Ultra-Precision Fluid Jet and Bonnet Polishingfor Next Generation Hard X-Ray Telescope Application
Anthony TH. Beaucamp1,2, Akihiro Matsumoto1,2 and Yoshiharu Namba1
1Department of Mechanical Engineering
Chubu University
Kasugai, Japan
2
Research and Development
Zeeko LTD
Coalville, United Kingdom
1 INTRODUCTION
Single-point diamond-turning is a wellestablished method to create precise axiallysymmetric forms on ductile materials, including
flats, spheres, aspheres and cylindrical shapes.
A typical example is the production of large offaxis aspheric mirrors for the three-mirror
astigmatic configuration [1]. Other applications
include various infrared mirrors, and mandrels
for producing the cylindrical forms of Wolter-type
X-ray mirrors. The main limitation of this method
has been the resulting micro-structure which iscyclic in nature (‘turning marks’) and which
produces diffraction and stray light effects. For
this reason, molding dies for X-ray mandrels inparticular are post-polished to achieve both theform and texture required.
Namba et.al have previously described theirwork on replication-mandrels for Wolter Type 1
mirrors used in soft X-ray microscopy [2] andhard X-ray telescopes [3]. The mandrels wereproduced by single-point diamond turning, butrequired post-polishing by hand to remove thehigh spatial frequencies on the surface. As they
pointed out, this is extremely difficult on
aspheres, and leads to an inevitable trade-off
between quality of the surface texture achieved
and destruction of the aspheric figure. Clearly anautomated method to remove the diamondturning signature without destroying form could
be an important step forward.
Fluid-jet polishing (FJP) is a method in whichslurry of polishing particles is pressurized andprojected through a nozzle towards the surface
to be polished. The jet impacts the surface of thepart directly, i.e. with no physical tool contact.
Booij et.al [4] have shown the linear dependenceof removal rate with slurry concentration. They
have also shown the non-linear dependencewith impact-velocity, due to the combined effectsof i) a minimum velocity-threshold below which
no removal occurs, ii) the increased rate ofparticle-delivery with increased velocity and iii)
the square relation between particle kinetic
energy and velocity. They conclude that, using
appropriate abrasive and particle size with theright flow velocity, FJP can achieve ductileregime removal with stable volumetric removalrate. L. Yang et.al have also investigated fluid jet
polishing and concluded [5] that the removal
process is a mixture of shear and collision
mechanisms.
Bonnet polishing (BP) is a method based on amore traditional approach. A reinforced rubber
tool is kept under constant back pressure, andpressed into a workpiece to generate a contact
area [6]. A polishing cloth is stuck on the bonnet,
and slurry sprayed onto the surface. The
abrasion rate can be controlled by changing
various parameters such as pressure, bonnet
spindle rotation speed, precess angle, and tooloffset.
Both of these processes can produce stablesub-aperture footprints, and can be controlled by
the Precessions corrective polishing software to
improve form-error on freeform surfaces [7].In
the work reported in this paper, the FJP process
has been used to attenuate diamond-turningmarks on plano electroless nickel coatedsamples. This process was complemented bysubsequent smoothing of the surface roughness
with the BP process, using slurry of nano-sizedparticles. The BP process has also been used tocorrectively polish sections of a borosilicate
mandrel, which is being investigated as analternative to electroless nickel for making XRay molding dies. The method described
potentially opens up new applications fordiamond-turned surfaces, as well as improving
the performance of glass optics.

2 EXPERIMENTAL PROCEDURE
2.1 Nickel and Borosilicate Samples
A7075 aluminum alloy was cut into 100mm
diameter and 30 mm thick plano samples andturned by a single-point diamond turning machine.
A layer of nickel-phosphorus alloy 0.1 mm thick
was deposited on the diamond-turned aluminumalloy samples by electroless nickel plating inindustry. The hardness of the electroless nickel
and aluminum alloy were 568 Hv and 183 Hv
respectively. All samples were single-point
diamond turned (SPDT) again in order to obtainconsistent surface conditions.
A hollow borosilicate glass cylinder wasobtained directly from industry, with dimensions
200mm diameter and 200mm length. Theprocess used to make the mandrels ensuresuniform surface texture across the sample,
around 0.5nm rms, but poor straightness in therange 3-5um. The cylinder was cut into
rectangular sections 50mm by 100mm.

ADE optical profilers. Form error on theborosilicate samples was measured with a Form
Talysurf profilometer.
3 RESULTS AND ANALYSIS
3.1 Fluid Jet Polishing of SPDT Nickel
The fluid was pressurized at 18bar, and expelled
in laminar flow through a small nozzle (1mmdiameter). The CNC machine was programmedto raster the workpiece with a track spacing of
0.1mm and feed rate of 500mm/min. The
resulting surface texture showed no residualmarks from the prior diamond tool machining, andimprovement in the surface roughness from
3.05nm rms down to 1.47nm rms (see Figure 3aand 3b). Figure 2 shows power spectral density
analysis performed on the surface texture before
and after FJP. The two high powered frequencies
associated with the SPDT processing step werecompletely eliminated.

