Fiddly Bit Fixtures

No great invention here, just applying some Engineering to the problem of holding small, round pins so that holes can be drilled and threaded radially.

The problem is illustrated in a YouTube video by John Saunders of NYC CNC where several, nominally identical parts are loaded into a fixture that is clamped using a single clamping tool; a vise.

In reality, parts are never exactly identical, not in length nor in diameter. A compliant fixture is required that has some give so that the clamping force is distributed more evenly over all the little parts in the fixture, holding each more successfully.

The other problem that John encountered was that round pins tend to rotate if the radial drill is not exactly on the supported axis. Very high axial clamping forces would be required on each pin to prevent rotation.

Looking at where the part is used, the body of the pin does not have to be round. It can be made from square stock allowing the threaded hole to be perpendicular to a flat face, with the edge of the opposite face supported to prevent rotation and to provide a reaction force during drilling and threading operations. The thread though the pin will also be stronger as more threads are fully engaged.

NYCpin.pngOne plausible fixture, using no dimensions directly useful to John, is illustrated above. Fixtures can be machined from aluminium. Stresses and strains should be calculated for anticipated clamping and machining forces.

The pins are the eight square items in grey. The fixture plates shown in green and blue are fixed to opposite vise jaws. The yellow part if screwed onto the blue fixture plate.

The green plate has 8 “fingers”, each with a slightly conical (tapered) hole to centre and to locate one end of each pin. Fingers flex individually, requiring about 1kN of force at the pin centre, for a deflection of about 0.25mm. Through-holes in the fixture plate allow any parts stuck in the plate to be removed easily after machining.

NYC-BY.png

Supplying the axial reaction is the blue fixture plate that has only a recessed shoulder on which the square edge of the pins rests.

Vertical and lateral constraint is provided by other finger plate fixture in yellow where a vee cutout in the tip of each finger centres the pin across that finger’s width and to the corresponding finger on the opposite fixture plate. The vee also stops the pin from popping up, out of the fixture after the vise jaws are closed.. The yellow plate is attached to the blue one with screws so that there is an adjustable preload on the fingers with the nominally-sized pin when snapped into the fixture. If any “undersized” (within tolerance) pins are placed into the fixture, they will still be held.

But it’s not all good.

A compliant fixture can also flex during machining, possibly resulting in poor surface finish, greater tool wear, wider variations in machined sizes and usually more noise during machining.

When designing a compliant fixture, it’s very important to maintain maximum rigidity against the direction of the main tool forces. In this case, the shoulder of the blue part and the vertical fingers of the green, resist the vertical forces of drilling and threading.

During e.g. drilling however, cutting forces that rotate about the drill bit centre can move the pin axially against the springing of the fingers of the green plate. Such movement may not be important but compliant elements should still be as rigid as possible; while providing the necessary clamping forces for the allowable range of dimensions.

Bonus Tip

When making fixture that are to be re-used (assume that they will); incorporate recessed internal corners so that the fixtures can be aligned more easily on subsequent runs. Faces of internal corners are less likely to be damaged when fixtures are (mis)handled or during unintended machine excursions.

Put some masking tape or similar over the internal corners to protect them when not in use and so that they can be identified easily when the fixtures are re-used, many moons later.

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