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WandrinAndy
06-08-2012, 08:48 PM
I'm having some difficulty getting to grips with lateral resistance and torsional forces and deflection, particularly on a Y-axis gantry of about 800mm unsupported, so am hoping that a discussion comparing the coloured cross-section pics I've attached will help me and probably others too.

Only now that I've drawn these up and can picture them properly, I think the varying degrees of ease-of-construction, alignment, ease-of-changing-pulleys, etc., are fairly self evident from the pictures, and know that the impact of the respective weights on acceleration needs to be considered, but the gist of my confusion is around rigidity and stiffness.

GREEN -- 45x90 ------- 3.9kg/m extrusion
PURPLE - 120x60x4 ---- 3.7kg/m rectangular tube
BLUE --- 152x51x6.35 - 6.5kg/m rectangular tube
RED ---- 152x51x9.53 - 4.9kg/m angle + 45x45 extrusion 1.9kg/m

If cost were to be ignored for now, how do these cross-sections compare to one another in terms of resisting the forces experienced during cutting aluminium?

Thanks,

Iwant1
06-08-2012, 10:38 PM
How come you didn't make a drawing of how JAZZCNC quoted in your build log.


The 45x90 will be better but I'd put it top & bottom and have the wide bit flat.
This way it ties into the sides better and gives more resistance to bending in lateral cutting directions.,

I've knocked up a drawing of this and also think this is a great way to go.

1. It gives a nice straight guide to fix the profiled rails onto.
2. Using the BK/BF blocks as spacers between the top and bottom extrusions, will help the profile rails being parallel with each other.
3. Makes a nice void for the ball screw to hide in and keeping the z axis close to the extrusion.
4. Better fixing to the gantry side plates by having 4 bolts in both extrusion ends, 8 for both ends.

Cheers

6525

WandrinAndy
06-08-2012, 11:09 PM
How come you didn't make a drawing of how JAZZCNC quoted in your build log. I've knocked up a drawing of this and also think this is a great way to go.

I don't think that's what Jazz was suggesting Adil... If anything I think having the two 45x90's flat like you've drawn would be lots weaker towards the middle of the gantry than the 45x45 which I was going to go for... similar to your original design... because the profile rails are so far removed from the back plate!

I think Jazz favours the GREEN option for an initial DIY build which I'm now thinking is a far better option that what I designed.... I was thinking of using the GREEN at one stage but somehow dropped it and went off on a tangent after seeing your design pics. lol

My thoughts are constantly evolving, and while I am looking at changing things, may as well look at the box-sections too, but I really need to understand things, hence the reason for this thread.

I suspect that the PURPLE is too thin, so have added another option which I've attached below.

ORANGE - 127x51x6.35mm

JAZZCNC
06-08-2012, 11:43 PM
I don't think that's what Jazz was suggesting Adil...

Adil's correct that is what I meant in that post. It will be hardly any weaker in the middle over this width but will add far better strength and rigidity to the gantry than 45x45 would in the lateral directions.

Your also correct that I favor the green design but didn't want to get into that in your previous post because it's slight different design.

I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.

WandrinAndy
07-08-2012, 12:07 AM
Adil's correct that is what I meant in that post. It will be hardly any weaker in the middle over this width but will add far better strength and rigidity to the gantry than 45x45 would in the lateral directions.

Your also correct that I favor the green design but didn't want to get into that in your previous post because it's slight different design.

I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.

Thanks Jazz and apologies for doubting your interpretation Adil. Afraid I still have nagging doubts though about the vertical strength of those sections lying flat like that, and reckon there must be extra sag in the middle when compared to the GREEN design.

Getting back to this thread, how would the design Adil listed compare to the GREEN design in terms of rigidity?

I wonder what size box section Jonathan's gantry is and particularly what wall thickness?

Iwant1
07-08-2012, 12:30 AM
How about using components from my yellow and your green, tada.
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I added a 60x45mm extrusion in between the top and bottom extrusions. These can all be bolted through from the top and also screwed into the 10mm back plate. Now your worry about the weaker wider section is dealt with, plus that extra piece for 800mm length is only 12, sweet.

irving2008
07-08-2012, 12:34 AM
Its quite hard to model these different designs without being able to calculate the moment of area. But to answer your last question, the L-shape created by two sections of extrusion, assuming they are rigidly joined will be slightly stiffer than the single piece in the X direction but two 45 x 90 in the X direction will be stiffer again.. without doing the math, I'm guessing the two 45 W extrusions bolted to a piece of plate in Jazz's design will be slightly more rigid in the vertical than the 45 x 45 bolted to a 45 x 90 horizontally. So on balance the yellow one I'd say is stiffer then the green one, certainly for static forces vertically or horizontally on the gantry, but it weighs more. Torsional stiffness is harder to assess. Box section is far better than extrusion in resisting torsional forces, but my gut feel is the green design is likely to be better than the yellow in this respect...

