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CNC router 'OTOCOUP'
- Construction -




Advices

Cabling / Wiring (separated page)

Adjustment

Practical equipment choice

Preliminary tests


Setting in a box

Stronger !

Improvments

 

Advices

There are a lot of nuts, bearings and tubes, but construction is relatively fast, because this is serial work.

For drilling tubes, it is compulsory to make a drilling guide, because tuning and soft working of the machine depends from good centering of holes. Hammer marking is long, and don't improve that much precision. You can correct bad drills with a small file, but that's long...

A fixed column driller is required to have straight bores.

See plans for the drilling guide. A good pecision for guide construction is required. Wood bars must be glued, in addition to screws (Epoxy).

Drill directly to diameter 8.5, without pre-drill, which will deviate the drills. Moreover, you will go faster.

For aluminium ruler drilling, another guide is also required. Du to center partition element, your drill will irremediably goes sideway, even if your drill is installed very short.

If you needs to redrill some parts (gears), it is required to use center-drills, to prépare a cone which will guide your drill. It is nearly impossible to maintain proper centering with usual drills, even if you drill by small diameter increase.
Proxxon distribute a set of 3 center-drills, with the good diameters, one being diam. 8 and the other being diam. 6

The nut of Z carriage is a wood insert, manufactured in brass. It have been ovalised with a hammer to reduce the play. Do that progressively and test on all length of the screw, because standard screws have not a good regularity.

To well center rods in bearings, roll a copper wire in the thread. This is compulsory for cables rods if your machine is driven by cables.
I've personnaly use tape for centering the rods, but that's not much precise. Others have tried plumbing teflon tape.

When you screw wood parts which constitute you support area, bury the head of 3mm inside the wood, in order not to cut headscrew when you will surface (with a bit) this area, which is compulsory to have a flat area aligned with the movements.

Router installation.
If your router is equipped with a neck, the best solution is to make a support which allows to tight/untight it quickly, to remove router, which is the fastest mean to change bits. If not, you can install the support on the carriage. This is what i have done, but this is a bit heavy.
Plans are drawn for smaller routers, like the Kress, so no problem.
On some routers, you will be obliged to open the machine to remove top abutment. This will allow to remove router from it's support. Don't forget to remove springs. I've left them temporarily, set the router on it's head, and when untightened, the support have been throwed in my teeth, which is somewhat unpleasant...

Control board.
If you don't have any experience in electronic, i suggest you to buy an already manufactured board, because realisation (and test, mainly !) of such a board is work, and there is important risk it will never work.
If you wish to build yourself the board, component cost can be estimated around 100 Euros.
To this cost, you must add connectors, wires, metallic box, fan, so almost 100 Euros more.

Electromagnetic radiation :
Set your board in a metallic box, a board based upon PWM give a lot of parasites and radiation, and your computer may dislike it. Use shielded cable for steppers and end-switches (4 x 0.22 mm2 + shield). Beware, the router also generate parasites.
Earth properly all your equipment.

By what to began ?
If you decide to do yourself your boards, began by this job. This is the longest and most delicate part.
Else, it is needed to order sooen belts, gears and steppers, because your machine may be modified by the equipment you can effectively supply.

How to install stepper connectors:
Generally steppers are equipped with connectors similar to Molex 'KK'. It is needed to install compatible male connector.
As these connectors does not have box, to have a clean installation it is needed to :
. Set thermoretractable tube on the wires
. Set thermoretractable larger tube on the cable
. Weld the wires
. Slide the thermoretractable tubes on the wires and restrain them with a hot air gun.
.
Slide the thermoretractable large tube on the enssemble and restrain

Emergency shutdown :
Attention, most of board emergency shutdown cut the power transistors. In the case of Otocoup router, it will drive to have the carriage falling down under it's own weight, so this is to be totally avoided.
It is only needed to stop clock signals between computer and board. On my own boards i cut the signals by shutting down the signal drivers. If you cannot do like that you must either :
- Send an emergency signal to the computer, some software have this function, but it is not very safe.
- Shutdown computer power. This is a bit savage, but result is guaranteed.

