Rounding Fir-Tree Slot Edges in Superalloy Disks with a Hybrid Diamond-SiC Filament
A chamfer is not a radius. That is the whole problem in a fir-tree slot. The drawing calls for a rounded edge; the cutter can only cut a straight face, and where that face meets the original surface it leaves two new micro-edges — sharp enough to make a blade root bind on the way in.
Polishing those edges out sounds like a finishing job. In a superalloy disk it is a material problem. Silicon carbide will not move the workpiece, and diamond coarse enough to round the edge leaves the surface dark and streaked even when the roughness number passes.
The answer our sales engineer, Frank, arrived at is one filament carrying both abrasives: diamond to form the radius, silicon carbide to bring the surface back. This article explains why that filament exists, where it applies, and what to send if you want to evaluate it on your own part.
1. What the Cutter Actually Leaves
A fir-tree slot is the seat for a blade root, machined into the rim of a turbine or compressor disk. The slot flanks are cut to a profile, and the edge break at the slot is usually specified as a radius, because a sharp edge there is a stress riser and a handling hazard.
The cutter cannot produce that radius directly. A chamfer or profile tool cuts a flat or angled face; where the new face meets the original surface, the intersection is a line, and on either side of that line the material still forms a corner. Under magnification the edge is no longer 90 degrees, but it is not a radius either — it is two new edges, each with its own included angle.
Those two edges are what the blade root touches first. Instead of sliding on a rounded surface, the root rides on two narrow contact lines. Friction rises, the root does not seat by hand, and forcing it risks galling the slot or damaging the blade coating. In an engine disk that is not a cosmetic complaint; it is an assembly problem that shows up before the part ever runs.
2. Why This Is a Material Problem, Not a Finishing Problem
The natural response is to brush the edge. On steel that works: a silicon carbide or aluminum oxide filament cuts the corner back and blends it into a radius in one pass.
On disk alloys it does not. Nickel-based superalloys and titanium alloys hold their hardness at the temperature a brush generates at contact, so a silicon carbide filament does not cut — it rubs. The edge may look polished and measure slightly better, but the two corners are still there, and the brush wears out while producing almost no geometric change.
Diamond is the abrasive that will cut these materials, which is why it is the default for hardened steel, tungsten carbide and superalloys. But cutting a corner back to a radius is a different order of removal from polishing a surface, and that difference is where the pure-diamond approach runs into trouble.
3. The Pure-Diamond Trap: Ra Passes, Appearance Fails
Frank describes the trial sequence as a dead end that looked like a solution. Diamond filament did cut the superalloy, but no single grit did both jobs:
- Fine enough to protect the surface — the filament would not move the edge. The corners stayed, assembly stayed difficult.
- Coarse enough to form the radius — the edge came back round, and the surface came back dark, with a random scratch pattern across the flank.
The second result is the one that surprises buyers, because the measurement can pass. Ra is an amplitude average over a sampling length; it does not describe direction, and it does not describe colour. A surface full of deep, randomly oriented scratches can return an acceptable Ra while reflecting light completely differently from the surrounding material — which is exactly what a visual inspection under a bench light will catch.
In aerospace work those two acceptance routes are often separate. A part can meet the roughness callout and still be rejected on appearance. Once that happens, the argument is no longer about grit size; it is about whether one abrasive can be asked to do two incompatible jobs.
4. One Filament, Two Abrasives
The hybrid filament is made at the raw-material stage, not at the brush. Both abrasives are compounded into the filament itself, so each individual filament carries diamond grains and silicon carbide grains together. It is not a brush built from two kinds of filament mixed together, and it is not a two-brush sequence.
That construction is what lets the two actions happen in the same pass, at the same point on the edge:
| Abrasive in the filament | What it does at the edge | Why it is there |
|---|---|---|
| Diamond | Cuts and rounds — removes the two micro-edges and generates the radius | Hard enough to move nickel- and titanium-based alloys |
| Silicon carbide | Polishes the track the diamond leaves behind | Softer and blockier, it blends the scratch pattern instead of deepening it |
The practical effect is that the edge geometry and the surface finish stop competing. Diamond removes material fast enough to form a radius; silicon carbide, working the same line immediately after, keeps the flank bright instead of streaked. Neither abrasive is doing the other's job.
5. Why the Blend Is Tuned, Not Catalogued
There is no published ratio for this filament, and that is deliberate. Proportion, grit size and filament diameter are set per workpiece, because the balance point moves with the job.
The variables that shift it are predictable even when the numbers are not:
| What the part needs | Which way the blend moves | What you trade away |
|---|---|---|
| Faster radius formation | More diamond, or coarser diamond | Higher risk of a visible scratch pattern |
| Brighter flank | More silicon carbide, or finer silicon carbide | Slower edge formation, more passes |
| Tighter radius tolerance | Finer abrasives both sides | Longer cycle, less margin on removal rate |
| Heavier incoming edge condition | Coarser start, then refine | Either two specifications, or one compromise |
This is why a hybrid filament is quoted against a drawing and a sample, not against a grit number. The first build is an estimate based on similar work; the final blend comes out of trials on the actual workpiece, where edge radius and surface appearance can both be checked.
