Square Nut for 8 mm T-Slot Aluminium Profiles M5 M6 M8

OEM / ODM Available Free Sample Available RoHS, REACH, material certificate for stainless steel

Square T-slot nut with four flat sides for 30 and 40 series aluminium profiles. M6 and M8 threads. Prevents rotation in groove, positioning accuracy +/-0.2 mm. For precision mounting.

Wholesale Price Quoted per piece, price breaks at 100 / 500 / 1000 per per piece

Material	Carbon steel
Alloy Grade	Carbon steel C1035, stainless A2 / A4
Temper	custom
Section Size	n/a (piece part)
Slot	Fits 8 mm and 10 mm groove widths (30/40 series)
Length	n/a (piece part, sold per nut)
Tolerance	Thread 6H, body width +/-0.1 mm standard / +/-0.05 mm precision
Surface	Zinc plated / stainless steel A2 or A4
MOQ	No MOQ for stock sizes; 500 pieces for precision tolerance
Lead Time	Stocked sizes ship in 2 to 3 days; precision and stainless about 2 weeks
Loading Port	Shanghai, Ningbo, Qingdao
Supply Ability	Stocked carbon steel sizes in volume; stainless and precision sizes produced to order
Payment Terms	T/T 30 per cent deposit, balance against copy of B/L; L/C at sight for larger orders
Trade Terms	FOB, CIF, CFR, EXW
Certifications	RoHS, REACH, material certificate for stainless steel

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01What is a square T-slot nut?

How is a square nut different from a hex nut in a T-slot?

A square nut has four flat sides that run along the slot axis. Two of those sides bear against the inner walls of the groove, preventing the nut from rotating when the bolt is tightened. A hex nut has six sides, and the contact points with the groove wall are two narrow edges — not enough surface area to prevent rotation. Under bolt torque, a hex nut will rotate in the groove and the threaded hole will shift position.

Why does rotation matter?

When the nut rotates, the threaded hole moves along an arc around the groove centre. For a 30 series M6 hex nut, the thread can shift up to 1.5 mm from its intended position. For precision mounting — sensor brackets, alignment fixtures, repeatable jig plates — that shift is unacceptable. The square nut’s flat sides lock the orientation, and the thread stays at a known position within +/-0.2 mm.

Can I use square nuts everywhere instead of hex nuts?

You can, but square nuts are slightly wider than hex nuts of the same thread, so they may not fit side by side in a narrow groove. They also cost about 20 per cent more because the square body is harder to cold-form than a hex body. Use square nuts where positioning accuracy matters and hex nuts where it does not.

02Square vs hex: the rotation test

We tested square and hex T-slot nuts in a 40 series groove with an M8 bolt torqued to 15 Nm. The nut was positioned at a marked point on the groove, and the thread position was measured before and after tightening. The hex nut rotated about 8 degrees during tightening, shifting the thread centre by 1.2 mm. The square nut did not rotate measurably — the thread centre stayed within 0.2 mm of its initial position.

Nut type	Rotation (degrees)	Thread shift (mm)	Positioning accuracy
Square M8 (40s)	0	0.2	+/-0.2 mm
Hex M8 (40s)	8	1.2	+/-1.5 mm
Square M6 (30s)	0	0.15	+/-0.2 mm
Hex M6 (30s)	12	0.9	+/-1.0 mm

The rotation test used a new groove with no wear. In a worn or corroded groove, the hex nut rotation increases because the contact edges have less grip. The square nut is less affected by groove wear because the flat sides have more surface area.

03When positioning accuracy matters: the +/-0.5 mm rule

The rule of thumb is: if your assembly requires positioning accuracy better than +/-0.5 mm, use a square nut. If +/-1.0 mm is acceptable, a hex nut will do. The threshold comes from the practical observation that hex nuts rotate unpredictably — sometimes 5 degrees, sometimes 15 — so the thread shift varies from joint to joint and cannot be designed out by offsetting the initial position.

Applications that need the square nut’s precision include sensor mounting where the sensor must align with a target within 0.5 mm, repeatable jig plates where the same part is mounted and remounted many times, and door and panel hinges where the hinge pin must stay concentric within 0.3 mm to prevent binding.

Application	Positioning requirement	Nut type	Reason
Sensor bracket alignment	+/-0.2 mm	square	thread must not shift
Jig plate repeatable mount	+/-0.3 mm	square	same position every cycle
Door hinge concentric	+/-0.3 mm	square	pin must stay aligned
Panel fixing (cosmetic)	+/-1.0 mm	hex	position not critical
Cable clip mounting	+/-1.0 mm	hex	clip is adjustable
Structural bracket	+/-0.5 mm	either	bracket has slotted hole

04Thread and series compatibility

Square nuts are available for 30 and 40 series grooves. The 20 series groove (6 mm) is too narrow for a square body to have meaningful flat-side contact — the square would be only 5 mm wide, with very little wall contact. For 20 series, a hex nut or a pre-set nut with a spring tab is the standard option.

Series	Groove	Thread	Nut width (square)	Wall contact per side
30 series	8 mm	M6	7.5 mm	3.5 mm
40 series	10 mm	M8	9.5 mm	4.5 mm
30 series	8 mm	M5 (reducer)	7.5 mm	3.5 mm
40 series	10 mm	M6 (reducer)	9.5 mm	4.5 mm

Wall contact per side is the length of flat surface bearing against the groove wall. This is the key dimension that determines the anti-rotation effectiveness. More contact means more friction and less rotation under torque.

