Steel Frame 8' x 5' CNC - Advice Sought

Hi All,

I have been reading this forum with interest and busily designing my CNC machine.

It will comprise of a steel frame, steel gantry and aluminium Z axis.

I want it to be as accurate as possible and will be cutting acrylic, plastic, wood and hopefully light aluminium.

Over all the machine size will be 2600mm x 1500mm (8' x 5') with a cutting area of 2200mm x 1150mm (7.5' x 4').

I have already purchased 2500mm linear rails for the X axis and will mate them to dual HGH20 linear blocks on each side.

The Y axis will use linear rails with HGH20 blocks also.

I have read with interest the threads that use bolted top rails for the X axis and I am thinking of using epoxy to level the top of the rail between the linear rail and the top rail.

I will have a rack and pinion system on either side of the X axis mated to NEMA34 stepper motors. I am going to use the R&P drive systems from CNCRouterParts.com

Here is the frame design as it stands at the moment:

http://www.mycncuk.com/attachment.ph...id=15369&stc=1



The Gantry will be made from a 250mm x 150mm RHS steel beam and I am going to add a flat plate inside the bean to strengthen it.

I am working out whether to use a ballscrew on the gantry or to use a R&P system here as well.

Here it is:

http://www.mycncuk.com/attachment.ph...id=15370&stc=1http://www.mycncuk.com/attachment.ph...id=15371&stc=1



I would like to hear peoples opinions on the frame as this is what I will weld up first.

Hi,
IMO:
- Dont add plate inside, add it at the back and bottom if you feel that its necessary
- Use ball screws as much as possible if budget permits it. So maybe even rotating ball nuts for both sides of the gantry.
- Add some more short supports left and right, 3 each side is too low for such long rails

Good luck with the build!

Ok, here is my updated table design with the extra supports in place.

http://www.mycncuk.com/attachment.ph...id=15416&stc=1

Hi,
when i said above for the plates i meant the gantry.

What would be the square steel box profile size? I calculated it many times and for steel machine/this size especially/ is best to use bigger profile 100x100x3 or 4mm and simplify the structure. I say always 100x100 as it is the max that could be handled and cut at home using dry saw or small band saw.
The benefits are:
-simpler
-easy to cut at home
-easier to solder
-stronger
-less metal for the same strength so cheaper

You could read more here and see changing the table design leads to substantial savings http://www.mycncuk.com/threads/5302-...highlight=tony

In fact if you have the means the bigger is better here.

Quote:

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Originally Posted by BrentonSpear

Ok, here is my updated table design with the extra supports in place.

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I personally would not put a rail all around the bottom and I would add triangulation in.

I would have it sitting on say six legs, that way it would be a lot easier to set up on an uneven floor. Also make it out of 4mm or 5mm box section then you will have enough thread depth for fixings.
.
It might be worth thinking about having heavier cross rails and ditching the centre supports to be able to use the underneath to store sheet materials. ..Clive

Hi Guys,

I was looking at using 50mm x 50mm x 4mm SHS steel tubing for the frame.

Silyavski, that is an interesting thought. I suppose I could do it in 100x100x4mm SHS. I will have to do the price calculations. I am planning to get all the pieces cut to the required lengths at the place I buy the steel so the size of the SHS is not really an issue.

Clive S, I had always planned to add feet to the frame.

Here is the updated design with the feet in place.

http://www.mycncuk.com/attachment.ph...id=15417&stc=1

For the gantry I am going to look at how I get a ball screw between the rails of the Y axis.

I think I will put an I beam inside the RHS beam and then remove material to make a void. I have seen other person do this and it looks doable.

What sort of span will I get with a 100x100x4mm beam? Could I get the full 2500mm?

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

http://www.mycncuk.com/attachment.ph...id=15418&stc=1

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

Quote:

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Originally Posted by BrentonSpear

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

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

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?

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

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.

Quote:

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Originally Posted by 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.

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

Quote:

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Originally Posted by routercnc

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.

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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?

Quote:

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Originally Posted by routercnc

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.

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

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.

http://www.mycncuk.com/attachment.ph...id=15441&stc=1

Yes, that was the basic idea on the spacer approach. I was looking into this a while ago (been thinking about a gantry upgrade) and you can reduce the spacer offset further with:
Custom ballscrew AC bearing housing +
Custom floating bearing housing +
Removing part of the flange on the ballscrew nut

You'll have to decide if it is worth the effort for a few more millimeters.

The diagonal plate down the centre is not worth it if you only weld at the ends. Better would be a shear panel set into each end to blank off the section. You could set the stepper motor with slotted adjustment holes into one of the plates (i.e. motor inside the section) to keep it tidy.

Quote:

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Originally Posted by routercnc

Yes, that was the basic idea on the spacer approach. I was looking into this a while ago (been thinking about a gantry upgrade) and you can reduce the spacer offset further with:
Custom ballscrew AC bearing housing +
Custom floating bearing housing +
Removing part of the flange on the ballscrew nut

You'll have to decide if it is worth the effort for a few more millimeters.

The diagonal plate down the centre is not worth it if you only weld at the ends. Better would be a shear panel set into each end to blank off the section. You could set the stepper motor with slotted adjustment holes into one of the plates (i.e. motor inside the section) to keep it tidy.

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I am planning to add the end plates to stiffen things up on the gantry.