Drill sizes Thread tapping - UNC

In the American system, each size comes in two different thread sizes. Depending on the application, you can choose from a fine or coarse thread. This way you always have the choice of a bolt that is optimally geared to its application. UNF (Unified National Fine) is the designation for the fine thread, where UNC (Unified National Coarse) is the designation for the coarse thread.

Unified National Coarse	Drill diameters for tapping threads

UNC No.2 - UNC No.12	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
No.2	56	1.85	2.00
No.3	48	2.10	2.30
No.4	40	2.35	2.60
No.5	40	2.65	2.90
No.6	32	2.85	3.20
No.8	32	3.50	3.80
No.10	24	3.90	4.40
No.12	24	4.50	5.00

Unified National Coarse	Drill diameters for tapping threads

UNC 1/4 - UNC 2	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
1/4	20	5.10	5.80
5/16	18	6.60	7.30
3/8	16	8.00	8.80
7/16	14	9.40	10.30
1/2	13	10.80	11.90
9/16	12	12.20
5/8	11	13.50	14.80
3/4	10	16.50	17.90
7/8	9	19.50
1	8	22.25	24.00
1.1/8	7	25.00
1.1/4	7	28.00
1.3/8	6	30.75
1.1/2	6	34.00
1.3/4	5	39.50
2	4.1/2	45.00

Thread pitch refers to the distance between threads and is expressed in millimeters. This measurement is taken along the length of the fastener. For example, if the distance between two threads is 1.75mm, this means the thread pitch is 1.75

Difference Roll tap - Cutting tap

Roll tap
A roll tap or forming tap usually has a longer life and works faster. It produces no chips, a stronger thread and is also suitable for blind holes. A disadvantage is the higher purchase price. In addition, much more machine power is required to let the forming tap do its job (approx. 50% higher compared to cutting tapping). Special attention is needed for the right lubricant because of friction and with a lot of heat development as a result.

Cutting tap
With a cutting tap - usually called a thread tap - the thread is cut by machining. A tap is usually made of HSS (High Speed Steel) and has sharply ground cutting teeth. A lubricant is also used for tapping. The supply of cutting oil and the removal of the chips takes place through the recessed grooves of the tap. A set of taps usually consists of: 3 taps for metric threads, 2 taps for gas threads, 1 tap for machine taps. There are taps for the generally used right-hand thread, but also for left-hand thread.

Lubricants
When tapping, a lubricant suitable for the material is used, which serves to lubricate and cool. Today there are excellent universal oils available, for example drilling oil, cutting oil, tap oil, even 3 in 1 in aerosol.

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