Larger steel drills straighter holes

Paul Boughton

A specialist, drilling contractor in New England in the US has recently switched to Sandvik’s new, larger Sandvik60 drill steel. This has performed very well with regard to their major drilling criteria – hole straightness. It’s also proved exceptionally easy to decouple and is making their rigs more flexible and competitive as to the hole sizes they can drill.

Gary Davidson started Yankee Drilling in 1984 in partnership with his wife Paula. The company specializes in drilling. “We work in three states in New England; New Hampshire, Massachusetts and Vermont. Almost none of the quarries in this area do their own drilling. Generally, they’ll make a contract with a blasting company, who will then put out a tender for the drilling operations as a sub-contract. The blasting companies are often explosive manufacturers. There are three major companies of this type operating in our area,” says Gary.

Yankee Drilling has 14 employees and 11 drill rigs, of which five were manufactured by Tamrock: “We’ve got three CHA 1100s, one CHA 660 and a Tamrock Ranger 700. I always buy used rigs with the expectation that they’ll run well for ten years. The key, of course, is good maintenance. We do this ourselves but with a lot of help and support from Capitol (Capitol Construction Equipment & Supply Inc. of Bow, New Hampshire, where Gary is a well-established customer) – they supply the parts we need and deal with any problems we can’t handle. We do our major maintenance in January and February when the quarries stop crushing for the winter. December and March are also generally slow months for us.”

Gary’s attention was first drawn to Sandvik’s new, 60 mm drill steel, Sandvik60, when he read about it in a trade journal. “I generally believe that the bigger the steel the better, but at the time we were working in a quarry with particularly hard rock, so I was particularly interested in trying a heavier steel.”

In fact, the rock was so hard that Yankee drilling had to lower the percussive pressure which it normally used. “We had been customarily running the CHA 1100s rigs equipped with T51 steel at 150 bar. When drilling a five-inch hole, this gives us a production of around 100 feet an hour. In this rock, however, which was both hard and seamy, we were getting pitting of the steel, unacceptably high hole wander and actual steel failures.” Tom Donovan of Capitol Construction explains that, “when we measured the temperature of the steel couplings it was registering over 350º F – we consider any temperature above 200º F as too high.”

As a result Yankee drilling had to lower its percussive pressure to 115 bar, which took care of the temperature problem but cut the production rate to only 60 feet an hour, and since Yankee Drilling’s contracts are calculated on a per-foot-drilled basis, this was a serious problem for Gary.

“Given this situation, I was very willing to try the new steel. We switched over to the Sandvik60s. Our CHA 1100 rig had to be adapted, but together with Capitol, we managed this very easily. With the Sandvik60s mounted, we returned to drilling at 150 bar. Our production went back to 100 feet per hour but this time without the problems.” Tom Donovan confirms that, “the temperature of the new steel’s couplings averaged under 200º F. This lower temperature is due almost entirely to the greater straightness of holes drilled with the Sandvik60 steel.” This demonstration left Gary convinced that the Sandvik60 was the steel he’s been looking for. “Given that we’re paid by the foot, the performance of the drill steel is crucial for us. In many ways, I believe that these days it’s the weak link in the equipment chain.”

When asked what criteria he uses to select drill steel, Gary replies, “first and foremost, hole straightness – hole deviation can cause enormous problems both for us and for the quarries: we get a drilling specification from the blasting company. If any of the holes we drill deviates more than a foot in terms of straightness, we have to drill it at our own expense. Then again, hole deviation can cause blasting problems: it can result in ‘fly rock’ – stray pieces of rock from the blast. If these, for example, hit some body’s car, we might end up getting sued. Another problem with crooked holes is getting the steel stuck in them. If we can get it out, the round may have to be blasted anyway. This is also dangerous, and, from the quarries’ point of view, the steel debris has to be got out of the rock before it’s processed otherwise it’ll damage the crushers.”

Gary’s second criteria for drill steel performance is minimizing downtime from production: “Obviously, from our point of view, the faster we can drill, the better. But, of course, there are trade-offs; one is hole straightness and all the problems associated with that which we’ve just discussed. The other is the relationship between the cost of the steel and its service life. Service life issues come in two forms; one is simple durability. It’s maybe too early for us to say how the Sandvik60 is going to perform on this, since we haven’t really been using it long enough, but the signs are looking good – I generally expect to get 20,000 feet out of a single T51 steel and 10,000 feet out of a T51 shank. I’ve now got a Sandvik60 shank that’s already gone beyond 10,000 feet and still looks very good.”

The other service life problem is random steel failures. “This is a special problem for us because we’re not operating on our own sites and we operate over large areas. If we have an unexpectedly large number of steel failures at a particular site, that can mean transporting replacement steel over long distances which is expensive in itself, but also means prolonging our downtime from production. It’s hard to give figures on this type of problem, but it’s my general impression that Sandvik60 is going to help us minimize such costly failures.”

Easier decoupling

Another feature of Sandvik60 which Gary is particularly enthusiastic about is its decoupling: “It’s the easiest steel to uncouple that we’ve ever worked with. Previously we were experiencing a major decoupling problem about once a week, meaning that the decoupling could take up to an hour. I believe that the easier decoupling is related to the cooler running of the Sandvik60. For example, we were getting pitting on the threads of the T51 steel, which made decoupling difficult.”

Competitive increase in hole range

Switching to the new larger steel from Sandvik has also made Yankee Drilling more flexibly competitive. Gary explains that, “in order to increase production there’s a general trend for quarries to want to drill the largest holes they can get away with from the blasting security point of view. The average hole size is still around 3.5 inches, but more and more sites want five-inch holes. Conventionally, holes this big will often be drilled by down-the-hole rigs, which have tended to produce straighter holes, but, on the other hand, are slower and consume more energy. Now that we’re using Sandvik60, we can compete on drilling five-inch holes. This solution has the added advantage for everybody that we can also drill smaller holes, e.g. 3.5 development holes, on the same site with the same rig – we just switch back to the lighter drill steel.” (3 5/8 inch is the smallest hole that can be drilled using Sandvik60 steel.)

Recent Issues