Notes 22

What is CNC? Introduction to CNC

Computer Numerical Control The process of manufacturing machined parts using a computerized controller to command motors which drive each machine axis.

Lecture 22 Engineering 475 Automated Production Processes

History of CNC

History of CNC 1947

1951

1952

(Continued)

John Parsons, Parsons Corporation, Michigan Developed a control system that directed a spindle to many points in succession Servomechanism Laboratory of Massachusetts Institute of Technology (MIT) – Added computer to Parson’s system Cincinnati Milicron Hydro-Tel Vertical Spindle Milling Machine First three-axis numerically controlled, tape-fed machine tool

1954

NC was announced to public

1957

First production NC machines were delivered and installed

1960

NC machine tools commonly available

A lot of the initial funding for the development of NC machine tools came from the United States Government. CNC Workbook

CNC Workbook

Why CNC machine tools? • Increase production throughput • Improve the quality and accuracy of manufactured parts • Stabilize manufacturing costs • Manufacture complex or otherwise impossible jobs – 2D and 3D contours.

Integration with Mathematics

y

x Complex curves and surfaces can be described mathematically Kalpakjian, Figure24.1

1

Integration with Mathematics Advantages of CNC

(Continued)

Vf ≡ feed rate along curve Tangent Or Slope

Vf

∆y

∆x

Curve to be followed Complex curves can be followed by breaking the curve into a series of small straight lines and using linear interpolation.

Vx = Vy =

∆x ∆x 2 + ∆y 2 ∆x ∆x 2 + ∆y 2

Vf Vf

• Flexibility of operation is improved, as is the ability to produce complex shapes with good dimensional accuracy, repeatability, reduced scrap loss, and high production rates, productivity, and product quality. • Tooling costs are reduced, since templates and other fixtures are not required. • Machine adjustments are easy to make with microcomputers and digital readouts. • More operations can be performed with each setup, and less lead time for setup and machining is required compared to conventional methods. Design changes are facilitated, and inventory is reduced. Kalpakjian, 1135.

Advantages of CNC

Limitations of CNC

(Continued) • Programs can be prepared rapidly and can be recalled at any time utilizing microprocessors. Less paperwork is involved. • Faster prototype production is possible. • Required operator skill is less than that for a qualified machinist, and the operator has more time to attend to other tasks in the work area.

• • • •

Relatively high initial cost of the equipment. The need and cost for programming and computer time. Special maintenance with trained personnel. High preventative maintenance since breakdowns are costly.

Kalpakjian, 1135.

Kalpakjian, 1136.

Anatomy of a CNC Machine

Anatomy of a CNC Machine

(3(3-Axis Vertical Mill)

(Horzontal Drilling Machine)

Chang, Figure 9.8 www.thomsonbay.com

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Operational Features of CNC Machine

Recirculating Ball Screws

A CNC control system includes a velocity loop within an axis drive system and a position loop external to the axis drive system.

A - Ball Screw B – Linear Bearings

Microcomputer Controlled X-Y Table

Degarmo, Figure 29-10.

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Recirculating Ball Screws

Recirculating Ball Screws

(Purpose)

(Continued)

Transform rotational motion of the motor into translational motion of the nut attached to the machine table. Advantages of Recirculating Ball Screws Pneumatic Ball ACME Fluid Belt, Rack & Cam Screws Screws Power Chain Pinion Followers Cylinders

Inexpensive Low Power Use Low Maintenance High Accuracy

Recirculating Ball Screws are generally preloaded to give zero backlash.

High Repeatability High Efficiency High Load Capacity Compact Size

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Recirculating Ball Screws

Recirculating Ball Screws

(Rotational to Linear Velocity Conversion)

(Positioning Resolution)

Lead – distance the nut moves parallel to the screw axis when the screw is given one revolution.

l

=>

Vf l in

[ min ] = [rev ] [in rev] min

l

l ≡ ball screw lead [in/rev] θ ≡ rotation angle [degrees]

Vf ≡ Feed Velocity n=

æ 1 ö u = l ⋅θ ⋅ç ÷ è 360 ø u ≡ axial displacement [in]

D

æ 1 ö ç ÷ ≡ converts revolutions è 360 ø

D

ù to degrees érev êë degreesúû

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Recirculating Ball Screws

Recirculating Ball Screws

(Positioning Resolution Example)

(Lead Screw Resolution)

l = 0.125 in θ = 15 degrees

( 360)

u = l ⋅θ⋅ 1

Example A ball screw has a lead of 0.125 in/rev. What is the distance that the nut will travel if the screw is turned 15 degrees?

ö u = (0.125 in ) ⋅ (15 degrees ) ⋅ æç rev 360 degrees ÷ø è = 0.0052 in

l = 0.125 in

(

u =l⋅ 1 360 θ

)

The positioning resolution of a ball screw is directly proportional to the smallest angle that the motor can turn.

u ö = (0.125 in/rev ) ⋅ æç rev 360 degrees ÷ø è θ u = 0.00035 in/degree θ

Stepping Motors

Stepping Motors

(Full Step Operation) Ø Hybrid stepping motors have a variable-reluctance rotor with a permanent magnet in its magnetic path, usually in the rotor. Ø The term hybrid refers to the use of two sources of magnetic field, the stator windings and the permanent magnet.

A stepping motor provides open-loop, digital control of the position of a workpiece in a numerical control machine. The drive unit receives a direction input (cw or ccw) and pulse inputs. For each pulse it receives, the drive unit manipulates the motor voltage and current, causing the motor shaft to rotate by a fixed angle (one step). The lead screw converts the rotary motion of the motor shaft into linear motion of the workpiece. Bateson, Figure 10.15

Ø Hybrid stepping motors are used when small step angles are required. Ø The 1.8 degree stepping motor is the predominant standard for industrial automation.

200 step hybrid motor cross section

Bateson, 386.

Parker, Figure 1.12

Stepping Motors

Stepping Motors

(Angular Positioning)

(Lead Screw Positioning Resolution)

360 degrees ∆θ = N steps N = 200 360 deg ∆θ = = 1.8 deg/step 200 steps

( 360)

u = l ⋅θ⋅ 1

∆θ =

360 N

Example

l ∆u = N Increased resolution can be obtained using a technique known as microstepping.

l = 0.125 in/rev N = 200 steps/rev 0.125 in/rev ∆u = 200steps/rev = 0.000625 in/step

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Stepping Motors

Stepping Motors

(Microstepping Operation)

(Lead Screw Positioning Resolution)

The rotor can be positioned in partial steps by simultaneously controlling the currents supplied to the stator phase windings. Microstep sizes of 1/10, 1/16, 1/32, and 1/125 of a full step are most commonly used.

∆θ =

360 N*m

m=number of microsteps per full step

Summary • The positioning resolution of a ball screw drive mechanism is directly proportional to the smallest angle that the motor can turn. • The smallest angle is controlled by the motor step size. • Microsteps can be used to decrease the motor step size. • CNC machines typically have resolutions of 0.0001 in or better.

( 360)

∆θ =

u = l ⋅θ⋅ 1

∆u =

l N⋅m

360 N*m

Example

m = 32 microsteps/steps l = 0.125 in/rev N = 200 steps/rev 0.125 ∆u = 200 ⋅ 32 = 0.0000195 in/microstep

Assignment A CNC mill has 0.125 inch lead recirculating ball screws that are driven by a 200 step hybrid step step motor with 125 microsteps per step. What is the position resolution of the machine? How many microsteps will the motor have to undergo in order to move the mill table attached to the ball screw 1.0 in?

Watch videotape at library on the mill.

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