Tweakie.CNC under construction.

(13) A lead screw thrust bearing has been built into the top section of the Z axis. This consists of two angular contact radial bearings mounted back to back.

 

(14) Z axis nearing completion.

 

(15) Limit switches have been fitted to each axis. These are standard size micro switches. They have been fitted to operate just before the mechanical stop is reached but in such a position that they will not be damaged in the event of an over run.

 

(16) Stepper motors fitted. There could be a problem with loss of residual magnetism if bipolar stepper motors are taken completely apart, but if you are careful, just the back plate can be removed to drill and tap to fit a terminal block as I have done here.

 

(17) Y axis stepper, flexible coupling and thrust bearing. Although the method of mounting the steppers is extremely basic it does work very well.

 

(18) Z axis stepper and flexible coupling. In the perfect world true alignment, squareness and concentricity would be possible but in reality flexible couplers are a must.

 

Some design considerations when using 200 Step (1.8 degree) Bipolar Stepper Motors as the driving force for a router.

 

 

Steppers, under no load conditions, have a maximum speed at which they will rotate (without loosing steps) and if steps are lost then the position and accuracy of the whole machine is compromised. It is important to decide on the accuracy required (maximum whole steps per mm) at an early stage and not just to opt for the maximum number of steps possible. Obviously the greater the number of whole steps per mm the greater the accuracy but the slower the finished machine will be able to traverse and the longer it will take to complete projects. This may not be an issue with a precision milling machine but for a router designed for wood, plastic and alloy parts a rapid feed rate of 100mm per minute would be painful.

 

An accuracy of better than 0.01mm would be wasted on wood and plastic parts although it may be a good working tolerance for accurate alloy parts. Some of the most commonly used stepper motors are 200 steps per revolution and one of the most commonly available ball screw is 5mm pitch therefore by direct drive a resolution of 5/200 or 0.025 mm (which relates to 40 whole steps per mm) is possible. Micro stepping at say 1/8 will increase the theoretical resolution to 5/(8x200) or 0.003125 mm (which relates to 320 micro steps per mm). Now micro steps are not that accurate but from tests which I have carried out on three different manufacturers motors, they are a lot better than +/- one quarter of a full step. Taking this into consideration a 200 step motor, micro stepping at 1/8 step, direct driving a 5mm pitch lead screw will give an accuracy of better than 0.00625 mm. Over long distances of travel the accuracy of the lead screw has to be considered in the equation but it can be ignored for short distances as the errors in the screw more or less cancel each other out.

 

So, I submit that an accuracy of better than 0.01mm can easily be achieved by direct driving 5mm pitch lead screws with 200 step per revolution steppers operated in 1/8 step mode.

 

The next consideration is to keep the enemy friction at its lowest possible value for all moving parts. Once fitted to the machine the steppers maximum possible rotational speed is reduced by the friction in the drive chain (lead screws, drive belts, couplings, slides etc.) and it is important to be able to keep the speed up. This is why the low friction linear slides and ball screws are favoured despite their high cost. The mass of the moving parts is also an issue with regard to speed and any reduction in their weight (whilst still preserving rigidity) is beneficial. Mach3 software incorporates acceleration and deceleration routines in it's motor tuning section (to allow for mass) but the longer the times which have to be selected, to achieve reliable operation, the longer it will take to complete a project. It is fairly obvious, and goes without saying, but there must be no binding or tight spots anywhere on the machine. Where precise alignment is not possible (almost everywhere) then some form of flexibility (without backlash) in the coupling medium between parts is necessary. I have no doubt that with expert skill and a fully fitted machine shop a precise machine could be constructed but for the rest of us careful design, to allow and cater for our inaccuracies, is essential. For a relatively small machine it makes sense to have a moving table and a fixed gantry to distribute the mass of the moving parts in a more even manner. But when considering the floor space requirement or footprint of the finished machine a moving gantry almost cuts this in half.

