Racks & Pinions with gear box

[eloid]

I was plan to build 5'x 9' cnc router 3 axis table with Racks & Pinions for x,y screwball for z. I need some advice for pitch size for my rack, want smooth cuts but want speed too from the x,y axis. How best to handle this? go with 20 pitch, about 20 tooth pinion, 20 degree pressure angle, giving nice smooth
travel then gear boxing it to get speed?

What ratio is common for x,y for this pitch size?

[judleroy]

I'm not sure what rack and pinion you should go with.
I do know that you will be needing to gear down not
up. The distance traveled in 1 revolution on a 1 to
one ratio for a 1" diameter wheel would be a little over
3 inches. That means if your drive motor runs at 3000 rpm you would get like 9000ipm rapids. You also would
have lower resolution from your motors. You would need to gear down 5 or ten to one. At 5 to one you would have a resolution of .0003 and 1800 ipm rapids with a
3000rpm drive motor. That assuming 2000 count encoder or I believe a 1/8 step. I hope this helps.
Oh yeah even at 1800ipm rapids you would need to
have a very rigid and accurate machine to handle that speed.

[Greolt]

Eloid

I used "Module 1" with 24 tooth pinion geared down 4 to 1 with timing belt and pulleys. Get about 7000mm per min

See mine here. http://www.cnczone.com/forums/showthread.php?t=28979

However mine is a smaller machine than you are describing.

I would have a good look at the Mechmate site. You may learn from their experience.

They are direct driving 20 tooth pinions or using 6 to 1 gear box reduction.

http://www.mechmate.com/index.html

[eloid]

Originally Posted by judleroy I'm not sure what rack and pinion you should go with. I do know that you will be needing to gear down not up. The distance traveled in 1 revolution on a 1 to one ratio for a 1" diameter wheel would be a little over 3 inches. That means if your drive motor runs at 3000 rpm you would get like 9000ipm rapids. You also would have lower resolution from your motors. You would need to gear down 5 or ten to one. At 5 to one you would have a resolution of .0003 and 1800 ipm rapids with a 3000rpm drive motor. That assuming 2000 count encoder or I believe a 1/8 step. I hope this helps. Oh yeah even at 1800ipm rapids you would need to have a very rigid and accurate machine to handle that speed.

can you explain more about how you calcualted rpm of 3000, and how you determine a resolution of .0003? Is rpm not determined from torque table?
can 270 oz motor do 3000rpm?


im using a HT23-400d
spec. sheet http://www.applied-motion.com/produc...s/sizeht23.php

[DocTanner]

Originally Posted by eloid I was plan to build 5'x 9' cnc router 3 axis table with Racks & Pinions for x,y screwball for z. I need some advice for pitch size for my rack, want smooth cuts but want speed too from the x,y axis. How best to handle this? go with 20 pitch, about 20 tooth pinion, 20 degree pressure angle, giving nice smooth travel then gear boxing it to get speed? What ratio is common for x,y for this pitch size?

Eloid,
I use 20 pitch 20 dp with a 35 tooth gear and PK296A1A-SG7.2 on a 4 X 8 router The 7.2 gear reduction gives nice smooth cuts and sufficient speeds to keep most people happy. MechMate.com has excellent plans for a router like what you want. Extremely well thought out and fantastic support. His rack and pinion Z axis design doesn't crash into your work when powered off.The MechMate is like a Shopbot on steroids.

DocTanner

[judleroy]

Eloid in answer to your questions.
I don't know alot about stepper motors. From what I understand they can run up to 3000 rpm. This doesn't
mean all steppers run at that speed but some can. I
don't think the ones your looking at will go that speed.
I also don't think those motors at 270oz in. are going
to be good for speed or torque on a 5' x 9' machine. You
should look at how much weight these motors are going to be moving. As for resolution. If you move 3 inches in one revolution then 3 is divided by how many count or microsteps your motor has. In a servo encoder you get
between 1000 to 2000 counts per revolution. I believe
a stepper motor gets about 200 steps per rev. With a
stepper driver you can multiply your steps to get better
accuracy. 1/2 step 1/4step 1/8step each dividing your
steps into smaller pieces. If a stepper has 200 steps
per rev. you get 400 at 1/2 step 800 at 1/4 step and 1600 at 1/8step ect. ect... So depending on your motor
resolution for a stepper at 1/8 step you get 1600 counts per revolution. If you divide 3 by your 1600 you get 0.001875. That would be your smallest increment you can move. If you gear down 5 to one then you divide
3 by 5 to get 0.6 inches per revolution. Divide your 0.6
by 1600 and you get 0.000375 as your smallest increment.
If your stepper runs at 500 rpm and you
gear it down 5 to 1 you get 100 rpm at the pinion. If
you move at 3 inchs each time you turn the pinion one
time at 100 rpm you would move at 300ipm which
is closer I beleive to a 270oz in motors capacity. That
is assuming it can drive your 5 foot gantry.
Sorry I don't know more about steppers but I'm sure
someone else here does and will help.