Iwant1
07-08-2012, 12:38 AM
I wonder what size box section Jonathan's gantry is and particularly what wall thickness?

Jonathan's gantry uses 80x80 box section probably 3-4mm wall thickness. But as he says in other posts, the ball screw is not in an ideal location, the distance from the ball screw to the cutter is too far.

JAZZCNC
07-08-2012, 12:46 AM
Thanks Jazz and apologies for doubting your interpretation Adil. Afraid I still have nagging doubts though about the vertical strength of those sections lying flat like that, and reckon there must be extra sag in the middle when compared to the GREEN design.

Obviously there will be some tiny tiny difference but when you have some profile in your hands you'll realise why it's not a problem.!! . . . . The difference you would struggle to measure with a dial gauge and I'm 99.9% sure you have far more pressing problems ahead to overcome than if the profile sag's 0.001mm more than 45x45.??
The extra lateral strength and rigidity the extra bolting into gantry sides allows will far far outway the very very minute difference any sag will make.

JAZZCNC
07-08-2012, 01:05 AM
I've built machines using the green design and please believe me you have nothing to fear the strength is easily over the top for anything but steel or harder.

If you use Bk/BF blocks then they line straight up with slots so easy fitting. It offers good strong attachment to gantry sides.
Yes it places the ball-screw slightly further back than what's considered ideal but again it's nothing and doesn't make any difference to how machine performs. The positive is the screw is away from the firing line and protected.

Not trying to sell it to you but I know it works with no detrimental side affects and plenty of positives.!!

WandrinAndy
07-08-2012, 01:45 AM
How about using components from my yellow and your green, tada. I added a 60x45mm extrusion in between the top and bottom extrusions. These can all be bolted through from the top and also screwed into the 10mm back plate. Now your worry about the weaker wider section is dealt with, plus that extra piece for 800mm length is only 12, sweet.

Thanks Adil, that looks incredibly strong in all directions... my gutfeel is it's even stronger than GREEN!

Jazz's GREEN looks by far the simplest to build and has better accessability for changing belts and pulleys... certainly if this is to be done within the gantry ends... as my current limited space dictates a compact design. And it moves the ballscrew away from the mess of the cutting chips, but also away from the profile-rails.

Your YELLOW looks the strongest, and brings the ballscrew between the profile-rails, but also closer to the mess of the cutting chips.

Anyway, my beddy beckons and tomorrow is but another day to decide....

WandrinAndy
07-08-2012, 05:15 PM
Not trying to sell it to you but I know it works with no detrimental side affects and plenty of positives.!!

Well, you have sold GREEN to me Jazz... even without trying! I have updated my design and am moving on, although I'm still curious about how the thicker-walled box sections compare, and the effect this may have on the machine.

For instance if Jonathan were to build a new machine and dosh wasn't a limitation, I wonder if he would still use box section, what size it would be, and why?

Thanks for the ideas guys.

JAZZCNC
07-08-2012, 05:58 PM
For instance if Jonathan were to build a new machine and dosh wasn't a limitation, I wonder if he would still use box section, what size it would be, and why?

If NO DOSH limit then I'm 99.9% confident he wouldn't and it would be a hefty lump of ground cast Iron or some fancy composite affair.!!

Can't hide from the fact box section sings like a canary and resonance and vibration are to be avoided so density wins every time, that's why cast iron's used so much.

This is the kind of thing you'd buy(New) if Dosh no limit.!!! . . . .
Here's the Y Axis carriage(pic1) which carries the Z axis(pic2) of my future 5 axis toy(Doubt I'll touch it this year thou.!).!! . . . It's one big cast iron lump with a counter balanced Z axis which is very long lump of cast and ground aluminium that runs along the roof of the main machine frame which is one big structure of steel and ground cast Iron.. . . Very very very strong and heavy 3.8ton the frame weighs.!!
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WandrinAndy
07-08-2012, 06:36 PM
Very very very strong and heavy 3.8ton the frame weighs.!!

