Kiran’s notes 👨🏻‍💻

Minirack with 2020 extrusions

8 min read

The DeskPi RackMate 10” server racks are very nice for organising smaller equipment. They’re available in 4U (model T0), 8U (T1) and 12U (T2). However, only the T1 is available in India, with the price inflated from USD 120 to INR 16,000, over 50%.

I’ve obtained a T1 and like the quality, but don’t like the price and need a smaller rack, so I’m attempting to make one from 2020 aluminium extrusions. First, the relevant specs:

  • Server racks use legacy imperial units instead of metric, reflecting their age (standards established 1910s–1930s)
  • 10” and 19” refers to the width of the mounted equipment including some margin. The actual distance between the centres of the screw holes is a (somewhat arbitrary) 236.525mm (9.312”) for 10” and 465.12mm (18.312”) for 19”
  • One unit height (1U) is 1.75” or 44.45mm
  • There are three screw holes per side per U, but the vertical alignment is irrelevant when using 2020 extrusions because we have a continuous channel
  • Screws are typically 10-32”, which is similar to M5 with 0.8mm pitch, but not the same. M5 is very slightly larger and will have a tight fit in 10-32 threading, potentially stripping it. Some racks have square holes that accept a cage nut with your choice of thread
  • There is no standard for the depth of the rack. RackMate T0 and T1 are 200mm deep, while T2 is 260mm (note the switch to metric)

We’ll use 2020 extrusions. Either T-slot or V-slot will work. They have a square profile of 20mm, with a central mounting channel that’s offset 10mm from the corners.

The first priority is to establish the spec width, so we’ll need one horizontal bar between two vertical bars, sized to have 236.525mm between the mounting channels of the vertical bars. The channels are 10mm from the edge, so we deduct 20mm and round down, because it’s not possible to get sub-mm precision in consumer-grade metal cutting. (Most rack mounts will have oversized holes to aid with centring.) This gives us 216mm for the horizontal bar. To make a cuboid, we’ll need 4× 216mm.

Next, the height. 1U is 44.45mm, 4U is 4×, 177.8mm. We’ll round this up to 180mm because extra height is not a liability to the spec, while reduced height will make 1U unusable. To make a T or L join between extrusions, one of them has to be sufficiently longer. Add 20mm at either end. For 4U, that gives us 220mm per vertical bar, 4× total. Similarly, for 200mm depth we need 4× 180mm bars.

These are our cutting specs:

  • 4× 216mm (width)
  • 4× 220mm (height)
  • 4× 180mm (depth)

Next, the joints. A cube has eight corners, so we have eight three-way joints. There are three ways to make this joint, with differing costs (using prices from reliable vendors here):

  1. Use blind joints. 2020 has a central hole that’s the correct size to have an M5 thread tapped into it. To access this thread, the facing extrusion will need a hole cut into it 10mm from the end.
    • ₹30 per tap ×16 = ₹480
    • ₹70 per hole ×16 = ₹1120
    • ₹3.40 per M5×16 BHCS screw ×16 = ₹54 (rounded down for bulk pricing)
    • Total: ₹1654
  2. Use corner cubes. These are 20mm³ and have screw access holes on all sides. All three extrusions will need to be tapped but won’t need holes themselves. The vertical extrusions have to be shortened by 20mm at either end.
    • ₹150 per corner cube ×8 = ₹1200
    • ₹30 per tap ×8×3 = ₹720
    • ₹3.40 per M5×16 BHCS screw ×8×3 = ₹80 (rounded down for bulk pricing)
    • Total: ₹2000
  3. Use three-way connecters. These have arms that slide into the extrusion’s channels and can be screwed down. The extrusions don’t need taps or holes. These joiners don’t have the strength and precision of blind joints, but we don’t need that. A server rack is not a 3D printer with its sub-mm precision, moving mass, and metal-warping temperatures.
    • ₹100-150 per joiner ×8 = ₹800–₹1200

Three-way connectors are the most cost effective. They add a 5mm cap to the vertical extrusions, so we reduce 10mm to get our new cutting dimensions for a 4U rack:

  • 4× 216mm (width)
  • 4× 210mm (height)
  • 4× 180mm (depth)
  • Total: 2424mm

Extrusions are typically sold in 1 metre lengths. This will need 3m and leave about 570mm spare (each cut will lose material corresponding to the cutting wheel’s thickness, typically about 1mm). To reduce waste, add height to make a 6U rack.