 

Stepper motors are specified as having a Voltage and Current rating. The current rating should never be exceeded as this is related to the capability of the internal windings to dissipate their heat generated under operation. The voltage rating, on the other hand, can be exceed and it is common practice to use an operating voltage five or more times that specified. (The theoretical maximum voltage would be just below the point where windings insulation breakdown or arcing between terminals occur.) Now the formulae for the torque of a stepper is Power (watts) = Volts x Amps and as we cannot increase the current (amps) without providing some form of elaborate cooling (liquid nitrogen would work !) it is necessary to increase the voltage to gain torque. As torque is opposed by friction, the higher the torque the better the speed, up to a point. (It perhaps should be noted that once magnetic flux saturation of the cores within the motor has occurred, no further improvements to performance can be obtained - so there are design limits to the maximum torque and speed that can be achieved no matter how high the voltage is increased.). Everything in life is a compromise !.

 

Stepper motor driver cards, readily available to the hobby constructor, also have design limitations as to the maximum Voltage and Current which they can handle (imposed on them by the type of the I.C chip used and whether or not they incorporate post chip driver circuits and heatsinks) and this is typically around 2.5 Amps at 30 Volts for StepMaster or 10 Amps at 37 Volts for Rout Out and 7 Amps at 80 Volts for Gecko Drives.

 

I intended building my own 30 Volt power supply but dropped the project when I was able to obtain a new Omron 24 Volt 6 Amp p.s.u. on eBay at less than the cost of the required torroidal transformer. This fixed my maximum voltage at 24 Volts and as my steppers were rated at 2.4 Amps the basic driver card would be satisfactory. I was never really certain if there would be enough torque to drive Tweakie until it had been built and tested but my design always allowed for Plan B of using a belt drive reduction gearing between the steppers and lead screws, accepting the associated maximum traverse speed reductions. (My plan was to use a 4:1 reduction with 10 and 40 tooth pulleys and a 10 mm wide belt - but in tests I found that there is a lot more friction associated with timing belt drives than I had first thought. Large diameter pulleys are OK but small diameter ones should be avoided.). As luck would have it, there was plenty of torque at 24 Volts with the driver card current limit set to 2 Amps - so direct drive it was to be.

 

The following figures are based on practical tests and the assumption of a 5mm pitch lead screw :- The available torque, from the stepper motors, being speed related (more speed - less torque) sets the practical limit for the fast traverse speeds of the finished router. Whilst the no load rotational speed, under Mach3 control, could be as high as 1300 rpm (which equates to a fast feed rate of 6.5 Metres per minute) the torque is so low that any reasonable resistance to rotation will stall the motor. It would be extremely difficult and very costly to build a machine with low enough friction and big enough stepper motors for this to be a practical feed rate to aim for. (It is this very limitation which leads professionals, where machine time is money, to chose servo motors as the driving force, but hey, this is another subject.). A practical speed of around half the maximum attainable would be a good starting point to aim at. I have found that a maximum motor speed of 600 rpm giving a fast feed rate of 3 Metres per minute is easily achieved and 100% reliable under all circumstances. (tweakie has been running at this motor speed, without issue for extended periods at workshop temperatures below 5 degrees C this winter). There is sufficient torque here to break quite sizeable cutting tools (if the block clearance heights have been set incorrectly) without stalling the motors. The current adjustment on the driver card comes into play at this point and adjustments should be made as a compromise between the maximum speed and maximum torque required. If you are going to be using an expensive vacuum table, a mechanical (adjustable) stop fitted to the Z axis can save a lot of grief, but to avoid damage to the axis components, if the stop is evoked, the current limiting should be set just right.

 

Studying others designs and reading from others experiences is, in my opinion, the best starting place for any project (it is far better and a whole lot cheaper to learn from others experiences than to learn from your own mistakes) but you must think outside the box to innovate or invent. The club of CNC followers is constantly coming up with new ideas and what ifs and the Mach3 forum is very active in this respect. If you have not yet dipped your toe into the waters of CNC then dive in, it is great fun, trust me.

 

In industry, your employer has an obligation towards ensuring your safety.

In your own workshop the health and safety regulations are not enforced it is up to you to keep yourself safe.

!

!

CNC is only limited by our imagination.

Tweakie.CNC

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