TPT March 2012

T echnology N ews

Tube coatings WHEN seeking high wear consumable tooling in rotary draw bending, the obvious items are internal mandrels and external wiper dies. These tools are in constant friction and under substantial load and pressure to support the tube in the bending process. As the severity of the bend increases (tube OD gets larger, wall thickness gets thinner and/or the centreline bend radius gets tighter) the support needed from these tools increases as does the pressure exerted on them. As these dies are in a fixed static position relative to the tube being bent, it is imperative that close attention is made to the materials these dies are made from and the type and amount of lubrication applied. As the tube materials change so should the materials for the mandrels and wiper dies as well as the lubrication type. Unfortunately, in many cases, especially in a high production environment, decisions on mandrel and wiper material type get based on an attempt at using one solution for a myriad of variables – the misconception goes, if we use a soft material for these dies, they cannot scratch the tube regardless of the material type, or even if the lubrication selection (or amount) is inadequate, intermittent or even lacking altogether. Enter the old stand- by, aluminium bronze. There is no question that the use of this material for wipers and mandrels works (and for forming certain materials such as titanium and nickel alloys it is the material of choice) but generally speaking it is never the most cost effective solution. It should be noted that as the bronze material requires no post machining heat treating, that is possible to get replacement dies more quickly. However, the raw material cost fluctuates wildly and in a high tool changeover environment this volatile pricing for a consumable item radically affects profit margins. Recent pricing of €7.50 per pound for Ampco (aluminium bronze) has been reported. There is little chance to salvage a mandrel due to wear once it is too undersized to properly support the tube; it is simply scrap material at that point. It stands to reason that a harder material that will hold up longer is potentially a better consideration. On the other end of the spectrum there are numerous coatings over hardened steel that can have merit in solving the tool cost versus bending production equation. Many of these coatings have been used effectively with a variety of tube materials but have other drawbacks that in many cases are also too costly to bear. It is not hard to understand that in addition to the base cost of the tool steel that all the process costs add up substantially. The other obvious issue is all the manufacturing steps take additional time to execute. In most cases a simple solution is the best. A plating process known as industrial hard chrome (hexavalent chrome) has been used effectively for bending tooling for almost as long as ball mandrels have been used in tube fabrication. Hard chrome should not be misunderstood as its cousin decorative chrome (trivalent chrome). Hard chrome when properly applied should have a minimum and uniform thickness of 0.001", with a preferred minimum of 0.002" on both sides of the tool.

C ONCEPTION ET F ABRICATIOND’ I NNOVATION M ECANIQUE

Specialist in tube machining.

PiPe Bevelling machine PiPe cutting machine PiPe facing machine

Join us at tuBE DüssElDorf, Germany HALL 6/Stand n°6H42 26-30 March 2012 CoME&sEEnEWtECHnoloGY inPiPE CuttinG&BEVElinG EQuiPMEnt

Tel : +33 476 656 389 - Fax : +33 476 659 472 www. COFIM.fr - info@cofim.fr

Tools for Bending – USA Website: www.toolsforbending.com

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M arch 2012