Voumard CNC 300: How This Internal Grinder Reaches Micron Bores
A plain explanation of the Voumard CNC 300 and the CNC Anal control line: what the workhead, wheelhead and gauging head actually do to a bore. Written for engineers and buyers who need to judge whether a ground bore is the right call for their part.
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What the Voumard CNC 300 Actually Is
The Voumard CNC 300 is an internal grinding machine. A small vitrified or CBN wheel spins inside a bore and removes material in many light passes until the hole hits size, roundness and finish. It is not a milling machine and it is not a lathe, so it does not cut a bore from solid. It refines a bore that already exists.
That distinction matters when you read a drawing. If a hole carries a grinding allowance and a ground finish callout, someone expects a machine like this. If the hole is only drilled and reamed, a machining center can finish it in the same setup as every other feature.
The 300 size class covers a wide bore range. Small hydraulic spools sit at the low end, larger bearing seats at the top. Bore length relative to diameter decides whether a standard quill reaches or a longer spindle is needed.
The CNC Anal control line sits on the same platform. It brings the axis interpolation, dressing cycles and gauging interface that let an operator run a grinding program instead of feeding a handwheel. Everything below applies to both.
Useful reading of the name: Voumard built its reputation on internal grinding for bearing and hydraulic work. The 300 and the Anal machines are the same family of bore-finishing tools, sized and controlled differently.
How the Workhead, Wheelhead and Gauging Head Work Together
A bore is ground by three motions at once. The workhead rotates the part, the wheelhead oscillates the small wheel in and out of the bore, and the wheel itself spins at cutting speed. Each motion has its own error contribution, and they add up on the finished diameter.
The workhead runs at low speed compared with a lathe spindle. Its job is roundness, not removal rate. Chucking error goes straight into the bore, so a worn collet or a soft jaw that is not bored in place will show up as ovality long before the wheel wears.
The wheelhead stroke sets how much of the bore length gets ground. Too short a stroke leaves a shoulder step or a taper. Too long a stroke wastes cycle time and can grind into a relief groove that was never meant to see a wheel.
The gauging head is the part most people underestimate. On an in-process gauge, two contact fingers ride the bore while grinding continues. The control reads the diameter and stops the cycle when the size window is reached, so the machine compensates for wheel wear as it happens.
Why the Grinding Allowance and Wheel Choice Decide the Result
Grinding removes very little material per pass. That is the point, and it is also the limit. Leave 0.10–0.30 mm of radial stock on a hardened bore and the cycle is short and stable. Leave 0.5 mm and you burn time, dress more often and risk heat.
Wheel choice follows the material and the finish target. Aluminium oxide suits hardened steel, while CBN holds form on long runs and hard alloys. Harder wheels hold size but load up. Softer wheels cut cooler and lose form faster.
Dressing is not optional housekeeping. A dull wheel rubs instead of cutting, which raises temperature and pushes material sideways. Dressing frequency depends on the finish callout, the wheel grade and how much stock each cycle takes.
Finish and tolerance pull against each other in the setup. A Ra 0.2–0.8 μm bore usually needs a spark-out at the end of the cycle, with the wheel still turning but no infeed. Spark-out costs seconds and buys roundness.
When Internal Grinding Is the Right Call and When It Is Not
Grinding a bore makes sense when the callout is tighter than a milling or turning process holds reliably. Hardened steel above roughly 45 HRC is the classic case. So is any bore with a roundness or cylindricity requirement measured in microns.
It also makes sense when the bore is the datum. A bearing seat that locates a shaft, or a hydraulic bore that meters fluid, carries tolerance that the rest of the part depends on. Grinding that one feature after heat treatment keeps the datum honest.
It is the wrong call when the hole is a clearance hole. If a fastener passes through and the fit is loose, grinding adds cost and lead time for nothing. Reaming or boring on a machining center finishes it in the same setup.
It is also the wrong call when the bore is deep and narrow. Wheel deflection grows with length-to-diameter ratio, and a small quill in a deep hole is a spring. Past roughly 5:1, expect to grind in steps or accept a looser cylindricity band.
Blind bores with a small corner radius need a wheel dressed to clear the corner. If the drawing calls for a sharp internal corner, no grinding wheel will produce it. The corner radius has to match the wheel, or the feature has to be relieved.
Grinding vs Boring vs Reaming: Which Finishes the Bore
Judge by tolerance, hardness and quantity, not by habit.
| Bore situation | Grinding | Boring on a mill | Reaming |
|---|---|---|---|
| Hardened steel above 45 HRC | First choice | Needs a hard-milling setup | Tool wear kills size |
| Tolerance tighter than ±0.01 mm | Holds it | Possible on a rigid machine | Rarely holds it |
| Roundness under 5 μm | Holds it | Setup dependent | Not the tool for it |
| Clearance or through hole | Overkill | Fine in the same setup | Fastest option |
| Length-to-diameter over 5:1 | Needs stepped passes | Deflection risk | Follows the drilled path |
| One-off prototype | Slow to set up | Good choice | Good choice |
| Runs above 1,000 parts | Wheel wear is managed | Tool changes add up | Size drifts with wear |
| Blind bore, sharp corner | Needs a radiused wheel | Can be milled | Cannot reach it |
The Short Answer
If the bore is hardened, is a datum, or carries a micron-level roundness callout, grind it. If it is a clearance hole in soft material, bore or ream it and spend the money elsewhere.
Questions Engineers Ask About the Voumard CNC 300
How much stock should be left for internal grinding?
For a typical hardened steel bore, leave 0.10–0.30 mm of radial stock after heat treatment. Below 0.05 mm the wheel has almost nothing to cut and the bore may not clean up if distortion moved the hole.
Above roughly 0.5 mm the cycle gets long, dressing frequency climbs and heat builds up in a thin-walled part. If the heat-treat distortion is large, rough grind first, then stress relieve, then finish grind.
Can the machine hold a size without an in-process gauge?
Yes, but the operator has to compensate for wheel wear manually, and the size drifts over a run. The gauge closes that loop by measuring the bore during the cycle and stopping at the target window.
On short runs the difference is small. On long runs it decides whether the last part still passes inspection without a rework loop.
What surface finish is realistic on a ground bore?
A well-set internal grinder reaches Ra 0.2–0.8 μm on a hardened steel bore with a spark-out at the end of the cycle. A general ground finish sits around Ra 0.8–1.6 μm.
Pushing below Ra 0.2 μm usually means a finer wheel, lower infeed and a longer cycle. Ask whether the function of the bore actually needs it before adding that cost.
Why does a ground bore come out tapered?
Taper usually comes from the wheelhead stroke, not from the wheel. If the oscillation does not cover the full bore length evenly, one end sees more wheel time than the other.
Other causes are workhead misalignment, a part held with uneven clamping force, or a wheel that has lost form. Check the stroke first, then the part setup.
Does grinding work on aluminium or bronze?
It can, but soft materials load the wheel quickly. The wheel glazes instead of cutting, and the bore can smear rather than shear.
For aluminium or bronze bores with a tight tolerance, boring or fine turning usually gives a better surface and a shorter setup. Grinding is reserved for the hard materials where it earns its place.
How do I specify a ground bore on a drawing?
Give the nominal diameter, the tolerance band, the roundness or cylindricity limit, the surface finish, and the grinding allowance on the pre-grind operation. State whether the bore is a datum.
Add the corner radius at the bottom of a blind bore. If you need a sharp internal corner, say so early, because it changes the process rather than just the wheel.
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