Steel Frame 8' x 5' CNC - Advice Sought

[BrentonSpear]

I have had a rethink on the steel size and now modelled it on 75x75x4mm SHS.

[Boyan Silyavski]

Add some more inner pieces for the worktop area and you have it nice and clean

[Clive S]

I have had a rethink on the steel size and now modelled it on 75x75x4mm SHS.

That is much better now with the triangulation bits put in and heavier steel section. Good luck with the build. ..Clive

[BrentonSpear]

I have been thinking about having the bed raised above the rails to make it easier to get materials on and off the machine as suggested by another person.

If I was to leave the design as it is, is it acceptable to have a lighter steel (say 25x25x3mm) frame inside the outer rails to raise the bed above the rails?

Obviously I would have to increase the height of the gantry to allow this and I would also lower the rails from the 80mm height they are at now to more like 20mm.

Thoughts?

[Boyan Silyavski]

No. Either do it like it is or redesign it completely for the other type of machine, where the bed is raised.

There is a reason most strong DIY builds use raised sides on the bed and the bed is bellow.

For a first build is not advisable to make the bed raised cause most possibly you will finish with a machine having a weak link somewhere at the gantry. Not that is not doable, just needs some careful considerations.

If you look at forums you will see all machines of these type are weak or crappy, even if they have ton of steel in it. Especially this is the case if you want to do aluminum.

Not to speak of the difficulties you wil encounter of aligning properly the long rails

[routercnc]

Hi Brenton,

As the table looks like its coming together I'll make a few comments on the gantry in post #1. If you simple space the Y axis rails further apart so that they mount nearer the corner you'll get a big stiffness increase. At the moment they are part way along the flexible face of the RHS. Also helps as bit with racking of the Y axis due to the wider spacing of the bearings.

When I've sketched out that type of layout I've aligned the upper & lower faces of the carraige with the top & bottom surfaces of the RHS, which puts the rails slightly inboard so that the fixing bolts don't hit the top or bottom surface.

You also talked about inserting an 'I' beam and chopping back to get clearance for a ballscrew between the rails. I can see where you are coming from but its alot of work. I remember some commercial machines welding a solid spacer under where the rails will sit (full length of gantry, wider than the rails, and say 6 - 8mm deep to suit) to give clearance to the ballscrew. Probably would need the seating face grinding true to recover any distortion from the welding prior to seating the rails. Although it's an offset from the gantry, some of the small loss of stiffness would be recovered by the spacers. Just another option that is relatively simple to implement.

The diagonal plate inside the RHS would be difficult to attach wouldn't it? Unless you are cutting periodic holes in the RHS to tack weld through.

[BrentonSpear]

Hi Brenton,

As the table looks like its coming together I'll make a few comments on the gantry in post #1. If you simple space the Y axis rails further apart so that they mount nearer the corner you'll get a big stiffness increase. At the moment they are part way along the flexible face of the RHS. Also helps as bit with racking of the Y axis due to the wider spacing of the bearings.

When I've sketched out that type of layout I've aligned the upper & lower faces of the carraige with the top & bottom surfaces of the RHS, which puts the rails slightly inboard so that the fixing bolts don't hit the top or bottom surface.

That is a good suggestion. I will incorporate that into my design.

You also talked about inserting an 'I' beam and chopping back to get clearance for a ballscrew between the rails. I can see where you are coming from but its alot of work. I remember some commercial machines welding a solid spacer under where the rails will sit (full length of gantry, wider than the rails, and say 6 - 8mm deep to suit) to give clearance to the ballscrew. Probably would need the seating face grinding true to recover any distortion from the welding prior to seating the rails. Although it's an offset from the gantry, some of the small loss of stiffness would be recovered by the spacers. Just another option that is relatively simple to implement.

Yes I have seen this done previously. I am going to adopt this approach.

I currently have 32mm clearance between the gantry face and the linear block face where the Z axis will attach.

I am assuming that a 1610 ballscrew and ends will need 60mm clearance between these two so I would have to make the spacer 15mm (after surface facing) and add another 15mm aluminium spacer onto each of the 4 linear blocks that attach to the Z axis to give me the clearance.

Will this thickness of spacer on the gantry and then also on the Z axis backing plate cause me issues?

The diagonal plate inside the RHS would be difficult to attach wouldn't it? Unless you are cutting periodic holes in the RHS to tack weld through.

I was thinking of welding this plate at two points either end.

[BrentonSpear]

Here is the revised gantry with the spacer.

I redid the measurements and a 15mm spacer from the Gantry face to the rails is enough to clear the ball screw.