Waste not, want not

Reducing scrap in bending tube, pipe

In today's cost-conscious times, anyone involved in the tube bending process may instinctively consider perishables such as wiper dies, mandrels, and lubricants to be their biggest problems. However, a quick look into a scrap bin next to a tube trimming machine reveals a far bigger expense: bending scrap.

For example, most vehicle exhaust systems are produced from stainless steel, the price of which has been known to double in 12 months. It is, therefore, straightforward to see that when the material costs 40 cents per inch, even 1 inch of material wasted per unit can add up to tens of thousands of dollars per year.

Although bending scrap is a necessary evil, several strategies can minimize it and thus cut costs.

Tube Dimension Calculations

Three main components that must be considered when calculating the length of material needed to produce a bent part (see Figure 1) are:

Clamping stock—The extra material needed to sufficiently grip the tube and form the first bend of the component.

Component(s) length—The developed length along the centerline of the component, including material needed for subsequent end forming operations and, if you are manufacturing several components from a single length of tube, material required for parting.

Collet stock—The extra material needed to sufficiently grip the tube to rotate and position it for bending.

In this example, the tube characteristics are 60.3 mm diameter, 1.75-mm wall thickness, and 409 stainless steel.

Clamping Stock Length

Several factors influence the length of the clamping stock. These include the bender's capabilities, specific product requirements, and subsequent cutting. Some of the relevant questions are:

Does the bender have boost capabilities?
Does the product require a square end?
What are the cosmetic requirements?
Will the extra clamping stock be removed with a saw or a nick-and-shear operation?

If the bender is equipped with boost and it is intended to use the minimum grip length (usually 1D), the end of the tube will be out of square. A good rule of thumb, therefore, is the first straight must be at least 1.5D so the end of the tube is not pulled out of square by the bending process. If boost is not used, the first straight should be at least 2D.

According to the sample data (see Figure 2), the first straight is 23.97 mm. This is a 0.40D straight section (23.97/60.3=0.40). Assuming the bender used does not have boost capabilities, this application requires enough extra stock to increase the first straight to 2D.

Figure 2


Bend Data
X	Y	Z	CLR	Y	B	C
150.70	67.60	-7.45
130.40	44.50	0.00	63.50	23.97		13.78
94.10	0.00	0.00	63.50	19.61	-98.62	50.79
15.50	0.00	0.00		48.45		0.00

Clamping stock formulas:

Clamping stock = (OD x 2) - SL_x (without boost)
Clamping stock = (OD x 1.5) - SL_x (with boost)

Where:

OD = Tube outside diameter
SL_x = Straight length (where x is the first straight)

Therefore, the amount of clamping stock (without boost) is:

Clamping stock = (60.3 x 2) - 23.97 = 120.6 - 23.97 = 96.63

Money-saving Strategy 1. In this example, it would be safe to start with 120.6 mm of total grip stock. After the tube is developed, you can shorten the actual grip length by 5 mm and rerun the trial. If the tube doesn't slip, shorten it another 5 mm. Continue to run tests and shorten the grip length until the tube slips during bending. Then add 5 mm to 10 mm so it no longer slips.

Note that when you are shortening the tube for bending trials, you don't have to physically cut the tube to shorten it relative to the first straight. Instead, you can simply tell the bender that the tube is longer than it really is and the bender will adjust the tube back, effectively shortening the first straight. After you determine the final length and you physically cut the tube to length, it is then necessary to tell the bender the actual length.

Component Length

To determine the length of stock required for the components, first calculate the distance along the centerline path; then add any material needed for subsequent end forming. If you are forming a single tube that eventually will be several components, include the material required for splitting or parting. If you plan to saw, you will need at least 3 mm plus the saw blade's width. On the other hand, a stab-cut process requires about 15 mm plus the blade's width.

SL_x = Straight length (x is the straight number)
AL_y = Length along the arc (y is the bend number)
NC = Number of components in tube
CW = Cut width
EF = Material length required for end forming

Arc length formula:

AL_y = (π x CLR / 180 x degree of bend)

Component length formula:

[(SL1 + AL1 + SL2 + AL2 + SL3 ... + EF) x NC] + [CW x (NC - 1)]

Using the sample data, and including 3 mm for end forming and 4.75 mm for the cut width, the component length is:

[(23.97 + 15.27 + 19.61 + 56.29+ 48.45 + 3) x 3] + [4.75 x (3 - 1)]
[(166.59) x 3] + [4.75 x 2]
[499.77] + [9.5]
509.27 mm

Collet Stock Length

The collet stock length is determined after evaluating the pressure die, wiper die, and collet.

Pressure Die. Pressure die calculations are based on the deepest bend. The pressure die is typically set to travel at a 1-to-1 ratio with the perimeter of the bend die.

Pressure die length formula:

π x CLR /180 x degree of deepest bend

If this were the pressure die's final length, it would leave a deep impression at the end of the bend. Therefore, it is necessary to add an amount to support the tube at the end of the bend. This typically is 2D.

