In Part I and Part II of this series I covered how to generate toolpaths that would fit on a smaller CNC such as The Machinist. I reviled a twist on I called Offset tiling and how I can reduces the amount space required. This as I will explain is key to my whole build.
Part III will explain how to overcome the physical problems of working with a large board on a small machine. I think if you stick though it you will find it interesting.
First Mechanical Issue to Overcome – Clearance Between Y-Rails.
The space between the Y-Axis rails is ~20″ which means my plan on working on 24″ wide project won’t fit with the standard Onefinity setup. I need to figure out a way to lift the entire CNC machine up out of the way of the large boards. No easy task.
Starting with a 3D CAD Model
The first thing I did was to develop a 3D CAD model in Sketch-Up. OneFinity published a dimensional drawing so I was able to scale my model pretty closely.
This drawing is a little busy but it shows my Sketch-Up model (RED) overlayed on the Onefinity dimensional drawing. My model is not perfect but it is quite close to the published dimensions from Onefinity. The important dimension is the 20.049″ clearance between the Y-rails. This is the limiting size that can be milled although the actual cutting area is only 16.25″ X 16.25″.
A Closer Look at the Onefinity CNC Design.
The Onefinity CNC design is I suppose an open rail design. It is not supplied with any type of base. The owner mounts the machine on his own table or material sheet. The machine is mounted down by the four corner feet to the support base. Initially, my concern was how the heck would the Y-Rails be set up in parallel. and how hard would that be? Well, I have seen a number of videos show how easy it really is and seems to work so no worries.
Looking from below the machine truly is only supported by the four corner feet. It would be very important to make sure the Y-rails are absolutely parallel during setup as any tilting would bind up the Z-Rail travel down. The last image shows the CNC mounted on a sheet of material. That material could be a waste board itself or an additional wasteboard mounting between the rails. The latter makes more sense to me as then the wasteboard can be changed out not affecting the alignment of the machine. This is the typical installation setup for the Onefinity CNC machines. If you choose it can be broken down and moved fairly straight forward in pieces. That is a plus feature I feel.
Lifting the CNC Without Messing Up the Alignment.
So how am I going to lift the CNC up off the bed allowing me to pass a large board without messing up the alignment? That’s a good question. Let me state I am not a mechanical engineer. I am an electrical engineer with just enough knowledge of mechanical things to be dangerous. But what I have is a good analytical mind and some common sense. So I think I am able to work things out and have some gut feel on how it will perform. Any mechanical engineer types are free to throw shade at me but I will lay out my thoughts on how I am going to accomplish my goal.
Mount the Machine in a Sturdy Lift Frame.
So here is my idea. Instead of mounting the CNC down on a base sheet I instead will mount it on a two ridged metal rails of angle iron (maybe aluminum for weight?) I will need an angle bar with 4″ of bottom width to allow mounting of the CNC feet. Plus some slop room for alignment. To keep the Y-Rails in parallel I am planning on two cross members with probably some sort of corner bracing to prevent any racking of the frame.
The beam would be an attached beam on both ends with a length of ~40 in. The angle iron would be 4″ wide and probably 2″ high.
The weight of the Machinist not counting the spindle is about 80 lb. I’m rounding up the weight to 100lb with the spindle and maybe a laser module etc. That would put approximately 25lb on each corner and 50lb on each beam.
The beam would be an attached beam on both ends with a length of ~40 in worse case not using a distributed weight model at all. Feeding all that information into a beam deflection calculator I saw defections less than 1 thousandth of an inch. In my book that is an insignificant deflection. and I am not even considering the stiffness of the Y-Rail it’s self in the calculation. Or the weight is really a distributed model and the beam length is shorter. It’s always safer to go with worse case analysis until that becomes a problem. Near as I can tell no worries mate. That’s why I am contemplating angle aluminum to keep the weight down. I am thinking aluminum would be more than sufficient.
So really my biggest concern would be any racking of the frame throwing the alignment out of wack. But I think maybe instead of a crossbar I could go with an L or U bar forming a corner brace preventing any twisting.
So How am I going to Lift the Lift-Frame Up off the Board?
I started off thinking I would build a really cool frame out of 80/20 excursions. Never working with 80/20 before I started hacking around finding I need to put in another piece and then another to keep something from tilting or deflecting. I present the “Mad Scientist” Version of my Onefinity CNC Lift ThingaBob. Probably a few hundred dollars of 80/20 crazy mess to build.
I really thought I had something. I made some animations showing it working. It was cool. But one long night at the computer.
I finally headed off to the sack only to wake up in the middle of the night with the thought, “Are you Insane?” OK don’t let my wife answer that question but if I learned one thing in engineering:
Clearly, my “Mad Scientist” Version would not win Albert’s approval. But my mid-night epiphany made me realize the lift rails are really all I needed. That and a simpler way of lifting them. I recalled the design of a wood planer and how it lifts the cutting heads while keeping it parallel to the bed. Sound familiar. Well, the head is lifted by cranking lead screws that are linked together with a belt or gears. So all the lead-screws turn in sync lift the head and staying parallel. To me, that sounds exactly how I should be lifting my CNC.
