[Riceburner98]

There are quite a few websites dedicated to the microstepping discussion.. I have some Parker Compumotor stepper drives that will let you select down to 256-microsteps. Is that at all possible with a standard stepper motor? I highly doubt it. Maybe with a 10" diameter pancake motor that has larger spaces between the poles; I hear they make special motors like that for things like medical applications where you really need tiny tiny movements - dispensing micro droplets of fluid, etc..

Thing is, when you're trying to hold a motor between full steps you're pulling with an electromagnetic force in opposite directions on a pole of the motor. With no load, sure it's easy to see that the shaft will stay exactly there. But if you start loading the motor in one direction, without some feedback to the stepper driver the thing has no idea that it's being moved away from its "microstep" position. Say you're trying to hold on the 9th microstep, and you have a force (even the screw friction) pushing the motor towards that full "10th" step. The motor really really wants to line up rather than sit 'almost' there.

In a test fixture I built at work, we use a large Nema34 stepper to focus a camera. There's a 1/2" x 9" steel disc mounted to the shaft, then a 1/2" x 1" steel bar sticking out from the face, to which bolts an electric gripper mechanism, with the gripper fingers concentric with the motor shaft. (it grips the camera lens, then the whole thing turns around. Hollow shaft motor with a rotary connector allows the electric gripper power wires to run through the motor and still spin) If the steel arm is at the top or bottom of rotation, everything is fine. But when it stops at 90* or 180* all the weight of the steel bar is pulling against the magnetic force, it definitely doesn't hold the microstep position and we're using $700 7A stepper drivers. (you can wiggle the arm +/- a few degrees by hand) We still use 8 microsteps, mostly to reduce the sound while the motor is spinning.

If you do need the equivalent of microstepping to get a particular resolution, you're better off with low-backlash gear reducers, though they can be $$$ and take up space.. I'd think for machining wood, .001" resolution would be more than fine, unless you're doing super-detailed engravings with a tiny bit. I'd love to try this belt drive system if I ever have the chance to build a larger machine!

[xshaper]

Hi Mike, I am finely getting around to putting this belt drive on. I had rack and pinion spring loaded to hold pinion to the rack. Do you think a spring will work to hold the pinion tension to the belt.
For the gear reduction check this site if you look there are a number of them.
MechMate CNC Router Forum

[diyengineer]

Hi Mike, I am finely getting around to putting this belt drive on. I had rack and pinion spring loaded to hold pinion to the rack. Do you think a spring will work to hold the pinion tension to the belt.
For the gear reduction check this site if you look there are a number of them.
MechMate CNC Router Forum

I used the mechmate design but with a twist. I had to rebuild an entire axis after my ball-screw died. Converted it over to rack and pinion from scratch. Have been happy ever since!! Under the plate the gear head and servo sit exists a small sheet of Teflon that is epoxied on. the entire plate and spur gear is held against the rack with a 20lb spring. The plate can "float" to accommodate for any irregularities in the straightness of the rack.

Rack and gear from macmaster. Gearhead and servo from Keling. Everything was made in solidworks, and sent off to be machined by a great fellow off cnczone.

[xshaper]

Same with mine I put my own twist on it. My rack is set up so the rack is up the pinion is held down with the spring. I can remove the and put the belt in its place. In my motor mount setup I should just have to drill the holes for the bearings that hold the belt down and a hole to make the spring pull the motor pulley up. I hope it work so I can use all the same motor mounts. I have the belt and pulleys on order so I will give it a try in the next week or two.

[ikhan42]

Hey everyone,

this is my first post so be patient. I have read a few books now last one being the cnc hand book. Most builds I have seen utilise a rack and pinion setup with linear rails. I thinks thats the wording for it hey its all new to me and im reading again LOL.

