Fundamentals of Precision Bar Straightness
Straightness in precision ground bar and rod type materials is the uniform, linear extension of the material across its entire length without any bends, curves, or twists. In precision materials, this attribute is considered alongside other measurements like diameter and roundness as marks of material quality. A perfectly straight bar is ideal, although actually achieving such a form can present an immense challenge. Although many suppliers will provide material that they have verified to be straight, their standard of straightness often fails to meet customer expectations.
Straight ≠ Straight
The reality is that not all bar straightness is measured equally. What’s most common across the industry is that a supplier will balance a bar between two V-rollers placed at each end, then the measurement of straightness will be taken from the middle of the bar. There are two issues with this method. The first is that it does not account for the material’s natural tendency to deflect between the points of tension exerted by the V-rollers that it’s propped against. The second is that it only measures straightness across one segment of the bar, leaving the remainder of its length totally unchecked. As a result, ‘straightness’ can often be inconsistent from one bar to the next, even when those bars come from the same manufacturer.
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Why Straightness Matters in Manufacturing
To start, let us reframe the narrative: what difference does it make if a bar is not straight?
A bar that is bent or off-centered in any way is referred to as being ‘out-of-straight’, or having an out-of-straight defect. An out-of-straight bar presents critical issues during CNC machining that can make precision a steep challenge, or in some cases flat-out impossible. To start, an out-of-straight bar will not rotate evenly– an issue which is exacerbated at high RPMs. This can result in two things: excess vibrations, and an uneven depth of cut. Excess vibrations mainly present an issue to the machine itself, as they can eventually dislodge internal components and damage the machine to the point where maintenance and repair related downtime will become necessary. Not only are repairs themselves costly– such an occurrence also puts the machine out of commission for an indefinite period of time, reducing productivity. The rotation of an out-of-straight bar will also result in an inconsistent depth of cut. This has two potential downsides: one to the bar itself, and the other to the cutting tool. First, if the depth of cut is not accurate across the entire bar, the final result could very well fall outside of the acceptable tolerance range. At the same time, the cycle of repeated increases and decreases in depth of cut can place excessive stress on the cutting tool, which can lead to poor machining quality or a premature tool failure. These issues also compound upon the first issue of excess vibration, meaning that an out-of-straight bar is bad news both for your machine and for your final part. Whether in the form of part rework, cutting tool replacement, or machine repairs, unnecessary downtime becomes more imminent the less straight a bar is. Poor straightness can be mitigated somewhat by working at slower RPMs, but that comes at the cost of machining speed and subsequently, productivity. The following video helps illustrate the value of bar material straightness in relation to Swiss machining:
The Skilled Material Advantage
What we’re trying to get at here is that all of the above issues can be avoided simply by using straight material. With round bars that have been properly straightened prior to machining, manufacturers can extend the life of their tools and machinery and get more done, faster, to produce a better quality end product. While straightness may seem like just another product dimension, a properly straightened bar can not only simplify machining but bring peace of mind for precision manufacturers across countless industries. We call this Skilled Material: the pinnacle of precision which allows our customers to offset the costs of premium material by relying less on skilled labor, minimizing material prep time, and streamlining manufacturing to maximize throughput. In the end, the benefits of bar straightness all come down to cost savings.
How BC Guarantees Bar Straightness
At Boston Centerless, we recognize that straightness presents a difficult challenge for precision bar customers, which is why we go the extra mile to ensure uniformity of every unit that passes through our facility. We measure each bar multiple times, one foot at a time, throughout the entire length atop a flat, level surface. This method enables us to consistently deliver precision bars with the greatest straightness in the industry, and empower our customers with material that works with them, not against them.
Our Bar Straightening Expertise
While proper measurement is important to verify a bar’s straightness, how the bar is straightened in the first place is equally important. Most suppliers will straighten bars of all diameters and compositions with a hydraulic or servo press to get the job done quickly. While this can be effective with the right conditions, not all materials take well to the press treatment, especially those made of softer metals or those of smaller diameters. In these cases when the integrity of the material is a concern, we take the less common approach of tapping the material into shape by hand. This manual processing technique requires extensive training to master, but our highly skilled technicians have the experience and know-how to get the job done right. In addition, bar straightening is always the first step of our process, regardless of the material. A properly straightened bar lays the foundation for precision in all aspects, and ensures that our centerless grinding process produces the highest quality results. It’s how we are able to deliver consistent diameter and roundness so that our customers can experience the most hassle-free machining possible.
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