Water Bottle Spring Cap Repair

One of our nice aluminum water bottles hit the floor and, of course, the tiny little hinge shattered. It’s some wonderful engineering plastic, but just look at the leverage you can apply to those few millimeters of material. This is the sort of repair that can’t possibly be economically justified, but it pisses me right off when something that should be rugged turns out to be this fragile.

The 2 mm steel hinge pin snapped the molded plastic center post of the hinge off the cap; we found the larger fragment, but the smaller one may lurk under the refrigerator for quite some time. Nothing bonds to this plastic and, if the post broke in the first place, adhesive isn’t going to help.

Broken hinge
Broken hinge

Some doodling showed that a replacement hinge post should be machineable. The general idea was to square up the remaining chunk of the post, then attach a replacement hinge pivot with a screw. The post is almost exactly 1/4-inch thick, call it 6.2 mm, which means the right-angle feature under the pivot ought to keep the whole affair from twisting.

Water Bottle Hinge
Water Bottle Hinge

I planned to leave the left side unmachined and cut it to fit by hand, but then figured, eh, just make it happen. I also expected to leave the area around the screw a lot thicker, with a neat counterbore around the head.

This being a bash-to-fit, file-to-hide kind of project, I wrote a snippet of G-Code (at the bottom of the post) to chew out the part from a sheet of Lexan, then did the perpendicular hole & countersinking with manual CNC.

No pix of that; I was working in a white-hot fury. Basically, I double-sticky-taped a slab of Lexan to a sacrificial sheet, clamped it to the tooling plate, and had at it with a 2 mm end mill. Cutting a 6.4 mm sheet with a 2 mm end mill is a bit iffy, as the flutes are just barely that long; the mill was armpit-deep in swarf and I was dribbling water into the cut to keep it cool.

By the time I stopped for a picture, the situation looked like this.

Replacement hinge part
Replacement hinge part

For what it’s worth, that’s the second part. I had to lower the screw head below the top of the half-round feature on the left end in order to clear the cap. That’s what CNC is really good for in my shop: make another one, just like the other one, only different exactly like that.

I drilled a #50 (2-56 tap) hole in the cap pretty much by eye, using laser targeting to touch off.

Laser aligning to hinge stub
Laser aligning to hinge stub

The hole wound up minutely too far inboard, but some filing cleaned up the stub edge and it was all good. I started the tap in the mill, held loosely in the chuck and turning it with my fingers, then finished up on the bench.

The screw hole goes all the way through the cap. I filed the screw down so the end sits flush at the bottom of the cap, where the silicone rubber gasket should seal firmly against it.

Here’s what the hinge looks like with all the bits assembled. The spring bears on the screw head, which makes the cap open with more snap than before. I put a little counterbore under the screw head, even after lowering it, to reduce the spring tension.

Rebuilt hinge
Rebuilt hinge

The cap has a spring-loaded latch that never worked very well in the first place and this repair didn’t improve it. As nearly as I can tell, the molded ledge on the cap has a rounded edge that the latch simply cannot engage. This is beyond even my level of interest; Mary was accustomed to using the wire snap to hold the cap closed and that practice will continue.

Works well enough for us and I got some Quality Shop Time on a rainy afternoon.

The G-Code uses a slightly modified & simplified version of the tool length probe routines. I’m not convinced that using the G59.3 coordinate system is the right way to go, but everything else seems worse.

(Water bottle hinge repair)
(Ed Nisley - KE4ZNU - June 2010)
(Rough-cut 1/4-inch plate with clamp at +Y)
(Sacrificial plate below, double-stick tape to secure)
(Tool change @ G30 position above length probe)

(-- Global dimensions & locations)

#<_Stock_Thick> =        6.5                    (overall thickness)

#<_Traverse_Z> =        1.0
#<_Safe_Z> =            30.0                (clamp clearance)

(-- Section controls)

#<_Do_Outline> =    1
#<_Do_Drill> =        1

(-- Initialize new tool length at probe switch)
(    Assumes G59.3 is still in machine units, returns in G54)

#<_Probe_Speed> =        250            (set for something sensible in mm or inch)
#<_Probe_Retract> =        1            (ditto)

O<Probe_Tool> SUB

G49                    (clear tool length compensation)
G30                    (to probe switch)
G59.3                (coord system 9)

G38.2 Z0 F#<_Probe_Speed>        (trip switch on the way down)

G0 Z#<_Probe_Retract>            (back off the switch)

G38.2 Z0 F[#<_Probe_Speed> / 10]    (trip switch slowly)

#<_ToolZ> = #5063                    (save new tool length)

G43.1 Z[#<_ToolZ> - #<_ToolRefZ>]    (set new length)

G30                    (return to safe level)

O<Probe_Tool> ENDSUB

(-- Initialize first tool length at probe switch)

O<Probe_Init> SUB

#<_ToolRefZ> = 0.0        (set up for first call)

O<Probe_Tool> CALL

#<_ToolRefZ> = #5063    (save trip point)

G43.1 Z0                (tool entered at Z=0, so set it there)

O<Probe_Init> ENDSUB


(-- Get started ...)

