Steel frame design. Opinions please

[MattH]

I'm in the design phase of building a new CNC gantry type Router to replace/augment my current machine. The existing machine was built using 80mm SHS and is running well. However when I try to push the cutting speed I get a lot of vibration and flex.
The new frame needs to be built in such a way as to reduce flex and vibration. So key factors here are mass and rigidity.
The machine will have a 600mmx1200mm cutting area and will be approx. 900mm wide and 900mm tall.
In the renders below the cross section plates are going to be 12mm laser cut steel, the Large box sections are 120mmx60mmx5mm RHS, the small box section is 60mmx30mmx3mm and the angle is 125mmx75xx10mm.
I will add some more 60x30 along the sides of the machine and perhaps the bottom as well.

Thoughts? Opinions? Flames?



For those interested the mechanicals will be X axis 25mm Hiwin rails and 1.25 Mod helical R&P, Y axis 20mm Hiwin rails and 1.25 Mod helical R&P, Z axis 15mm Hiwin Rails and 10mm 4mm Pitch THK C3 ballscrew. 3HP 24KRPM ATC Spindle, 400W AC Brushless servos (Leadshine EL5), CMSIO-IP/6S Controller with Mach3.
Rapids on X and Y are 24M/Min, cutting: 5M/Min engraving and 15M/Min profiling (all in wood).

 

[MattH]

Nobody got an opinion?

 

[alistairlee]

since no one else has mentioned it, diagonals?
Al

 

[MattH]

since no one else has mentioned it, diagonals?
Al

I was planning on get some triangles cut by the laser cutter, maybe 6"x6" and welding them in as gussets.
I was also thinking about skinning the sides with 3mm sheet to brace everything in that direction.

 

[Shox Dr]

since no one else has mentioned it, diagonals?
Al

Beat me to it

 

[Spark plug]

In days gone by I used to build small CNC machines and 25 linear rods over 1200mm sounds a bit light IMO.

This is one of the models I used to build:

This might give you some inspiration, also the added beauty of this type of design is you can index a 8'x4' sheet though if you need to. Also I've never used a rack and pinion drive a dim a little sceptical as to the potential for backlash.

 

[MattH]

The rails are profile rail and are fully supported along the entire length. Commercial 8x4" machines use 25mm rails of this type.
Most people have switched to R&P for wood routers, if set-up correctly with the right mesh and tension then backlash is as good as ballscrews.
This will be my third CNC of this size, the mechanicals and electronics are tried and tested on my current machine which is running at least 4 hours per day every day.

The issue is how to best make the frame heavy and rigid but still be able to build it with the required accuracy. That's the idea behind the laser cut cross sections, they will allow me to mount the main x axis box sections as parallel as possible to each other.

The 120x60 RHS will have something like 40mmx20mm flat bar stitch welded along the length, this will be machined flat to then mount the X rails on.

I like the idea of the machine you posted, being able to feed a full sheet through. However, I couldn't even manoeuvre a full 8X4 sheet in the workshop this machine will be in!

 

[Spark plug]

Ahh right I thort you ment rods with ball bushings running on them. Those rails you pictures will be fine.

 

[Maker]

However, I couldn't even manoeuvre a full 8X4 sheet in the workshop this machine will be in!

Given how small your workshop is, have you considered a vertical machine, like a really sturdy version of a panel saw?

 

[mike os]

have you tried fillig your existing frame with sand?

the above design fills me with doubt as to its real stiffness in X, thin sheet wont cope well i dont think, i would look at a robust well braced RHS frame

 

[MattH]

Given how small your workshop is, have you considered a vertical machine, like a really sturdy version of a panel saw?

No, I haven't. At the moment we get our sheets rough sawn into 4'x2' at the wholesalers and I have an area setup to store about 3 weeks worth of stock in different thickness's. If I went to 8'x4' then I'd struggle to store the stock and moving the sheets around would be nigh on impossible.

My current 4'x2' machine works very well for my needs. Part of the reason for building a new machine is that I had planned on adding an ATC to the existing machine but I can't get enough down time to retro fit it. It's easier for me to build the new machine with the ATC and then swap them over. Later in the year we hope to move into premises so then I can install the old machine, transfer production back to it and then move the new machine and run both.

I could then use all the electronics and half of the motion control gear from the old machine to build into a 10'X5' machine once I have the space.

 

[MattH]

have you tried fillig your existing frame with sand?

the above design fills me with doubt as to its real stiffness in X, thin sheet wont cope well i dont think, i would look at a robust well braced RHS frame

Sand might help deaden some vibration but it's not going to make if stiffer.

The cross section pieces are going to be 12mm but I think you're right that stiffness is going to be an issue. This is where I need it the most and the gantry which will weigh around 50Kgs is going to be flying back and forth in this direction.

 

[ukracer]

Given how small your workshop is, have you considered a vertical machine, like a really sturdy version of a panel saw?

It's not in there... I never replied as what he already has blows my mind. Lol

 

[MattH]

It's not in there... I never replied as what he already has blows my mind. Lol

Lol, CNC is in workshop two, even smaller than workshop one!

 

[MattH]

Existing machine:

 

[ukracer]

No, I haven't. At the moment we get our sheets rough sawn into 4'x2' at the wholesalers and I have an area setup to store about 3 weeks worth of stock in different thickness's. If I went to 8'x4' then I'd struggle to store the stock and moving the sheets around would be nigh on impossible.

My current 4'x2' machine works very well for my needs. Part of the reason for building a new machine is that I had planned on adding an ATC to the existing machine but I can't get enough down time to retro fit it. It's easier for me to build the new machine with the ATC and then swap them over. Later in the year we hope to move into premises so then I can install the old machine, transfer production back to it and then move the new machine and run both.

I could then use all the electronics and half of the motion control gear from the old machine to build into a 10'X5' machine once I have the space.

And there was me hoping to save enough to buy your existing machine off you. Lol

 

[MattH]

I've got a hell of a lot into it! Electronics and servos costs me about £3K in parts alone.

 

[ukracer]

I've got a hell of a lot into it! Electronics and servos costs me about £3K in parts alone.

I know....that's why I said hoping to save.....lol

 

[gaz1]

I think id do exact opposite to yours

the cutting frame is renewable and sits in the middle for stability id use a triangle frame

drawn a simple drawing but note ive never built a cnc so i could be way out here



it could even be run by rollers at the edges on something like box section or like a I beam

if it was split into half then an inserted piece could make it much wider

 

[Maker]

Do you run a CNCing business?