Constructional Notes for Hot Wire Cutting System

Transcription

1 Constructional Notes for Hot Wire Cutting System David J Haigh December 2010 This article describes the construction of a simple hot wire foam cutting system suited to making foam wing cores for model aircraft. The system will cut white and blue foam cores up to about 500mm in length, and can easily be adapted to cut larger cores if necessary. Modern rechargeable batteries are used as the power source reducing both cost and complexity, as well as removing the need to use dangerous mains electricity supplies. Copyright David J. Haigh All rights reserved. 1

2 Introduction There are many articles and videos on the internet which describe schemes for cutting foam wing cores. The various methods differ widely in their degree of complexity, ranging from the most basic setups to full blown CNC machines. One thing I have learned about foam core cutting is that very high quality cores can be produced with the simplest of setups, as long as three basic rules are followed. These are: Do not use fancy wire. Nichrome, constantan and manganin are not very suitable. This is because they are either too malleable and cannot be tensioned properly, or they have a near zero temperature coefficient. Music wire (top E guitar strings or very fine piano wire), and stainless steel wire are best. The positive temperature coefficient of these wires helps stabilise the wire temperature to some extent. Use the correct wire tension. Too slack and the wire will sag and produce an inaccurate cut. Too tight and the wire will stretch and snap. Use the correct wire temperature/operating current. Too low and the cut will be very slow. Too high and the wire will cut beyond its own radius and affect the core profile. Copyright David J. Haigh All rights reserved. 2

3 I have tried to keep the design of this hot wire system as simple as possible. There are no mains voltages, no bulky car batteries, and no complicated electronic circuits. I built mine, and was cutting foam cores in a single evening. Power is supplied from eight rechargeable AA cells. These are small, light, and easily capable of supplying the 3 or 4 amps required. NiMh cells can be purchased with capacities exceeding 2000mAh nowadays, meaning that the system can be run continuously for 30 minutes before recharging becomes necessary. The wire current (and hence the temperature) is controlled by a simple rheostat made from the same wire as is used for cutting the foam. Not only is this very effective, but it is cheap and simple to fabricate. The cutting bow is simple to construct from a single piece of wood and a couple of lengths of 8swg piano wire. Wire tension can be adjusted both quickly and easily. The cutting wire is 0.5mm stainless wire, which is both easy to obtain, and is ideally suited to the job in hand. Stainless wire is strong and has a positive temperature coefficient of resistance. It is recommended that you don t attempt to use Nichrome wire for foam cutting. It is just not strong enough to take the tension required without stretching when it gets hot. Copyright David J. Haigh All rights reserved. 3

4 Materials/Parts List Wood Dimensions Quantity Grade 12mm x 38mm x 650mm 1 off Softwood (pine) 18mm x 38mm x 650mm 1 off Softwood (pine) 15mm x 150mm x 800mm 1 off Contiboard Maplin Components (suitable alternatives widely available) Item Maplin Part Number Quantity 4mm banana plug (red) JB24B 2 off 4mm banana plug (black) JB25C 2 off Batteries (8 x AA NiMh) L32BJ 1 off Crocodile clip (black) FK34M 1 off Crocodile clip (red) FM34S 3 off Fuse holder GU73Q 1 off Fuses (5A pack of 10) GJ96E 1 off Indicator LED (12V) NO5BB 1 off Plastic enclosure LH22Y 1 off Switch (SPST, 6A) N19CL 1 off 4mm terminal Post (black) N51AQ 2 off 4mm terminal post (red) N50AQ 2 off 2 core 6A mains cable XR62S 4 metres I use mains wire and remove the outer sheath to make strapping wire. It is much cheaper this way. Alternatively, any multi-strand insulated copper wire rated at 6A or more should be ok. The box should be approximately 120 x 100 x 45mm. Copyright David J. Haigh All rights reserved. 4

