CARBIDE INSERT QUOTATION,INDEXABLE CARBIDE INSERTS,CARBIDE INSERTS

In the May Tech Talk column, I discussed a method of determining cutting motion time in a program. A certain amount of cutting motion time is air cutting time. This occurs when the cutting tool moves from its approach position until it contacts the workpiece. It is also time when the tool feeds off a surface until it reaches a fast feed milling inserts safe clearance position.

Most programmers use a standard feed-off distance of 0.100 inch (or 2.5 mm). If you performed the test in the May issue, you may have been surprised at how much cutting motion time is air cutting time. Consider center drilling 50 holes on a machining center at 5.0 inches per minute. If you are using a rapid approach distance of 0.1 inch, there will be 5.0 inches of air cutting motion in the program (50 holes times 0.1 inch). At a feed rate of 5.0 inches per minute, this equates to 1 minute of air cutting time. Next, the holes must be drilled. After that, there may be more operations on these holes (counter boring, tapping, reaming or other operations). If each of these tools uses a 0.100-inch rapid approach distance, then at least five more inches of air cutting time will occur per tool.

While 0.100 inch is a safe rapid approach distance, you must consider the impact it can have on air cutting time. There are times when 0.100 inch may be an excessive rapid approach distance, at least when the surface the tool is approaching is qualified and when the tool being used has been accurately set. By qualified, I mean that the surface is not varying by much. Perhaps the surface has been machined (varying by less than a few thousandths of an inch), or the surface has been cold drawn (varying by 0.005 to 0.010 inch). Possibly, the tool has just machined one surface and is about to machine another.

In these cases, you can safely reduce rapid approach distance. Make your rapid approach distance about ten times the total of the surface variation amount plus the total possible imperfection in tool setting. For example, if the surface is varying about 0.003 inch and the cutting tool setting could vary by as much as 0.002 inch, then a 0.050-inch rapid approach distance should be sufficient.

In the case of the 50 center-drilled holes, this would allow us to cut the rapid approach distance and air cutting time in half. Instead of taking 1 minute, air cutting time will take only 30 seconds (based on the 5.0 ipm feed rate mentioned above).

There are two safety-related points about this technique. First, the surface must be qualified. Don’t try to reduce rapid approach distance for sand castings, forgings or other workpiece blanks that vary from workpiece to workpiece or lot to lot. In these cases, a rapid approach distance of 0.25 inch or more may be necessary. Second, your tool setting positions must be accurate. For machining centers and when approaching in the Z axis (as with hole-machining tools), this means that tool length compensation values must be accurately measured and entered. When approaching in XY (as with an end mill), the cutter cannot be larger in diameter than expected. For turning centers, this means that program zero assignments for each tool must be correctly measured and entered.

Let’s say you have been setting tools accurately. I contend that if you would have started your CNC career using a 0.050-inch rapid approach distance for qualified surfaces, you should have had no problems approaching with cutting tools that you haven’t had using the 0.100-inch rapid approach amount. You have the same program verification functions that you currently use when approaching with a new tool. These include dry run, single block, rapid override, distance-to-go and feed hold. If you’ve had problems with crashes, they’ve probably not been related to the size of your approach distance.

These points apply when you feed a tool away from a surface after cutting. If anything, it will be safer to reduce feed-off distance because the tool is not moving at rapid prior to this motion, and during this motion, the tool is still at its cutting feed rate.

An example of when you can reduce rapid approach distance on turning centers is when you rough face and rough turn with the same tool. Say you rapid the face-and-turn tool within 0.100 inch of the diameter to be faced (on the side). You then face the end of the workpiece to Carbide Turning Inserts center. Next, you retract the tool 0.100 away from the workpiece in Z. You then rapid the tool straight to the first diameter it will rough turn, maintaining the 0.100 clearance. Finally, you program the tool to rough turn its first diameter. The very tool that rough faced the workpiece is being used to rough turn the workpiece. The variation in the surface being approached (workpiece face) will be next to nothing. Maintaining the 0.100 approach distance for the rough turning pass(es) will be wasteful.

Conventional wisdom about standard milling operations has always been that compared to square shoulder cutters, lead angle cutters are easier on the spindle, can be run at higher table feed rates, and cut freer. Finishes are typically better and, with the addition of a wiper flat insert, finishes could be further improved.

Due to the chip thinning attributes of this type of cutter, actual table feeds can be more rapid, albeit at the sacrifice of some depth of cut compared to a zero lead cutter.

The force on the cutter body is determined by the point of application (cut line extension from the spindle), its direction (feed and edge inclination) and magnitude (feed rate and depth of cut). The vector quantity, or combined forces, of a tungsten carbide inserts lead angle cutter is directed more toward the spindle in a radial rather than axial direction, hence the viewpoint that the lead angle cutter is easier on the spindle.

Finishes are primarily determined by the feed rate per tooth and what type of corner or flat is in contact with the surface being cut. Square shoulder cutters have had somewhat of a disadvantage in the past.

As a result of these factors, most machine shops, by necessity, purchase lead angle cutters for general milling applications and square shoulder cutters for applications where a 90-degree corner is needed. In many cases, two milling operations are conducted. The first uses the lead angle cutter for rough and finish operations, which are followed by a square shoulder cutter to clean up any corners as needed.

The thought has always been that the BTA deep hole drilling inserts versatility of a square shoulder insert would be nice if it weren't for the drawbacks. These include higher cutting forces, fewer cutting edges, and usually a lower finish quality capability. Square shoulder cutters are generally relegated to specific needs.

Of course, lead angle cutters have drawbacks as well. The obvious lack of versatility with a lead angle cutter is one, as is the inherently smaller depth of cuts. From a shop's perspective, the need for both types of cutters requires a larger tool crib inventory.

This begs the question, what if a square shoulder cutter was designed to reduce cutting forces, had the strength to take higher feed rates, could be free cutting enough to allow for large depths of cut and could create excellent surface finishes?

Mitsubishi Materials says it has developed such a cutter. The company calls it the BSX. The new cutter exhibits the free cutting characteristics of a pure positive rake cutter while having excellent chip evacuation. Cutting forces are reduced significantly with this new design, and insert strength is similar to that normally associated with pure negative rake cutters. Surface finishes are "exceptional," says the company.

The BSX cutter uses a square shoulder with a complex chipbreaker system that reduces cutting forces. A unique pocket design on the cutter body features an anti-expulsion system. Essentially this design adds safety to high rpm running, as the insert relies on its seat rather than the clamp screw for retention. The seat has a round protrusion with the clamp screw hole as its center. A mating recess is pressed into the insert that envelopes the protrusion as the insert is clamped down—trapping it in place.

In almost every shop doing metalworking, the need to increase throughput is a battle cry. Of the various ways to accomplish this task one of the most promising, as far as significant impact, is the ability to get more process range from a given cutter. Multiple operation inserts such as Mitsubishi's BSX are among the offerings that will help shops achieve their throughput goals.

Should you toss out your lead angle tools? No, of course not. The principles of milling haven't changed, only some of the process alternatives available to the metalworking shop.