Button Bit Rotation Speed

Where drill bit life and cost is a major consideration rotation speeds should be carefully monitored.The following table and the chart shows how it needs to be operated

	Bit Head Diameter in mm
	90	95	100	105	115	127	140	152	165	190	216	240	254
100	25.3	24	22.8	24.5	22.4	20.5	18.6	17.1	15.8	16.8	14.7	12.4	11.7
150	31	29.4	27.8	30.1	27.5	25.1	22.7	21	19.3	20.5	18	15.1	14.3
200	36	34	32	34.7	31.7	29	26.2	24.2	22.3	23.7	20.8	17.5	16.5
300	44	42	40	42.6	38.9	35.5	32.1	29.7	27.3	29	25.5	21.4	20.2

	Bit Head Diameter in mm
	280	305	311	381	445	508	559	610	660	745	813	866
100	10.7	9.8	10.9	8.8	7.5	6	4.7	4.3	4	3.5	3.2	3
150	13.1	12	13.3	10.8	9.2	7.4	5.8	5.3	4.9	4.3	3.9	3.7
200	15.1	13.9	15.4	12.4	10.6	8.5	6.7	6.1	5.6	5	4.6	4.3
300	18.5	17

The purpose of percussive 'Down the hole button bits' is to the fracture the rock being drilled and immediately carried away by the exhaust air. Button bits have no cutting or tearing action as such and the effect of over rotation can have a serious effect on the life of the bit especially in abrasive rock. This can wear away the outer carbide insert buttons or in more solid or dense material can cause the inserts to over heat causing micro-cracks which can lead to fracturing. If the drill string is rotated too slowly this will cause the button bit to re-drill previously cut chippings in the hole causing a drop in penetration rate. As a guideline, the harder the rock and larger the bit diameter, the slower the rotation speed required. However it may be necessary however to increase the RPM if in broken ground as to prevent the bit sticking in the hole. However it should be remembered that this could also be caused by a badly worn bit and increasing RPM in these circumstances would only accelerate the problem.