Graphite Moulds for Geological Drill Bits: Teeth Must Hold Their Ground, Waterways Must Flow Freely – Why Are These Two Accounts Opposed?

XRD Graphite · Graphite Moulds for Geological Drill Bits · Technical Column

Graphite Moulds for Geological Drill Bits: Teeth Must Hold Their Ground, Waterways Must Flow Freely – Why Are These Two Accounts Opposed?

This type of component is an annular forming piece with a ring of teeth: an annular body, a central through-hole, and a ring of raised working blocks distributed by angle on the end face. The top of each block is engraved with several fine slots, and gaps are left between blocks. It forms the contour of the drill bit end face—the raised portions correspond to the blocks that take abrasive on the end face, while the fine slots and gaps correspond to the passages for powder discharge and water flow. This article explains four things: where it sits in the drill bit manufacturing process, how the opposing accounts of teeth and waterways are calculated, why the fine slots on the tooth tops deserve separate discussion, and several points worth specifying in advance on drawings and acceptance.

Written by: XRD Graphite · Technical Column

I. First, Positioning: What Stage of the Drill Bit Manufacturing Line Does It Handle?

A geological drill bit uses a “matrix” to cut rock. The so-called matrix is a working layer formed by mixing diamond particles with metal powder, pressing them together, and sintering them into a ring. This working layer must also be divided into “places that take material” and “places that give way”: the places that take material are the raised working blocks on the end face, and the places that give way are the gaps and fine slots between them, used to discharge rock powder and deliver cooling water to the working face.

The job of the graphite mould on this line is to frame this ring of contour. It is not a cup for holding material, nor a regular cylinder, but a forming piece with shapes distributed by angle around a ring: the raised working blocks on the end face define the tooth profile of the drill bit end face; the fine slots on the block tops and the gaps between blocks define the passages; the outer diameter and inner bore define the inner and outer boundaries of the drill bit. Powder is loaded inside, and after pressing, sintering, and demoulding, this ring of end-face contour is transferred to the green body.

It becomes clearer when separated from adjacent component types: support components worry about “holding it up, holding it flat, and not contaminating it”; a single-cavity mould worries about “the dimensions and fit of this one part”; this component worries about “how shapes are distributed around a ring of contour”—which section protrudes to take abrasive and which section recesses to pass water and discharge powder is all determined by it. The accounts differ, and the following discussion covers only the last type.

II. Why Graphite Is Used for This Category

Not many materials can frame this kind of contour at high temperature and still be repeatedly inserted and removed. Graphite is widely used in this category because several properties happen to line up:

• Dimensional stability at high temperatures. Low deformation within the sintering temperature range means tooth width, height, and ring indexing hold, and only then can the tooth profile on the end face of a ring of green bodies be consistent.
• Thermal shock resistance. Entering and leaving the furnace and heating and cooling often involve rapid thermal cycling; the material must withstand this repetition without cracking.
• Demoulding-friendly. Graphite does not easily bond with green bodies pressed from diamond particles and matrix metal powder, and has a degree of self-lubrication, so it pulls less on tooth walls and slot openings during demoulding.
• Chemical inertness. Under vacuum or protective atmospheres, it does not react with matrix powder or introduce metal contamination into the matrix, making it friendly to products with strict appearance and bonding requirements.
• Relatively uniform thermal conductivity. The ring body distributes heat relatively evenly to the ring of teeth, keeping temperature differences between teeth small so that a ring of green bodies sinters to a consistent degree.
• Ability to produce features difficult for metal moulds. Narrow, dense fine slots, fillet transitions at corners, and equally divided tooth positions around a ring can be turned, milled, drilled, and engraved to drawings. The fine slots on tooth tops in particular have width and depth on a very small scale, and the material’s grain size and machinability directly determine whether these features can be produced.
• Lightweight. With dozens of tooth positions cut into a ring, it can still be handled during loading and unloading, and the component can be made in larger formats.

These advantages share a common prerequisite: vacuum or protective atmosphere. Under oxidising atmospheres at sustained high temperatures, graphite loss accelerates noticeably—this is discussed specifically in the boundaries section later.

