[ScottDamman]

Im just starting the process of building (design stage) a 5 Axis CNC router. I already have a Y axis from a machine that had been torn apart.
As seen here. https://youtu.be/BdTr0OTAqx4
This was a quick and dirty way to flatten some large slabs. My plans are a steel frame/bed with linear rails for X axis. Use the existing gantry for the Y access and build a Z axis.


The first part I am designing is the 4th and 5th Axis for spindle. I am still looking for/finding a spindle with a taper tool changer (BT30 or something similar).

Here is what I have so far, I am open to critique on this design. All of the purchased components (bearings, flanges, etc) come from Mcmaster. The opening in the lower arm is so I can see the belt and my plan is to fill that with some acrylic or the like. Also the end of the arm I plan to cover with sheet metal to keep the mech inside clean. Everything else will be 1/2" or 3/8" aluminum.





I have a few Noob questions...

1, the stepper motor drives for A and B, do the steppers hold position when the machine is running (I assume so but I dont understand this). So it will hold the spindle at the B axis while X and Y are moving for instance.
2, In regard to the gantry, the gantry that I have is is an 1 1/8" thick by 7" tall piece of Aluminum. Is this substantial enough for a Z axis that will be ~ 28" (16 inches of travel plus 12 inches for the 4/5 axis head) inches from the gantry when fully down? Im thinking I might need to add to this to make more like a C channel possibly. Im not so much worried about flex in the vertical direction more so twist of the rail because of the long moment arm. Thoughts on this are greatly appreciated.
3, I am planning to add 0 position prox switch to A and B axis, however should there be positive physical stops on the max limits? Also how far should they go? I was thinking 360 deg on A and maybe 200 on B (100 in each direction)?


I will keep adding pictures as I draw more of the system any help is greatly appreciated!

Thanks
Scott

[ScottDamman]

I got a bit more complete, this is just a rough draft of the Z axis. Needs lots of refinement.

[Eddard_Stark]

You need to tell us what the machine will be used for. Everything flows from that.

Depending on what it is, 16" can be a lot of Z travel. Remember your Z acts as a lever. Relatedly, your gantry of 7 x 1.125" aluminum bar? is not suitable at all. You need a tube. The moment that is imparted by the tool tip will cause the gantry to flex, as such you want your gantry to have enough torsional stiffness to resist the moment imparted by the cutting tool. As it is you have a pretty long lever (16" Z) with a gantry that will twist like a noodle under fairly light loads. (All relatively speaking of course).

Again it all depends on your use, but if you need the occasional Z capacity I suggest making a part of your table removable to expose a sub table of sorts. Meanwhile the majority of your cutting can be done with a more manageable Z height. Say half what you have specced.

[ScottDamman]

Thank you for the reply, the machine will be used to cut plywood, slab work like shown in the video flattening and notching mostly. The Z axis requirement came from wanting to be able to machine large chunks of EPS foam to make stand up paddle board blanks.

I started doing some math on the gantry for sizing, I am learning about feed rates and tool load to get a good starting point for load on the Z axis. Is there a rule of thumb for this? Also a safety factor of 2 is that enough?

I agree with your quick assessment that the gantry I have is not nearly strong enough for any kind of force on this long of a Z axis. Im going to try the load case with 4 x 8 x 1/4 wall box tubing. But to your point, maybe it would be much better to have a table that can be changed and just machine 1/2 of the board at a time....

Much more calculating needs to be done on my end.

[lcvette]

For foam it should be fine as there is very little resistance to cut foam, wood might require some light finishing passes depending on tolerance required to keep the moment low enough to avoid deflection and get you closer to your desired tolerance.

Sent from my XT1080 using Tapatalk

[wizard]

Thank you for the reply, the machine will be used to cut plywood, slab work like shown in the video flattening and notching mostly. The Z axis requirement came from wanting to be able to machine large chunks of EPS foam to make stand up paddle board blanks.

It might be cheaper to make interchangeable heads. One for 3 axis work and one for 5 axis foam work. The problem is keeping things stiff enough for conventional work in wood. I really think your correct design will suffer vibration issues in wood working. Done right you could install a 3HP spindle for conventional work, something that wouldn't be out of place on a large machine like this and a factional HP spindle on the foam cutter head.

