Disc-Shaped Graphite Heating Elements: What Heats Up Are the Narrow Strips on Either Side of the Groove, Not the Whole Disc

XRD Graphite · Disc-Shaped Graphite Heating Elements · Technical Column

Disc-Shaped Graphite Heating Elements: What Heats Up Are the Narrow Strips on Either Side of the Groove, Not the Whole Disc

There is more than one way to make a disc-shaped heating element. Another common form has a continuous serpentine groove machined into the disc face. Current enters through one hole, travels along this groove around the entire disc, and exits through another hole. It looks as though the whole disc is generating heat, but in reality the heat is concentrated in the solid narrow strips on either side of the groove. This article breaks down this “long resistance band hidden on the disc face,” then covers how the groove is machined, how the holes are positioned, and which items left blank on drawings and during machining tend to cause disputes at both ends.

Written by: XRD Graphite · Technical Column

Let us put the conclusion first: A disc-shaped heating element has two different “current paths.” One is radial slotting on the disc face, cutting the disc into several sector blocks that each loop back and forth. The other is a continuous serpentine groove machined into the disc face, dividing the entire disc into a single conductive band connected end to end. In the latter, heat is not spread evenly across the whole plate; it is concentrated in the solid narrow strips between the grooves—the groove is the “gap” for current, and the solid material on either side is where resistance and heat generation reside. Once this is understood, the precision of groove machining, corner geometry, and hole positions have a proper standard to follow.

I. Positioning: Same Disc, Different Current Paths

Disc-shaped heating elements are usually chosen for three reasons: a large-area planar heat source is needed, an axisymmetric temperature field is required, and space must be freed at the centre for a temperature measurement passage or support. But within the same circular outline, the current path can be arranged in two ways, and the geometry and machining logic differ.

Both are commonly used for planar radiant heating that supports or surrounds workpieces in vacuum furnaces and atmosphere furnaces. The difference is that radial slotting has multiple independent branches connected back by “bridges,” while the disc-face serpentine is a single continuous path with no “bridge” concept; structural continuity relies on the solid material left at the disc rim, inner bore, and groove bottom so that the part does not separate. Only the latter is discussed below.

II. Essence: It Is Still a Resistor, but the Resistance Is Hidden on Either Side of the Groove

Heat output is determined by power, power by resistance and current, and resistance by material resistivity, path length, and cross-section. This relationship holds for any shape. The special feature of the disc-face serpentine groove is that the solid narrow strips separated by the groove have a very small cross-section, and current must travel along this narrow band around the entire disc. The path is therefore long and the cross-section small, so resistance is naturally high and heat generation is concentrated in this band.

This leads to an easily reversed point: the disc face looks like a solid plate, but what actually “conducts and heats” is only the narrow strips between grooves, accounting for part of the total area. The denser the grooves and the more turns, the longer this band, and at the same voltage the lower the power and the gentler the temperature rise. Therefore, the power of this type of component is not determined by “how large the plate is,” but by “the developed length and cross-section of that band.” What is really being adjusted in design is the shape of this band, not the outline of the disc.

III. The Continuous Groove: Length Equals Resistance, Number of Turns Equals Power

The continuous serpentine groove divides the entire disc into a band connected end to end. The current path length is approximately equal to the total developed length of this groove. Several things are therefore directly linked:

• The denser the groove and the more turns, the longer the path and the higher the resistance. With the same material and voltage, power is lower, making it suitable for applications requiring gentle temperature rise and distributed thermal load.
• Groove width and disc thickness determine the cross-section of that band. Only with consistent groove width and consistent disc thickness is the cross-section consistent, and only then is heat generation relatively uniform along the length. If the groove is wide on one side and narrow on the other, the cross-section changes and heat generation shifts accordingly.
• The remaining wall thickness from groove bottom to disc back is an independent control value. It is the lower limit of mechanical strength for this band and the thermal and structural barrier between the disc back and the disc face. If slot depth is fixed but remaining wall thickness is not specified, the machining end stops at slot depth and wall thickness may be a matter of chance.

