Economics Of Drilling

Labor is the largest expense in a machine shop. Because of this, every shop is working to reduce the amount of labor that is in machined parts, and reducing cycle times is the fastest way to reduce labor cost in a part. For machinists and engineers, this typically means cutting faster, which usually motivates them to start looking for more advanced cutting tools.

While cutting faster can reduce labor cost, it can also increase tooling expense, which can offset the labor savings. Worse yet, it can increase the cost of the part.

Understanding the true cost of cutting tools is critical to achieving the best overall cost of a part. Drilling is the most common machining operation and therefore the most common place to see an imbalance between tooling cost, cycle reductions and labor costs.

To illustrate this situation, let’s consider a 5/16" diameter drill making a ½" deep hole in steel. While there are several styles of drills available, the most common choices in this size are going to be high-speed steel (HSS) or solid carbide. Most machinists and engineers are going to select the carbide by default because we all know that carbide will drill a hole faster than HSS. However, that does not guarantee it is the most economical.

We must calculate total cost per hole to understand if carbide is the best choice. For this comparison let’s use the following cutting parameters.

For a carbide drill:

• Cutting speed is 300 sfm.
• Feed rate is 0.005" per revolution.

For HSS drills:

• Cutting speed is 50 sfm.
• Feed rate is 0.005" per revolution.

Using these parameters, we see that the carbide drill can penetrate the steel at roughly 18" per minute, whereas the HSS drill penetrates the steel at 6" per minute. The carbide drill is three times faster, so it should be the better choice, right? Maybe.

The new SUMO L DRILL family from ISCAR features exchangeable carbide heads, designed for challenging conditions like slanted surfaces and cross-hole drilling. Iscar

We must also consider the cost of each tool. According to a cutting tool catalog, the carbide drill cost is $50 and the HSS drill cost is $20 (both from a respected manufacturer). Some people might be inclined to use the acquisition cost as a determining factor, which would make the HSS drill a better choice because it is cheaper. But that is not always the case.

Using the cutting parameters and the standard calculation of rpm times feed per revolution, we can determine the time it takes to make the hole with each tool. If we apply the hourly rate to this time, we can see the total labor cost, which is constant. The drill cost is variable and depends on the number of holes that are being drilled. Using an arbitrary, but reasonable, labor rate of $75 per hour, and factoring in the time it takes to drill 50 holes using the parameters above, the HSS cost per hole is $0.50 and the carbide cost per hole is $1.03. In this scenario the HSS drill is more economical.

However, as the number of drilled holes increases, the cost per hole for the carbide begins to decrease and the carbide drill becomes more economical. Likewise, as the shop rate increases, the labor cost per hole using HSS begins to increase, which eventually would make the HSS drill less economical due to labor costs.

This scenario considered the cost for drilling 50 holes. If that number were increased to 300, the cost per hole would be equal because the higher tool cost for carbide is amortized over more holes.

Our first example considered a 5/16" diameter drill, and we found the difference in cost between HSS and carbide to be rather small. When we look at larger diameters like a ¾" drill, the difference in tool cost is much greater.

The cost for a carbide ¾" drill is $400 while a ¾" HSS drill is $95. Using the cutting parameters above with the ¾" drill yields an $8 cost per hole for carbide and a $2 cost per hole for HSS. It is necessary to drill 1,500 holes before the cost per hole for carbide is equal to that of HSS. Neither of these tools will successfully drill 1,500 holes in steel.

So, which one is the right choice? Probably neither. In this case I would look for a different tool.

Now that I have all the sales guys fired up and picking through my calculations for mistakes, I need to point out that some of my parameters for the examples were arbitrary and others based on experience. Every drilling operation is different, and there is no single calculation that can determine which tool will deliver the best cost per hole.

There was no consideration for other tools like indexable drills, spade drills, drills with removeable carbide tips or any of the other options available in the market today. Also, most shops have a tool sharpening program which extends the useful life of drills. Sharpening drills reduces the overall cost of ownership and cost per hole. However, the costs and life span of sharpened tools varies greatly from shop to shop so they were not considered in the above example. In practice, a robust sharpening program will significantly reduce the cost per hole.

Variation in things like machining processes, machine tools, drill costs, labor rates and other factors make every drilling process special. Optimum cutting speeds and chip loads are often reached through experimentation, which causes variation between estimated cost and actual cost. For these reasons the cost per hole must be calculated for each scenario.

It is important to remember that every drilling operation is different, and defaulting to the more expensive tooling option or the latest technology does not guarantee a cost-efficient operation. Each drilling operation needs to be considered as a stand-alone operation.