Weiss VM32 Mill Conversion

[picclock]

Continuing with the saddle Y axis and some electrical work.
Limit switches, soft limits and home (zero return, grid 0, origin, etc.) position. These systems are designed to prevent the machine trying to go to areas outside the set envelope. Hard limits are set with switches that are triggered when the axis exceeds its normal working range and are used to set home coordinates. Soft limits are often set to confine the work area to a subset of the total area. With stepper motors if the mechanical limit is exceeded all that happens is the motor stalls with few other consequences. If there is a reduction drive it will be subjected to the maximum force the motor can supply. Forunately no smoke or fires occur. However, if this occurs it means that things have gone seriously awry, and the work may or may not be recoverable. There is little to be done to prevent the Z axis hitting the bed during an error as the tooling used is of indeterminate length, so no mechanical switch can be used. When a CNC controller starts, usually the first thing it does is to go to the axis home and detect the position with the limit switches. From that point on all coordinates are calculated relative to these positions.

As this conversion will use glass scales I have used the Y scale bracket to trigger the microswitches. The rear of the saddle has the X limit microswitches, and the left of the saddle the Y. I will upgrade these at a later date to improve the homing accuracy using a latching optical disc sensor I read about here :-
https://www.cnczone.com/forums/uncat...mit-works.html. The switches will be wired normally closed to ensure that any break will stop the machine.

Sketch with dimensions is really just my crib sheet so take it with a pinch of salt. Also check dimensions of your machine before assuming my drawing is correct. Initial test install of the ballscrew showed an issue with the hole in the front base of the machine. This was approx 1mm too small for the thrust bearing I was going to use. This was opened out using a 33mm hole saw. The position of hole saws is tricky to maintain without a pilot hole to guide. Positioning was achieved by use of a metal plate turned to the outside diameter of the saw cup. This was a simple and successful way of enlarging and centering the hole. The table and saddle were then marked out using a scriber and engineers blue, then drilled and tapped. As shown on one of the pictures its easy to use a metal piece with a right angle cut out to align the drill/tap perpendicularly. The base was similarly treated. To get good right angle holes and threads tapped into the base the drill needs to be held horizontally. This can be simply achieved with a few books or magazines under the drill body, piled to the correct height. I have used std V5 microswitches, the Y ones are mounted on a carrier plates for simple adjustment. Last pictures show the current state of play.
Best Regards
picclock

[routercnc]

Nice work. Following with interest !

[picclock]

As requested my take and decision making on :-
Controllers,
I have chosen to go with a standalone option, the DDCSV 2V1. This is a 4 axis controller integrated into a single unit which runs Gcode. It incorporates a colour screen and keypad, providing the signals for the stepper drivers, and inputs for the limit/homing switches. The advantage of this approach is cost, simplicity, reliability and mechanical robustness, as its all solid state. The gcode is written to a USB stick and plugged into the controller. There are no software licence fees or internet connection needed for licence verification, no spinning hard disks to fail or corrupt, no fans to fail, no external interface compatability issues . Additionally the performance of the box is outstanding with stepper rates on all axis up to 250KHz and pendant support thrown in. In size terms the box is about the same size as two cd rom drives ~ 160x100x50mm. It requires an 18 - 32V supply which can be derived from a dedicated supply or the stepper psu. Power requirements are very modest at up to 0.5A max (12 watts or so). Inputs are opto isolated, and it incorporates a VSO for motor speed control. I have no financial or other interests in this company, just a user.

Motors and ballscrews
How much force do I need ? How quickly do I want to go ?.
Mill cutting forces* for metal under maximum load measured at ~ 50-60kgs, with smaller cutters around ~20kgs or less.
How fast can the motor turn while producing this force?. Stepper motor specifications generally only quote the holding torque - useful only if the motor is not turning. Getting a torque curve for the motor you are going to use is paramount - if you cant get one don’t buy it !!. Hybrid stepper, bipolar 4 wire versions are generally best. A torque curve for the popular longs motor 23HS9442 indicated I could get 1Nm (140 ozin) at 450 rpm.

