property of roger cortesi, mit precision engineering research group. do not copy or transmit without...

Property of Roger Cortesi, MIT Precision Engineering Research Group. DO NOT COPY or TRANSMIT without written permission.

The Ceramic Samurai

Ceramic Lathe Concepts by

Roger Cortesi Alex

Roger

Precision Engineering Research GroupMassachusetts Institute of Technology, Mechanical Engineering Department

Phone: (518) 248-6923Fax: (703) 991-5353

http://pergatory.mit.edu/

Room 3-47077 Massachusetts Ave.Cambridge, MA 02139

[email protected] http://pergatory.mit.edu/rcortesi/


There are many unanswered questions.

Close collaboration with marketing and the customer will be required

What follows are our ideas. These could be used as a starting point…


When designing a machine to grind ceramics there are two (2) major types.

Machines that grind parts that HAVE been fired.

and

Machines that grind parts that HAVE NOT been fired


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.

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)

Roger in the desert and very dry(just like the dry machine)


We have ideas for both…

However, our instinct is that there is a larger market for machines that grind the finished part.

So we will stay focused on the wet machine for now.


Basic Machine Requirements(this is our guess as to what the customer wants)

• Turn Ceramic Parts of up to 15” diameter and 12” long• Turn Ceramic Parts with a Maximum Weight of 100 kg• Turn Inside and Outside Diameters• Target Part Accuracy of 1 micron (0.00004”)• Target Cost of LESS THAN $50,000• Robust with respect to Ceramic Swarf• Minimal Assembly• Last ?????? Cycles (5? Years)


Init

ial E

rror

Bud

get

Errorrms 0.923 mErrorrms MEwps2

MEwpc2 MEtps

2 MEtpc2 CEenc

2 TEstruc2

Errorsum 1.65 mErrorsum MEwps MEwpc MEtps MEtpc CEenc TEstruc

Total Machine Error:

Tallowed 0.205 KTallowedTEstruc

L

length of the machine structure we are concerned withL 12in

coefficient of thermal expantion for aluminum oxide 8m

m K

From this value we are able to determine the allowable change in machine temperature during machining operations.

maximum allowed thermal error.TEstruc 0.5m

Thermal Errors: These errors are due to the thermal contraction and expansion of the structure.

CEenc 0.75 mCEenc 3 resminControl errors are approximately 2 to 3 times encoder resolution.

minimum encoder resolutionresmin 0.25m

Control Errors: These errors are due mainly to the errors in the position encoders.

motion error in toolpiece carriage. This is assumed to be the amount of errors than can't be removed by mapping the controller.

MEtpc 0.1m

non-circularity motion error in toolpiece spindleMEtps 0.1m

motion error in workpeice carriage. This is assumed to be the amount of errors than can't be removed by mapping the controller.

MEwpc 0.1m

non-circularity motion error in workpiece spindleMEwps 0.1m

Motion Errors: These include errors in the bearings, straightness, and non-circularity of part/tool motion


Concepts Considered for the Wet Machine


Initial Double L Concept

Con’s•Low Natural Frequency•Potentially Very Heavy Structure•Alignment of Both Spindles w/ axis at

COM is tricky but possible

Pro’s•Excellent Access to Part and Spindles•Easy to Offer Multiple Configurations•Simple Structure


Double “C” Concept

Con’s•Lower Natural Frequency•Potentially Very Heavy Structure•Alignment of Both Spindles w/ axis at COM is NOT possible

Pro’s•Excellent Access to Part and Spindles•Easy to Offer Multiple Configurations•Simple Structure


Alternate Double L Concept

Con’s•Low Natural Frequency•Potentially Very Heavy Structure

Pro’s•Excellent Access to Part and Spindles•Easy to Offer Multiple Configurations•Simple Structure•Alignment of Both Spindles w/ axis at COM is possible


“C” on Pipe Concept

Con’s•Low Natural Frequency•Potentially Very Heavy Structure•Complicated Structure

Pro’s•Excellent Access to Part and Spindles•Easy to Offer Multiple Configurations•Alignment of Both Spindles w/ axis at COM is possible


The Pipe Lath Concept

It is difficult to get the 1st Non-Solid Body Mode greater than 200 Hz with the Double L structure.

