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alanb
03-10-2012, 10:58 PM
Hi,
I am just finishing building a small tool post sat on top of a stepper on my cnc based rose engine. The idea is that when I cut a pattern on a curved surface I can rotate the cutter so that it is always normal to that surface, the allowable range would be 0 - 180 degrees.. I need to buy a controller and was was looking into the new digital ones such as the DM320C from Zapp. I understand that you can program the micro-step revolutions so I am wondering how this is achieved. Also would it make sense to set the micro-step to 36000 pulses per rev or 0.01 degree per pulse, is this possible.

Anyone done this yet?

Alan

jonnydeen
04-10-2012, 02:17 AM
i think you might find this article of interest


MICROMO - Microstepping: Myths and Realities (http://www.micromo.com/microstepping-myths-and-realities.aspx)

irving2008
04-10-2012, 08:32 AM
Thats a good article. Though I knew the info already it explains it in a really nice and succinct way.


In answer to the OP. Yes you can use some degree of microstepping as long as the cutting forces tangential to the stepper axis are less than the % of the holding torque.

By way of example:

The force required is dependant on the specific cutting force of the material and the area of the chip. Unlike metals where the specific cutting force is a constant, in wood it varies with the grain direction and hardwoods can approach the forces needed for machining aluminium when going across the grain.

Aluminium is 500N/mm2 cutting force, common woods would appear to be around 50-150N/mm2. A 3mm dia cutter taking a side cut of 0.1mm deep would experience/generate a 75N force in aluminium, maybe 7-25N in wood.

A common 3Nm Nema23 stepper, detent torque of .2Nm. 1 step = 1.8deg, resolution required 0.5deg therefore use 4 microsteps. %holding torque available ~ 40% = 1.2Nm less detent torque = 1Nm.

Radius of item being cut, say, 5cm (0.05m), means that the cutting force cannot be larger than 20N (and for safety probably 50% of this) before the work will move the stepper more than the microstep size.

The best solution to improving resolution, since rotary speed isnt an issue, would be to gear it down. A 10:1 ratio would give a resolution of .18deg and 30Nm of torque which would be good enough for aluminium at 5cm radius.

Like all things, this is theory, you need to try it in practice to find out what combination of cutter, cut depth, workpiece size and angular resolution you can truely achieve.

alanb
04-10-2012, 10:26 AM
Thanks for that, I didnt want to gear it down but it seems I will need to.

Alan

Jonathan
04-10-2012, 11:05 AM
If you gear it down with a worm gear drive then far less torque is required from the stepper motor since (the majority of) worm drives do not drive backwards - if you apply a significant force to the worm-wheel the motor will not spin. Backlash might be a problem, although there are ways to reduce it to a negligible magnitude.

Also, since you only require 180 of movement, you could use some sort of lever arrangement with a ballscrew which can have very low backlash.

irving2008
04-10-2012, 03:43 PM
If you gear it down with a worm gear drive then far less torque is required from the stepper motor since (the majority of) worm drives do not drive backwards - if you apply a significant force to the worm-wheel the motor will not spin. Backlash might be a problem, although there are ways to reduce it to a negligible magnitude.

Also, since you only require 180 of movement, you could use some sort of lever arrangement with a ballscrew which can have very low backlash.

Very true Jonathan, in a static situation where you are holding the work steady while cutting. If however you want a true 4th axis to rotate the workpiece while cutting you'll still need the same stepper torque to drive the worm chain. That said, a worm arrangement is probably better than using belts for this, although I have seen it done both ways. I modified a 4" rotary table to do the same thing. With a 9:1 worm reduction it gives 0.2 degree resolution and a 1Nm motor seems more than happy with the small workpieces I have used, e.g. gear hobbing on 50 - 60mm dia gears..

Jonathan
04-10-2012, 04:27 PM
Very true Jonathan, in a static situation where you are holding the work steady while cutting.

Fair enough, I thought the plan was to just index the cutter.

I've also converted a rotary table - vertex HV6 which has a 90:1 worm drive and 1Nm stepper. It works well for timing pulleys pulleys, but there is a bit of backlash. A harmonic drive would be ideal, although rather expensive.

Also if you've got a lathe it's not difficult to make wormwheels, here's one I made a few years ago:

7044

This was the process:

7045

irving2008
04-10-2012, 04:43 PM
Nice. Is that a steel bolt you are using as a cutter? and letting the ali gear wheel free rotate? Only issue that would worry me is how much torque you can put through the final piece, its a fine thread and not much contact area.

Jonathan
04-10-2012, 05:16 PM
Yes, it's just a piece of steel M12 threaded rod that I hardened a bit and ground slots into to encourage it to cut instead of rub, although it does work without doing that. The gear wheel does rotate freely in the toolpost. I should have used a coarser thread since, as you say, the torque is limited otherwise. This gear is part of the mechanism to control the pitch of the blades in the wind turbine I made, so I chose to use a worm drive for the same reasons I gave earlier - it means I can reduce the current supplied to the motor significantly when stationary to save power. I then made the worm gear, which is just an M12 thread, from brass, which I know is not the best material but better than steel:

7046

It's probably easier to just use a 4" rotary table. How much backlash do you get with yours?

irving2008
04-10-2012, 05:35 PM
...It's probably easier to just use a 4" rotary table. How much backlash do you get with yours? Of the top of my head I think it was around .07 - .08mm, mainly in the in/out motion of the main shaft rather than the gear meshing itself. Not great but usable and a lot better than the stock table had. There were some lightly loaded Bellevue washers in there but making it too tight meant the motor struggled. I had planned one day to up the motor to a higher spec but I dont use it enough to justify the spend and it was good enough for the purpose I needed.