Threading

Our CNC machining service provides the ability to easily add threaded features to milled and turned parts.

Threads are specified within our automated interactive quotes. When a quote is returned, the 3D display will show which thread types are possible for each feature. Different threads (where possible) or no threads can be selected on a feature-by-feature basis. View a demo quote to explore a 3D model with threads. For those having trouble with the interactive display, a 3D PDF is included in the demo quote that shows the thread options available.

After an order is placed, an order confirmation will be emailed with the thread sizes selected. Please review the order confirmation for accuracy. We offer a limited selection of UNF, UNC and coil inserts (but do not supply or install the coil inserts). See the table below for all of our available selections.

Milled Parts
Threaded holes on milled parts must be modeled at the proper diameter and oriented along one of the three primary axes. Automatic external threads are not available for milled parts at this time. Keep in mind that it is possible (but not recommended) to obtain external threads on milled parts, but in this case, the threads must be modeled. External threads are machined using our standard ball and flat end mills. The smallest external thread that produces reasonable results in our CNC milling process is roughly 1/2-13 (M12).

Turned Parts
Threaded holes on turned parts must be modeled at the proper diameter and oriented axially or radially. External threads are available on the axial diameters of turned parts. Simply model the nominal diameter — do not model the threads.

Please Note
  • We may thread a through hole or continuous outer diameter from either side of the part. If the depth of the hole or length of the outer diameter is deeper than our tools can reach, we will offer to thread both ends of the feature. It should be assumed these threads will not be continuous. See the Maximum Reach tab on the table below.
  • All threads will be right handed. Left handed threads, and diameter and pitches not shown in the table cannot be produced with our machining processes at this time.
  • We may be unable to thread features that are obstructed by other elements on a part or that are obstructed by our fixturing. If a feature cannot be threaded, no thread options will be offered for that feature on the quote.
  • Threaded features are not available for parts loaded in the .STL format.

Threading options differ for milled and turned parts, so make sure the right process is selected in the table below. If you have any issues or questions please contact our customer service engineers at customerservice@protolabs.com or 877.479.3680.
Standard Holes
Coil Inserts
OD threads
Key Inserts
Maximum Reach
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UNC threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) 75% hole drill size (in.) Maximum thread depth (in.) (note 1)
#2-56 0.067 0.074 0.070 0.20
#3-48 0.076 0.084 0.082 0.20
#4-40 0.085 0.094 0.089 0.30
#5-40 0.098 0.104 0.102 0.30
#6-32 0.105 0.114 0.109 0.40
#8-32 0.130 0.139 0.135 0.40
#10-24 0.145 0.156 0.151 0.60
#12-24 0.171 0.181 0.176 0.60
1/4-20 0.196 0.207 0.202 0.65
5/16-18 0.252 0.265 0.259 0.65
3/8-16 0.307 0.321 0.314 1.00
7/16-14 0.360 0.376 0.372 1.00
1/2-13 0.417 0.434 0.426 1.00

UNF threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) 75% hole drill size (in.) Maximum thread depth (in.) (note 1)
#2-64 0.069 0.075 0.072 0.20
#3-56 0.080 0.084 0.083 0.20
#4-48 0.087 0.099 0.093 0.30
#5-44 0.100 0.104 0.104 0.30
#6-40 0.111 0.119 0.115 0.40
#8-36 0.134 0.142 0.138 0.40
#10-32 0.156 0.164 0.160 0.60
#12-28 0.177 0.186 0.182 0.60
1/4-28 0.211 0.220 0.216 0.65
5/16-24 0.267 0.277 0.272 0.65
3/8-24 0.330 0.340 0.335 1.00
7/16-20 0.383 0.395 0.389 1.00
1/2-20 0.446 0.457 0.452 1.00