Optimising cost

The RackMate racks skip the bottom horizontal bars front and back, to make an arch shape instead of a cuboid. This reduces weight and cost, but also reduces structural integrity as it depends on the rack mounted equipment to reinforce the frame at the bottom. This is fine for metal rack mounts but probably not for 3D printed plastic mounts.

If we make the same choice and skip 2×234mm bars, the bottom three-way joiners can be replaced with cheaper two-way joiners. The total extrusion length for 4U is now 2028mm, unfortunately still needing 3m of source material. However, it works for a taller 8U rack:

  • 2× 216mm (width)
  • 4× 386mm (height)
  • 4× 180mm (depth)
  • Total: 2696mm

This optimisation makes an 8U rack cheaper than a fully reinforced 4U or 6U rack! Note that the cost savings are from the cheaper two-way joiners, so an unreinforced arch-shaped 4U, 6U and 8U will all cost the same, with differing wastage.

We’ve discussed the cost of the joints. Extrusions themselves are available at a range of prices varying by quality, surface finish, and supplier logistics (imported vs local). Once again, we’re not building a 3D printer or milling machine, so the cheapest material is good enough, as long as it’s in your preferred surface finish. We’ll typically have two choices: raw aluminium (labelled “silver”) or anodised black.

If choosing black, note that the cut surfaces are not anodised. Joiners are also only available in raw finish, so colouring them black becomes an extra DIY project. Screws are available in both stainless steel and alloy steel with black oxide coating, but the backing nuts (see below) are typically only stainless.

Mounting equipment

The mounting channels in 2020 extrusions can host a T-nut with M3, M4 or M5 threading. T-nuts come in three varieties:

  1. Slide-in t-nut: These must be inserted from one end and slid to the required position. They have a fairly strong grip but are a pain to work with because you have to pre-insert them before closing the frame.
  2. Hammerhead t-nut: These are rectangular and can be dropped-in at any time. They rotate 90° to grip the channel when screwed down. They can fail to rotate sometimes and their grip strength is determined by whatever is on the other side. Plastic may not compress well enough for a good grip.
  3. Spring-loaded t-nut: These cost more but have none of the disadvantages. They can be inserted at any time and hold their position in the channel using a spring-loaded ball bearing.

M5 is preferred as rack mounts are cantilevered, unsupported on the rear. Screws are typically M5×8 as there is only about 5mm depth in the channel.

Power supply unit

I have not found a PDU for sale that fits 10” racks, so the options are to use a regular power strip, held down with cable ties or double-sided tape, or to build one using wall sockets and 3D printed casing. Fitting them within a 1U (44.45mm) height limit is tough as Indian sockets are typically taller:

  • Anchor Roma: 55mm
  • Legrand: 45mm
  • MX: Unknown

2U height clearance will allow for bulky adapters, and space usage can be optimised by mounting the PDU inward-facing on the rear side.

Sourcing

I’ve only listed vendors I’ve personally purchased from.

2020 extrusions (price per metre)

Connectors (price for each, lower in bulk)

M5 screws and nuts (price for each, lower in bulk)

Acronyms:

  • BHCS: Button Head Cap Screw (their cap size profile is necessary for cube corners and blind joints, and it has more surface area than socket-head screws for gripping rack mount plates)
  • SS: Stainless Steel, followed by a grade number. The austenitic family (200 and 300 series grades) is non-magnetic

M5×16 is for cube corners and blind joints. M5×8 should be sufficient for all rack mounts. Thinner plates may need M5×6.

Acrylic sheets

Prices vary by colour and dimensions. 3mm sheet should be sufficient for side panels.

Graph View

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