How to Choose the Best Drill Type for Your Next Job

Holemaking is a pretty common procedure in any machine shop, but selecting the best type of cutting tool for each job isn’t always clear cut. You want a drill that caters to the workpiece material, produces the specs required, and ultimately provides the most profit for the job at hand.

So, which drill provides the most value? When it comes down to the variety of jobs manufactured in your machine shop, there is no “one-drill-fits-all.” According to experts from Allied Machine & Engineering, you can simplify the process by considering five criteria when choosing between two big players in the world of metal-cutting: solid drills and replaceable insert drills.

Is your next contract long-term or a short run?

Ask yourself if this particular job is a high production run of the same part or is it a custom prototype or small batch of components?

If you are running a long-term, repeatable process, invest in a replaceable insert drill, like the T-A Pro^™ drill manufactured by Allied Machine and Engineering. Commonly referred to as a spade drill or replaceable tip drill, these drills are engineered so that machine operators have the ability to change out the worn cutting edge quickly. This reduces the overall cost per hole in these types of high production runs. The initial investment of the drill body (insert holder) is recouped quickly by the reduction of cycle time and cost of replacing inserts versus the cost of new solid tooling. Simply put, speed of changeout coupled with a lower long-term cost of ownership makes replaceable insert drills the better choice for your high production jobs.

If your next project is a short run or custom prototype, then a solid drill is the better choice predominantly due to the initial low cost. Since you’re not likely to wear the tool out while machining smaller jobs, the ease of cutting edge replacement is irrelevant. For a short run, the replaceable tool is likely to have a higher initial cost than its solid cousin so it may not pay dividends to invest. Lead time can be better for the solid tool as well, depending on your source for these products. Between solid carbide drills like the ASC 320^® and Superion^® lines of solid carbide tooling, you’ll be able to machine a wide range of holemaking applications with efficiency and cost-savings.

How much stability is required for this job?

Consider the dimensional stability of a reground solid tool vs replacing the worn cutting edge with a fresh blade. Unfortunately, with a reground tool, the diameters and lengths of the tool no longer match the original version. When you regrind a tool, it is smaller in diameter and the overall length is shorter. The reground tool is used more often as a roughing tool at this point, and a new solid tool is needed to meet the required finished dimensions in the workpiece. By using the reground tool, you add another step to the manufacturing process to make use of a tool that no longer satisfies the finished dimensions. This ultimately increases your cost per hole in each part being made.

Are there inventory requirements to consider?

Tool float is the total number of tools in the machine shop environment needed to avoid downtime. In order to keep the job running, you need to maintain a certain amount of tool float inventory. This includes tools at the machine, back ups in the tool crib, and even tooling waiting to be reground. Let’s look at two scenarios.

Scenario one – Currently you need to keep three solid drills at the machine to maintain uptime for this job. You have 12 on hand in the tool crib in case of catastrophic failure as well as six out for regrind at any given time. In this situation you need 21 solid carbide drills in the system to keep the machine running.

Scenario two – For the same job in scenario one, you only need two replaceable drill bodies and four inserts at the machine and four bodies and 12 inserts in the tool crib. Since you won’t have to accommodate for tools out for regrind, you can keep the same job running smoothly while significantly reducing the amount of inventory needed.

How important is performance for this particular job?

Any machine operator knows that solid drills can be run at higher feeds than indexable tools of the same diameter. Solid cutting tools are stronger and more rigid as they have no connection to fail over time. At times though, machinists opt to use uncoated solid drills in order to reduce time invested in regrinds and lead times on reorders. Unfortunately, using uncoated tools reduces the superior speed and feed capabilities of a solid cutting tool. At this point, the performance gap between solid drills and replaceable insert drills is almost negligible, so take this into consideration when making your decision.

Finally, what is the overall cost per hole?

Let’s talk about tool life and cost of ownership again. The job size, initial cost of the tool, downtime for changeouts, regrinds and touch-offs, and number of steps in the application process are all variables in the cost of ownership equation.

Solid drills are a smart choice for short runs due to their lower initial cost. Generally, small jobs don’t wear a tool out before they are complete. This means there isn’t any downtime from changeouts, regrinds and touch-offs.

A drill designed with replaceable cutting edges can offer a lower cost of ownership over the life of the tool for long-term contracts and high production runs. The savings start when the cutting edge is worn or damaged because there is no need to order the whole tool, you only pay for the insert (a.k.a. blade).

Another cost savings variable is the amount of machine time saved or spent when changing out cutting tools. The replaceable insert drill’s diameter and length are not affected by changing out the cutting edge, but since the solid drill needs reground when it’s worn, solid tools should be touched off when replaced. While this only takes a minute, it’s a minute that you aren’t producing parts.

The last variable in the cost of ownership equation is the number of steps in the holemaking process. Replaceable insert drills can usually complete the process to spec in a single operation. As was mentioned earlier, many applications that incorporate solid drills add a finishing operation after using the reground tool in order to meet the job’s requirements. In reality, it may be an unnecessary step that adds machining cost to the part produced.

Overall, most machine shops need a good selection of drill types. Most industrial tooling suppliers offer guidance in selection of the best drill for a particular job and tooling manufacturers have free resources like worksheets for determining the cost per hole to help aid in the decision process as well.