Drill resistance measurement with a resistograph

Miller Florian, 28.01.2015

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The drill resistance measurement, also known as resistography, is a hand-guided, non-destructive testing method to determine the density and density distribution of living trees or engineered woods. With this active testing method, a calibration curve is generated that shows drill resistance in relation to drilling depth. This article discusses the drill resistance measurement of Rinn who invented and patented this method in the late 1980’s.


Fig.1 Performing a drill resistance measurement

Operating principle of a resistograph

Resistography is a wood testing method which measures penetration resistance of a material via a device-integrated drilling needle. The long and very thin drilling needle is automatically driven into the wood and captures the motor’s torsional moment during penetration. This torsional moment results from both the shank friction of the needle inside the material and chip removal at the drilling needle tip.

$\begin{array}{l}M_{machining } + M_{shaft friction} = M_{engine}\end{array}$

Thus, the appliance can determine the bulk density of the material along the length of the bore and evaluates the result in a visual measuring curve. This process allows the determination of the relative strength of the wood and the detection of cavities, cracks and decay of the test object.


Fig. 2 Resistograph from the company IML

Drilling needle

The drilling needle which is made of a very robust special steel alloy, has a diameter of 3.0 mm (0.12 in) at the tip and of 1.5 mm (0.06 in) at the shank. The 378/497mm (15/19 in)long exchangeable drilling needle is surface-coated and has a reinforced tip. Depending on application and correct use, 100 to 150 bores can be made in different woods with a single needle. Heavily used and worn drilling needle tips can lead to false measurements.

Drilling depths depend on the type of resistograph and can vary between 150 mm (6 in) and 1000 mm (39 in). The feed rate of the needle of most resistographs on the market is between 5 and 55 cm/min (2 and 22 in/min). Newer resistographs can achieve a feed rate of up to 250 cm/min (98 in/min).


Fig.3 Drilling needle

Control panel

The majority of resistographs on the market have an integrated display. With this display, an operator can analyse a particular drilling in further detail. Via this control panel, additional user specific settings such as feed rate in relation to the type of wood can be set. It also provides the operator with access to the internal memory of the resistograph and thus to all previously taken measurements.


Fig.4 Operator control panel

Measurement results

An electronic measuring plot converts the measuring results into a measuring curve.

The y-axis describes the drilling resistance in % (amplitude height).
The x-axis describes the drilled depth in cm.

These recorded measuring curves can be sent to a computer via a USB data transmission cable and then evaluated and analysed.


Fig.5 Measuring curve	Fig.6 Diagram of processed measuring results

Advantages

A major advantage of drill-resistance measurement is the easy and quick evaluation of the measuring results on-site. Measuring curves can either be viewed on the resistograph’s display right after measuring or evaluated on the computer. The measuring results allow an experienced and correspondingly trained operator a quick and qualitatively precise assessment of the wood quality in the tested area. For example, with a resistograph it is possible to test playground equipment or supports in the crucial area where they enter the ground. Digging out the wooden supports is therefore not necessary.

A resistant and robust housing adequately protects the device for daily use in nature or on a construction site.

Disadvantages

Due to the drilling the wood is minimally damaged. This damage is of minor importance in the field of construction and can be neglected. The generated drilling chips remain in the bore and therefore block it. However, it has to be considered that using the resistograph on trees, that each drilling causes damage and this might lead to a strong fungal infestation of the tree’s organ system.

Measuring only shows a localised image of the wood quality and should therefore be carefully considered and performed at the right location. This method can only assess the relative density distribution along the bore. Thus, the measuring does not provide the precise bulk density of the wood.

Applications

Living trees:

Detection of decay, cavities and cracks
Determination of wood quality
Analysis of annual rings and growth rates

For construction timber, poles and playground equipment

Wood inspection
Determination of wood quality
Detection of cracks, cavities and decay

Literature

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