Drawbar (machine tool)

A drawbar or spindle drawbar is a clamping mechanism for toolholders on machine tools. The toolholder or machine taper itself is held by the draw bar and applies force to the spindle, especially when spinning at low speeds.

The drawbar pull is an important component that allows the milling machine to keep the tool in place while it’s being used. The force from the drawbar pull supplied to the tool has to be just right because not enough force would cause the tool to wobble leading to inaccuracy, and too much force would apply a lot of stress leading to shorter tool life. Regularly checking the drawbar pull using a drawbar force gauge would ensure that the spindle and tool stay in tune so that your machine stays running efficiently.^[1]

Choosing the right material for a drawbar is important because the milling machine needs high durability. Some commonly used materials are High carbon steel, Stress-proof steel, Heat-treated alloy steel, and Black oxide steel.  

• High Carbon Steel- Used for its strength and durability, this material can survive through the wear and tear applied by the drawbar.

• Stress Proof Steel- Known to be more adaptable under high stress and corrosion resistant this material is a reliable option for a drawbar.

• Heat Treated Alloy Steel- Providing a drawbar with more strength by being resistant and flexible this material is used in many milling machines within the industry.

• Black Oxide Steel- When looking for a drawbar material that adds an extra layer of protection this material is the one because this steel has a coating of black oxide that provides a thicker more resistant aspect that slows corrosion.^[2]

Threaded drawbars are less common in milling machines but still get the job done. This drawbar has a more hands-on approach because it requires the operator to use a wrench and hammer to perform some twisting and tapping so the cutting tool can be removed.

Power drawbars are starting to become more popular in milling machines because they are a fully automated version of the drawbar. The way it works is by using compressed air to tighten and loosen the spindle giving the operator an easy way to change tools.^[2]

Choosing the right drawbar is important when setting up a milling machine. The right drawbar can be the difference between a good or bad milling machine by affecting its lifespan and efficiency. When choosing a drawbar things to consider are how it fits in the machine, the material of the drawbar, finding a drawbar that is easy to install, and making sure that the drawbar is easy to maintain.

• Fit- Making sure that a drawbar fits properly in the milling machine is always important because a poor fit will damage the drawbar and how the machine performs.

• Material- A selection of drawbar materials can be used for different operations, so picking the drawbar material for the end goal of the milling machine is more than necessary.

• Install and Maintenance- Installing a drawbar needs to be efficient and the maintenance on the drawbar needs to be easy to understand and done often to keep the milling machine in order.^[1]

• Drawbar Force Gauge
• Milling machine
• Machine tools

• Don’t Forget The Drawbar, Modern Machine Shop magazine, March 2006, By Peter Zelinski

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