Thread lead anglein mm

Due to the nature of the operation, boring operations required positive inserts, since the tool was extended and had a tendency to chatter. During this same era, machine tool builders were making lighter duty machines with less horsepower.

Lead anglesymbol

Insert pressing technology evolved and this technology was quickly implemented into turning inserts. By producing a deep, positive chip groove on an insert it was discovered that a negative insert could have positive cutting forces. This technology meant turning inserts could be double-sided, and have lower cutting forces.

These factors increased the popularity of positive geometry inserts. These inserts drew less horsepower, created less chatter, were easier on the part due to freer cutting forces, and reduced “built up edge” on the inserts since they created less heat.

Depending on the connection type and thread form, lead error can affect the connections differently. Many manufacturers control the threading process by measuring pitch diameter or functional diameter with gauges, but do not explain how lead error will affect the standoff. Now, standoff is using ring or plug gauges to verify if the part’s size is within tolerance.

Insert manufacturers began to differentiate themselves with the use of proprietary chip breakers. No longer were turning inserts labeled CNMG or TNMG, with generic breakers — the new chip-breaker designations were added to the description, such as CNMG 432-TH or TM.

Thread lead anglechart

With chip-breakers, a small “single bump” or groove would reduce the amount of contact the chip had with the insert, and thereby reduce the amount of heat transferred into the insert, extending tool life.

Lead angleturning

Obviously if the thread element is called out in API specs, then it is a dimension you must measure, but how critical is this dimension? We defined thread lead above as: the distance from a point on a thread to the corresponding point on the next thread, typically measured over 1” intervals. Thread lead has a direct effect on functional diameter and is one thread element that will vastly affect standoff measurements. In order for the male and female connections to mate properly, you must make sure the threads line up. If you lead is not within tolerance, it will result in the threads not lining up which will then cause galling of the threads on both connections. Depending on the form and type of connection, lead error will affect the functional size or standoff differently. Aside from pitch diameter and taper error, thread lead error accounts for most incorrect ring or plug measurements.

So, for a rotary shouldered connection with 2” taper per foot, a 0.001” change in pitch diameter affects the standoff about 0.006”; but a 0.001” lead error correlates to a 0.0035” change in pitch diameter which correlates to a 0.0208” change in standoff. So, for RSC connections, a 0.001” lead error can cause you to exceed your standoff tolerance while the part in question is actually still good.

No longer is the term “top form geometry” applicable since this new technology applies to more than just the top form. Perhaps a better term may be three-dimensional pressing technologies?

Lead anglevs helixangle

Thread lead is essentially how fast or slow the machinist is threading the part. If you receive a positive measurement when measuring lead, it is because there is too much space between your threads which means you are threading too quickly. On the opposite side, if you receive a negative lead measurement, then there is not enough space between the threads which means you are threading too slow. The simple solution is to then adjust the feed rate depending on the lead measurement you receive.

Cutting forces were generally controlled by the rake angles of the cutter. That is axial rake and radial rake. These rake angles were built into the cutter bodies, so in order to run a negative insert and take advantage of multiple edges (double-sided) the tool needed to be double negative — that is negative axially and negative radially. This allowed for clearance on the 90-degree edges.

Another factor to take into account is the thread form of the connection. Lead error affects various thread forms differently. When calculating the effect of lead error on standoff for 60° threads or “V” threads, you use the following formula:

When measuring thread lead, you are measuring from load flank to load flank inside the thread form. All lead measurements are taken by using a specific contact point size, depending on the pitch of the thread, which sits inside the thread contacting each flank tangent to where the pitch line is located. The pitch line of a thread is an imaginary line that runs through the middle of the threads so that along the pitch line, the distance of the thread is the same distance as between the threads.

Another feature the chip-breaker provided was its ability to keep the heat from the chip out of the insert. With a flat-top insert the hot chip would slide across the face of the insert and the heat from the chip was transferred into the insert, shortening the tool life.

Lead angleof worm gear

On RSC connections, lead error has a greater effect on pitch diameter and standoff as well. The effect that standoff has is much more significant than people realize. We will use the example of a 0.001” lead error on a 2” TPF rotary shouldered connection.

