (f) Practicability of stretch-forming heated and clamp in stretch press. sheet metal over an unheated form block since (6) Stretch form to proper contour 0∙50 min. (6) Remove from stretch press and stack 0∙50 min. the glass blanket acts as a heat resistant insulator, retarding transfer and loss of heat from material (7) Remove from stretch press and stack 0∙50 min. to form block. (8) Remove graphite lubricant by 60∙00 min. Use of the glass cloth thus far at the Martin scrubbing in chromic acid-calcium nitrate bath and stackt Company indicates that it (1) facilitates produc­ tion, (2) eliminates operations, (3) produces more Total time per part 70∙1 min. Total time per part 4∙9 min. satisfactory parts. With further experience in this practice more data will become available on the •In many cases healing of the form block is unnecessary because of Ihe insulating effect of the glass cloth which permits retention of the heat in the part being formed. life expectancy of the spun glass blankets when †AI1 lubricant must be removed from the part or the graphite particles will form individual galvanic cells o n the magnesium surface and utilized under these conditions. thereby induce corrosion. October 1948 315

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Oct 1, 1948

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