One point to note: this groove is a continuous path completed in one pass. A broken tool midway, deflection off course, or a corner that fails to connect means the path is broken—at best that section does not heat; at worst the entire band is discontinuous somewhere and power drops significantly. Therefore, this type of component places higher demands on tool path continuity and clamping stability than radially slotted parts.

IV. The Two Holes: Current Inlet and Outlet, and the Two Endpoints of the Entire Path

This type of component often has an inward-opening slot on each of the left and right sides, with a circular hole at the end of each slot. They look like mounting locations but are essentially the two endpoints of the current path: one inlet, one outlet. The two ends of the groove connect to these two holes respectively. Once the hole positions change, the start, end, and direction of the entire serpentine path change accordingly.

• Distance from hole to groove end: This “terminal connection” is also part of the conductive band. Too close, and current converges into a bundle at the hole edge, causing local overheating and eventual enlargement and chipping at the hole opening. Too far, and an extra “cold section” that should not generate heat by design appears, leaving a low-temperature area near the disc edge.
• Hole and groove ends must align: The two ends of the groove must land accurately in the side slots and connect to the holes. If the drawing does not define an angular datum, the machining end starts the cut according to its own habit, and the actual connection point may differ from the design intent—at best power deviates; at worst that small section does not connect.
• Edge distance: The distance from hole edge to disc rim, to inner bore, and to adjacent groove sections cannot be corrected at the machining stage. If edge distance is too small, cracking propagates from the hole edge under assembly or thermal cycling load.
• How the hole wall bears load: If the hole only provides clearance for a bolt, the graphite does not bear thread force. If made as a threaded hole, graphite threads do not withstand repeated assembly and disassembly and are suitable only for light loads and infrequent assembly.

Therefore, for these two holes, the drawing should specify three things: hole diameter, hole position (relative to the centre with an angular datum, such as the angle relative to the groove start), and the net distance from hole edge to adjacent groove section. Of these three, the angular datum is often omitted—and this is precisely the basis for “whether the holes and grooves line up.”

V. Several Points Often Underestimated at the Machining End (Key)

The continuous serpentine groove on the disc face is the main source of machining difficulty for this type of component. Several points are easily treated as “just follow the drawing” but actually involve trade-offs:

• One continuous pass, no interruption: The path is continuous; a broken tool or clamp change midway risks a broken circuit. Tool, tool path, and clamping must succeed in one attempt, and the clamp should not be released or flipped midway—flipping hands the datum of “which hole to start from and which hole to end at” to luck.
• Corners are not sharp angles: At the “S” bends of the serpentine, the milling cutter sweep naturally leaves sharp corners or abrupt changes in groove width. Sharp corners are stress concentration points and current concentration points at turns. The corner radius should be specified as a dimension and made as a fillet, not left to the tool to form naturally.
• Long-path tool deflection: Lateral deflection along the path makes groove width wider at the top and narrower at the bottom—measuring only at the groove opening passes, but the groove bottom may already be narrower. At dense corners, tool deflection is greater and groove width drifts more easily. Countermeasures include layered milling, climb milling pressing toward the solid side, smaller depth of cut with faster feed, and using the thickest possible tool shank.
• Remaining wall thickness at groove bottom: The distance from groove bottom to disc back is not infinite. This is often an independent value missing from drawings. For measurement, depth gauges measure groove depth in segments, then ultrasonic thickness measurement is used point by point to derive the remainder—more practical than “reaching a gauge down to the groove bottom.” If only slot depth is specified, the operator stops at depth.
• Thin-disc rigidity and flatness: After grooving, the entire disc is divided into a single band, rigidity drops noticeably, and residual stress release causes the disc face to warp slowly. Grinding flat first and then grooving means it will move after grooving no matter how flat it was; for large-diameter thin discs, rough grinding—grooving—resting—finish grinding is advisable.
• Chipping and dust cleaning: Tool exit points and corners are prone to chipping; graphite powder in the grooves must be blown clean. Residual powder after furnace loading may be carried by hot gas to groove openings and accumulate, potentially causing contact short-circuiting.

VI. Two Current Concentration Points: Corners and Hole Openings

As current travels along this band, two locations become “crowded” and deserve attention on the entire component.