The trade-off was the inserts were single-sided and had half the number of cutting edges. The popular milling inserts of this era were SEKN and TPKN.

The trade off with getting double sided inserts with multiple corners was the double negative geometry created tremendous cutting pressure and heat. (Double negative inserts by nature, such as aluminum, stainless or low carbon steel.)

Thread lead anglecalculator

In conclusion, you can see how exponentially lead error can affect standoff as well as pitch diameter across the various connections. Lead error is a huge problem for companies using ring and plugs to quantify functional diameter. What most people don’t realize is that ring and plug gauges aren’t taking a true pitch diameter measurement, they are taking a cumulative measurement of diameter, lead, taper, flank angle and form error. Each of these dimensions will have a profound effect on the standoff measurement, which is why lead should always be checked separately. With the equations in this newsletter, you can now calculate the affect your lead error is having on your P.D. or standoff to ensure your connection will accept the mated part.

Initially, chip-breakers were added to positive milling inserts and the term chip-breaker was aptly changed to “top form geometry” since its main function in milling was not to break a chip, rather it was designed to change cutting forces, direct chip expulsion, reduce heat and increase tool life.

The primary function of these early chip-breakers was to curl the chip and force it to break. However, the only chip-breakers available were “G” type. For example, CNMG, TNMG, SNMG etc.

On the other hand milling was considered an interrupted cut and chips were not as long as turning due to the insert only cutting for half of a revolution. Naturally, turning inserts were the first to be developed with chip-breakers.

Again, a more common issue is confusing threads per inch (TPI) with the thread pitch. Now, while they may be the same dimension, they are in fact the inverse of each other. If you have an 8 round thread with an 8 TPI, then the pitch of the thread is the inverse or 1/8 = 0.125” pitch.

So, a 0.001” lead error will affect the functional diameter 0.0035” for this example. In order to calculate the effect standoff will have on the pitch diameter, you must know how they correlate to each other. The correlation between pitch diameter and standoff is expressed by dividing 12 by the taper per foot (TPF). This will yield the effect on standoff for every 0.001” change in pitch diameter you have.

It is very common in the industry for individuals to refer to thread lead as thread pitch. While these measurements are the same, they are expressed differently. It is more common for individuals to confuse thread pitch and threads per inch. While TPI and Pitch are directly related, they are actually the inverse of each other. Thread pitch is defined as:

Lead angleformula

So, thread lead is the distance from thread to thread measured over a 1” distance, while the pitch is the distance from thread to thread measured over one thread. In either case, you are checking the distance between each thread, just in a different interval. Thread lead is expressed at a + or – measurement from nominal, ex. +.001”. Thread pitch is expressed as a direct measurement, ex. 0.125” pitch.

As pressing technology evolved, insert manufacturers were able to press highly positive top form geometries on double-sided inserts. This allowed multiple cutting edges on inserts that were free cutting.

So, while your lead error may be within allowable tolerance, your standoff gauge will sometimes measure badly because of the effect lead error has on the standoff. That is just with round threads. When you get into RSC connections, the lead error has a profound effect on diameter and standoff. In order to calculate the lead error effect on the functional diameter for all threads types, use the following formula:

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Some of these geometries allow for double sided positive inserts. These inserts actually have clearances built into the insert itself. Rather than a traditional 90-degree angle on a double-sided insert this new technology allows for inserts to have a positive clearance angle on the insert such as 5 or 11 degrees and still be double-sided.

This week’s gauging category takes a step back and focus on Thread Lead as a whole. Sometimes when we focus on a specific detail, we can lose sight of the big picture. This week we will define thread lead, discuss the importance of lead and the effect lead error has on standoff.

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The dovetailed pockets adds more stability, and less chance of any insert movement. This has allowed double-sided inserts on very small boring bars and indexable drills with 6 cutting edges per insert, milling inserts with 16 cutting edges or grooving tools with 4 edges.

This created a lot of issues with chip were rigid, the machine had ample horse control. Turning was a particular issue, since in many power and the material was not “sticky” cases turning was a continuous operation and created long, stringy chips.

Pressing technology has evolved even further. Today some insert companies are capable of pressing unprecedented types of complex geometries.