Slot 6, 8 or 10 mm? Choosing the Groove Size for Aluminium Profiles
An 8mm slot aluminium profile is the most common choice for machine frames, workbenches and guarding. That does not make it the right choice for every project. Slot 6 and slot 10 profiles also exist for good reasons. If you order T-nuts, brackets or bolts for the wrong groove, they will either drop straight through the slot or refuse to go in at all. This guide explains what the slot number actually means and which profile series use which groove. It also shows how to check an existing profile with a caliper, so the accessories in your next order fit on the first try.
What "slot 8" really means on an aluminium profile
The slot number is the width of the groove opening, measured between the two lips at the surface of the profile. It is not the depth of the groove or the width of the cavity behind the lips. A slot 8 profile has an opening of about 8 mm, usually 8.0 to 8.2 mm. Slot 6 and slot 10 follow the same rule.
The opening width decides three things:
- Which T-nuts fit. The neck of the nut must pass through the opening, and the wings must rest under the lips.
- The maximum bolt diameter. A wider groove allows a bigger thread and higher clamping force.
- Which brackets and connectors fit. Many brackets have a locating tongue that sits in the slot, so the tongue width must match the opening.
Profile size and slot size are related, but they are two different numbers. Most manufacturers use the following pairing:
- 20 series (2020, 2040, 2060, 2080): slot 6
- 30 and 40 series (3030, 3060, 4040, 4080): slot 8
- 45 series (4545, 4590): slot 10
There are exceptions. Some systems put a slot 10 groove on a 40 mm profile, and some 30 mm profiles have a slot 6 groove. Always check the specification or measure the profile before you rely on the series number alone.
Profile series, slot size, bolts and T-nuts at a glance
| Profile series | Slot | Bolt sizes | Typical T-nut | Typical use |
|---|---|---|---|---|
| 20 (2020–2080, V-Slot) | 6 mm | M3, M4, M5 | Hammer nut / drop-in M5 | 3D printers, small CNC, light frames |
| 30 (3030, 3060) | 8 mm | M4, M5, M6 | Hammer nut / sliding M6 | Enclosures, medium CNC, jigs |
| 40 (4040, 4080) | 8 mm | M5, M6, M8 | Sliding / spring-loaded M6–M8 | Machine frames, workbenches, guarding |
| 45 (4545, 4590) | 10 mm | M6, M8 | Sliding / heavy-duty M8 | Heavy frames, gantries, production lines |
As a rule of thumb, use the largest bolt the slot allows wherever a joint carries load or vibration. On a 4040 frame, M8 with slot 8 nuts gives much more clamping force than M5. If your design uses 40 series profiles, the T-nuts for slot 8 cover all common thread sizes. For 45 series frames, use the T-nuts for slot 10, which are built for M8 and higher loads.
V-Slot (slot 6) vs T-Slot 8: are they compatible?
In short, no. A 2020 V-Slot profile has a slot 6 opening. Its lips are bevelled at 45° so that V-Wheel rollers can run along the groove. A slot 8 T-nut is too wide to pass through that opening. A slot 6 nut placed in a slot 8 profile will either spin freely or pull straight out under load.
Accessories within slot 6 are a different story. Most slot 6 hammer nuts and drop-in nuts fit both 20 series V-Slot and standard 20 series T-Slot profiles. The V-shaped lips do not prevent a nut from seating underneath them. The real differences are:
- V-Wheels only run on V-Slot. A flat T-Slot lip gives the wheel no track to follow.
- Brackets with a locating tongue may sit slightly differently on V-Slot than on T-Slot because of the bevelled edge.
Mixing series is common in practice. A typical example is a 20 series V-Slot gantry mounted on a 40 series base. In that case you need two separate sets of accessories. You can join the two series with plates or angle brackets that have holes for both bolt sizes. Do not try to share nuts between them.
B-type vs I-type profiles
Two profiles can both be "40x40 slot 8" and still take different T-nuts, because the groove geometry differs between systems. B-type profiles follow the Bosch Rexroth style. The lips are relatively thin, and most low-cost profiles and accessories on the market are B-type. I-type profiles follow the Item style. They have a different cavity shape and lip thickness, and their T-nuts are shaped to match. A B-type nut in an I-type profile may not lock properly, may rotate, or may sit too low. The reverse problem happens too. Before you buy accessories, find out which type your profile is, or compare its cross-section with the drawing in the product listing. All T-Slot profiles in our range are B-type, and their accessories are matched to them.
How to measure the slot of an aluminium profile with a caliper
If you are extending an existing frame and have no documentation, a digital caliper will tell you what you need in about two minutes.
- Outer dimension. Measure across the profile face, for example 40.0 mm. This tells you the series.
- Slot opening. Use the inside jaws of the caliper between the two lips, at the surface. A reading of about 6.2 mm means slot 6, about 8.0–8.2 mm means slot 8, and about 10.0–10.2 mm means slot 10.
- Lip thickness. Use the depth rod or the tips of the jaws to measure how thick the lip is from the surface to the cavity. You need this number to choose bolt length.
- Cavity width. Measure the width of the space behind the lips, if you can reach it from the profile end. This helps you tell B-type from I-type and shows whether a large sliding nut will fit.
- Lip shape. Look at the profile end. A 45° bevel means V-Slot. Flat or slightly rounded lips mean T-Slot.
For bolt length, use this sum: bracket thickness + lip thickness + full thread engagement in the nut. Then check that the bolt does not bottom out against the profile core. A bolt that is too long feels tight even though the joint is not clamped.
Checklist before ordering accessories for an 8mm slot aluminium profile
The same points apply to slot 6 and slot 10. Go through them before you place an order:
- Series and slot confirmed. Measure the opening and do not rely on the series number alone.
- Profile type known. Check whether it is B-type or I-type. This matters most for 40 and 45 series.
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Nut type chosen for the assembly order:
- Sliding nuts go in from the profile end, so they must be inserted before the frame is closed.
- Hammer nuts and drop-in nuts can be added later through the slot opening.
- Spring-loaded nuts stay in position on vertical members.
- Thread size matched across the joint. Nuts, bolts and bracket holes must all use the same thread, for example M6 or M8.
- Bolt length calculated from the lip thickness and bracket thickness.
- Spare nuts counted. Add 10–20% extra for later additions such as panels, sensors or cable ducts.
- Brackets and connectors checked for the correct slot tongue width. See the T-Slot 8 mm accessories or the T-Slot 10 mm accessories.
- Profile lengths specified to the millimetre. Cut-to-size profiles save time and reduce the risk of square-cut errors.
Summary: which slot should you choose?
Choose slot 6 for compact 20 series builds such as 3D printers, laser engravers and small CNC machines, especially when you need V-Wheel motion. Choose an 8mm slot aluminium profile in 30 or 40 series for most machine frames, workstations and guards. It offers the best balance of stiffness, accessory choice and M6–M8 clamping. Choose slot 10 with 45 series profiles when the frame carries heavy loads, long spans or significant vibration. Whatever you choose, measure the profile first, confirm whether it is B-type or I-type, and keep nuts, bolts and brackets within one slot standard.
We cut every profile to length and ship profiles and accessories to customers across the European Union. If you are not sure which slot your project needs, or want us to check a parts list before you order, send us the profile dimensions and a short description of the build.
Need help matching profiles and accessories? Write to us at shop@bsv-slot.pl. We will recommend the right slot size, T-nuts and connectors for your frame.