Back Rake, Side Rake, and Exposure: Choosing PDC Cutter Geometry and Density in Numbers

Two quotes can carry the same bit diameter, the same blade count, and the same cutter size, and still deliver a two-fold difference in metres and bit life. The difference lives in four numbers on the drawing: back rake, side rake, cutter exposure, and placement density. None of them shows up in a price table, and all of them show up in the hole.

Why Two Identical Bits Drill Differently

Two quotes rarely disagree on diameter or cutter size; they disagree on geometry. The quotation lists what is easy to measure, while the metres and the bit life are decided by four numbers that only the design drawing carries: back rake, side rake, cutter exposure, and placement density. Through 2026 the industry has treated cutter placement and geometry as the main efficiency lever in PDC bit design, and the working consensus is that the four parameters have to be evaluated together, because each one changes how the load lands on the diamond table. This article turns each of the four into a number range a buyer can check against a drawing, before the price line is read.

The mechanics behind it are simple to state. A 19 mm cutter cutting 0.4 mm deep presents a contact area of 15-25 square millimetres to the rock; on a 6-1/2 in bit under 25 kN of weight on bit with six blades, that load spreads across 12-14 cutters at 1.8-2.1 kN each. Back rake decides how much of each cutter force points forward into the rock and how much turns into thrust; exposure decides whether the diamond or the blade behind it takes the first hit; density decides how many times that hit is repeated per revolution. Mechanical specific energy, the energy needed to remove one unit volume of rock, is the field yardstick: in clean cutting it runs near two to three times the rock UCS, and departures from that band point back at the geometry.

Back Rake Sets How Hard Each Cutter Works

Back rake is the angle between the cutter face and the vertical, and it is the single strongest penetration-rate lever on the drawing. A low back rake of 8-12 degrees bites deep, takes a thick chip, and suits soft clay, claystone, and unconsolidated sand, where the risk is not wear but balling and torque spikes. A medium range of 15-20 degrees is the default for siltstone, sandstone, and marl between 50 and 90 MPa. Above 20 degrees, in hard limestone, dolomite, and anything carrying chert, the geometry trades bite for protection: a thinner chip, less impact on the diamond table, and a flat wear crest instead of a chipped one.

The trade is measurable. In customer post-run reports across abrasive sandstone sections, raising back rake from about 12 to about 20 degrees cost crews 15-25 percent in penetration rate and returned 30-50 percent more metres per bit. The reverse error is the expensive one: a soft-formation angle run in hard rock loads the diamond edge past its limit, and the dull comes out of the hole with fresh fractures instead of a wear flat. Heat follows the same logic, because a deep bite at high rotary speed concentrates friction on a small face; past roughly 700 degrees Celsius the diamond table itself degrades. We publish the recommended back rake band for each of our cutter grades, and crews that log the band they ran can read the next dull grading against it.

Table 1. Back rake bands follow formation strength; the last row carries the bit-type crossover worked out in the November 3 article on chipping.

Side Rake Steers the Cuttings Off the Face

Side rake tilts the cutter face sideways so the chip flows across the bit face toward the open blade channel instead of piling at the cutting edge. The working range is narrow: 5-10 degrees covers most water well sediments, and 10-15 degrees earns its place in sticky clay where the chip smears instead of breaking. The direction has to alternate with the blade side and match the rotation, because a right-hand chip pushed toward a closed channel is worse than no side rake at all. Cuttings that stay on the face get recut, torque grows erratic, and the clay eventually builds into a ball.

In bedded formations side rake turns into a steering problem. Where bedding dips at a shallow angle, a cutter with a large side rake aligned down the dip can slide along the bedding plane instead of cutting across it; the dull grading shows polished faces at low penetration, and the crew reads it as a worn bit when the geometry is at fault. Orienting the high side rake against the apparent dip, 10-15 degrees, restores a true cutting action. This is one of the checks our engineers make when a customer sends drawings back asking for more penetration in a bedded section; changing the side rake direction costs nothing in tooling and sometimes removes the whole complaint.

Table 2. Side rake direction must alternate with the blade side and follow the rotation; the values are set per blade in the design drawing.

Exposure Is the Diamond Working Height

Exposure is the height the diamond table stands proud of the blade or body face, read as a percentage of cutter diameter. Soft formations want 30-35 percent, general sections 25-30 percent, and impact ground such as conglomerate drops to 18-25 percent so the tungsten carbide substrate takes the first hit from a pebble instead of the diamond edge. On a 19.05 mm cutter those bands are 5.7-6.7, 4.8-5.7, and 3.4-4.8 mm of exposed height; on a 13.44 mm cutter, 4.0-4.7, 3.4-4.0, and 2.4-3.4 mm. A gauge at goods-in is enough to check the number against the drawing.

Exposure couples with the chip the bit is trying to make. At 120 rpm and 12 m/h a 6-1/2 in bit advances 1.7 mm per revolution; if exposure is smaller than the chip thickness, the blade face starts dragging, torque rises, and penetration stalls at the body instead of the cutter. Raise exposure too far and the unsupported overhang grows: the braze joint holding each cutter, a silver alloy rated at 280 MPa shear in our process, then carries a longer bending arm at the moment a pebble arrives. That is why impact designs lower the exposure rather than thicken the diamond layer alone, and why the 3.3-4.0 mm thick-diamond option is usually paired with the 18-25 percent band, not added on top of a 35 percent exposure.

Table 3. Exposed height equals the band percentage times the cutter diameter; verify at goods-in with a depth gauge against the drawing.

Density, Coverage, and Reading MSE in the Field

Density decides how many cutters share the work and how often each one is hit. Nose cutters sweep the longest path per revolution and run the hottest, so they get the tightest spacing and the best cooling; shoulder cutters take the scraping load; the gauge rows hold the diameter and need backup support behind them. Spacing and hydraulics work as one system: the chip has to leave the face before the next cutter arrives, or the bit recuts its own debris. On a 6-1/2 in bit that means keeping 600-900 L/min across the face; a design with generous open area but weak flow to the inner rows will ball before it wears.

Mechanical specific energy closes the loop in the field. MSE is the energy spent per unit volume of rock removed, computed from weight on bit, torque, and penetration rate; in clean cutting it tracks two to three times the rock UCS. The absolute number matters less than the trend: when MSE climbs more than about 50 percent above its own baseline while penetration drops 20 percent, the cutters are overloaded, and the fix is geometric — drop the exposure one band or add a blade and redivide the load. Torque that swings more than about 15 percent at interbeds is the impact signature described in the November 3 article; polished faces at low penetration point back at side rake. The table below puts the four parameters in one lookup.

Table 4. The bands assume a 6-1/2 in water well bit at 25 kN; scale with diameter and keep the four parameters together when adjusting any one of them.

Table 5. Signals are read against the baseline recorded on the same bit run, not against another well; one change at a time, then re-read.

Read in order, the drawing stops being a black box. Back rake answers how hard each cutter works, side rake answers where the chip goes, exposure answers what takes the first hit, and density answers how the load divides. We put the recommended bands for each of our cutter grades on the specification sheet, and for custom orders we work from the formation log rather than a depth figure, because the four numbers only make sense together. A buyer who checks these four lines before the price line is comparing bits, not packages.

 Back rake, side rake, exposure, and placement density are the four numbers that separate two identical-looking PDC bits. The working bands by formation, plus an MSE trend check in the field, turn the drawing into something a buyer can read.

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