Design Tips

There are many factors you should consider in choosing what thread mill Design is the best for your application. The question is of all the different Designs and manufacturers available, which is the best for your application? Which is the most cost effective? Which is the most productive? Assuming that you have narrowed down your selection to an indexable thread milling system, this article's goal then, is to inform you of the Design elements that can be important for your operation.

Of course, cost effectiveness and productivity on your end needs to be defined by you - the end-user - the user of the equipment: What is the cycle time that you need for the application? What is the tool life? Is the quality of the thread important? How many holes will you need to thread using these thread mills? What is the material that you are threading? What is the cost of the tooling and its future capacity, if applicable? All of these factors will be part of your equation for defining productivity.

A more versatile system will allow us to use the same tool for several different applications, while a dedicated system is optimized for a special use or purpose. Our job in evaluating the Design for our particular needs is to determine which takes priority - versatility or dedication? Will it only be used for this application or other ones when we are finished? Or does this application allow us to use a more optimum Design specifically for it?

What are the particular Design elements we need to evaluate for defining the best thread mill Design for our application?

• The diameter of the thread mill itself in relationship to the thread's diameter

• Production requirements - i.e. number of threads per day / month / year

• The material that we are machining

• The number of flutes of the thread mill / number of inserts

• Rigidity of the tool holder Design and clamping

• Coolant through capability, but also what type of coolant through capability, either straight through the body, or at the flutes?

Thread Mill Diameter

The tool's cutting diameter will determine how versatile of a tool it will be, but also how rigid it will be for our particular application. We could select a tool with a very small tool cutting diameter that will allow us to do our application, and in addition, many more larger diameters in the future. Or, we can maximize the cutting diameter for optimum stability, and as a result, better productivity, and still be able to use it in the future for larger threads, but not smaller. In addition, with thread milling, the programmed RPM is determined by the tool's cutting diameter, calculated using the manufacturer's suggested Cutting Speed (SFM) for the applied carbide insert grade / coating. In effect, the smaller the diameter the higher the RPM is, and conversely, the larger the cutting diameter, the slower the RPM is.

Production Requirements and Workpiece Material

In general, the more versatility Designed into a system, the more types of threads and materials can be threaded. But obviously, this limits efficiency and the threading cycle times will be increased.

Also, keep in mind that best tool life may not be the most important consideration when it comes to threading just one work-piece. Of course, a low priced thread mill is probably good enough to produce one thread, even in hard materials. But when threading difficult-to-machine materials or manufacturing thousands of workpieces, you'll end up using a lot of "low priced" tools. Thus, often tooling that gives you the lowest initial cost proves to be more costly to the manufacturing process when you calculate the actual cost-per-workpiece.

Number of Inserts

Feed rate in thread milling is a linear calculation of millimeter per minute (inch per minute). This is calculated by using a function of RPM (discussed above) and chip load (feed per insert / per flute). With RPM, chip load and the number of inserts / flutes known, we can calculate our linear feed-rate. With that said, the more inserts the faster the linear feed rate and the result is a reduced cycle time.

With one insert, the programming calculation provides us with an approximate cutting cycle time of 14 seconds, using 2620 RPM @ 60 mm/min. (2.36 in/min).

Using two inserts with the same cutting speed and feed per revolution increases the linear feed-rate to 120 mm/min. (4.74 in/min.) which now gives us a cycle time of 7 seconds - a 50% reduction!

Insert Design and Clamping Method

Simply stated, the tool holder is more long term as far as investment while the inserts are part of your perishable tooling cost. The main elements affecting your operation:

1) Insert Size - thinner will usually cost less, but will also be less rigid. Thicker, of course, will cost more, but also come with added rigidity.

2) Length of the Insert - see above note about cost. But more importantly, the insert length is the determinate factor in the maximum thread length that can be produced efficiently. A longer insert can machine a longer thread with fewer passes. However, the load on the insert is usually higher.

3) Number of Inserts Needed Per Tool holder - defines how many inserts are needed for a job, in other words, the cost to load the tool holder.

4) Clamping Method - the need to clamp the insert rigidly, but retain tool holder integrity.

Coolant-Through Capability

The benefits of an option of coolant-through the tool should not be underestimated. In the case of thread milling, the tooling is rotating, and at recommended SFM's, the tool's RPM will be quite fast. The centrifugal forces of this high rotation, will attempt to negate the benefits of the applied coolant. Coolant-through Design straight through the tool may be a benefit for many applications such as a blind, vertical hole. The coolant going through the tool will hit the bottom of the hole and be forced upward and outward thereby providing for cooling, lubricity, and assistance with chip evacuation. A limitation of this Design though, is with through holes, where other than making a mess, the coolant does not get applied where we need it - at the insert's cutting edge. Another affected application will be with horizontal jobs where gravity will tend to pull the coolant towards the bottom of the hole, as a result varying the amount of coolant applied. This will have different effects as the tool travels around the circumference of the hole.

