[cancruiser]

Hello,
I would like to build a fixed-gantry, moving-table router. I am looking to get a 12"x12"x4" work envelope. I plan on using a set of of linear motion/bearings (SBR16,RM1605 and associated components).

Before I do anything, I would like to answer a very basic design question: How to determine the location of the linear rail nuts (SBRUU) to give the required travel distance on the Z-axis while keeping the overall height at a minimum. I tried to follow this guide Building a CNC Router Step 4: The Z-axis assembly but could not come to any concrete numbers. Also, how can I calculate the z-axis mounting plate dimensions (the plate where the router will attach to). Probably same goes for the Y-axis.

Basically, what method to follow in order to come up with the geometry and dimensions for a given work envelope.

I could go out an buy a ready-made kit like a Shapeoko or something similar, but I would rather build my own. I believe it will be better in terms of rigidity and accuracy. I have a welder and will use 1"x2" tubing for the frame. Just need to decide on the dimensions.

Thanks for the help in advance.

[grubscrew]

Order your motion and drive components, router mount first. when i built my first machine, i just layed all the components out and did the calcuations at that point. Was muchasier to visualize and determine the correct mounting at that point

[ger21]

Draw everything up in a CAD program and play around with everything until you get it the way you want it.
You need to determine how much travel you need, and where the top and bottom of the travel will be. The location of the table in relation to the Z axis, and how the Z axis mounts to the gantry may play a role here.

There are no hard and fast rules, as every machine is different.
If this is the first machine you're building, plan on building another when you finish it. You'll learn a lot.

Once you get some CAD drawings done, if you post them here, people can give you much more specific advise.

[datac]

If you are not already drawing things up in Cad, you really will need to at the very least have the items you want to use in your hands so you can mock up some variations. If your really a newbie, a few things you should be aware of is that there are for the most part, two different techniques regarding travel of a Z axis. One is where your linear rails move with the z axis and the bearings are fixed to the Y axis, the other is where the bearings are in motion with the Z axis and the linear rails are fixed to the Y axis.

The two methods create very different functionality when regard to the location of the tip of your tooling, and in relation to overall gantry clearance.

Generally, if you build with linear rails that move, there is a likelihood that your tooling tip always hangs down lower than the gantry clearance when it is the fully up position, requiring your design to have a higher gantry clearance to start with. There are certain reasons why you might want to use this method. Any attempt to get the tooling tip higher can only be done by keeping the lowest bearing slide much higher than preferred.

When you design such that the bearing slides are in motion and the rails are fixed, you can design such that the tooling tip can be retracted fully from the gantry clearance.

Either way will require a similar height for the same amount of travel, so you need to always consider what your goal is for final products, where the tip of the tool will end up when it is retracted (up) in comparison to the thickness of the material you want to cut.

If your not Cad familiar, and one wouldn't really have to be to build a machine (but its going to be a requirement to use it later), you can mock up the possibilities in wood, then build the final after approval.

Now a few more "issues" are:

You have to consider where you drive mechanism will reside within this mix. And there too are an awful lot of variables depending on the mechanism used.
Lead screws, belt drives, rack and pinion, even air cylinders are all methods to raise and lower Z axis. If the end design lends itself to not being overwhelmingly rigid with lots of mass riding on THK type systems, the best possible location to drive the axis is dead center between the slides as close to the slides as possible. On the other hand, if you do design in a great deal of rigidity (which means lots of metal plate or castings that do not flex unlike wood or alum extrusions and THK style slides), you can drive a z axis from one side without too much difficulty or inaccuracy.

Finally, You have to consider what "axis overhang" means and does when it relates to FLEX. During design, always consider the location of the tip of the tooling when loaded/cutting. The perfect compilation of X,Y,Z would be to have the slides and bearings of one bolted directly to the slides and bearings of the next without anything in between them. Of course this is not possible, and everything you add between those axis transitions creates an axis overhang. Always try to keep your linear rail centerlines as close to each other as possible, with the most rigid materials you can.

Obviously, there are specific designs for specific needs that can dictate major variance to that rule, but the more anything cantilevers out from the linear centerline, the more flex becomes an obstacle.

One last thought..... in order to keep your machines overall height as low as possible, consider mounting the motor behind or off to one side of the axis and drive the mechanics via timing belt and pulleys rather than for example, just hanging the motor sky high on the end of a leadscrew.

Good luck !

[cancruiser]

@datac, thanks for the very thorough reply. It is very detailed and every bit makes sense. I'll sketch something up and show it here for more detailed advice.
@ grubscrew, I have the parts already, just received them last week. What I'll do, is to make a cardboard template to resemble the Z-plate and Y-plate and see how it should be scaled to give the proper work envelope. Before I jump to the saw and start cutting metal..

[wizard]

Hello,
I would like to build a fixed-gantry, moving-table router. I am looking to get a 12"x12"x4" work envelope. I plan on using a set of of linear motion/bearings (SBR16,RM1605 and associated components).

