The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

  • Thing-O-Matic: Revised start.gcode and end.gcode

    SJFW supports the Dimension plugin with the E extruder axis and doesn’t implement the older M101/M103/M108/M113 direct extruder commands. So I rewrote start.gcode to take advantage of that, compensate for Z-axis backlash, and clean up a few other nits; an older version (for MBI firmware) is there. The new files are at the bottom of this post.

    The M351 disables dry-run mode, which I often engage while manually tweaking things: running the whole start.gcode file without heating helps get all the coordinates just right.

    Coarse homing sets the axis positions to zero, rather than their real values, to simplify the fine homing process. I could switch into G91 relative motion to back off the switches, but this is easier. The fine homing now runs dead slow through the 2 mm backoff distance and produces very consistent results.

    The first wipe gets rid of any residual snot before touching off on the Z-minimum platform switch, although I generally pick it off with a tweezer as the extruder heats.

    Unlike the MBI firmware, SJFW’s G92 can set each axis independently, which means the Z-min touchoff need not simultaneously reset the XY position. Note that the G92 distance gets smaller to raise the Z=0.0 position: a thinner switch means a smaller distance to the platform, so when you make the value smaller (for a constant switch height) the nozzle doesn’t travel as far downward to Z=0.0.

    I’d hoped to use SJFW’s M220/M221 commands to auto-set the home positions (including Z-min!), but they don’t quite work for this “in development” version. At least, feeding the appropriate commands directly through pronsole produces error messages indicating that the home positions aren’t set after a single-axis move, so I’ll continue using G92 for a while.

    I ram 10 mm of filament into the extruder with the nozzle positioned in front of the wiper blade, which generally does not poot out a giant tangle of thread because the previous print ended with a retraction that leaves the extruder depressurized. The point here is to squirt out at least some thread to get the extruder pressure close to normal, then do a retraction and wipe to prevent drooling on the way to the start of the Skirt outline.

    The Z axis has about 0.2 mm of backlash, which I compensate in two places. The mechanical Z=0.0 position works out to be 0.2 mm above the platform with the Z axis descending, so I made the G92 value slightly too small. The next two Z motions after the G92 descend to Z=0.0 in the left rear corner, then ascend to Z=0.2 to take up the mechanical backlash without causing any mechanical motion. After that, the nozzle is exactly (kinda-sorta) 0.2 mm above the platform and that matches the firmware’s notion of where it is. All successive Z motions (including to the first layer at Z=0.25) will be upward, so the backlash stays compensated and every layer comes out just about exactly correct.

    The last G92 presets the E axis position so that the first anti-reversal at the start of the Skirt thread doesn’t produce a giant blob. This is totally empirical; I think my manual reversal after priming doesn’t result in exactly the same extruder pressure as Skeinforge’s reversals.

    SJFW doesn’t implement the G0 rapid-motion command at all (which, for me, is better than implementing it wrong), so all the motion commands use G1 with an appropriate F feedrate parameter.

    However, extruder speed control using a G1 F before the G1 E doesn’t produce consistent results. I think the extruder normally runs in inverse-time mode, where the speed makes the E distance come out right for the prevailing XY speed. All I know for sure is that changing the F parameter doesn’t work the way it should: it does not set the subsequent filament speed in any predictable way.

    The new start.gcode file:

    ;---- start.gcode begins ----
; TOM 286 - Al plates + Geared extruder + Zmin platform sense
; Ed Nisley - KE4ZNU - Dec 2011
; SJFW 1.11 without G0 and G28/G16x homing
; Not yet -> Requires M220/M221 endstop positions
; See TOM286-sjfw.gcode for EEPROM setup
;
; Set initial conditions
G21                 ; set units to mm
G90                 ; set positioning to absolute
;----------
; Dry run?
M351 P0             ; P0 = normal P1 = no heat
;----------
; Begin heating
M104 S200           ; extruder head
M140 S110           ; HBP
;----------
; Coarse home axes
; Set zero at limits for convenience
G1 Z999 F1500       ; home Z to get nozzle out of danger zone
G92 Z0
G1 Y-999 F4000      ; retract Y to get X out of front opening
G92 Y0
G1 X-999 F4000      ; now safe to home X
G92 X0
;----------
; Fine home axes
; Set actual offsets!
G1 X2 Y2 Z-2 F5000  ; back off switches
G1 Z999 F50
G92 Z116.3
G1 Y-999 F50
G92 Y-58.5
G1 X-999 F50
G92 X-53.5
;----------
; Initial nozzle wipe to clear snot for Z touchoff
G1 X0 Y0 Z3.0 F1500     ; pause at center to build confidence
G4 P1000
G1 Z10                  ; ensure clearance
G1 X39 Y-58.0 F10000    ; move to front, avoid wiper blade
G1 X55                  ; to wipe station
G1 Z6.0                 ; to wipe level
M116                    ; wait for temperature settling
G1 Y-45 F500            ; slowly wipe nozzle
;-----------------------------------------------
; Z platform height touchoff
; Make sure the XY position is actually over the switch!
; Home Z downward to platform switch
; Compensate for 0.05 mm backlash in G92: make it 0.05 too low
G1 X55.5 Y8.2 Z3.0 F6000     ; get over build platform switch
G1 Z0 F50                    ; home downward very slowly
G92 Z1.35                    ; set Z-min switch height
G1 Z6.0 F1000                ; back off switch to wipe level
;-----------------------------------------------
; Prime extruder to stabilize initial pressure
G1 X55 Y-45 F6000   ; set up for wipe from rear
G1 Y-58.0 F500      ; wipe to front
G91                 ; use incremental motion for extrusion
G1 F4               ; set slow rate
G1 E10              ; extrude enough to get good pressure
G1 F4000            ; set for fast retract
G1 E-2.0            ; retract
G90                 ; back to absolute motion
G1 Y-45 F1000       ; wipe nozzle to rear
;----------
; Set up for Skirt start in left rear corner
; Compensate for Z backlash: move upward from zero point
G1 X-50 Y55 Z0.0 F10000     ; left rear corner -- kiss platform
G1 Z0.2 F1500       ; take up Z backlash to less than thread height
G92 E1.5            ; preset to avoid huge un-Reversal blob
;G1 X0 Y0
;---- start.gcode ends ----

    The matching end.gcode on the other end of the file simply retracts a bit more filament, then positions the stage front-and-center for easy build plate removal:

    ;---- end.gcode starts ----
; TOM 286 - Al plates + Geared extruder
; Ed Nisley - KE4ZNU - Dec 2011
; SJFW 1.11 without G0 and G28/G16x homing
; Not yet -> Requires M220/M221 endstop positions
; See TOM286-sjfw.gcode for EEPROM setup
;- inhale filament blob
G91
G1 E-5 F900
G90
;- turn off heaters
M104 S0         ; extruder head
M140 S0         ; HBP
;- move to eject position
G1 Z999 F1000   ; home Z to get nozzle away from object
G92 Z117.2      ; reset Z
G1 X0 F6000     ; center X axis
G1 Y35          ; move Y stage forward
;---- end.gcode ends ----

    The replace.csv file that squashes commands that SJFW doesn’t implement:

    M101	;-- M101 no Extruder Forward
M103	;-- M103 no All Extruders Off
M108	;-- M108 no Extruder Speed
M113	;-- M113 no Extruder PWM

  • SJFW: Initial EEPROM Configuration File

    Starting with acceleration and speed values based on those initial estimates, I hand-fed G-Code moves directly into the printer via pronsole while bouncing among these choices:

    • Increase acceleration until motor stalls
    • Decrease acceleration until motor starts
    • Increase velocity until motor stalls
    • Decrease velocity until motor runs

    The X axis runs fine at 333 mm/s = 20 m/min with 20 m/s2 acceleration; that little motor isn’t an impediment at all. As expected, the Y axis can’t accelerate that hard; it eventually started at 10 m/s2 to run at 250 mm/s = 15 m/min. I set X = 15 m/s2 and Y = 5 m/s2, with different maximum speeds.

    With those values in place, the printer can run the Smooth Axis Test at 250 mm/s, which is breathtaking and surprisingly noise-free: acceleration control eliminates the jarring start-stop motion. I modified Jackson’s G-Code to remove position testing, which uses codes that SJFW doesn’t implement.

    It’s worth mentioning I haven’t adjusted the motor currents at all.

    The Z axis can run at 2000 mm/min = 33 mm/s with acceleration around 1500 mm/s2. I backed that off to 1500 mm/min = 25 mm/s with 1000 mm/s2 acceleration. It’s noticeably faster than before, but that really doesn’t make much difference; there’s no point in replacing the stock MBI high-resistance / high-inductance motor.