6. Where Hybrid Filament Makes Sense — and Where It Does Not
The filament is worth considering when three conditions overlap: the workpiece is hard enough that silicon carbide cannot cut it, the requirement includes a geometric radius rather than only a cleaned edge, and the surface is judged visually as well as by roughness.
Outside that overlap it is usually the wrong purchase:
- Cast iron, carbon steel, aluminium — silicon carbide or aluminum oxide filament handles the edge at a fraction of the cost.
- Deburring with no radius requirement — the goal is removing a burr, not forming geometry; a standard abrasive filament is enough.
- Appearance not judged — if a directional pattern is acceptable, a single coarse abrasive will be faster and cheaper.
- Very tight access — where the brush cannot present filament to the edge at a consistent angle, no blend will fix the kinematics.
7. Case: Fir-Tree Slots in an Aero Engine Disk
An aerospace manufacturer machines fir-tree slots into an engine disk in a heat-resistant alloy. The slot edge is chamfered, and after chamfering the blade root would not seat smoothly — the two micro-edges left by the cutter were catching and binding during assembly.
The first attempt used silicon carbide filament. It polished the edge without changing its geometry, because the alloy would not yield to that abrasive. The second attempt used diamond filament, and ran into the grit compromise described above: fine enough to keep the flank acceptable, it would not form the radius; coarse enough to form the radius, it left a dark random scratch pattern that failed the visual check even though the roughness reading was acceptable.
The part that shipped is a hybrid filament with diamond and silicon carbide compounded into the same filament, built for this workpiece and adjusted across trials until the edge radius and the flank appearance both held. It has been delivered and is running in series production.
Two details are worth stating plainly. The blend, grit and filament diameter are specific to this workpiece and are not published; the filament is a custom build, and at present it is produced for this application only. And the lesson transfers better than the specification: when a requirement is geometric and cosmetic at the same time, one abrasive is usually being asked to do two jobs, and the fix is often in the filament rather than the machine setting.
The customer is identified by industry only.
8. What to Send for Evaluation
A hybrid filament is developed against a part, so the first conversation needs more than a brush size. Six items shorten the loop considerably:
- Workpiece material and hardness, including the alloy designation if it can be shared.
- The edge specification: radius value, where it applies on the slot, and the tolerance.
- The incoming edge condition: chamfer angle, and whether the two micro-edges are visible under magnification.
- Acceptance criteria: roughness value plus the visual standard, ideally with a photo of an accepted and a rejected surface.
- Machine and mounting: spindle or robot, speed, brush format, and the angle at which the filament meets the edge.
- Volume, so the trial build and the production build can be planned together.
Expect at least one round of adjustment. The first filament is a starting point chosen from similar applications; the final blend is set once the edge and the surface can be measured on a real part.
FAQ
Q: What is a hybrid diamond–SiC filament?
A:It is a single abrasive filament with diamond grains and silicon carbide grains compounded into the same nylon. Every filament carries both, so the diamond can round an edge while the silicon carbide polishes the track it leaves — in one pass, at one point on the edge.
Q: Can I use a finer diamond instead of a hybrid filament?
A:Usually not. Fine diamond protects the surface but does not remove enough material to form a radius on a superalloy. The edge stays sharp, and the assembly problem stays. The hybrid exists because those two requirements pull in opposite directions.
Q: Does the silicon carbide scratch the superalloy?
A:It leaves a finer, more uniform pattern than coarse diamond, and it is present to blend the diamond track rather than to cut the workpiece. Whether the result passes is decided by your visual standard, which is why a sample and a rejected-surface photo matter more than a grit number.
Q: Does this replace the chamfer operation?
A:No. The chamfer is cut first and stays. The brush only addresses what the chamfer leaves behind: the two micro-edges at the junction, which have to become a radius before the blade root will seat.
Q: Is the blend available as a standard product?
A:No. Proportion, grit and filament diameter are set per workpiece and adjusted during trials. The filament is a custom build, and the specification for one application is not published or reused as a catalogue item.
Round the Edge, Keep the Surface
Longguang manufactures abrasive filament brushes in silicon carbide, aluminum oxide, diamond, ceramic and hybrid formulations, with filament diameter, grit, brush diameter and mounting matched to the part. For edge work on hard alloys, the starting points are our abrasive filament brushes collection and the ultra-fine diamond wheel brush; for slot and edge access on a spindle or robot, see the wheel brushes range.
If the requirement is a radius rather than a cleaned edge, send the drawing with the radius value, the alloy and the visual acceptance criterion. We will tell you whether a standard abrasive will hold it or whether the edge needs a hybrid build.
Related reading: cutting tool edge passivation covers the same distinction on a different part — where the goal is a controlled edge, not a sharper one.
Note: Longguang is a manufacturer and exporter of industrial brushes. We do not provide on-site deburring services or local installation.
Henry Zhu
International Sales Manager
Phone / WhatsApp / WeChat: +86 13818514531
Email: info@shlgbrush.com
Email: zhulei@shlgbrush.com
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