III. The Opposing Accounts on This Component: Teeth Must Hold Their Ground, Waterways Must Flow Freely

Almost all trade-offs on this mould arise from a single contradiction.

One side is that teeth must hold their ground. Teeth are the parts of the end face that take abrasive. Tooth width, tooth height, and tooth root determine whether a block can carry material and bear force. Making teeth wider and thicker allows more material to be loaded, gives the root sufficient strength, and distributes abrasive more widely on the end face.

The other side is that waterways must flow freely. Between teeth and above tooth tops, passages must be left for rock powder to travel and water to pass. If the passage cross-section is too small, rock powder cannot be discharged during drilling and smears on the end face, so the working face ends up grinding against itself. If cooling water cannot reach the tooth tops, their temperature cannot be reduced.

Putting the two together, they form a pair of opposing accounts: widen the teeth and the passages narrow; narrow the teeth and the passages widen, but the area on the end face that takes material decreases. Tooth height is also part of this account—making teeth taller can enlarge the passage cross-section, but the tooth root is thinned, and it is often the root that deforms first during pressing and deflects first during machining. Therefore, this mould is neither about “the more teeth, the better” nor “the wider the waterway, the safer.” It must be determined by what rock is being cut, how much water is intended, and how much material each tooth block must carry.

This brings out a second relationship: tooth height also determines the thickness of the end-face working layer. How much the working block protrudes is the thickness of usable working layer on the end face. If the tooth is set high, the usable face is thicker, and the bending that the root section must bear is also more pronounced. When the drawing says “tooth height of X,” it is actually determining two things at once—the passage cross-section and the thickness of the usable working layer.

IV. A Ring of Tooth Positions Tied to the Same Axis: Concentricity and Indexing

The shapes on this component are distributed around the centre, so two things should be established before individual dimensions.

First, concentricity. The outer diameter determines its position in the furnace and on the press, and the inner bore defines the centre. Only when the two are coaxial can a ring of tooth positions be uniform. If the two are eccentric, the entire ring of tooth positions shifts to one side—in some places teeth are squeezed narrower and passages narrow accordingly, while in other places teeth are released and passages widen. This differs from flat-plate components: cavity positions on a flat plate are a row-and-column relationship, and deviations can be traced row by row. Tooth positions on an annular component are an angular relationship, and deviations are distributed around a centre. When measuring, it must be clear from which circle the measurement is taken and whether angle or arc length is being measured.

Second, indexing. How many teeth the ring is divided into and how much angle each tooth occupies are basic parameters of this component. There is an easily ambiguous point here: the same angular deviation corresponds to different arc lengths when it falls on the circle near the inner bore versus the circle near the outer diameter. Therefore, writing “uniform indexing around the ring” on the drawing is often insufficient. It must also specify whether indexing is marked along the inner circle, outer circle, or pitch diameter, and which circle is used for measurement during inspection.

These two matters show directly on the drill bit: if a ring of tooth positions is offset or indexing is uneven, each tooth block on the end face receives a different amount of abrasive and force. During drilling, the end face is unevenly loaded, manifesting as uneven wear and runout. Such deviations are not easily seen on the mould by appearance alone; they must be measured along a circle.

V. The Fine Slots on Tooth Tops: A Place Easily Treated as a Detail

On a ring of teeth, everyone notices the raised portions, while the fine slots on the tooth tops are often treated as an accessory. They actually handle three things.

They define the passage cross-section. The fine slots on tooth tops form the powder discharge and water passages on the drill bit end face. A slightly wider or deeper slot enlarges the passage cross-section; conversely, the passage becomes smaller. The width and depth of these slots on the mould become this cross-section dimension on the drill bit—it is not an appearance feature but a passage parameter.

They also divide the working face. Several fine slots divide one tooth block into several smaller material-taking surfaces. The working face is no longer a single block but is cut into strips, shortening the powder discharge path on each strip and making it harder for rock powder to accumulate in the middle of the end face. The cost is that multiple edges appear on the same tooth block, and wear at these edges is naturally faster.