This spawned the Pipe Lathe Concept

The minimal pipe lathe structure (left) showing the workpiece pipe and tool carriage half-pipe. The pipe lathe with stiffening members and support plates (right). Workpiece carriage motion

shown in blue-dashed arrow. Tool carriage motion shown in red-solid arrow.


More Images of the Pipe Lathe

Grinding Wheel

Workpiece

Tool CarriageHydrobushing Rails

Workpiece Carriage Inside Pipe (not shown)

This finger necessary to allow grinding the inside surface


Components


Major Component Costs

Major Component Cost Estimates

Ballscrew Hardware 1,000$ per axisLinear Encoder 1,000$ per axisMotors 500$ per axisMotor Amp 500$ per axis

Per Axis Sub Total 3,000$

Hydrostatic Pump 3,000$ Grinding Spindle 8,000$ Workpiece Spindle 12,000$ CNC Controler 4,000$ Thermal Management Equipment 1,000$

Major Component Sub Total 34,000$

The machine structure is NOT included in these estimates


Linear Guide SelectionSliding Bearings

•Cheapest•Ruined by Swarf•Standard Replacement Part

Rolling Element Trucks and Rails•Ruined by Swarf•High Stiffness•Standard Replacement Part

Aerostatic Bearings•Non affected by swarf•Low stiffness stiffness•Affected by coolant•Difficult to Replace (once installed)

Hydrostatic Bearings•Unaffected by swarf•Very High Stiffness•Not affected by coolant

Star Linear™ Steel Truck

Newway™ Aerostatic Bearing Pad

Hydrobushing™

Pacific Bearing™ Sliding Bearing


Actuators

The machine should give the customer the option of being driven by Ball Screws or Linear Motors.

The customer should be able to switch between them once the machine has been purchased.

For example: If the ball screw is not meeting the customer’s accuracy requirement, then they could upgrade to the linear motor.


Actuator Selection

An open face linear motorTypical ballscrew assembly

Ballscrew

•Cheap ($700 per axis w/o motor & encoder)

•Will be destroyed by ceramic swarf, therefore must be super easy to replace.

•Allows the use of cheap rotary position encoders

Linear Motor

•More expensive ($2000 to $6000 per axis)

•Will not be destroyed by ceramic swarf

•May have to be protected against water

•Requires use of linear position encoder

The image below is of an open face linear motor. These have a lot of cogging so they would not be used in the ultra precise version of the machine, rather a “closed face” linear motor, which has the coils run between a part of magnet tracks to eliminate the cogging.


Encoder Selection

• Must not be affected by coolant or swarf• A Rotary encoder on ballscrew allows high

resolution and low cost• linear position encoder is needed if linear

motors are used, more expensive• We need more cost data• We need estimates on how much variation

in ballscrew length due to force loading and thermal effects (hollow ballscrew?)


Linear EncoderA rotary position encoder on the ball screw will NOT detect expansion in the ball screw. Internal cooling of the ballscrew may be needed to minimize the thermal expansion of the ballscrew.

A linear encoder on the carriage with the scale on a Super Nilvar plate will detect and correct changes due expansion of the ballscrew. This means that an internally cooled ballscrew is not needed.

Star Linear™ Integrated Measuring System


Minimizing Motion Errors

•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 from 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 Importance of Error Mapping

The machine MUST be designed for easy error mapping.

Especially if ballscrews or contact bearings are used. (since these will have to be replaced and the machine remapped)

Pitch and Yaw Mapping are easy

Straightness and Roll Mapping require the use of a straightedge and take more setup and work.

Mapping Fixturing MUST be included in initial design.

Mapping yaw errors on the Axtrusion required minimal fixturing.

Mapping vertical straightness errors on the Axtrusion required more fixturing (cap. Probe and

straightedge).


Spindle Questions

I have about a million spindle question that I need help with before the design can go forward


Thermal Management

The Thermal Errors is the largest source of error in the error budget.

The method of thermal management will affect the following subsystems:

•Base Structure•Actuator selection and design•Position Encoders

Techniques for thermal management include:•Thermally Centered Base•Aluminum Oxide Base (same as part)•Internally Cooled Base•Internally Cooled Ballscrew•Linear Position Encoders (measuring actual carriage pos.)


