Lowering Your Grades

If you’re up to your ears in carbide inserts, it may make economic sense to eliminate some of them.

Thumb through a typical tooling catalog or talk to a typical tooling salesman and you will probably come away with the same feeling-that you need more carbide inserts.

It is overwhelming how many different grades, shapes and styles of carbide inserts are available today. There seems to be an "ideal" insert for just about every job and material imaginable. One well-known toolmaker offers 54 different grades and 22 styles of chipbreakers.

What are all those inserts for? More importantly, how many different types of inserts do you really need? Probably fewer than you think and fewer than what makes sense economically.

Less Can Save More
It’s easy to understand how the average machine shop can overdose on carbide inserts. To illustrate this point, consider the case of a fictitious shop, which we’ll call XYZ Precision.

XYZ operates a small lathe department and machines a wide variety of workpieces made from a handful of different materials. The shop’s toolcrib contains CNMG inserts for rough turning, DNMGs for finishing and VPGRs for profiling. Each style is available with a variety of nose radii and assorted chipbreakers.

Additionally, XYZ stocks several widths of cutoff and grooving inserts, as well as threading tools for both internal and external work. And, like most shops, XYZ drills and bores holes. These applications require it to carry an assortment of inserts for indexable drills and boring bars.

Wait, there’s more. On the advice of its local tooling salesman, XYZ buys a different grade of carbide for each of the five materials it machines.

Given the materials it cuts and the many operations it performs, XYZ found that, in a fairly short time, it had acquired hundreds of different carbide inserts. This means that it has thousands of dollars invested in inserts and thousands more tied up in the necessary toolholders, boring bars, shims, screws, clamps and storage cabinets.

But the money tied up in inventory is only part of the picture. At first glance, there are plenty of good reasons to use the best insert for each operation you perform. Modern cutting tools and machinery allow you to optimize cutting speeds, feed rates and tool life.

What is rarely considered, however, is that changing an insert consumes valuable machining time. And making tool offsets and program adjustments to compensate for the new insert eats up even more time.

Let’s revisit XYZ Precision to demonstrate how much time-and money-can be lost when a shop always uses the optimal grade of carbide insert.

Most of the jobs that pass through XYZ’s six lathes are repeat orders. But since the shop delivers parts on a just-in-time basis, lot sizes are kept fairly small. Because of this, each lathe averages three setups per shift. Each setup takes 30 minutes. Approximately half of all setup time is spent changing inserts and touching off tools.

Several years ago, XYZ implemented a simple tool-life-management program. The aim was to get maximum usage and efficiency from its inserts. Every time a lathe is being readied for a new job, the machinist replaces the inserts used for the previous job with the grade of inserts recommended by XYZ’s cutting tool supplier. The operator puts the replaced inserts into tooling kits by his or her machine. With the tool-life program, the company is able to track when inserts are worn out and in need of replacement.

There is no doubt that XYZ is making the most of its cutting tools. Its tooling cost per lathe is around $18,000 annually. The company runs two shifts per day and spends about 10 minutes per shift replacing worn inserts. At a rate of $75 per hour, this equates to $6,250 a year in downtime per machine.

By always using the optimal insert grade, the engineers at XYZ are confident that tool life is as good as it gets and that they employ the most efficient feeds and speeds possible. And they’re right. However, they fail to realize one thing: Inserts may cost a lot, but an idle machine costs even more.

XYZ would save money by equipping its lathes with general-purpose-grade inserts that stay on the machines from job to job. Since these inserts wouldn’t need to be changed out every time a new job had to be set up, the shop would save roughly 15 minutes per job. At three setups per shift and two shifts per day, this totals more than 375 hours annually, or $28,125 per machine. For the shop’s six lathes, the yearly savings would be $169,000.

Granted, using a less-than-optimal grade would probably increase tool wear. And that would require inserts to be replaced more often, thereby raising XYZ’s tooling and downtime costs. But even if we assume that optimal-grade inserts last 50 percent longer than a general-purpose grade, XYZ would still save more than $16,000 annually per machine by using the latter (Table 1). Multiply that by the shop’s six lathes and the yearly savings totals $96,000.

XYZ would realize other benefits by switching to a general-purpose grade, too. These include a smaller tooling inventory, fewer purchase orders and less confusion among machinists trying to select the proper inserts for jobs.

Additionally, XYZ could receive a better price on the inserts it orders. The reason is twofold: The shop would order a limited number of grades in greater quantities. And because general-purpose grades tend to wear faster, it would order more inserts overall.

Select Inserts Based on Lot Sizes
None of the preceding is meant to imply that you should never optimize your cutting processes. Demanding tolerances and surface finishes, for example, might require you to consider high-price inserts, such as those made of polycrystalline diamond, ceramic or polycrystalline cubic boron nitride. Also, some materials are just too hard to cut with anything less than the best insert, regardless of the quantity of parts being machined.

For long-run jobs-say, 5,000 parts-it usually pays to cut every inefficient second of cycle time out of the machining process by using the best insert possible. The reason is that time spent on engineering and setup is amortized over many thousands of parts. In these cases, it makes sense to fine-tune the process to obtain the very best tool life and cutting performance possible.

But an average shop-if there is such a thing-probably runs parts in batches of a few dozen,which makes setup time a significant factor, and machines steel, aluminum and maybe some stainless. It could carry a lot fewer grades than shops that do production work.

Imagine for a minute that you are assigned the task of buying carbide inserts for the lathe department of an average shop. What would you choose? Generally, two or three general-purpose grades should cover the lion’s share of materials that pass through such a department. You should look for a good general-purpose C-2 grade for roughing and semifinishing; CVD- or multiphase-coated tools work well for most materials. Also, order a supply of PVD-coated or possibly even uncoated C-5 or C-7 inserts for finish turning, grooving, boring and cutting off.

If the results from a particular grade prove unacceptable, try another.The goal should be to find a grade that covers as many machining needs as possible.

Buying general-purpose grades isn’t the only way to reduce the number of inserts you stock. Another is to use multipurpose tools. The ones available today will allow you to turn, groove, face and cut off with the same insert. This type of tool will drastically reduce your tooling inventory and shorten your cycle time.Multipurpose tools might cost

50 percent more than conventional tooling, but sometimes it’s much easier to maintain one tool than two or three.

Also, consider using the same insert in your turning and boring holders. You might have to use a smaller insert, but with carbide, smaller is cheaper.

Finally, beware of salesmen bearing gifts. Many distributors offer so-called specials. You know the ones—buy a pack of inserts and get a free holder. Every time you let a new holder into your shop, you are making a commitment to stocking inserts and spare parts that might take up shelf space for years. So when these guys come knocking, remember that you get what you pay for.

Remember, too, that life is full of choices. Take a trip to the grocery store and you’ll find 15 brands of toothpaste, seven kinds of baked beans and nine types of tissues. All of them are supposed to be the best. But the truth is, most products do a fine job.

It’s the same with carbide inserts. Even though it’s great to be able to select the perfect insert for every possible cutting application, a less-than-optimal grade will usually do the job and, often, will save your shop some money.

About the Author
Kip Hanson is a regular contributor to CTE.