They are also where machining difficulty concentrates. Slots are narrow and dense, tools are thin with long overhang, and rigidity cannot be good. Tool deflection and chatter marks easily emerge here. The transition between slot opening and slot bottom is also a high-risk area for chipping—if a piece chips at the slot opening, the passage shape in that small section changes. Feeding must be light and divided into multiple passes, so efficiency naturally suffers, and labour time concentrates in this section.

One point deserves separate mention: however many such fine slots there are on a mould, their widths and depths should be consistent with each other. If several slots on one tooth differ slightly, the difference looks small on the mould, but on the drill bit it means powder discharge capacity differs at different points on the end face. Therefore, when re-inspecting this component, slot width and depth should be checked together with tooth width and tooth height, not just the external contour—measuring only the contour easily lets through the case of “contour correct, slots uneven.”

VI. How It Is Consumed: Not Through Appearance, but Through Tooth-Top Dimension Drift

A mould does not serve only one furnace run. It lives through repeated powder loading, pressing, furnace entry, demoulding, and slot cleaning, and during this process the tooth tops, slot openings, and inter-tooth gaps gradually change.

There are two ways it changes, both on the tooth-top side. Tooth tops become shorter—during pressing, the tooth top directly transmits pressure against the powder, and during demoulding the green body is pulled out against the tooth top, so wear occurs here first. Fine slots become wider and shallower—slot openings are dulled and small pieces chip off, slot bottoms are pressed with marks, and on the other side, residual powder and scale left when slots are not cleaned properly occupy depth.

Together, the result is that with the same mould and the same pressing parameters, the end-face contour and tooth height of the parts from this run begin to differ from the previous run. Therefore, it does not appear in the form of “the mould cracked or chipped”—a slightly lower tooth top or slightly wider slot still allows the part to be loaded into the furnace and pressed, with no visible abnormality in appearance. What shows first is the end-face tooth profile and working layer thickness of the output, or the difference between two batches from the same mould.

Two actionable points follow. One is to include the width and depth of tooth-top fine slots and tooth height in re-inspection items, and tie the re-inspection rhythm to furnace runs or usage counts—this is easier than waiting for output to fluctuate. The other is routine tooth and slot cleaning—how thoroughly each cleaning is done directly determines the next slot depth. Careless cleaning effectively makes the mould shrink step by step.

To conclude this section in one sentence: consumption of this type of component is gradual, not sudden. It is a consumable and is better managed on a consumable rhythm—self-inspection by furnace run, replacement based on drift trend, rather than waiting for it to crack. The cost of replacing one mould is usually lighter than the batch of green bodies it affects.

VII. At the Machining End: Several Underestimated Points on Annular Components

From a graphite blank or tube to a toothed forming piece ready for the furnace, the process roughly follows these steps: determine outer diameter, inner bore, thickness, and tooth profile plan according to product and furnace type; first grind both end faces, establishing flatness and parallelism as subsequent datums; turn the outer diameter and inner bore, achieving concentricity in this setup; mill a ring of tooth positions by indexing, cut inter-tooth gaps, and engrave fine slots on tooth tops; chamfer and clean dust; verify tooth width, tooth height, and slot width and depth tooth by tooth, then re-check concentricity of inner bore and outer diameter and parallelism of the two faces.

The process looks conventional, but five points are underestimated.

• Concentricity depends on one setup, not on flipping and aligning. Tooth positions on an annular component are measured from the centre, and one alignment may shift the entire ring. The reliable approach is to first produce either the inner bore or outer diameter as the datum, then measure all tooth positions from this axis, rather than indicating separately in each operation.
• The relationship between the two end faces must be established first. The pressing direction is the relationship between the two end faces. If the end faces are not parallel, the force on the ring of teeth is uneven when the press head descends, and the degree of compaction between teeth differs accordingly. This is basic work at the machining end but a direct consequence at the pressing end.
• Fine slots require light cutting and multiple passes. Slots are narrow, tools thin, and overhang long. Cutting to size in one pass easily causes deflection, chatter marks, and slot opening chipping. Dividing into multiple passes with smaller feed preserves the condition of slot walls and bottoms—and the smoother the slot walls, the easier the green body is pulled out, and the slower the slot opening wears.
• Transition between inter-tooth gaps and tooth roots. The bottom of gaps and the transition between tooth root and ring body are stress concentration locations and high-risk chipping locations. Rounding corners is not for appearance but to prevent cracking from starting there. Staged machining and controlled feed are far more reliable than cutting to size in one pass. A chip at the tooth root also affects adjacent teeth.
• Dust cleaning and slot bottoms. Teeth are small and numerous, slots narrow and deep, and places where compressed air cannot reach are hidden risks. Residual powder at slot bottoms makes the actual slot depth insufficient and occupies space that powder should fill. This is the same matter seen from two ends—the machining end must blow clean and verify once, and the use end must clean thoroughly after each furnace run.

