Straightness of steel/alumunium box section

[Jonathan]

Wow' thanks for the drawings! That looks like a good way of going about things - much less expensive aluminium plate which is a bonus.

I've been trying to get the rails to fit on 3"x1" angle mounted too the plate...but it's just generally not working. I'll post more when I've got a drawing done.

[routercnc]

I'd also been playing around with a few ideas recently. Some of them are very similar to yours Ross!

First off, tried to get Z axis as close to gantry as possible (using profile rail, which I'm fortunate to have) and came up with the scheme below (LH image).

Also, playing with gantry sides idea using RHS, with an offcut from the gantry as a spacer to get the loads well into the RHS and avoid just locally distorting the wall (centre and RH image). Lower joint down to the X bearing is always an area for many ideas - thought using these bars each side were a bit neater than the usual double back to back plate arrangement.
Option of TIG-ing a closer plate over the sloping part of the RHS, but probably not worth the effort.


p.s. Looked at some of the steel box and RHS in the stores, and the ERW stuff would need grinding. The sides tend to cave in slightly relative to the corners.

[Ross77]

I like the idea of rhs with the spacer, nice and neat, you could probably get away with just gluing a closer stip over the angle to keep dirt and chips out.

Where would the ball screws go on the first one? with those rails could you wrap the carridge around the beam and mount the srews and motors on the back or would the leverage be all wrong? not pull ing from the centroid etc...

Some of them are very similar to yours Ross!

I guess there are no new designs its all been done before and when you start leaning out or designing close to the wire you end up at the same point any way. I'm sure I ve seen elements of my design else where and thought if i just change that a bit and move that over there it will work how i want it too.

[Jonathan]

Well, I tried 'borrowing' your design Ross but couldn't get it narrow enough for my requirements. Great ideas though.

This is what I ended up with:



Here's my current design. Just trying to work out how to fix the 4 blocks to the aluminium plate that slides on the Y axis. There's not much room and it's made more difficult because the bolts thread into the bearing blocks...I don't really want to start drilling those out.





One plus of having the Z rails this way round is that that aluminium angle will stiffen up the 20mm aluminium plate significantly. Maybe I can replace the 600x160x20mm plate with 15mm to save a few ££'s?

[Jonathan]

Right, hopefully this will be the final final design!:



I've rendered it with the Y axis off the blocks on the Z as otherwise you wouldn't see much.

Have I done anything silly...does it look OK?

The mass of all that is 25.9Kg, that's not including the stepper motor, screws, bearings and spindle.

[routercnc]

Some good development and ideas, but where were you thinking of putting the Z axis leadscrew? Looks like there's no space now.

I wonder if you just turned the Z axis rails through 90deg and mounted the support rail onto the router plate, and mounted the blocks back onto the main Y axis carriage, you would be better off. Stiffness wise it must be similar, and you are no further away in terms of spacing from the gantry than the above dwg (judging by the dwg above). You could even include the angle pieces in there as well if you want.

[Jonathan]

where were you thinking of putting the Z axis leadscrew?

I forgot to put a hole in the blocks to fit it, it fits now:

I wonder if you just turned the Z axis rails through 90deg and mounted the support rail onto the router plate...

Are you describing what I drew in the first post on this thread, or something different? I rotated the rails to even up the load on the bearings. Do you think this isn't an issue? If so I'll quite likely go back to the previous version as it's easier.

[routercnc]

If I understand the dwg in post#17, then the router plate mounting footprint onto the Y axis carraige is now very narrow. I assume the blocks with the hole in for the ballscrew are the mounting feature into the bearings and into the Y axis carraige.

If this is the case I personally would go back to you original drawing. Ross can make this work, but your geometry requirement is obviously a bit different. Don't think the bearing load would be a problem for the original dwg, especially compared to the compromise on mounting in post#17. If you keep the angle piece you can always add these to the router plate if required. Any further views on that Ross?

The supported rails do have bulk, more so than profile, so it looks like you've done all you can.

[edit: Wonder if you should allow for (i.e. drill the holes for) a second pair of bearing blocks at the lower Z support. Because of your long Z axis reach the loads will be high at the bottom bearing, trying to open it up on the post#1 dwg. Another one next to it would help. Bit like some heavy doors have 3 hinges, with 2 close together. Never tried 3 in a row - don't think there would be a tolerance problem unless someone knows different. ]

[Jonathan]

Narrow mounting, yes I had just been thinking that. I'll wait and see what Ross thinks. I could separate them a bit more if I make the axis wider but I don't really want to sacrifice the Y travel. At the moment the travels are 1700x740x400 (XYZ).

Adding the L section to the previous design is perfectly possible so I may well do that. Do you think if I did I could use 15mm aluminium plate instead of 20mm for the bit they mount on?

[Ross77]

Ross can make this work, but your geometry requirement is obviously a bit different. Don't think the bearing load would be a problem for the original dwg

I just based mine on ease of construction and cheap Ali box. Trying to keep it simple to build is also worth concidering. I also based the bearing strength from this thread I posted earlier in the year
http://www.mycncuk.com/forums/showth...supported-rail

Dont forget that I was aiming to get the most out of the rails and therefore designing for the smallest possible size, again to save cost and weight. But if your 2 rails and blocks can still handle the load at 25% capacity then its not worth worrying about and save your y travel


It dose look a bit narrow to me tho. dont forget that making it wider will reduce the torque arm caused by the long z axis. (fun this design stuff eh....)