Project 10: French Curve

This project teaches the following skills:

Figure A.25: French Curve

Plate parts can present unique work holding challenges. Thin and flexible material tends to be lifted and torn by the cutter. This exercise uses the fixture plate created in Project 9 and works with parts that have good rigidity.

This same process can be used with very thin and flexible parts that would normally require stamping or laser cutting by sandwiching the material between a backing plate and the fixture plate, as shown in Figure A.26 below.

Figure A.26: Thin Part with Backing Plate (Finished)

The following instructions show how to use the fixture plate created in Project 9 to machine a sheet metal part.

1. Clamp Part Into Fixture Plate

Use the dowels pins to locate the stock material as shown in Figure A.27 and tighten the clamp bolts firmly.

If using steel clamps on aluminum or other soft material, adjust clamp pressure carefully and consider placing a piece of aluminum between the clamp and part to prevent marring.

Figure A.27: Clamp Part

2. Remove Dowel Pins

Any contact between a hardened dowel pin and tool will destroy the tool. Remove the dowel after clamping if there is any chance the tool could come in contact with it.

Figure A.28: Remove Dowel Pins

3. Drill Hold-Down Holes

Drill Holes for the #6-40 screws that will secure the part after the stock being gripped by the clamps is cut away. The drill depth should be just enough so the shoulder clears the bottom of the part, but does not cut deep into the threads in the fixture plate.

French Curve: Job-1 Setup

Datum: Use same datum as fixture plate.

Op-1
CTR Drill 		Tool (in)
.25 Ctr Drill 		Speed (rpm)
1800 		Feed XY (ipm)
18.0 		Feed Z (ipm)
9.0
  Rough XY (in)
1.0 		Rough Z (in)
.25 		Feed XY (ipm)
1.0 		Finish Z (in)
.005
Op-2
Drill 		Tool (in)
.156 (5/32) Drill 		Speed (rpm)
3200 		Feed XY (ipm)
36.0 		Feed Z (ipm)
18.0

Table A.10: Fixture Plate Job 1

4. Install Hold Down Bolts

Figure A.29: Install Bolts

5. Machine OD

Cut the outside profile of the part as shown in the figure in Table A.11.

French Curve: Job-2 Setup

Op-1
Contour 		Tool (inch)
0.25 End Mill 		Speed (rpm)
1800 		Feed XY (ipm)
18.0 		Feed Z (ipm)
9.0
  Rough XY (in)
.005 		Rough Z (in)
0. 		Finish XY (in)
.005 		Finish Z (in)
-.005

Machine the OD taking one rough and one finish pass. Start the tool in an area where it is away from the clamps, and use line/arc lead in/out. Cut into the fixture slightly so there is no flashing on the bottom of the part.

Table A.11: Fixture Plate, OD Machining Parameters

6. Move Clamps, Remove bolts

First move two of the clamps to the alternate positions shown so they secure the OD of the part. Then remove the hold down bolts so the ID of the part can be machined.

Be careful when tightening the clamp bolts. The part is thin and excess pressure might cause the part to deform. Of course, insufficient gripping force might allow the part to shift under the cutting forces and ruin the part. Using the custom clamps (Project 8) machined for this part maximize the grip surface area.

Figure A.30: Move Clamps

7. Machine ID

Machine the ID Profile to complete the part.

French Curve: Job-3 Setup

Op-1
Contour 		Tool (in)
0.25 End Mill 		Speed (rpm)
1800 		Feed XY (ipm)
18.0 		Feed Z (ipm)
9.0
  Rough XY (in)
1.0 		Rough Z (in)
.25 		Feed XY (ipm)
1.0 		Finish Z (in)
.005
  Stock to Leave XY (in)
0.0 		Stock to Leave Z (in)
0.0 		  

Table A.12: Fixture Plate, ID Machining Parameters

To observe the effects of data starvation, set the tool length offset so the tool runs above the part. Run the program, increasing feed rate until the machine begins to shudder. Notice that the actual machine feed rate is less than the programmed feed rate.

The feed rate at which this occurs depends on the block execution time of the machine tool. Reduce or eliminate data starving by reducing the feed rate at the machine, or by using the tool path filter function in the CAD/CAM software.