SMALL DIAMETER TUBE BRANCHING
By William H. Holyoak,
President, T-Drill Industries, Inc.
, GA, USA
THE HOLE STORY (Small Diameter Tube Branching )
The tube and pipe fabrication industry, like most others, is facing new challenges to provide a product or service of higher quality than those supplied in the past, but still meet today's ecological and safety standards. One facet of tube fabrication is branching - the process to obtain an alternate flow path in a system.
Up until recent times the plumbing industry, and others utilized purchased fittings. Industries such as automotive or HVAC used saddle adapters that can be formed on the mating tube or purchased like a fitting. In larger pipe applications for the chemical, food and other industries, the connections were made by use of a nozzle weld or ball pull (extrusion process).
In order to remain competitive and comply with the new regulations, industry has had to utilize available alternatives. This paper will address the basic process reviewing their merits and limitations to help you decide the best process for your specific application.
A branch outlet is a right angle (or smaller) connection made in tube to provide a means of distributing a system flow to more than one point.
We can categorize the types of connections into three basic systems:
These are either fittings, saddle connectors or
weld-o-lets depending on the application.
A hole is produced in the tube or pipe and the
mating branch tube is inserted into the hole and
brazed or welded. Sometimes the mating tube
utilizes a flange or saddle shape to obtain more
A flare, flange or collar, as they are commonly
called, is formed on the main tube. The branch
tube is either inserted for a lap joint or butt
welded to a faced collar.
Reviewing the methods, this paper will provide a better understanding of the application of each process.
The typical tee fitting is one of the most common methods of obtaining a branch connection in all but manifold applications. This method is ideal for low volume applications, since it provides a quality joint designed to be equal to or stronger than the tube into which it is being inserted.
While it is a popular approach the apparent advantages are far outweighed by the other considerations.
- Does not require special machine tools - only a brazing capability
- Quality connection
As stated above, the fitting provides a short run solution. However, higher production volumes warrant investigation of alternative methods, since the limitations of this method, can significantly increase your operating costs.
- Requires inventorying and in-process control
- Odd sizes used by the HVAC and other industries are
not usually available for short delivery. This
increases their cost
- Connection requires three (3) joints with the
applicable brazing times and potential for leaks
- Requires up to 50% more braze material than other
- Creates a rigid member that does not have the
vibration strength of other systems.
- Requires multiple tube cutting operations and
Similarly a purchased saddle connector which is a fabricated form of a fitting provides similar results with one less connection. It does eliminate the multiple operations such as tube cutting, 2 brazed joints and is less expensive that most purchased fittings - in fact most companies produce their own on the branch tubes in-house. While it does reduce some operations it requires the production of a hole in the tube with it's limitations as described in the subsequent section.
Most companies that produce a moderate volume of branch connections, particularly when manifolds are a consideration, develop their own in-house production systems. Usually this takes the simplest form of production method, the producing of a hole in the main tube in which a branch tube is inserted and brazed in place.
The hole can be produced by either drilling or punching..the punching can be with or without an internal mandrel for support.
This approach usually utilizes an available (in-house) drill press that is fixtured for the part. The fixture is moved around to obtain multiple holes in a given part. If there is a volume application with multiple holes in the same part - such as a manifold - the part can be set-up on a multiple spindle drill press to minimize the part cycle time.
The main considerations are:
- The drill core flute design. The most successful
seems to be a sheet metal configuration to minimize
swath and burrs.
- Drill speeds and feed rates. It is most important to let the drill core cut the metal which means the feed must be at a rate in keeping with the drills cutting capability.
- Drilling speeds vary with the metal, typical values:
Copper 1000 - 1500 rpm,
Steel - approx. 500 rpm,
Stainless Steel - 375 rpm.
Note: these will be dependent on hole diameter and
- Tool life has been improved by applying coatings for
Copper - flash chrome plate,
Stainless - TiNi coating,
Steel - TiNi (but not as much as advantage as
There are a wide variety of coatings available, it
would be advantageous to contact one of the coating
suppliers for recommendations.
- Lubricants. Lubricant is essential to
long tool life. For copper, aluminium and mild steel applications
there are now available mist lubrication systems that uses biological
oils. This is a big improvement because such oils are evaporative and
requires no washing.
An alternate method of achieving the same hole is to punch it in the tube. This method usually requires a specialized machine. However, many companies have achieved the desired results using a standard press.
The advantage of punching is important particularly in today's market. It requires minimum or no lubrication and burrs can be almost eliminated if the tooling is maintained.
The principal considerations in punching are:
- Punch to die (mandrel) clearance usually is 10% of
the metal thickness; for harder metals such as
stainless steel, 15% is used (clearance is the
difference between punch to die diameter)
- Punch alignment to die (mandrel)
- Support of the tube during punching.
For the best results the use of a mandrel (internal die) is advised to assure a clean, burr free hole with minimum tube distortion. Use of a mandrel is nominally limited to tubes with a 5/8" or larger ID.
