WK08

[sweckard]

I thought I would start a thread not only to give back for what I have taken but also for any helpfull insight that can be given during this build. This is my second router, the first one worked; but not the way I wanted it to. The main problem I had with it was jitter in the x-axis. I went through several rebuilds before realizing I was spending way to much time on a router that isn't going to suite my needs.

So now I am going to do a complete design, put up the cash, and use the right tools for the right job. The router is mainly going to be aluminum, use professionally made bearings, and is going to be bolted together like a tanker.

The cutting envelope is greater than 24" x 48", and Z-axis travel is greater than 5.5".

Parts purchased so far:

56" THK SSR20 Linear Bearings
32.5" THK RZR15ZM Linear Bearings
11" THK SRS12 Linear Bearing
1/2"-10 5 start acme screws
4 - 1500 in-oz servo motors with 1024 cpr encoders (big motors for a small router but I feel I will be upgrading to a bigger router in the future)

The main structure of the cnc is aluminum extrusion. There is also a aluminum I-Beam on the Y-axis to increase stiffness.

Before making the build I plan on purchasing a SX3 and CNCing it. This way I can make parts for the router that are accurate and then hopefully the whole thing goes together like a puzzle. I have been in the design and purchasing phase for several months now and I still have awhile before I can even begin to start cutting. I would like you guys to give me any thoughts or ideas on how can make this router better. I have a e-drawing file and will be uploading it soon.

[ger21]

If your spindle mounts to the bottom of that Z axis plate, you'll probably need to stiffen it up with some bracing or something.

[sweckard]

Originally Posted by ger21 If your spindle mounts to the bottom of that Z axis plate, you'll probably need to stiffen it up with some bracing or something.

You are right Gerry, as shown in the drawing the router sits low on the z-axis plate. This location of the router mount is approximate because I have not put the router and the router bracket (k2 cnc, hopefully purchase soon) in solidworks. Once I get them drawn up I should be able to locate the router in the right position. But there still needs to be some bracing for the 1/2" aluminum, so I have put some 3/8" aluminum brackets on the side to help stiffen the plate up. I also have room down the center on the back of the plate to put some more brackets if necessary.

One of my big worries about the CNC is the gantry. I have included a picture of the center of mass of the gantry, I think (hope) that the center of mass for the gantry will be in the center between the two x-axis bearing after I put the router and router bracket on. But one thing I wanted to achieve is the lowest possible COG (center of gravity) for the gantry to help decrease inertia. With my COG being so high right now, do you think I should raise the height of the x-axis acme screws. Having the moving force closer to the center of gravity should help with inertia, but is it really necessary; economics and footprint wise? Would adjusting acceleration in Mach be good enough?

[sweckard]

Click Here for the edrawing file.

[georgeburns]

Originally Posted by ger21 If your spindle mounts to the bottom of that Z axis plate, you'll probably need to stiffen it up with some bracing or something.

How about extending the pillow block standoffs down the length of the Z axis plate?

[harryn]

Perhaps consider including an enclosure as part of the design. Once it is stiff enough, it certainly will be too noisy.

[sweckard]

Originally Posted by georgeburns How about extending the pillow block standoffs down the length of the Z axis plate?

Thats a great idea. I will update the drawing with longer spacers.

Originally Posted by harryn Perhaps consider including an enclosure as part of the design. Once it is stiff enough, it certainly will be too noisy.

The aluminum extrusion is suppose to be great damping material. I don't think I will design for a enclosure yet due to cost and I already put the order in for the extruded aluminum. Maybe the louder the router is the more neighbors will drop by and what some work done, help pay for the router.

[ger21]

Originally Posted by sweckard With my COG being so high right now, do you think I should raise the height of the x-axis acme screws. Having the moving force closer to the center of gravity should help with inertia, but is it really necessary; economics and footprint wise? Would adjusting acceleration in Mach be good enough?

I'd move them up a little, but not high enough to interfere with loading and unloading parts. As for accel, typically, you'll want to set it as high as possible for best overall performance.

[mhasting2004]

Hi sweckard

Your design looks in a lot of ways like mine. I'm curious why most people mount the rails to the fixed part and the bearings to the Z slide. I've done the opposite on my design and figure that when the router is high I have the best rigidity (bearings close to router) and when its low its no worse than if I did it was the other way around. The rails also add stiffness to the Z slide. Thats the theory anyway.... I'm still building.

To reduce my COG I am opting for a rotating nut and a fixed ball screw so that the Z axis motor is stationary and off the z slide.

Note my Z axis travel is quite long (530mm).

Cheers

Mark


BTW how did you select all your parts to get the COG. Mine are sub assemblies and it doesn't seem to like doing more that one at a time.

[sweckard]

Originally Posted by mhasting2004 Hi sweckard Your design looks in a lot of ways like mine. I'm curious why most people mount the rails to the fixed part and the bearings to the Z slide. I've done the opposite on my design and figure that when the router is high I have the best rigidity (bearings close to router) and when its low its no worse than if I did it was the other way around. The rails also add stiffness to the Z slide. Thats the theory anyway.... I'm still building. To reduce my COG I am opting for a rotating nut and a fixed ball screw so that the Z axis motor is stationary and off the z slide. Note my Z axis travel is quite long (530mm). BTW how did you select all your parts to get the COG. Mine are sub assemblies and it doesn't seem to like doing more that one at a time.

Mark, sorry for not getting back to you sooner, I have been offshore. Your have a very nice machine, I am jealous of what you have; the foot print, the table, the rotating nut, the whole shu-bang. I will be keeping watch of you post and hope you post a couple videos when your done.

I tried your idea on the reverse linear rails and came to the conclusion that the only way I could do it and for it to lower my COG would be to use a rotating nut like yours. Unfortantly I need to use the parts off of my first router, that way I can keep the cost down. If I ever build a another router though, I would like to put rotating nuts on both the y and z axis.

I had to reassemble my whole drawing with sub assembles due to having one whole assemble was a little to much for the computer. I have drawn the x-axis as one assemble, then the y axis, and now I am working on the Z. When I do do my COG on select parts I hold down the control key and select the parts. Holding down the shift key will work too, but it also select all the parts in between. Hope that helps ya.

[sweckard]

While I was gone my extrusions showed up. I was like a little boy on christmas day when it came to opening the boxes.

Also included in the picture are the linear bearings. The really large bearing were a big ebay mistake. I under-estimated how big 35mm wide linear rails are. Should of drawn them up before purchasing.

[mhasting2004]

Hi Sweckard

I assume you did the COG calc when you had one big assembly? So far I have not found out how to do several assemblies (multiple parts is easy).. not a big problem was just curious. Thanks for the kind comments hopefully my machine will turn out as good as I hope it will but there is a long way to go still. This forum is really a gold mine for ideas, support and inspiration.

Keep up the great work on yours... looks like we have a race

Cheers

Mark