Thread Milling Coarse Pitches

Thread milling, still in some minds a relatively “new black box” technology, continues to develop with further applications being added to an already exciting and productive method of machining.

With the continuous development in CNC machining centers and the resulting expansion in sales of these machine tools, manufacturers now have great opportunities to reduce manufacturing costs. Most machine centers are designed for applying helical interpolation (an essential feature for thread milling) in both clockwise and counterclockwise directions, so that left and right hand threads can be machined with the same cutter.

Development in software aids to programming take the uncertainty out of the decisions to use thread milling as a key component, not only to reduce the direct cost of producing threads but also significant costs in unnecessary handling and set up costs.

Still today, tapping many types of hole is fraught with problems, the biggest of these being the potential risk of scrap or rework should the tapping operation result in breakage of the tap or due to wear, the thread is undersize. The flexibility of the thread milling operation with indexable inserts eliminates these potential problems entirely.

Advantages of Thread Milling

– short cycle times due to rapid feed rates and high cutting speeds

– no thread relief groove needs to be machined, thus saving an unnecessary operation

– for a given set of tooling, there is no limit to the maximum bore diameter that can threaded.

– complete machining can be finished in a loading up thus eliminating costly set up charges

– the same toolholder can be used for both internal and external threads

– the same insert can be used for both left hand and right hand threads.

– inserts are interchangeable, if it is necessary to replace an insert there is no need to reprogram the machine tool.

– in the case of BSW, BSP, BSPT, NPT, NPTF the same insert will produce both internal and external threads.

– since a small range of tooling covers a very wide range of applications, the amount of money tied up in stock can be significantly reduced.

– the quality of the threads have a high surface finish

– the technique is suitable for machining hard materials.

– chips are always short, thus there are no problems in chip control.

– threading of large diameters requires no extra horse power compared to smaller diameters.

– due to very low cutting forces, components with thin walls can be machined without risk of component distortion.

– as wear takes place on the insert, compensation in the program can be carried out without any waste of time.

What is Helical Interpolation?

The pre requisite for generating a thread form with a thread milling indexable inserts is the CNC machine tool function for guiding tool movement along a helical path.

Helical Interpolation creates a tool movement in a helical path from point A to point B on the surface of an imaginary cylinder. This helical path involves the simultaneous movement in axes X , Y, and Z. In thread milling, the circular movement of the X and Y axis creates the thread diameter while the simultaneous linear change in the Z axis creates the pitch of the thread.

Thread Distortion

Thread milling inherently creates a level of thread distortion, however, in all the standard operations, this distortion is designed and confined within the limits of the tolerance of the thread standard.

Why do we get distortion? The thread is formed by applying a tool with a rectilinear profile of its teeth and the resultant profile being curvilinear. In other words, if we want to mill a slot in a piece of metal using a T-slot cutter, the width of the groove is a direct result of the width of the cutter being used.

If, however, the slot to be machined is inclined to the normal axis of machining, then the width of the groove becomes wider than the width of the T-slot cutter itself. Thus the slot is distorted.

With the complex form of thread inserts, this distortion can affect the radii of the thread form, the depth, the flank angles, flank form and width of the thread form.

Threading of coarse pitches creates the potential hazard of distortion and therefore, special designs of tooling must be made in order to compensate for each of these distortion factors.

What Are Coarse Pitch Threads?

Coarse pitch threads are a combination of a small thread diameter and a relatively large pitch. Since the thread milling operation is based on 3-axis simultaneous movement, so the profile shape machined on the workpiece is not an exact copy of the insert profile. In other words the profile is generated as in hobbing operations and copied as is the case with thread turning operations.

What Causes Distortion?

In addition to combination of thread pitch and diameter, two other factors can cause distortion; the diameter of the tool used for generating the profile and also the flank angles of the thread form.

The potential for distortion is usually confined to threading internal holes although thread standards such as ACME and TRAPEZE with flank angles of 29 deg and 30 deg respectively must be checked on a case by case basis.