Because every ring follows the drawing—change the drill bit and the tooth profile plan, tooth count, and slot layout change—these components are usually custom-machined to drawings, not standard stock items.

VIII. Material Side: These Items Should Be Considered Together with the Grade

For the same external shape, changing the graphite grade can produce considerably different performance. When selecting a grade, the following items are worth putting on the table:

• Purity and ash. Ash corresponds to inorganic inclusions in the material. At high temperature and in contact with matrix metal powder, it is a variable that must be controlled. For products sensitive to appearance and bonding, this item ranks before strength.
• Density and strength. Teeth and tooth roots must withstand lateral force during pressing. With grades of insufficient strength, it is often the tooth root that deforms first, not the tooth top.
• Structural uniformity. A ring of teeth is pressed and heated simultaneously. If the material itself is not uniform, deformation and wear around the ring will not be consistent, and such deviations appear in finished products as individual parts differing from others in the same ring.
• Grain size and machinability. This determines what condition fine slots and corners can achieve, how prone slot openings are to chipping, and how stably slot width and depth can be produced.

The specific levels of these items vary with grade and specification. When using them, refer to the technical conditions for the corresponding grade. Specific values are not given here—the same drawing paired with different grades should be selected according to the application, not by a fixed level.

IX. Loading and Use Side: Several Things On Site That Determine Results

The mould provides consistency of shape and position; the other half lies in loading and use.

• Powder loading and distribution. Tooth tops and inter-tooth gaps are uneven surfaces. The distribution of material and the technique of scraping directly determine how much each tooth block actually holds and whether the powder layer on tooth tops is level. With the same mould and the same pressing parameters, a change in powder loading technique changes the tooth height of the output—this cannot be solved at the machining end and must be stabilised by fixing the actions on site.
• Pressing and pressure transmission. Pressure first passes through the press head, tooth tops, and inter-tooth gaps, then reaches the powder. If the condition of tooth tops and gaps changes, the force around the ring changes accordingly, which is why tooth tops and slot openings were emphasised earlier as re-inspection items.
• Demoulding and part removal. Graphite is brittle. Tapping and point contact should be avoided during removal, especially at tooth tops and slot openings. Prying or forcing will pull and chip tooth tops, and a chipped section makes the end-face contour in that section non-conforming.
• Tooth and slot cleaning and storage. Clean teeth and slots according to the furnace-run procedure after each run. If powder and scale are not removed, the mould is effectively shrinking step by step. Store against moisture and impact; a mould that has absorbed moisture should be treated according to site procedures before first use.

Then there is atmosphere. Under oxidising atmospheres at sustained high temperatures, graphite loss accelerates noticeably. Locations with high specific surface area, such as tooth tops, slot openings, and gap transitions, are often damaged first. Once tooth tops become shorter and duller, the end-face tooth profile and working layer thickness of the output drift accordingly. Vacuum or inert gas protection avoids this problem. If use in an oxygen-containing environment is required, loss must be anticipated and corresponding protection provided.

X. Where Are They Mainly Used?

Wherever a process requires “distributing a ring of working blocks and passages by angle, pressing a ring together, sending it into the furnace together, and taking it out together,” this type of annular toothed component has a role. For a specific piece of equipment, the outer diameter, inner bore, number of teeth, tooth width and height, and slot width and depth must all be determined based on product drawings, powder characteristics, shrinkage behaviour, and pressing method. This type of component basically does not offer universal values.