Error Analysis


The Double L Machine Abbe Errors

Lathe #3

Contact PointX, Y , & Z Axis CO M

Other Benefits

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.

Abbe Errors are Eliminated in: Work Carriage Roll & Tool Carriage Pitch(Because the axis of error motion is collinear with axis of workpiece and tool rotation.)

Tool Carriage Roll is in a Non-sensitive Direction

Minimizing the Effects of carriage roll allows for faster mapping of motion errors.

Yaw is the sensitive error motion for both carriages!


Double L Machine Evolves…

So an Alternate Double L is designed to allow the tool carriage to travel more inboard (direction of red arrow).

Alternate Double L without Fillets Alternate Double L with Fillets

The original Double L configuration does not allow the tool to center on the workpiece (due to not enough travel in tool carriage).


Modal Analysis


Double L Machine Modal Analysis

• The Double L structure was the best at minimizing Abbe errors of the structures considered to date!

• Were its resonant frequencies high enough? ( greater than 200 Hz)?

Lets find out…


Alternate Double L Modal AnalysisCast Iron Alternate Double L Base with No Carriages

Without Fillets With FilletsWeight 2095 kg 2238 kgMode 7 115 Hz 126 HzMode 8 175 Hz 197 Hz

When the base was sized to accommodate the maximum workpiece size it became very heavy and didn’t have a very high modal frequency.

Modes 7 and 8 are the first nonsolid body modes.

Mode 7 for a Cast Iron (40) Alternate Double L with fillets


More Alternate Double L Modal AnalysisBy making the walls thinner and making the base from Aluminum Oxide the base is

1/5 the weight and improves the resonant frequency.

Aluminum Oxide Alternate Double L BaseWeight 403 kgMode 7 168 HzMode 8 266 Hz

These results are for the base alone. Analysis shows that adding the carriages can cause the resonant frequency to go up or down.

The thinner walled Alternate Double L configuration. Path of tool carriage red-solid arrow, workpiece carriage blue-dashed arrow.


Pipe Lathe Modal Results

244 Hz 325 Hz

321 Hz 424 HzThese results are for the base only, when the workpiece carriage is installed (filling a portion of the tube) the tube will be prevented from collapsing and the frequency for modes 8, 9, and 10 should all increase.


Pipe Lathe Basic Structure

• 16” OD Steel Pipe for workpiece tube

• 18” OD Steel half-pipe with ½” steel plate for tool half-pipe

Steel Pipe Lathe, Base OnlyWeight 426 kgMode 7 244 HzMode 8 327 HzMode 9 325 HzMode 10 425 Hz

There is a dramatic improvement in resonant frequencies


Proposed Plan of Action

• Work with customer Coors-Tech to determine machine specs

• Work with Hardinge Marketing to determine market size.

• Build and test a 1m/$50K machine

• Build and test a machine to validate interchangeable concept


Our Crude Market Estimate

• Coors-Tech buy or rebuilds 100 cheap lathes per year.


A marketing idea…


Bicycle Marketing Model

When buying a bike the you can…

Choose from a variety of frames

and

A variety of components

to

To get a bike that meets

your needs within your budget.

Lets apply this same idea to a machine tool…


Machine Combinations

ALO2

Base

PolymerConcrete

Base

CastIronBase

AluminumBase

Base Bearings

Hydro-Bushing

Aero-Static

RollingTruck &

Rail

SlidingTruck &

Rail

ActuatorsClosedFace

Linear M.

OpenFace

Linear M.

Ballscrew

EncodersLin Pos.w/ SuperNilvar

Lin Pos.

Rotaryon

Ballscrew

Controllers

High End

Low End(PCI based)

OtherControlled

TempBath

DressingStation

SinglePoint

Turning

These options alone could provide several hundred possible machines.But some combinations would not make sense.

Not shown above are spindle options, and size variations…


Likely Machine Combinations

ALO2

Base

CastIronBase

AluminumBase

Hydro-Bushing

Aero-Static

RollingTruck &

Rail

SlidingTruck &

Rail

ClosedFace

Linear M.

OpenFace

Linear M.

Ballscrew

Lin Pos.w/ SuperNilvar

Lin Pos.

