Mandrel bending technology
Mandrel is one of the main elements for tube bending and machines to work with mandrel are called "mandrel tube bending machines". There are other machines called "mandreless" which means that they cannot use mandrel.
Mainly historically bending of tubes were performed with filler, usually sand. Sand was filled inside the tube, both ends welded or plugged and after the bending was perfomed. Mandrel plays a role of the filler in tube bending technology and helps to make more controlled deformation and prevent the problems and ovality of tube after the bend. Also mandrel stabilizes the tube pressure of external wall during the bending process.
Position of mandrel is adjusted on tube bending machine and various types help to perform even very complicated bends, thin wall tubing, small radiuses and hard materials. Important also to note that the bending with mandrel is used in "draw bending" tube technology.
Based on various tube materials different material of mandrels is used. Today for technology and all the producers for all hard materials such as stainless steel mandrels made of aluminium-bronze alloy are used.
Types of mandrels
MAN-F: Solid mandrel flat nose
MAN: Solid mandrel classic shape
MAN-R: Solid mandrel rounded nose (US style)
MAN-PL: Solid mandrel with 1 side shape
MAN-1B: 1-ball mandrel classic
MAN-1BS: 1-ball mandrel spheric
MAN-MB: Multi-ball mandrel classic
MAN-MB-SP: Multi-ball mandrel with small pitch
Multi-ball mandrels could be also called "flexible" mandrels
Mandrel inside the tube
Why mandrel is so important?
Generally mandrel is considered as the element for use for any tube bending less than 3D (which means radius of bending on centerline Rm is less than 3 outer diameter of tubes (for example for tube OD 16 mm Rm is less than 48 mm. Anyway even for bigger radiuses mandrel is the element to make the quality of bend better.
Possibility of mandrel or use of mandrel tube bending machine is the only one active technology for most of bends and used in all industries. Moreover as we already told it depends of the appliccation - for example for any fluid or air tubes with pressure the quality of bend is very important.
Quality of bended tubes (outside of question of final geometry) are based on quality outside the bend and quality inside the bend. For quality inside the bend wiper die, mandrel are main elements to consider + possibilities of machine and material physical characterstics. For ovality, humps, bellows and areas deformation mandrel is the first element to install and to adjust for the quality of bending.
How necessary to have a correct mandrel? Depends of the requests and for hard tasks even the small tenths of millimiters positioning can change the quality of bending. Also the fine adjustment effects wear of the main elements of tube bending mandrel.
Formulas for mandrel choice
There are several possibilities to study the hardness of application and we will use the theory that is mostly common in Europe. The mandrel request is based on 2 parameters which are HB (Hardness of bending) and WFI (Wall factor index). Indication both of them on table allows to verify the correct mandrel:
WFI = OD (Outer Diameter) / WT (Wall thickness), for example for tube OD 16,0 mm with thickness 1,0 mm the WFI = 16 / 1 = 16
HB = Rm (Bending radius on centerline - Radius medium) / OD (Outer Diameter), for example for tube OD 16,0 mm with Rm 50 mm the HB = 50 / 16 = 3,125
-- - mandrel is not necessary (anyway the bending with mandrel will make only positive effect)
S - solid mandrel
1, 2, 3, 4 or more - ball mandrels with identification of number of balls
SP + number - ball mandrels with identification of number of balls with small pitch (SP)
Wall factor index
In US market for diameter of mandrel mainly additionally used the formulas:
MN (mandrel nose / working part) diameter = OD (Outer Diameter) - (WT (Wall thickness) * 2,21), for example for tube OD 16,0 mm with thickness 1,0 mm the MN = 16 - (1,00 x 2,21) = 13,79 (it means 0,105 gap from each side)
In general usually we make the solution with 0,15-0,2 gap between internal diameter and working side of the mandrel (each side).
Also is used recommended nose radius as 10% of MN, for example for MN 13,79 recommended nose radius will be 1,379
We have classic construction and do not use this formula.