Nah... doesn't match my needs... my touring caravan frame wouldn't handle it... ;- )

JAZZCNC
07-08-2012, 06:47 PM
Nah... doesn't match my needs... my touring caravan frame wouldn't handle it... ;- )

You could live in the control cabinet put wheels on it and it's touring CNC machine. .:joker:

Jonathan
07-08-2012, 08:50 PM
For instance if Jonathan were to build a new machine and dosh wasn't a limitation, I wonder if he would still use box section, what size it would be, and why?

The simple answer is wait and see ;)

That is an extremely difficult question to answer since it depends primarily on the intended purpose and size of the machine. If the machine was required to do everything I could ever want, i.e. 5-axis able to cut most metals, then I would consider a steel and epoxy granite construction since it offers great versatility and strength in the design. Next step down perhaps would be a gantry made from aluminium plate. Neither of these would follow a style remotely similar to what has been posted. If the machine was only ever intended to cut woods and plastics with a short Z-axis then I would use multiple pieces of steel box section for the gantry, after appropriate FEA to ascertain and suppress potential resonance issues. None of this really helps you though since your budget is regrettably finite, so I'll leave it at that.


Jonathan's gantry uses 80x80 box section probably 3-4mm wall thickness. But as he says in other posts, the ball screw is not in an ideal location, the distance from the ball screw to the cutter is too far.

Mine is 80x80x3mm since that was cheap at the time. It is adequate since the SBR20 round rails introduce a far greater error than deflection of the box section. With profile rails then something stronger is warranted.

I would discount purple and blue due to the potential problems with accurately mounting the rails on material that may not be made to sufficient tolerances to maintain an accurate rail spacing. Additionally the ballscrew position is sub-optimal on these, since there is a horizontal distance between the ballscrew an the rails which allows the bearing blocks to move (slightly since they are mounted on something that is not perfectly stiff) without the ballnut moving. This is a significant limitation on my machine, made more significant by presently not having profile rails on the Y-axis.

It's harder to say between green and red, in my opinion they are both fit for purpose. I'll post more about them later.

Edit: In fact, if you send me or post the drawings (ideally just the cross section as a .dxf) I will run a FE stress analysis simulation since as irving has pointed out it would be good to get some idea of the torsional stiffness and calculate the second moment of area.

WandrinAndy
07-08-2012, 09:23 PM
Edit: In fact, if you send me or post the drawings (ideally just the cross section as a .dxf) I will run a FE stress analysis simulation since as irving has pointed out it would be good to get some idea of the torsional stiffness and calculate the second moment of area.

Thanks Jonathan.... Interesting as always.

Second-moment-of-area is beyond me, but thankfully I probably only need to understand that it's different to having a senior moment.

Afraid I currently only have SketchUp, where one can do a section plane if this would help, but haven't used it before so cannot promise... I've just recovered the files as is from the recycle-bin, let me know if it would be worthwhile me sending or posting them... they're 300-600KB each (Green and Red?). Would like to see the results for Adil's most recent Yellow too if that's possible.

Jonathan
07-08-2012, 09:41 PM
let me know if it would be worthwhile me sending or posting them... they're 300-600KB each (Green and Red?). Would like to see the results for Adil's most recent Yellow too if that's possible.

Yes I should be able to work from those.

WandrinAndy
07-08-2012, 09:58 PM
They are invalid file types for this forum so have uploaded them to Models by MacTavish - 3D Warehouse Search (http://sketchup.google.com/3dwarehouse/search?uq=0990694220207768883659336&scoring=m)

Iwant1
07-08-2012, 10:18 PM
Would like to see the results for Adil's most recent Yellow too if that's possible.

If Jonathan doesn't mind I will send him a section of my yellow design.

I'll send two, my first yellow one which I designed how Jazz advised in your build where I have the z axis to the right.
6551

And a newer one I've just made which is very similar to your green 'L' but a 'C' shape. This allows us to fix the Y axis motor mount easily and also have a surface to run energy chain. Have a look remember the z axis is now towards the left, and the ali plate is removed. I wonder if having this plate will increase its strength?


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wilfy
07-08-2012, 11:17 PM
I would discount purple and blue due to the potential problems with accurately mounting the rails on material that may not be made to sufficient tolerances to maintain an accurate rail spacing.


am i reading this right that mounting on steel box section is a no no?? if so how big of a problem are we talking here?

