40mm finnish birch plywood CNC-router

[hautamak]

Hi!

Starting my first build and I was planning to use 40mm birch plywood as frame. would it be sturdy enough to hold just flat without torsion box system?

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[fretman_2]

40mm is pretty thick...but my guess is that it could flex especially with torsion loads. Go ahead and bite the bullet and design a torsion box. I did and it was worth the extra effort. It's absolutely rigid and mine was constructed out of MDF. Even with 40mm, I'd double the thickness of the end supports for the gantry. They'll flex with the side loads of cutting in the X.

[ger21]

Depends on how big, and exactly how you are building it. But if it's anything other than a small desktop machine, it will still flex a fair amount at a minimum.

[wizard]

I have to agree that there is a danger of flexing. How bad depends upon the machines size and design.

Given that even a small machine can benefit from construction from box sections. On a gantry for example you get stiffness from the box sections but also benefit from area to run things like drag chains, limit switches or other parts based on your designs need. You benefit similarly from torsion box sections for the main table. You can make the entire table a torsion box or fabricate box beams and frame up a table. Making the entire table a torsion box is probably easier but maybe slightly more materials intensive.

In any event 40mm plywood has to be pretty expensive. Just using box sections everywhere, with thinner plywood, should save you some money. Just avoid going too thin.

[Biggs427]

Most of my machine is made of 12mm and 18mm BB panels and I use torsion box for the frame and gantry. Way cheaper than thicker panels.

[hautamak]

Well, maybe I'll do the torsion boxes. I Wonder if they hold together with glue only, or do I need to use screws? Have you attached torsion box to other with bolts or screws?
I tried to search some cam programs but didn't find anything good for free, have you suggestions for free or cheap cam software. CNC will be running on linuxCNC or mach3

[Biggs427]

I used a 16ga nail gun and carpenter glue to build my torsion boxes. To assemble the parts together, I used thickened epoxy and wood screws. The screws are to hold the parts while the epoxy cures. Basically, the structure is held with glue and epoxy.

You can have more info in my build thread:
https://www.cnczone.com/forums/cnc-w...rch-frame.html

[harryn]

The baltic birch that I have used had a maple surface ply and needed to be drilled first before a screw would go in. Those laminations of glue and birch are pretty tough and the boards that I have purchased have very decent thickness tolerance.

As an alternative to a torsion box build up, for a moderate size machine you could laminate a few of them together to make something in the 120 mm range for the base as an example. It would use more wood, but might be much easier to build up accurately if you have minimal tools.

My wood skills aren't terrible, but they aren't as good as a pro and neither are the tools that I have access to. You don't have to be off very much to have a distorted torsion box.

[wizard]

Well, maybe I'll do the torsion boxes. I Wonder if they hold together with glue only, or do I need to use screws? Have you attached torsion box to other with bolts or screws?
I tried to search some cam programs but didn't find anything good for free, have you suggestions for free or cheap cam software. CNC will be running on linuxCNC or mach3

I often run into structures that I'm build where I simply don't have enough clamps or clamps with the reach required. In these cases I make a point to screw the works together from the center out. Sometimes I use an alternative method such as lots of weight (Barbell weights, old motors, castiron chunks, railroad rail & etc) stacked up on the glued together surfaces. The screws I leave in after the glue sets but the real strength comes from the glue up.

One of the biggest issues you run into with glue ups, (torsion boxes, laminated beams or whatever), is that you can end up with a slight distortion in flatness. It can be advisable here to have an outer layer of composite materials like particle board or MDF that can be mill flat after the glue up dries and stabilizes. I ran into this sort of a problem on a manual router table build with a bit of plywood (laminated plywood) that apparently had more built in stress than I imagined.

This is perhaps where a torsion box is probably easier to build to a high degree of flatness. The flatness is based on how well you set up the build and the ability you have to cut straight piece of plywood. If you want a good primer on how to do a torsion box well, "King's Fine Woodworking" on youtube has a well done set of video covering a bench build. The guy does have a fine set of wood working tools to accomplish the build so he may have advantages many of us don't.

[Biggs427]

I wouldn't use the torsion box for flatness but for the rigidity it provides.


For example, on my machine I used epoxy to ensure flatness.

[hautamak]

I have planned to start drawing solidwork models tomorrow, so what would be good thickness for torsion boxes? Linear rails are, X-rails sbr20 and Y and Z is mgn12 rails. So those are not the sturdiest ones. It would be nice If the machine could somehow be able to work with aluminium. though I'm happy with plywood.

Oh, and thanks for replies. really bring ideas to me!

[BalloonEngineer]

For free or cheap CAD and CAM check out Fusion 360.

[ger21]

How big is the machine?
The thicker the torsion box, the stiffer it is.
My gantry torsion box is about 75mm thick, with 12mm skins.
My table is thicker.

[hautamak]

How big is the machine?
The thicker the torsion box, the stiffer it is.
My gantry torsion box is about 75mm thick, with 12mm skins.
My table is thicker.

Cutting area is 400mm x 600mm.

[Biggs427]

Mine have a 75mm gantry with 12mm skins and ribs. (700mm long)
100mm for base with 18mm skins and ribs. (1200mm long).

Cutting area of 500mm x 800mm

[hautamak]

Some progress made. Don't know if this design works or not.

[Biggs427]

How will you attach the gantry to the linear trucks?

[hautamak]

Now I have drawn rest of the machine, not very detailed though.



For the attachment I was thinking something like this.

[wizard]

A couple of things based on what I see on my cell phone.

1. I’d seriously consider putting the X axis lead screws on the outside of the frame. This simply to make maintenance and cleaning easier. If I out have the screws where it is easy for swarf to accumulate it can impact screw operation.

1a. Those screws also look to be small diameter threaded rod. That can be a problem. It f you must use threaded rod it should be of fairly large diameter. Better yet consider ACME or trapzodial threads.

2. You need to separate the gantry support bearings by more. Remember round rails are not that stiff and generally the bearings fit loosely. Also how those bearings are attached to the gantry supports is very important. At the moment I don’t have a lot of confidence in the bearings I got attachment.

3. I don’t have a lot of confidence in that gantry beam. If you insist on using 40mm ply, use two layers to thicken and make it wider. Twitter layers would also help with mechanical strength at the gantry supports. Think half lap at the gantry supports. Gantrys are commonly underengineered even with your modest span you will want the stiffest gantry beam possible. I’m not sure what your goal is with respect to the number of sheets used but it is important to allocate enough materials to the gantry.

That is it for now.

[Biggs427]

Maintenance will prpbabpy be a pain with the holes. If the holes are bigger to help maintenance, it will make your gantry side less stiff.

+1 to put the leadscrews outside.