CT Scanner Frame Sheet Metal Fabrication

• 1
  What a CT Frame Has to Do Before Anything Else
• 2
  Material Choice for CT Scanner Frame Sheet Metal Fabrication
• 3
  Where Weld Distortion Actually Comes From
• 4
  Why Datum Recovery Is the Step That Decides Accuracy
• 5
  Inspection and Medical Device Documentation
• 6
  Step by Step: From Blank to Inspected Frame
• 7
  Material and Process Trade-offs at a Glance

A CT gantry frame is a stiff box that holds a rotating mass. The tube, detector array, slip ring, and power electronics all sit on or inside it, and the whole assembly turns at 0.3 to 4 revolutions per second. Any flex in the frame becomes ring artifact in the reconstructed image. That is the first requirement, and it drives every other decision.

The second requirement is positional. Detector-to-tube alignment is set by a small number of machined interfaces, often no more than six or eight. Everything else on the frame is structure. This split matters because it tells you where to spend money: cut and bend the structure economically, then machine only the datum pads and interface faces to tight tolerance.

The third requirement is stability over time. A frame that measures well on the CMM in January and drifts by 0.1 mm in August has failed. Weld residual stress, coating thickness variation, and thermal expansion all move the geometry after inspection.

Weight matters too, but less than most teams assume. A fixed scanner can tolerate a 200 kg frame because the gantry bearing carries it. A mobile or point-of-care unit cannot. That single constraint usually decides the material before any other factor is discussed.

Mild steel is the default for fixed-installation scanners. Grades such as A36 and 1018 cut fast on fiber laser, bend predictably, and weld with stable parameters. Specific stiffness is high enough that 2.5 mm to 4 mm wall sections with formed ribs give the rigidity you need without machining pockets. The trade-off is corrosion, so the frame gets powder coating or zinc plating after welding.

Stainless 304 and 316L appear when the frame sits in a humid room, a mobile trailer, or a cleanroom where paint inside closed cavities is not acceptable. Passivation gives corrosion resistance without coating, which removes a particle source. The cost is real: stainless bends with more springback, needs higher tonnage, and its low thermal conductivity concentrates weld heat, so distortion control takes longer.

Aluminum 5052 and 6061 fit mobile CT and portable imaging. They cut the frame mass by roughly two thirds against steel, which changes the bearing and drive sizing. The penalty is lower stiffness, so aluminum frames need deeper ribs or thicker sections, and weld strength in the heat-affected zone drops unless the design accounts for it.

Titanium and Inconel are not practical for a large frame. They are used for small brackets and inserts where corrosion or non-magnetic behavior is required. If a supplier proposes a titanium gantry frame, ask what problem it solves, because the cost and lead time will be extreme.

• 1
  Fixed scanner, dry roomMild steel, powder coated, ribs formed into the blank.
• 2
  Humid or cleanroom304 or 316L, passivated, no internal paint.
• 3
  Mobile or portable5052 or 6061 aluminum, deeper ribs, revised weld design.

A gantry frame is welded from laser-cut blanks and formed channels, and welding is the single largest source of dimensional error. Heat input shrinks the weld bead as it cools, pulling the surrounding material toward the joint. On a 1,200 mm long frame rail, a 3 mm fillet weld can pull the ends 0.3 mm to 0.8 mm out of position if the sequence is not controlled.

Three controls do most of the work. First, balanced sequencing: weld opposite sides of a symmetric joint in alternating passes so shrinkage cancels instead of accumulating. Second, fixturing that holds the part in the welded position, not the free position, so the fixture resists the pull rather than measuring it afterward. Third, tack density. Short tacks every 80 mm to 120 mm hold alignment before the full pass and cost almost nothing.

Material thickness plays a role that is easy to miss. Thin sheet distorts more because there is less material to absorb the shrinkage stress. A 2 mm panel welded to a 6 mm rail will bow toward the weld. Designers who add a formed flange along the weld line cut that bow substantially without adding mass.

After welding, the frame goes through stress relief and straightening. For steel, this is usually a thermal cycle followed by press correction. For aluminum, thermal relief is limited by the alloy, so mechanical straightening and re-machining do the job.

Welded frames do not arrive at the machine tool in a known position. The as-welded geometry may be within 1 mm, which is fine for structure and useless for a detector mount. The machining operation exists to create a new coordinate system on the finished part and cut the interface features from it.

The practical approach is to establish three datum pads first, machine them flat, then use those pads to locate the part for every subsequent operation. On a frame up to 4,000 mm long, this is done on a large travel machine with the part set on adjustable supports and indicated in. Removing the part and re-setting it mid-process is where tolerance is lost.

Tolerances on those interface faces typically land between ±0.02 mm and ±0.05 mm for position, with flatness in the same range. Our general machining capability reaches ±0.005 mm on smaller features, but on a welded frame the limiting factor is thermal and mechanical stability of the part itself, not the machine. Claiming ±0.005 mm across a 2 m welded frame without thermal control is not credible.

Surface finish on datum pads usually runs Ra 0.8–1.6 μm. That is fine enough for a seated interface and coarse enough to hold a lubricated or bonded joint. Mirror finishes on a gantry frame serve no purpose.

• 1
  Machine datums firstThree pads, cut flat, then used to locate everything else.
• 2
  One setup where possibleRe-clamping mid-process is the main tolerance leak.
• 3
  Realistic limits±0.02 to ±0.05 mm position on welded frames is normal.

A frame that is dimensionally correct but undocumented is a problem in a medical device supply chain. The drawing tolerance is only half the requirement. The other half is evidence: material certificates, weld records, dimensional reports from a CMM or portable arm, and coating thickness measurements.

Our inspection sequence starts with raw material verification, continues with in-process checks during welding and after machining, and ends with a 100% inspection before shipment. Reports are available on request. For a gantry frame, the dimensional report usually covers datum flatness, interface position, hole diameters, and overall squareness.

Certification scope is worth checking carefully. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. ISO 13485 is the one that matters for medical device work, and it applies to the quality system, not to a specific part. Ask any supplier to state which site holds the certificate.

Confidentiality is part of the same conversation. Scanner geometry and detector mounting details are sensitive. NDAs are available on request, and uploads are handled as confidential by default.