Plywood Hydraulic Arm with Two Control Handles
I was so impressed with the cardboard hydraulic arm by Technovation
(https://www.instructables.com/id/CARDBOARD-Robotic-Hydraulic-Arm/) that I built one of my own from plywood, with some small modifications:
- I wanted to have more grip on the arms, so I moved the pivot point. (see photos)
- To get neater and stronger linking wires, I used bicycle spokes. Disadvantage: they are very strong, and small bending errors cause the arms not to meet in the middle. - Note that I had to bend one of them to make the arms meet.
- I used a round base and disc shaped retainers on the ends of the bamboo skewers to hold them in.
I won't re-hash this construction, as it was very well covered in the instructable.
I have seen other versions of hydraulic arms with 4 controller handles, but I wanted one where the operator could have their hands on both handles and control the 4 functions of raising & lowering, slewing & grabbing without the need to let go of the handles.
Up & down movements are controlled by pushing the handles back and forwards
The grabber is engaged by a sliding pull-up handle within the left controller
Slewing left and right is done by see-sawing the control handles left and right.
I have attached a video of it working:
Step 1: MATERIALS AND TOOLS:
Bill of Materials:
7mm plywood 300 x 300 mm
Wood: ( I used radiata pine)
1 x 15mm x 115 x 70mm
2 x 20 x 40 x 140mm
1 x 20 x 40 x 63mm
1 x 30 x 65 x 100mm
2 x 30 x 45 x 60
1 x 16mm dowel x 135mm
1 x 16mm dowel x 260mm
2 x 16mm dowel x 60mm
1 x 16mm dowel x 59mm
8 x 10cc syringe bodies. On Ebay 6 for $1.39(us)
2 metres of Aquarium air tube
1 x bicycle spoke (U- connector on the moving handle)
Screws, panel pins, PVA wood glue
2 x Rubber bands,
4 x 2.5mm bamboo skewers.
Nails, screws, pva glue.
Drill & bits up to 25mm
Side cutters, pliers, vice
Files/rasps (round and square)
Extra tools (make the job easier)
Hole saw with 5/32 centre drill
Step 2: Summary of Steps:
if the Fusion 360 drawings & photos are enough for you, skip the details and make:
- The handles
- The centre handle support
- The right hand bracket
- The slew bracket
- The middle support
- The semicircular support
- The end caps
- The sliding handle
- Install the Syringes
- Connect to the arm & have fun!
Step 3: Detailed Steps: 1) the Handles
Decide which handle will be the left one, and before assembly, cut matching slots in each side (as per fusion 360 model)
The slot is 32mm long, 4mm wide and starts 25mm from the top of the handle
Drill 2.5mm holes for the bamboo on centre, 5mm from the bottom of each handle
Drill 3mm holes on centre 8mm from the top of each handle to screw the dowels on.
Drill pilot holes for the screws in the centres of the 70mm dowels.
Glue and nail the sides onto the centre blocks, and glue and screw the handles. Avoid putting nails anywhere near the line of the pivot hole, which is on centre, 65mm from the handle bottom.
When the glue is dry, Carefully mark the position of the pivot hole, and drill a 3mm pilot hole towards the centre of each handle, from each side. (Drill press is very handy for this)
Using a spade bit, or a forstner bit, drill a 16mm hole through the centre of each handle from each side. If all is well, when you break through into the first hole, they will be perfectly aligned.
Use a file/rasp to smooth out any irregularities in the hole and test fit a 16mm dowel into the hole. It should slide easily in and out. If it is tight, the hole needs to be rasped out until the dowel fits and moves easily.
Make the bracket that fits onto the left handle to hold the syringe, and glue it on to the left handle. (see drawings) Be careful if you nail it that you don't allow the nails to enter the axle hole.
Bracket dimensions: 40mm long x 40mm wide x 25mm sides from 7mm plywood, with a 30mm x 25mm base, holes in each side, 30mm from lower part of the bracket, on centre.