I started doing some math on the gantry for sizing, I am learning about feed rates and tool load to get a good starting point for load on the Z axis. Is there a rule of thumb for this? Also a safety factor of 2 is that enough?

There are different numbers thrown about but you need to carefull consider how your geometry is set up. The leverage applied to your Z also impacts the gantry uprights. I'd suggest load values of several hundred pounds with less than 0.005" deflection overall. In reality you want the frame strong enough so that if a significant crash happens nothing get damaged frame wise.

I agree with your quick assessment that the gantry I have is not nearly strong enough for any kind of force on this long of a Z axis. Im going to try the load case with 4 x 8 x 1/4 wall box tubing. But to your point, maybe it would be much better to have a table that can be changed and just machine 1/2 of the board at a time....

Honestly, with a 16" clearance you will probably want around 12" square tubing. That is for the gantry beam the uprights also need to be rather massive and spread the bearings to it to at least 16" on the X. That square tubing will have to be gusseted inside also.

I'd strongly suggest against a half length machine. You will quickly regret it when you end up constantly screwing around with setups.

Much more calculating needs to be done on my end.

It takes awhile when you move outside the canned designs often dealt with here. Deciding upon what is most important to you with the described uses can help with decision making. For example my gut feeling is that if you where doing foam only, for paddle boards, you could get buy with a very light machine that isn't all that accurate. On the other hand if you want to process big slabs of wood, in a timely manner, you need a stiffer more accurate machine. You can build a machine to do both but it will be more expensive than a single machine optimized for one task.

Edit: forgot to mention that you need to be concerned about machine structure when it comes to supporting large chunks of wood. Wood can be very heavi so you need to ask just how much weight do you want to be able to support. If somebody comes into the shop with a 16" diameter log or a ten inch thick slab, will the machine safely and precisely support that load. The reason I brings this up is that once you are known to have a large capacity machine people will come knocking at your door from time to time.

[ScottDamman]

Sorry for the delay in replying to this thread Thank you for the responses. I have been busy learning much more about CNC routers and doing the proper calculations for the gantry to have this big of a Z height. I needed to do more learning before posting my questions

Here are some new designs I have been working on. The gantry is 2 6 inch by 4 inch by 1/4 wall steel box tubing. The uprights are 4 x6 x 1/4 box. The spread on the X axis is 16 inches although they still need more support added. I have also changed (however havent updated the drawing) to square profile 20mm linear rail. The Y axis will be ballscrew and the X will be rack and pinion (planned or possibly timing belt?)





Heres the gantry with 45 deg braces

[ScottDamman]

Finally some updates to this build.

I have started on the gantry with many major design changes.

Attachment 310244

Here are the legs partially welded up. Notice how much the weld pulls that 1/2 inch plate! I bent it back before welding in the gusset. I plan to use epoxy to level and get those legs on the same plane.

Attachment 310242

Here is the gantry partially welded up. The ball screw will live inbetween the 2 4 x 6 x 1/4wall box tubings. The block is in there to keep the spacing until I weld bracing on the back of it.

Attachment 310240

So for the Z axis I am still playing around with the idea of making an A axis for the router motor. The trouble is the length makes things a little tough. I have 18 inches of clearance between my grantry and the final bed height.

Anyway here is a design I have been working on using 3in x 6in x1/4 wall box tubing. Will this work? My motor mount for the A axis gets in the way a little but I think with a long end mill in that shouldnt be too much of a problem. The other option would be to go with an even longer belt and put the A axis motor all the way ontop of the Z axis but I think that gets a little long for the belt? Will I have trouble with how long the belt will need to be for that A axis?

The box tubing will be cutout on the backside so the ballscrew can live inside there. On the one picture you can see some bracing that I want to put inside the tube. I think a couple of these will be sufficient. Then after its all welded up I can face the 2 sides.

Attachment 310246
Attachment 310248
Attachment 310250

https://youtu.be/Lq3Ei8oALXY