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Offline ART

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Size of a table..
« on: October 10, 2006, 03:11:58 PM »
Table - Continued..

   Well, Size is something to be considered. Not alot to say on it other than it must be large enough to hold what you need held.
Dont forget to consider though how to connect your gantry to the table. Some youll see overhang the table anf literally hang from
bearings under or on the side of the table. Those are nice, keeps the material away for the bearings. I didnt do that , of course,
I went for the more obvious method of bolting the bearings to the table and having the gantry ride on top of them. Works well,
and I dont really regret the decision, but it is somethign to consider.

Plan the size to hold the gantry. Personally, I think making the gantryand then making a table to fit it is the easier way to go. Our gantry was a 3/4 aluminum
side wall with an Aluminum beam riding between them.  The sides were gussetted for strength, and had flat bottoms put on that would just sit on the bearing
carriages when done. That part worked very well, we may even keep it that way.  Still hasnt been decided to start again from scratch or resuse the old table,
but I lean toward reuse.

   That made a very tight gantry and the X axis was a dream, the Y axis was tougher. We quickly found that keeping it square was difficult. Luckily, we had lots of linear rails
hanging about, so we just doubled them up, so with two sets of rails on each side, everyhting became much more rigid.  We bolted a 1/4 steel plate about a foot wide on each
side of the table to hold all this, and that too worked pretty good. Its easy to level a steel plate bolted to a wooden top, and the thing IS quiet when moving. Originally we
put rack and pinion on the Y axis, but found it noisy and abit too slow to accelerate. A belt drive fixed that, and we found the belt tended to dampen vibration to the extent
that higher acceleration was available without having a resonance problem. (Resonance will lose you steps if the coupling between motor and carriage is too tight.. and belts flex enough
to greatly aid in that. ).

   We used about 16" centers on the wooden joists, and it has incredible strength. The total cost for the table was about 200.00-300.00 I think with an extra 50.00 for the steel plates.
We have found it very nice to work with... Id match its performance to a steel table any day, and I suspect no difference would be seen. So if your building a table for yourself, I do suggest
a wooden one. Might defy convention, but hey, its relatively cheap, works well, and youll have lots of other places to put your money.. might as well spend high when needed , and keep it low otherwise..

More later, I need to get some photos of this tabel Im describing so you can see what I mean on construction..


Offline ART

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Re: Size of a table..
« Reply #1 on: October 10, 2006, 11:10:21 PM »
Ok, so Bobs decided he really wants a metal table. (I suspect he just doesnt want to screw with the wooden one, it does work pretty well.) So Ill gather photos
of the older wooden table so you can see how that one worked out, and some of the new planned metal table as its built. We'll try to post at least once a week
 till the project is done, and likely several times a week when we get wiring and such.

  We will post photos of the process along the way so you can see what youd be up against. So I guess this tabel will be pretty much new from the ground up.. it'll
be interesting to see just how we make out..

Art....& Bob..
Re: Size of a table..
« Reply #2 on: October 14, 2006, 04:13:05 PM »
thanks. Please continue with this. My main problem is putting it all together when I only get bits and pieces through other sources. Like the boxing cat, too. GS
Re: Size of a table 18X36 polymer concrete
« Reply #3 on: October 14, 2006, 07:58:12 PM »
Table idea for a moving table 18X36 made out of polymer concrete


Re: Size of a table..
« Reply #4 on: October 28, 2006, 12:33:50 PM »
Hi Art,
I am going to join you on your journey, I am going to build my own I but I think I am going to need your help, so your going to hear alot from me. You have been talking alot about gears and pulleys what about direct drive from the steppers/servos?
"Ideas have consequence" Ravi Zaccharias

Offline ART

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Re: Size of a table..
« Reply #5 on: October 28, 2006, 01:09:05 PM »

  Sorry, Ive been a bit slow on this project, but this will likely take some time.
We have decided to simply upgrade the the table we currently have to work for us,
it does work well in most things, but cant do a small photo for example as the gantry will skew
too easily. So Im getting some new Tompson bearings for the linear rails on the X (or Y whatever. :) )
  SO first, we'll fix the skew, then we'll change the long axis drive system.

  The problem with direct drive from a motor, is usually the gearing is too low. You want to aim for about 500 steps/mm,
, at 25Khz  thats  about 2inch per second, or 120IPM, not bad..but still slow.  At 45Khz thats about 212IPM,  which woudl be
much nicer. We'll work out some numbers as we get closer and choose the best compromise. The thing you need to worry
about with direct coupling, is the resolution. A 4inch diameter belt drive direct coupled, would give you only about 500 steps/inch,
which isnt very good. Hi speed, yes, but low resolution. That is the balance you need to look at.