[massajamesb]

Don't count on those 270 ozin steppers to spin any faster than 1000 RPM, if you even get that much. If they do spin faster, the torque they do have will be all but useless.

I suggest 20 pitch, 20 degree p/a rack, and 20 tooth spur gears. That is what I run, and it works for me. One revolution of the spur gear gives 3.14 inches of movement.

I would either get gearboxes or run chain/sprockets or timing belts and pulleys to get a more effective gear ratio, and increase resolution. You should never rely on microstepping to better your resolution. Make sure the mechanics allow for the accuracy you want.
I would suggest a 1:5 ratio for starters, give or take. Check with the manufacturer of the motor for a torque chart, find out at what speed your motor makes the most torque, and gear your machine to that. If the motor does not make sufficient torque at that speed, or you need the torque at a different RPM than that motor allows for, get a different motor.
If you are not that picky, try the belts and pulleys approach, and change out the drive pulleys to suit the best balance of torque-speed.

At a 1:5 ratio, when your motor spins 5 times, the spur gear will rotate once.
This gives you 3.14 inches of travel for every 5 revolutions of the motor.
At 500 RPM, you now have 314 IPM. This is more than fast enough for a hobby machine, and cutting speeds will probably be half of that or less. My bet being less, as the 269 ozin motors just simply do not have enough torque to push a router through hardwood or even MDF at 150-200 In/min.

I think you will find your options limited with that size of motor, at least for routing. You can increase the torque by going up in ratio of your gearbox,as well as gain accuracy (resolution) but then you lose speed.

As far as figuring out your resolution, take the number of motor steps (200 or 400, most these days are 200) and the number of teeth on your pulleys or sprockets (I will use a 10 tooth and a 50 tooth here, for the 1:5), etc. or the ratio of your gearbox and the final diametrical pitch of your spur gear (I will use 3.14 for example, what I have.) and

200 x (50/10) / 3.14 = 318 steps per inch. That is without microstepping.
Basically, your motor will turn 1.59 full revolutions to move one inch.
Figure out your microstepping amount on the drivers, and do the math from there.

[eloid]

Do you think slaving would help ie twinning (2x 270 oz motors), on x,y if not what would be the min size motor one would use with a gear box then, from here I can start doing the math for gear reduction. im think of 7.2:1 or 10:1 gear reduction, for the larger single motor solution.

[massajamesb]

Yes, slaving would help, but only so much. You would have more torque.

I would suggest a minimum of a 400 ozin motor, and if possible, one on either side of the gantry, in other words- slaved.

getting the rapid travel speeds is not a problem when using spur gear and rack, it is trying to cut at higher speeds without losing steps.

either one of the gear ratios you listed should work fine with the motors you listed, but you will have to resign yourself to a lower cutting speed than you would see on a production level router.
With a 10:1 ratio, at 500 RPM (which still shows a bit of torque for those motors, according to the link), with the same spur gear mentioned above, you are moving 157 in/min (2.6 in/sec). At the outside of the motors' RPM range (say 800 RPM), you are moving at 251 In/min (4.2 in/sec), which is not bad. The only problem is, I don't think you will be able to cut at that speed. I could be wrong, I don't really know the answer to that one.
I don't know that I would want to cut that fast, you would have to have a pretty strong router, sharp bits, and a pretty tough machine to cut that fast.
What kind of travel speeds do you expect? How fast do you want to cut/rapid? You need to know that information as well.

