[hawkmoon77]

Preface

After assembling an enourmous amount of data on the project, I thought it best to funnel it all to one place, and I can think of no better place than CNCzone. My intention of this thread is to completely follow the process of a home-built CNC machine from start to finish (and by start, I mean the start of design, not the start of assembly).

• Machine Summary: All steel construction, with precision linear motion parts. Precision carving of wood, and light metal work. Cost range of 4K to 6K.

• Machine Design Criteria: X travel of 5 to 6 feet, Y travel of 3 to 4 feet, Z travel 9 to 12 inches. 600 to 900 pounds or less. Accuracy of .003 inches or better. Modest speeds, but speed is second to accuracy. Relative ease and repeatability of assembly with minimial avanced tools.

• Scope of Thread: While my goal is to have all information about my machine in one organized place, there are limitations. I will not be walking through the basics of CNC parts and construction. There are so many wonderful and helpful guides on that subject, that I would just be re-inventing the wheel. I will consider all such material to be introductory. I will also not focus on any software, modeling, CAD or CAM theory. Such will be considered outside the bounds of this thread. This guide will begin with machine design and end with completed construction and testing.

I thank you all in advance for taking an interest in this thread, and hope to be able to give back a bit to CNCzone. I'd also like to thank Haydn and Madvac for the unmeasurable amount of guidance and information they unknowingly contributed to my project.

*** The information I post throughout this thread is for archival purposes only, and only intended as a historical account of the decisions I made, and the actions I took. I AM NOT QUALIFIED TO GIVE ANY ADVICE ON THESE TOPICS. All information is provided without guarantee or warranty. You assume all risks. Please be safe and smart in your endevours.

[hawkmoon77]

Table of Contents

Section I: Design

Chapter 1 - Introduction, POST #5

Chapter 2 - Xaxis Support Design, POST #6, POST #7, POST #15

Chapter 3 - Bed Design, POST #16, POST #17

Chapter 4, Table Bracing, POST #24

]
Section II: Table Build

Section III: Gantry Build

Section IV: Electrical

Section V: Final Details

Sectino VI: Testing

[hawkmoon77]

RESERVED 1

[hawkmoon77]

RESERVED 2

[hawkmoon77]

SECTION I: Design

Chapter 1, Introduction

Design is, at the very least, as important as the build. I do not want to waste money or time building a machine that does not function as I intend. It is with that premise that I begin this very critical phase.

My overall design objectives include a balance of precision and budget, with liberties taken to reduce weight, size and cost where possible. I also plan on using readily available parts, and will not rely on any dumpster finds. My goal includes a certain potential for repeatability in construction and cost.

Throughout this section, I'll refer often to the weight differences between alternative configurations. Building a lighter machine isn't just about transportability. Most metal suppliers determine their pricing by the pound, and not by the shape. The final weight of the strutural components of the machine, multiplied by the price/pound figure, will give me a reliable cost estimate. Bottom line is that saving weight is saving money.

Another consideration is the general availability of the tools required to construct the project. While I will require more than a hammer and drill, my plans have included special consideration for minimizing the types of advanced tooling that is not generally available. I know noone in a machine shop, and I don't own a metal lathe, mill or welder. Nonetheless, the ultimate design of the machine takes into account this disadvantage. Other than leveraging the machine itself to aid in its own construction, few advanced tools should be required.

I rely very much on CAD software for both modeling and (more importantly) simulation and testing of loads, forces, and other design stresses. I will not bolt any parts together because it seems like the right thing to do. Rather, my design will be thoroughly and methodically tested and simulated to achieve the objectives I laid out. If for example, I need the deflection of a part to stay within .001 in. at a given load of 300 pounds, then I will rely on my simulation software and actual mechanical testing to demonstrate that capability. Over-engineering (in the way of extra braces or thicker materials) only adds cost and weight, and so my design seeks to meet its design objectives using the minimum amount of readily available parts.

Within the next few chapters, I'll share my design choices, as well as the results of some load tests and simulations. Further refinement is required, but only few design choices remain.

