For long-term rigging, stainless stud anchors are superior to self-drives . But you need a drill, and in some situations a hammer drill with SDS bits is not available. Lev Bishop has built a hand drill holder for SDS bits. Here is his report on how it works and how to build one if you are interested. They are also available commercially. -- Editor.
by Lev Bishop, email@example.com
I took a 8mm hand-drilling system on expedition to Spain and Austria with me. Unfortunately it didn't really see any proper underground action due to the way things worked out (the places where it ended up weren't the places where bolts needed to be placed). However I did a few comparative tests...
SDS bit (courtesy of Rawl, www.rawl.com)
In Spain two of us placed 2 spits [self-drives] each and two 8mm thrubolts (Note 1) each [using SDS bits] in the same piece of limestone.
It seemed that it was about 1 minute faster to place the thrubolt than it was the spit, with the thrubolt taking about 10 mins and the spit about 11 mins. This is for drilling the hole and setting the anchor, but not dressing the rock or attaching the hanger (if the latter were included then the thrubolt would come out further ahead because you can have the hanger pre-attached).
In Austria we were in an area where the limestone is extremely hard and placing spits can take anything from 25mins to an hour, and often requires you to use more than one spit to drill the hole since all the teeth get worn off during such a long drilling session. I placed only one bolt with the new system, which took around 20mins, which would seem to be a significant improvement, though there are places where the rock is softer than others so this may have just been luck.
There were only two problems with the system as it stands. Firstly, the plastic handle had a tendency to rotate relative to the metal shaft, even though I didn't need to apply much torque. A couple of layers of insulation tape wrapped around the metal seemed to stop this but a better solution would probably be to use a piece of octagonal or hexagonal steel stock to machine the handle from (the hole through the plastic handle is octagonal cross-section).
Secondly, you have to keep rotating the drill clockwise in order to clear the dust out of the hole and this means that any tether you attach to the drill to prevent you from dropping it down the pitch rapidly gets horribly tangled. In the end I just didn't bother with a tether but it would be nice to have some kind of slip-ring arrangement instead.
Dimensions are in mm. I actually machined it out of the nearest standard stock to 16mm, which was 15 point something. It is a piece of cylindrical steel stock, with a 10mm hole turned into it to a depth of 45mm. There are 4mm tapped holes on opposite sides of the cylinder 30mm from the end. These accept 4mm hex-head grub screws [US: set screws], which sit in the indents in the SDS bit to stop it rotating, but the end of the bit actually contacts the base of the hole, so the grub screws are not taking the impact force.
The length of the drill is 130mm. I covered it with a plastic hand guard I bought from the hardware store (they sell them for use with masonry chisels. This stops you hammering your thumb and gives a comfortable grip on the drill. I intend to melt a small hole in it to take the hex key for the grub screws and keep it safe.
The length of the drill was chosen so that the plastic cover would just cover the grub screws, preventing the hex holes from filling with mud, but allowing you to slide the drill out a cm or two if you needed to change a broken bit.
Unfortunately the hammer end of the drill has already started to "mushroom" by cold-flow from the hammer impacts (despite the 2mm bevel I included to try and reduce this), and this prevents you from being able to slide the handle over the drill far enough to access the screws (instead you have to remove the handle, sliding it off at the bit end).
I think next time I will use more bevel, a harder steel, and make the drill a couple of cm longer, so you don't actually have to slide it over the end of the drill to access the grub screws. I was worried the handle might rotate relative to the drill and stop you turning the bit, but it is a snug fit and anyway you don't need anything like as much torque as you do with spits. And a single wrap of insulation tape around the drill makes the fit even snugger still, so I doubt this will be a problem.
I got some advice from some people who've sharpened their own bits before and they claim that grinding the bit to a sharper point will make a very significant improvement to drilling speed.
If you were using it in an electric drill then you may have to resharpen quite regularly but by hand the bits should stay sharp for quite a while. Apparently it's quite easy so long as you have a silicon carbide grinding wheel. The Hilti double-flute bits that I was using (because several people recommended them from their experience using them in electric drills) would be a bit tricky to sharpen because the tips have a fairly complicated profile, but apparently most other bits are much simpler.
Note 1: A thrubolt is a specific make of stud. The ones I was using required an 8mm hole and were 50mm long (in total, i.e. including the threaded part). I think you'd call it a collar stud. See http://www.dragon-speleo.co.uk/contents/bolts.html for a picture.
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