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View Full Version : pulley ratios on 3 pulley axis



Swarfing
20-07-2011, 01:44 AM
Ok guys just got myself very confused. I have the following configuration setup on my X axis
5mm pitch HTD15 belt used:

16 tooth pulley on stepper going to a 72 toothed pulley, on the same shaft as the 72 is a 24 toothed pulley driving the axis with matched 24 toothed idler. What by your calculations should the ratio be? Things are cutting slightly larger than expected in EMC2 :whistling:

Thanks in advance

John S
20-07-2011, 02:01 AM
Doesn't make sense.

16 driving a 72 with a 24 on the same shaft, OK, now is the final pulley a 24 in which case it's not an idler but driven.
And if the 24 is an idler then you haven't told up the final driven pulley.

If the first then it 4.5:1 reduction

Swarfing
20-07-2011, 02:12 AM
Hi John

I only mentioned the idler for completion. It 16 to72 then 72 to24 which my reckoning is 4.5:1 then 3:1. the total being? this is where im lost :-(

John S
20-07-2011, 09:21 AM
Still doesn't make sense.
In the first post you state " on the same shaft as the 72 is a 24 toothed pulley driving the axis with matched 24 toothed idler."

Now you are saying "16 to72 then 72 to24"

So have you got a 72 and a 24 on the intermediate shaft or two 72's ?

AdCNC
20-07-2011, 11:55 AM
So am i right in thinking (starting with the stepper end) you have geared it down then back up again? :S 16---72----24

Swarfing
20-07-2011, 01:29 PM
ok a picture speaks a thousands words

42484249

this should make it easier to understand

Jonathan
20-07-2011, 01:31 PM
( Deleted what I had posted here as posted at same time as previous)

It's 16 to 72 then, which is (as JohnS said) 4.5:1 or 0.2222=2/9

The 24T pulleys just link the two screws so are irrelevant.

John S
20-07-2011, 01:35 PM
Right so looks to me like 16 driving 72 with a 24 on the same shaft driving another 24 ? [ not in picture ] ?

If so the two 24's cancel out at 1:1 and the 16 to 72 gives a 4.5 : 1 reduction x by 1 : 1 is still 4.5 reduction.

Swarfing
20-07-2011, 01:39 PM
Cool that is what i was hoping you would say, shall change the config tonight and give it another go. As you can already guess i was trying to do something with the gear up scenario which does not exist.

If all is well i will take some proper pics and post the results :-)
Cheers guys

Swarfing
20-07-2011, 09:41 PM
Here's the update so far, the config set up as 4.5:1 got me closer. over a distance of 150mm on screen the actual travel was 173.5. I've never set up belts before so what next? play with the ratio or the number of steps per mm? it seems a lot to play with the steps. Im using quality pulleys an belt.

Jonathan
20-07-2011, 10:21 PM
Here's the update so far, the config set up as 4.5:1 got me closer. over a distance of 150mm on screen the actual travel was 173.5. I've never set up belts before so what next? play with the ratio or the number of steps per mm? it seems a lot to play with the steps. Im using quality pulleys an belt.

That's odd ... it implies the ratio is actually 3.8905:1
You could just enter that ratio and see ... then check it with a DTI to get a more accurate value. There must be more to it though. Are you confident you have the pitch of the screw correct?

John S
20-07-2011, 11:02 PM
What are you entering as steps per?
Are you using microstepping ?

Jonathan
20-07-2011, 11:04 PM
Also what pitch are your screws?
It could be a combination of the microstepping and screw pitch causing it ... if it's just one of those then the factor would most likely be a more round number. The other possibility is the number of steps per rev. you used in the calculation is too low, causing the motor to move further and the step signal to the driver is poor making it 'loose' some of those steps. Pretty unlikely though. Is the distance consistent/repeatable?

John S
21-07-2011, 12:05 AM
Jonathan,
It's my take that he's using belt drive, no screws.

