by Mary Scianna
Toolholder considerations for drilling deep holes 10 to 30 times the hole diameter
Toolholders provide the link between the machine tool spindle and the cutting tool. When you’re machining holes deeper than ten to 30 times the hole diameter, toolholder-spindle connections become crucial.
“Toolholders are often overlooked in the cutting tool story and with deep hole drilling you need a good rigid interface between the machine and the drill, and that can sometimes be a problem to achieve,” says Brendt Holden, president of Haimer USA, Villa Park, IL.
The biggest problem with deep hole drilling? Runout. Runout occurs when the tool or the part is being rotated off centre and doesn’t correspond with the main axis, known as a radial runout. If the tool or the part is at an angle to the axis, then it’s known as axial runout.
“People don’t realize that runout problems are glorified at 10 to 20 times the diameter,” says Mike Roy, general manager of Schunk Intec Corp., Mississauga, ON. “If you’re drilling 12 times the diameter, you could have zero runout where the tool is being clamped, but at the tip of the tool you can see extensive runout. Selecting the proper toolholder will allow you to achieve the best runout accuracy possible.”
Cory Cetkovic, application engineer and product manager for the Sphinx line of tooling at BIG Kaiser, Hoffman Estates, IL, advises manufacturers select tooholders that “emphasize accuracy and balance. We publish runout accuracies of our holders to help customers make the right choice.”
To overcome runout, Holden suggests shrinkfit type holders.
“Shrinkfit toolholders are very rigid so they’re able to keep runout at the tip to a minimum. With traditional style ER collet chucks, you introduce operator variables: did the operator over-tighten the tool, did he clean out the collet properly? If the toolholder fails, it is a potential source of runout.”
Hydraulic chucks are an option too, adds Holden, “but they have some maintenance issues, which you don’t get with shrinkfit holders; they’re right all the time.”
One reason why someone might choose a shrinkfit toolholder over milling chucks or hydraulic chucks is that their rigidity ensures that the tooling systems remains balanced during deep hole drilling operations.
Balance, say suppliers, is critical in deep hole drilling, particularly when you’re dealing with special deep hole drilling tools in which the holders and tools are one body.
“Especially in deep reach applications, even at slow rpms, if you’re off balance even a bit, you can scrap out a part. So in cases like this, it’s important to use balancing machines to balance toolholder assemblies,” says Haimer’s Brendt Holden.
Assess toolholder component quality
No matter what type of drill you’re using for your deep holes, if you don’t have a high quality toolholder, you will compromise your deep hole drilling process.
Tight taper tolerances, for example, are critical.
“Making sure runout accuracy is repeatable and consistent requires a high precision toolholder that meets precision specs, such as grinding shanks to an H6 tolerance,” says Schunk’s Mike Roy. “That’s because tools have to fit the exact size of the toolholder and if it doesn’t, you’re not going to get the grip force and accuracy required for your deep hole drilling application.”
With collet chuck type holders, you should consider smaller collet angles for tighter tolerance runout, say suppliers. Also, retention knobs or pull studs, a replaceable part of the toolholder, should be of high quality as they’ll resist wear and deformation better, says BIG Kaiser’s Cory Cetkovic.
You should also consider the design of your toolholder, advises Schunk’s Mike Roy.
“First thing you should look at is accessibility. Is the nose diameter in clearance? Typically, the nose diameter of the toolholder has to be long and slender. You also need to consider flexibility. How often do you need to change the tools? For example, with our Tendo toolholders, there’s no peripheral equipment for tool changes. We have a manual hex key and a full range of intermediate sleeves. So you can purchase a 20 mm holder and you can clamp a full range of different clamp diameter sizes; there’s no obligation to have a specific holder for every size. With other systems, you need a hydraulic pump or heat shrink machine.”
Selecting the right toolholder
When it comes to selecting a toolholder for your deep hole drilling needs, you have three choices: hydrualic, collet and shrinkfit type holders. Each has its place, says BIG Kaiser’s Cory Cetkovic. In deep hole drilling for holes 20 times the diameter and deeper however, he suggests high precision collet holders and hydraulic holders.
Collet holders are versatile; an ER type holder can clamp 2 to 10 mm just by interchanging the collets, but the drawback is that they’re not capable of handling high coolant pressures beyond 1,500 psi. “In long reach applications, you may require coolant pressures of 2000 psi, so that limits you to a hydraulic or shrinkfit type toolholder,” says Cetkovic.
Another issue with collet type holders, according to Cetkovic, is how they’re made. Since they are an assembly of components versus a solid body, single piece construction of hydraulic or shrinkfit holders, they are typically less rigid and will “introduce more variables, which does lead to a risk of not being able to control runout. As well, maintaining a good balance level with this holder can be more difficult.”
Despite some issues, collet holders can be a good choice for deep hole drilling because of the design and angle of the taper, adds Cetkovic.
“The shallower taper angle of the collet will increase the gripping strength on the cutting tool, so using a collet with a smaller taper angle will give you a higher gripping strength.”
The biggest advantage of a hydraulic holder is the simplicity of clamping of the cutter in the holder. It offers ease of operation, says Cetkovic.
Ultimately, say suppliers, the best toolholder for deep hole drilling applications will depend on the machining environment.
If you’re a shop drilling only a handful of deep holes periodically, a collet holder may be your best choice. If you’re drilling deep holes consistently hydrualic or shirinkfit holders may be more suitable.
One company that selected shrinkfit technology is Baker Hughes INTEQ, an oilfield evaluation service company that manufactures instruments for challenging drilling situations, such as deep sea. The 4 to 6 m long tools for deep hole drilling rods are placed behind the drilling head and deliver information about the local conditions, while simultaneously steering the drilling head.
The company uses toolholders and extensions with Haimer shrinkfit technology, which produces runout accuracy of three microns. Haimer’s system is based on what it calls New Generation (NG) coil technology, which the company says provides high speed shrinking in and out. An adjustable coil adapts to the length and diameter of the chuck so only the clamping area is heated, shortening shrinking and cooling time, important factors for Baker Hughes, which handles 60 to 80 shrinking processes a day. With its success on drilling applications, the company is now using shrinkfit for its milling applications too. SMT