Corners: Current turns at bends, the cross-section is geometrically narrowed here, and heat from both sides accumulates here. Thus a corner is simultaneously a point of resistance concentration, heat concentration, and stress concentration. If the corner fillet is too small, it cannot spread the concentration; if too large, it encroaches on the heating length. This is a set of values that must be given together.

Hole opening convergence: The current of the entire band converges into and out of the two holes at the ends, and the hole edges are local high points of current density. If edge distance is too small and hole opening dulling is insufficient, scaling and enlargement start here over time.

These two locations are easy to observe: during heating, watch which location glows first—if corners or hole openings clearly turn red before the band body, concentration there is excessive. After a period of use, inspect the surfaces at these two locations—if they thin first or edges round off first, temperature and atmosphere attack are heavier there. This deviation cannot be compensated by adjusting power, because it comes from geometry.

VII. Temperature Field: Why Disc-Face Serpentine Is Often More Uniform, and Where It Is Still Uneven

Compared with radial slotting, one advantage of the disc-face serpentine is that if groove width is consistent and disc thickness is consistent, the band approximates a long strip of uniform cross-section; resistance along its length is relatively uniform, and heat generation is relatively uniform. It does not easily produce the systematic “inner ring runs hot” deviation of radial slotting. This is why it is often used where disc-face temperature uniformity is required.

But “relatively uniform” does not mean “no non-uniformity.” Two sources remain:

• Different arc lengths on inner and outer turns of the spiral: As the serpentine runs from the inner ring to the outer ring, the arc length corresponding to an outer section is longer. If arranged at equal pitch, the outer band is more “stretched” and the inner band more “compressed,” so the effective width of the band differs between inner and outer rings, causing slight radial temperature differences.
• Local overheating at corners and hole openings: The concentration points discussed in the previous section make local temperature higher than the band average. The more corners and the stronger the convergence at hole openings, the more pronounced these local high points.

Therefore, the statement “uniform disc-face temperature” is only enforceable at acceptance if the drawing specifies “at which radii to measure, what temperature difference is allowed, and whether corners and hole openings have separate limits.” If it says only “uniform,” and measurement afterwards shows differences, responsibility cannot be clearly assigned.

VIII. Special Risk of Open Grooves: Powder Accumulation and Contact Short-Circuiting

Radial slots are narrow slits, while disc-face serpentine grooves are open channels. Both can collect powder, but open channels have a larger, shallower containment surface; material powder, condensate, spalled coating, or graphite debris falling in more easily bridges adjacent band sections.

Contact short-circuiting is a local failure of this type of component, not a whole-component failure. It manifests as one section running particularly hot while other areas cannot heat up; in severe cases that section ablates first. For prevention, cleaning powder from grooves should be a step before furnace loading. During use, if a fixed “hot spot–cold spot” alternation appears on the disc face, suspect contact short-circuiting first rather than insufficient power. This differs from the “overall power decline” caused by oxidation; the two must be distinguished before judgement.

IX. How to Select the Material

For components with disc-face serpentine grooves, material selection has a shape-related special requirement: isotropy. Because the band circles around the centre, the same component contains solid sections running along one direction and across another. If the blank has pronounced directionality, the strength and resistance of band sections with different orientations will differ, causing systematic differences on the disc face. Isostatically pressed fine-grain material is more suitable in this respect, with uniform structure and small directional differences.

• Purity and ash: The heating element itself is inside the furnace chamber; impurities migrate at high temperatures and contaminate workpieces and furnace internals. For semiconductor, optical, and high-purity material applications, this item is more sensitive than power itself.
• Resistivity consistency: Beyond the nominal value, batch consistency is more important. If resistance differs along the band, the temperature field cannot be uniform.
• Flexural strength: The band after grooving should be considered as a cantilever; what matters is flexural strength, not compressive strength. Handling, clamping, and thermal shock all test this item.
• Thermal expansion and thermal shock resistance: The band is thin with many turns; materials with higher thermal expansion and weaker thermal shock resistance more easily crack from corners.