FIGURE 1. Nickel and Borosilicate samples.
2.2 Fluid Jet and Bonnet Polishing
The diamond-turned plano samples werepolished by FJP on a Zeeko IRP200 CNC
polishing machine. CeO2 slurry was used, withparticles ranging between 0.6-1.0um, and aconcentration of 80g/L. The samples weresubsequently polished by BP on the samemachine. Slurry of nano-sized SiO2 particles (730nm diameter) was drip fed onto the surface
directly above the bonnet, while the rest of thesurface was regularly fine sprayed with purewater to prevent crystallization of the slurry overthe sample.
The borosilicate samples were polished by FJP
and BP using the CeO2 slurry described above.
The objective of FJP was to assess the effect ofpressure on surface texture, while that of BP wasto assess corrective polishing capability.
2.2 Surface Texture Measurement
Surface micro-topography of the samples wasmeasured with three-dimensional NewView andFIGURE 2. PSD analysis of electroless nickel
surface before and after FJP.
The edge of the zone polished by FJP wasimaged with a stitching optical profiler. Figure 4
shows very clearly the progressive overriding of
directional tool marks from prior diamond turning(left hand side) with a random FJP texture (righthand side). This effect is very desirable since it
produces a surface with no strong frequency that
could lead to diffraction or stray light effects on
the final replicated mirror.

Rz :14.72nm rms :3.05nm Ra :2.37nm

Rz :22.10nm rms :1.47nm Ra :1.17nm

Rz :4.61nm rms :0.64nm Ra :0.52nm(a) Diamond-turned(b) FJP polished(c) BP polished
FIGURE 3. Surface texture of electroless nickel surface at various machining stages (optical profiler).

FIGURE 4. Surface texture of electroless nickel surface at edge of FJP zone (stitching optical profiler).
3.2 Bonnet Polishing of SPDT Nickel
The surface texture obtained from FJP was not
sufficiently smooth for hard X-Ray application.
Subsequent smoothing by BP was thus
investigated. A powder of ultra fine SiO2 nanoparticles with average size 7nm was mixed withpure water at a concentration of 20g/L. Theslurry was delivered above the bonnet at a rateof 12L/H. To prevent drying of the slurry on thesurface, which can cause crystallization, thesurface was sprayed with a fine mist of purewater every few seconds. The slurry and water
were thus delivered in total loss mode.

FIGURE 5. Surface texture variation onelectroless nickel sample.
Bonnet pressures in the range 0.2 to 3.0bar andspot sizes in the range 5mm to 15mm wereexperimented with. The best pressure was foundto be 1Bar, and the best spot size 15mm. The
surface texture was evaluated after each rasterpass of the tool on the surface. Using FFT Autofiltering, the evolution of waviness (low-pass)
and roughness (high-pass) could be assessed.
The result is shown on Figure 5. The roughnesswas very effectively reduced by the polishing tocirca 0.3nm rms. The waviness was slower toremove, and reached a lower limit of 0.65nm
rms.
The best surface texture obtained on a sample
was 0.64nm rms (see Figure 3c). The main issue
was identified as residual waviness from thebonnet polishing tool, in the region of 0.6-0.7nm
rms. This value is not yet sufficiently low for Hard
X-Ray applications, so future research willconcentrate on reducing the waviness induced by
the bonnet polisher. The effect of hardness of thetool in particular will be investigated. In themeantime, it is possible to use this automatedprocess to reduce the final hand polishing time byas much as 75% [3].
3.3 Fluid Jet Polishing of Borosilicate
The Borosilicate samples were polished at
different FJP pressures and the effect on surface
texture assessed with the same method as
described in the previous section. Figure 6 showsthat the effect of pressure on roughness wasalmost linear, while waviness was degraded only

slightly at lower pressures. The removal rates
were very low because of the small size of thenozzle (only 1mm diameter). Removal rates
3
were in the range 0.0004-0.008mm /min.

FIGURE 6. Surface texture on borosilicate
sample as a function of FJP polishing pressure.
A new slurry management system capable ofhigher flow rate was recently delivered that will
enable the use of larger diameter nozzles infuture experiments. It is hoped that with3
sufficient removal rates (circa 0.04mm /min) FJP
will be used in the future at pressures of 8-10Bar
to correct form error on the surface while leaving
waviness in the range 0.3-0.4nm rms androughness in the range 0.9-1.0nm rms.
Subsequent polishing by BP with nano-particles
may then reduce the roughness back to circa
0.3nm rms (see 3.2).
3.4 Bonnet Polishing of Borosilicate
Form correction was carried out on a borosilicate
sample with BP using the 0.6-1.0um CeO2 slurry.
The precessions software was used for thispurpose [7]. After 3 corrective iterations, the form
error was reduced from PV 3.2um down to
0.18um (see Figure 7).

FIGURE 7. Improvement of form-error on
borosilicate cylindrical sample.

4 SUMMARY
A study of automation of hard X-Ray molding dies
polishing by FJP and BP was carried out. The
following conclusions were drawn:
1. Surface roughness in the range 0.6-0.8nm
rms can be obtained on electroless nickel
samples by a combination of Fluid Jet
polishing with micro-particles followed by
Bonnet polishing with nano-particles.
2. Automatedcorrective
polishingwasdemonstrated on a borosilicate cylindricalsample, with form error improvement fromPV 3.2um down to 0.18um.
3. By using the automated process to getsurface texture down to circa 0.6-0.8nm rms,
it is possible to reduce the time of final handpolishing to 0.3nm by as much as 75% [3].
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