All posts by rui

Attaching an Encoder to a Stepper motor

The voind robot will use encoders to provide feedback on the position of 4 different axles. These optical encoders attach directly to the motor shaft and were supplied disassembled with the encoder disks carefully packaged inside a sheet of protective foam.

CAM00370

The encoder uses a mylar disk with the optical grid almost invisible to the naked eye. This component attaches directly to the motor shaft.CAM00371_detail

The motor producing the movement on the X direction was the first to have its encoder attached. However, unlike some other motors which already have mounting holes for encoders, this particular motor required an additional arrangement to securely attach the encoder.

CAM00015

An option would be to drill very shallow (2-3mm) mounting holes on the motor backplate hoping that these holes would not damage the motor. However I was not able to determine if the backplate was sufficiently thick to drill a hole without ruining the motor.

The first option was to build a bracket using aluminum sheet that would be secured at the front of the motor using the existing mounting screws and extend to the back of the motor to provide the mounting point for the encoder.

CAM00350

Two bolts would prevent it from moving sideways and provide additional adjustment.

CAM00357

This design had however a fatal flaw: the encoder needs at least 13.5 mm of shaft to properly attach the encoder wheel. This arrangement would let only 8mm of shaft exposed. The part could be redesigned to sit more closely to the motor backplate, giving the extra shaft exposure needed.

We decided to try something simpler: using the very shallow M3 threaded holes on the motor backplate to fix an adapter plate to which the encoder would then be attached. The challenge here was to fix the adapter plate using only 2mm of thread engagement and at the same time providing M3 threaded holes to fix the encoder, all this on 1.8 aluminum sheet.

CAM00366

This arrangement proved to be adequate for fixing the encoder.

CAM00375

After a final cleaning to remove the dust particles that were sitting on the disk, the encoder cover was screwed in.

CAM00378

Strong-arm tactics

The whole purpose in life for the voind robot is to actuate the physical controls of vintage studio equipment, effectively allowing the remote operation of the equipment.

To accomplish this the end effector must be quickly and precisely positioned over the controls and for that a strong arm is required.

arm

The main requisites for the arm assembly were:

  • the weight (all the assembly should weight less than 750g)
  • the arm’s structural rigidity (resistance to torsion on all axes)
  • the rigidity of its clamping to the shaft

The arm structure is made of aluminum which was first machined using a CNC router.

 

OLYMPUS DIGITAL CAMERA

After cleaning up the parts their dimensions were checked and minor adjustments made so they would fit precisely into one another.

Initial assembly allowed us to mark the exact locations for the fixing screws, which were then carefully drilled through the temporarily joint parts.

 

CAM00280

The lower end of the arm attaches directly to the “wrist” motor, making it effectively a key element of its structure. These holes were also carefully marked and drilled.

CAM00289

 

CAM00291

The arm was then temporarily clamped to a dummy shaft to access its overall stiffness.

CAM00285

CAM00284

Finally the arm was clamped to the aluminum shaft to test its integration with the robot.

Our tests so far demonstrated that the arm requisites are being met. However the aluminum shaft on which it is clamped has proved to be too flexible causing an unacceptable degree of torsion to the whole assembly.

 

Aluminum sheet fabrication

There are several parts on the voind robot made from aluminum sheet, most notably the cable supports which route the cables and keep them out of the way of moving parts.

YUpperCableSupport

Although their basic shape and position were defined early in the design, only after initial assembly we were able to determine their correct dimensions and integrate them with the other parts.

We started by creating a cardboard mockup of the part.

CAM00148

This allowed us to check if it would support the cables effectively and the clearances we’d have for driving the screws during assembly and disassembly.

CAM00087

Once we were satisfied with the cardboard mockup we traced the part over 1.5mm aluminum sheet.

CAM00152

Using a Dremel cutting wheel we then cut the part out of the aluminum sheet.

CAM00154

Bending was done manually using a vice and steel guides.

CAM00159

Final tests guided us on tracing the locations of additional features like holes for screws and cables ties.

CAM00170

Finally the part sitting happily in the overall scheme of things and doing exactly what it was designed for.

CAM00232

DIY combined Milling/Lathe machine

The voind robot design is based on a swivel arm that rotates on a horizontal shaft driven by a worm/wheel combination gear. This shaft has a very simple design with a 10mm main section and a 20 mm narrower section with 6mm diameter.

To fabricate the narrower section we decided to build a small combined Milling/Lathe machine from MDF board. This contraption basically holds firmly a small rotating tool with a small drill and at the same time allows a 10mm shaft to be fed and rotated against the mill.

CAM00246

After a quick test we were good to go at machining the end of our aluminum shaft. Check the actual footage below:

After the operation the machined surface shows a scaled aspect due to the manual feeding of the shaft.

