Gantry Weight, The Hol(e)y One

[Mrayhursh]

I have heard in the past that you want to keep the total weight of the gantry as low as possible. Less strain on the stepper motors and maybe quicker response. If this is true would it be a good Idea to drill a series of varies size holes to lessen the weight of the cross member tubes. I know they do this in the aircraft industry. Would it be a good idea here.

[lonestaral]

Personally I would not bother.
Extra work and somewhere else for the dust to go.
Probably fill up with spider webs as well.

[smreish]

Don't do it. The machine is balanced and works within parameters as designed.
If you need hyperfast speeds (like in plasma cutting), you may want to look a something really light, but otherwise don't bother.

I think somewhere in the past when KIM was building, this was addressed by Gerald and it was non-issue. If I remember correctly, it actually was detrimental in some mechanical aspects.

[KenC]

If you are that paranoid, why don't you drill whatever holes you fancy after you get one running.

[zumergido]

i have made some test on the tubes and they bend on the middle when the spindle pass. like 0.5mm and i have 4mm thick tubes.
maybe take some material from the gantry parts. but the machine is fast enough mine run 12000mm /min with no isues no reduction. but i feel more confortable on 9000mm or less.
in fact i consider on fill the tubes with concrete to make it more rigid. the steppers have plenty of power, more if i put some 4.1 reductions.

[Mrayhursh]

Well. because I do not need anymore spiders in the garage. Lord, I have enough already. I still have the scars from when I was bitten 52 times. I will not proceed, but I appreciate the input.

[zumergido]

lucky you don't live in australia...

[Mrayhursh]

yeah the spiders you have there can kill and eat birds. Where I live I have never heard anybody say "a dingo took my baby":)

[Tom Ayres]

Too funny:D. I think you should wait for such modifications, steel flexes more than one thinks, add inertia and other movement factors, could make a 'hole' lot of flex IMHO

[lonestaral]

Snakes and scorpions here.

The cat brought a snake into the house this afternoon.:eek:
The King Cobras can grow 5M +

[Mrayhursh]

Yeah reptile lovers are bringing those critters to Florida. We now have large lizards and snakes. Not to mention the government saved the gator from extinction. Now tourist can have their toy poodles eaten as a snack. My neighbors wanted me to take out the gator in the pond behind their house. No thanks,

[Tom Ayres]

Is she a real dog?

[swatkins]

This caught my attention..

i have made some test on the tubes and they bend on the middle when the spindle pass. like 0.5mm and i have 4mm thick tubes.


As 0.5mm = 0.020 inches I was surprised that the MM would flex that much...

Is this amount of flex normal for most people?

[Mrayhursh]

That is a great test. I am not ready to try it yet but I look forward to trying it. Thanks for doing it.

[danilom]

My 2070mm cutting width gantry and 4mm thick tubes bends 0.5 with some 50kg applied on the car Y-car on top.

[smreish]

I used 4.76 mm tube, never really noticed a .020 deflection (72" wide gantry)
Of course, I didn't put a run out gauge on it either.

[swatkins]

A long time ago we hashed out the "light v heavy" problem... http://www.mechmate.com/forums/showthread.php?t=232

I had 6.4 mm, or .250", wall thickness material on hand when I started the build. It was determined that size was going to be ok for my machine. Since then I have acquired some 1/8 wall 4 x 2 material and was considering using that instead.

This talk of the gantry flexing .020" has pushed me back to the thicker material!

[domino11]

Sean,
I believe you used 1/4 inch tube on one of your machines didnt you?

[smreish]

Heath,
I did use 1/4" wall tube on #28 with 7.2 geared motors.
The machine is not lighting quick, but respectable 250-280 IPM cuts with decent acceleration. The FAST jogs are slower due to the mass, but I found it an easier build by tapping all the rail holes and eliminating the clamp strip.

Worked just fine and is working fine still to this day.

Sean

[Mrayhursh]

I measured mine today and it was 1/8 inch wall. I am going to stress load it tomorrow with 25, 50, 75 lbs. and see what deflection I get.

[Tom Ayres]

Mine is .180 inch thick (just under 3/16"). It may be a good balance between the two. I'll check it out once I get it together.

[darren salyer]

Mine is 3/16".
All this talk about fast speeds.....most of the cutting I do is with a .25 end mill.
If I go over 100" per minute, bits start breaking too frequently.

[KenC]

If the bits don't break, something else will break :)

[dbinokc]

I am not a mechanical engineer, but the bending of the beam should be a pretty straight forward beam deflection problem. A freely supported beam, with two point forces applied. Calculate the deflection.

If the deflection of the beam were too much, instead of going with a thicker walled beam, perhaps a piece of T-section could be bolted or welded to the bottom to provide addition resistance to flexing while minimizing added weight.

Anyone here with some real ME experience?

Edit: There is a calculator here
http://www.engineersedge.com/beam_bending/calculators_protected/beam_deflection_4.htm

Tube properties
http://www.engineersedge.com/calculators/section_square_case_10.htm

[Tom Ayres]

DB, I have zippo ME experience. But I did stay at a Holiday Inn Express last night.

[lonestaral]

Hey Hurshy,
I told you the spiders would invade.
The first one moved in last night.:eek:

[Mrayhursh]

Mechmate spiders they are the worse kind.