Jonathan
07-08-2012, 11:22 PM
I will have a go at the analysis tomorrow evening and report back...approximately what distance do you expect between the tool tip and the bottom of the gantry?


am i reading this right that mounting on steel box section is a no no?? if so how big of a problem are we talking here?

Yes, mounting on opposite sides of steel box section is a no-no since the distance between those sides when the steel is manufactured has a much greater tolerance than for aluminium. This means that there is a high probability of the dimension being sufficiently even for the bearings to run smoothly upon the rails. Profile rails especially tolerate very little misalignment, so it is important to ensure they are mounted accurately to avoid premature failure due high stress.

wilfy
07-08-2012, 11:42 PM
Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

Jonathan
07-08-2012, 11:46 PM
Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

Yes, unless the box section is really bad - for instance if it has been welded it would distort somewhat.

WandrinAndy
07-08-2012, 11:53 PM
...approximately what distance do you expect between the tool tip and the bottom of the gantry?

Could you use some hypothetical distance at this stage... I'm more interested in a general comparison and haven't got to the Z-axis of Mk1 design yet and things may well change if the recent past is a good yardstick... lol

Are you able to volunteer a distance you would like to be used Adil?

irving2008
07-08-2012, 11:55 PM
Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

there are two separate issues - parallelism of the oppsite sides of the box and flatness of each side of the box; neither are guaranteed in the manufacturing process. The flatness issue means you might have to shim the rail to keep it straight when you bolt it down.. even if you have 2 boxes you still have this problem. The only sensible solution is to surface grind or face both sides of the box section to make them flat and parallel.

JAZZCNC
08-08-2012, 12:55 AM
am i reading this right that mounting on steel box section is a no no?? if so how big of a problem are we talking here?

Depends which world you live in.? The theoretical or the real.!! . . . . . FFS wish folks would stop over complicating things.!!

Here's the real world deal.!! . . . Irving is absolutly correct that unless you can surface the two sides parallel and flat then you can't be 100% sure every things correct.! . . . BUT . . . This is true in one way or another for ALL the surfaces the rails mount onto whether they be 1 rail on top & 1 on front, or both on front etc and unless you can put these surfaces complete and assembled onto a milling machine and accurately machine all surfaces flat and parallel or perpendicular to each other then you are going to have to accept they will need some adjustment or tweaking.!!

How you do this will depend on the design But having the rails on 2 separate sections, no matter if that be profile, boxsection, ali plate will make very little difference and It's FACT that some shimming or filing or grinding will be needed to bring the rails into perfect alignment and parallelism either way.!

Now I'm not going to blow smoke up your arse's saying this is easy and quick because it's not and doing this takes patience and a lot of measuring and masses of time if you want good accuracy.!. . . BUT . . . If your just cutting wood,foam and bit of plastics then even box section will be close and with slight bit of clean up and shimming you'll be there in no time.!

I know it's hard if you haven't hands on experience of the forces cnc impart on frames and accuracy required but really for the application of mainly wood use then any of the designs shown here would be plenty strong enough.!! . . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

Don't fry your noddles trying to create the perfect flex free gantry for cutting woods etc because your just wasting braincells and time when it's not needed.!

Iwant1
08-08-2012, 01:15 AM
Are you able to volunteer a distance you would like to be used Adil?

Erm, if I'm understanding it correctly the vertical distance is about 5 inches, but depends on the length of the tool also.
6554

Iwant1
08-08-2012, 01:30 AM
. . . If your just cutting wood,foam and bit of plastics then any of the designs shown here would be plenty strong enough.!! . . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

Don't fry your noddles trying to create the perfect flex free gantry for cutting woods etc because your just wasting braincells and time when it's not needed.!

Thanks for the comforting post Jazz. Since this thread started, I've been thinking I got my machine drawings all wrong, and thinking about it day and night. I'm building my machine for general use and for a multitude of materials. I definitely won't be cutting steel, but would like to know it could do the odd bit of aluminium such as the 'Aztec calender', lol.

Is there any chance we can see your machine's gantry or get a description, as I know you mostly work with aluminium.

Thanks

WandrinAndy
08-08-2012, 01:40 AM
. . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

The trouble in my case is that my (human-oid) mind, although not (and maybe because of not) having any experience of this field, wants to understand at least the basic concepts of why some things are bad and others good.