Metric threads Minimum modeled diameter (mm.) Maximum modeled diameter (mm.) 75% hole drill size (mm.) Maximum thread depth (mm.) (note 1)
M2 x 0.4 1.57 1.68 1.60 5.08
M2.5 x 0.45 2.01 2.14 2.05 5.08
M3 x 0.5 2.46 2.60 2.50 7.62
M3.5 x 0.6 2.85 3.01 2.90 10.16
M4 x 0.7 3.24 3.42 3.30 10.16
M5 x 0.8 4.14 4.33 4.20 15.24
M6 x 1.0 4.92 5.15 5.00 16.51
M8 x 1.25 6.65 6.91 6.80 16.51
M10 x 1.5 8.38 8.68 8.50 25.40
M12 x 1.75 10.00 10.44 10.20 25.40

Note 1: Obstruction by other features of your part, lead-in treatments or fixturing requirements may reduce the maximum depth of threads.
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UNC threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) 75% hole drill size (in.) Maximum thread depth (in.) (note 1)
#2-56 0.0899 0.0961 0.094 0.30
#3-48 0.1053 0.1104 0.107 0.40
#4-40 0.1175 0.1252 0.120 0.40
#5-40 0.1305 0.1373 0.134 0.40
#6-32 0.1450 0.1527 0.147 0.60
#8-32 0.1708 0.1781 0.173 0.60
#10-24 0.1990 0.2080 0.203 0.65
#12-24 0.2250 0.2340 0.228 0.65
1/4-20 0.2608 0.2704 0.266 0.65
5/16-18 0.3245 0.3342 0.332 1.00
3/8-16 0.3885 0.3987 0.397 1.00
7/16-14 0.4530 0.4639 0.453 1.00
1/2-13 0.5150 0.5313 0.522 1.00

UNF threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) 75% hole drill size (in.) Maximum thread depth (in.) (note 1)
#2-64 0.0897 0.095 0.093 0.30
#3-56 0.1029 0.104 0.104 0.30
#4-48 0.1166 0.123 0.120 0.40
#6-40 0.1440 0.150 0.147 0.60
#8-36 0.1701 0.177 0.173 0.60
#10-32 0.1968 0.204 0.201 0.65
1/4-28 0.2577 0.265 0.261 0.65
5/16-24 0.3215 0.329 0.325 1.00
3/8-24 0.3840 0.391 0.388 1.00
7/16-20 0.4483 0.456 0.452 1.00
1/2-20 0.5108 0.519 0.515 1.00

Metric threads Minimum modeled diameter (mm.) Maximum modeled diameter (mm.) 75% hole drill size (mm.) Maximum thread depth (mm.) (note 1)
M2 x 0.4 2.0879 2.1387 2.11 5.08
M2.5 x 0.45 2.5476 2.6492 2.60 7.62
M3 x 0.5 3.1090 3.2487 3.18 5.08
M3.5 x 0.6 3.6830 3.7897 3.70 15.24
M4 x 0.7 4.1529 4.3332 4.20 15.24
M5 x 0.8 5.1740 5.3746 5.27 16.51
M6 x 1.0 6.2179 6.4059 6.31 16.51
M8 x 1.25 8.2702 8.4836 8.38 25.40
M10 x 1.5 10.3251 10.5588 10.44 25.40
M12 x 1.75 12.3800 12.6441 12.51 25.40

Note 1: Obstruction by other features of your part, lead-in treatments or fixturing requirements may reduce the maximum depth of threads.
For turned parts only.
UNC threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) Maximum thread depth (in.) (note 1)
#4-40 0.106 0.113 0.22
#5-40 0.119 0.126 0.25
#6-32 0.131 0.139 0.28
#8-32 0.157 0.165 0.33
#10-24 0.182 0.191 0.38
#12-24 0.208 0.217 0.43
1/4-20 0.241 0.251 0.50
5/16-18 0.303 0.314 0.63
3/8-16 0.364 0.376 0.75
7/16-14 0.426 0.439 0.88
1/2-13 0.488 0.501 1.00
5/8-11 0.611 0.626 1.25
3/4-10 0.735 0.751 1.50

