Evaluate/critique my design

[esm]

I'm getting my ducks in a row to start building a 4'x8' gantry router table. Will you evaluate my design so far? I could use suggestions/help on motion, too.

It's 4'x8'x7"
Steel 1.5" x 3" x .125" frame with corner bracing and center supports.
Skate bearing glides on Sched 40, .840" OD pipe.
Gantry is 3/4" Baltic Birch construction.
Y axis, pipe is bolted to metal 1.5"x 3" frame along sides
X axis, wood torsion box w/top and bottom .840 OD pipe rails. skate bearings.
Z axis, 2 .840" OD pipe rails with skate bearing glides.

I'd like Y axis to be driven under the table by acme or belt. Acme for the other 2 axis's.

How fast should I attempt to go? What will I need to get me there?

I was inspired by the simplicity of www.buildyourcnc.com.
My skills:
Woodworking and basic metal working.
Mig Welding
Measuring accurately

I have:
10" Table saw
12" Miter/chop saw
Router
Mig welder
Grinder
Drill press
taps and dies
lots of steel and wood

I've attached several versions of the same drawing. Give it to me straight

[ger21]

You'll never be able to drive that gantry with one screw. It'll skew a lot. You can probably put a screw on each side, and still keep them under the table.

If you plan on cutting 4x8 sheets, you'll need 9 or 10 ft screws. Most DIY machines that size use rack and pinion to avoid the long screws. If you want to use screws, you'll probably need to use 1" dia.

How fast should you go? as fast as possible, or as fast as your budget permits. You're cutting speed will be limited by your router, but you can always take multiple passes as deep as the router will allow.

I'd shoot for at least 200ipm, preferably more. Here's some info on choosing motors courtesy of GeckoDrive.



Q.) Should I use servos or steppers in my machine?

A.) If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or servo) first and then figure out if it will fit what you need.

Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.

Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

To separate what motor need (neglect the gerbil), is the power your mechanism needs.

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but you’re not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531


Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feed rate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist's experience on a manual machine. The hand crank is about 6" inches in diameter. How much force would you place on the hand crank before you figure you're not doing something right? I hear about 10 Lbs.

10 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) of torque on the leadscrew.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

[tpworks]

You might want to check out mechmate.com for a tried and proven table. can be built for under $4000.00 and you get a professional quality build with lots of support. This is my next build soon as I get the space.

[esm]

Wouldn't a 1/2"-1 turn per inch work? critical rpm at 110" is 227 = 227 IPM.
Is that correct?

[ger21]

Wouldn't a 1/2"-1 turn per inch work? critical rpm at 110" is 227 = 227 IPM.
Is that correct?

I see about 190ipm with a fixed screw and rotating nut, and about 150ipm with double angular contact bearings at each end. Based on the charts at Nook.

You're resolution will be a bit low, but should be adequate for what you plan on doing with it.

[esm]

I didn't see the chart @ nook. I used their critical speed calculator. (see screen shot)
I'll go back through through the forums and find the calculation for resolution and determine if it'll work.
The machine needs to be at least as accurate as an highschooler with a hangover.

[Gir]

The machine needs to be at least as accurate as an highschooler with a hangover.

So about +/- 1/2" good? Great.

To calculate the resolution you take 1 / [(steps per turn)*(turns per inch)] and you get inches per turn. Using your numbers you get 1 / (1 * 200), where 200 is full stepping the motor (most motors have 200 steps per revolution), which will give you 0.005". This seems good from what you've mentioned.

[esm]

.005 will work just fine! 1/16" is good, 1/20" is great!
Now about the racking ger21 mentioned. what about this? wyldesyde007's idea about the ol' drafting table cable, pulleys and spring. My old drafting table has fallen apart more times than I can count. It was my grandfather's, he got it in the Navy during WWII and although I've replaced several pieces and adjusted it from time to time, it's still square. Using that would eliminate complexity and cost.

[Med-Pac]

esm,

we're in the process of building a table of the same size. we've posted it on the diy router thread, might give you a few new ideas. not sure how to put the link on this post.

Mike

[Gir]

Now about the racking ger21 mentioned. what about this? wyldesyde007's idea about the ol' drafting table cable, pulleys and spring. My old drafting table has fallen apart more times than I can count. It was my grandfather's, he got it in the Navy during WWII and although I've replaced several pieces and adjusted it from time to time, it's still square. Using that would eliminate complexity and cost.

It's an excellent way of eliminating racking, but don't absolutely depend on it. Build the router the best you can, and if there is racking after all is said and done apply that fix (or just use it to be sure).

[esm]

Thanks Gir, I'll do that. Maybe I'll beef up the gantry sides a little more.
Med-Pac, great machine. I've been scouring your pics and posts for ideas already

[Med-Pac]

Thanks esm,

We do have a few extra brackets left over. It took us a few trys to get it right, and cnc joe's help. If you could make these work on your table, I'd part with them pretty cheap. They're just collecting dust now anyways.

Mike