3D WERK
CAD DESIGN · RING SIZING
RING SIZING GUIDE
From finger size
to a ring that fits.
A ring that comes out the wrong size rarely has a single cause. Size systems get mixed up, wide bands are sized like narrow ones, and castings come out a little smaller than the model. This guide gives you an accurate Australian size chart and a simple test that measures how your own printing, casting and clean-up change the size, so the rings you model are the rings that fit.
Before you start
This guide is general information to help you get the most from your WaxJet. It does not replace the Flashforge manuals, the safety data sheets for your materials or your own safety checks. Where they differ, follow the manufacturer's instructions and safety data sheets. Isopropyl alcohol (IPA) is highly flammable, and warm IPA gives off more vapour. Keep it away from heat, sparks and open flames, heat it only with equipment suited to flammable liquids, use it in a well-ventilated area, and wear suitable PPE such as gloves and eye protection. As a business, you are responsible for work health and safety in your workshop, including risk assessment and how you store, use and dispose of chemicals. See our Terms and Conditions.
THE NUMBERS AT A GLANCE
1.25 mm
of inner circumference per whole letter size (UK letter scale)
0.2 mm
of inner diameter is one half size
D = C ÷ π
inner diameter from inner circumference
½ size up
for wide bands, a common starting point
3 test bands
to measure your own shrink, never borrow it
WHAT YOU NEED
Finger sizers
narrow and wide, one system
Precise CAD
jewellery or general
Ring mandrel
marked in AU/UK letters
Test band file
three known inner diameters
Digital callipers
reading to 0.01 mm
Sizing log
one line per test
Before you start
One size system for everything. Sizers, mandrel and the size table in your CAD all in AU/UK letters, or all in millimetres. Mixing systems is the quickest way to a ring that is a size out.
Think in inner diameter. Letters are for talking to customers. In CAD and at the bench you work in millimetres of inner diameter.
Check your tools against each other. Put a sizer ring on your mandrel. If it does not read its own size, find out which one is wrong before you trust either.
1
Measure the finger
Australia uses the British letter scale: A to Z in half sizes, then Z+1, Z+2 and up (also written Z1, Z2). The former British Standard BS 6820:1987, withdrawn in 1993, defined each letter by the ring's inner circumference: size A is 37.5 mm, and every whole letter adds 1.25 mm. Jewellers still use that scale.
Use a finger sizer, not string or paper. String stretches and paper curls with humidity, so both mislead.
Measure more than once. Fingers swell in the heat, shrink in the cold and change through the day. For an important ring, measure on two occasions.
Check the knuckle. The ring has to slide over it, then sit snug without spinning or falling off.
Match the sizer to the band. A wide band fits tighter than a narrow one of the same size. Use wide sizers for wide bands. With narrow sizers only, a common starting point is about half a size up for bands of about 5 mm and wider.
Comfort fit feels looser. A domed inside slides over the knuckle more easily. Some makers size it smaller, often half a US size (about one letter), but not on very wide bands. A comfort fit sizer of the same width takes out the guesswork.
MEASURING AN EXISTING RING
Round it first. An oval ring reads small. True it on the mandrel, or measure both ways at 90° and average.
Callipers at the narrowest point, which on a comfort fit ring is the crown of the dome. Half a size is only 0.2 mm of diameter, so read to 0.01 mm.
Check the mandrel now and then. Mandrels wear in the sizes you use most, and sizers do not always agree with them. Use a gauge ring of known size.
Why published charts disagree
A UK half size (0.625 mm of circumference) is a touch smaller than a US quarter size (0.64 mm). Charts that pair each letter with a US size drift from the standard, by up to half a size or more at the large end. Keep your sizers, mandrel and CAD table in one system, and check them against the chart in this guide, which is computed from the standards.
AUSTRALIAN RING SIZES (UK LETTER SCALE, FROM BS 6820:1987)
SIZE
CIRC. MM
DIAM. MM
US
F
43.75
13.93
2.8
F½
44.375
14.13
3.1
G
45.00
14.32
3.3
G½
45.625
14.52
3.6
H
46.25
14.72
3.8
H½
46.875
14.92
4.0
I
47.50
15.12
4.3
I½
48.125
15.32
4.5
J
48.75
15.52
4.8
J½
49.375
15.72
5.0
K
50.00
15.92
5.3
K½
50.625
16.11
5.5
L
51.25
16.31
5.8
L½
51.875
16.51
6.0
M
52.50
16.71
6.2
M½
53.125
16.91
6.5
N
53.75
17.11
6.7
N½
54.375
17.31
7.0
O
55.00
17.51
7.2
O½
55.625
17.71
7.5
P
56.25
17.90
7.7
P½
56.875
18.10
8.0
SIZE
CIRC. MM
DIAM. MM
US
Q
57.50
18.30
8.2
Q½
58.125
18.50
8.5
R
58.75
18.70
8.7
R½
59.375
18.90
8.9
S
60.00
19.10
9.2
S½
60.625
19.30
9.4
T
61.25
19.50
9.7
T½
61.875
19.70
9.9
U
62.50
19.89
10.2
U½
63.125
20.09
10.4
V
63.75
20.29
10.7
V½
64.375
20.49
10.9
W
65.00
20.69
11.1
W½
65.625
20.89
11.4
X
66.25
21.09
11.6
X½
66.875
21.29
11.9
Y
67.50
21.49
12.1
Y½
68.125
21.68
12.4
Z
68.75
21.88
12.6
Z½
69.375
22.08
12.9
Z+1
70.00
22.28
13.1
Z+2
71.25
22.68
13.6
Inner dimensions in mm. Letters from the former BS 6820:1987 (A = 37.5 mm, plus 1.25 mm per letter), diameter = circumference ÷ π. US from inner diameter in inches = 0.458 + 0.032 × size, to one decimal, so N is about US 6¾.