So here is is
The “Simple” Lift Version.
So here it is. I still am working on a few details but the overall concept is there are 4 lead-screws in each corner. Each screw is linked to the others via a toothed belt and pulley so they all turn in unison.
Here are some Animation Videos Of the Lift In actions.
The lead screws sit into bronze bearings recessed into the base. There is a lock collar on the shafts to keep them from sinking into the bearings and I think it will be better to have the lock collar sit on top of a fender washer. Not a perfect low friction bearing but I think good enough. There is a linear bearing i.e. screw nut recessed into the bottom of the lift frame. I added a block on top to give me plenty of vertical space to recess the nut and to give some additional stability to the rods. I am not sure what to expect yet for the tolerance of the lead screw bearing seats so I am planning on added for alignment guide pins to keep the frame from shifting. I think when the frame is down I do not expect any significant movement. When it is up I am not sure but we will see.
I uploaded the 3D model so you can view it yourself in a Web Browser. Take a Look
Design Features and Goals
From the beginning I had some design features I wanted from my Machinist upgrade:
- Be able to work on larger project up to 24″ wide by 24″ +
- Be able to set the CNC up to mill Dove Tails and Box Joinery.
- Make it possible to transport to another surface such as a dining room table for engraving.
Item #1 is check!
Dove Tail Configuration
Because the CNC sits on Vertical Lift Rails it is also possible to shift horizontally forward to the edge of the base or tabletop. Set up a jig to hold the workpiece board vertically and you are in business cutting joinery. You will only be able to ~ 18″ wide boards but that is probably ok for most work. Dovetail Jigs typically only work on ~12″ stock anyway. I make a lot of boxes so this feature will be awesome for me.
Just pull the guide pins and back off the lead screws and the frame can be slide forward. Probably a good idea to have some new locator inserts for the guide pins to locate the position. Also maybe something to guide the rails sliding. I am thinking of a cutout area in front to minimize the forward position keeping the balance on the tabletop.
Item # 2 – Check!
Item 3 I think is obvious as showing in the Dovetail configuration the frame is free to move anywhere. Aside from the weight which is probably a two-man job but it can be quite easily moved and placed anywhere to engrave a surface.
Item #3 – Check!
I am really happy with my design so far. But the skeptical side of me does have a few concerns.
Milling Forces Causing Shifts and Errors
Milling forces causing the CNC to shift during cutting. I mentioned this earlier and how I will be putting some guide pins along with the lead screws. I think in the full down position the frame should be quite stable. The locator pins and the leadscrews should hold the frame in place. My gut tells me it should be about as good as the normal mounting of the feet with screws. Of course, I could also screw down the rails for normal operations. In the raised position I suspect there will be a trade-off. There will be some movement with the lead screws and guide pins. It makes sense to only lift up just enough to clear the workpiece which will 3/4″ plus the wasteboard 1/2″ At this point I just don’t know how much movement to expect. I might have to limit the feed rates. I will need to experiment to determine the limits.
In the lift position the rails and the linear nuts are holding up the frame. It is important the Z height does not shift after zeroing the CNC. I don’t think it will but I thought of a idea to make sure. I could slip in a pair of lift blocks front and back and lower the frame on to them. The block would then take the weight and would hold the Z height perfectly. An additional benefit is it would add a fair amount of lateral stability with the guide pins running through the blocks. This would greatly help with my concern for milling forces.
Tramming Errors With Lift
Tramming errors happen when the router is not perfectly perpendicular to the milling surface. The router can be adjusted by tilting the CNC lift plate to minimize these errors. When lifting the frame I suspect I will introduce some tramming errors due to the slight differences of the lifting leadscrews. But I think the best solution is again using my lift blocks which if milled accurately should place the frame is about the same level position as in the full down position. As I said in the lift position there will be some trade-offs to accommodate working on larger pieces.
What did I forget? Is this idea awesome or crazy? In theory, my project design seems to meet all of my goals. It seems to be very doable. Also quite inexpensive. And scale-able. For those with the Onefinity “Woodworker” CNC who want to work on plywood sheets 48″ wide, I think it could be done.
I would love any feedback on my project ideas. I am not scheduled to receive my new Onefinity Machinist CNC until Mid-October so I have time to work out the final details and improvements.
One being the enclosure. I am sure some of you are wondering if this all will fit in an enclosure? I am still in the contemplation mode with the enclosure but I did work up a quick one just to see.
I need to finish up the wasteboard design and the Tile Shifting Indexing pins/guide. I want to make this somewhat convenient so I can work on the larger projects without too much setup and hassle.
And I need to finish up my design for an enclosure. I will be setting up my machine in my upstairs office. It is separated from the rest of the house but I sill want to keep the noise down. And of course dust control.
In case you were wondering how all this could fit in an enclosure I did work up a concept. I am still up in the air for what I am going to do but it will be on along the ideas of this.