The Clearance Check: Verifying Your Part Fits Before You Buy
A specification sheet gives you travels, table size and maximum workpiece. It is entirely possible to satisfy all three and still be unable to machine the part, because none of those numbers describe the configuration your process actually needs.
The failure mode is consistent. A part is quoted against a machine on the strength of its bounding box, the machine arrives, and the first angled setup reveals that the tool holder collides with the cradle at the inclination the feature requires. The purchase order is signed; the problem is now a fixturing problem.
Four clearances decide whether the part fits. All four are checkable before the order.
Clearance 1 — spindle nose to table, at the angle you need
This is the number most often missing from a comparison. It is not travel. It is the vertical distance between the spindle gauge plane and the table surface, and on a trunnion machine it defines the vertical window your part and fixture must live inside.
|
|
G300 |
G630 |
G800 |
|
Spindle nose to table |
≈145–445 mm |
175–675 mm |
225–825 mm |
Read the G300 row carefully. A total vertical window of 300 mm sounds generous until you subtract the fixture. A 90 mm fixture stack leaves 210 mm of usable height, and the part also has to clear the cradle as the A axis rotates. Parts that measure comfortably against the travel figures on paper routinely fail here.
The check is not the part height. It is: part height plus fixture height, measured along the machine's Z axis, at the maximum tilt angle in the process plan. On an inclined feature the effective height is greater than the part's nominal height, because tilting rotates the diagonal into the vertical.
The large-format tilting-head machines state this differently, because the spindle swings rather than the part. U1000 and U1250 give a spindle nose to table distance of 200–1200 mm, U1600 gives 50–1250 mm, and U2000 gives 200–2000 mm. The same three also publish a spindle centre to table distance for horizontal mode — 80–1080 mm on U1000 and U1250, 150–1350 mm on U1600 — which is the figure that governs interference once the spindle is swung below horizontal. If you are comparing a trunnion machine against a tilting-head machine, these are not the same measurement and cannot be read across.
Clearance 2 — the swept envelope at the extremes
Maximum workpiece is a single number on a datasheet — Ø390 × 250 mm, Ø700 × 400 mm, Ø900 × 500 mm in the G series. That figure describes the envelope at one condition. At full A-axis deflection the swept volume changes shape, and corners that sit inside the envelope upright can swing outside it when the cradle rotates.
The A-axis ranges differ across the series, which is why this matters more than it appears:
● G300 — A axis ±120°, symmetric. The cradle swings equally in both directions.
● G630 and G800 — A axis +30° to −130°. Asymmetric, with substantially more range below horizontal.
If your process needs the part rotated far positive, the symmetric G300 range is the more permissive one. If the critical features face downward, the deeper negative range on the larger machines wins. Compare the axis range against the angles your process plan actually calls for, not against the range that looks widest.
On the U series (https://www.depu.com/products/u-series), the geometry question is different in kind. The workpiece does not rotate; the spindle swings through a tilting range of roughly 210°, so interference is driven by the spindle head sweeping around a stationary part rather than by the part sweeping inside a cradle. That is generally a more forgiving arrangement for large parts, and it is the reason the series reaches parts up to 8,000 kg. The structural side of that design is covered in how U series machine structure achieves micron-level accuracy (https://www.depu.com/blog/depu-u-series-machine-structure-micron-accuracy).
One caution here as well: rotary ranges are quoted against a datum, and different documents can state the same physical range from different zero positions. Confirm the convention and the direction of positive rotation with the machine builder and in your post-processor before the first setup.
Clearance 3 — the tool, not just the part
The part is only one of two objects moving through the work envelope. Long tools at steep angles are frequently the first thing to interfere, and the tooling limits that govern this are separate from the travel figures.
|
|
G300 |
G630 |
G800 |
U1000 |
U1250 and above |
|
Tool diameter |
Ø75 / Ø100 mm * |
Ø75 / Ø120 mm * |
Ø125 / Ø150 mm * |
110 / 200 mm |
110 / 200 mm |
|
Max tool diameter |
— |
300 mm |
400 mm |
— |
— |
|
Max tool length |
200 mm |
300 mm |
300 mm |
350 mm |
400 mm |
|
Max tool weight |
5 kg |
8 kg |
14 kg |
20 kg |
15 kg |
\* The larger figure applies with the adjacent pocket left empty.
Three practical consequences follow.
Tool length is a hard limit, not a preference. On the G300, 200 mm is the maximum. A long reach tool for a deep cavity may simply be unavailable for the machine, which changes the process plan rather than the machine selection. Check the longest tool in your CAM setup against this figure.
Tool weight limits feed rates indirectly. A heavy face mill or boring bar at the top of the weight range restricts how aggressively you can cut, which shows up as cycle time. Note the inversion on the U series: U1000 accepts a 20 kg tool while the larger U1600 and U2000 accept 15 kg, because the two are built to different priorities.