[JerryFlyGuy]

I would [did] approach this problem a little differently on my own project. The real question is, how much force do you need to cut at the speed you want? If your router can rapid at 5000ipm, but can only cut at 1ipm it's not going to make the operater to happy. Figure out what type of force you want at the tool tip and then work backwards from there. As Massajamesb has said, a slightly larger drive motor might be a good plan, however I disagree on the 1000 rpm. If your using a 23 size motor you'll get well over 1000rpm. I've got 640oz-in 34 size motors and can get up there. I don't know as yet what my torque is at that speed. According to graphing available on the net, a 400oz-in 23 size motor will deliver about 100oz-in at 1200rpm. Results will vary depending on motor sizes and power supplies.. etc..

I'd think a 5:1 or 10:1 reducer would be fine. I prefer the larger reducer 'cause you can always come back up on your speed by using a larger pinion. However, you if you use too small a reduction, you can only get a smaller pinion to a point.. If you build with a larger reduction and don't get the speed or force you planned on, you just tweak it by changing the pinion [and its cheap to do as well] and voila, you've tuned your system w/out all the hassel of changing reducers or any of that..

Also, check the spec's for the rack and pinion, they have ratings which should be used to specify what size of rack you should use. It would seem that your system is a bit smaller and lighter and using lower powered motor's this might not be a huge consideration. However, you might as well use the same work/design flow as if you were building a larger system.
20 deg gearing is prefered over the 14.5

Fwiw

Jerry

[massajamesb]

As Massajamesb has said, a slightly larger drive motor might be a good plan, however I disagree on the 1000 rpm. If your using a 23 size motor you'll get well over 1000rpm. - Quote

You are right,Jerryflyguy, I hadn't looked at the specs for the motor when I wrote that. It does indeed seem to top out around 1200 RPM, though the torque available at that speed at 24-36 volts is almost nil. Hence my reasoning to go to a larger motor, with the 5:1 reduction.
Yes, some Nema 23's will hit 1000 RPM, my PowermaxII's are supposedly 1500 RPM- capable motors. I am not sure what performance to expect from them at that speed, but they will "do it".
What I should have said, is that 1000 RPM may be a bit much for some steppers. I have seen some in that size range that were limited to 300-400 RPM max. That you are getting that high of a speed from a Nema 34 definately shows proof of what you say.
So, in short, you are 100 % correct. Sorry for the misunderstanding, my mistake. Thanks for helping me realize my error .

[JerryFlyGuy]

Mariss could shed more light on it [ I consider him to be one of the premier experts on the subject of motors ] but apparently he had a stepper running in the 100k range once.. [I think it exploded at some point there.....] Your correct about the reliable torque at speed. It is suprising, however, how much those little guy's can put out. It's entirely dependant on how much power you put to them, but if you use name plate amp's and 25x the listed voltage they REALLY scream! [ literally ]

I've read of people who couldn't get any power outta their steppers and had given up on them, they were going to get 'servos' which had some 'proper power' over those stupid steppers, yet when you query them on their power supply it's a 400w computer supply or something, not deliverying anywhere near enough power/voltage for the application. Power supplies are a huge part of getting a 'balanced' system.

I know of one fella who is running his 640's at 630rpm. He has 600ish stepps per inch. I asked him how much force he was putting out at those speeds and he couldn't tell me exactly. But he said he could stop it if he really tried to [only one 640 stepper per axis]. This makes me think that the torque is still more than one would expect at those speeds. The best bet is to be a bit conservative and then maybe tweak it back a bit to get it where you want it. Thats the beauty of using rack pinion. If you get a slightly higher reduction than you need, you can always put a larger pinion onto the system. Typically all single stack planetary reducers cost about the same money so there shouldn't be any extra cost to go w/ a 10:1 over a 5:1 reducer. You'll have to buy a pinion which is twice as large which might cost you an extra $5/gear but who cares.. This just gives you that many more options when it comes to tweaking the system.

My understanding is also that a 400oz 34 size motor won't put out as much 'power' as a 23 size 400oz motor due to the electrical 'black magic' stuff that goes on inside of them.. reluctance or inductance or .. well something like that.. [I'm not no electronic's/electrical expert if you hadn't noticed..]

In the end there's only one way to find out what it'll do..and that's to build it..

Jerry