TPT March 2020

T E CHNOLOG Y

‘Reading the chips’ to gain valuable process information while cutting tubes By Christian Mayrhofer, R&D, AME

Depending on the material size the gullet size of the saw blade needs to be properly sized in order to avoid filled up gullets. If the gullet is too small for the cutting parameter, or has not been designed properly to shape, the gullet will fill up and the chips will put too much pressure on the tooth, which will cause premature failure of the teeth. If the gullet shape is generally acceptable you can increase the cutting speed or

The length of the chip will shrink accordingly:

WHEN operating a high production circular carbide saw quite a bit of process information from the cut chips on the shop floor can be gathered. One fact, which is not intuitive, if one has never thought about it, is the change of the chip thickness. When measuring the chip thickness with a calliper, the value is usually higher than the set chip load per tooth. Which brings up the question why does that happen and what are the practical implications for circular sawing? The difference in the chip thickness is due to the shear strength of the work piece, the friction between the chip and the tool and the rake (cutting) angle γ. The shear angle Φ, which can adjust itself, is between the cutting direction and shear plane. Resulting from the trigonometry the chip thickness is bigger than the set chip load per tooth.

λl….chip length ratio; l1….length of chip; φ e ….penetration angle of cut; r….saw blade radius Generally, the chip width ratio is a lot less compared to the other two dimensions. So, for further discussion is can be neglected. For designing the tooth geometry, even this small amount of growth has to be considered to avoid jamming of chips in the cutting slot. This is usually solved by the notch grind geometry or triple chip geometry. The chip volume of an individual chip has to be constant. This concludes when chip thickness will grow as a result and the length will be shortened. From the velocity plot the relation between the chip thickness ratio, the rake angle and the shear angle can be derived.

reduce feed rate to avoid overfilling the gullet. Let us say the chip thickness ratio for carbon steel is in the range between two and five. The length will shrink

Figure 3: Filled gullet

by the same factor and this determines the appropriate pitch for the blade. For stainless steel the factor is lower, which means that the chip does not grow so much in thickness, but on the other hand stays longer too. As a result, a bigger spacing for cutting stainless steel should be used and fewer teeth are engaged in the cut. As an alternative, chip breakers are used too. Generally, as a ball park rule, the sawblade is about three times bigger than the material diameter. Depending on the drive hub diameter this factor is reduced to minimise tooling cost. Under this situation the user will end up with about five teeth in the cut for solid steel. P….pitch; D….saw blade diameter; d….material diameter Z….number of teeth This is only one criterion for selecting number of teeth, besides required cutting power, guidance, vibrations and others. Advanced Machine & Engineering | AMSAW www.amsaw.com

Figure 2: Velocity plot

with γ….rake angle; v sp ….speed of chip flow; v….cutting speed Based on the formula above the shear angle is determined empirically, but it can be calculated through mechanics of plastic deformation Φ = f (y, , ...); ...Friction angle . Together with the corresponding shear cross-section A = and the shear strength gathered from material tests the required shear force as well as the direction can be estimated (figure 1). How does this chip thickness ratio apply to your saw blade?

Figure 1: Chip formation

λt….chip thickness ratio; h1….measured chip thickness; h…. chip feed rate per tooth

The width of the chip will increase too:

λw….chip width ratio; w1….width of chip; w….width of saw blade kerf

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MARCH 2020