The neighbour-pocket rule costs capacity. The larger tool diameter figure only applies with an adjacent pocket left empty. If your program alternates between a large face mill and many small tools, an oversized tool at a class of the magazine can cost several usable pockets.
Clearance 4 — fixture height, and the datum chain
Fixturing is the clearance most often left out of the sizing conversation, and it consumes capacity twice.
Vertically, a fixture stack reduces the available spindle-to-table window directly. A hydraulic or zero-point system can occupy 90 to 150 mm before the part is mounted.
By mass, workholding counts against the table load rating, because the rating describes rotating mass on the rotary axes, not lifting capacity:
|
|
G300 |
G630 |
G800 |
U1000 |
U1250 |
U1600 |
U2000 |
|
Max table load |
50 kg |
850 kg |
1,200 kg |
1,800 kg |
2,500 kg |
4,000 kg |
8,000 kg |
On the G300 the interaction is severe. A 12 kg fixture leaves 38 kg for the part, and a part that was specified at 45 kg now exceeds the machine. Reserve 20 to 30 percent of the load rating for workholding at the sizing stage and the problem does not arise.
There is a second-order consideration on multi-face work. A single-setup strategy is often justified precisely to avoid re-datuming a heavy part, which is covered in why use three setups when 5-axis can do it in one (https://www.depu.com/blog/why-use-3-setups-when-5-axis-cnc-can-do-it-in-one). But that benefit depends on the fixture being able to hold the part through the full range of motion without re-clamping — which in turn depends on the clearances above. A single-setup plan that runs out of clearance halfway through the tilt is not a single-setup plan.
A worked example, and the number that is missing from every datasheet
Take a block 350 × 350 × 220 mm, mounted on a 90 mm fixture plate, on a G300. The dimensions look comfortable: upright, the assembly stands 310 mm tall, and the G300 offers a spindle-to-table window of 145 to 445 mm. There is 135 mm of clearance above the part with the spindle retracted. Nothing about that looks like a problem.
Now tilt it.
The section that rotates about the A axis measures 350 mm by 220 mm. The diagonal of that section is:
√(350² + 220²) = 413 mm
So the vertical extent of the part changes from 220 mm to 413 mm as it tilts — an increase of 193 mm. Add the 90 mm fixture and the top of the part reaches 503 mm above its mounting plane, which is beyond the G300's 445 mm maximum spindle-nose height. The part that fitted comfortably upright no longer fits once the process plan tilts it, and the limit was reached by geometry rather than by load.
There is a catch in this calculation that deserves attention. The part rotates about the A-axis centre, not about its own bottom face, and that centre height does not appear on any standard specification sheet. The calculation above therefore gives you the magnitude of the problem, not a final verdict: enough to know that a 193 mm growth in vertical extent needs checking, but not enough to sign off.
The reliable method is to request the A-axis centre height with the layout drawing, work in CAD from the actual geometry, and check the assembly at every angle in the process plan. It is an afternoon of work against a machine purchase, and it is the step where an under-specified order is caught.
The pre-purchase checklist
Run these eight checks against the largest and the most geometrically difficult part in the book, not against the average:
13. Part plus fixture height against the spindle-nose-to-table window, at maximum tilt
14. Swept envelope at the maximum A or B angle in the process plan, not at zero
15. Tilting axis range against every angle the process plan requires
16. Longest tool in the CAM setup against the machine's maximum tool length
17. Largest tool diameter against the magazine limit, allowing for the empty neighbour pocket
18. Heaviest tool assembly against the tool weight limit
19. Part plus fixture mass against the table load rating, with 20 to 30 percent reserved
20. Rotary axis convention, direction of positive rotation and zero datum against your post-processor
If a machine fails checks 4 through 8, the fix is process planning. If it fails checks 1 through 3, it is machine selection — and it is far cheaper to discover now.
For process-level technique on keeping the tool clear of the workpiece at extreme orientations, tool orientation optimisation in complex surface machining (https://www.depu.com/blog/tool-orientation-optimization-in-complex-surface-machining) goes deeper. For machine-level sizing, the G series (https://www.depu.com/products/g-series) and U series (https://www.depu.com/products/u-series) pages carry full travel and envelope data, and how workpiece size and weight drive machine selection (https://www.depu.com/blog/workpiece-size-weight-5-axis-machine-selection) covers the framework.
Send the geometry, not just the price bracket
A quote request that includes the part drawing, the intended fixturing and the angles in the process plan can be checked against the work envelope before a machine is proposed. Send them through the DEPU inquiry page (https://www.depu.com/inquiry), and ask specifically for the spindle-nose-to-table distance and the swept envelope at the angles you need. Model data: G300 (https://www.depu.com/product/g300), G630 (https://www.depu.com/product/g630), G800 (https://www.depu.com/product/g800), U1000D (https://www.depu.com/product/u1000).
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*Specifications shown are for configuration guidance and may vary by option package. Confirm the final datasheet for your build before quoting. Workholding design and CAM post configuration should be validated against the actual machine drawing.*
