With over 80 years of service, Bridgeport is a classic machine that's dearest to the manufacturing industry.
It's a popular machine among machinists. Every professional machinist would have come across a Bridgeport mill at least once in their career.
This article discusses how a Bridgeport mill works including its parts, versions, and applications.
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Bridgeport Mill: How does it Work?
A Bridgeport mill is a powerful multipurpose knee-type vertical milling machine that is capable of performing various operations like drilling, milling, slotting, tapping, reaming, shaping, etc., which otherwise require multiple machines. It is a manual mill that has been dominating the machining industry for over 80 years.
Why is it called Bridgeport?
The name Bridgeport originated from a series of vertical milling machines released by Bridgeport Machines, Inc., located in Bridgeport, Connecticut.
These machines were so popular among machinists in the 1950s that they even started calling every knee-type vertical milling machine "Bridgeport."
Later Bridgeport was acquired by Hardinge, and they started making different variants of the Bridgeport mill, including CNC-integrated models.
Machines Under Bridgeport
Bridgeport Series I:
Series I is the introductory machine of Bridgeport. It's also their most favored machine because its average size is equally suited for small and large shops.
This original design dates back to 1938. Interestingly, even today, Hardinge is selling this model of Bridgeport. You can find one on their website.
|Parameter||Bridgeport Series I|
|Work area||31" x 12" x 16"|
|Footprint||7 x 10 ft.|
|Table size||49" x 9"|
|Throat distance||6.75" - 18.75"|
|Max workpiece Load||750 lbs|
|Speed||500 - 4200 RPM|
|Overall weight||2,075 lbs|
Bridgeport Series II:
Series II was made for heavy metal working jobs allowing it to process large metal pieces faster.
|Parameter||Bridgeport Series II|
|Work area||31" x 12" x 16"|
|Footprint||9 x 5 ft.|
|Table size||58" x 11"|
|Throat distance||10.5" - 26.5"|
|Max workpiece Load||750 lbs|
|Speed||50 - 3500 RPM|
|Overall weight||5,000 lbs|
The original mills under series I and II are manually operated and work with similar toolings.
You can also retrofit these machines with accessories like a power drive, variable frequency drive (VFD), digital read-out, etc.
The newer models of Bridgeport made by Hardinge have CNC (Computer Numerical Control) capability, meaning they can automatically drive the machine transmission with the help of program codes.
V-Series, GX-Series, and XR-Series are the newer models of Bridgeport mill. Unlike Series I and II, these machines have an enclosed build and have much more advanced features.
Parts and Features of a Bridgeport Mill
The main casting on the Bridgeport mill, called its column, supports all the machine components with a stable base.
It's the heaviest single-piece part on a Bridgeport mill, and it alone weighs over 800 lbs.
The saddle and table components sit on the knee of the Bridgeport mill. You can move the knee up and down by turning the elevating screw using the vertical feed crank.
It moves along a dovetail alignment, providing it with a stable and precise movement.
Once you move the knee to the desired location, you can lock it in place using the locking lever. This prevents it from vibrating during the machining process.
Like the knee, the saddle is also driven by a screw, and a dovetail alignment supports it.
The sliding surface of the knee that touches the saddle is hand-scraped. This creates little pockets on its surface, allowing the lubricants to stay in place much longer.
It is also a good indicator you can look at when purchasing a used Bridgeport mill. If these hand-scrapped pockets have worn out, it's a clear indicator that the machine is overused.
You can move the saddle along the Y-axis by turning the cross-feed crank and use the locking lever to fix it at the desired position on the bed.
The table sits on top of the saddle. To move the table along the X-axis, you can turn either of the two handwheels on both sides of the table.
Like every other moving part of the machine, it also has a locking lever.
The main casting of the machine connects with the ram through a turret. You can rotate the turret by loosening up a series of nuts.
It helps the cutting head reach locations on the worktable that are otherwise hard to reach.
Some machines have a protractor marking on the turret that allows you to precisely position the turret along the worktable.
Because of the marking, you can repeat the same movement and zero it easily. So if you are planning to buy a Bridgeport mill, check for this feature.
The ram mechanism of the machine is locked into the dovetail assembly on top of the turret.
You can move it in and out through the dovetail assembly by turning a crank that moves the ram using a rack and pinion mechanism.
The back of the ram consists of a large hole along the turret setup.
Using this, you can fit additional attachments to the machine. Since it's on a turret, you can turn them 180 degrees left or right.
Bridgeport mills are available with different heads. It houses the spindle and the quill.
The original design of Bridgeport had a C Head, but J Head is by far the most used head on a Bridgeport.
The spindle speed range will vary depending on the type of head and motor attached to the machine. Some even have back gears that reverse the spindle rotation.
You can turn the head at an angle of 360 degrees left and right, and also 45 degrees up and down.
The tilt joints will have protractors. You can use it as a reference to precisely tilt the head at any desired angle.
The quill on a Bridgeport mill allows it to act as a drill press by moving the spindle up and down using the quill feed handle.
For boring (machining) operations, you can unlock the quill handwheel to raise and lower the quill slowly.
There is also a knurled knob above the handwheel. It allows you to set automatic forward and reverse feed for the quill to perform smooth boring cycles.
You can set the depth stops on the quill's feed screw to automatically reverse the quill till it reaches a position.
Oilers are placed on different parts of the machine. There is a one-shot oiler system for the column that pushes oil to all the other parts located on the main casting using a series of internal tubes.
Similarly, all other moving parts of the machine have oiling holes for fast lubrication.
You can upgrade Bridgeport Mill with motorized transmission systems (also known as power feed). It allows you to move the axes faster than manually turning the feed crank.
You can also retrofit it with digital readouts to display the position and speeds of the mill.
Applications of Bridgeport Mills
Bridgeport mills are versatile machines. Its turret, ram, tiltable head, quill, knee, and gear systems, allows you to machine almost anything.
Milling, facing, drilling, tapping, boring, reaming, etc., are some of the regular applications of a Bridgeport mill.
You can creatively use the capabilities of Bridgeport to machine projects at different angles.
Since the release of Bridgeport mills, Bridgeport has become a universal brand in the machining world.
In the 1950s, when Bridgeport was at its peak popularity, many manufacturers tried to copy the design of the original Bridgeport mill.
Though some of them made low-cost alternatives to the Bridgeport mill, none were able to deliver similar quality.
Even today, the original Bridgeport mills have good demand. Newer ones can cost around $22,000, but used machines can be had for under $10,000.
Frequently Asked Questions
Are Bridgeport mills still made?
Yes, Bridgeport mills are still made. They are currently made by the brand Hardinge. They've been making the Bridgeport series of mills since 2004 when they acquired Bridgeport.
What is the difference between a knee mill and a bed mill?
The difference between a knee mill and a bed mill lies in the Z-axis setup. In the case of a knee mill, its work table moves along the Z-axis, whereas on a bed mill, the spindle moves up/down along the Z-axis.
How does a milling machine quill work?
The quill on a milling machine allows you to move the spindle assembly up or down using a quill lever on the mill's head. It helps the mill act like a drill press to perform tapping and drilling operations.
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