CAM00266

But after a quick sanding with fine sand paper the result looks very smooth:

CAM00271

You can see below the wheel firmly clamped to the new shaft and being driven by the worm.

CAM00273

Update:

This aluminum shaft proved to be too much flexible. A steel shaft will have to be used instead.

Cutting a threaded shaft

One of the motors on the voind robot is an External Linear Stepper Motor, more precisely an SY17STH0404-300A6.35X2X2-P1-EX. It has a threaded shaft engaging a threaded nut that drives the robot arm perpendicularly to the equipment face.584[1]

This motor is conveniently supplied with a 30 cm shaft which should be more than enough for every application. However in our case we needed to cut the shaft shorter.

YMotor

We first wrapped the motor in plastic film to protect it from any metal chips and used a small drill press vice to hold it firmly by the shaft, having wrapped it in a generous quantity of soft paper to protect the thread.CAM00251

We then used a Dremel EZ456 cutting wheel to make an initial cut one centimeter outside the mark to be able to position the tool perpendicularly to the shaft for the final cut. To prevent overheating the shaft and motor we cut in 10 to 15 seconds intervals followed by 45 to 60 seconds periods of pause for cooling.

Interestingly, although the shaft seems to be made from stainless steel, not a single spark was produced during the cut…

Ideally we would have a 6.35×2 nut screwed in to help restore any damaged thread start by removing it after the cut. However we did not have any suitable nut around and we had to resort to a magnifying glass and some fine sand paper to make sure the threads started smoothly.CAM00258

Finally the motor with the correct thread length and ready for assembly.CAM00261

First cables installed

We just installed the first cables that will connect the control electronics to the various motors and sensors.

CAM00225

At this stage this is only for checking if all cables are correctly routed and clear any moving parts.

CAM00232

Cabling

 

Bend over (and over and over…)

Some of the parts for the voind robot project are built from bent sheet metal. This is the case for the two stainless steel chassis side plates on which the main ballscrew is mounted:

CAM00015

Fortunately, having sourced the chassis construction to a very competent expert on metal fabrication, I never had to account for the material along the bending lines and only specified the final dimensions of the finished parts:

Chassi_Assembly_10

However as I soon discovered while fabricating the X Assembly cable guides supports from aluminum sheet, correct calculation of both bend allowance and bend deduction are paramount for obtaining a precisely dimensioned part from metal bending.

The failure to understand how to correctly incorporate the bending into the flat part design was partially responsible for multiple attempts at this part fabrication:

CAM00075

We finally got it right after using a mockup of the part which included the main bends but using only a narrow cross section of the finished part (the part on the upper right image above). Taking precise notes of the bending lines and adjusting them accordingly on the production part landed us with a precisely dimensioned part (almost) ready for integration!

CAM00080

More bending action lies ahead as we prepare to fabricate the more complex Y Assembly Cable Guide supports.

CAM00087

Screws and more

For the voind robot project we are using nearly two dozen screw references, mixing types (DIN 7984, 912) and sizes (M2 to M5, 6 to 35 millimeters lengths).

ScrewList

The screw types and positions were established during the design phase taking into consideration the dimensions of the parts, the forces involved, the clearances for the screw heads (and nuts) and also how to reach the screws during assembly and maintenance.

XAssembly_screwdraft

Most of times the screws had mating threads cut into one of the parts using a manual tap.

OLYMPUS DIGITAL CAMERA

Some tap operations proved to be very challenging, specially when cutting M3 threads into stainless steel beams and in particular the one where we could not use a tap handle due to the part geometry:

OLYMPUS DIGITAL CAMERA

In the end we assembled quite an impressive array of screws, nuts and washers references!

CAM00085

X Assembly cable guides support fabrication

This assembly supports the main cable guides carrying power and data for the X Assembly carriage and also provides a mounting point for the micro switches used to detect the Y movement limits.

XAssembly_Cables_blender

CAM00048

This part is being built from aluminum and acrylic sheet.

CAM00040

CAM00041

Several arrangements for the micro switches are still being evaluated.

CAM00047

CAM00062

One technique we are using here to save precious space and minimize exposed metal near electric contacts is embedding the nuts into the acrylic using a small soldering iron to heat the nut and press it into the acrylic part.

CAM00069

The final design for the aluminum sheet part is not yet decided. Until then we keep fabricating several versions of it 🙂

CAM00067

Notes on aluminum fabrication

This was the first time we ventured into aluminum fabrication using a CNC milling machine. The majority of the voind robot parts were designed to be fabricated from aluminum bar using a CNC milling machine.

