User:Jstapko/EngLab/ScrollSaw/WireAnchors

Heating Element Supports
The heating element supports serve to firmly attach the heating element to the. Additionally, they serve as electrical connections from the heating element power supply to the heating element wire. They have been designed with simplicity and convenience of both replacing the heating element and converting the scroll saw back to a wood cutting blade as secondary, but important goals. There are two supports, the upper support, which includes features such as an electrical insulator and a thermal expansion compensation spring, and the lower support which consists of a guitar string ball end and a paper clip, designed to be compatible with the scroll saw's original lower blade holder so that the lower blade holder does not need to be removed/installed when changing from wire to wood/metal cutting blade and back. This was an important feature of the lower element support because removing the lower blade holder requires substantial disassembly of the scroll saw, a process requiring extended time and specialized skill. Each support will be discussed in detail seperately.

Preliminary Designs


Several designs were originally considered for the upper element support, including:

'''Option 1:

Modify the original upper blade holder to electrically insulate it, and connect the heating wire directly to the upper blade holder.'''


 * Advantages:
 * It would prevent the need to remove the upper blade holder when changing between foam cutter mode and scroll saw mode, simplifying the changeover procedure
 * it would make use of existing materials, reducing cost and environmental impact


 * Drawbacks:
 * making the necessary modifications, such as drilling holes to make electrical connections or add insulating bushings, could have destroyed the original part
 * the blade holder is hard spring steel, which could cause damage to the equipment used to do the modifications
 * the stock blade holder is a very stiff spring, which gives little elasticity to compensate for thermal expansion of the wire or imperfections in the scroll saw's mechanical linkage (for an explaination of spring stiffness, see Hooke's law)

''' Option 2:

Use a in series with the heating element, connecting the support to the upper blade holder.'''

This approach was not examined in depth, but major issues included:
 * the availability of very small guy wire insulators - it may have been necessary to custom fabricate them from machinable ceramics
 * the electrically isolated wires and insulators would have taken up linear space in the cutting area, reducing already limited thickness of foam which could be cut

Option 3:

Build an adapter out of non conductive material such as wood that could be bolted onto the frame in the same position of the upper blade holder


 * Advantages
 * could be built with existing parts, cheaply
 * based on pictures and videos of prior work by others, it seemed very likely to be effective without the need to purchase special materials or tools
 * could be fitted by hand to a radius on the scroll saw frame


 * Drawbacks
 * wood can absorb moisture, which can give it poor electrical characteristics
 * to mount the wood insulator, the upper blade holder would have to be removed and the wood adapter bolted onto the frame in its place. Thus, this option makes the process of changing between wire cutter mode and scroll saw mode more complex and time consuming, contrary to the secondary design goals described above

First Prototype
After exploring initial options, it was decided to build a wood prototype that would bolt on in the place of the upper blade holder, with a radius to precisely match the curvature of the scroll saw frame. It was hoped that a tightly fitting adapter would effectively resist torsion forces despite being secured with only one screw.

Problems with First prototype While the first prototype was rigid enough to hold the wire tightly, and worked well to electrically insulate the power supply from the frame, the support point was further from the pivot of the scroll saw linkage than the lower element support. This caused two problems:
 * 1) The wire passed through the hole in the table at an angle, with the top end closer to the operator than the bottom end.  This worked well when cutting in a straight line parallel to the plane of the frame, but turning the part would cause a cone to be cut out of the foam, and cutting sideways would bevel the edge.  It was desired that the cuts be square to the piece in both directions so that turning the part in a cut would not deform the part.
 * 2) With the saw turned on to reciprocate, (desirable to even out the temperature along the wire and increase cutting speed) the top arm would move a greater distance over each stroke than the bottom arm.  This caused variations in the compensator spring's force on the cutting wire, changing the tension in the wire during the stroke.  This stressed the wire unnecessarily, and tensile strength was already a problem so it was desirable to remove this extra stress.

