LN Valve Stem Creation

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Introduction

LN valve stem

The LN Valve stem is formed from 1/4" diameter FEP rod. It fits into the LN Manifold body, so that in the opened position, the passage hole cross-drilled in the stem aligns with a port hole through the bottom of the manifold's reservoir, allowing liquid nitrogen to flow through. Rotating the stem 90° blocks the flow of liquid.

Cut blanks

1. Cut 2.7" long blanks from 1/4" OD FEP rod stock.
   • Use the micro table saw with a 0.006" blade.

Machine the ends

Machine the workpiece on the CNC lathe:

1. Machine the stem tip (program LN valve stem step1 tip).
2. Machine the drive end (program LN valve stem step2 drive end hole).
   • Use the same 5C collet stop position as for the stem tip.

Machine passage and position mark

Machine the workpiece on the CNC mill:

1. Setup the 2" vise 2" right of the left extreme and 1" back from the front face of the build plate.
   • Note: While this uses the 2" vise, this is not the standard setup.
2. Install the LN valve stem port drilling fixture (TL-LNVSPDF) flush against the fixed jaw and clamp the fixture to to the build plate.
3. Insert the workpiece with the stem tip flush with the far side of the fixed jaw.
   • Use a flat stop, such as a 123 block, against the far side of the vise to ensure it's flush.
4. Machine the passage and position mark (program LN VALVE STEM)

Form the spline

Hot-form the drive end using the servo drive spline form (TL-SDSF) and die (TL-SDSD) tools on the CNC lathe (program LN valve stem step3 form drive end). Run the program in single step mode:

1. Repeatedly press Start to advance the program until the spline form is positioned 3/4" from the slowly rotating part.
2. Use a high-temperature heat gun with a narrow nozzle to warm the end of the part for ~10 revolutions.
3. Slide the die (TL-SDSD) onto the workpiece up to the collet.
4. Advance the program until the spline is buried in the workpiece.
5. Slide the die over the spline and against form (TL-SDSF).
6. Wait a five seconds for the part to cool.
7. Advance the program until the turret retracts.
8. Remove the die workpiece from the collet and separate from collar.

Polish the OD

After polishing

Surface before polishing

After polishing

Surface before polishing, focused on edge

The stem's cylindrical surface must be smooth to prevent it from sticking mechanically in the LN manifold containing liquid nitrogen. When cold, the foam reservoir shrinks and hardens sufficiently to tightly grip tiny surface irregularities on the valve stem. See photos at right (click to view full size): the improvement in surface finish is perceptible but difficult to detect even with magnification. However, the improvement in performance is significant.

1. Chuck the spline-gear driveshaft (TL-SGD) into the hand drill.
2. Snap the stem onto the driveshaft.
3. Dry-sand stem OD at 400, then 600 grit.
   • Use pre-cut 1" x 2.5" strips of sandpaper.
   • Spin in both/alternating directions for each grit.
   • Control the speed to avoid overheating the work-piece.
   • Clean the sandpaper surface frequently with a sandpaper eraser and compressed air.

Color the position mark

Using a black marker capable of writing on Teflon, color the inside of the position mark hole near the drive end to increase its visibility.