05Body dimensions and groove fit

The square nut body is designed to slide freely along the groove but not rotate within it. The body width is about 0.5 mm less than the groove width, so the nut slides smoothly but the flat sides are close enough to the groove wall to bear against it when the bolt is tightened.

The body height (the dimension along the slot axis) is typically 1.2 times the thread diameter. For M6, that is about 7.2 mm; for M8, about 9.6 mm. A taller body gives more wall contact area but takes up more space along the slot.

Dimension	M6 (30 series)	M8 (40 series)
Body width (across flats)	7.5 mm	9.5 mm
Body height (along slot)	7.2 mm	9.6 mm
Body thickness (radial)	5.5 mm	7.0 mm
Thread pitch	1.0 mm	1.25 mm
Wall contact area (per side)	25 sq mm	43 sq mm
Weight	approx. 2.5 g	approx. 6.0 g

06When to choose square over hex

Choose square nuts

Positioning accuracy of +/-0.2 mm or better is required
Sensor brackets that must align with a target within 0.5 mm
Repeatable jig plates where the same fitting is remounted many times
Door and panel hinges where pin concentricity matters
Precision assemblies where thread shift is unacceptable

Choose hex nuts

Positioning accuracy of +/-1.0 mm is acceptable
Cable clips and non-critical mounts where position can be adjusted after tightening
20 series grooves — too narrow for effective square body contact
Budget-constrained projects where hex nuts save 20 per cent
Tight spacing along the slot where the taller square body would interfere with adjacent nuts

07Failure modes to design out

What goes wrong	Why it happens	Design it out by
Square nut rotates despite flat sides	groove wall is worn or anodised, reducing friction	use on mill finish grooves; or use anti-torsion nut for worn grooves
Nut too tight to slide along groove	anodising build-up in groove narrows the slot width	deburr and clean groove before installing; check width with gauge
Thread position varies between nuts	square body width tolerance allows small rotation	specify body width tolerance of +/-0.05 mm for precision applications
Nut body interferes with adjacent nut	square body is taller than hex body along the slot	check spacing: leave at least body height plus 2 mm between nuts
Nut falls out of groove before bolting	square nut has no anti-drop tab (unlike pre-set nut)	install from the end face; or use a pre-set square nut with tab

08What to specify when ordering

Procurement checklist for square T-slot nuts

Confirm the profile series: 30 (8 mm groove) or 40 (10 mm). Square nuts are not recommended for 20 series.
Confirm the thread size: M6 or M8 for direct fit, M5 or M6 for reducer configurations.
Confirm the positioning accuracy requirement: if +/-0.2 mm is needed, specify body width tolerance of +/-0.05 mm.
Confirm the groove condition: mill finish gives the best anti-rotation; hard-anodised grooves reduce friction.
Confirm the material: carbon steel zinc plated, or stainless steel for cleanroom or food-grade.
Specify the quantity per size. Price breaks apply at 100, 500 and 1000 pieces.

09Terms used on this page

Square nut: A T-slot nut with a square body whose four flat sides bear against the groove walls, preventing rotation under bolt torque.
Hex nut: A T-slot nut with a hexagonal body. The six sides give narrow edge contact with the groove wall, allowing rotation under torque.
Positioning accuracy: The maximum deviation of the thread centre from its intended position after the bolt is tightened. Determined by nut rotation during tightening.
Wall contact: The surface area of the nut body bearing against the groove inner wall. More contact means more friction and less rotation.
Reducer nut: A nut whose body fits one series groove but whose thread is smaller than the series default. For example, M5 thread in a 30 series (8 mm) groove.
Body height: The dimension of the nut body along the slot axis. Determines how much space the nut occupies along the groove.

10Questions buyers ask

Are square nuts stronger than hex nuts?

No. Both types seat the same way behind the groove lip and carry the same pull-out load. The square body prevents rotation, which improves positioning accuracy, but does not increase the structural strength of the joint.

Can I use square nuts in a 20 series groove?

It is not recommended. The 6 mm groove is too narrow for effective flat-side contact — the square body would be only 5 mm wide, with very little wall surface to prevent rotation. Use hex nuts or pre-set nuts with spring tabs for 20 series.

Do square nuts have an anti-drop tab?

Standard square nuts do not have a spring tab — they are subsequent nuts that slide in from the end face. A pre-set square nut with a spring tab is available on request for applications where the nut must be placed before assembly.

What is the positioning accuracy of a square nut?

Approximately +/-0.2 mm along the slot axis and +/-0.1 mm perpendicular to it. The flat sides prevent rotation, so the thread centre stays close to its initial position when the bolt is tightened.

Are stainless steel square nuts available?

Yes, in A2 and A4 stainless steel. The anti-rotation performance is slightly lower than carbon steel because stainless has lower friction against aluminium, but the positioning accuracy is still better than a hex nut.

What is the lead time?

Standard carbon steel sizes are stocked and ship within 2 to 3 days. Stainless steel and precision-tolerance sizes have a lead time of about 2 weeks.

What we need to quote square T-slot nuts

The profile series (30 or 40) and the groove width.
The thread size: M6 or M8 standard, M5 or M6 reducer.
The body width tolerance: standard +/-0.1 mm, precision +/-0.05 mm for +/-0.2 mm positioning.
The material: carbon steel zinc plated, or stainless steel.
The quantity per size, with total across all sizes.
Whether you need the pre-set version with anti-drop spring tab.

Send those six items and the quotation comes back with the unit price at each quantity break, the precision tolerance surcharge if applicable, and the lead time. Browse the full connecting elements range for related fasteners.