Calculation of flat blank length in press tool design
In any press tool design, the first thing to consider is how to determine the flat blank length of the folded component. I won’t go into the boring theory of how the material behaves during bending and how the individual material property affects the length and spring back. To be frank, I hate all these theories!
During the bending process, the material being bent results in forces: tension and compression. The external portion of the sheet will undergo tension and stretch to a greater length, while the internal portion experiences compression and shortens. The neutral axis is the boundary line inside the sheet metal, along which no tension or compression forces are present. As a result, the length of this axis remains constant. This is the bend allowance that we are interested to know to determine the flat blank length.
There are tones of materials and formulas out there in the cyberspace teaching people how to make calculations. I personally did not use any of these formulas’ during my last thirty years of tool making career.
I applied only one formula for any flat blank length calculation.
@ Pi D/360
where D= circumference of neutral line
@ = Bend angle
Any radius bend smaller than 2x the material thickness, the neutral line will be 1/3 thickness
Any bend radius equal or larger than 2x material thickness, the neutral line will be = 1/2 material thickness.
90 degree sharp bend radius.= 0.4 x material thickness.
+/- 0.05 mm accuracy can be achieved by applying the above method. Of cause, the bending clearances and accuracy of the bend radius does affect the end result.
Spring back in sheet metal bending
All metal has a certain degree of spring back after its being bent. During sheet metal bending, the interior radius of the bent metal is under compression and the exterior bend radius is in tension and stretched. Due to the metals’ elasticity, it tends to go back to its original shape after its being bent and this is know as spring back. All metals, when subjected to deformation, exhibit a certain amount of elastic recovery. The amount of spring back in sheet metal bending depends on the following factors.
Factors affecting spring back in sheet metal bending.
- The greater the yield strength of the material, the greater the spring back will be.
- The thinner the raw material, the greater the spring back.
- The larger the bend radius the greater the spring back.
- Raw material manufacturing specification different from batch to batch and coil to coil.
In metal stamping, bend angle will never able to get it perfect every time, a press tool can be designed and built to make acceptable bend angles though it will never be perfectly consistent.
There are many different methods adopted by different tool makers to overcome the spring back in sheet metal bending. The key is how to make it easily, quickly, safely while making adjustment within the press while having the ability to compensate minor differences in coil thickness and mechanical properties at the same time.
Overcoming spring back in progressive tool
I tried many methods in the past. Such as coining the external bend radius, over bend the angle, reduce bending clearance etc. All these can’t really provide an effective solution to it until I tried the following method which I’m happily using for almost all the high speed progressive tools in my design.
In Fig 1. Let’s say you are bending a 1.0 mm thick steel sheet with a R 2.0 internal bend radius. The length of the neutral line is 3.927 mm. Refer to flat blank length for how to compute the neutral line.
In Fig 2. A 45 degree pre-bend is to be placed before the 90 degree bend, the bending is suppose to start from the 1.9635 calculated position. ( length of the 45 degree art ) To overcome the spring back, the location has to move away by 0.05 mm. See Fig. 2a.
In Fig 3. A 90 degree bend as usual. Due to the off-set start line, the bent angle will be at near 90 degree or lesser after the metal springs back.
Fig 4. Showing the design of an adjustable side cam to fine tune the bent angle for any minor differences in thickness and properties.
Spring back in metal bending (II)
Some readers wrote to me after reading my previous post on “Metal forming spring back”. The technique I mentioned in the post was used on a progressive tool it is not suitable in the single forming tool.
Some of the factors for successful bending are the size of interior bend radius and the metal grain direction or direction of rolling. In general, the interior bend radius should be minimum a metal thickness. Good way bend has more spring back than the bad way.
The most common technique to control the metal spring back is by setting or coining the outer radius of the bent metal by a punch. The punch radius must be machined to the metal radius plus 0.07mm to 0.012mm to ensure full metal contact at the outer radius. We can compute the size by using exterior Radius x 4/3. (See Fig. 1)
There are also some toolmakers that make the die insert slightly smaller than the bend width to avoid cutting burr interfere with the form to control the spring back of the metal.
Another alternative method is to machine a 45% angle at the punch face, allowing adjustment to the 90-degree bend. The punch can be adjusted downward to further collect the bend angle. Drawback of this method is that it might produce a flat dent across the bend metal and it may not be acceptable. ( Fig. 2)
For “U” shape bending, the punch is machined with a recess at both sides of the edges of the punch. The depth of the recess is about 10% of the material thickness, the recess coin into the material thickness to overcome the plastic deformation of the metal, thus, spring back can be controlled. (Fig. 3)
For sharp edge bending, a coin line can be machined along the bend lines. The depth of the coin line is about 0.3 of the metal thickness. (Fig. 4) This technique produces an accurate and level flange, especially so in short edge bending. Furthermore, this technique requires lesser force to make the bend.
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