The Ceramic Samurai - PowerPoint PPT Presentation

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http://pergatory.mit.edu/

The Ceramic Samurai

Alex

Ceramic Lathe Concept by

Roger Cortesi

Roger

rcortesi@alum.mit.edu http://pergatory.mit.edu/rcortesi/

Precision Engineering Research Group

Massachusetts Institute of Technology, Mechanical Engineering Department

• Turn Ceramic Parts of up to 15” diameter and 12” long
• Target Part Accuracy of 1 micron (0.00004”)
• Provide a “Dry” option for turning green parts
• Robust with respect to Ceramic Swarf
• Minimal Assembly

• Simplest to Manufacture

• More complicated to manufacture
• Swarf can fall away from bearing surfaces

• A possible design if it is not practical to move the spindle & work piece

A few more variations on these concepts were considered in the 1st pass sketch models

The Dry Machine and Wet Machine have sufficiently different requirements, that they have developed into separate machine concepts.

• Dry Machine
• Much lower accuracy since it will be rough turning parts while they are still green.
• No liquid coolants or lubricants to trap swarf and cause problems in firing the green part.

Roger in the desert and very dry

(just like the dry machine)

• Wet Machine
• A much higher stiffness and cooling requirement since it will turning finish parts to their 1 micron target accuracy.
• Liquid coolants and lubricants are not a problem since the part has already been fired.

Roger underwater and wet

(just like the wet machine)

The Axtrusion is a linear motion concept developed and implemented by Prof. Slocum and Roger Cortesi.

It uses porous air bearings and linear motors to make an easy to assemble, non-contact linear motion system.

Because it is a non-contact air system it will should be very robust with respect to the ceramic swarf

Rough Error Abbe Errors were calculated for this design with the following assumptions:

• Average errors in Prototype Axtrusion used for the work piece and grinder carriage’s pitch, yaw, and roll errors.
• No MAPPING CASE: Error magnitude is the actual error in prototype, 10 mradians (2 arc sec)
• PERFECT MAPPING CASE: Error magnitude is the repeatability of prototype 2.5 mradians (0.5 arc sec)
• No errors in spindles
• Rough machine size based on a 15” dia. by 12” long work piece
• All Magnitude Abbe errors are added for a worst case

• Maximum Total Abbe Error
• With NO Error Mapping
• Radial 13 mm (0.0005”)
• Axial 10 mm (0.0004”)

• Maximum Total Abbe Error
• With PEFECT Error Mapping
• Radial 3.2 mm (0.00013”)
• Axial 2.5 mm (0.0001”)

The previous error estimates are for an Axtrusion with a permanent magnet linear motor. A coreless linear motor would dramatically reduces these error motions further.

• Total RMS Abbe Error
• With NO Error Mapping
• Radial 6.6 mm
• Axial 5.0 mm

• Total RMS Abbe Error
• With PEFECT Error Mapping
• Radial 1.6 mm
• Axial 1.2 mm

• Estimated Total Abbe Error
• With NO Error Mapping
• Radial 9.8 mm
• Axial 7.4 mm

• Estimated Total Abbe Error
• With PEFECT Error Mapping
• Radial 2.4 mm
• Axial 1.8 mm

Hydrobushing Rail

Ballnut and Drive Motor Assembly

Ballscrew

Hydrobushing

• This machine was designed to maximize accuracy at a minimum of cost
• The motive force could come from either a coreless linear motor or a ballscrew.
• The linear guides could be either traditional ballrails or a Hydrobushing
• It should be very easy to assemble and error map

Better dynamic stability due to heavy (and varying) workpiece mass mounted between rails

It is easier to integrate wheel “dressing” station, since grinder is moving orthogonal to its axis.

Better Access to Work Piece

The machine was designed to bring the Center of Motion (COM) of the two axes as close the contact point as possible.

If the COM for an axis can be brought inline with the axis of the spindle (either the grinder or work piece), then all the errors associated with either roll, pitch, or yaw of that carriage will vanish

It is preferred to use the above technique to eliminate error due to carriage roll, since mapping the carriage roll over the length of travel is much more difficult.

The Abbe errors in the X direction for the wet machine are estimated to be about half than the dry machine.

The Abbe errors in the Z direction for the wet machine are about one third that those for the Dry Machine

• The motive force for the two axis can be applied through the COM for each axis. This minimizes errors as the carriages are accelerated.
• The Ballscrew and Ballnut/Drive Motor Assembly is designed to be removed straight of the end of the machine, making it very easy to replace when the swarf kills it.
• A similar implementation could also be used for a coreless linear motor in place of the Ballscrew (eliminating the wear problem)

Ballscrew acting on spindle carriage COM

The wet machine is base on a very simple structure. The two main pieces are shown below. These could be aluminum extrusions, a casing (polymer concrete, cast iron, etc) or pressed aluminum oxide.

A third piece not shown will be needed to join the structure.

Load

Fully Round Hydrobushing

Partially Round Hydrobushing

Sides flex outward, under load conditions,

Fluid gap widens, stiffness lost

Fully round End Support

Hydrobushing Rail

Completely Bottom Supported

Hydrobushing Rail

Because the axis only require a short range of motion the Hydrobushing rails can be completely round.

This means that fully round Hydrobushing can be used. This eliminates the compliance due to a partially round Hydrobushing.

