PUSH’em, PULL’em, PACK’em or NONE at ALL Tapping Chip Management
Those of you who follow my blog, have grown to understand I view metal cutting from a slightly different perspective than most. First off, I don’t call it machining. I call it chip making. I do not say making parts either. The part is the product of the chip making process. Your productivity, quality and profitability rely on how well you make chips. The faster you can make and manage quality chips the more parts you can ship out the door. With that let’s look at the tapping process from the chip’s perspective.
I don’t know about you; but when I started out as a rookie machinist, I hated tapping. Why; because I broke a lot of taps, scrapped a lot of parts, and had to learn how to run a burnout machine. I swore that tap manufacturers designed taps to brake, to sell more taps. Alas, I was wrong. Years later I learned that taps are one of the most highly engineered cutting tools, and when they broke, it was usually my fault. More accurately said, it’s the responsibility of the individual who selects the tap to pick the right one for the material alloy and type hole being tapped. It’s all about the type of chips produced and where you need them to go.
If you are making a lot of chips very quickly, chip management becomes a critical part of the process. You need to get the chips away from part and the tool, so they don’t damage anything. Remember chips are just jagged little pieces of heat-treated steel. Re-cutting and packing chips are the biggest contributors to poor quality threads and tap breakage. But it’s OK you have choices on what to do with the chips. You can PUSH the chips, PULL the chips, PACK the chips or produce NO CHIPS at all. It all depends on the part material and type of hole to be tapped. The above illustration provides a quick reference to basic tap application as it relates to chip management.
The first method of chip management for tapping is to PUSH the chips. Let’s face it through holes are the easiest to tap. If we do it right, the chips never get re-cut. They are pushed out the bottom of the hole and never encounter the cutting edge once they are formed. This pushing effect is accomplished by modifying the spiral or helix angle of the flute or gash ground at the start of the tap. This angle is usually between 5-7 degrees. Looking at the tap with the shank up and cutting edge facing down, you will notice that the helix angle of the flute gradually curves around to the left. Once the chip is separated from the base material the clockwise rotation of the tap in conjunction with the left-hand helix pushes the chips down and out in front of the chamfer teeth and eventually out the bottom of the hole. These taps are known in the industry as GUN taps or LHS (left-hand spiral) taps. LHS (left-hand spiral) or PUSH taps are the most efficient taps for managing chips. They typically require smaller flute space which creates a larger core diameter providing more core strength. More taping fluid or lubricant flows to the cutting edge as the chips are being pushed in front of the tap and not fighting with the lubricant coming in from start of the hole. The extra strength and better lubrication allow speed and feed rates up to 40% faster than other tap designs. As with every good thing in life there are drawbacks, LHS or PUSH taps can only be used in thru-hole applications. When used in blind hole applications the chips will pack up in the bottom of the hole and obstruct and break the tap.
The second method of tapping chip management is to PULL the chips. PULL taps do just that, they pull the chips up and out of the hole. They perform this task by using a RHS (right-hand spiral) or helix angle. Again, holding the tap with the shank up and cutting edge down the spiral or helix angle; usually between 15-52 degrees, rotates up and to the right. This angle coupled with the clockwise rotation of the tap creates a screw lift effect and pulls the chips up and out of the hole. RHS or PULL taps are used on blind and deep holes. They are also often recommended for soft gummy materials which product long stringy chips. Given the chips must ride the chip flute all the way up the length of the hole to evacuate the flutes are larger than that for LHS or PUSH taps. This produces a smaller core diameter which makes the tap weaker. In addition, tapping fluid or lubricant must also fight the chip coming up and out of the hole to get to the cutting edge. These combined factors require RHS or PULL taps to be run as speeds and feeds 30%-40% slower than that of straight flute taps.
As stated above RHS or PULL taps have spiral or helix angles that range between 15 to 52 degrees. The lower the spiral or helix the larger the core diameter and the stronger the tap; however, you sacrifice the chip lift provided with higher spirals. Higher spirals or helix angles provide greater lift but have smaller core diameters sacrificing strength. It’s always a give and take, but you can select the best combination for your material. Tougher alloys require a slow spiral or helix of 15-20 degrees. The stronger design can withstand the demands of higher strength materials. Soft gummy materials which produce long stringy ships require a high spiral or helix. Angles of 45-52 degrees are used. The ultra-high spiral creates a curly or corkscrew type chip. Unfortunately; the higher spiral makes this the weakest of all tap designs.
The ultra-high spiral or helix does have one unique and counter intuitive advantage over other designs. The higher spiral design creates 360-degree gage of contact between the tap the workpiece. Usually; more surface contact creates more tool pressure, stress and heat in both the workpiece and tool. This is typically not a good thing. However, in cases where the hole being tapped has an interruption, cross hole, recess, or exits on an uneven surface, this added surface contact stabilizes the tap. The high spiral or helix angle creates 360-degree gage of contact above and below the interruption and acts like a guide bushing stabilizing the tap and preventing it from deflecting. In many interrupted cuts or cross hole applications this stabilizing effect outweighs the added tool pressure and heat and prevents tap breakage.
The next option in chip management is to PACK or store the chips. To PACK or store the chip the tap utilizes straight flutes. These taps are often referred to as “Machine” or “Hand” taps and are one of the most common and mis-applied. Straight flute taps should only be used on short chipping materials such as cast iron, brass, or harder short chipping alloys. With no spiral or helix, the chips are not pushed out or pulled up and out of the hole. They are packed or stored in the flute space. To provide space for the packed chips the flute space is enlarged sacrificing core strength. Coolant or air blast must be used to assist in the evacuation of the chips.
The last method of tapping chip management is to create NO CHIPS at all. This is accomplished by using a FORM TAP. FORM TAPS differ from CUT TAPS in that they do not displace the material for the thread profile by cutting it out of the way, FORM TAPS create the thread profile by “extrusion displacement” of the material. Thus, producing no chips to manage. Often this process is referred to as “thread forming” or “roll tapping). Form Taping has many advantages. The design of the tap is very strong with a large core allowing them to be run at faster speeds and feeds. The thread form produced with a Form Tap is stronger than that of a Cut Tap as well. Form Taps do have their limitations. Not all materials can be Thread Formed. Materials with a hardness rating above 28 Rc and tensile elongation rate less than 12% should not be thread formed. In addition, hole size and lubrication requirements are different than cut tapping. In short, Thread Forming is a whole new subject that will have to wait for another day.
In summary, when it comes to tapping proper chip management must be addressed to create a reliable, productive, and quality tapping process. It’s your choice and selection is easy if you apply these principles to determine if you should PUSH’em, PULL’em, PACK’em or create NO CHIPS at all.