Rig size should effect Hammer selection not hole size!
Quite often a Drill manufacturer will specify the hole size capabilities of a drill rig based on the capability of the drill. They base this on whether the drill can hold the drill rods needed to match the hole size, the ability of the drill to be able to pull back that sized rod and bit and in most cases they have tested this in different grounds.
Although these statements are true and the drills will have the capability to do this it does not mean this will be the most cost effective means in which to drill the hole sizes you require. A lot of us when selected the drill we require try to gain the most versatile drill for the job, a drill with the capabilities to not just do wall control but help with production where we need it.
The issue i have seen most is that the rigs versatility is based of the manufacturers specifications of the drill. But it should be made of the basis of ground conditions and the ability of the drill to reach depth at the size you want in the most cost effective manor. Spending extra on a drill can save you in the long run through less maintenance, drill consumables used and downtime from issues created by maxing out your drill.
Lets say you have purchase a smaller drill and you would like to drill from 115mm to 203mm holes (Pre-Splits, Wall control and production holes) which is a common choice for many companies. To drill 203mm holes ideally a 8" hammer is used but this hammer would be to big for a smaller drill so the selection of a 6" hammer can be used. Commonly we look at the PSI required to use this hammer (350-500Psi) which these drills have the capability to do. But we often forget that prior to running these hammer we need to fill the hammer with volume (CFM) and this is where the issue begins.
A 6" hammer can use anywhere from 972 CFM to 1041 CFM to operate efficiently and a lot of the newer small track drills only offer 995CFM. This volume is fine initially with this size and work extremely well with 5" hammers and down. But as soon as you get a small amount of wear in your hammer your pen rate will drop as the pressure in the hammer drops. If you have no spare air above what the hammer requires this will show a low hammer life and rapid drop in penetration rate. On the other hand there are drills that offer more than enough volume to run a 6" hammer with plenty of volume to support the larger hammer size with 1120-1320 CFM compressors.
The point of this article is to give you the information to make an informed decision on your drill choice and when looking at increasing your hole size on site first ensure your compressor can support the goal you are after. If it doesn't support your goals your cost of production and product use will increase dramatically and not give you the reduced production time you would like to expect from your increased spacing and minimising hole amounts.
A good supplier will be able to inform you of the hammers requirements prior to use so you can make an informed decision on the direction in which you want to head with you hole and hammer size selection.
Danv Tools Australia