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Monday, 12 May 2014

Homemade weighing scale - Construction and Plans

This post provides a tutorial and plans for a homemade weighing scale, similar to a classical letter scale. You obtain a scale that you can easily adjust to have the range of your needs, be it for only 20 grams like my prototype shown on the left, or for 100 or 200 grams.
Accompanying this post, I provide a tutorial and JavaScript tool that allows you to easily calibrate the weighing scale and that produces a fancy readout scale that you can print and attach to the device.
The video below demonstrates usage and accuracy of the prototype.


Homemade weighing scale.



Built from only four major plywood parts, these weighing scales are a simple project that can be completed by everyone with the most basic handcrafting skills. Below, required materials and tools are listed and the handcrafting steps are explained: cutting the plywood parts, drilling a few holes and glueing and screwing the parts together.
After assemblage, the scale has to be coarsely balanced to the desired range of mass to measure. Finally, it must be calibrated and equipped with a readout scale. This process is explained in another post, which contains a convenient software tool that deos the calculations and graphics for you.

Contents
1. Materials and Tools
2. PDF Plans for download
3. Construction Manual

1. Materials and Tools

Materials

  • Plywood, thickness 6 - 8 mm, beech or poplar (optionally: plywood 4 mm, poplar, for a very sensitive scale with small mass range)
  • Brass or steel wire (⌀ ≈ 2 mm)
  • Large washers (outer ⌀ ≈ 30 mm, min. 4 pieces)
  • Small nuts or washers (inner ⌀ ≈ 3 mm, min. 2 pieces)
  • 2 wood screws, 10 - 15 mm long
  • A dish-shaped item for the weighing pan (max. ⌀ 8 cm, e.g. a tin can base)
  • Wood glue (PVA)
  • Superglue
  • Paper glue stick
  • Double faced adhesive tape

Tools

  • Jigsaw or scollsaw
  • Drilling tool
  • Drill, ⌀ fitting the brass or steel wire
  • Sandpaper (fine ≈ P100, very fine ≈ P220)
  • Black-white printer

2. PDF Plans for download

Plans for the parts of the weighing scale are available for download as PDF.
Plans for homemade weighing scale
Figure 1: Plans for the weighing scale. Use high quality PDF for printing. Download PDF.


3. Construction Manual


Step 1: Print the plans


Download the PDF plans. Print the plans on A4 paper.

Step 2: Glue plans onto plywood


Cut off the text parts of the plans sheets. Glue the first page (parts 1-3) onto the 6 - 8 mm plywood using the paper glue stick. If you want a very sensitive weighing scale, glue the second page (the indicator, part 4) on the light, thin poplar plywood. Otherwise, glue it onto the 6 - 8 mm plywood, too.

Step 3: Saw out the parts


Saw out all 4 parts using a scrollsaw or jigsaw.
Additionally, saw out two rectangles sized 2cm by 1cm from the thick plywood or some other wood of similar thickness. You finally want a small block of wood, sized 2cm by 1cm by 2-3 times the thickness of your indicator's material.

Step 4: Drill


Drill holes with the diameter of your brass or steel wire (≈ 2 mm) everywhere indicated on the plans. Try to drill as perpendicular to the material as you can. Slightly enlarge the holes in parts 1 and 3.

Step 5: Remove paper from the parts


Remove the paper from the sawed out parts, except for part 1 (body), as you will need the calibration scale later. To remove the paper, moisten it, wait half a minute or so, and peel it off. From part 1, only remove the paper from the top part, around the two upper holes. Remove remains using the very fine sandpaper.

Step 6: Sanding


Sand the parts you removed the paper from, to have a smooth surface and rounded edges and corners. Start with the coarser sandpaper for corners and edges and end with the finer for the surface.

Step 7: Prepare and attach the rotation axis


Cut a piece off the steel or brass wire, approx. 4 cm long.
Using superglue, fix that piece of wire in the central hole of the indicator part (4), as perpendicular as you can. For the desired result, see Figure 2, below.

Rotation axis of homemade weighing scale, glued into the indicator
Figure 2: Rotation axis, glued into the indicator part.


Step 8: Attach the indicator mark


Glue a piece of your brass or steel wire, or of 1 mm craft wire, onto the back of the indicator using superglue. Place the wire horizontally centered over the large opening of the indicator, as shown in Figure 3.

Indication mark of the weighing scale
Figure 3: Indication mark, seen from the back of the indicator.


Step 9: Prepare the bearing


Glue the two small, 2cm by 1cm (ply)wood pieces with the flat sides onto each other using wood glue. After drying, glue the resulting block onto the upper left corner of part 3 as shown in Figure 4, below. The shifted position of the block is important to avoid collision with the indicator.

Front part of bearing of the weighing scale
Figure 4: Front part of the bearing.


