How to make a square and straight bar from nasty hot-rolled steel

We machine a lot of long bars from 1044 hot-rolled steel where the finished product must be square and straight along the length.   That sounds pretty easy until you try it!    Getting a good result repeatedly is difficult.   Commercial-quality hot rolled steel is as crooked as a dog’s hind leg – it has a belly on the sides, it is twisted and it is bowed along the length of the bar.   It is common to see 1/8-inch variation in 4 feet of bar and even more if the sheared bar end is included.

Our initial approach was to clamp as tightly as possible in several vises as tightly along the length while we milled the tops and sides.  Then, we would flip the bar and do the opposite side.    We would end up with a substantial bow in the finished bar and irregular witness lines on the sides where the milling passes overlapped, regardless of how accurately we indicated back jaws of the vises, how we oriented the part or how carefully we machined it.    The biggest problem was that the results were inconsistent – sometimes the part was perfect and sometimes it was awful. 

After struggling with the inconsistency, we realized the obvious – we were flattening the bar in the vises and it was relaxing back to its previously bowed state when we released the pressure after machining. Even a large bar will flex much more over a long length than one might imagine.    

We learned that eliminating the bow was as simple as locating the in the same way but clamping it differently.   After perfecting the technique, we can repeatedly make bars that are square and straight within 0.0005 inches over 60 inches – basically, as accurate as our CNC mill.  For the critics in the crowd who will say this is not possible, keep in mind that the objective is a straight and square bar.   In our application, the absolute dimensional accuracy of the length and width is not critical.   The absolute accuracy of the thickness is critical and machining the part from both sides in the same fixture helps immensely.  Even if other dimensions were critical, this method removes the inconsistency introduced by the natural bow in the raw bars.

TaperT and Simploc modular components make it easy to fixture long bars accurately.   

The reference point (G54 in our case) is located by the virtual corner defined by the vertical faces of Stop #1 (X₀), Stop #2 (Y₀) and the horizontal face of Stop #1 (Z₀).

Indicate the vertical faces of Stops #2 and #6 to be in line with the X-axis of the mill.

Loosely locate the bar against the Stops #1, #2 and #6.  Snug Clamps #3, #7 and #8 but do not tighten them.    Tighten Clamps #3 and #8.    Leave Clamp #7 snug but not tight.   Snug Clamps #4 and #5 but do not tighten them.  These clamps support the center of the bar will reduce chatter and deflection but it is critical not to push on the sides of the bar. Do not tap or pound the workpiece into place at any point in this process.   

Mill the top and side surfaces of the bar in the first operation. Rotate the part on the fixture to expose the unmachined surface.   Do not flip the bar end-to-end.    It is important that End A be placed against Stop #1.   Place the bar against Stops #1, #2 and #6.   Snug Clamps #3, #7, and #8.   Snug Clamps #4 and #5.   Tighten Clamps #3, #7 and #8.

Machine the remaining material off of the bottom and side surfaces.