    The E axis seems to require setting its speed with a separate G1 Exxx command (which is how Skeinforge does it) to get consistent results, although I confess to not taking good notes. I disconnected the filament drive to run the motor / gears without a load, got a workable speed & acceleration combination, reconnected the drive, fired up the hot end, and squirted filament all over the place to get actual numbers. It turns out that the little stepper on the extruder can actually ram a few millimeters of filament into the hot end at 4000 mm/min = 66 mm/s, but with acceleration down at 250 mm/s2. That’s dramatically peppier than the previous pace, which should reduce the Reversal Zittage problem.

    The resulting SJFW config file, with many unchanged default entries, will not work on a stock Thing-O-Matic:

    ; TOM286 with Z-min switch
M402                            ; Write this config to EEPROM
M309 P1 S0                      ; Endstops
M300 X28 Y25 Z22 E15            ; STEP pins
M301 X27 Y24 Z17 E14            ; DIR pins
M302 X26 Y23 Z16 E3             ; ENABLE pins
M304 X12 Y10 Z8                 ; MIN pins
M305 Z7                         ; MAX pins
M307 X1 Y1 Z0 E1                ; Axis Inversion
M308 X0 Y0 Z0 E0                ; Disable After Move
M200 X47.06985 Y47.06985 Z200  E48.30 ; Steps per MM
M201 X1200     Y1200     Z1000 E60    ; Start Speed mm/min
M202 X20000    Y15000    Z1500 E4000  ; Max speed mm/min
M203 X1800     Y1800     Z1500 E90    ; Avg speed mm/min
M206 X15000    Y5000     Z1000 E250   ; Acceleration mm/s^2
;M220 X-52.5    Y-58.5    Z1.50        ; Home - min
;M221                     Z117.2       ; Home - max
; LCD setup as per sjfw page on reprap wiki.
M250 P47     ;set LCD RS pin
M251 P43     ;set LCD RW pin
M252 P41     ;set LCD E pin
M253 S4 P39  ;set LCD Data 4 pin
M253 S5 P37  ;set LCD Data 5 pin
M253 S6 P35  ;set LCD Data 6 pin
M253 S7 P33  ;set LCD Data 7 pin - always set last of all LCD pins OR ELSE!
M255 S2 P48  ;set Keypad Col 3 pin
M255 S1 P46  ;set Keypad Col 2 pin
M254 S3 P42  ;set Keypad Row 4 pin
M254 S2 P40  ;set Keypad Row 3 pin
M254 S1 P38  ;set Keypad Row 2 pin
M254 S0 P36  ;set Keypad Row 1 pin
M255 S3 P34  ;set Keypad Col 4 pin
M255 S0 P44  ;set Keypad Col 1 pin - always set last of all Keypad pins OR ELSE!
M104 S0                          ; Turn off hotend heat (Important with EC!)
M140 S0                          ; Turn off platform heat (Important with EC!)
M350 P1                          ; Enable lookahead
;M211 P5000                       ; Report temperatures every 5 seconds
M84                              ; Disable all Motors
M400                             ; End EEPROM write

    I haven’t connected an LCD or keyboard to the thing; for me, printing is fire-and-forget.

  • Acceleration Doodles

    The SJFW firmware applies acceleration limiting to motion along all four axes, so I did some doodling to come up with reasonable starting values, based on various measurements and estimates.

    Some useful background, with lots more tucked away in odd corners around here:

    Relevant equations for uniform linear acceleration a, velocity v, distance x, time t:

    • v2 – v02 = 2·a·x
    • x = (1/2)·a·t2

    X and Y Axes

    Current values with low-resistance / low-inductance steppers:

    • Decent printing at 30 mm/s = 1800 mm/min
    • Not-so-good printing at 60 mm/s = 3600 mm/min
    • Point-to-point non-printing motion at 100 mm/s

    Given that the (beefed up) XY motors can accelerate their respective stages to 100 mm/s without acceleration, assume they can reach a top speed of 200 to maybe 250 mm/s within 1 mm of travel. That’s half of the belt pitch and seems like an overestimate of the actual distance.

