Tolerancing

Here are some general tolerancing tips, picked up from work experience, in no particular order. Anyone is welcome to add to these or correct them if you see something inaccurate.

• Tolerances should always be as large as possible for the part to still function

  • Overly tight tolerances are expensive, time consuming, and unnecessary

• 3D printed parts will shrink. A lot.

  • Online estimates are ~8% for ABS and ~3% for PLA but the actual amount will vary drastically based on the printer, settings, and the part itself.

• If possible, part corners should be chamfered or filleted, especially for sharp/hard materials

• For machined parts, a surface finish of 125 microinches (3.175 micrometers) is standard

• Any holes to be tapped should be first made one size smaller than the tap size

  • I.e. for a #6 screw, one should drill a #5 hole before tapping with a #6 tap

• Holes should be larger than the fasteners that go in them. The smallest diameters for a "normal" fit by ASME standards are listed below.

  • See ASME standard 18.2.8 for more information, or go to: https://mechanicalc.com/reference/fastener-size-tables

Designation	Nom. (in)	Nom. (mm)	Min. (in)	Min. (mm)
#0	0.060	1.524	0.076	1.930
M1.6	0.063	1.600	0.071	1.800
#1	0.073	1.854	0.089	2.261
M2.0	0.079	2.000	0.094	2.400
#2	0.086	2.184	0.102	2.591
M2.5	0.098	2.500	0.114	2.900
#3	0.099	2.515	0.116	2.946
#4	0.112	2.845	0.128	3.251
M3.0	0.118	3.000	0.134	3.400
#5	0.125	3.175	0.156	3.962
#6	0.138	3.505	0.170	4.318
M4.0	0.157	4.000	0.177	4.500
#8	0.164	4.166	0.196	4.978
#10	0.190	4.826	0.221	5.613
M5.0	0.197	5.000	0.217	5.500
#12	0.216	5.486	N/A	N/A
M6.0	0.236	6.000	0.260	6.600
1/4"	0.250	6.350	0.281	7.137
M8.0	0.315	8.000	0.354	9.000

GD&T should be added at some point