Robot Joint Connecting Tube Thin Wall Aluminum Concentricity 0.01mm Machined

A connecting tube looks like the simplest part in a robot joint: a cylinder with a bore, two end faces and a bolt pattern. It is usually the part that causes the most assembly trouble, because a tube is thin, it is long relative to its diameter, and every operation tends to bend it. A tube that is 0.02 mm out of straight shows up as a joint that binds at one end of its travel and not the other.

Dongguan Changxinda Precision Technology Co., Ltd. has turned and milled tube geometry 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.

Why a Tube Is Harder Than a Bushing

On a short bushing, wall stiffness is high and deflection is negligible. On a tube with a 60 mm bore, a 3 mm wall and a 200 mm length, three things happen at once: the wall deflects under clamping, the wall deflects under cutting force, and the part relaxes when it is released. Each of those becomes an ovality or a bend that appears only after the part leaves the chuck.

• Clamping distortion: a three jaw chuck pushes a thin wall into a triangle. Soft jaws bored to the tube diameter distribute the load around the circumference.
• Cutting deflection: a boring bar pushing on a 3 mm wall springs away, so the bore tapers and the tool marks show chatter. Light radial passes and a supported bar fix this.
• Release spring back: a part machined round under clamp is not round when unclamped. The finish pass has to be planned for the free state, not the clamped state.
• Thermal drift: a long thin tube grows measurably during a roughing cycle, so measurement has to wait for the part to stabilise.

Two Stock Routes, Chosen by Ratio

The route is quoted from the ratio, not from habit. For a plain tube with a standard bore, hollow bar is cheaper. For a tube that needs 0.01 mm concentricity between OD and ID, boring from solid in one setup usually wins because the second setup is where concentricity is lost.

Process Route for a Thin Wall Tube

• Rough both ends and the OD with 0.8 mm stock, leaving the part long enough to hold in soft jaws.
• Stress relief after roughing, because a bored tube redistributes residual stress from the bar.
• Finish the bore first, using a boring bar with a damped or carbide reinforced shank, light radial depth of cut, and a finishing pass at constant surface speed.
• Mount on an expanding mandrel to finish the OD from the finished bore, so concentricity is inherited rather than chased.
• Face and chamfer both ends in the same setup as the OD, keeping overall length to 0.01 mm where the stack requires it.
• Mill cross holes or flats last, from the finished OD, so position follows the true axis rather than the as-roughed one.
• Deburr and mask before anodizing, with bore and end faces masked if they carry bearings or seals.

Precision Declared for This Family

• Bore diameter 0.01 mm, measured at three depths and two planes to catch taper and ovality
• OD to bore concentricity 0.01 mm, checked on the CMM with the tube supported on V blocks
• Straightness 0.02 mm per 100 mm, measured on a granite plate with a dial indicator along the length
• Wall thickness variation 0.02 mm around the circumference, checked at both ends
• End face squareness to the axis 0.01 mm, which is what actually sets the joint stack height
• Overall length 0.01 mm where specified, 0.05 mm by default
• Bore surface roughness Ra 0.8 as standard, Ra 0.4 available for seal surfaces
• Metrology room at 20 degrees Celsius, CMM resolution 0.001 mm, ballooned first article report

Non Standard Customization

• Bore from 15 mm to 250 mm, outside diameter to 300 mm, length up to 1200 mm depending on ratio
• Materials: 6061-T6 and 7075-T6 aluminum, 304 and 316L stainless, 40Cr and 42CrMo for loaded tubes, titanium on request
• Features: cross holes, radial ports, keyways, flats, ears, threaded ends, internal shoulders and snap ring grooves
• Flange ends machined integral or supplied as a matched pair, with dowel fits proven at the plant
• Finishes: clear, black and hard anodizing, nickel plating, chromate free conversion coating, with masking on seal surfaces
• Wall thickness down to 2 mm, with the process reviewed before quoting rather than after the first scrap part
• Drawing input in STEP, IGES, X_T, DWG, DXF or PDF, or a sample tube for measurement
• Prototype quantity from one piece, design feedback within 24 hours

Confidentiality When the Customer Needs It

Most tubes are not sensitive, but some carry a customer's joint architecture. On request, a tube program can be run on confidential terms: mutual NDA before drawings transfer, 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. Tell us at enquiry stage and the quotation is issued under those terms.

Delivery Stability

• Prototype tube: 5 to 8 working days including first article inspection
• Batch of 10 to 200 pieces: 10 to 18 working days including anodizing
• Long tubes above 600 mm: add 3 to 5 working days for stress relief and stabilisation
• Recurring releases: reserved machine capacity, same mandrel and same program, so the second batch measures like the first
• Named project engineer per account, weekly status update with photographs of work in progress
• Written notice at least 5 working days before the ship date if any step is at risk
• Packing in individual tubes with foam end caps, so the bore cannot be damaged in transit

Questions From Design Engineers

How thin a wall can you turn on a 100 mm tube?

2 mm is achievable in aluminum with an expanding mandrel and light finishing passes. Below that we would review the design, because handling and packing damage become more likely than machining error.

Do you bore from solid or start from tube?

It depends on the ratio and on the concentricity requirement. Below 6:1 with a standard bore, hollow bar is cheaper. For 0.01 mm concentricity between OD and ID, boring from solid in one setup usually holds the tolerance more reliably.

Can you certify the bore for a bearing fit?

Yes. Bore diameter, roundness, taper and roughness are measured and reported, and the CMM data can be shipped with the parts. If your bearing supplier needs a specific measurement plane, tell us where to measure.

Can you keep our design confidential?

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