I am confused with this post is this meant to be a replacement to the rack and pinon setup. I am looking at doing something similar to whats in the cnc machining hand book but for a wood/metal mill rather then a plasma cutter. My wife has a lot of projects around the house and this would hopefully make these projects easier. I am also looking at building a filght simulator (initially a cessna but ultimately a 737 simulator) and a plasma cutter, wood/metal mill will come in hand for building analog dial instruments, dash board and the like.

So what exactly is happening in this post is this meant to be a alternative to rack and pinion that is more a proof of concept.

thanks
ikhan42

I just wanted to put up a little photo of how basic it gets on a linear. This is the smaller version with 15mm bearing rails, meant for nema 23 size motors only.

[dmatsumoto]

Hi ikhan42,

The problem with rack and pinion, or anything that ends up meshing gears is that there's backlash. In other words, when you move forward in one direction by X, and then move back in the opposite direction by X, you're not exactly where you started because there is "play" in the interfacing between gear teeth. You can get around with anti-backlash gears that are basically two "slices" of the same gear, coupled by a spring that takes out the play. Other ways to do linear motion w/o backlash problems include ballscrews with anti-backlash nuts, linear motors, or belts and pulleys.

I believe one of the cool things about Mike's design is that it is a novel way to use belts and pulleys to remove backlash from linear motion, but it also avoids some of the weird resonant frequencies that would otherwise occur when you're traveling long distances with a typical belt and pulley setup. This makes motor tuning more complicated, and for me, tensioning multiple setups similarly with belts and pulleys is tough.

I haven't personally played with the servobelt, but if I ever come up with enough money to make a linear axis at home, this is the first thing I would go with, provided that it is okay to have the motor become part of the payload for that axis. At the moment, the only two disadvantages I see include that (i.e. the motor moves with the thing that it's moving, so it has to move its own weight as well), and the fact that all requisite wiring has to pass through the axis of travel.

[diyengineer]

My dial indicator will not pick up even .0005" backlash, and thats with a 10:1 gearhead attached to the servo. I do not have a better dial indicator to measure less than that at the time. I believe the 20lb spring helps A LOT especially at the 45 degree intercept angle to hold it against the rack. That is without backlash compensation on in mach 3!

Hi ikhan42,

The problem with rack and pinion, or anything that ends up meshing gears is that there's backlash. In other words, when you move forward in one direction by X, and then move back in the opposite direction by X, you're not exactly where you started because there is "play" in the interfacing between gear teeth. You can get around with anti-backlash gears that are basically two "slices" of the same gear, coupled by a spring that takes out the play. Other ways to do linear motion w/o backlash problems include ballscrews with anti-backlash nuts, linear motors, or belts and pulleys.

I believe one of the cool things about Mike's design is that it is a novel way to use belts and pulleys to remove backlash from linear motion, but it also avoids some of the weird resonant frequencies that would otherwise occur when you're traveling long distances with a typical belt and pulley setup. This makes motor tuning more complicated, and for me, tensioning multiple setups similarly with belts and pulleys is tough.

I haven't personally played with the servobelt, but if I ever come up with enough money to make a linear axis at home, this is the first thing I would go with, provided that it is okay to have the motor become part of the payload for that axis. At the moment, the only two disadvantages I see include that (i.e. the motor moves with the thing that it's moving, so it has to move its own weight as well), and the fact that all requisite wiring has to pass through the axis of travel.

[JD68]

Hi ikhan42,

The problem with rack and pinion, or anything that ends up meshing gears is that there's backlash. with a typical belt and pulley setup. This makes motor tuning more complicated, and for me, tensioning multiple setups similarly with belts and pulleys is tough.

I have read the ZONE a lot and rarely hear 'helical rack and pinion' mentioned. Years ago when my employer was trying to eliminate backlash they went to helical cut gears. Made a big difference. Here is a link to study. ATLANTA Gear Racks

Is there a way to use servo-belt in a 'dirty' environment?