Step 4: The Sliding Handle:
A 16mm dowel 59mm long secured by screws through the slots in the left handle. The slots need to be filed and smoothed out so that the handle moves freely up and down. The screws are not screwed right in, but they have a 10mm washer between them and the handle, and a rubber band on each side, down to another screw on the side of the handle, with a washer, (See photos) to act as a return spring. The distance between the lower screws and the handle is trial and error, depending on the length and strength of the rubber bands. I opted for about 40mm. The washers help retain the rubber bands and prevent them rubbing on the sides of the handle & jamming.
The linkage between the syringe and the sliding handle is a U-shaped piece of bicycle spoke, 10mm wide with legs about 15mm long. I used a bicycle spoke because it is extremely strong, yet can be bent and sharpened. Each leg is sharpened, and pilot holes are drilled into the handle so the bracket can be easily hammered in after threading through a pre-drilled hole in the syringe plunger. This is far from ideal, because the joint is not rigid, and must be supported to work effectively. I just wrapped it in insulation tape, which works, but is not elegant. Heat shrink tubing is another possibility. This joint is not assembled until the syringes are being installed.
Step 5: The Centre Handle Support:
Mark out the 16mm hole which runs along the length of the wood. on centre, 58mm from the top.
Drill as per the handles, starting with a 3mm pilot drill from each end.
If the hole is straight, then mark and drill a 16mm hole, perpendicular to the first hole, centred, 55mm from the bottom. (see drawings)
These holes do not have to be a sliding fit. It's better if they are not.
Glue the 260mm dowel in the long hole, leaving 75mm protruding on the left hand side. Check that the left handle fits onto this with about 1mm of dowel sticking out past the handle (for clearance).
Glue the 135mm dowel into the other (perpendicular) hole, making sure that it is oriented correctly. IE: Looking at the back of the rectangle, if the long dowel is lower down than the one you are installing, and the 75mm length is sticking out on your left, then the 135mm dowel must protrude away from you. (see drawings)
Fit the right handle, and check how much dowel is sticking out. Adjust it so there is about 8mm protruding past the handle, and cut a vertical key in the end of the dowel that is 6.5mm deep and 6mm wide. This Key is to fit inside the right hand bracket which has a corresponding slot.
Step 6: The Right Hand Bracket:
This bracket supports the syringe which slews the arm left and right, and as such needs to be firmly fixed to the end of the axle that the handles pivot on. I didn't want to glue it on, because I wanted to be able to remove the handles if required, but it needs to be rigid. I opted for a slot to mount the handle on to the end of a key on the axle. The key is 0.5mm shorter than the depth of the wood, so there is pressure exerted on the bracket by the end cap which is screwed to the axle.
The size of the bracket is 80mm x 30mm in 7mm plywood. The 2.5 holes for the bamboo are drilled in the sides, 18mm from the bracket base, 50mm from axle centre. Axle centre is at 18mm from the lower end of the bracket and has a vertical slot 6mm wide and shaped to fit the curves of the 16mm dowel. (see drawings)
Step 7: The Slew Bracket:
This U-shaped wooden bracket supports the end of the syringe that is used to slew the arm left and right. It can clearly be seen in the photo above.
Dimensions: 45mm high by 20mm wide with a 25mm space in between the arms of the U with 2.5mm holes drilled in each arm, on centre, 10mm from the top. The top looks better if it is rounded over with a 20mm radius.
Step 8: The Middle Support:
This wooden bracket is made from 20mm x 45mm pine. The Bracket is 140mm high x 100mm deep x 45mm wide, with a 16mm hole through both arms of the bracket centred 125mm from the bottom. I glued and screwed this bracket together. (see Fusion 360 file)
Step 9: The Semicircular Support:
This component supports the handle syringes, while still allowing the handles to be tilted left and right. I made this part fairly robust to reduce any deflection caused by the syringes being pushed in and out, and to provide a sturdy mounting for the 16mm shaft.
It has 3 main parts: 1) The semicircle, cut from 7mm plywood, radius 120mm but with the top of the arc cut off parallel to and 115mm from the base (2) a centre strengthener made of 15mm x 70mm x115 pine, (3) wooden brackets on the front side of the semicircle, either side of 25mm holes drilled 70mm either side of centre and 20mm from the diameter of the semicircle.