Using a 5mm(0.2”) pitch ballscrew with this calculator :-
http://www.cncroutersource.com/linea...alculator.html
indicates a useable force of 109Kg whilst cutting at a speed of 2.3M/min (90 inches/min), more than enough for my requirements. Smaller pitch = less speed more force, larger pitch more speed less force.
The speed of a stepper motor is limited by the driver voltage and its inductance. The inductance resists the change of current which produces the magnetic force. But its the magnetic force which produces the torque. So low inductance = fast motor, low torque. High inductance = slow motor, high torque. The maximum voltage for a motor is reckoned to be 32*sqrt(inductance in mH)**. For my 3.8mH choice this gives 63V. DM860 drivers meet this and seem to perform well without fan cooling.

https://www.hobbytronics.co.za/Conte...ALENZ_rev1.xls
https://www.allaboutcircuits.com/too...0motor%20info/

Tests on my Y axis shown below. 1st picture shows controller, driver, motor and steel cable attached to scales. Bathroom scales indicate 130kgs @ 0 (having gone round once) and a further 25kgs before they hit the stop at 155kgs. Calculations indicate that with 4.2A max force should be over 200kgs.

Best Regards

picclock

* the cutting forces I estimated are simply using the same bathroom scale method and remembering how much force I had to apply to the axis wheel. At the high load figure with a big cutter, deep cut in steel the machine was overloaded - so just my estimate, marginally better than no idea :-).

**Although this formula is widely quoted I cannot find any derivation of it. IMHO the voltage should be limited only by the breakdown voltage of the windings.

[Hulksmash]

Thanks for the great post.

I think choosing your operating system / hardware is so difficult. Everyone has a different opinion as to what their preference is. For a complete noob like myself trying to get my head round the pros and cons is so hard.

I was only recently chatting to one of the forum members re Linuxcnc, and 4 and 8 wire motor wiring.. series connections vs parallel.

There is CNC operator near me who swears on 4 wire stepper motors, however he really only works with servo's. So it's good to see the reasoning behind using a 4 wire motor and who supplies them.

Thanks again for the detailed information, I look forward to your future posts.

[takeda]

Unfortunately I believe the owner of Toolco lost the contract with Weiss Machines - The owner of Weiss said he went in for an operation and not been heard from since :( I hope he is OK.

That's terrible news.

Their website is down some time now.

[takeda]

And some comments,

Thrust bearing won't work well for a bs application, as stated, no radial support.
I see you use a couple per axis, maybe you can find a drop-in replacement angular contact bearing instead.

For me it's important to have speedy tool changes, so I went with R8 and TTS tooling.

I was considering DDSCV in the past, but having used PlanetCNC software I can't switch to anything else. Everything else just wastes my time (haven't used DDSCV but beeing chinese.....)

But most important, is DDSCV supporting a tool table with tool offsets? Even if it does, editing a tool table in such small screen using arrow buttons would drive me nuts.

Maybe it's nice and will work OK but, I don't know.

About the stand, you will propably need steady feet or some retractable casters (https://www.ebay.co.uk/itm/Retractab...frcectupt=true).

When you'll start accelerating with the motors the whole thing will start dancing with these wheels (except you use them temporarily only for moving arround).

Also, how will you cycle coolant, is it the mid compartment designed for a tank ?

Nice work with the ballscrew machining :) I had a different experience than you, machining the nut was OK, machining a ballscrew was a pain. So much interrupted cut in the beginning and broke many carbide inserts. Finally I made a jig to grind the ballscrew a bit before machining. Also machining long ballscrews is a pain, needs support on the other side of the lathe spindle (we have the same lathe or close).

For the limit switches, is there any reason you prefer mechanical switces instead of inductive proximity sensors? They will be flooded with coolant and might get sticky due to chips (chips can prevent them from switching). Also they are bulky.


As for the strength of Y axis motor, try to mount the table on the carriage, move the table to one of the positions and try to accelerate the Y axis back and forth. I found that in my case you need multiple times the force than in mid position.

Last, you could consider filling the base epoxy granite. I am most definetely doing so.

[dazp1976]

I'm just finishing a 25 size mill.
Thrust bearings? ooooh no. Never again.

Did thrusts on my X2 and they did not sit right at all on the screw ends. Wobble and excessive wear all over the place.

Looked at thrusts on the 25 to see and then tried AC ones instead and the difference is massive.
Had to machine some mounts myself to suit them but worth the effort in the end.

Thrust bearings NOT recommended imo.