Roller Cutter Forces
Authors: Peterson C.R. 
Published: December 2007

Abstract
An experimental technique is described in which three component forces are measured while a typical toothed cutter is rolled in a straight line over a rock sample. The technique includes the attainment of a steady state in which volume-averaged penetration is correlated with average force during penetration is correlated with average force during the removal of several layers from the rock surface. Simple rolling and skewed rolling forces are measured. The cutter was artificially dulled for some of the measurements. Surprisingly little variation in force requirement is noted. A qualitative explanation is suggested. The normal force requirement is substantially reduced when the cutter is skewed. A theoretical description of the force reduction is presented, showing reasonable agreement with the observed behavior in terms of cutter radius, tooth width, penetration and skew angle.

Introduction
Toothed roller cutters have long been in use on tricone bits, and they are in common use on boring machines. Yet the designer of boring machines is still faced with a dearth of good design information on the performance of such cutters. For example, what are the relationships between thrust, power, and penetration rate? How are these relationships influenced by rock properties and cutter configuration? While the data presented here provide answers to more specific questions than those mentioned above, these data are necessary for arriving at solutions to the broader questions. This work is restricted to one tooth type, typical of the wedge-shaped steel teeth used on medium rock. A limited range of rock types was tested; this coupled with the extreme variation of rock drillability, renders the data of limited value in predicting penetration rate. But the designer must predicting penetration rate. But the designer must answer questions even more important than the prediction of absolute penetration rate. For example, prediction of absolute penetration rate. For example, the cutter normal force is usually known in terms of the thrust to be applied to the cutter head. What is the torque or power required to rotate the cutter head? For an answer, one need know only the ratio of normal force to the tangential or rolling force. This ratio may be estimated from the present data. Variation of this ratio is reasonably small from one rock to another so that, lacking more specific information, these data can provide at least rough design estimates for other rocks. Tricone bits for soft to medium rock usually are constructed with skewed cutter elements that provide a "gouging and scraping action". Whatever the explanation, skewed cutters do provide increased drilling rate or, for a given drilling rate, a decreased thrust requirement. To my knowledge, skewed cutter elements have not been used on boring machines. If they were, bearing load could be reduced at a given penetration rate, or, conversely, an increased penetration rate could be obtained at the same penetration rate could be obtained at the same bearing load. Of course, a side load is introduced to the cutter bearing and this must be provided for. As for the rolling force, the designer really needs only the ratio of side-to-normal load. The present data indicate that this ratio is quite independent of rock type. The magnitude of the force reduction to be expected with skewed cutters is also of interest. The present data indicate that substantial reductions might be expected. A simple analytical model predicts the observed reduction reasonably well on the basis of the limited data available.