G40 G49 G54 G80 G90 G92.1 G94 G97 G98        (reset many things)


(msg,Verify clamp to +Y, stock taped down)

(msg,Verify X=0 at left edge, Y=0 on finished centerline)

(msg,Verify tool touched off at Z=0 on surface)

O<Probe_Init> CALL

T0 M6                            (ensure first tool change pauses)

(-- Drill the hinge pin hole)

#<Pin_X> =                7.0
#<Pin_Y> =                0.0

#<Drill_Dia> =            2.06    (Drill diameter)
#<Drill_Num> =            46        (Drill number)
#<Tool_Num> =            146        (Tool number)
#<Drill_Radius> =        [#<Drill_Dia> / 2]
#<Drill_RPM> =            3000
#<Drill_Feed> =         [#<Drill_Dia> * 100]

#<Drill_Depth> =        [#<_Stock_Thick> + 2 * #<Drill_Dia>]

O<Doing_Drill> IF [#<_Do_Drill>]

(debug,Insert Num #<Drill_Num> drill)
T#<Tool_Num> M6

O<Probe_Tool> CALL

(debug,Set spindle to #<Drill_RPM>)


G0 Z#<_Traverse_Z>

G83 X#<Pin_X> Y#<Pin_Y> Z[0 - #<Drill_Depth>] R#<_Traverse_Z> Q[2 * #<Drill_Dia>]

O<Doing_Drill> ENDIF

(-- Mill outline)

#<Hinge_Radius> =        3.75                        (half-width of hinge body)
#<Cutout_Base> =        2.75

#<Cutout_Screw> =        1.50
#<Cutout_Screw_Y> =        [#<Hinge_Radius> - #<Cutout_Screw>]
#<Cutout_Screw_A> =        ASIN [#<Cutout_Screw_Y> / #<Hinge_Radius>]
#<Cutout_Screw_X> =        [#<Hinge_Radius> * COS [#<Cutout_Screw_A>]]

#<Passes> =                3

#<Mill_Dia> =            1.98            (end mill diameter)
#<Tool_Num> =            20
#<Mill_Radius> =        [#<Mill_Dia> / 2]
#<Mill_RPM> =            3000
#<Mill_Feed> =            100

#<Entry_XL> =            [0 - #<Mill_Dia>]
#<Entry_YL> =            [0 - 2 * #<Hinge_Radius>]

O<Doing_Outline> IF [#<_Do_Outline>]

(debug,Insert #<Mill_Dia> mm end mill)
T#<Tool_Num> M6

O<Probe_Tool> CALL

(debug,Set spindle to #<Mill_RPM>)


G0 X0 Y[0 - 2 * #<Hinge_Radius>]    (get to comp entry point)
G0 Z#<_Traverse_Z>

G42.1 D#<Mill_Dia>                                    (cutter comp right)
G1 X#<Pin_X> Y[0 - #<Hinge_Radius>]

#<Step_Z> = [#<_Stock_Thick> / #<Passes>]
#<Current_Z> = [0 - #<Step_Z>]

O<Outline_Passes> REPEAT [#<Passes>]

G2 J[0 - #<Hinge_Radius>] Z#<Current_Z>                (ramp down to cutting level)

G3 Y#<Hinge_Radius> J#<Hinge_Radius>
G3 X[#<Pin_X> - #<Cutout_Screw_X>] Y#<Cutout_Screw_Y> J[0 - #<Hinge_Radius>]
G1 X0
G1 Y[0 - [#<Hinge_Radius> - #<Cutout_Base>]]
G1 X#<Pin_X>
G1 Y[0 - #<Hinge_Radius>]

#<Current_Z> = [#<Current_Z> - #<Step_Z>]

O<Outline_Passes> ENDREPEAT

G0 Z#<_Safe_Z>


O<Doing_Outline> ENDIF

G30                    (back to tool change position)