Typical production, as discussed earlier, can utilize a horn die or a press for single hole applications.
Many companies have multiple outlet applications requiring indexing of the part to obtain the desired outlet pattern.
There are machines available for this work and they incorporate all the essential items for processing the tube including interchangeable mandrels, an integral punching unit and/or indexing headstock to hold the tube with an axial and radical positioning capability.
The merits and process limitations are:
- Relatively inexpensive machine - sometimes available
within the company.
- Can be multiple tooled
- Requires no or minimum lubricant (Punching)
- Fast cycle times
- Joint has minimum strength due to one metal thickness
of contact between branch and main tube
- Can result in intrusion of branch tube in main tube
- Requires 35-40% more braze material to obtain a level
of joint strength
- Can not be used with a mandrel on tubes less than
- May require internal deburring of the tube
- The work area must be accessible (straight) to allow
the mandrel to be inserted
- Can not be used for punched holes above 60% of the
main tube diameter
Approximately 30 years ago the extruded outlet was adapted for use on small diameter tubes (it had been used on large pipe for over 80 years).
The extruded outlet provides a means of obtaining the benefits of the fitting, without the cost.
A hole is produced in the tube, then the material around that pilot hole is formed outward to produce a collar around the hole. This collar provides the support in a lap joint or a butt weld connection when faced.
There are two methods of forming the extruded outlet: An internal method, and externally.
INTERNAL PUNCH FORMING
This method utilizes a mandrel mounted punch assembly to punch form the collar. In some systems, the collar is free formed resulting in a conical shaped extrusion. While internal punching is an improvement over just using a hole, providing more joint strength, it is still not up to the AWS standards which specify a 3X metal thickness contact area for maximum strength. This standard can be met by forming the outlet into a die (clamp).
Similar to the hole punch system described earlier, the internal punch system is limited to 5/8" or larger ID tubing and must be used on a straight tube to allow accessibility of the mandrel. In addition, this system can provide outlets only up to 60% of the main tube diameter.
Typical systems are either home-made or provided commercially, such as the unit described below.
A tube is inserted onto the mandrel against the stops and
after each punch extrusion, the tube is moved to the subsequent stop for like operations. The internal punch is actuated by a pneumatic or hydraulic actuator.
EXTERNAL SPIN FORMING
To overcome the limitations of internal systems, a process was developed over 30 years age to form an extruded outlet from outside the tube. The advantage of this system is the tube does not have to be straight and the outlet can be size on size.
The tool rotates, driven by a special machine that feeds the tool at a controlled rate for drilling a pilot hole. Then when the tool is at bottom dead center, forming pins are extended into the tube. As the tool is extracted the forming pins spin and form the collar.
Extruded outlets offer many of the advantages of a fitting, but like any process you have to evaluate all the considerations.
- Maximum joint strength (provides 3X the metal
thickness in the contact area when a spin form or a
punch form with external die is used)
- Reduced braze material. (Copper clearances .004"-.010". Steel clearance, a press fit)
- Collar acts a fixture (Collar is holding the branch tube during brazing)
- Can get a size on size outlet (external spin forming only)
- Fast cycle times
- Can be faced for butt welding
- Minimum number of joints (1 Lap joint - with TEE fittings there are 3 joints)
- Minimum or no lubricants required (punch form)
- Can be made on bent tubes (external spin forming)
- Optimum flow characteristics
- Limited to straight tube sections (punch form only - with external spin also bent tubes can be collared)
- Limited to 5/8" or larger ID tubes (punch form only - with external spin the minimum collar I.D. is1/4")
- Limited to outlets 60% of the main tube diameter (punch form only - with external spin also 1:1 joint can be done)
- Requires lubricant (spin form)
Each of the methods used for branching has its benefits and limitations. Therefore it's essential that you properly apply the process to your specific requirements. This procedure is not as simple as it may appear since your production may be made of a variety of applications that do not fit a single branching technique. Usually it isn't practical to have a variety of processes in your facility to cover each requirement, so a compromise is essential. The following is a summary in broad terms that can be used to make the initial decision, the final choice requires an in depth analysis of all your needs to properly choose the equipment best suited for your company.
In a similar manner to the process selection, the equipment can be applicated to establish to the most effective configuration.
You should consider an automatic machine if your requirements include one or more of the following:
- A wide variety of applications produced in short runs
where set-up time is a consideration
- Limited to 5/8" or larger ID tubes (punch form only)
- A high volume application, where more than one
machine can be run by the operator
- A multiple operation that can be combined into one
machine to eliminate interim handling
- An operation that requires precision positioning that
is not possible with manual headings.
For more information, please contact :
William Holyoak, President, T-DRILL Industries Inc.
Norrcross, GA, USA
Tel: (+1) 770-925-0520
Toll free. (+1) 800-554-2730
Fax. (+1) 770-925-3912