    • (250 mm/s)2 = 2·a·(1 mm) → a = 31000 mm/s2
    • (200 mm/s)2 = 2·a·(1 mm) → a = 20000 mm/s2

    The 1 kg Y axis probably can’t accelerate as fast as the 0.4 kg X, despite having a bigger motor.

    Z Axis

    Currently traverses at 17 mm/s = 1000 mm/min, OK at 25 mm/s, fails at 33 mm/s = 2000 mm/min. That’s with the original high-resistance / high-inductance MBI stepper without acceleration limiting.

    Typical motion will be 0.25 mm, which is 15 ms at 17 mm/s: it’s not a performance limitation.

    It’s a 4-start leadscrew that moves 8 mm/rev, so 0.25 mm = 0.031 rev. At 1/8 stepping = 1600 step/rev, that’s 50 steps. Allow 20 steps = 0.1 mm for acceleration to top speed:

    • (33 mm/s)2 = 2·a·(0.1 mm) → a = 5400 mm/s2
    • (17 mm/s)2 = 2·a·(0.1 mm) → a = 1400 mm/s2

    E Axis

    Currently runs at about 2 rev/min = 0.033 rev/s with a 9.6 mm effective drive diameter = 1 mm/s for the incoming filament. Reversal runs at 15 to 25 rev/min for about 100 ms, figure 20 rev/min = 0.33 rev/s = 10 mm/s, without acceleration limiting. The extruder has 7:51 geardown, so the motor runs at 14.6 rev/min and reverses at 146 rev/min = 2.4 rev/s = 500 step/s, none of which seems particularly challenging even in 1/1 step mode (due to using a defunct MBI stepper driver).

    That reversal speed tends to leave blobs at the end of threads, but it’s not clear the motor is up to much more acceleration. It’s a relatively small stepper, so a larger one with with more current may be needed for enough torque for faster reversal action.

    Allowing 0.1 mm to reach full speed:

    • (1 mm/s)2 = 2·a·(0.1 mm) → a = 5 mm/s2
    • (10 mm/s)2 = 2·a·(0.1 mm) → a = 500 mm/s2

    None of these numbers should be cast in stone!

    The original doodles:

    TOM286 Calibration Doodles
    TOM286 Calibration Doodles

  • Thing-O-Matic: Software Shuffling

    Having installed the 0.4 mm nozzle, being desirous of turning on Skeinforge’s Dimension plugin, and being therefore faced with recalibrating everything, I figured I might as well update all the software to the current versions before commencing. While this adventure turned out well in the end, it required fitting together a large number of moving parts; this is an overview / core dump of how I picked the pieces.

    Note: I’ve certainly gotten something wrong in this mess, perhaps drastically so. Let me know, but consider the entire assembly before suggesting a different part.

    ReplicatorG is the default Thing-O-Matic printer interface and consists of two parts: the Java-based Arduino-IDE-oid program on the PC and the firmware inside the printer (which is, itself, in two parts divided: Motherboard and Extruder Controller). Their mutual interfaces have become sufficiently tangled that they must upgrade in lockstep, as no versions have backwards or forwards compatibility.

    RepG 29 bundles Skeinforge 35 as its default STL-to-G-Code converter, with 40 and 41 as experimental (i.e., largely unsupported) options. Skeinforge 35 is now ten full clicks behind the current version, came out on 6 November 2010, and has a number of fairly well-known problems. Although I understand the need for upstream stability, SF35 long ago fell off the thick edge of the wedge and even SF41 is 8 months old.

    I have been using RepG with SF40 for much of the last year, having figured out the parameters essentially from scratch to suit my admittedly oddball configuration & preferences. Regressing to SF35 lacks appeal and, frankly, going just one click up to slightly less obsolescent SF41 isn’t in the cards, either. I have no particular aversion to using bone-stock Skeinforge, fetching the most current version as needed, and controlling the update process myself.

    RepG manages Skeinforge profiles that collect its myriad parameters into named groups that can be selected for a particular build. RepG also includes a Print-O-Matic function that pre-sets / computes key SF parameters based on desired extrusion parameters within a given profile, but (apparently) only for SF35. Given that I want a single printer configuration that produces known-good results, putzing around with multiple profiles isn’t of interest and I’m unwilling to use an obsolete version of Skeinforge to sidestep them.