JD

[ikhan42]

Hi JD68,

Thanks for the reply so are you saying the helical cut gears are a good idea or a bad idea to avoid backlash. The cnc machining book I read had said to use i guess what is classified as helical rack and pinion as they mention 20degree angles in the teeth. They also mention preloaded split pinion and dual pinion drive systems to eliminate backlash. What exactly is dual pinion ?

Is there a way that 2 gears can be attached to a plate maybe 1 inch apart with a 1:1 ratio gear between them and have both gears sit on the rail in a rack and pinion setup to help reduce the backlash or is this basically what dual pinion is ?

I have read the ZONE a lot and rarely hear 'helical rack and pinion' mentioned. Years ago when my employer was trying to eliminate backlash they went to helical cut gears. Made a big difference. Here is a link to study. ATLANTA Gear Racks

Is there a way to use servo-belt in a 'dirty' environment?

JD

[ger21]

Atlanta gears sells split helical pinions to remove backlash, so I don't think helical r&p by itself eliminates backlash.

Back on topic.

I got my belt in, and while I don't have anything to mount it to, it sure doesn't seem like it'll have any backlash. But I'll have to wait until I start throwing 50 lbs back and forth to see. Unfortunately, it's going to be a while before I'm able to test it.

[Ken_Shea]

Gerry,
You still planning to use an additional support bracket?

[ger21]

My first reply was yes, I am.

But, I just spent 45 minutes researching, and think I'm going to go in a different direction. I think I might switch to using 1/2" shoulder bolts instead of 5/16", and switch to a 1/2" drive shaft. The shaft will probably need to be turned down to fit the pulley on as the pulley hub is only 14mm. The pulley is bonded to the shaft already, as it's not big enough for setscrews.
It'll probably cost about $75-$100 more per drive (mostly bearings, and thicker plates), but then I wouldn't have to mess with a bracket.

What I'll probably do, is test it as is, and see what happens.

[HomerSimpson]

Gerry, your hardware looks fantastic. Can you snap a picture of the backside so we can see how you did the gearing/pulley ratio?

[ger21]

Post #181

[HomerSimpson]

oops! Forgot you had posted those shots. Thanks. Let us know how it turns out. I suspect I will need to gear mine down too but I hope to have my direct drive design working by this weekend so I'll let you know.

[heavycnc]

My concern with this design is payload. Has one tested how much this design can take in payload. I'd like it to handle at least any 400lbs gantry at 1200ipm with dual motors on the x axis

[Bear5k]

I know it's been a couple of years since it was first mentioned, but throw me into the mix of people interested in a DIY version. I think that I'd just need the carriage assembly, sans motor. The rest of it seems straight-forward enough to source myself.

[ger21]

My concern with this design is payload. Has one tested how much this design can take in payload. I'd like it to handle at least any 400lbs gantry at 1200ipm with dual motors on the x axis

See post #145.

[heavycnc]

Hi Mike,
It's more about acceleration load on the belt than speed. While we've taken this T-5 belting to 150 acceleration force (32mm width), I would not use it over 100. So, with your 300 lb payload, you'll want to limit it to .3g acceleration/deceleration. At 100 lb, any one spot will be good for approximately 1.5-2 million cycles (start and stop on this exact point) before tooth failure occurs, and the steel reinforcement shortly thereafter. Plenty of life for a random motion. we test it as a bang-bang actuator, which is worst case. Life is a non-linear thing, so for comparison's sake, a 25 lb acceleration force will do 30 million hits on the same spot.
Speed will not be an issue, since your rotary to linear conversion is 75mm/motor rev. Where you may get in trouble is inertia matching your motor to the reflected inertia of the mass in motion. You can push the limits on that with ServoBelt. With great amps, 200:1 will work, decent amps, 100:1 (at a guess).

I wonder is that 100lb per axis or 100lb for both X axis

[Bear5k]

I wonder is that 100lb per axis or 100lb for both X axis

It seemed pretty direct that you don't want more than 100lbs. per axis accelerated at 1G. Hence, 300 lbs ~= 0.3G max acceleration.