5 Other Components & Hardware Item 0.5mm stainless steel wire, 2m length 8swg/4mm piano wire, 500mm length Wooden beads, 10mm diameter Wood screws 3mm x 12mm 25mm countersunk M5 bolts 70mm countersunk M5 bolts M5 hex nuts M5 wing nuts M5 washers Spring approx 45mm long, 8mm diameter Quantity 1 off 2 off 2 off 2 off 4 off 2 off 6 off 4 off 10 off 1 off There should be no problems obtaining any of these components. Reels of 0.5mm stainless wire can be bought on Ebay. Search for lock wire. It is important that the wire is of the correct diameter. The wire resistance is inversely proportional to the square of the diameter. Using the wrong diameter will change the voltage and current required for cutting. The spring does not need to be very powerful and its dimensions are not very critical. Aim for one with a wire diameter of about 0.8mm. These are easily obtainable on Ebay, or alternatively, Maplin sell a spring assortment part number NO6CT. Copyright David J. Haigh All rights reserved. 5

6 Tools No special tools are required for the construction of this project. Most workshops will have all that is required. However, for the sake of completeness, I have listed all the tools I used to complete the hot wire cutting system. Wood saw Hacksaw (with a good quality metal cutting blade) Drill A selection of drill bits (3mm, 5mm, 8mm 12mm) A countersink bit Spanner Screwdrivers (crosshead and flat head) Needle files (for shaping the rectangular hole for the switch) Soldering iron & solder Wire cutters Wire strippers Glue (Araldite Rapid is ideal) Electricians insulating tape I also used a reamer and a Dremel drill to cut out the front panel of the power supply, though these are not strictly necessary. Copyright David J. Haigh All rights reserved. 6

7 Construction This cutting system predominantly uses parts available from Maplin, a UK based company. However, since most dimensions are not very critical, and identical/compatible electrical components can be purchased from other suppliers, you should not have any problems sourcing suitable bits and modifying the unit appropriately. The system comprises two main components the power supply and the cutting bow. Copyright David J. Haigh All rights reserved. 7

8 The Power Supply 1. Obtain a plastic box of approximate dimensions 100 x 120 x 40mm deep. Drill the lid to take the switch, indicator LED, fuse holder and four 4mm terminal posts. Figure 1 is a drilling template suitable for the Maplin components specified in the parts/materials list. Figure 1. Drilling template for the power supply box 2. Drill two 3mm holes in the base of the box so it can be mounted on a back board, and a 4mm hole in the right hand side for the connection wires to the rheostat. Copyright David J. Haigh All rights reserved. 8

9 3. Fit the various components in place as shown in Figure 2. Figure 2. Component Placement 4. Make up a flat pack of eight NiMh AA cells. Wire the cells in series and configure the pack so it will lie in the bottom of the box and not interfere with the components mounted in the lid. The cells should be soldered together with short pieces of wire. Rub the terminals with fine emery paper and tin them before attempting to solder them up and be sure to minimise the time solder heat is applied to the cells to reduce the risk of damage. Hold the pack together by binding with insulating tape. Copyright David J. Haigh All rights reserved. 9

10 Figure 3. Completed Battery Pack 5. Study Figure 4 overleaf, and wire up the box, making sure to wire up the LED indicator with the correct polarity. Connections to the stainless wire are made via leads fed through the hole in the side of the box. One lead needs to extend about 100mm, and the other 600mm. Both should be terminated with crocodile clips. Note: Be very careful not to short out the battery pack. Modern rechargeable batteries are capable of discharging very high currents, which could generate heat and cause burns or start fires. Copyright David J. Haigh All rights reserved. 10

11 Copyright David J. Haigh All rights reserved. 11 Figure 4. Power Supply Wiring Diagram

12 Figure 5. Completed Power supply Box 6. Make the baseboard by cutting a piece of 15mm x 150mm Contiboard to a length of 800mm. Drill two 5mm holes in the board as shown in Figure 6 below, then countersink them on the underside. Figure 6. Baseboard Layout 7. Feed M5 x 70mm countersunk bolts through the baseboard and secure them with nuts and washers. Then screw the power Copyright David J. Haigh All rights reserved. 12

13 supply box to the baseboard in the position indicated in Figure 7 using 3mm x 12mm woodscrews. Figure 7. Baseboard Construction 8. Make a 6mm diameter loop at one end of a length of 0.5mm stainless wire. Bind with fine copper wire and solder. The picture sequence over the page (Figure 8) describes the process. Make sure the wire end is bent over exactly as shown in the first picture. Copyright David J. Haigh All rights reserved. 13

