Engineering Expert Excluded From Saw Injury Case For Failing to Conduct Proper Testing


Court: United States District Court for the Eastern District of Missouri, Eastern Division
Jurisdiction: Federal
Case Name: McMahon v. Robert Bosch Tool Corp.
Citation: 2019 U.S. Dist. LEXIS 191666

In this product liability case involving a saw injury, testimony from the plaintiff’s engineering expert witness was excluded because the expert failed to properly test the saw in question and lacked sufficient experience in the power tools industry to offer a reliable opinion without proper testing.


The plaintiff sustained damage to his hand when the auxiliary handle of a hand-held spiral saw separated from the saw’s body, causing the saw to fall while the blade was still in motion. The plaintiff filed a suit against defendants on the basis of strict and negligent product liability for design defect, negligent failure to warn, and negligent supply of dangerous instrumentality.

At the time of the accident, the plaintiff had mounted and was using a wood carving blade with deep-grooved teeth in a saw attachment, even though the user manual of the saw explicitly cautioned against the use of wood cutting toothed blades in the attachment. Immediately before the incident, the plaintiff was balanced on a stepladder holding the saw in one hand by the auxiliary handle and supporting himself with the other. Several user manual pages advised users to always use two hands to hold the saw.

The saw was designed to allow two movements before the auxiliary handle could be removed. First, the user must slide a lock pin to either side and, while holding the lock pin, depress the separate handle release button. The plaintiff testified that the auxiliary handle of the saw had detached even though the handle release button had not been pressed, leaving the saw to dangle across his hand and cut his fingers. After the incident, damage to the part of the locking system holding the auxiliary handle to the saw was discovered because part of the plastic container shelf broke off.

Engineering Expert Witness

The plaintiff retained a mechanical engineering expert witness to testify to defects in the design of the saw. The engineering expert’s report provided the following opinions: (1) the handle connections on the saw was faulty and caused the saw’s release, injuring plaintiff; (2) the handle attachment design was defective, and its failure was highly predictable because (a) the connection system on the movable latch was susceptible to wear-induced tolerances that decreased the holding strength of the handle, (b) wear and tear reduces the available holding force, and (c) in some cases, the two-factor locking scheme is reduced to a one-factor locking scheme. The mechanical engineering expert also asserted that the positioning of the handle release button on the saw was defective because it promoted the positioning of the user’s thumb on the button and that the saw should have had an interlocking device to stop the engine once the auxiliary handle has been detached.

The defendants argued that the opinions of the plaintiff’s mechanical engineering expert were unreliable and inadmissible under Rule 702 and Daubert because the expert had not tested his opinions. Furthermore, the defendants argued that the engineering expert had not prepared any alternate designs and had only relied on inadequate facts and data to form his opinions. The defendants further alleged that the expert’s opinions did not conform to the facts of the case. The defendants argued that the expert was not qualified to give critical opinions on the materials from which the saw was made, but did not otherwise question his qualifications.


The mechanical engineering expert used the term “testing” to describe the videotapes he took of detaching and attaching the auxiliary handle of the subject saw. The court noted that the engineering expert did not carry out any testing to determine that the product was in an unreasonably dangerous condition at the time of its manufacture and sale, nor did he do any testing to rule out other potential causes of the accident. The court observed that the expert had only confirmed the fact that the auxiliary handle of the saw could be detached from the broken latch receptacle shelf when someone pressed the release button of the handle.

The engineering expert’s report proposed the design of a screw-on detachable handle to provide a very significant safety factor for the tool control in use. The court found that the expert had not provided a factual basis to support that his proposed amendments were feasible or that they would not interfere with the utility of the saw, much less that they would have prevented the accident. The court noted that the engineering expert had never worked in the power tool industry and was thus unqualified to offer an opinion on the defects of the saw without any testing.

The court also noted that the expert’s opinions did not fit the facts of the case as he ignored the facts in evidence without sufficient scientific justification to depart from them. The court enunciated portions of his opinion that contradicted the plaintiff’s testimony and observed that he had failed to show proper justification for ignoring the plaintiff’s factual testimony.


The defendants’ motion to exclude the testimony of the plaintiff’s engineering expert witness was granted.

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