Mitsui Seiki J7CN
Key Specifications
X Travel
Y Travel
Z Travel
Spindle Taper
Tool Capacity
Table Size
Overview
The Mitsui Seiki J7CN is the larger sibling in Mitsui Seiki's renowned CNC jig borer lineup, extending the J-series' sub-micron precision to a 700 x 600 x 450 mm (27.6 x 23.6 x 17.7 in) working envelope. For shops that need the J6CN's legendary accuracy but on larger workpieces — full-size mold bases, large aerospace bearing housings, progressive die plates — the J7CN delivers +/-1.0 micron positioning accuracy across a workspace that handles the majority of precision boring applications without compromise.
The construction philosophy mirrors the J6CN: heavily ribbed Meehanite cast iron castings that are thermally aged, stress-relieved, and then hand-scraped and hand-lapped on all guideway surfaces. The additional mass of the J7CN's larger castings — the machine weighs in at 8,500 kg (18,739 lb) — actually benefits precision work by providing greater thermal inertia and vibration resistance. The box guideways are precision-ground and then hand-scraped to achieve the geometric accuracy that defines Mitsui Seiki jig borers.
The BT40 spindle runs to 8,000 rpm with 11 kW (15 HP) of continuous power, providing a modest bump over the J6CN's 7.5 kW for handling slightly heavier cuts. Glass scales with 0.01 micron resolution are standard on all axes, and the Fanuc 31i-B control reads these scales with the interpolation accuracy needed for nanometer-level positioning. The precision rotary table provides +/-1 arc-second indexing accuracy for angular bore patterns.
The J7CN's larger table — 1,000 x 600 mm (39.4 x 23.6 in) — accommodates workpieces up to 1,200 kg (2,646 lb), a significant step up from the J6CN. This means full-size mold bases can be bored in a single setup without repositioning, eliminating the accumulated error that comes with multiple setups. For mold shops, aerospace manufacturers, and gauge makers working with larger components, the J7CN provides the working envelope to machine complete parts while maintaining the accuracy that has made Mitsui Seiki jig borers the industry benchmark for over half a century.
Full Specifications
| Parameter | Value |
|---|---|
| X-Axis Travel | 700 mm (27.6 in) |
| Y-Axis Travel | 600 mm (23.6 in) |
| Z-Axis Travel | 450 mm (17.7 in) |
| Table Size | 1,000 x 600 mm (39.4 x 23.6 in) |
| Max Table Load | 1,200 kg (2,646 lb) |
| Spindle Speed | 8,000 rpm |
| Spindle Taper | BT40 |
| Spindle Motor | 11 kW (15 HP) continuous |
| Positioning Accuracy | +/-1.0 um |
| Repeatability | +/-0.5 um |
| Scale Resolution | 0.01 um (10 nanometers) |
| Rotary Table Accuracy | +/-1 arc-second |
| Rapid Traverse Rate | 10 m/min (394 ipm) |
| Tool Capacity | 20 tools |
| Guideway Type | Hand-scraped box guideways |
| CNC Control | Fanuc 31i-B with high-resolution scales |
| Machine Weight | 8,500 kg (18,739 lb) |
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Strengths & Limitations
Strengths
- Sub-micron positioning accuracy of +/-1.0 um on a larger 700 x 600 mm envelope handles full-size mold bases and large precision components
- 1,200 kg table capacity accommodates heavy workpieces that exceed the J6CN's capacity, enabling single-setup precision boring
- Greater machine mass of 8,500 kg provides improved thermal inertia and vibration damping compared to smaller jig borers
- Hand-scraped and hand-lapped box guideways deliver the surface contact quality essential for sub-micron boring accuracy
- 11 kW spindle motor provides adequate power for both precision boring and light milling operations on the larger workpieces
- Nanometer-resolution glass scales with 0.01 um interpolation ensure position feedback matches the machine's mechanical precision
Limitations
- Premium pricing of $500K-$650K for a machine with limited material removal rate requires consistent high-precision work to justify
- 8,000 rpm spindle speed and BT40 taper limit the machine to precision boring and light milling — not a general-purpose machining center
- 10 m/min rapid traverse is extremely slow by modern standards, adding significant non-cutting time to cycle times
- 20-tool ATC provides minimal tool storage, though jig boring operations typically require few tools per setup
- Larger footprint and 8,500 kg weight require more floor space and foundation engineering than the J6CN
Best For
Frequently Asked Questions
01
New J7CN machines typically price between $500,000 and $650,000. The premium over the J6CN reflects the larger castings, bigger table, and greater mass. Used J7CN machines are rare but command $200,000-$400,000 depending on vintage and condition.
02
The J7CN offers larger travels (700x600x450mm vs 600x500x400mm), a bigger table (1,000x600mm vs 800x500mm), higher table load (1,200 kg vs 800 kg), and more spindle power (11 kW vs 7.5 kW). Both share the same +/-1.0 um positioning accuracy and construction philosophy.
03
For precision boring operations on medium-sized parts, yes. However, horizontal boring mills handle much larger parts and heavier roughing. The J7CN is specifically designed for bore location accuracy, not heavy material removal. Many shops use both — the HBM for roughing and the J7CN for final precision boring.
04
With proper maintenance, environmental control, and periodic recalibration, Mitsui Seiki jig borers routinely maintain sub-micron accuracy for 20+ years. The hand-scraped guideways actually improve in geometric accuracy over the first few years as the surfaces wear in.
05
For sub-micron accuracy, the J7CN should operate in a temperature-controlled environment of 20+/-1 degree C with minimal vibration. Shops achieving the best results isolate the machine from traffic, presses, and other vibration sources, often on a dedicated foundation pad.
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