anyone using Rack and spur gears

[kfranklin]

Hello, I am new to cnc also to cnczone and i was wondering if anyone has used rack and spur gears for their cnc machines that they have built themselves or if have converted one to a rack system. I am hoping to get some info on accuracy and gear sizes and such,also if they can be direct driven by a stepper motor or do they have to be geared down. I will be using this for a cnc plasma cutter with a 5' X 5' table so there is no force needed to push the gantry through material just to move the gantry but i also plan on being able to put in a small hand held type router or dremel type tool for engraving purposes later after i have it working as plasma cutter.

[Greolt]

I believe R&P drive is a good option for a plasma. Good for a dirty and gritty environment.

I also would look very hard at the possibility of direct driving the pinions if you're using steppers.

For an example of this look at the Mechmate router.

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

That machine pushes a 5hp spindle through 18mm ply at good speed.

Plasma demands are less and gantry will be lighter. Speed is important in thin material.

The router I built has R&P on the X and Y axis with a 4 to 1 belt reduction driving 24 tooth pinions on "module 1" rack.

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

It is a router and not producing parts for the space shuttle. I am very pleased with it's accuracy and repeatability.

Greg

[massajamesb]

I, too, use R&P, but for my plasma cutter. I have no real cutting drag as a router would, but I see no reason why R&P wouldn't work for some light routing as well.
I use 20 pitch, 20 degree PA, with a 1:2.66 timing belt and pulley reduction, and a 20 tooth final driver spur gear.
It works well and is fairly inexpensive, and survives well in the dirty environment of plasma cutting.

[kfranklin]

I very much appreciate the info that y'all have provided and i hope to get more. Can anyone direct me as to how you calculate the accuracy or the resolution of certain sets of chosen steppers and gears, and i am a little confused if you use optical encoders with steppers or not if not how does the computer keep up with the position of the x and y axis. Sorry for the random questions."so many questions so little time". Thanks to all who contribute knowledge.

[rustamd]

Originally Posted by kfranklin I very much appreciate the info that y'all have provided and i hope to get more. Can anyone direct me as to how you calculate the accuracy or the resolution of certain sets of chosen steppers and gears, and i am a little confused if you use optical encoders with steppers or not if not how does the computer keep up with the position of the x and y axis. Sorry for the random questions."so many questions so little time". Thanks to all who contribute knowledge.

1.Can anyone direct me as to how you calculate the accuracy or the resolution of certain sets of chosen steppers and gears,

Greolt can probably answer that question best, as he was the one who helped me.


2. i am a little confused if you use optical encoders with steppers or not

You do not use encoders with steppers. Thats why system that used steppers is called "open loop". Some one can correct me if i'm wrong


3.If not how does the computer keep up with the position of the x and y axis

With steppers(with open loop system) computer doesnt get any feedback. Pretty much if computer tells the driver/stepper to move 14 steps, it expects it to do so. Again, someone can correct me if i'm incorrect

[Greolt]

Originally Posted by rustamd 1.Can anyone direct me as to how you calculate the accuracy or the resolution of certain sets of chosen steppers and gears, Greolt can probably answer that question best, as he was the one who helped me.

I dropped out of school at 14 but fortunately it is pretty simple maths.

You need to work out the following things when calculating this type of drive.

Steps needed to turn the motor one rev. Usually 200 x whatever microsteps.............. 200 x 10 microsteps = 2000 per rev

Belt reduction. I used 15 and 60 tooth pulleys which gives me 4 to 1 reduction ..........2000 x 4 = 8000 steps per rev of pinion.

Then you need to know about the pitch of your gear rack. I have a 24 tooth pinion which travels 75 mm per rev........... So 8000 / 75 = 106 steps per mm

Not these last numbers are rounded. I actually go to lots of decimal places.

I'm no teacher so I hope that makes sense.

Greg

[kfranklin]

Greolt i appreciate the math lesson on my above questionand i was wonderin your reason for the 4 to 1 reduction is t for strength or incresed resolution (more steps per rev). Rustamd Thanks for the encoder explination i had gotten lost on the difference in servos and steppers. Thanks to all.

[Greolt]

Both increased resolution and better acceleration.

When I decided on the 4 to 1 ratio there were a few reasons.

It just seemed a good place to start. The pulleys were what I could get off the shelf.

I could fiddle with ratios pretty easily and cheaply just by changing the pinion tooth count.

Now the outcome is that I have 7500 mm per min rapids.

1000mm/sec/sec acceleration which is zero to rapid in about 0.1 sec

However remember this is a router and it's only 1000mm by 750mm. You're building a big plasma. Quite a different beast.

I wish I had room in my shed for a plasma but I don't.

If I did I would be looking pretty hard at about a 650 oz inch Nema 34 stepper on each side direct driving a 20 tooth pinion.

Hope I haven't muddied your waters.

Greg

[JerryFlyGuy]

I'm just getting my mill running and it's using a 12DP rack pinion. I went to 10:1 planetary reducers and a 2" [12 tooth] pinion. I drive it all w/ two 640oz-in steppers. My Gantry weight's over a ton and can hit 800ipm in well under a second. It's suprising how well it all works. I was planning on a 5:1 bet reduction and then a much smaller [20DP] rack and pinion. However I realized that my torque numbers were exceeding the spec's on the pinion's. This forced me to go up to a 12DP gear set and a pinion which was twice a big. This ment that I had to double the reduction to 10:1. I'm now wishing I'd gone even farther w/ the reduction and had more like 20:1 as I plan to jump over to a servo system here in the near future.

If your using a plasma only and not routing or milling your main concern is going to be getting as high a acceleration and rapid/cut speed as possible. I would think a 5:1 w/ a 20DP rack and a 1" pinon would work very well for you.
Are you driving your gantry with one or two steppers? I would think on a plasma of that size you'd be ok with a 400 oz-in nema 23 stepper.. you'll get a higher RPM and lots of power for what your doing.

HTH