    FWIW, I eventually figured out that having one master set of start.gcode, end.gcode, and alterations.csv files with symlinks from the profiles helps keeps the clutter under control, which is particularly important given the complexity of my homing routine. RepG doesn’t create symlinks in new profiles, but after you’re used to it, you just create a profile, blow away the copies, and install the symlinks.

    So RepG really doesn’t provide what the B-school gurus called a compelling value proposition for my use case. The STL models I cook up using OpenSCAD emerge properly scaled, properly located, properly oriented, and ready to build. All I need is a way to convert those STL models to G-Code, then send G-Code to the printer. Everything else RepG does simply gets in the way.

    The dealbreaker, however, was having RepG 28 occasionally freeze up solid, to the extent of requiring a killall java in a console window to dispose of the corpse. RepG 29 misbehaved the same way and both failed on two different machines with two different versions of Ubuntu. The hole may have been in my end of the boat, but I didn’t devote much time to diagnosing / reporting the problem, given the attention given to the last batch of tickets I opened.

    Freed from the confines of RepG, Skeinforge turns out to be not nearly so intimidating as one might be led to believe. Admittedly, a bit of option pruning helps, but after that you’re left with knobs controlling those things that need controlling.

    Slic3r seems to be the up-and-coming alternative G-Code generator. The key problem, at least for the objects I create, is the lack of an equivalent to the Skeinforge Cool setting that enforces a minimum time for each layer. Printing exactly one of those caliper repair parts at 15 seconds per layer worked perfectly: no fans, no slumping, no hysteria. One could, I suppose, slow the motion throughout the entire object to make the top come out right, but that’s not appropriate for large parts with small towers. Slic3r is under heavy development, so who knows what the New Year will bring?

    Incidentally, my experience with those earlier caliper parts explains why I’m unwilling to regress Skeinforge just to use RepG.

    Kliment’s Printrun wins, hands down, as the RepRap UI that does what I need and very little else. The pronterface GUI presents a reasonably clean, single window printer interface. Even better, from my perspective, is the pronsole command-line interface; I generally do everything except actually print while sitting upstairs in the Comfy Chair, so being able to drive the printer with a command-line interface through a simple SSH session (shared keys, an oddball port, no root logins) is wonderful.

    The pronterface G-Code preview pane has its origin at the lower-left corner, presumably from its RepRap lineage, while RepG puts (0,0) at the build platform’s dead center. Centering the origin avoids baking the platform dimensions into the G-Code and greatly simplifies the overall alignment, but the mismatch is not insuperable: I can ignore the preview and the printer will be perfectly happy.

    However, MBI firmware expects to receive a binary version of the G-Code file, known as S3G and documented there, from the PC through the UI. As nearly as I can tell, nobody else does it that way and none of the other UIs do S3G translation / compilation. Not using RepG means ditching the MBI firmware inside the printer in order to use any other UI.

    The current state-of-the-art open-source 3D printing firmware seems to be the Marlin branch of the Sprinter family tree. Its main appeal, at least for me, is motion control with acceleration limiting, which should resolve most of the problems with the MBI stock firmware and greatly enhance the printer’s performance & print quality. For more details on that topic, search herein for acceleration. Alas, Marlin runs on “single processor electronics” controllers, categorically excluding MBI’s Motherboard + Extruder Controller configuration.

    While I could junk the entire contents of the Thing-O-Matic’s electronics bay and pop in a RepRap RAMPS 1.4,  Generation 6, or Generation 7 electronics package just to use Marlin, that bears a strong resemblance to bad craziness, even by my relaxed standards (although, should another MBI stepper driver board go toes-up, it’ll make considerable economic sense). That comparison of various electronics packages may be helpful. The temperature sense hardware for most of those boards uses thermistors, which means tearing apart the Thermal Core to replace a thermocouple that delivers perfectly accurate results with a thermistor requiring fiddly calibration, which I’d be willing to do, but …

    As it turns out, ScribbleJ’s SJFW firmware runs on both RepRap and MBI electronics, includes acceleration limiting, features automagic endstop position settings for both min & max positions, and seems reasonably stable. It has some quirks (no G0 rapid motion, no G28 homing, weird G-Code parsing assumptions / failures), but on the whole it does what’s needed.

    So the software stack, from the top down, consists of:

    • OpenSCAD
    • Skeinforge
    • Printrun UI — pronsole / pronterface
    • SJFW Motherboard firmware
    • Bone-stock MBI Extruder Controller firmware

    Everything requires configuration / tweaking before plastic starts oozing out of the nozzle. Then I can begin retuning the printing process.