   Each person will have their own resolution/speed mix, but Ill probably end up driving this with a G100 or ncPod, so in the end speed
wont matter as much. ncPod does 75000, G100 does 4Mhz, so if I need more speed, Ill bump it up that way, in other words, Ill probably aim for
a reduction of quite a bit to make the servos go almost full out in rapid, thats the sweet point from my understanding of all the motor magic
involved.  So if I can reduce the 2000-3000RPM down to 500IPM, Ill feel pretty good about it, and will use an appropriate speed pulse engine
to get the end required speed. Dont know if that answers your questions, but its the type of thing Id consider when it comes to motor coupling..

Soon, we'll begin to show what we did to kill the skew, and keep the gantry square, it may take two motors, but we'll try it with one for the sake
of simplicity..

Re: Size of a table..
« Reply #6 on: November 04, 2006, 03:13:33 AM »
Edited to say that the post below does not apply to servo motors, and does not apply for stepper motors driven with coarse pulses

Maybe I can describe the resolution issue in practical end-result terms.... (and bring the thread back to table size)

If you are engraving the faces of medals and coins, you need a very fine resolution and lots of gearing (lots of pulses per inch).

If you are cutting background props for a Wild West movie set then you absolutely don't need any gearing!

Gearing is expensive, complicated. So, the natural question is, "How good can one cut without gearing?" I have lots of experience with a biggish (8'x4') router running 600 oz.in steppers directly to 1" diam pinions running on open racks. (The drives are 1/8 microstepping.) This combination makes a rather good machine for cutting up MDF and Plywood sheets with typically a 3/8" cutter.

When cutting rectangles out of boards (eg. Kitchen cupboard doors), the "low" resolution of the direct motor/pinion/rack drive has no noticeable effect - the edges are smooth and free of chatter marks. When cutting curved counter tops, a tiny "chatter" can be seen on the edge - still far better than a jigsaw or bandsaw. These "chatters" are easily sanded out by hand. We do cutting on a job-shop basis and the boatbuilders and shopfitters have no problems with the edge quality. www.camcraftsa.com But we won't do small plastic parts for glossy promotional gifts. (A larger cutter also gives a much smoother finish). If you are willing to apply a little sandpaper afterwards, you can also do the high-gloss smaller artsy stuff

So, I guess what I want say is that one can build a productive CNC router with direct drive stepper motors, but this is for big cutters, big sheets of wood, big space, big income :) Anyway, this is a table size thread and I thought I needed to colour in the big table, low resolution end of the scale. The "tractor" end versus the "sports car" end - they both have their place, and they are not interchageable. You could drive to church on your tractor, but you can't plough a field with your sports car! :)

All the best
GeraldĀ  (www.mechmate.com)
« Last Edit: November 04, 2006, 05:30:23 AM by Gerald_D »
Re: Size of a table..
« Reply #7 on: November 07, 2006, 10:59:27 PM »
Art wrote:
>>A belt drive fixed that, and we found the belt tended to dampen vibration to the extent
that higher acceleration was available without having a resonance problem. (Resonance will lose you steps if the coupling between motor and carriage is too tight.. and belts flex enough
to greatly aid in that. ).<<

About the coupling what did you mean by that?  I have a coupling between the motor and the lead screw and I have been having problems with my machine loosing steps or loosing location. I'm not too sure what is going on but I keep wrecking parts.

What do you think?

"Ideas have consequence" Ravi Zaccharias
Re: Size of a table..
« Reply #8 on: December 13, 2006, 12:13:20 PM »
Hi Art
Thanks for a great article; a couple questions though. Has anyone considered turning the table on its side in order to reduce the floor space footprint? I would have thought that as long as the table is slightly angled and some thought was given to the machining operation it could give many advantages. Do you know if anyone has done this?
Re: Size of a table..
« Reply #9 on: February 16, 2007, 11:55:18 AM »
Hi Guys

Another newbie joins the fray  :)
Interesting post Brian I am designing a 4x8'x6" router table at the moment and I plan to have the bed at a 60 degree angle to the floor so it sounds very similar to your question.

I am very fortunate that i have been able to save from the tip some thk linear bearings, 5mm pitch ball screws, 500oz/inch steppers and a bunch of slosyn drives and a big vacuum pump...so I've at least got all the expensive stuff for free :). We were decomissinoning so gear here at work and it was all going to the scrap dealer so I "saved" it.

As far as I can see the only real drawback to the vertical design is the x-axis platform will be exerting all its weight onto this axis drive unless I counter balance it. On the plus side the gantry will not suffer from sag as much if it is not as stiff since it is nearly vertical. Should be better for dust extraction too.

I am confused about a previous post saying direct drive was not giving enough resolution and one needed 500 steps/mm!!! sure you don't mean inch?

I am also building the gantry and rest of the bed of MDF reinforced with fibeglass and carbon.