Step 10: Install the bearing


Put the indicator part (4) onto the front of the body part (1), plugging the rotation axis into the bottom-most hole of the body. Add part 3 on top, with the attached block downwards, again plugging the axis into the hole (see Figure 5). Indicator (part 4) and part 3 are now in front of the body.
Thoroughly adjust the position of part 3 until the indicator is exactly parallel to the body and swings freely. Keep everything fixed in this position while drilling through the two existing holes of part 1 into the block on part 3. Do not drill through part 3, just drill as deep as necessary for the wood screws!
Disassemble the bearing again.

Bearing of the weighing scale
Figure 5: The bearing. Here, already with the holes to be drilled in the next step.


Step 11: Attach the foot support


Glue part 2 onto the back of the body (part 1) as shown in Figure 6, below. You may use superglue here, as it can be hard to fix the parts for the drying of wood glue.
If necessary, glue some stabilizing weights, like large nuts, onto the foot support, to prevent the device from falling to the front.

Foot of the weighing scale
Figure 6: Foot of the weighing scale, with stabilizing weights.


Step 12: Assembling the parts


Assemble the bearing as explained in Step 10, but adding a small washer or nut on each side of the indicator (see Figure 5). Screw the two wood screws through the holes on the back of the body, into part 3.

Step 13: Make the weighing pan


Find something sufficient for making the weighing pan. It should be shaped like a shallow bowl or dish and not be too heavy (the lighter, the better). Furthermore, it has to fit under the side arm of the weighing scale, i.e. have a maximum diameter of 8 cm.
I made the pan from a tin can base that I hammered to a bowl-like shape, and attached a piece of brass wire to it to hang it onto the scale. Below are two photos of the pan. It can be easily attached to and removed from the scale by plugging the upper end of the wire into the arm's drilling.

Weighing pan from top and bottom
Figure 7: Weighing pan, made from a tin can base.


Step 14: Crude balancing


Plug the pan to the weighing scale.
Place the scale on an even, horizontal surface.

You will now coarsely balance the scale to fit the weight range of your needs, and to have the indicator point approximately downwards under zero weight. For that puropse, find some items of known weight that fit your desired range. If you have no access to standard weights, coins of your local currency will be sufficient in most cases. You will find a list of their weights in the internet. Euro coins are listed below:

Coin1 ct2 ct5 ct10 ct20 ct50 ct1 €2 €
Mass2.3 g3.06 g3.92 g4.1 g5.74 g7.8 g7.5 g8.5 g

Completed homemade weighing scale with balancing weights
Figure 8: The assembled scale, with balancing weights..


Start making the indicator point downwards. Make sure the pan is attached to the scale! Using double faced adhesive tape, add large (30mm) washers to the left arm of the indicator until the indicator' main arm points approximately downwards, showing a value between 0° and 10° on the calibration scale. For the prototype, I needed 2 washers.

Now, you adjust the device for the upper bound of the measuring range. Successively add washers to the lower end of the indicator (using double faced adhesive tape), and weights to the scale's pan. Continue until the indicator shows something between 70° and 90° on the calibarion scale with your desired maximum weight on the pan.

Remove the weights from the pan to check the zero weight position again. It should have moved closer to 0°. If necessary, experiment by adding and removing weights from the left arm and the indicartor's tip, and by moving the weights closer to and further away from the rotation axis of the indicator until you are satisfied with the range.
Doing so, remember that the weights on the left arm act as counterweights for the pan itself, while those at the lower end of the indicator determine the maximum of the scale.

Step 15: Calibration


Finally, it is necessary to determine the exact relation between the mass on the pan and the inclination of the indicator, and to create a respective readout scale in grams.
For that puropse, I wrote another tutorial with calibration tools, that does all the math work for you and creates a fancy, configurable readout scale that you can glue onto your device.

Step 16: Attach the scale


Disassemble the brearing to remove the indicator.
Mark 0° and 90° of the calibration scale on the underlying wood of the body using a needle or knife.
Remove the plans paper from the body as explained in Step 5.

Print and cut out the readout scale you obtained here. Apply paper glue to the part of the body where the scale will be, i.e. on the entire range from 0° to the 90° mark. Glue the readout scale onto the body, fitting it exactly from 0° to 90°.

Congratulations, you completed your scale!


Yours sincerely,
Turnvater Janosch

4 comments :

  1. I'm impressed that you were able to make that yourself. I don't think it would work with heavy materials with that type of supply though. I would recommend just looking into commercial weighing scales. http://www.nuweigh.com.au

    ReplyDelete
  2. This is exactly the kind of weighing device I was looking for. I'll have a go. Thanks for all the info!

    ReplyDelete
  3. Nice art. It's an art, appreciated. Do you appreciate Electronic Weighing Scale ?

    ReplyDelete
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