    The overall workflow looks like this:

    • Edit/save OpenSCAD program in external editor on right-hand portrait monitor
    • Watch/examine OpenSCAD 3D rendering on left-hand landscape monitor, iterate
    • Export to STL on file server
    • Convert to G-Code using Skeinforge on PC at printer via SSH
    • Examine proposed G-Code paths with Skeinlayer (set to auto-display), iterate
    • Load/print with pronsole / pronterface via SSH/VNC
    • Trot downstairs to watch the show

    For the relatively simple models I build, CPU load generally isn’t a big deal. I’ll move the Skeinforge config from ~/.skeinforge to the server and add symlinks to it from both PCs, so as to run SF from either PC with the same settings and eliminate synchronization hassles.

    I’ll be writing up my scattered notes over the next week or so…

  • Sony NP-FS11 Batteries: After the Aftermarket

    The batteries I rebuilt for our much-beloved Sony DSC-F505V camera back in early 2010 have faded away with constant use. Having already sawed the cases open, rebuilding three of them didn’t pose much of a challenge; this time I added a short tab of Kapton tape to help extract them from the camera socket.

    Rebuilt NP-FS11 batteries
    Rebuilt NP-FS11 batteries

    Three batteries seems to be about the minimax for ordinary use:

    • One in the camera
    • One in the carrying case
    • One in the charger

    You (well, we) can’t keep track of more than three: it always seems one battery gets overused and another gets lost in the dark. We’ll see how three works in practice; there’s a set of six more raw cells lying in wait.

    The new batteries produced these results on their first two charge-discharge cycles:

    Sony NP-FS11 2011 Packs - First Charges
    Sony NP-FS11 2011 Packs – First Charges

    One battery didn’t come up to speed on the first charge, but after that they’re all pretty close.

  • Skeinforge: Simplified Plugin Selection Page

    The Skeinforge Craft window presents a formidable array of buttons, one for each possible plugin:

    Skeinforge standard
    Skeinforge standard

    I’ve disabled many of those plugins because, for example, limiting Z-axis speed isn’t relevant on my printer. If you’re sure you won’t use some of the plugins, remove them by editing /where-it's-installed/skeinforge_application/skeinforge_plugins/profile_plugins/extrusion.py thusly…

    In getCraftSequence(), located at about the midpoint of that file, duplicate the line that lists the plugins and add an octothorpe (OK, a hash) to make one line a Python comment, then remove the plugins you don’t care about from the other line:

    def getCraftSequence():
	'Get the extrusion craft sequence.'
#	return 'carve scale bottom preface widen inset fill multiply speed temperature raft skirt chamber tower jitter clip smooth stretch skin comb cool hop wipe oozebane splodge home lash fillet limit unpause dimension alteration export'.split()
	return 'carve scale preface inset fill multiply speed temperature raft skirt jitter clip smooth skin cool dimension alteration export'.split()

    This being Python, do not change the indentation. If you get overenthusiastic and toss something useful overboard or just pine for the Good Old Days, swap the octothorpe to your modified line to restore the original plugin assortment.

    Save the result and you’ll see only the useful buttons:

    Skeinforge simplified
    Skeinforge simplified

    There, now, wasn’t that easy?

  • Enabling Remote Desktop Sharing in Xubuntu

    I set up Xubuntu 11.10 on the Dell 531S driving the Thing-O-Matic, as the Unity UI seems surprisingly like crippleware: every feature that isn’t mandatory is prohibited. However, Xubuntu’s XFCE UI also has a long list of things that should be easy and aren’t, such as enabling remote desktop sharing. Gotta have that so I can fire up the printer and monitor progress from upstairs.

    It turns out that the Vino server is installed, but not enabled, so you must start by firing up vino-preferences in a terminal to set some preferences:

    This is a local machine behind a firewall, so a moderately secure password with no confirmation will suffice. Your paranoia may vary.

    Then drill down through the menu from Settings → Settings Manager → Session and Startup to the Application Autostart tab, then Add the Vino VNC Server to the list: /usr/lib/vino/vino-server. You can start it manually if you have the hots for immediate sharing.

    This seems to be impossible in Unity, trivially easy in GNOME, and unduly mysterious in XFCE.