Safety :
. After having adjusted belt tensioners, put screw-locking adhesive on the nuts.
. All the nuts of pulleys and belt bearings must be blocked with screw-locking adhesive.
. It is a good idea to lock similarly all the nuts of the machine.

. Check the electrical box and the electrical insulation between power (230 V) and electronic signals, before wiring it to the boards.
. Don't start the machine before having installed the emergency shutdown :
   - On the router/drill head
   - On the computer/control board
. Before running the router for the first time, have a look on everything
. Safety glasses and earplug required


Cabling / Wiring (separated page)


Adjustment

Bearing lean :
All bearings must lean on the ruler. If your holes are not well centered, some bearings may not touch the ruler. In that case, you have to tighten somewhat. This will render the carriage movement a bit hard. In the worst cases, you will have to dismount and adjust the holes with a file.
To check proper tightening, try to turn the bearing with fingers, with the carriage stopped. They must be able to turn, but with some difficulty if properly tightened.

Play on gears :
It is very important to have no play on the gears, and even they must be a bit tightened. If you fail to redrill the gear center hole well centered, buy a new gear, and do it again (...)
If you have play :
. On X carriage, you will have at low speed resonances giving much noise and vibrations. It is impossible to completely eliminate it, but with proper tightening (especially on plastic gears), you can significantly reduce it.
. On Y carriage, while inverting motion direction or on speed transitions, the back movement due to play will give a shock to the stepper, increased by the fact that the carriage is suspended to cables/belts, and the stepper will lost a step, which given the carriage weight will drive to lost many steps and your carriage will fall down, though the speed rate is lower than if you cut power on the boards.

My carrying system is based upon cables, and i have important slipping on Y carriage (weight..). I need high tension on the cables to reduce it.
This is one of the reasons present drawing only propose belt system, even if much higher cost. The other reason is stiffness, a bit insufficient on cables.

Practical equipment choice

If for the mechanical part, the drawings are fairly straightforward, a lot of questions arise about electronic, steppers and power supply choices. You will find some theory, explanations and metaphysics in the Technical chapter.
Here, i will try to give you more practical advices. But, as always, use your judgement.

What kind of stepper board ?

Static, with resistances for current regulation, or PWM driven 'choppers'.
For low speed, low torque need, any simple board can do the job. So, for hotwire cutting machines, of for small routers used to drill PCB or to machine balsa wood, simple boards with resistances and no current control are used.
If you need speed, and maintaining the torque at that speed, you have to give high voltage to your motors (at least four times the nominal voltage). With a static board, that need high power resistances, which generates a lot of heat, and your board electrical consumption will be high. So, it is recommended to use a PWM current regulated board, as soon as you want exceed 2 rotations per second, with minimal torque.
For Otocoup machine, it is my firm recommendation.

What kind of steppers ?

If you use a PWM board, you will have the ability to run fast with yours steppers. So you can obtain power, not by having a high torque, but by having medium torque and higher speed, which will drive you to have larger gear reduction.
So, using Nema23 size (57 x 57 mm) steppers is possible for large machines, provided you choose good quality steppers, with PWM boards.
I'm using on Otocoup machine Nema 23 bipolar steppers, with a torque of 6.2 kg.cm (86 oz.inch). Regarding the machine size, they look tiny, but that works !
Anyway, if you salvage steppers, you may find old Nema34 steppers (in phptocopy machines, for example), where the big size can compensate a technology less advanced. The weight will be higher, but as you have strong stepper, this is not a problem.

If you bought your steppers new, my recommandation is bipolar steppers, 2.6V, 2x 2A, torque 6.1 kg.cm. They costs 10 Euros each. Don't go for Otocoup machine below that torque, or you will have to increase the gear ratio. As these steppers are bipolar, they must be driven by bipolar boards. A lot of bipolar boards are using L297/L298 circuits, which are perfectly appropriate for the given steppers.

