While touring collision facilities in the southwest, we were asked by a shop owner our thoughts about the difference between the popular method of repairing a unibody vehicle structure with the Universal Anchoring and Measuring methods compared to the up and coming Fixture Method, also called Fixed Point Anchoring or the Jigs. The shop owner asked if we might be entering the age of the NETbuilt vehicle of zero tolerance. This was a topic we were not totally knowledgeable about, prompting us to do some research on the subject. You know the old saying: “if you think you know, you don’t know; but if you know you know, then you know.”

    Let’s begin with some basic information on the various types of pulling devices and measuring equipment.

    Going back thirty years, tolerances were close enough for government work – or one quarter of an inch. With the advent of the unitized vehicles, tolerances became the industry buzz word. We pulled out some old frame books (we really are two old farts) and found that some points were as much as plus or minus 5 mm. By comparison, the thickness of a dime is 1 mm.

Importance of tolerances

Why are tolerances so important? An inward movement of 4 mm at the top of the strut tower can change a camber reading as much as a half a degree. Moreover, manufacturers are competing against one another based on their vehicles having closer tolerances. The truth of the matter is that as tolerances get smaller, the manufacturing costs decrease. How is a body shop going to  duplicate these tight tolerances when changing a frame rail on a late model sedan? With self-centering gauges and a tape measure – we don’t think so. Let’s get back to tolerances.

    It’s not uncommon for today’s vehicle to have a zero structural tolerance at the torque box control points and not more than 1 millimeter at the (front) strut towers. Now with these tighter tolerances, it is required to have a more precise method of repair than the vehicles of yesteryear, with more and more manufacturers now stating how their vehicles must be repaired and with what types of equipment – more particularly the pulling and welding equipment they approve or require.

    Therefore tolerances do interact with the ability and type of equipment that a technician uses; more notably structural correction equipment. We’ll continue by looking at equipment types for correcting structural misalignment on damaged vehicle, putting the pulling devices into three different categories: frame machine, dedicated bench, and universal bench..

    The traditional frame machine, as most of us know it, has a large platform with some sort of anchoring clamping system for attaching the unitized vehicle to it. The towers can be attached to the unit or they can be portable. Due to their size and weight, the machines are usually in a fixed location in a shop, but there are some portable units on the market. These machines are loaded by driving the vehicle onto them or using some sort of winching system for non-driveable vehicles. Most of these machines have a clamping system available for mounting trucks and SUVs.

    The dedicated bench is next. These machines are smaller than most of the traditional frame machines. Their tread assembly is machined to very high tolerances. Most are portable and require extra equipment (either special high lift jacks or an asymmetrical hoist) to mount the vehicle. The pulling towers are usually portable. The thing that distinguishes these machines is that they use dedicated fixtures for mounting and measuring.

    The last category of frame machines is the universal bench. Like the dedicated bench, their treads are also machined to very tight standards, which allows for a true and precise platform which enables it to measure accurate datum heights. They can be portable as well as fixed. Again, the pulling towers are either portable or fixed to the machine. Vehicles are mounted similarly to the dedicated bench. The difference in this category of machines is that they do not use dedicated fixtures, but some sort of universal fixture system or mechanical/electronic devices to measure the damaged vehicles.