Effect of Forces on Gantry Cross-Sections?

[wilfy]

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]

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]

Ok but mounting rails on 2 smaller pieces of box section is ok I presume because you can make some adjustment??

[Jonathan]

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]

...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]

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]

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]

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.

[Iwant1]

. . . 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]

. . . 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.