It is always helpful to know what your uses will be. When I see a small machine like this being built I imagine that it could be anything from a printer circuit board (PCB) router to a machine routing patterns for dental appliances.

Before I do anything, I would like to answer a very basic design question: How to determine the location of the linear rail nuts (SBRUU) to give the required travel distance on the Z-axis while keeping the overall height at a minimum. I tried to follow this guide

You have already gotten good answers here however I will try to give you a little more to think about. As mentioned above usage counts, if you expect to use longish drill bits for example you may need a lot more vertical movement than say a machine design primarily for PCB routing. So one way to look at this is how high up does the head have to go so that the cutting tool completely clears the work area. Ideally you want to clear the cutting area with enough clearance to do a cutter change.

Don't think just in the case of Jobber type drill bits either. If your intentions are to carve up a 4" thick work piece you may need a long ball nose endmill or a cutter with some sort of extension. Generally you want your cutters as short as possible to get the job done but this goes back to needing to understand what the machine will be used for.

Finally you will most likely need clearance added to your 4" work area requirements for the Y & Z axis mechanics.

Building a CNC Router Step 4: The Z-axis assembly but could not come to any concrete numbers. Also, how can I calculate the z-axis mounting plate dimensions (the plate where the router will attach to). Probably same goes for the Y-axis.

There are multiple valid answers to these questions. Some guys have a rule of thumb that the y axis saddle should be as high as your working height. So if you want a 4" work Z then the saddle bearings should be at least 4" apart. That is all well and good but an engineer would do the mechanical calculations to get the accuracy he wants at the cutter with the goal being to minimize tool deflection.

Basically, what method to follow in order to come up with the geometry and dimensions for a given work envelope.

Actually someplace back in time in this section there was an excellent thread that covered the general engineering behind basic gantry beam design. In simple terms a big square beam generally wins over lesser solutions. If you want to learn more you will need to search backwards almost a year I believe. I forgot the authors name so off the top of my head I can't offer a lot of help. There is also a PDF that floats about the Internet called Rapdi Machine Design, by a guy called Bamburg (if I remember this correctly).

In the end these machines, especially the little ones are well understood with regards to their mechanical requirements so good results can be had with seat of the pants designs. Your primary goal should be to produce a solid mechanical structure, frame if you will. If the frame isn't solid it will lead to poor results.

I could go out an buy a ready-made kit like a Shapeoko or something similar, but I would rather build my own. I believe it will be better in terms of rigidity and accuracy.

THis is certianly true. When dealing with these smaller machines understand that it doesn't cost a lot more in materials to make a machine that is much stronger than one design only with cheapness in mind.

I have a welder and will use 1"x2" tubing for the frame. Just need to decide on the dimensions.

Well maybe not 1x2 tubing. I say "maybe" because again it depends upon your goals but I certainly wouldn't use this tubing for the gantry by any means. The tubing might be suitable for the rest of the machine but even here I'd want to see at least 2" square tubing for some parts of that frame.

Also welding up a frame can be tricky. If you already have a welder you are probably aware that welding distorts things. The best thing you can do in this regards is to start out with precise parts before welding the frame up. This means machining the ends of the tubing square or at least bring in and filling to an extreme level of squareness and length accuracy. Precise fit up will help control distortion from welding.

In a perfect world your frame would be stress relieved after welding. You may be able to get away with not doing so but keep that in mind.

Thanks for the help in advance.

No problem! There are two keys to a successful build, one is knowing what the machine is to be used for. The other is to have a realistic budget and time allotment.

About that gantry beam there are other issues beyond the mechanical strength ones to prefer a bigger beam even if it might be excessive on a amchine this size. Here are a couple:
1. You need to be able to separate the two linear rails by a rational amount. I'd suggest 4" of separation in this case if you can fit up all the parts in that space. It is easier, in my opinion, to do so one a single beam in a tool poor work shop.
2. You need a flat surface for you linear rails to mount to. There are a number of ways to accomplish this but again I see a single square beam as the easy way to a flat mounting surface for your gantries Y axis rails.

[ger21]

Actually someplace back in time in this section there was an excellent thread that covered the general engineering behind basic gantry beam design. ...............

I made it a sticky so it's easy to find.

http://www.cnczone.com/forums/diy-cn...cad-posts.html

[wizard]

O

I made it a sticky so it's easy to find.

http://www.cnczone.com/forums/diy-cn...cad-posts.html

Hey thanks ger21!

[cancruiser]

@wizard, thanks very much for your usual help and knowledge sharing. Now that I am armed with the info I have a better idea to move forward. By the way, I think I found the PDF file you are referring to. It is this one, isn't it?
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

@ger21. Thanks for posting. I am reading it as we speak..

Thanks for all who posted and replied my questions. Will post a design sketch here for evaluation when I come up with the dimensions.