14 The solder does not attach to the stainless steel, but flows around the copper wire. This sheaths the stainless wire tightly. The fold in the very end of the wire prevents the loop pulling out of the sheath. Figure 8. Forming a Loop End with Stainless Wire 9. Make an identical loop in the other end of the stainless wire, but this time Feed the loop around one end of a light spring before binding and soldering. The spring should be approximately 45mm long, and 8mm diameter, such that the total length of the wire and spring is 550mm. Stretch the wire and spring across the bolts on the baseboard. Copyright David J. Haigh All rights reserved. 14

15 Figure 9. Fitting the Rheostat Wire 10. Now cut a 650mm length of 12mm x 38mm strip-wood and drill to match the holes in the baseboard as shown in Figure 10. Figure 10. Rheostat Guard Dimensions 11. Fit the strip-wood over the top of the wire and spring using M5 nuts and washers. The wire is the rheostat for adjusting the cutting wire current. The strip-wood is a guard to prevent accidental burns, as the rheostat gets hot in use. Figure 11 shows the completed rheostat. Copyright David J. Haigh All rights reserved. 15

16 Figure 11. The Completed Rheostat Connections to the rheostat wire are made with crocodile clips. As the crocodile clips are moved closer to one another, so the cutting wire will get hotter. The fuse prevents excessive current flowing which could overheat the wire, or damage the batteries. Your power supply is now complete, and the batteries can be charged via the charging terminals. Copyright David J. Haigh All rights reserved. 16

17 The Cutting Bow 1. Cut a 650mm length of 18 x 38mm strip-wood and drill/countersink 5mm holes. Cut 4mm deep slots between the holes. See Figure 12. Figure 12. Cutting Bow Dimensions 2. Cut two 500mm lengths of 8swg (approx 4mm diameter) piano/spring wire. 3. Cut a shallow notch a few millimetres from one end of each piece of wire as shown in Figure 13. Copyright David J. Haigh All rights reserved. 17

18 Figure 13. Notch for Cutting Wire 4. Glue wooden beads onto the other end of each piece of wire to protect against eye injury. 5. Secure the wires in the bow with M5, 25mm countersunk bolts, washers and wing nuts so that the slots in the wires face outwards. 6. Make up a 600mm length of stainless wire with 5mm loops on both ends in the same way as you made the rheostat wire. Stretch it between the notches in the piano wire. The cutting bow is now complete, and can be connected to the power supply with fairly long (2m) lengths of strapping wire. Connection to the bow is made via crocodile clips, and connection to the power supply, by 4mm banana plugs. See Figure 14. Copyright David J. Haigh All rights reserved. 18

19 Figure 14. Cutting Bow Connection The tension of the cutting wire can be adjusted easily by sliding the pieces of piano wire in and out of the slots. Copyright David J. Haigh All rights reserved. 19

20 Setting Up Your Hot Wire Cutting System To set up the foam cutter, first charge the batteries via the two charging terminals. Then connect the cutting bow and set the crocodile clips on the rheostat as far apart as possible. Switch on and lay the cutting wire over a scrap piece of foam with the bow dangling below. Adjust the wire current by slowly moving one of the rheostat crocodile clips closer to the other until the wire cuts through the foam steadily under the weight of the bow. It will be necessary to adjust the rheostat periodically as the batteries discharge. Cutting Larger Cores The battery pack voltage using 8 cells is only really suitable for cutting cores up to about 500mm in length. For larger cores, the battery voltage has to be increased by using more cells. This is relatively easy to contrive, and the power supply box has the capacity to hold at least another 4 cells. If the number of cells is increased, it will be necessary to increase the length of the rheostat in proportion to provide adequate control of the wire current as the battery pack discharges. The electrical components do not need to be changed, and the wire current needed to cut the foam does not change either, so do not increase the fuse rating. 5A is more than adequate no matter how long your cutting wire is. Copyright David J. Haigh All rights reserved. 20