Using the sample data, the pressure die length is:

[(π x CLR / 180) x deg. of deepest bend] + [2 x tube dia.]
[(3.1416 x 63.5 / 180) x 50.79] + (2 x 60.3)
56.29 + 120.60
176.89 mm

Money-saving Strategy 2. If the tube in the collet is gripped through the last bend, the end of the pressure die will normally be the first point of interference the collet housing encounters (see Figure 3). This is especially likely if the deepest bend is not the last bend. Shortening the pressure die allows you to shorten the tube length. Although 2D support is normal, in some cases 1.5D is suitable.

Wiper Die Length. Because many tooling manufacturers produce wipers in high volumes, they have standardized lengths. Typically, insertable tip lengths are:

100 mm for tube diameters < 76 mm
127 mm for 76-mm tube
1.5D for tubes > 76 mm
1.5D for custom-made tips

Using the sample data:

Wiper die length = 100 mm

Money-saving Strategy 3. If the wiper length is greater than 2D, the wiper die may be the first point of interference (see Figure 4). As with the pressure die, this is a safe dimension, but it may not be necessary to produce an acceptable bend.

Collet Depth. The factors that influence the collet depth are the rotational torque from the tube and the bender manufacturer's standards. It is commonly accepted that the collet depth minimum is 0.5D.

Collet depth formula:

Tube OD x 0.5

Using the minimum (0.5D):

Collet depth = 60.3 x 0.5 = 30.15 mm

Collet Stock Length. After determining all the tooling lengths, calculate the additional material needed for the collet end. At this point it is necessary to decide whether to use boost pressure.

Collet stock length with boost (collet will hold tube during the entire bending cycle):

PL_t = Pressure die length (where t is the total length)
PL_r = Pressure die length (where r is the remaining length from tangent after the last bend is made [see Figure 5].)
WL = Wiper die length
CD = Collet depth
SL_x = Straight length (where x is the last straight)
CF = Clearance factor (5 mm)
AL_y = Length along the arc (where y is the last bend)
LL = Limiter length (distance from tangent at which the collet housing will encounter interference)

Arc length formula:

AL_y = (π x CLR / 180 x degree of last bend)

Pressure die length remaining formula:

PL_r = PL_t - AL_y

Limiter length formula:

LL = WL or PL_r (whichever is greater)

Collet stock length formula:

LL + CF + CD - SL_x

Using the sample data:

WL = 100 mm
PL_t = 176 mm
SL_x = 48.45 mm
CD = 30.15 mm
AL_y = (π x CLR / 180 x 50.79) = 56.29
PL_r = 176 - 56.29 = 119.71
LL = 119.71 (because PL_r is greater than WL

Collet stock with boost:

LL + CF + CD - SL_x
119.71 + 5 + 30.15 - 48.45
106.41 mm

In some cases the calculation results in a negative number. This isn't necessarily an error; it indicates that no additional collet stock material is required.

Money-saving Strategy 4. For part configurations where the deepest bend is not the last bend, the pressure die is typically extended past the wiper at the end of the bend cycle (see Figure 5). In this situation, a split pressure die is a solution. Before the last bend is in position, the rear pressure die is lifted out of the way, allowing the collet housing to move farther toward tangent (see Figure 6).

When bending without boost, the pressure die length is not a consideration (see Figure 7).

Collet stock length without boost (collet will release the tube during the last bend):

WL = Wiper die length
CD = Collet depth
SL_x = Straight length (where x is the last straight)
CF = Clearance factor (5 mm)
AL_y = Length along the arc (where y is the last bend)
LL = Limiter length (distance from tangent at which the collet housing will encounter interference)

Arc length formula:

AL_y = (π x CLR / 180 x degree of last bend)

Limiter length formula:

LL = WL (pressure die length is not considered)

Collet stock formula:

LL + CF + CD - SL_x - AL_y

Using the sample data:

LL = 100 mm
CD = 30.15 mm
SL_x = 48.45 mm
AL_y = (π x CLR / 180 x 50.79) = 56.29

Collet stock without boost:

= LL + CF + CD - SL_x - AL_y
= 100 + 5 + 30.15 - 48.45 - 56.29
= 30.41 mm

If the calculated collet stock is a negative number and the last straight is less than 2D, then add just enough material to make the last straight 2D. If the calculated collet stock is a negative number and the last straight is greater or equal to 2D, then no additional collet stock material is required.

Theoretical Tube Length

The theoretical tube length is the sum of the three main elements: clamping stock, component stock, and collet stock:

Theoretical tube length formula:

Clamping stock + component(s) stock + collet stock

From the sample data:

Theoretical tube length (with boost) = 96.63 + 509.27 + 106.41 = 712.31 mm
Theoretical tube length(without boost) = 96.63 + 509.27 + 30.41 = 636.31 mm

Implementing Waste-reduction Strategies

When estimating the amount of material required to bend a component or series of components, fabricators often run the numbers by the book and play it safe. A little waste is better than ordering a few truckloads of tubes that turn out to be too short.

That said, in many cases the initial tube length is accepted and never re-evaluated. The opportunities for cost saving are real and they are easy to accomplish.

The first two steps in cutting waste are taking a close look at a process and asking questions about it. Look at the excess tube coming from the first clamped end and ask yourself, Does it need to be this long? Run the bender to the end of the last bend, pause it, and look again. Then ask yourself, What's stopping me from shortening the tube?