For Salvaged Unipolar steppers, which may have higher currents, boards based upon L297 + 4 Mosfets can be more appropriate, though boards with L297/L298 can also do the job.
Note : even if the nominal current of your stepper exceed the maximum current of the board, the stepper may have a sufficient torque for your use, as the torque decrease less fastly than the current.

What kind of power supply ?

If you choose a PWM board, more costly than 'resistance' board, you will have the big advantage that, the current being continuously controlled by your board, there is no need for a stabilised power supply.
So, a simple transformer, with rectifiers and a big capacitor (10 000 microFarads) is sufficient.

What voltage for that power supply.
Being fed by a chopper drive, your stepper will need a supply voltage at least four times equal to the nominal voltage.
The maximum supply voltage is generally considered to be 20 times the nominal voltage.

Higher voltage give higher speed ability (provided your stepper is able to go fast)
Higher voltage give higher the torque at high speed (but low speed torque is unchanged)

Though, there is a drawback. When you increase the supply voltage, it tend to increase the losses (iron losses) in the stepper, so it heat more. That heat increase is progressive with the voltage ratio, with a higher rate at high ratio. It became significant around 10 times the nonimal voltage. Also, most of our machines does have steppers installed on wood support, so are not very well cooled.
So, a good compromise can be a voltage supply ratio between 8 and 12.
Beware, it is normal the steppers heat up. A temperature of 50-60°C (122-140 °F) is common.

But beware, voltage after rectifiers and filter downstream your transformer will be significantly higher than the mean voltage on transformers wires. It will approach the ondulation peak voltage.
Approximatively :
Voltage after rectifier = 1.4 * Transfo voltage -1.6V (1.6V is voltage drop in the rectifier)
At full power, we will still have an ondulation which will decrease the voltage, but at low power, this tension will be present.

For a 24V transformer, filtered and rectified voltage is so 24V x 1.4 -1.6V = 32.1 V

So, for the recommended stepper (2.6V), maximum voltage of the transformer is 24V, so 32V after rectifiers, which give a voltage ratio of 12.4.

But beware, your board have maximum voltage.
You must maintain a margin between your power supply voltage and your board.
If your power supply is stabilised, it is recommended to not exceed 80% of the maximum board voltage
So, if the board have max voltage of 46V, it is recommended to not exceed 46V * 80% = 37 V
A 24V transformer giving output at 32V is so near the maximum.

To defined the power of your transformer feeding a PWM board, as a rough estimation, you will need half of the Amps needed on the stepper. This is true if all stppers can be used at full speed all at the same moment. That is way rare, because if it's possible to have X and Y at full speed at the same moment, Having Z also is fairly not probable. Moreover, if you do have two heads and so two Z axis, they will never run at the same moment.
Coonsumption of stopped steppers is not zeron but it is much lower than at full speed.

Here we have steppers 2x2A, so for 4 steppers, total current is 4 x 2 x 2A = 16A.
The Amps 'saving' of the PWM board will drive to a maximum consumption of around 8A.
(a PWM board behave sort of a voltage transformer)

That will drive with a transformer of 24V, to 24 x 8A = 192 VA, so practically a 200 VA transformer will be used.
Stepper stopped, the electrical power consumption of the stepper is
2x2A*2.6V = 10.4W. L297 brige (or another bridge) having a not negligible consumption, we can estimate the stopped consumption upstream the power board between 15 W to 20W.
For 4 steppers, that gives a total from 60 to 80 W.

If your fourth axis is that of a second head, never working simultaneously with the first head axis, you most probably can be ok with a transfo of (2x2A * 3 steppers)/2 * 24V = 144 VA, so a transformer of 150VA is sufficient.


You must take care that, over 24V voltage, electricity is dangerous for human beings, so beware not to touch naked wires in service, and take care of the quality and isolation of your cabling.

