Short on Vibrations of Boring Bars

It is well known that vibrations will result in a poor surface texture. It will also cause the cutting edge to wear out too fast - thus reducing production. There are several reasons and the most frequent are:

• Inferior mounting of the boring bar and other tools in often worn tool holders. The diameter always differ between tool and tool holder due to the difference in tolerance between h and H. The boring bar will rest against the holder along a line opposite the screws. Vibrations will start at the utmost screw about 12 mm from the end of the tool holder. Moreover, the difference in tolerance will intensify these vibrations. Thus, the boring bar will vibrate inside the tool holder as well as outside and at very high cutting forces the boring bar will move to a new position resulting in an incorrect cutting angle.
   
• Poor machine stability - there might be a play when using the VDI-system
   
• Incorrect cutting angle
   
• Too high a feeding rate and, consequently, too high a cutting pressure

In order to better understand why a boring bar will vibrate even at the presently best available clamping methods you should be familiar with the formula for the deflection of a bar with a one side fastening - as with a boring bar mounted in a tool holder. The formula is:

 

F x L3 ------------- E x l x 3

Where F = cutting force in kp. L = overhang in mm E = module of elasticity; 21,000 for steel and 63,000 for cemented carbide l = moment of inertia 0.049 x d4, with d = the diameter of the tool

With a distance of 12 mm to the first screw and an overhang of 30 mm corresponding to 2.5 x the boring bar diameter 12 mm and a cutting force of 150 kp on the insert and a factor of 1.10 for a worn tool holder, you will get for a steel boring bar:

150 x 42 x 42 x 42 x 1.10 ------------------------------------------------ 21,000 x 0.049 x 12 x 12 x 12 x 12 x 3

=

12,224,520 ------------ 64,012,032

= 0.19 mm    deflection

The corresponding calculation with a hydraulic bushing type HydroFix that locks against the boring bar as well as against the holder, you will get

With HydroFix the overhang can be increased to about 75 mm if a deflection of 0.19 mm will be accepted.

The above formula along with extensive production experience makes it clear that the best available method to secure a very good fit of a boring bar is to use HydroFix.

The lifetime of the cutting edge will increase.

HydroFix is available from shelf up to 100 mm inner diameter

 

Libell/Spirit Level with a strong magnet

Most NC operated turret lathes have a slide tilting 30 to 45 degrees. See illustration showing a machine with two turrets (left).

When the lathe is in operation the turret will move along its slide. Thus, if the tilt angle is 40 degrees, then the milled down reference surface of the boring bar must have this tilt angle.

The HydroFix hydraulic bushing, with or without a collet, does not have any reference surface. To overcome this a quick and reliable method has been developed with a simple adjustable spirit level with a strong magnet (right).

This is what you have to do:

• Calibrate your Libell through placing it on the surface of the slide

• Turn the Libell housing to align the air bubble within the two markings

• Lock the Libell housing with its four screws. Now, the housing cannot be unvoluntary altered. The Libell has now been adjusted to a particular machine

• Fasten the Libell on the milled down reference surface of the boring bar

• Turn the boring bar to align the air bubble within the two markings

• Lock the boring bar by tightening the pressure setting screw in the flange of the HydroFix and, finally,

• Check with the help of the Libell

If the boring bar should lack a reference surface - check the cutting angle in the manufacturer's catalogue and adjust the Libell housing with this cutting angle. Do like this:

• Attach the magnet direct on the insert and turn the boring bar to align the air bubble within the two markings and follow the above procedure.

HydroFix S: A method to preassemble HydroFix, a standard tool and a collet. 

The S-types which are available with id 12, 16, 20, 25, 32 and 42 mm have been developed to bring down the time required to change any tooling with shaft-diameters from 3 to 40 mm.

– outside the machine and with the help of a fixture with a location pin you assemble the fixture with for example HydroFix NSC/NSE-20/25-46 plus a tool plus a collet

– you pressurize the HydroFix and the above three parts will become one unit

– then you depressurize slightly the HydroFix but not more than you can take away the unit from the fixture - i.e the HydroFix with tool and collet

– the unit is then placed in a ”wait position” and is ready for use

– the unit is mounted in the holder which has to have a similar location pin

– pressurize fully, i.e. with the piston brought to a full stop

For all S-types, see the products page.