Load

L

Stiffness of an end clamped round beam

• Where does one get the workpiece spindle? (This component will heavily affect the design.)
• Are there suitable, infinite stiffness, and infinite life spindles available?
• Should we build our own with the Hydrobushing.
• Is there a standard grinding spindle that you like to use?
• What the tradeoffs between a linear motor and an easily replaceable ballscrew?
• What is the manufacturing cost goal?

• Aesop Inc.’s self-compensating water hydrostatic bearing technology can be used to enable machines where:
• Bellows can be eliminated
• Bearing maintenance and wear can be virtually eliminated
• Bushings can run on standard hard ground 2” diameter Stainless Steel shafting
• Self-cleaning action can enable a friction drive to be used, thereby also eliminating the ballscrew
• Modular: all features injection moldable or lost-wax castable (using rapid prototyping)

Compensator-to-pocket connection grooves on OD

Upper compensator for lower pocket

“Picture frame” upper pockets for greater damping

Supply groove

Lower compensator for upper pocket

• The RodWay system minimizes the need for precision alignment of bearing ways
• Accommodates change in way parallelism if machine foundation changes
• Machine accuracy is not affected by self-aligning function
• Enables low-cost use of water hydrostatic bearings in rugged industrial environment
• Old US Patent (4,637,738) OMAX can improve on with damping materials and rod-end bushing to create RodWay design and trademark
• Operating principle:
• One hard ground stainless steel round shaft (the fixed shaft) is mounted in a normal manner using supports that can be replicated to structural steel so the shaft remains straight
• The second round shaft (the pivot shaft) is mounted as parallel as is reasonable to the first
• One hydrostatic bearing carriage rides on the fixed shaft, and it is bolted to the machine structure (e.g., bridge structure risers)
• One hydrostatic bearing carriage rides on the pivot shaft, and it is connected to the machine structure (e.g., bridge structure risers) by a spherical bearing
• A steel hemisphere stud mounted in the machine structure and resting in a replicated spherical seat in the bearing carriage, where the center of the hemisphere is a distance H above the round shaft center
• Alignment errors (pitch and yaw) between the round shafts are accommodated by the spherical bearing
• Center distance errors (d) between the round shafts are accommodated by roll (q) of the bearing carriage.
• Vertical error motion (D) of the hemisphere is a second order effect
• Example: d = 0.1”, H = 4”, q = 1.4 degrees, and D = 0.0012”

The slides that follow contain:

• Background information on the Axtrusion
• Performance Data on the Prototype Axtrusion

Not Shown: Position Encoder and Position Encoder Scale

The attractive force between the motor coil and magnets preload the air bearings.

Changing the values of q, ym, and zm the values for Fside, Ftop1, and Ftop2 can all be set independently

Magnet Adjustment Screw

Magnet

2 Rollers

(side surface)

Way side surface

Way top surfaces

3 Rollers

(top surface)

The first Axtrusion ever built! It uses 5 CAM rollers as bearings

Load Applied Here

Vertical Displacement Measured Here

The basic setup for the pitch, yaw, and accuracy measurements. A laser and a variety of optics was used to make each measurement.

The basic setup for the vertical straightness measurement. The probe is suspended above the straight edge on the carriage.

The basic setup for the vertical stiffness measurements The load is applied in the center of the carriage and measurements made above each of the top bearing pads.

Raw Accuracy: 2.44 arc sec

Raw Repeatability: 0.50 arc sec

Period of Variation 29.9 mm

This period is the distance between alternating poles on the magnet track!

Measurements every 10 mm

Carriage Speed of 10 mm/sec

6 passes (3 in each direction)

The way also appears to be slightly curved

(from –0.5 arc seconds to +0.5 arc seconds).

Pitch data was also taken at a carriage speed of 40 mm/s, after incremental movements, and as a function of time. There were no major differences. Please see the appendix for the complete data.

Raw Accuracy: 1.59 arc sec

Raw Repeatability: 0.56 arc sec

Raw Yaw Data

Yaw Data with Linear Slope Removed

Measurements every 10 mm

Carriage Speed of 10 mm/sec

6 passes (3 in each direction)

Period of Variation 30.1 mm

Pitch data was also taken at a carriage speed of 40 mm/s, after incremental movements, and as a function of time. There were no major differences. Please see the appendix for the complete data.

Raw Accuracy: 1.34 micron

Raw Repeatability: 0.33 micron

Raw Linear Accuracy Data

Linear Accuracy with Linear Slope Removed

This is the difference between the actual carriage position and the position that the controller thinks it is in.

Carriage

Carriage errors in pure translation

Abbe error

Carriage

Carriage errors in pitch translation

An Abbe error of 1.2 microns vertical displacement at both edges of the carriage.

Data for displacement of carriage center

Period of Variation is 28.6 mm

Notice that the hourglass shape of the data is due to pitch errors measured away from the center of pitch rotation.

To get better data, the test must be rerun with the probe mounted in the middle of the carriage and the mirror suspended above it.

The data from the center of the carriage shows the pure translation of the carriage to be about 0.3 microns.

Data for displacement of carriage edges.

Two passes are shown for a carriage speed of 10 mm/s

Because the plane mirror could not be leveled perfectly, any linear slope in the data was removed.

Load Applied Here

Vertical Displacement Measured Here

Masses were place on the top center of the carriage and the displacement above the four (4) top air bearings was measured.