Swarfing
21-07-2011, 12:16 AM
I played around with the ratio tonight and set it to 5.35 which brings it almost correct after some trial and error adjustment. I did this by measuring distance travelled etc. There is no screws on this axis just belts, i've set the drives at 16 microsteps (3200). definitely confident that it is not loosing steps as it will always return to point zero every time. Still managed to cut a few sample pieces which turned out rather nice in comparison to my old machine. I will spend some more time tomorrow playing with the ratio. I will rig up the DTI rather than judge by ruler which is what i did tonight. the belt is 5mm pitch, if you look at the pics it shows the end of the X axis. It runs with belt both sides powered by the single stepper (4amp bi parallel). Using PM542's on a Uniport V2 bob, kept to 5mm pitch screws on Y and Z. The Y has 24T pulleys 1:1 config and these axis are absolutely spot on

Jonathan
21-07-2011, 12:19 AM
Jonathan,
It's my take that he's using belt drive, no screws.

I see, that makes the last picture make more sense. Since it is therefore the belt that is moving, not the pulley/stepper you need to use the circumference of the 24T pulley to work out the steps per unit distance. Since the belt is 5mm pitch, and 24T pulley, one revolution moves the axis 120mm ... so including the reduction that's effectively 26.66..mm pitch.

John S
21-07-2011, 12:21 AM
Jesus H Christ this is like pulling teeth, we have all the details including what you had for dinner but not what was asked.

HOW MANY STEPS PER ARE YOU SET TO ?

John S
21-07-2011, 12:23 AM
I see, that makes the last picture make more sense. Since it is therefore the belt that is moving, not the pulley/stepper you need to use the circumference of the 24T pulley to work out the steps per unit distance. Since the belt is 5mm pitch, and 24T pulley, one revolution moves the axis 120mm ... so including the reduction that's effectively 26.66..mm pitch.

Nothing to do with the circumference.
24 teeth, 5mm pitch 1 rev = 24 x5 = 120mm per rev
can't be anything else.

Swarfing
21-07-2011, 12:25 AM
200 as per the motor?

Jonathan
21-07-2011, 12:28 AM
[QUOTE=2e0poz;22816]I played around with the ratio tonight and set it to 5.35 which brings it almost correct after some trial and error adjustment.

4.5*173.5/150=5.20 which is close to 5.35 ... doesn't make sense to me at this time of night though.

With 3200 steps per rev, and using 26.66mm per revolution as worked out in my last post that's 120 steps per mm.

Oh just notices, 5.35/4.5 is 1.188 ... i.e. very very close to 1.2, which is the factor the 24T pulley adds in. 1.2*4.5=5.4

So in conclusion try 5.4 and I bet it will be right...I'm not going to bet much though as it's getting late :wink:

Jonathan
21-07-2011, 12:29 AM
Nothing to do with the circumference.
24 teeth, 5mm pitch 1 rev = 24 x5 = 120mm per rev
can't be anything else.

120mm is the circumference at the pitch diameter so it has everything to do with the circumference.

Jonathan
21-07-2011, 12:36 AM
Jesus H Christ this is like pulling teeth, we have all the details including what you had for dinner but not what was asked.

HOW MANY STEPS PER ARE YOU SET TO ?

Using EMC2, not mach3 so you don't set the steps per directly... you enter motor steps per rev (so 200 is correct), microsepping (so 16 in this case), pulley ratio (4.5 as discussed) and leadscrew pitch (120mm).

It looks like at the moment you have the leadscrew pitch set to 100mm not 120mm .... so either change that or put the ratio to 5.4 and it'll be fine.

Swarfing
21-07-2011, 12:37 AM
Jonathan i tried 5.4 already and it was close but not quite. I will try again tomorrow with the DTI, i think sometimes we can only let the math get us most of the way there then rest is probably a bit of trial and error. i will set out a drawing to draw out markers, I have about 1300 cutting distance which is a large distance to loose on.

John S
21-07-2011, 12:42 AM
Ooops my bad didn't read EMC.

Note to self Read all the damn thread...........

Jonathan
21-07-2011, 12:44 AM
The difference between 5.35 and 5.4 is less than 1% ... I wouldn't be too surprised if there's that much error in the belt, particularly as it's quite long. I would definately try the DTI, or even attach digital calliper as that gets you 5 significant figures (unless your DTI is digital too). Digital calliper is not perfectly accurate, it will say the tolerances in the instructions. Either way it's better than a ruler which could account for the 1% error.


