High vacuum chamber
High vacuum technology is a new learning curve for most, UHV chambers go down to $ 10^- $$ ^1 $ $ ^1 $ torr, but these chambers are tricky; we need to get to $ 10^- $$ ^4 $ torr, which is much less demanding on skills of welding and machining. You can even use glue sealants for leaks on $ 10^- $$ ^4 $ chambers.
For the bootstrap prototype using the old fashioned tried and tested simple TIG welded Nominal pipe size 14inch sch 20? length gun tube XX length build chamber XX with filler pipe sleeve NPS XX sch XX length XX, with an aluminum hopper box thick plate thickness XX Width XX Length XX with inside removed to make top and sides of hopper box, bottom of box will be a 304 plate thickness XX mm Width XX length XX.
Eventually thes fittings will be self printed and the chamber may be half this size but for now that's the challenge. Test instruments will also required creativity for price reduction.
As far as we know all existing commercialized vacuum products are based round permanently sealed designs, and those people who want to have pump down chambers have traditionally had million euro budgets Labs etc, so this is where the creativity comes in to get below this 1600 euro plus fittings and welding.
New Ideas to save cost
Given build platform is circular, build electron gun & build platform in similar tube with smaller diameter tube inserted in ends to fill gap between self printed parts max diameter and build platform tube diameter.
Free standing bolt together structure put inside chamber so use thinner plate, no virtual leaks problems as no welds on free standing structure. hack existing pressure chambers?
Research which vacuum device supplier catalogs are best information wise! ( we need to help each other up the learning curve) high vacuum 10^-5 Torr Limited outgasing required units Torr Liters Per Square Cm. Per Second,
Design build & test conventional manufacturing and Self print manufacturing route ;
CF or KF or Bimetal flange - to be welded on stainless plate, Consisting of two parts plus copper ring seal a) Short pipe and flange, b)an end CF flange fitting Standard Dimensions , 12inch flange won't fit as a self print as OD is same as build chamber OD, So 10inch flange is the biggest standard size self printable. Nominal pipe sizes  mean that the spacer pipe sleeve between ID of 14NPS shc 20? and OD 10" flange matching pipe with OD ??? should fill this gap with tolerance of XXX is NPS XX sch XX ( holding ; windows or electrical connectors or pipes to pumps or electron gun or etc...)*** TOP Priority ( as expensive to buy so need to design and a challenge to self manufacture) pump pipes, 3x electrical feed through , 3 rotational feed through, window, arm glove hatches/ port, (9) no special heat treatment can be 304 or 319
vacuum chamber rotational motion feed-through magnetic type with inner wall ( not seal type or ferra fluid type too unreliable) See here - or solid shaft O ring types leaky but high torque See page 21 Thread Mount Solid Shaft–KJLC Standard example  *** TOP Priority
air filter subµ- to protect roughing pump and turbo pump from metal powder
pipe fittings to pumps & gauges normal guage baritron ( down to 10 -4, ) cold cathode guages( 24 Volt supply cathode chamber with anode inside 2.7Kv chamber, powerful magnet or micro pirani element ( no magnet) ). flexible hose to connect roughing pump so avoid vibration viton ring conectors KF 16 flange .
valves- to close electron guns from main chamber inner diameter min 12mm- and on connections to pumps. kf16 falnge both ends,
low voltage electrical connectors / electrical conductor feedthroughs optical passthrough , High-vac electrical feed-through glue name check ceramic name check TOP priority
leaded window Min diameter 12cm ( should be equivalent X ray shielding to 2mm lead or 6mm stainless, but could bodge with stainless shutter if too expensive.)Example window
door locks ( Door will be whole front side of chamber, ie H 1.6m x W 1m) *** TOP Priority
Viton O ring Main door seal or alternatives
Solenoid casings to avoid the need for tefelon coated wire.
approx total wieght. 25 kg fittings, 25 kg pipe 2K Euro to 4K euro.
Special case of for sub 15µ powder active metals i.e.Titanium, Nitrogen use reduced oxygenated cleanup. See nitrogen separation tube here also needs compressed air supply, and an atmospheric pressure (1 Bar) arm glove entry lockable port holes, for sand blast & suction/vacuum cleaner clean up (using metal powder not sand in blaster & Vacuum cleaner) Sieve metal back to metal powder hopper from vacuum cleaner and needs compressed air supply here.
Use airlock for build while pump down contained in vacuum chamber door,
Cleanup external Arm glove Inner door Port hole- So for 1µ size active metals (aluminum , Titanium ) - after print flood chamber with nitrogen and Sandblast/hoover parts inside chamber via arm gloves ( to avoid exothermic reactions as the nuclear industry refer to them.).
Vacuum airlock so print while repump
pumps initially purchase
Essential reading before you weld/construct your vacuum chamber, "Basic Vacuum technology" workbook version is best by Varian
Maths behind vacuum processes ( Not for the faint hearted )
All parts should be made of low out-gassing materials see here  Stick to this list / or materials with values like these and you won't go far wrong. High vacuum 10^-5 Torr or high Vacuum , This will be achieved through Diffusion pump or turbo pump system, along with a roughing vane pump The connection order is first gauges then turbo pump or oil diffusion pump next to chamber and then roughing pump last. ( Oil diffusion pump is cheap but slower pump down See pump down time discussion and with oil vapor back-stream that can affect some build metals ) ( maybe later upgraded to CRYO pumped systems for solar deposit modes as no back-streaming but requires regeneration / can cause contamination from helium oil , and difficult to self print)
Construction tips ; Welding; avoid dirt accumulation nooks and crannies, weld from inside full depth ( i.e. not both sides leading to trapped internal air pocket). Can use Brazing ( little local differential heating , as heats whole part therefore less leaks from residual stress cracks) , TIG welding (a argon) Ideally only inside / vacuum side See weld speed caculator, If must weld both sides only intermittent welds on the outside, So no empty trapped voids at plate joints. Avoid gaps between plates inside on the bottom surfaces and corners of the chamber , where dirt could accumulate leading to virtual leaks ( virtual leaks are created by out-gassing of trapped dirt or voids behind screws or voids between double sided welds, this type of leak is recognized by a change rate of leaking overtime (curved time vs log pressure plot) , normal leaks have a constant leak out rate within stable pressure situations ( straight line on log plot).
Avoid grease touching metal, all inner screws must have venturi vent holes ( ie hole down middle of screw axis)
Seals 1)stainless steel knife into OFHC copper plate ( relocation on reseal creates leaks) , 2)Aluminum gasket with ridge pressed on flat stainless steel seat( only low temp),3)gold or silver plated ridge pressing on stainless flat plate( can bake to 400C and position changes uncritical and most robust solution).
Sealing force ; via bellows, roller ramp mechanism, Varian elastomer-sealed gate design.
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding. Alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. Ferritic chromium stainless steels do require preheating to prevent cracking in the heat-affected zone. . A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases.
Can use nitrogen for venting chamber as thick easy to pump out, ( unlike water argon oxygen difficult to pump out) High vacuum sealant leaks spray 
list of traditional high vacuum chamber machining methods 
6mm plate with internal bolted structure
Further reading ; Vacuum chamber principles; Essential reading before you weld/construct your vacuum chamber, Basic Vacuum technology by Varian