Quite sure that I will get a massive buzz with the first proper movement of the cutting tool... maybe just scribing something... but the REAL joy for me is in learning new stuff and putting ideas together, albeit mostly cribbed from others!

My journey started with the intention of only machining wood, and then the idea evolved to...

- maybe it could also be used for at least accurately marking layouts on aluminium parts which would surely be better than marking them manually

- and maybe it could be used to machine aluminium parts like the guys on here have made for the next upgraded version of their machine, albeit at slow speeds and not to the greatest levels of accuracy

Don't think it will evolve further than that... certainly not on the initial machine.

WandrinAndy
08-08-2012, 01:48 AM
Since this thread started, I've been thinking I got my machine drawings all wrong, and thinking about it day and night.

Blame it on Jazz... lol... It was he who got me questioning that my drawings were all wrong, although I thought I had roughly copied my gantry design from your gantry design.

But... I am extremely pleased that he sold me his simpler green design!

LOL and night night.

JAZZCNC
08-08-2012, 12:58 PM
Blame it on Jazz... lol... It was he who got me questioning that my drawings were all wrong, although I thought I had roughly copied my gantry design from your gantry design.

It's OK my shoulders are broad has well as my stomach.!! . . . .But never said wrong just could be better with wider profile.!!



Is there any chance we can see your machine's gantry or get a description, as I know you mostly work with aluminium.

Well actually this will seem ironic in that my gantry is kind of like box section.? I'm a bit of "Womble" and hate waste. So It's rescued from scrap bin and started life has a Stanner stair lift real HD bespoke aluminium profile that the chair rails ran along.
It's shaped like an "H" but imagine the bridge being box section with the sides sticking above and below about 30mm. Cant show pics because I'm not at home and my pics are on external drive.
The void the top and bottom sticking up bits create have had 60x30mm profile inserted and bolted to gantry which the profiled linear rails fasten onto.
The boxsection void is filled with sand to dampen resonance.! This was done afterwards because I wasn't happy with the tune it was Singing.!!

I wouldn't recommend anyone copy it because it's a rubbish why to do it and I only did it because I had the profile for free. It took me along time "Shimming" to get it working to an acceptable level of accuracy and I'm still far from happy with it regards strength and vibration so while It's strong there's easier and better ways to do it.

The secret to cutting aluminium successfully on a "Router" type machine is to limit tool and Z extension to the minimum possible and reduce resonance/vibration to minimum thru using dense gantry/machine materials.
That's why box section is not ideal for gantry's cutting aluminium unless filled (Made denser) but for woods then it's not really a problem if a decent wall thickness is used and like I say the rail alignment/parallelism etc has to be dealt with thru shimming or surface adjustment regardless if rail mounting surfaces are on individual pieces of profile/B-section or one big boxsection/profile.!!

If I was in the position lots are where they have to use off the shelf materials that can only be worked with hand tools then for a "Gantry" I'd choose Profile over box section every time.!

irving2008
08-08-2012, 01:38 PM
Well actually this will seem ironic in that my gantry is kind of like box section.? I'm a bit of "Womble" and hate waste. So It's rescued from scrap bin and started life has a Stanner stair lift real HD bespoke aluminium profile that the chair rails ran along.
It's shaped like an "H" but imagine the bridge being box section with the sides sticking above and below about 30mm. Cant show pics because I'm not at home and my pics are on external drive....

This made me smile Jazz... I have nearly 7 metres of the stuff, in two 3.5m lengths, rescued from my late fathers house, had to cut it into two bits to get it in the car. I had plans to use it for 'something' but never did figure out how to get round the odd shape, I'd describe mine as like two round rails attached to a box section with fins (see the pic below, the end cap is roughly the shape of the profile). I keep thinking about chopping it up and turning it into ingots for future needs but its just looks too good as a profile. Also got the rack (that was already in 2 lengths) and pinion and the original drive motor and batteries still.

http://www.mobilitynation.co.uk/productimages/stairlift/top_folded_250.jpg

JAZZCNC
08-08-2012, 03:52 PM
This made me smile Jazz... I have nearly 7 metres of the stuff, in two 3.5m lengths,

Yes I've seen this type on the newer models Irving but it's completely different to the stuff I had given, it was 3.5mtr thou and yes I cut it In half to get into car that's why my machines 1500mm wide. . Lol . .. . I actually didn't have a use for it when given just knew it was too good to go in scrap bin.!!
My machine came about by watching A U-tube vid and thinking "I can do that.!!" then looking at this piece of profile and pile of spare box section and hey presto 6hrs later the frame was roughed together.!!

