Cutting force in cross-sawing spruce: carbide versus HSS circular saw blades

Introduction

Circular saws are the workhorse of cross-cutting in woodworking, from trimming lines to joinery shops. The force the teeth must overcome decides how much power the machine needs, how much stress the blade body carries, and how quickly the teeth wear. Blunt teeth raise the cutting force, which in turn changes stress in the blade and the quality of the cut. For blade makers and mill maintenance teams, knowing realistic force and stress values for different tooth materials is therefore valuable.

The 2023 paper "Analysis of Cutting Forces in Cross-Sawing of Wood: A Study of Sintered Carbide and High-Speed Steel Blades", by Jozef Krilek, Jan Melichercik, Tomas Kuvik, Jan Kovac, Arkadiusz Gendek, Monika Aniszewska and Jan Marecek, was published in the journal Forests. The authors, working with a test rig at the Technical University in Zvolen, Slovakia, compared measured and calculated cutting forces for two 600 mm circular saw blades cutting Norway spruce across the grain, and then used finite element modelling to see how stress is distributed in the blades.

Research methodology

Test rig

The cutting drive consisted of a 7.5 kW electric motor, a belt transmission, a safety clutch and an HBM T20WN torque and speed sensor feeding the saw spindle. A separate 5.5 kW motor moved a sliding support through a threaded rod to provide feed, with the workpiece clamped by a lever mechanism. Signals were recorded with a Conmes Spider 8 measuring centre connected to a PC. Torque was measured, and cutting force was derived from it.

Wood

The workpieces were Norway spruce prisms of 150 x 150 x 1500 mm, cut from fresh 3000 mm logs on a horizontal band saw, and sawn transversely (perpendicular to the grain). Moisture content was determined gravimetrically on two small samples (50 x 50 x 20 mm) dried at 110 degrees C, although the paper does not report the resulting values.

Blades

Two coated blades from Pilana Tools were compared:

Test conditions and calculation

Cutting speeds of 60, 70 and 80 m/s were combined with feed speeds of 6, 8, 10 and 12 m/min. For each condition, six runs were recorded; the five highest torque values in each run were averaged, then the six run averages were averaged, and the result was converted to force. The blades were swapped at the end of the sequence.

The authors also calculated theoretical forces from the specific cutting force, which depends on tooth width, chip thickness and speeds. An example calculation used a specific cutting force of 30.19 N/mm2. They computed how many teeth are in the cut at once: about 6.95 for the SC blade and 7.2 for the HSS blade, so at most seven teeth are engaged.

Finite element analysis

Stress in the blades was simulated using PTC Creo Parametric software (the paper cites two different versions in different places) with hexahedral and tetrahedral mesh elements. Only one condition was modelled, 80 m/s cutting speed and 12 m/min feed, with forces applied at the tooth tips and with gravity and centrifugal load included. Statistica 12 was used for the measurement statistics.

Key findings

Measured cutting force

Across 60 measurements the average cutting force was about 41.0 N, with a minimum of 38.4 N, a maximum of 43.8 N and a standard deviation of 1.31. (The table in the paper prints the unit as "W", but the text indicates newtons.)

Measured force per tooth in the cut rose from the first to the seventh engaged tooth:

Theoretical (calculated) forces were higher, especially for the SC blade, which has wider teeth: the seventh tooth gave 9.83 N (HSS) and 15.87 N (SC). For the SC blade the paper reports a theoretical total cutting force of 76 to 94 N against a measured 40 to 44 N. Our own summation of the paper's tables gives roughly 40.5 N (HSS) and 43.7 N (SC) measured, and 51.2 N (HSS) and 82.7 N (SC) theoretical; these totals are not stated in the paper.

Speed effects

The authors conclude that neither cutting speed nor feed speed significantly affected cutting force. The detail is more nuanced: for the SC blade, force fell noticeably as cutting speed rose, while for the HSS blade it dropped at 70 m/s and rose again at higher speeds. No statistical test supporting the "no significant effect" conclusion is reported.

Stress in the blades

The simulations show stress concentrated at the tooth tips, with the highest values on the last tooth in the cut. For the HSS blade, theoretical and real stress were both in the range of 14 to 22 MPa. For the SC blade the real maximum stress was about 14 MPa and the theoretical about 25 MPa. In a narrow zone around the tip peaks, the stress rises to roughly 50 to 70 MPa. The authors explain the large gap for SC between theory and reality mainly by the idealised assumption of perfectly sharp teeth.

Practical applications for sawmills

Points from the paper

• Real cutting forces measured on spruce were substantially lower than force predicted from a specific cutting force model for the wide-toothed carbide blade; theoretical calculation overestimates the load.
• Stress is highest at the tooth tips of the last teeth in the cut, which is the zone where blade tips are most at risk.
• Within the tested range, changes in cutting speed (60 to 80 m/s) and feed speed (6 to 12 m/min) did not change the force by a large amount, according to the authors.

Portal interpretation (not claims made by the authors)

• Mills sizing motors or checking blade loading for spruce cross-cutting may use the measured force range as an order-of-magnitude reference, but should not rely on a calculation that assumes sharp teeth for a blade that will be used while worn.
• Because the stress peaks sit at the tooth tips, tooth geometry, tip brazing and regrinding quality are the points to watch in maintenance, rather than the overall blade body.
• The paper compares only two blades that differ in several ways at once (material, tooth width, tooth count), so it does not show that carbide is better or worse than HSS in cutting force. Purchasing decisions should rest on tool life and cut quality data, which are not covered here.

Limitations

The authors acknowledge that the simulation assumes perfectly sharp teeth, whereas real teeth blunt after a few cuts, which would lower the real stress. For the HSS blade the mesh was too coarse to apply the force at the small contact triangle of the tooth tip, so the whole edge was loaded instead; the HSS and SC simulations are therefore not fully comparable. The finite element analysis covered a single cutting condition, and wood anisotropy was mentioned but not modelled.

From our reading, further limits apply. Only one wood species (spruce), one blade of each type and a single test rig were used. Moisture content values are not reported, although wood moisture strongly affects cutting force. The unit mix-up in a results table and the two software versions quoted are minor editorial inconsistencies. The raw data are available only on request. We read the open-access full text, but the effect of tool wear, which motivates the study, was not itself measured.

Conclusions

The study shows that, for transverse cutting of fresh spruce with 600 mm blades, measured cutting forces were around 41 N on average and well below the theoretical values for the sintered carbide blade. Stress concentrates at the tips of the teeth that leave the cut last, with simulated values of roughly 14 to 25 MPa in the blade for the conditions tested. The finding of little sensitivity to cutting and feed speed should be treated cautiously given the data presented. The paper is a useful reference for researchers and tool engineers, and sawmill users can draw cautious lessons about the gap between idealised and real loads, but the evidence base is narrow.

Original research

• Authors: Jozef Krilek, Jan Melichercik, Tomas Kuvik, Jan Kovac, Arkadiusz Gendek, Monika Aniszewska, Jan Marecek
• Title: Analysis of Cutting Forces in Cross-Sawing of Wood: A Study of Sintered Carbide and High-Speed Steel Blades
• Journal: Forests, 2023, volume 14, issue 7, article 1395
• DOI: 10.3390/f14071395
• Link: https://www.mdpi.com/1999-4907/14/7/1395
• Licence: Creative Commons Attribution (CC BY), open access

This article is an independent summary by the portal; the original paper should be consulted for full details.