Rack frames always have same-style mounting rails at the front and back.
In addition to the front holes, there are also identically spaced holes in a flange at 90 degrees to the front face. These are used to mount support rails running from front to back of the rack. You put in rails to support each piece of gear that is too heavy to be supported just by the front bracket screws. The rails also make it possible to slide gear out, without having it interfere with gear above or below.
There are many kinds of fancy (=costly) rack-rails, but you can make your own simple ones from plain galvanized rolled-steel L-section. 2.5mm thick, 40mm sides. Work out how long they have to be to bridge between your rack's front and rear struts, cut a bunch to length (eg with a drop-cut-off-grinding wheel saw), then drill one hole at each end to suit the strut holes spacing.
The _height_ of the holes in the L section sides needs to be carefully worked out. Depends on the equipment bottom level, when the front brackets are screwed in. And get them all accurately the same, or the gaps between front panels will be annoyingly skew.
You'll need to buy a lot of the cage nuts and matching screws. The metric ones are M6. But some old racks have UNF #10 hardware.
For the M6, screws are typically 16mm long. I strongly recommend pan head philips head. Slot screws are a pain when you are trying to juggle a bit of gear with one hand while getting a screw to thread in with the other hand.
For inserting and removing cage nuts (you'll be doing this a lot) you can use a plain flat blade screwdriver, but I recommend making a thing like a bale hook. See pic.
There are special-purpose nylon washers that fit M6 screws, designed for racks. They have a raised lip and friction-clips that grip the thread so they don't fall off when you remove the screws. These protect the equipment front panels from being scratched by the mounting screws. I had found some of these, but so far haven't located a supplier to buy more.
Some of my writeups that include rackmount-related stuff.
http://everist.org/NobLog/20121017_the_last_rack_bender.htmhttp://everist.org/NobLog/20121029_hp_racks.htmhttp://everist.org/NobLog/20130923_hd_procrastination.htmhttp://everist.org/NobLog/20130924_rack_roller_base.htmhttp://everist.org/NobLog/20141212_racked_serendipity.htmhttp://everist.org/NobLog/20160228_bench_rack.htmhttp://everist.org/NobLog/20170224_summer_vacuum_odyssey.htmCastors: need to be very heavy duty. A full rack can weigh many hundred kilos.
For safety, bear in mind that castors can turn in, which puts the load point back from the front of the rack. Beware the rack tilting forward and falling on you. My rack base frames put the front castors forward of the rack front to avoid this risk.
Castors with rubber rims, when left under load, develop permanent flat spots. So it's best to have some system to take the load off the castors when the rack is not moved for a while. I use screw-down bolts. Some racks have facility for these built in, others don't.
You don't need freely rotating castors on all four corners. Two fixed wheels, and two rotating castors, works fine. (Fixed ones at the back.) But some racks only have single bolt-holes for castors at all corners, so then use 4 rotating castors.
Buying racks-
Unless you _really_ want fancy looking ones, it's almost certain you should be able to get very cheap 2nd hand ones. Some of mine were free, some $50 each from a place that does telecoms work. Carriers etc often rip out old systems and discard everything, including the racks.
When they have front and back doors, these are always easily removable, by just lifting them off the pin hinges. So you don't _really_ have to have glass doors, just doors you can leave off when the equipment is in use.
Try to get racks that include adequate power outlet rails at the rear. You need an outlet per piece of equipment.
Having a master power switch at the front of the rack is very useful.
Other useful info
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19" rack
1 U = 1.75" = 44.45mm (by the standard.)
Opening width: 450mm
Screw H spacing: 465mm
Facia width: 483mm (282.6mm / 19")
Usable equipment face width: 422mm
On the vertical mounting rails, the square holes for the cage nuts are 9.4mm square.
The metal between the holes has a repeating pattern: thin, THICK, THICK, thin, THICK, THICK, thin, etc.
ie it repeats every 3 holes. This is the 'single unit height', or 1U.
The lines on which the 1U modules (and all larger multiple Units) break is the _thin_ metal piece.
The three-hole group pattern repeats every 44.45mm.
Actual instruments will be about 1mm less than their n x U module height, to give clearance.
Rack unit heights Blanking panels
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1 U = 44.45 mm 43.2
2 U = 88.9 mm 88.6
3 U = 133.35 mm 132.9
4 U = 177.8 mm 177.4
5 U = 222.25 mm 222.0
6 U = 266.7 mm 266.2
7 U = 311.15 mm
8 U = 355.6
9 U = 400.05
10U = 444.5
11U = 488.95
12U = 533.4
13U = 577.85
14U = 622.3
15U = 666.75
16U = 711.2
17U = 755.65
18U = 800.1
19U = 844.55
20U = 889.0
http://www.brenclosures.com.au/19-inch-rack.htmSpecifications
A rack's mounting fixture consists of two parallel metal strips (referred to as "rails") standing vertically. The rails are separated by a gap of 17.72 inches (450 mm), and are a minimum of 0.657" (16.7mm) wide giving an overall rack width of 19 inches (482.6 mm).
The rails have holes in them at regular intervals, with both rails matching, so that each hole is part of a horizontal pair with a centre-to-centre distance of 18.3 inches (465 mm). The holes in the rails are arranged vertically in repeating sets of three, with centre-to-centre separations of 0.5 inch (12.7 mm), 0.625 inch (15.875 mm), 0.625 inch (15.875 mm). The hole pattern thus repeats every 1.75 inches (44.45 mm).
Racks are divided into regions, 1.75 inches in height, within which there are three complete hole pairs in a vertically symmetric pattern, the holes being centred 0.25 inch (6.4 mm), 0.875 inch (22.2 mm), and 1.5 inch (38.1 mm) from the top or bottom of the region. Such a region is commonly known as an "RU" or "U", for rack unit or unit respectively.
http://www.brdatasystems.com.au/PDF/Server-Rack-Specification.pdfthe SSI specifications visit
www.eia.org/https://www.eevblog.com/forum/projects/neat-control-cabinets-control-panel-wiring/https://www.server-racks.com/rack-screws-10-32-12-24-m6.htmlhttps://www.eevblog.com/forum/chat/what-are-these-old-style-rack-slide-out-rails-called/ Chassis trak. A division of General Devices Co, Inc. PO Box 39100-Indianapolis, In 46239 - Phone (317) 897-7000.