Drill sizes Thread tapping - BSW

BSW threads are standardized from 1/8”-40BSW up to and including 6”-2.5BSW. Because some BSW diameter-pitch combinations are the same as those with UNC threads, they can in principle be interchanged. However, due to the apex angle configuration and some other minor differences, this is not recommended.

British Standard Whitworth	Drill diameters for tapping threads

BSW 3/32 - BSW 1/4	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
3/32	48	1.85	2.10
1/8	40	2.55	2.90
5/32	32	3.20	3.60
3/16	24	3.70	4.30
7/32	24	4.50
1/4	20	5.10	5.80

British Standard Whitworth	Drill diameters for tapping threads

BSW 5/16 - BSW 2	Thread pitch (P)	cutting tap	roll tap
ø ”	TPI	ø mm	ø mm
5/16	18	6.50	7.30
3/8	16	7.90	8.80
7/16	14	9.20	10.20
1/2	12	10.50	11.60
9/16	12	12.00	13.20
5/8	11	13.50	14.70
3/4	10	16.50	17.70
7/8	9	19.25	20.75
1	8	22.00	23.75
1.1/8	7	24.75
1.1/4	7	28.00
1.3/8	6	30.50
1.1/2	6	33.50
1.5/8	5	35.50
1.3/4	5	39.00
1.7/8	4.1/2	41.50
2	4.1/2	41.50

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.

Extra