Friday, 31 August 2007 17:00

Frame Systems Special Section: Anchoring, Measuring, Fixture Methods of Repair

Written by Toby Chess and March Taylor
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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.


Measuring the vehicles

For now, let’s concentrate on the items used today beginning with the universal measuring system, which utilizes a track that is centered beneath the vehicle. Three points are located in the torque box areas of the vehicle, which is needeed to center track. A measuring tape is located along the edge to measure the length of the vehicle. Cross bars are attached and placed at specified lengths. The cross bars are calibrated with markings to determine the width of the various points. A height tube is attached to the fixture along with some sort of locating cone or cup. With the measuring system in place, any point on the bottom of the vehicle can be checked against a data sheet to determine the extent of damage. The system also has a bridge to check the upper strut towers and other upper points in the engine compartment.

    Next up is the universal laser system, which is used primarily on the bottom of vehicles. They are universal and can be used on a frame machine as well as with an overhead vehicle hoist. Some systems will furnish a computerized print out both before and after repairs. These systems are not recommended for use during the pulling and repair process.

    The sonar or ultra sonic system utilizes probes that are placed at selected reference points under the vehicle. An ultrasonic signal is generated by the probes at which point the information is transmitted to the central beam where data is converted to measurements. This data is then transferred to a computer where that data is compared with known measurements of the vehicle being measured and a comparison of the two sets of data are made.

    An electromechanical system employs a mechanical arm that rides on a bridge under the vehicle. The bridge is centered underneath the vehicle to create a datum plane. A pointer is attached to the arm and after the system is locked into position, every reference point can be measured for length, width and height. Various attachments to the arm are employed for measuring door opening and strut tower locations. After the data is a accumulated, it is sent to the computer by way of wires or wireless. The data is analyzed and compared with standard values to determine the extent of damage. Data can be received during the pulling process in order to monitor the current status of vehicle dimensions. The computer also stores images of the various reference points so that the technician does not measure the wrong reference points.

    Computerized Laser Measuring systems employ a laser generator under the vehicle and a  series of flags or targets placed at selected reference points to measure length, width and height. Measurements are taken and these dimensions are transmitted to a computer where they are compared with computer-stored data bases. A misalign is diagnosed and a document is created showing any changes to norm.

    [For more information on past measuring devices – self-centering gauges, tram bars, tape measures, see I-CAR’s Measuring Class for details.]

Anchoring methods

Now with a better understanding of the equipment currently being utilized , we’ll return to the anchoring method question. Here is some information on the fundamental differences of the two repair methods – of the four designs of pulling machines or design types that are most common in the market today.

Frame rack

The drive-on frame machine design, commonly called a frame rack, has evolved through the years to not only offer versatile multiple pulling capabilities (with as many as five pulling towers), but also offering universal measuring systems that work in conjunction with the frame machine. This traditional machine was originally designed for the body over frame vehicle, commonly called a “perimeter frame.”

    A number of years later as the unibody vehicle became more prevalent (with the onset of different anchoring requirements), the ability of this pulling machine was broadened; adding pinch weld clamping capabilities – where a specially designed clamp attaches to the rocker panel flange anchoring the vehicle to the frame machine. These adaptations to the unibody vehicle led to the term the “Universal Anchoring & Pulling Method.”

    The traditional frame rack is larger with some machines having a total length of 25 feet or more and a total width of 14 feet. They were designed in this manner to be wider and longer than the vehicle, offering some access to the underside of the vehicle. At last count, there are more than a dozen manufacturers of drive-on rack systems in the market today, with pricing to meet the needs of any size collision facility. Moreover, these machines usually have a minimum of two towers and often come with three or four. This widely-used method of repair has proven to offer time savings in set up and pulling, allowing the user to repair a wide mix of vehicles and damage severity.


Universal bench

    The universal drive-on bench entered the U.S. collision market from Europe a few years after the introduction of the traditional drive-on frame machine. In Europe, the cars were of unibody design, being much smaller than the traditional American vehicle, negating the need for a full-size frame rack.

    This drive-on bench (fixed or mobile) is much smaller in overall size than its larger cousin, designed primarily to anchor the vehicle at the rocker pinch-weld flanges. Surprisingly it offers many of the same pulling capabilities as a drive-on full-sized universal frame rack while consuming less floor space. Most drive-on benches are designed to be narrower than the vehicle, with a removable platform allowing the technician to work comfortably standing closer to the vehicle. This design allows for portable units as well as in ground units.