2
Model to the inner diameter
Inner diameter, in millimetres. Take the finished size from the chart, then apply your own factor from step 3. Never size by the outside or the centre line of the band.
Check your software's size table. If your CAD has a ring size tool, check which system and table it uses and compare a few sizes with this chart. If not, draw the inner circle yourself.
Width and profile change the fit, not the number. Decide on any half size up or down when you measure the finger, then model that size.
Comfort fit: size at the crown. The narrowest point of the domed inside sets the fit, so put the size diameter there.
Put the numbers in the file name, such as "N 17.11 finished, modelled 17.26".
Band sections on the finger, to scale
3
Calibrate your own process
WHY CAST RINGS COME OUT SMALL
Example numbers for one test band. Yours will differ, which is the point of the test.
The pattern. Flashforge specifies the WaxJet 51C at ±0.04 mm per 20 mm. Wax changes size with temperature far more than metal, so measure it at room temperature.
The metal. Metal shrinks as it freezes (the sprue feeds that), then keeps contracting as it cools, by an amount that depends on the alloy.
The investment. A cristobalite investment expands roughly 1% or a little more as it sets and heats, offsetting much of that. In one published test (Austin, Ganoksin), castings from acrylic patterns came out 0.71% and 0.86% smaller than the patterns.
The clean-up. Every 0.1 mm you file or polish off the inside wall adds 0.2 mm to the diameter, a half size.
The net result is yours. The net result is yours. Published figures run from under 1% (Austin, acrylic patterns) to about 2.5% plus 0.25 mm (Shapeways, gold), which on an N ring is under half a size to more than a letter, so measure your own. It depends on alloy, investment, burnout, flask temperature and clean-up. Rubber moulding figures do not apply.
THE TEST
1
Print three plain bands at known inner diameters across your usual sizes, such as 16.00, 17.50 and 19.00 mm, in your usual width. Mark each size on the outside.
2
Measure the wax at room temperature, both ways at 90°, and log the average.
3
Cast them like normal work: same alloy, investment, water ratio, burnout and flask temperature.
4
Clean up the inside as you always do, then measure again both ways.
5. Work out your factor
scale factor = CAD inner diameter ÷ finished inner diameter
Example: 17.50 ÷ 17.34 = 1.009. The other bands gave 16.00 ÷ 15.86 and 19.00 ÷ 18.83, also 1.009. To finish at N (17.11 mm), model at 17.11 × 1.009 = 17.26 mm. Example numbers, not WaxJet data.
Same factor at all three sizes: scale the whole ring about its centre, in CAD or in WaxJetPrint, not both, because WaxJetPrint has its own shrink compensation setting. Print the test bands where and how you print production rings. Same loss in millimetres at all three: it is mostly clean-up, so add that amount to the inner diameter instead. Test again after any change of print material, alloy, investment, burnout or print software settings.
4
Cast, clean up and check
Measure every finished ring before you set stones, on the mandrel or with callipers, both ways.
Clean up the same way every time. Your factor includes your clean-up. Heavier polishing inside makes rings bigger than your test bands.
Small fixes on plain bands. Anneal, then stretch on the mandrel, up to about half a size. Bigger changes mean cutting out metal or adding it, and soldering.
Some designs are hard to resize: eternity bands, and pavé or channel settings, above all with stones all round. Get these right in CAD, and send a plain try-on band of the same width first.
COMMON MISTAKES
Mixing size systems
WHAT IT CAUSES
A size or more out
DO THIS INSTEAD
One system, tools checked against each other
Trusting any online chart
WHAT IT CAUSES
Up to half a size out
DO THIS INSTEAD
Check it against the standard
Sizing by the outside of the band
WHAT IT CAUSES
DO THIS INSTEAD
A ring far too small
Model the inner diameter
Wide band, narrow sizer
WHAT IT CAUSES
Ring too tight
DO THIS INSTEAD
Wide sizer, or about ½ size up
Oval ring measured once
WHAT IT CAUSES
Reads small
DO THIS INSTEAD
Round it, measure both ways
Wax measured while warm
WHAT IT CAUSES
DO THIS INSTEAD
A wrong factor
Measure at room temperature
Borrowed shrink figure
WHAT IT CAUSES
Sizes off
DO THIS INSTEAD
Cast your own test bands
Process changed, no re-test
WHAT IT CAUSES
Sizes drift
DO THIS INSTEAD
Re-run the test bands
BEFORE YOU PRINT: CHECKLIST
One size system for every tool
Measured twice, knuckle checked
Wide band: wide sizer or ½ size up
Comfort fit sized at the crown
Inner diameter modelled
Your own factor applied
Wax measured when cool
Finished ring measured both ways
Test re-run after any change
Make it easier in the design file
Keep the three test bands as a ready-to-print file, and put your workshop's factor in your CAD template or on the job sheet, so every designer applies the same number.
WHERE TO NEXT
GUIDE 4 OF 13 · DESIGN
THE BIG PICTURE
1 · DESIGN
2 · PATTERNS
3 · CASTING
4 · AFTER CASTING