UNF threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) Maximum thread depth (in.) (note 1)
#5-44 0.120 0.126 0.25
#6-40 0.132 0.139 0.28
#8-36 0.158 0.165 0.33
#10-32 0.183 0.191 0.38
#12-28 0.209 0.217 0.43
1/4-28 0.243 0.251 0.50
5/16-24 0.304 0.314 0.63
3/8-24 0.367 0.376 0.75
7/16-20 0.428 0.439 0.88
1/2-20 0.491 0.501 1.00
5/8-18 0.615 0.626 1.25
3/4-16 0.739 0.751 1.50

Metric threads Minimum modeled diameter (mm.) Maximum modeled diameter (mm.) Maximum thread depth (mm.) (note 1)
M3.5 x 0.6 3.35 3.53 7.00
M4 x 0.7 3.84 4.03 8.00
M5 x 0.8 4.83 5.03 10.00
M6 x 1.0 5.79 6.02 12.00
M8 x 1.25 7.76 8.03 16.00
M8 x 1.0 7.80 8.03 16.00
M10 x 1.5 9.73 10.03 20.00
M10 x 1.25 9.76 10.03 20.00
M10 x 0.75 9.84 10.03 20.00
M12 x 1.75 11.70 12.02 24.00
M12 x 1.5 11.73 12.02 24.00
M12 x 1.25 11.76 12.02 24.00
M12 x 1.0 11.79 12.02 24.00
M14 x 2.0 13.68 14.03 28.00
M14 x 1.5 13.73 14.03 28.00
M15 x 1.0 14.79 15.03 30.00
M16 x 2.0 15.68 16.02 32.00
M16 x 1.5 15.73 16.02 32.00
M17 x 1.0 16.79 17.03 34.00
M18 x 1.5 17.73 18.03 36.00
M20 x 2.5 19.62 20.03 40.00
M20 x 1.5 19.73 20.03 40.00

Note 1: Obstruction by other features of your part, lead-in treatments or fixturing requirements may reduce the maximum depth of threads.
For milled part only.
Threads Minimum modeled diameter (in.) Maximum modeled diameter (in.) Maximum thread depth (in.)
#8-32 0.133 0.137 0.40
#10-32 0.16 0.164 0.60
#12-28 0.186 0.191 0.60
1/4-28 0.227 0.231 0.65
5/16-18 0.271 0.276 0.65
3/8-16 0.331 0.336 1.00
7/16-14 0.396 0.401 1.00
1/2-13 0.452 0.457 1.00

Notes: Any countersinks required by the specification for the insert you plan to use should be in the CAD model.
Thread size shown is the external thread of the key insert.

Example 1:


This is a cross-section view of a through hole that is threaded from one side. In this case, the hole depth exceeds the reach of the tool. The result is an unthreaded section that is opposite the side selected.

Example 2:


This is a cross-section view of a through hole that is threaded from one side. In this case, the hole depth does not exceed the reach of the tool. The result is the threads will be formed across the entire length of the hole.

Example 3:


This is a cross-section view of a through hole that is threaded from two sides. In this case, the hole depth is too long to reach from one side only. However, it can be reached past the center from either side.


If this hole is selected to be threaded from both sides, there will be a discontinuity in the middle of the hole. This means that a bolt can only be threaded to roughly half the depth of the hole.


Example 4:


This is a cross-section view of a through hole that is threaded from both sides. This is similar to Example 3, but the hole is longer and the tool can not reach to the center from either side. In this case, the hole will be threaded to the maximum reach from both sides, leaving an unthreaded section in the middle.

Example 5:


This is a cross-section view of a blind hole (a hole that has a bottom/doesn’t pass through the part). In this case, the hole is less than the full reach and threading will stop roughly 0.010 in. off the bottom.

If the hole is deeper than the tool can reach, threads will be cut as deep as possible.

Example 6:


A designer may add a point at the bottom of a blind hole. In most cases, this represents the point left if a pilot hole is drilled with a drill bit. It may also serve a function to their design, or provide an area for chips that are generated during the threading process to fall.


If this point is added to the model, the threading will stop roughly 0.010 in. above the start of the point angle. Pilot holes are machined with end mills, not drills, so this feature is not required to be added to the model.


If a point at the bottom of a hole is present in a model, then the closest possible replication will be attempted based on the geometry and reach of the end mills used.