[pippin88]

If you haven't poured the epoxy yet, consider having all three crossbeams as bridges. That will give you a level table base (once you knock off the meniscus) and should level better. Just need a bit of extra epoxy to cover those extra bridges.

I found quick setting thick epoxy the best thing for sealing my forms.

[1Jumper10]

The table bed won't touch the epoxy. It's supported on cross members that span between the side I beams. I will use the center epoxy bridge to set the gantry level.

I sealed my forms with silicone caulk. In hindsight, using weather stripping around the perimeter and then levering the forms against it and then sealing with standard epoxy is better. I'm afraid it's going to look cobbled around the edge when I remove the forms. It will be flat though.

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[1Jumper10]

I made end caps for the side tubes and sealed up one end yesterday. Filled the tubes with sand today. Between the two tubes they held about 150lbs of sand.

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[1Jumper10]

I filled the last open space between the sand and the end cap with spray foam. I wanted to fill up as much room as possible to minimize settling. The end caps are just epoxy encapsulated plywood. They are held in place with gorilla glue and 4 screws. The gorilla glue wasn't totally expanded when I took the pic. But it will form a tight seal when it does. Regardless, the spray foam will hold back any sand from even touching the end cap.

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[NIC 77]

I finally got my work done that has kept me occupied for the past few days.

I'm working on some graphs for you at the moment, I haven't forgotten.

Nice work as always.

[NIC 77]

Sample Calculation:

Pinion pitch circle radius 0.0199m

Pinion mass = (pie x r^2) x width x density

Pinion mass = [3.14 x (0.0199m x 39.37 in/m)^2 x 0.6in (guess) x 0.283 lb/in^3] / 2.2 = 0.149kg

Jpinion = (Pinion mass x r^2) / 2 = (0.149kg x (0.0199m)^2) / 2 = 0.0000294 kg*m^2

Jload = Mload x r^2

For a 250lb gantry, there are two motors driving it, so that is 56.8kg per motor
or (250/2) / 2.2 = 56.8kg

Jload = 56.8 x 0.0199^2 = 0.0225 kg*m^2

Jmotor = 0.0001456 kg*m^2 (rotor inertia)

Jreducer Input = 0.00009154 kg*m^2 (for your specific planetary)

Jreducer Output = single stage planetary, output inertia considered small, used zero.

Gear Reducer Efficiency = assumed 94% or 0.94

Input torque x Gear Ratio x Gear Efficiency = Output torque.

Rack and Pinion Efficiency = assumed 90% (mechanical efficiency of the R&P system)

Reflected Load Inertia (Jrfl) is the total inertia of the system as seen by the motor.

Jrfl = Jmotor + Jreducer Input + [Jload x (1/GR Eff) x (1/R&P Eff) + Jreducer Output x (1/GR Eff) + Jpinion x (1/GR Eff)] x (1/Gear Ratio)^2

Jrfl = 0.0001456 kg*m^2 + 0.00009154 kg*m^2 + [0.0225 kg*m^2 x (1/0.94) x (1/0.9) + 0.0000294 kg*m^2 x (1/0.94)] x (1/10)^2

Jrfl = 0.0001456 kg*m^2 + 0.00009154 kg*m^2 + 0.000266 kg*m^2 = 0.0005 kg*m^2

So you can see that the motor armature and reducer have a large impact on the inertia as seen by the motor.

For the example we can use 3.7 Nm peak torque at 500 RPM from the motor graph provided by the manufacturer.

Torque = Inertia x Angular Acceleration

Let’s use 0.74Nm of the available torque, you’ll see why later.

0.74 Nm / 0.0005 kg*m^2 = 1479.3 radians / second^2 Angular acceleration at gear input

At gear output, the acceleration is 1479.3 rad / sec^2 / The gear ratio (10) = 147.9 rad / sec^2

The resulting linear acceleration is 147.9 rad / sec^2 x 0.0199m (pitch circle radius) = 2.94 m/s^2

2.94 / 9.81 = 0.3G

This leaves us with 3.7Nm – 0.74Nm = 2.96 Nm of peak torque at the motor unused for acceleration.

2.96 x 10 x 0.94 = 27.85 Nm

(27.85 Nm / 0.0199m) x (0.90 R&P Eff) = 1259.9 Newtons of Force per motor

(1259.9 / 9.81) x 2.2 = 282.5 lbs

282.5 lbs x 2 motors = 565 lbs

At 500 RPM:

[500 RPM x 2 x 3.14 x 0.0199m x 39.37 in/m] / 10 (GR) = 246 Inches per minute.

So that is where 565 lbs of force available at 0.3G acceleration and 246 IPM comes from and you can see this point on the graph for a gantry weighing 250 lbs and driven by two SDSK3421P motors.

I included two gantry weights so you can see how the different weight affects the graph. Also, I can’t see how everything will weigh 250, so you now have another reference if you end up a bit over that. And if you’re spot on, then there’s your data.

If I’ve made any mistakes (very possible) feel free to point them out. I won’t be offended.

I don’t see how you will ever make use of the peak torque at lower RPMs as obviously you will never need this amount of cutting force.

I also don’t know what balance to use when considering peak vs continuous torque and acceleration. I’ll provide you with the graphs and leave you to figure that out. Perhaps Tom from Teknic can make some suggestions in that area. I honestly don’t know. I think it depends on the geometry of what you’re cutting.