Preliminary tests

14 Feb. 2003 Tested with manual command (not completely finished).

Stepper Astrosyn 2.6V, 2x 2A, 6.2 kg.cm
Power supply 13.8V
Board current adjusted to 1.42 A, results being better than with max current (2A)

Maximum usable speed is 2400 half steps per seconds (under load), say 6 rpm on motor, 44 mm/sec, 2600 mm/min.
The carriage (weight 10kg) have no counterweight, and so, is suspended to cables and give permanent torque on stepper, so the load is important and continuous. Good conditions for a test.

Work only in half steps and control set on 'phase chopping'

Stepper have been tested without load at 4300 half steps per second, but that significantly reduce the torque (which is 1/4 to 1/3 of low speed torque)

The fact the system is working better with lower current tend to suggest that a much better speed can be obtained with higher voltage.
Maximum voltage for the L298 bridge is 46V, so practically 40V for a regulated power supply, or 30V for a transformer with rectifiers and big capacitor.

Some photos of test installation :


Setting in box

Franck Aguerre have used former PC power supply boxes to install his power supply and also his boards.

Herebelow the photos of power supply.:



You can also admire the Setting in box of power and control boards.

Stronger !

This machine is studied for amateur use, and don't pretend to a good durability, especially because of the marks which wil occurs on aluminium rulers.

So, as there is always furious guys who want more, find herebelow some ideas to reinforce this machine.

Reinforcment of horizontal rulers

Simply replace it by steel tube 80 x 80. They are fixed, so weight is not a problem. Cost is comparable, but there will be corrosion. So use protective oil (avoid any contact between that protective oil and bearings internals). See plans.

Belt reinforcment

It is possible to use more recent belts than HTD, as Polychain GT, but availability either of open belts or sprockets is a problem. That may change in the future.

Turbo !

Taking into account the demultiplication, speed depends mainly from your electronic and the possibility of your software to manage acceleration.

Anyway, if you want to go faster, further, stronger..., you can install bigger steppers. In Nema 23 size (57 x 57), it is possible to find steppers having 18 kg.cm torque, which allow use of unmodified planset, just by changing sprocket diameter.


Though, a NEMA 23 stepper of high torque might be more costly than equivalent torque NEMA 34 stepper. Moreover, if you salvage parts (from photocopy machines, in particular), chance is that you get NEMA 34 steppers.
Up to you, knowing that supplying high torque steppers is not easy, neither economic.

Improvements

Router(s)

It is possible to think installing another head on the other beam side, with a different tool on each router. Or with a router on one side, and a driller on the other.

Vacuum table

To maintain your pieces, it is possible to install a vaccum table.
Two plates of OSB, separated by wood beams (cutted, to left air passage), but with a tight frame, can with a powerful vacuum cleaner, make a vacuum table able to maintain your parts. It is useful if you mill/route thin plates. Top plate, thickness 12 to 15 mm, might be drilled with holes diameter 1.5 mm, every 50 mm.
Please note these are theoretical values, and that i have not tested such table.
But, as you already have the automatic machine to drill the holes, that will not be a great deal.
We can be tempted to make a derivation from main vacuum cleaner, but i'm afraid it will significantly reduce it's performances.

Pushing skid

Another alternative to maintain panels is to have, concentric to the bit, a skid pushed by springs. It must be done in slipping plastic, as polyethylene. You can see such a device on Haase site, in accessories page.

Automatic tool changer

For building a cutting head, a good solution is to have a naked shaft installed on bearings, with the motor aside, installed upward, with a belt transmission. Considering the fast price decrease of brushless motors and associated electronics, it can be thinkable to do that with modeller equipment, giving very light equipment. Durability (of bearings especially, which have a permanent side load), must be checked. For information, professional routers used high frequency generators.
That will allow to have a bored shaft to install an automatic pinch, and so an automatic tool changer. The router already have transversal and axial movement, only remains pinch problem...
Though, it seems that professionnal changers are based upon magnetic bit handling, with keys to stop rotation. We can make a permanent magnet hold, with a pneumatic ejection ? Or electromagnetic ejection ? In any case, for safety reasons, the bit must be maintain when power shutdown. To be studied...