Note to self Read all the damn thread...........

There was only one post to read when you replied :lol:

Swarfing
21-07-2011, 12:48 AM
Here is an example (not my config by the way)
4250

Swarfing
21-07-2011, 12:51 AM
my config is set up in metric, it is very straight forward to use

I think John you may have been too long on the dark side (Windoze euphoria)

Jonathan
21-07-2011, 12:52 AM
It would be good if they let you set the derivative of acceleration ... that was going to be in the successor to mach3 which never happened. I wonder if EMC2 supports it.
Just out of interest what have you set the acceleration to?

Swarfing
21-07-2011, 12:57 AM
only 30 at the mo, still lots of testing the limits as i am confident it runs in the first place. So far i'm pretty pleased with it though

Jonathan
21-07-2011, 01:01 AM
30mm/s^2? That's very low ... I've got mine at 2000mm/s^2 on X-axis and it can do more. The moment of inertia of all those pulleys, particularly the 72T ones, will be quite high. That will of course limit the acceleration, but it probably won't be as high as my rotating a ballnut and pulleys.

I'm beginning to think though that high deceleration is actually bad as it causes the Z axis etc to flex very slightly (due to it's kinetic energy...) and 'overshoot'. Would be nice if you could set different values for acceleration and deceleration.

Swarfing
21-07-2011, 01:06 AM
Velocity is et to 30 as well, full speed in less that a second sounds ok to me? i will be putting it higher but remember i only have a single motor on that axis (double stack neme34) around about 1000oz of torque

Swarfing
21-07-2011, 01:09 AM
Note to myself - buy a laptop with a better keyboard, i type faster than it responds

Jonathan
21-07-2011, 01:11 AM
It depends what you're doing. For engraving where the machine has to change direction rapidly and regularly you will certainly want more.
Increase the velocity until it just stalls (30*60=1800mm/min seems a bit slow but maybe not as I don't know the mass of your gantry) then back it off by about 10-20% just to be safe. Then increase the acceleration untill it stalls whilst accelerating, and do the same.


Note to myself - buy a laptop with a better keyboard, i type faster than it responds

Then change the keyboard layout to Dvorak like I have then you'll be touch-typing faster than it responds...

Swarfing
21-07-2011, 01:24 AM
Gantry must be somewhere near about 20kg and thats without the dc spindle motor i will need to mount later when I've finished modding the ER16 end. A soon as can i will post some pics up on my log which i have neglected for a very long time (family stuff takes over). I have a list of changes for it already but need the machine to make the changes. i am glad to see that you have come a long way since you started Jonathan and thanks for the help, you too John

Swarfing
21-07-2011, 01:25 AM
PS

Give EMC2 a try, you WILL (look into my eyes not around my eyes) like it better that Mach3 me thinks

Jonathan
21-07-2011, 01:43 AM
PS

Give EMC2 a try, you WILL (look into my eyes not around my eyes) like it better that Mach3 me thinks

I will ... I remember reading JohnS saying that you can't jog the machine whilst in feed hold with EMC? If that's still the case then that alone is probably enough to stop me using it again as it will make tool changes more difficult. That's the only reason really I haven't already used it extensively. I'm not put off with it using Linux as I do use Ubuntu for occasionally. The GUI is much nicer in EMC, in my opinion there's too much eye-candy in mach3.

Forgot to say in last post ... don't just test the feedrate by jogging. The worst case is probably when the X-axis changes direction rapidly. So for instance try something like this:

G0 X100
G0 X0
G0 X100
G0 X0
etc

(and since EMC does support loops in Gcode you can do that much more efficiently than in mach3! I remember once spending a while writing the Gcode to do pulleys using loops and stuff only to realise mach3 does not easily support conditional Gcode). Also make it move the Y and Z at the same time as you're testing X as the extra load on the power supply will make a difference. It has on my machine ... I connected oscilloscope to the power supply recently and that showed the ripple increase markedly when all the steppers were running.


i am glad to see that you have come a long way since you started Jonathan and thanks for the help

No worries...yes I have learnt a fair bit since starting my router. If you compare what it looked like in the first post of my build log to now there's quite a difference! A lot of that was down to trying to cut costs though.