The materials for whole main frame including gantry of my machine (1500mm,sq) cost 77 and that includes Ali for sides and profile on gantry, Z axis Ali cost 25. The complete thing including Z axis but excluding Ali bed cost less than 102. The profiled linear rails cost another 175 second hand of ebay. The ballscrews from chai/ebay and for roughly 500-600 I'd got a working machine that after plenty of tweaking cut's Ali fine and doesn't even know it's cutting wood.

D-man
08-08-2012, 08:49 PM
Sorry bit off topic but JAZZ trying to get hold of you but pm's just bouncing back as your hoarding you must be collecting them lol

JAZZCNC
08-08-2012, 09:21 PM
Sorry bit off topic but JAZZ trying to get hold of you but pm's just bouncing back as your hoarding you must be collecting them lol

Sorted mate.!!

wilfy
08-08-2012, 11:15 PM
Depends which world you live in.? The theoretical or the real.!! . . . . . FFS wish folks would stop over complicating things.!!

Here's the real world deal.!! . . . Irving is absolutly correct that unless you can surface the two sides parallel and flat then you can't be 100% sure every things correct.! . . . BUT . . . This is true in one way or another for ALL the surfaces the rails mount onto whether they be 1 rail on top & 1 on front, or both on front etc and unless you can put these surfaces complete and assembled onto a milling machine and accurately machine all surfaces flat and parallel or perpendicular to each other then you are going to have to accept they will need some adjustment or tweaking.!!

How you do this will depend on the design But having the rails on 2 separate sections, no matter if that be profile, boxsection, ali plate will make very little difference and It's FACT that some shimming or filing or grinding will be needed to bring the rails into perfect alignment and parallelism either way.!

Now I'm not going to blow smoke up your arse's saying this is easy and quick because it's not and doing this takes patience and a lot of measuring and masses of time if you want good accuracy.!. . . BUT . . . If your just cutting wood,foam and bit of plastics then even box section will be close and with slight bit of clean up and shimming you'll be there in no time.!

I know it's hard if you haven't hands on experience of the forces cnc impart on frames and accuracy required but really for the application of mainly wood use then any of the designs shown here would be plenty strong enough.!! . . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

Don't fry your noddles trying to create the perfect flex free gantry for cutting woods etc because your just wasting braincells and time when it's not needed.!


thats a really great post and it goes someway to help people realise what they need. thank you :D

Ross77
22-12-2012, 10:34 PM
Bit late to the topic and most of it has already been discussed but here's my take on it:smile:

Forces on a gantry are difficult to judge because they can change direction and magnitude and are mainly dynamic. To get an ideaof the problem of dynamic forces look in to why Marching Armys have to break step when going over bridges! :chuncky: At a basic level most beams only have to deal with a force in one direction and as such have a larger dimension in that axis, hence rectangular beams. however gantry's are also subjected to twisting so need to be able to resist torsion. circular sections are best for this and square the next best. In comparison rectangular beams are not so good especially if the are slender. Therefore a square beam would be the best option.

To get an ideaof the problem of dynamic forces look in to why armys have to break step when going over bridges:chuncky:

Box vs Profile

A made up beam using tee slot extrusions will have an new 2nd moment of area but the calculation would assume that all the faces are bonded together to make one solid section. any movement in the bolted joints will reduce its torsional stability.

Box sections are deemed the most efficient as they have all their mass at the extreme edges but can be affected by resonance. this can be damped or designed out.

Extruded sections can have the same parallelism issues as box and I have seen builds that have milled the top of the extrusion to get the accuracy they need.


So as Jazz and so many others have said its horses for courses but it does seem easy to get carried away and dismiss simple solutions. Just using linear rails an ballscrews wont result in a precision machine, they have to be correctly mounted and tweaked so they work correctly. so why not use an easy option and extend this tweaking to the mounting structure as well.

I had a go at designing a composite beam a few years ago but not had time to progress it any further. Planned to use extruded Ali box and channel to made a sort of closed I beam that had a recess for the ball screw. Top and bottom sections filled with epoxy granite to provide damping and compressive strength.

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