(See drawings) Construction is straightforward. Cut out the semicircle, glue on the 115 x 70 x 20mm block, drill the 25mm holes for the syringes and the 16mm hole for the shaft (centred, 25mm from the top) In the 4 x 38mm x 7mm x 15mm brackets which will go either side of the 25mm holes. Drill 2.5mm holes centred 30mm from the bottom of each bracket, then glue and clamp them in position.
If you don't drill the holes first, it may cause some drill clearance difficulties later....
Step 10: Three End Caps:
Any 4mm thick piece of durable material will be fine. I used plywood. (see above photo) You need one for each handle, and one to retain the 16mm tilting axle. To cut them out, I used a hole saw which has a 5/32 centre bit. If your hole saw has a larger centre bit, you may have to cut them out manually, because the centre hole has to be small enough for a retaining screw.
I made 2 x 40mm x 1mm plastic washers (from an ice cream bucket) to space the handle bracket off the centre bracket – one in front of the handle bracket and the other in front of the semicircular bracket. It reduces friction , provides clearance and allows free movement. This is visible in the Fusion 360 "centre handle support" model.
Step 11: Assembly:
Put rubber feet on the base board, then screw the middle support onto it about 200mm behind the grabber arm.
I pre-drilled the holes and screwed it on from the bottom, because it looks better, and at this stage I had assembled the main arm support, and access was limited.
Screw the handles and the right hand slew bracket onto the handle support bracket, using the end caps and 25mm 10g screws.
Put one 40mm x 1mm plastic washer on the short dowel, and thread it through the support bracket and also through the semicircular support. (see diagram) put another 40mm x 1mm washer behind the semicircular support before you push the 16mm dowel through other leg of the middle support bracket.
Screw and end cap to the 16mm dowel, and check for clearance & end float. Adjust as necessary.
Line up the semicircular support so that it is level with the handles, and so that it is hard up against the plastic washer, but still able to move freely with the whole assembly.
Drill a pilot hole through the top of the 15mm re-inforcement block on the semicircular support,, and put a 25mm screw into it to hold the it in position relative to the handles.
Step 12: Installing the Syringes:
It's best to install the grabber syringe first. This means taking the sliding dowel off the left hand side, threading the u-shaped bicycle spoke through the hole drilled in the plunger, and hammering it into the pilot holes in the dowel. Tape around the joint to prevent movement, then re-install it in the handle. I used the same technique as Technovation who designed the grabber arm: Cable ties and bamboo skewers. I found that for reliability, it was best, after the syringes were installed and filled, to put super glue on the cable ties, as it permanently stops them sliding, (they do, sometimes). With all the syringe bodies, it is important to allow the cable ties to be loose initially, until you find the optimal stroke for each one, then each cable tie can be tightened up, and super glue applied. I found it best to trim ALL the protruding parts (where your fingers usually grip the syringe) off the syringe bodies & plungers. Leaving even small remnants sometimes caused interference with the surrounding woodwork, causing jamming & other problems. If you look carefully at the photos you will see that the red tube (to the grabber) passes through a hole in the semicircular support. I did this to shorten its path and to keep it from dangling around and possibly catching on something.
The next syringes to install are the ones connected to the handles. To ensure that the syringes didn't get off centre on the handles, I used drinking straws as spacers. (red bits in photo 3)
An alternative to this would be to build the handles with a small wooden divider to create a space of about 25mm where the syringe will be, so that the syringe is supported between this divider and the side of the handle.
The last syringe to install is the slew syringe on the right. Work out where the U-bracket should go on the base-board, to get sufficient movement, then screw it down and attach the syringe. I used fish tank air hose to connect all the syringes up, and it worked fine, even though it is very thin walled. I coloured the water in each syringe differently as a visual illustration of what is happening & used a spare syringe to fill each section up, and added a drop or two of vegetable dye to each tube before filling.
Once it's all done & working, have fun!
Participated in the
Design For Kids Challenge