"DeusEx"

[mocha]

[edit: added pics 2 and 3, x axis in yellow! that might be clearer and make more sense!!]

Apologies for poor quality drawings!!!

Thanks Jazz!

Yes, the image probably doesn't make too much sense as it is, LOL, this might be clearer, although not necessarily better! :-)

In answer to your points;
Why? I'm thinking that the closer to the spindle that the supported rails of the Z are, the less leverage that can be applied to them, so the spindle mount is also the supported rail bearing mount.... This of course assumes that fitting the rails to the spindle is a good idea! lol

The way you have it drawn you are still relaying on just 2 rails/bearings and if the bit with rail on top thats jutting out from the Y axis with rails top n bot is not directly mounted to the gantry sides or X axis bearings plate then your in big trouble.!

yep, ammended pic above

central Z axis (Ala Mech mate)

Not familiar with that design, will go find it.

...can be very strong it just cost's MORE, more expense, more real estate, more work.! .

. .

Yep, the real estate is going to be my main issue on an 800 x 800 table,

trick is working out if it's worth that much MORE.

LOL, and therein lies the answer! I'm guessing that if it was worth it, evolution would have saved it and it would have been done by now!

Interesting exercise in getting my head around some of the things that I've only read about so far.

[mocha]

So what you have there is a copy of the MECHMATE? that is the type of arrangement they use.

Thanks, will go have a look at it!

:-)

[JAZZCNC]

Thanks Jazz!

Yes, the image probably doesn't make too much sense as it is, LOL, this might be clearer, although not necessarily better! :-)

Nope certainly wouldn't go with that layout.! . . . .Just think of all that weight levering on the Y & X axis bearings.? . . . Bad idea.

Go check the Mechmate out and you'll see how it could be done, thou it will make short work of your 800mm.!!

[mocha]

I can't pretend I understand the reasons, but I do agree with the conclusion! I'll stick with using a "traditional" Z axis if only because of time and cost restraints. :-)

Apart from the length of the Z ball screws, 400? 450?? I don't think I need to worry about the Z just yet. Still got other areas to sort in the next three weeks!

[JAZZCNC]

Mocha sorry didn't see this before but Doing it this way means the X axis is unsupported from under neath and could flex.?

I would still do it with this configuration but lift the whole thing up and sit it on the X axis bearings, basicly making a rectangle box section frame.
Like I say Spindel supported on four sides would be better but I see why you have done it this way to keep the width down, thou I think the extra strength would be worth the width loss.!

[Swarfing]

Mocha take a look at Rogers build, this is similar to what you want to do, he told me he would have taken a more traditional approach if he were to do it again.

http://www.mycncuk.com/forums/showth...-5-Years-in%21

[mocha]

there's some serious work gone into that! He mentions some sort of design flaw??

[JAZZCNC]

there's some serious work gone into that! He mentions some sort of design flaw??

Ye I big design flaw.??. . . . He used springy unsupported round rail.!

[Jonathan]

The location of the supported rails on their side is unusual isn't it? Most of the ones I've seen seem to be in the Z; up/down axis usually one facing up and one facing down. Any reason for doing it that way?

Putting one up and one down evens up the load ratings. Clearly the supported rail bearings are weaker in one direction as that portion of the bearing is 'missing'. Sure I've mentioned this before, but nevermind. I calculated it once by measuring the angular position of each of the rows of bearings in a SBR25 block and from that calculated the relative force rating. The figures I ened up with for the SBR25 blocks was 980N (from datasheet) and -340N (calculated). So clearly if both rails are the same way you get 1960N in one X direction and only 680N in the other, however if they are opposite it's 1320N for both up and down, and even for X directions.

But... there's more to it than that. The round rails are connected to the aluminium support by a series of bolts into the rail from underneath. This connection isn't actually that strong (though clearly it's a whole lot better than nothing), so the rail will slill bend on the support with a force (if the beraings are mounted facing) parallel to X, or if the bearings are mounted on the same side they'll deflect with forces parallel to Z.
So it's debatable which is best...the force ratings above are a lot greater than what you will actually encounter, so perhaps deflection is more important because as the force increases the deflection will be greater. One day maybe I'll measure it.

Mocha take a look at Rogers build, this is similar to what you want to do, he told me he would have taken a more traditional approach if he were to do it again.

http://www.mycncuk.com/forums/showth...-5-Years-in%21

Interesting that he used a worm drive...

This is similar to what was thinking of doing, and may still do, for my machine. The difference is I also considered having two ballscrews on Y:

http://www.mycncuk.com/forums/showth...h-X-and-Y-axis

Adding a third rail would help, but when the two rails are profile rails I don't think it will make much difference, as long as what the rails are mounted on is strong, as they are rated for an equal load in all directions.

[mocha]

No, I think he said something about it binding somewhere?? and having to "ease it" with a hammer (?I think...)

Although I don't intend to try it, I do still wonder if mounting the rails directly onto the spindle, parallel to the Y axis has any merit, especially if they were profiled rather than supported rail... file that one under mark 2 version ideas! :confused:

However, I've been working on the work area, (bed / table element) I wanted it to be adjustable to minimise the extension of the Z axis but still have the flexibility to accommadate different work heights.

The thought of driving the table up or down was discarded as too costly, handcranking, better but too complex, but I think I've found a way that might just work. Any comments welcome!

My work area, inside the frame, is something like 850 x 900. By making the bed bolt into place inside that frame, I get the bed as in the example on the left of the pic. Undo the 4 bolts, turn the bed other way up and rotate through 90 degrees and the bed is now 70mm higher, now supported by the box section on the other sides and locked back in place with the bolts again, now on a different side. By using a couple of 70mm box section "spacers" and longer bolts, I could get another 70mm too.

I'm not expecting to be changing heights every day... do those of you with some sort of adjustment on the bed height find that there is a sweet spot where it stays most of the time, or are you constantly fiddling with the height of the bed?