There is no universally acceptable standard for measuring straightness for a TGP or TP Cold Finished Bar nor is there any established testing method.
Steel mills use an antiquated standard for hot rolled bars tolerance which is one quarter (1/4) inch over five (5) feet. Years ago, before continuous casting, long products were manufactured starting with a large ingots which were reduced in thru a series of blooming, billets and bar mills. Utilizing only horizontal stands, it was very difficult to maintain straightness while utilizing so many horizontal stands. Thus, 1/4" over five feet was a reasonable standard under such circumstances. Today, long products are produced by mini-mills which reduce cast billets that are rolled thru a series of vertical and horizontal stands with sizing blocks on the last few stands. All of the factors permit the bar to be produced to a much closer tolerance. Quality Bar expects to receive bars from the mills no worse than a one tenth (1/10 or .100) of an inch over the length of a twenty foot bar.
One method commonly used by cold bar producers to test for straightness is to roll a bar down an incline plane. If wobble of the bar is minimal, the bar is found to be acceptable. Bars start to wobble when their straightness is worse than .050 inch or fifty thousands over the length of the bar. Many commodity bar producers use this as their in house standard for acceptable straightness.
One can test straightness by using two dial indicators on opposite sides of the bar or by using a minimum of four dial indicators and test around the full diameter of the bar (T.I.R.). Obviously one gts more meaningful results testing T.I.R. which is the way QB does its testing.
Quality Bar randomly tests its bars off line on a specially designed testing machine with an accuracy of one one-thousandths (.001). For mid carbon 1045 bars, Quality Bar expects their bars to be straighter than .030 over the length of a twenty-four foot bar.
Quality Bar has designed and built a precision bar straightnesss tester. In the tester 24 ft. long bars are supported on precision built resting points set six feet from each end of the bar. Dial indicators are placed 6 inches from the ends of the bar and another in the center of the bar. The bar is measured at rest at 0 degree and then rotated and tested in 90 degree increments. Readings are taken at 90 degrees, 180 degrees, 270 degrees and again at 360 degrees which is the starting point. The initial reading at 0 degree has to equal the reading at 360 degrees or else the results are not valid. The bar is then lifted from the supports rotated end to end and reset on the supports. What was the north end of the bar now becomes the south end of the bar. The bar is retested and the results are compared. The results from the first test for the north are expected to be the same as the results from the second test for the south end.
Bars under 1 1/4" tend to sag or droop during testing and their test results are more suspect, Test results for bars greater than 1 1/4" are more accurate.
On the larger diameter bars straightness can be measured down to one thousands of an inch over the length of the bar.