Rolling Tolerances of Rounds as per BIS Specification

On Size of Round

Table 6.2

Sizes

Tolerance mm

Over mm

Upto including mm

-

25

±0.5

5

35

±0.5

35

50

±0.8

50

80

±1.0

80

100

±1.3

100

-

± 1.6% of diameter

Ovality or Out of Square

The permissible ovality of rounds is measured as the difference between the maximum and minimum diameters. It should be less than 75% of the total tolerance (plus and minus) specified on the size.

Weight

The tolerances on weight per meter for round bar will be as follows:

Table 6.3

Sizes

Tolerance percentage

Over mm

Upto and including mm

-

10

±7

10

16

±5

above 16

-

±3

Design of Passes

Design of Finishing Pass

The reduction in finishing pass is recommended from 5 to 15%. The reduction varies according to shape of pre-finishing pass, surface finish desired, size of round, and tolerance of the finished product. Smaller reduction is recommended for bigger sizes rounds. Reduction is generally fixed around 12%.

It is also to be noted that before, designing the finishing pass; tolerances on size specified by the customer or as specified in reputed specification are to be taken into consideration for designing finishing pass.

Design of Finishing Passes

Fig. 6.4 Design of Finishing Passes.

Considering tolerances on size and ovality for finished product, the elements of finished pass is designed as follows:

Height of Finishing Pass

Roll pass design is always made on hot dimensions. As metal will get shrink in cold condition, a collection for shrinkage is to be made in the design, taken into the account of the co-efficient of expansion of rolled product and then depending upon the tolerances assigned, a part of tolerance value is subtracted from the obtained value.

D hot = (1.010 - 1.015) D cold, as finishing temperature varies from 850-950°C. Roll pass designer generally takes height of finishing pass equals to the size of the finished round.

Width of Pass

Horizontal diameter of finishing pass should be taken little more than the vertical one. It necessitate because if pass is made of collect round size, then even the smallest overfilling connected with the continual change of condition of widening (may be changed due change of co-efficient of friction), leads to formation of fins. It may cause rejection of material or production of inferior quality products.

Other reasons are:

  • • Shrinkage in horizontal direction is always more than the vertical direction.
  • • After wear out of the pass, it will remains as a good shaped round.
  • • It will also take care of the widening of the width of metal, due to change in tension in the continuous group.

Following are the methods for computing width of round pass.

  • (a) В = 1.015 x Dc, for round 6-30 mm
  • (b) В = (1.005 - 1.025) x Dc
  • (c) В = Dc + Tolerance on size + co-efficient of contraction

Roll Gap

It is generally taken equivalent to 1% of the mill size. The computation is based upon

Roll Gap or t = Nominal roll gap + spring of mill + Wear of pass allowed + Roll turning defect + Safety allowance

Radius of Finished Pass

Radius of Finished Pass

Fig 6.5 Radius of Finished Pass.

There are three methods, called as single plug, two plugs and three plug methods depending upon number of radius used for designing round. Application of one of these methods depends upon how much accuracy desired in finished products and need for longer run of pass without the dressing of the rolls. The three plug method is not generally used, unless high precision rounds are required.

Single Plug Method for Designing Rounds

Fig. 6.6 Single Plug Method for Designing Rounds.

For single plug method

It is recommended that R should be taken as 1.01 Dr /2 for rounds of size of 16 - 36 mm, (1.012 - 1.015) x Dc/2 for size 40 - 45 mm and (1.015 - 1.017) Dc/2 for rounds 50 - 75 mm.

Design of Pre-finishing Oval Pass

Single Radius Oval

Fig. 6.7 Single Radius Oval.

Selection of the form and dimensions of pre-finishing oval pass is based on the following considerations:

The more nearly the shape of oval pass to the round pass, the better will be the quality and will have close control on the size of finished product, as less draught will give less spread and hence there will be less chance of incorrect filling of round pass.

Double Radius Plum Oval

Fig. 6.8 Double Radius Plum Oval.

Plum ovals are used for higher diameter of rounds and slim oval are used for smaller diameter of rounds. This will facilitate to avoid the turning of work piece in pass, lead to twisting of the bar. The co-efficient of elongation in the oval pass is generally recommended to 1.13 -1.50.

Height of Oval

It is determined on the basis of spread in the finishing pass.