Rotaryon

Ballscrew

High End

Low End(PCI based)

Base Bearings Actuators Encoders Controllers Other

ControlledTempBath

Air PurgeSystem

High Accuracy, LongLife, Ceramic Working

High End Dry MachineFor “green” parts

High EndALO2

Base

Low End Dry MachineFor “green” parts

High Accuracy forMetal Working Ballscrew

Lin Pos.w/ SuperNilvar

High EndControlled

TempBath

The configurations listed above are only to give an idea of how a family ofbases and components could spawn a series of specialized machines.

When Marketing these variants of the machine the customer must be provided with accurate data on: cost, final part accuracy, machine lifetime, etc. so they can make an informed decision.


Machine Accuracy Map

1.2 micron envelope

0.8 micron envelope

Part of this marketing method involves giving the customer real performance data that is a function of which machine components they choose.


Background Slides


The Double L “Wet” Machine

Ballscrew

Ballnut and Drive Motor Assembly

Hydrobushing

Hydrobushing Rail


Thermally Centered

A lum inum O x ide P a rt

P a rt C en te r L ine

T he s truc tu re is cons tra ineda long the cen te rline o f the pa rt

The part and base are kept at the same temperature by flooding both with coolant.

If the part and base are both Aluminum Oxide they will expand at the same rate.

If they are of different materials the difference in expansion causes a radial error.

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Granite

Cast Iron

Base Granite

Cast Iron

Granite

Cast Iron

Granite

Cast Iron

22

11

0.5

0.5

0.25

0.25

Bas

e M

ater

ial

and

Tem

p T

ole

ran

ce (

C)

Radial Thermal Error (microns)

Radial thermal errors as function of increase in part and base temperature for granite or cast iron bases

An Aluminum Oxide Base has the same as the part, therefore no radial errors due to uniform heating and cooling


Preliminary Work on a Dry Machine


Axtrusion Based Concept

Con’s•Large Error Gains (Abbe Errors)•Potentially Very Heavy Structure

Pro’s•Simplest Structure•Excellent Access to Part and Spindles•Easy to Offer Multiple Configurations


The Dry Machine and the AxtrusionThe 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.

Linear M otorPermanent M agnets

Carriage

W ay

Top PrecisionSurface

Side PrecisionSurface

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


Axtrusion Components

Linear M otorPerm anent M agnets

Carriage

W ay

Top PrecisionSurface

Side PrecisionSurface

Linear M otor CoilTop OutboardPorous GraphiteAir Bearings

Top InboardPorous GraphiteAir Bearings

Side PorousGraphite AirBearings

Not Shown: Position Encoder and Position Encoder Scale


How the Axtrusion™ Works

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

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

z

zm

y2

y1

ym

FmFside

Ftop1 Ftop2

B earingB earing

Bearing

Motor

Y Axis

Z Axis

Preload ResultsFside 504 NFtop1 393 NFtop2 641 N

Prototype ParametersTheta 26 degreesFm 2300 NY1 30 mmY2 260 mmYm 145 mmZ 81 mmZm 25 mm


2nd Generation Sketch of Dry Machine

Dry Lathe

Contact PointX & Z Ax is COMYAxis COM

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 radians (2 arc sec)–PERFECT MAPPING CASE: Error magnitude is the repeatability of prototype 2.5 radians (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 ErrorWith NO Error Mapping

•Radial 13 m (0.0005”)•Axial 10 m (0.0004”)

Maximum Total Abbe ErrorWith PEFECT Error Mapping

•Radial 3.2 m (0.00013”)•Axial 2.5 m (0.0001”)


Notes on the Dry Machine Concept

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.

Estimated Total Abbe ErrorWith NO Error Mapping

•Radial 9.8 m•Axial 7.4 m

Estimated Total Abbe ErrorWith PEFECT Error Mapping


Total RMS Abbe ErrorWith NO Error Mapping


Total RMS Abbe ErrorWith PEFECT Error Mapping


property of roger cortesi, mit precision engineering research group. do not copy or transmit without...

Documents

property of roger cortesi

wet machine roger

written permission

wet machine slide

dry machine slide

roger underwater

ceramic parts

separate machine concepts