What I am hoping you see after getting this far, as with many different tools, there are trade-offs between versatility and productivity - it is difficult to achieve both simultaneously. Is there a happy medium, though?

MiTM - Multiple-flute Indexable Thread Milling

This new product line includes indexable thread milling inserts that provide a high degree of versatility, but still offer the customer productivity gains and cost effectiveness.

Some of the unique features of this new Design are:

• Multiple flutes per tool for faster cutting

• Tool cutting diameters as small as 17mm (.67")

• Long inserts for threading deep holes

• Inserts with 2 cutting edges

• Excellent stability

• Indexable insert system

• Two standard coated grades, perfect for stainless steel and general use

• Tool holders that can be used with multiple inserts or just a single insert

• Coolant through - right at the insert cutting edge!

Currently there are eight standard tool holders, each having a common shank diameter - 25mm (1"). The tool holders are divided into four groups - in each group there are two tool holders with the same cutting diameter, but two different tool overhangs ("nose" length). One tool holder has a shorter overhang and the other a longer overhang. For example, there are two tool holders with a 17mm (.67") cutting diameter, but the first tool holder features a 26 mm (1.10") overhang and the other a 36 mm (1.45") overhang. The different overhang lengths provide for the most stable tool holder Design for your application.

In addition, the MiTM line features three tool holders for producing common conical (tapered) threads.

The MiTM family allows for maximized rigidity but is not limited in versatility, as each tool holder has a wide range of thread diameters that it can produce. Depending on the cutting diameter, the MiTM line allows you to use the maximum number of inserts so that optimum linear feed-rates can be utilized.

To add to the versatility of the Design, not all the insert pockets are required to be used. Should a job present itself with lower volumes, one insert would be in one pocket and one or more - "plug inserts" can be used. These plugs, are necessary to provide proper balance and prevent chips from damaging the pocket(s). Plug inserts are not worn out by use and are a one-time investment.

This option can add versatility to the tool holder investment, by reducing your insert costs for lower volume work, but still retain more pockets available for future higher volume jobs where productivity can be better realized.

Since deep insert pockets can weaken the tool, MiTM insert pockets are intentionally not too deep to retain the rigidity of the thread mill. Each pocket supports a relatively thick insert to maximize the rigidity of the carbide. A thicker insert also allowed Design engineers to incorporate cutting profiles on both sides of the insert - an exception is with tapered thread inserts that have one edge. In effect, this almost halves the cost per workpiece of the insert Design, with the added benefit that it is a stronger insert. Since the insert is thicker and stronger, it can support longer insert length as opposed to thin inserts. This adds to the versatility of the thread lengths that can be generated by MiTM tools.

Looking at the clamping system of the inserts, you will note that it uses multiple screws that are applied perpendicular to the insert's axis, pulling the insert rigidly into the pocket. This allows for maximum holding power, while at the same time, not diminishing the tool holder's rigidity. A rigid clamping system, along with maximum insert rigidity, allows for the use of the most aggressive speeds and feeds, which directly affects your bottom-line- fast cycle times with low cost-per-thread insert costs.

By using two standard carbide grades and coatings, insert inventory can be minimized for the various jobs you may see. VBX is provided with a TiCN coating and is applicable for the majority of steel jobs and general purpose work that a shop may see. While VTX is provided with a TiAlN coating - this carbide grade, along with a coating that is very heat resistant, is an excellent combination for stainless steel, and other high temperature alloy jobs.

When the cutting diameter gets very close to the thread's diameter (greater than 0.7 x diameter), chip evacuation becomes much more of a factor in the thread milling process. As stated earlier, coolant through can be a major benefit to many applications, but with some it may not matter at all. An example is with through-hole applications where the coolant will not assist at all with the chip evacuation through the hole.

This newer MiTM line provides coolant through capability that is angled at the inserts - the coolant is directed right where we want it the most, the insert's cutting edge. This maximizes the chip evacuation in the thread milling process, allows for maximum lubricity and cooling, all of which provides for maximum speeds and feed-rates to be achieved.

Lastly, any thread milling operation is very dependent upon the CNC program used in the process. With that in mind, VARDEX TM Gen, thread milling CNC code generator - Designed for "Making Threading Easy." This is a very useful resource in not only helping the customer in selecting the right tooling for their application, but also generating an accurate thread milling program for their controller. The easier a thread milling tool is to use, the more effective results the customer will achieve, thereby creating a win/win scenario for all involved. The latest multilingual version of VARDEX TM Gen (available on web site below) supports 9 different languages in addition to English including: Chinese, Japanese, Korean, French, German, Italian, Spanish, Russian and Polish. As with previous versions, the software lets you work in either inch or metric units.

As you already knew or learned from this article, versatility and productivity in most cases are not intertwined. The key is to work with a supplier that not only has more options available to address your present needs, but will also provide you with more options in the future.

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