In general,

a) as the pitch of the thread increases with all other parameters constant, then the level of distortion increases.

b) as the profile angle of the thread decreases, as with ACME, for example, the level of distortion increases

c) as the pitch diameter decreases so the level of distortion increases

d) and finally, as the diameter of the toolholder being used increases, so does the level of distortion. For internal threads, a general rule is that when the ratio of the cutting tool diameter and the minor diameter is 70% or above, the risk of profile distortion is high.

Most manufacturers of thread mill tooling will specify minimum bore diameters for their toolholders and it is important to use these reference table

Mathematical modeling techniques allow simulation of the effects of these parameters in order to provide tooling to solve machining problems which previously could only be machined by turning or tapping.

Whilst a major advantage of the existing tooling is its flexibility to cope with a wide of diameters, in the case of the coarse pitch applications, the toolholder and insert combination is dedicated to a limited number of threading applications and care must be taken to refer to the manufacturers recommendations.

In conclusion, further applications can no doubt be found for this highly cost effective technique. It is always recommended to contact your supplier if you don’t find a solution in their catalog. You never know what tricks they may have up their sleeve to provide you with solutions you thought may not be possible.

Author: Stuart Palmer – Marketing Consultant

VARGUS – At the forefront of the tooling industry for more than 40 years. Vargus is a world-leading manufacturer of high-quality cutting and deburring tools for the metals and plastics industry.

Established in 1960 in Nahariya, Israel, VARGUS is the tooling division of the NEUMO Ehrenberg Group, one of Europe’s largest privately owned manufacturers and distributors of stainless steel products and metal cutting tools.

Since the company’s inception, VARGUS engineers have pioneered breakthrough tooling solutions such as the first triangular laydown threading insert, the first indexable thread milling system, and the original hand-deburring system.

This tradition of innovation continues with industry-leading solutions from the company’s two best known product lines – VARDEX threading solutions and SHAVIV hand-deburring solutions.

Today VARDEX is the undisputed market leading threading program, with tens of thousands of thread turning and thread milling tooling solutions, available from stock or tailor-made special tools. VARGUS also makes the MINIPRO line of miniature tooling for machining small-diameter parts, and a range of cutting tools for boring, groove turning and milling applications. The SHAVIV line of hand-deburring tools rounds out the company’s offerings with professional solutions for finishing metal and plastic components.

Finding the Right Manufacturer

Unless you plan on manufacturing your product yourself, or plan on licensing your product, you will need to find a quality manufacturer to handle production of your product. So whether you are contract manufacturing, private-label selling, or forming partnerships, you will need to find someone who can cheaply and effectively make your product. Of all the inquiries I get from inventors, finding a manufacturer is probably the inquiry I get first.

Step 1. Be sure you know the production term for your product. For example, is it extruded plastic, thermoset plastic, machined, stamped and a host of other terms that distinguish different ways to make products. You will have trouble finding the right manufacturer without being able to describe the production process accurately. If you don’t know the process name itself try contacting these groups:

  • Your local inventors club, which you can find on the Inventor’s Digest website. These groups typically have monthly meetings and they often have protoypers or other engineers who attend who can help you understand the process needed to make your product.
  • Your local SCORE (Service Corps of Retired Executives) organization. This organization typically has several retired plant engineers or related people that can help recognize the type of production you need to make your product.
  • Your local small business development center. If your local group can’t identify the process they normally can direct you to someone who can.

Step 2. Find potential manufacturers. I found the best ways to do this are by:

  • Check the MacRae’s Bluebook which lists manufacturers by state by category.
  • Check the Thomas Register which also list manufacturer by category.
  • Look for trade associations for the industry which will typically have a membership directory. For example do an Internet search for plastic injection molding manufacturing trade associations. Most of the time you find a trade group that most of the manufacturers belong too. If that doesn’t work you can also search for trade shows for your type of manufacturing. These are typically run by the trade association, where you will find a member directory. If those tactics don’t work, you can also check at larger libraries in your area who might have the Book of Associations to find the right group for your product.

Step 3. Shortcut – Rather than contacting companies and evaluate whether or not they could make your product, I’ve found it useful to contact instead companies that make the equipment needed to make your product. For example, if your product requires injection molding equipment, contact manufacturers of injection molding equipment and ask the salesperson there to recommend people in your area that have the right type of equipment. You can also tell the person that you are looking for companies that would produce small run. The salespeople selling the equipment you need a manufacturer to own often give you the best list of manufactures to contact.