H oval = Dc - spread in the finishing pass

There are other formulas to calculate the spread in the finishing pass, the most commonly used formula is

Spread in finishing pass = (0.03 - 0.04) x^j~D x ^~d~

Where D and d are diameter of roll and size of round desired. Higher value is used for smaller size rounds and vice versa.

Width/Height Ratio of Oval Pass

B/H, varies with the size of oval, it is recommended to use such derived ratio for oval pass :

Size of Rounds

B/H Ratio

16-28 mm

1.80-2.20

32-40 mm

1.50-1.70

45-75 mm

1.40-1.50

Radius of oval

It is suggested to use single radius i.e., R{ for the size upto 36 mm. Double radius i.e., Rl and R1 are recommended for size above 36 mm.

Computation of Rx will be as follows:

Calculation for R2 radius for double radius ovals for round size above 32 mm, is derived from the drawing itself, as it will connect outer point of the pass with Rx radius.

Outer radius r of Oval Pass

It is generally taken equal to the roll gap of oval pass.

Radius r = t, where t is roll gap of oval pass.

Roll gap t: Roll gap of pre finishing pass is generally taken 2 mm, more than the finishing pass.

Area of Oval Pass

Area F oval is determined by the formula

Where В and H are the width and height of pass respectively.

Design of Strand Pass

Strand pass is a pass, which follows the oval pre-finishing pass. It may be square, round or plug /edge oval.

Generally, plug/edge oval is preferred due to following reasons:

  • 1. Uniform wear out of pass due to big radius.
  • 2. Self centralizing of out coming bar from this pass, due to big radii, and whatever position bar comes out of the plug oval; it falls on its bailey, causing smooth entry to the next oval pass. It means no need of tilting device is required.
  • 3. Quicker setting of pass due to its shape.
  • 4. Because of uniform wear out in the plug /edge oval, roll life is increased. It is observed that the plug oval pass gives 1.5 times more life than the square pass. Moreover, the roll life of oval pass also gets increases due to uniform reduction in oval pass.
  • 5. The product obtained by this system will have fewer defects.

Only disadvantage of plug oval-oval system is that plug/edge oval which have not a high co-efficient of elongation. The maximum co-efficient of elongation will be in the range of 1.15 to 1.35 in plug/edge oval.

Strand Plug oval/edge Oval

Strand Plug oval/edge oval

Fig. 6.9 Strand Plug oval/edge oval.

First step is to determine the p oval, as it is mentioned earlier, the value of p oval is generally taken between 1.15-1.35. Higher value is selected for lower size and vice-versa. Once the value is assigned, then area can be ascertained by the following formula i.e.,

AH oval can be determined, keeping in view of В /Н of the pre-finishing pass. The less reduction is recommended for flatter oval than for plumber oval.

Generally, the design of plug oval with one radius is used for the round size of 20 - 25 mm. For bigger rounds i.e. from 28-75 nun, double radius edge oval are used.

Strand Square

Square pass is preferred for strand pass, when a higher reduction is required to be given in strand pass. It is important to note that to avoid the possibility of surface defects, strand square should give a correctly filled oval. The computation of side of square can be related with the size of finishing round.

Generally, side of square is computed by multiplying the size of round by 1.1, thus giving a overall reduction of about 35 %. For size of round above 12 nun, following formula can be used for computation of the side of square

Table 6.4

Round size in mm

Side of square

12

1.125 x Dc

22

1.16 x Dc

25

1.15 x Dc

28

1.13 x Dc

For size above 20 nun, a '‘Gothic shaped” square is preferred to give a better entry, better riding and less wear out of pass. In the Gothic square pass, the distance across the sides is slightly more than the square pass, but area is slightly less due to its shape.

Strand Round

A round is used as the strand pass, when round-oval-round sequence is applied. This implies a light reduction from strand round to finishing round maybe upto 20-30% only.

Design of Following Passes in Rolling Sequence

After the design of finishing, pre finishing and strand passes for rolling of particular round, pass design of following passes in rolling sequence are to be selected. Pass sequence can be oval-square, diamond-square, round-oval or box-barrel, depending upon the size of finished round, input billet size, type of mill, finishing temperature of finished bar and other consideration already been discussed.

Generally a square-rectangular sequence is used for round above 20 nun in a structural mill.

 
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