Soon into the project we realized how expensive it would be to have these parts sourced from specialized shops. We were quoted with hundreds of euros for a single aluminum part and we would need to fabricate more than 20 different parts!

Proposta_Laser

Online metal fabrication shops like eMachineShop were also evaluated but were still too far outside our budget.

OnlineShopQuote

Apart from this we feared we would end up doing several iterations before we obtained exactly the required part, specially because we lacked the knowledge and experience to spec the parts correctly.

So we decided to try and do it ourselves all the way from the 3d files up to the actual part machining.

We started by visiting a local fablab shop where we met knowledgeable people who introduced us to the basic tools and techniques used to create the parts drawing files and prepare them to be submitted to the milling machine.

FabLabEDP_Notes

Unfortunately this particular fablab shop had their milling machine already booked for the dates we planned. However they kindly introduced us to another fablab shop where we could book the milling machine for specific dates which were compatible with our calendar.

OLYMPUS DIGITAL CAMERA

Isn’t it wonderful to have access to such an array of resources, tools and knowledge?

If you happen to have a project or idea and you are stuck with how to realize the physical aspect of it, search for the closest fablab and pay them a visit.

Mills and Milling parameters

After several broken mills we set on 3mm HSS + 8% CO end mills:

  • Spindle: 5500 rpm
  • Feed rate: 5mm/s
  • Stepover: 1.2mm
  • Stepdown: 0.75mm

OLYMPUS DIGITAL CAMERA

You can buy these mills from your local specialist tool shop. I bought mine here and here.

Lubrication and cooling

During the milling operation we manually sprayed cutting lubricant and also water to cool things down, specially during the most aggressive phases of the cut, for example when the tool had to start a new cut into the material.

OLYMPUS DIGITAL CAMERA

Sometimes we could feel the generated heat just by touching the aluminum up to 15 cm from the cutting action. The only time we tried to save on the lubricant and water we ended up with a mill literally welded to the aluminum. Not wanting to ruin another mill we diligently kept the spraying action throughout the whole operation.

Material fixing

We used MDF as sacrifice boards on which we fixed the aluminum bars using screws. The exact positions of the fixing screws were determined during the layout of the files in preparation for the milling operations. This way we could plan for an affective support of the material during the mill operation and at the same time check that the tool path would not touch any of the screws.

FixingScrews

The MDF board was then securely fixed to the milling machine bed using additional MDF scrap boards and clamps.

OLYMPUS DIGITAL CAMERA

For long sequences of operations where the MDF board would be exposed for significant time to water and lubricant, we first covered the board and edges with adhesive tape.

OLYMPUS DIGITAL CAMERA

However, any drilling operation that drilled into the MDF board acted as a direct injection point of water into the MDF causing it to swell and slightly push the aluminum up, enough to ruin all accuracy in the Z dimension. This led to some ruined parts and on two occasions left parts dangerously loose due to overshooting the cuts in the z axle.

The software we used to create the toolpaths did not offer enough flexibility to let us specify where to place brigdes to hold the cut parts in place. As a result we decided to leave a half millimeter skin holding the parts and separated them afterwards.

OLYMPUS DIGITAL CAMERA

Workflow and Planning

The richness and openness of the environment at a fabrication shop like a fablab can quickly draw your focus away from your main objectives. There will be plenty of experiences to share and projects to admire and, unless you prepare yourself with a robust work plan, you might end up not using your time and available tools in the most efficient way.

Before a visit to the fabrication shop I usually try to:

  • have a detailed plan for the tasks I want to execute;
  • mentally rehearse and visualize all the tasks, the tools needed and all the contingencies which are likely to happen (like a broken mill, unavailable PCs, a corrupted file, etc.);
  • prepare a checklist on the day before and make sure I am carrying everything I might need;

OLYMPUS DIGITAL CAMERA

 Finishing

Most of the times the freshly cut parts needed extensive finishing before being ready for assembly, specially because we insisted on leaving a half-millimeter thick “skin” at the bottom of the cuts to hold the parts together.

OLYMPUS DIGITAL CAMERA

For that we used a Dremel and a mill tool to trim away the skin with the assistance of a steel guide to avoid cutting into the part. A final touch with a file and sand paper gave the part an acceptable finish to move it to the test assembly phases.

OLYMPUS DIGITAL CAMERA

Fixing parts together

We looked for some time into the possibility of welding the aluminum parts. However due to design constraints which dictated that some assemblies would have to be disassembled for maintenance, we drill and tapped holes and used mostly DIN 7984 M4 screws to fix everything together.

OLYMPUS DIGITAL CAMERA

Only time will tell if the joints are strong enough to withstand vibrations during normal operation.OLYMPUS DIGITAL CAMERA