XI. Drawings and Acceptance: Several Points Worth Specifying in Advance

These components are custom-machined to drawings, and how clearly the drawing is written directly determines whether there will be disputes later. On paper, it is recommended to state the following items all at once: outer diameter and inner bore diameters and tolerances; concentricity requirement between them; component thickness; parallelism and flatness of the two end faces; number of teeth and indexing (noting along which circle it is marked and measured); tooth width and tooth height and their respective tolerances; number, width, depth, and tolerances of tooth-top fine slots; transition fillets at slot openings and slot bottoms; width and depth of inter-tooth gaps; transition between tooth root and ring body; surface condition requirements for tooth walls and slot bottoms; quantity and matched grouping.

For incoming acceptance, it is recommended to go through the following sequence:

1. Re-inspect tooth width and tooth height tooth by tooth, focusing on whether tooth tops have chipped corners and whether tooth roots have chipping;
2. Re-inspect the number, width, and depth of tooth-top fine slots, checking whether each slot is consistent with the others and whether uncleaned residue remains at slot bottoms;
3. Re-inspect the inner bore and outer diameter and calculate their concentricity;
4. Re-inspect parallelism and flatness of the two end faces, measuring from the face specified on the drawing;
5. Re-inspect ring indexing, measuring angles from the datum circle specified on the drawing;
6. Blow clean and visually inspect slot by slot to confirm no powder residue remains. Where multiple pieces are used as a set, check indexing and tooth width consistency by group to avoid the situation of “individual pieces qualified, set not qualified.”

A more reliable approach is to first clarify the product drawing, matrix powder characteristics, pressing method, and loading and demoulding techniques, then determine the graphite grade and tooth profile plan accordingly. If drawings or operating conditions are not yet clear, single-piece or small-batch verification first, running through tooth height, slot width and depth, and demoulding condition once, then scaling up the full batch, keeps trial-and-error cost relatively controllable.

XII. About XRD Graphite’s Graphite Moulds for Geological Drill Bits

With 30 years of experience in graphite moulds for geological drill bits, XRD Graphite has accumulated extensive process expertise in custom machining of annular toothed forming components. We machine precisely to customer drawings and strictly control critical tolerances such as concentricity and indexing, tooth width and tooth height, dimensions of tooth-top fine slots and inter-tooth gaps, end-face flatness, and two-face parallelism. Our products offer relatively long service life under normal use and maintenance and have gained recognition from many industry clients. The factory is located in Baofeng, Pingdingshan, Henan, with both graphite material production capacity and machining lines. Material preparation and machining for such components can be completed in-house.

We can custom-manufacture graphite moulds for geological drill bits and various annular toothed graphite forming components, and support custom orders from drawings, samples, and small-batch trials. For applications involving superhard abrasives such as diamond products, it is especially recommended to specify the operating atmosphere, powder loading method, and slot cleaning rhythm together, so that a suitable point can be selected between purity and strength of the grade.

Final Thoughts

A graphite mould for geological drill bits may look like a ring of grooved teeth, but the difficulty lies at the junction of two opposing accounts: one is the present—teeth must hold their ground, and tooth width, tooth height, and tooth root must all withstand pressing and demoulding; the other is the future—passages must flow freely, and the cross-sections of fine slots and gaps determine powder discharge and water flow, and these cross-sections change little by little with use. Unlike structural components, it does not give feedback in the form of “cannot be installed,” but in the form of drift in the end-face tooth profile and working layer thickness of the output. Using it well is half at the machining end, through single-setup concentricity, circular indexing, light cutting to preserve slot shape, and dust cleaning to preserve slot depth; and half in management and use, fixing the re-inspection rhythm for tooth-top fine slots and tooth height, the actions of tooth and slot cleaning, and the technique of powder loading. In actual procurement, presenting the product drawing, matrix powder characteristics, operating atmosphere, and pressing method first, and then discussing the tooth profile plan, grade, and inspection conventions, is the more reliable path. For specific operating conditions, confirmation based on actual parameters and drawings is still recommended.