    Drawbacks include some size limitation as most are smaller in the size of truck or SUV it can accommodate and limited access to the underside of the vehicle. However, there are some machines that are longer and larger.

    Generally speaking, the popularity of the drive-on bench is with collision facilities wanting to do light- to heavy-hit vehicles, and where floor space is at a premium, allowing the repairer to process a high volume of work. Although it should be noted, there are some universal drive-on benches that have the capability to repair larger trucks and SUVs, it is done by using specialty anchoring adaptors that attach to selected locations on the frame. Some universal benches also offer electronic measuring systems, and/or universal ladder measuring systems, allowing the technician to pull and measure simultaneously with speed and accuracy.

Fixed-point anchoring bench

The dedicated fixed-point anchoring bench and the universal bench also offer two types of benches – a fixed location drive-on bench or a vertical-mount mobile design (requiring a two-post lift to lower the vehicle onto the bench). Again originating in Europe, it was developed to address the demands of a few European vehicle manufacturers who required the dedicated fixture anchoring method of repair on their vehicles and with specific manufacturers of benches. Based on the premise that this method is more precise and removes any technician guess work, it allows a less than highly-skilled technician to produce exact repairs.

    The dedicated fixture bench is in many respects similar to the universal drive-on bench, but is manufactured only to be used with dedicated fixtures, which both anchor and measure simultaneously. Instead of the universal anchoring method that attaches only to the rocker panel pinch-weld, in conjunction with electronic or ladder type measuring systems, fixtures are manufactured specific to each vehicle model, with the fixture dimensions taken from the manufacturer’s blue print specifications – identical to the fixtures used in the assembly line. 

    The technician installs the fixtures to the vehicle and bench, guided by a set of instructions that are worksheet specific to the vehicle, showing the technician where each fixture is located throughout the underside of the vehicle. Some benches also offering side-mounted fixtures to measure and secure side pillars. Depending on the vehicle model and severity of damage, there can be as many as 16 different fixtures attached throughout the underside of the vehicle, securely holding each anchoring/ measuring point in place.

    With multiple pulling capabilities, the dedicated bench design can be limited, coming equipped with only one pulling tower. Additional pulling towers can be purchased. But with the dedicated anchoring repair method, a technician would not normally need more than one pulling tower. Although this fixture method of repair has been around for many years, only in the last few years has the collision industry seen a slight shift in its direction. Repairers are beginning to hear what more and more vehicle manufacturers are publicly saying – that with certain types of damage, the fixture method can offer a precise method of repair.


Universal dedicated fixed-point anchor

The universal dedicated fixed-point anchoring design has also been around for several years, again originating in Europe to meet the demands of OEMs. A few of the European manufacturers also require a specific bench manufacturer. In principle, there is not much difference between the dedicated fixture and the universal fixture methods. Both benches are similar in design while having the same pulling capabilities. Although the dedicated and universal bench methods are fundamentally the same, there is a basic difference between them. The two differ in their individual design of fixtures, with the universal fixture method being used on all models of vehicles, compared to the dedicated fixture method being limited to a specific model of vehicle. With the universal fixture method, the individual fixtures have to be assembled, with the technician following a vehicle model specific set of instructions.

    The set-up instructions are detailed and show the technician how to assemble each fixture from a numbered set of universal brackets and attachments that are supplied with each bench; these instructions are produced by the manufacturer following manufacturer’s blueprints. The fixture is mounted to the bench and to a known specific holding/measuring point, visually showing the technician the precise location of the datum height, length and width, as does the dedicated fixture design. Similarly with the dedicated fixture design, there can be as many as 14 fixtures attached to the underside of the vehicle, both anchoring and measuring simultaneously.

    Part 2 of the article will show some real life situations dealing with pulling and measuring and how misunderstandings can affect the total repair process.

    Toby Chess has more than 30 years of industry experience. Chess is an ASE Master Certified Technician, an Accredited Automotive Manager, an I-CAR instructor, the Los Angeles I-CAR Chairman, and a technical presenter for CIC.

    Recently deceased, March Taylor was a shop owner in Hawaii and had been active in the collision repair industry for many years, often working alongside Chess.

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