Additionally, tests with the first prototype showed that the hole in the spring shaft for supporting the wire (visible in side view picture, below) was too small to allow convenient removal of broken pieces of wire, and that the broken piece had to be removed to replace the element. There were also problems with making the electrical connection to the wire, so efforts were made to resolve both issues. The current solution is a paper clip threaded through the spring shaft hole, soldered to a copper wire for electrical connection to the power supply, with the heating element being wrapped through the lower end and back on itself for mechanical and electrical connection. The soldered contact adds mechanical strength to the paperclip, preventing it from uncoiling under high tension. The paper clip provides the additional benefit of reducing heat transfer from the element to the plastic end cap that guides the spring rod, reducing the chance of damage to the cap by melting the plastic.



Second Prototype
To address the unequal distance from pivot point problem, the first prototype was cut into two pieces. The bottom piece held the anchoring screw and custom fit radius, while the top piece held the compensator spring and and associated parts. A pocket was milled in the upper piece to accommodate the head of the anchoring screw, and a concave curve was carved into the lower piece to allow clearance for the spring shaft in the center, while retaining as much mechanical support as possible on the outer edges. If the bottom piece had been cut straight across, there would have been less material to support the top piece. The two pieces were then strapped together with a piece of scrap sheet metal and some wood screws. As currently configured, the heating element is much more nearly square to the table, but torsional forces move the top piece and dislocate the spring shaft, so improvement is needed here. Ideas for improvement include:
 * A wood or sheet metal screw passing vertically through the top piece and securing it to the bottom piece. Ideally, there would be a curved slot in the top piece to allow for rotational adjustment of the top piece before tightening the screw.
 * another sheet metal strap on the other side, secured with wood or sheet metal screws. This could also possibly include slots in the sheet metal to allow for rotational adjustment.

Lower Element Support
The original lower element support was modeled on the pin that was in the original blade. It was a piece of a printer roller with a ridge sculpted into it, (with a dremel bit) with the intention of having the support fit in the detents of the lower blade holder in such a way that lateral and longitudinal motion would be prevented, ideally keeping the lower end of the wire in place. Difficulties with the electrical connection similar to those above, the success of the paper clip for the upper support, and the serendipitous discovery of the guitar string ball end lead to the current design. The lower support now consists of a guitar string ball looped into a paper clip, with a wire being then soldered to the paper clip to make the electrical connection. The solder was applied so that it would form the paperclip into a solid loop, and prevent it from uncoiling under high wire tension. The guitar ball end fits into the detent in the lower blade holder just as the first support was intended to do, and keeps the wire securely in place, even with the saw reciprocating.

Possible Next Steps

 * add a screw or tie strap to the upper element support to keep it from twisting sideways under torsional forces
 * use a hinge instead of the tie strap on the left side so the top piece can be flipped up conveniently for converting to scroll saw mode
 * try to redesign the upper support so that it has quick release devices for rapid and simple change over
 * put some tape marks or lines somewhere on the machine to indicate the ideal length of new cutting wire elements
 * perhaps design a collet or clamping device and electrical connection/insulation system that is compatible with the original upper blade holder

Things This Page Needs
(aluminum tab crimped on and connected with ring lug)
 * images of the supports, including detailed shots of the lower support both in good light and in position
 * a link to a tutorial or video on converting between scroll saw and wire cutter modes and back
 * a picture of the lower spring cap to clarify the melting from element problem
 * a picture of the current configuration of the upper element support (second prototype), showing tie strap and curved recess
 * possibly a picture of the pocket cut into the upper part to allow space for the Allen screw head
 * possibly a link to a page on using the drill press and Forstner bit to mill the pocket
 * a picture of the lower element support, showing use of guitar spring ball end
 * a detail picture of the upper element paper clip, showing soldered electrical connection and the hot wire wrapping back on itself
 * a link, possibly in the caption of the top view pic of the first prototype, to a page on modifying the RC car spring
 * pictures of the original electrical connection effort

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