Step 4. Call companies, see if they can produce your product and get an estimate for small and medium volume production appropriate for your part. If could be an order of 500 and 2500 parts for one product, and 50 to 5,000 for another. This way you can see if the companies might be a good fit for you. Don’t be discouraged if companies don’t want to quote you. Just keep calling till you find one that wants your business.

Step 5. Try to determine if a manufacturer has an underused plant. Every manufacturer has overhead, or fixed, costs (i.e. salaries, rent, and phone bills) that they need to pass on to the products they produce. So the fewer products they produce, the higher overhead cost per product. Usually these will be the manufacturer that can offer you the shortest lead time to fill your order.

Now it may seem that you will want a manufacturer that is running their plant near capacity to have the lowest overhead cost per product, but what is good in finding a manufacturer with an underutilized plant is that they will want your business and should be willing to make concessions. For instance, if you can have extended terms for the first six months to a year you will need much less operating capital. Or you could get whatever start up costs they have amortized, which mean that for the first run of your product, the start up costs are spread out on each product produced as a small fee. All of these concessions can make a big difference for an underfinanced inventor.

Step 6. Consider the manufacturer’s financial status. While you want a manufacturer with an underused plant to offer you concessions, you don’t want a company that is about to go bankrupt. Request a financial statement from all of your potential manufacturers. Then find an experienced business person to review this document with you. If the company is in financial trouble, it is probably too risky for you to produce your product with them.

So in a manufacturer you want to find a plant with all of the right equipment that is not running near capacity, but not so slow that they are in financial trouble. This may seem difficult to ascertain, but by following the above steps you should be able to find out all of this information from them. Don’t be shy in calling companies, they usually tell you much more than you would expect.

Tips of CNC Machining

Machinists want to safely speed up cutting processes to reduce cutting time and keep costs low. The ultimate goal is to machine efficiently and reduce waste of material and cutting tools. For this purpose, processes that allow the cutting of more parts per hour are generally beneficial both to the machinist and the overall bottom line.

Use the Right Cutting Tools and processes

1. Use of the correct cutting tools can increase production. If you do not have the knowledge or experience to know the correct speeds and feeds, consult with the tooling Manufacturer to maximize efficiencies. Today’s tooling is highly engineered and it definitely pays to know the correct speeds and feeds to use. Using the correct speeds/feeds will mean your parts are cutting faster, it will mean that your tooling will last longer!.

2. CNC machines are very rigid. Nonetheless, they do have limits. Improper tooling or processes can be dangerous or damaging. Changes should be carefully considered to prevent accidents. Take care to safely hold the workpiece. Once you know the material is safely held and the tools are correct and correctly positioned, it is time to consider optimizing the CNC program.

3. After you have carefully considered your setup and selected the proper tooling, it is important to put it all together. Start by using the mid range of the speeds/feeds provided by the tooling manufacture. Then adjust your speed/feed to find the optimum cutting conditions for your job. Keep close and careful record of your adjustments. Record how long the tooling lasts for each speed/feed setting. This information is critical to good tool life management. By optimizing the cutting process, you will see longer tool life and lower overall tooling costs.

Cooling

1. Give proper consideration to coolant. Good coolant can also reduce tooling costs, depending on the operation. Coolant accomplishes three objectives. Lubrication, Cooling, and moving chips away from the cutting process.

2. Lubrication can be critical in keeping cutting tools working properly. With HSS(High Speed Steel) cutting tools it is important to keep them lubricated when cutting metal materials.

3. Cutting processes can produce a lot of heat, so it is also important to keep everything cool. Generally, but not always, you want to keep the tool and work piece cool during cutting. Coolant is used for this and it is important that you have a consistent stream, as coolant turning off and on can “shock” some cutting tools, causing them to break, and shorten tool life.

Re-Sharpen Drills and End Mills

1. Keep tooling sharp and in good condition. If you do not know how, learn to re-sharpen your drills. It is good to know how to sharpen drills by hand, but many shops use special drill grinders, or end mill grinders to re-sharpen tooling. Sharp tools cut faster, give a better finish and leave smaller burrs to remove.