The above indicators vary with grade and specification; the technical conditions for the corresponding grade apply. Under the same phrase “high-purity graphite,” fine grain versus coarse grain, and isostatic versus moulded, produce groove opening condition and service life on entirely different levels. During selection, the grade should be written on the drawing.

X. After Installation in the Furnace: Drift, Short-Circuiting, and Oxidation

Failure of this type of component is mostly not “sudden breakage” but several slow losses appearing together.

• Resistance drift: Groove openings slowly widen in service—oxidative loss, high-temperature sublimation and migration, and repeated thermal expansion and contraction wearing away edges. As grooves widen, the effective cross-section of the band decreases, resistance rises, and power drops at the same voltage. The manifestation is often “heating gets slower and slower, and the same process curve can no longer reach the original temperature.”
• Contact short-circuiting: Powder falling into grooves or thermal expansion bringing band sections into contact causes local short-circuiting, with one section running particularly hot.
• Cracking from corners: Corners and hole openings are stress outlets; when heating and cooling are too fast or local overheating occurs, cracks start from these locations. Once a crack penetrates to an adjacent groove, that section of band fails.
• Atmosphere attack: Graphite is stable under vacuum and inert atmospheres; if the furnace contains trace oxidising components, oxidative loss still occurs at sustained high temperatures. This type of loss is faster on thin-walled, multi-grooved components than on thick solid components because there is more surface available for attack.

The criterion for replacement should be established before furnace loading. Measure the resistance of a new component in the cold state, or the voltage drop at a known current, and record it as a baseline; thereafter, re-measure at each maintenance and judge by an agreed ratio. Several other items can be checked together: whether groove openings have noticeably widened, whether the disc face has warped enough to affect furnace loading, whether corners or hole openings have noticeably thinned, and whether there are through cracks. If any one reaches its limit, continued use is uneconomical—not because it will fail immediately, but because its power is no longer at the design point and the temperature field in the furnace has shifted accordingly.

XI. On Drawings and Orders, It Is Advisable to Specify the Following

For components with disc-face serpentine grooves, the common drawing problem is not “cannot be drawn” but “several key numbers are missing.” Based on experience, the following are often left blank:

1. Inner and outer diameters and disc thickness, including thickness tolerance and measurement point distribution.
2. Definition of the groove path: developed pitch or number of turns, start and end angles, and the datum relative to which hole—drawing only a serpentine line without marking start and end leaves the supplier to guess the connection method.
3. Groove width and depth, including whether corner radii and groove bottom fillets are required.
4. Remaining wall thickness at groove bottom, listed as an independent control value with the measurement method noted (e.g., ultrasonic thickness measurement or equivalent).
5. Diameters and positions of the two holes, and the net distance from hole edge to groove; hole positions require an angular datum.
6. Disc face flatness and two-face parallelism, including measurement method and support condition (free state or installed state, support point locations, measurement point distribution).
7. Material grade, and requirements for data to be supplied with the goods, such as resistivity and ash content.
8. Marking and packaging: part number, batch, orientation marks; transport upright and separated to avoid pressure on the band body.

If only two items can be added, add items 2 and 6—the groove path definition and the measurement method. Without these two, the supplier starts the cut according to their own habit and measures flatness according to their own habit; the resulting component may be “dimensionally correct but functionally wrong,” a dispute that cannot be resolved by post-hoc measurement.

About XRD Graphite’s Disc-Shaped Graphite Heating Elements

With 30 years of experience in disc-shaped graphite heating elements, XRD Graphite has accumulated extensive process expertise in single-pass machining of continuous disc-face grooves, corner fillets, and control of remaining wall thickness at groove bottoms. We machine precisely to customer drawings and strictly control groove width, groove depth, corner radius, and disc face flatness. 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.

If your component is stuck on several unclear points—how dense the groove should be, what corner radius to specify, whether the two holes need an angular datum, and in what state flatness should be measured—you can send us the existing drawing and operating conditions, and we will review them together along the current path of that band. Graphite undergoes oxidative loss under oxidising atmospheres at sustained high temperatures; groove openings widen accordingly and power declines. This is more pronounced on thin, multi-grooved discs, and both selection and replacement cycles should account for it.