Also, the Torque vs speed for these motors was taken at 75 Volts from the teknic website.

Bearing friction for the linear rails wasn’t considered, and probably a few other things that I can’t think of at the moment. Also, no idea what the rack and pinion can handle. You probably need a bit a wiggle room if the assumptions of 0.94 reducer efficiency and 0.9 rack and pinion efficiency do not provide enough.

Have you checked your friend’s router to see what the acceleration settings are in the control software? I’m assuming that you will be happy with those settings as you mentioned it was really fast? I’m thinking anything over 0.4G is considered very fast?

I made some graphs for you for the SDSK-3432-P. The motor rotor inertia is 0.0002087 kg*m^2 for this one. I also tried increasing the pinion pitch circle diameter to 2 inches. Graphs for all to follow in the next posts.

If you want me to try something else, let me know. These calculations are a learning experience for me too, and I would like to know how the results correspond to the theoretical.

I really can’t see you being unhappy with what you are building, and I do enjoy seeing your progress.

If for any reason you feel that I have delved too much into this subject in your thread, please tell me so via PM, no harm, no foul.

[NIC 77]

SDSK3421P Graphs

Attachment 367176

Attachment 367178

Attachment 367180

Attachment 367182

[NIC 77]

SDSK3432P Graphs
Attachment 367184

Attachment 367186

Attachment 367188

Attachment 367190

Attachment 367192

Attachment 367194

[1Jumper10]

Looks good NIC-77. I wont have time to look the results over carefully for a little while but thanks for taking the time to graph them all out. I havent been over to my friends router for yet to check on those accel settings. Havent forgotten...

[ger21]

There's a much simpler method to calculate available force with rack and pinion.
With a 2" pinion, force = torque.
With a 1" pinion, force = torque x2.
With a 10:1 gearbox, multiple by 10.

Take off 10% for efficiency losses, and you are good to go.
Close enough for our purposes.

[1Jumper10]

Form is ready for the epoxy. I just picked it up but now I'm on a weather delay. Light rain and I want it to warm up a little more. Next few hours look promising.

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[1Jumper10]

Got it poured. That black pigment helps remove bubbles and makes it look like a black mirror.

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[NIC 77]

Wow, that is a shiny black!

I don't know how everything will fit together in the end, and by that I mean all the parts of your design. I'm sure I will find in out your next series of pictures. The bridge in the middle seems like it allows for the perfect leveling for the two sides to be exactly the same, as long as it is viscous enough, so that's kind of a cool way to do it. Is that the reason for it?

Nice work as always.

[1Jumper10]

Yep, that's the reason. Just like water it seeks its own level. It's a low molecular weight, low viscosity, slow curing epoxy with some incredibly small shrinkage percentage of cured versus uncured. So now in effect, everything black in the pic is on the exact same geometric plane.

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[1Jumper10]

Couple more pics of the pouring operation. I poured it out in puddles then skipped around to the dry areas and connected them. The last of what was still in the bucket I poured in the middle so it would flow equally to both sides. You can see it flowing together in the center bridge.

[1Jumper10]

It was about 90% humidity when I did this and it looks like that caused a blush to form on the top of the epoxy. Theres no room for water molecules in there so as it cures any water that got entrained during the mixing process get's squeezed to the top. Its only a cosmetic defect that wont even be visible. You can see it in the second to last pic.

Now I've got some clean up to do around the edges, knock off the meniscus and get it ready for painting. It takes 168 hours to fully cure. Will try to get it painted before that process is done.

[CNCMAN172]

Jumper10,
Looks like the pour turned out really well. Did you take a precision straight edge and lay it on the epoxy to see how precise it ended up. I tried a small pour long ago but and was surprised it was not perfect, but I think my issue was the epoxy isn't really viscus like water which would go level very quick. Anyway looks like yours appears pretty flat.

Russ

[1Jumper10]

Hi CNCMAN

Well, the only checking I've done so far is with an ordinary level. Before I poured it I spent a good amount of time getting the base as level as you can get a slightly twisted piece of steel. I reckon there was about 1/8" variation from high point to low point across the entire base. And depending on where you put the level it would always be a little off. The levels consistently indicated the high point was at the center where the bridge connects. Checking with the level after the pour it makes no difference where you put it, its exactly dead nutz level. So that was one check.

A second indicator of its flatness is how accurately it reflects light. 99% of it is as shiny as a black mirror and its easy to see where its not perfectly flat because it distorts a reflected image. its evident as you get close to the meniscus and in one spot where the blush meets itself. Its like how ripples in a pond distort light only these waves are no longer fluid.

The 3rd and final check will be the machined edge of the I beam's that will sit on top of this epoxy. They have been machined flat on a horizontal boring mill. Any surface protrusion in the epoxy will be evident when I place them on it.

I wish I had a machinists level to check with but I dont.

[CNCMAN172]

Well it sounds like it is pretty darn close to flat, based on what you checked. I am sure you will find it was really worth all the effort. Nice build.

Russ

[1Jumper10]

Thanks. I forgot how much work these things are. Its hard to believe I've been working on this since about February and I still dont have a semi-working machine. Just an assemblage of parts. I guess that's how it happens when your design is not finalized at the beginning.