Why steppers?

I understand the reason we use stepper motors is becouse as long as they dont miss a step we have good accuracy, but if we have encoders on each axis then would dc motors work? would they have to slow and speed up, or do people think that they could just stop?

Sorry not explaning myself very well but you get the idea, any input would be great

encoded dc motors are just like servos, not?
Servos are way harder to control than steppers.
Even though servos are usually cheaper than steppers, the set of controller+motor is cheaper for steppers (as far as I'm aware of)

'sid

There have been quite a few threads on this matter. It is worth doing a little searching to check... here are some I vaguely remember.

[forums.reprap.org] which has Forrest talking about his attempt to use home-brew servos and
[forums.reprap.org] which I mostly remember cos I spoke in it.

Ever so roughly speaking, For low speed, low load projects like the reprap cartesian robot, a servo system is likely to prove more expensive than a stepper system, and quite possibly you will end up with poorer performance. Best case scenario appears to be equal costs with no particular benefit either way.

Don't let this discourage you if you're keen to press ahead with this idea... one bad experience is just an anecdote after all.

Anyway, a precreated search for you. Enjoy. [tinyurl.com]

Looks good, Think i'll keep to the steppers, might even put one on for the extruder, Im hoping to keep the motors the same but able to transfer power to differnt heads , like the stepper powering the extruder with a head change could power the milling head.

on that note, anyone know off the top of there head the max rpm the steppers used normally run at?

... i have normal 1,8

i'll have a look at good speeds for milling, but think i'll try to use steppers for most things , the way im making a head changer i should be able to use gears to at least get double rpm on the milling head.

hope my idea works, means a couple more motors but in theory should make a good tool head system and no extra motors will be needid , eg no motor for each tool head.

thankyou soo much viktor, you and ru have been good to me, and hopefully soon i'll be bringing in a new mutatoin into the fold and maybe some bits that could prove usefull to you

Hi Noz,

... for a milling head you should use a much faster DC-motor - my millhead has 11.000 to 25.000 rpm compared to 300 to 600 with a stepper!

Use a dremel for first tests, but when you want to do serious milling with the repstrap, you should use motors/millheads with better bearings and more then 20.000 rpm - ye more rpm's, the lower are the applied forces for the frame ...

Viktor

My dremel gets 30,000 rpm.

at least that's what is written to it, yes.

Wasn't the mythbusters measuring those spindels once ? I can't remember really

'sid

Well, whatever it's getting, it cuts HDPE beautifully.

... the speed isn't the only thing to watch: - 'serious' milling is mostly performed with lower speeds (<3000rpm), thicker mills (6mm - 120mm! diameter) and therefore much sturdier frames ...

On the other side stands HSC (HighSpeedCutting) - here you have speeds above 50.000rpm (until 200.000rpm!) so the mechanical system could be flimsier or lighter.

With a milling-repstrap you have to select your desired properties wise - a frame as sturdy as possible with motors as strong and fast as possible and a higher millhead-speed, so the cutting forces wouldn't resonate the frame ...

Viktor

I think my frame will be pretty sturdy , 50mm angle iron with 6mm thickness all weldid into a strong cube, Also im not using belts im using threads, so i can apply a heck of alot of force. Im also guessing if i cant get enough speed for the head i can just cut/mill slower. i know its overkill , but i would rather go on the side of cautoin !

Also the steppers have high tourque so hopefully i should be able to put some gears in for the mill head maybe something a bit silly like 1:12 ratio. 400*12=4800.
maybe i'll have a look at getting a 1:100 gear ratio working. i think the stepper will have enough grunt.

if anyone can poke holes in this idea please say. i would rather know now then having a gear fly off and hit me in the head.

Noz

I did a cube design out of 2 inch .120 wall steel square tubing. TIG welded. Recirculating ball screws. Standard Gen 2 electronics and steppers. Doesn't move particularly fast--doesn't print right at the moment either--but it is sturdy as hell! And weighs in at about 140 lbs. Not dainty. Haven't hooked anything up to do any milling yet. Made it for school. They wanted it beefy.

Take a look at it here:

oitreprap.googlepages.com

Demented