Robot Joint Flange Housing Bearing Seat G7 Target Range 7075 Aluminum Machined

G7 appears on a lot of drawings and is understood on very few shop floors. It is not a diameter, it is a tolerance range: for a 50 mm bore, G7 spans 9 microns. A housing bored anywhere in that range passes inspection, yet a bearing that drops in at the low end of G7 can be a light press at the high end. The fit the customer actually feels depends on where in the range the bore lands, and on how round and straight the bore is, not on whether it passed.

Dongguan Changxinda Precision Technology Co., Ltd. has bored bearing seats for robot joint housings to drawing since 2008. The plant runs 4-axis and 5-axis machining centers, turn-mill combined lathes and precision CNC lathes, employs more than 90 people, holds ISO 9001:2015, and exports 50 to 60 percent of output to robot, automation and medical buyers.

Where in the G7 Range Should a Bore Land

The range matters because the clearance for the mating bearing is calculated from it. For a housing bore with a G7 tolerance and a bearing with a standard shaft or outer ring tolerance, the fit is decided by the pair of limits, not by a single number.

Where the drawing allows, the target is agreed at quotation stage: a housing destined for a press fit is bored towards the lower part of G7, and a housing assembled by hand at room temperature towards the upper part. Two identical drawings can need two different targets.

Diameter Is the Easy Part of a Bearing Seat

• Roundness: a bore can measure 50.000 mm on a two point gauge and still be three lobed, because a two point measurement cannot see odd lobing. Roundness needs a three point or CMM check.
• Taper: a boring bar that deflects cuts a taper, so the bore is right at the entry and out at the bottom. Measured at three depths, not one.
• Cylindricity: round at both ends but waisted in the middle is common on thin wall housings, and it is what makes a bearing feel loose after assembly.
• Squareness to the flange face: if the bore axis is not square to the mounting face, the bearing tilts, and the shaft runs out even though the bore itself is perfect.
• Surface finish: a rough bore collapses the effective interference by crushing the peaks, so Ra has to be controlled along with the size.

Aluminum Housing, Steel Bearing: The Fit Changes When It Warms Up

7075 expands about 23.6 microns per metre per degree Celsius, bearing steel about 11.7. A 100 mm aluminum bore grows roughly 2.4 microns per degree, while the steel bearing inside it grows about 1.2. The result is that an interference fit measured at 20 degrees in the shop becomes looser at 60 degrees in a running joint, and a housing that was correct on the bench can develop a creeping outer ring in service.

• Where the joint runs hot, the bore target is moved towards the tighter side of G7 so the running fit lands where the design intended
• Where the joint runs at shop temperature, the target stays central, because tightening it only makes assembly harder
• Housings with steel or cast iron inserts follow the insert, not the aluminum, and the compensation is calculated on the insert material

Process Route for a G7 Bearing Seat

• Rough the housing, including the bore with 1.5 mm stock, then stress relieve before any precision boring.
• Finish the mounting face and its bolt pattern first, so the bore has a square reference to inherit.
• Bore in two stages: a semi finish leaving 0.1 mm, then a finish pass with a rigid bar, light depth of cut and constant surface speed.
• Measure at three depths and two planes before removing the part, so any taper is caught while it can still be corrected in setup.
• Check roundness with a three point or CMM method, never a two point gauge alone, since two point measurement hides lobing.
• Mask the bore for anodizing, or compensate the diameter if the customer wants a coated bore, because 20 to 40 microns of coating will destroy a G7 fit.
• Final inspection at 20 degrees Celsius after the part and the gauge have soaked, with a bore report showing size, roundness, taper and roughness.

Precision Declared for This Family

• Bore diameter held within the G7 range, reported to 0.001 mm with the actual value, not just pass or fail
• Roundness 0.005 mm on bearing seats, cylindricity 0.01 mm
• Taper 0.005 mm over the bore length, verified at three depths
• Bore to mounting face squareness 0.01 mm
• Bore surface roughness Ra 0.4 for bearing seats, with a trace supplied on request
• Flange face flatness 0.01 mm and bolt pattern position 0.01 mm
• Material certification for 7075-T6 included, since heat treat condition affects both strength and stability
• Metrology at 20 degrees Celsius, CMM resolution 0.001 mm, bore report shipped with the part

Non Standard Customization

• Bore from 10 mm to 250 mm, housing envelope to 400 mm, flange patterns to the customer's joint module
• Materials: 7075-T6 and 6061-T6 aluminum, 304 and 316L stainless, 40Cr or cast iron inserts for loaded seats
• Fit classes other than G7: H7, K7, M7, N7, or a customer specific interference band, quoted with the target position stated
• Features: oil grooves, seal grooves, snap ring grooves, set screw flats, dowel pairs, spigot diameters
• Finishes: clear, black and hard anodizing with bores masked, chromate free conversion coating, nickel plating
• Bearing brands and part numbers can be specified; the bore is matched to that bearing's actual tolerance class
• Laser marking of bore size, batch and assembly position on a non functional face
• Drawing input in STEP, IGES, X_T, DWG, DXF or PDF, with the mating bearing part number if available
• Prototype quantity from one piece, design feedback within 24 hours

Confidentiality When the Customer Needs It

On request, this family runs on confidential terms: mutual NDA before drawings move, restricted file access with named staff, no photographs of the part for marketing, sub suppliers bound by the same terms, neutral marking and neutral packing, and programs and fixtures reserved to that customer. Mention it at enquiry stage and the quotation is issued under those terms.

Delivery Stability

• Prototype housing: 5 to 8 working days including first article inspection
• Batch of 10 to 200 pieces: 10 to 18 working days including anodizing
• Recurring releases: reserved machine capacity with the same boring program and tool, so bore targets repeat batch to batch
• A named project engineer per account, weekly status update with photographs unless the program is confidential
• Written notice at least 5 working days before the ship date if any step is at risk
• Packing with a bore plug and face protection, since a knocked bore edge ruins a bearing seat faster than any machining error

Questions From Design Engineers

Can you bore to a specific value inside G7 rather than anywhere in the range?

Yes, and for bearing seats we recommend it. Tell us the mating bearing and the assembly method, and we will hold a target inside the range rather than accepting any passing value.

Why does the bore need a three point roundness check?

Because a two point gauge cannot see three lobed bores, and a three lobed bore is exactly what a thin wall housing produces when it is clamped. It passes diameter inspection and still runs noisy.

Will anodizing ruin the fit?

It will if the bore is coated. We mask bearing bores as standard, and if a coated bore is required the diameter is compensated before machining so the finished size lands in G7.

Can the housing be supplied under NDA?

Yes, on request, including restricted drawings, no marketing photographs, neutral marking and packing, and fixtures and programs reserved to your program.