[hawkmoon77]

Chapter 2: Xaxis Support Design

I will be walking through my design method for the development of each component. Rather then just simply show or describe my decisions, I will present the methodology used to make them as well. Below describes my progression through the Xaxis design.

Picture #1 shows a basic side profile of the longitudinal side of the table that will support the Xaxis linear slide (visible in the first and second picture). The two corner legs are extended above the rail and will support the bed. Although there will be some interaction between the bed and the Xaxis rail, I am treating them seperately at this stage of the design.

Notice that there exists a longitudinal brace connecting the two outside legs. This is rather standard protocol, as it will prevent the legs from kicking out when weight is applied. Analysis of the benefits of the bottom brace showed that it added negligable benefit to any actual load bearing capabilities of the Xaxis rail (the beam at the top that will hold the Xaxis linear slide).

A major concern is overall stability when the gantry moves across the Xaxis. Even with bottom brace, there exists a danger that the machine will shutter and shake with quick changes in gantry direction. Acknowledging that the machine will have to be installed on a floor surface made truly level, I am making the decision (for various benefits) to anchor the machine directly to the floor. In my case, it will be bolted into a cement floor. Because of the floor's rigidity, the bottom brace becomes completely unnecessary, saving approximately 23 feet of structural steel tube and 126 pounds, while making gains in overall stability. See Picture #2, which introduces a steel plate at the bottom of each leg that will be bolted to the floor.

Initial load analysis is presented in Picture #3. Discplacement is over-exaggerated for better visualization; actual discplacement measurements are provided in the chart. At the center of the Xaxis rail is a load of 300 pounds applied over 1 square inch. I am assuming a gantry weight of 300 pounds, spread across two rails (150 each), with a safety factor of 2... thus my decision to model 300 pounds. Displacement is predicted to be .426mm at its worst (the center of the Xaxis rail). Note that the longitudinal beam that supports the top of the bed will add additional stability. The tops of the corner legs currently bend toward one another, but an additional bed beam will add compression strength to stabilize the legs, resulting in less Xaxis rail displacement. Modeling of that beam isn't necesary at this point.

I will need to keep maximum displacement at nearly .1mm to stay within the tolerances of the linear rail and my overall design criteria. Clearly I will need additional support along the Xaxis rail, but I want to avoid unnecessary steel. I may need multiple braces, a third center leg, or both. I will present my findings in the next update.

[hawkmoon77]

Chapter 2: Xaxis Support Design, cont.

In this section, I'll walkthrough three bracing configurations that do well to summarize the dozens of configurations I modeled. I've added the bed longitudinal beam (the gray beam visible in Picture #1) as it's presence is relevant for these tests. Note also the addition of the corner leg braces. Load testing is the same as that described in the previous portion of this chapter; the scale is normalized for all trials.

Picture #2: Brace centered to leg, narrow angle to Xaxis Rail
Combined Weight = 8 pounds per side
Combined Length = 28 in. per side
Displacement = .266 mm.

Picture #3: Brace near bottom of leg, narrow angle to Xaxis Rail
Combined Weight = 17.5 pounds per side
Combined Length = 53 in. per side
Displacement = .192 mm.

Picture #4: Brace near bottom of leg, wide angle to Xaxis Rail
Combined Weight = 22 pounds per side
Combined Length = 65 in. per side
Displacement = .182 mm.

The results do not fare well for the the side braces. All three trials showed displacement greater than .1 mm.

Also worth noting is the diminishing returns of added material with regards to the reduction in displacement. As the braces elongated, weight and length jumped up, while displacement remained materially similar. Additionally, when the brace length is maximized so that each brace extends from the floor of the corner leg to the center of the Xaxis rail (I did not include a picture of this configuration), weight is increased to 30 pounds per side, length reaches 86 in. per side, and displacement along the Xaxis rail is in some places reduced by .002 mm., while increased in other places by as much as .04 mm. This arrangement is hardly beneficial.

It is clear that at these lengths, the Xaxis rail will need a third leg.

[garagon127]

Hawkmoon77,
Thank you for taking the time to share this endeavor with this community. I will be watching with great enthusiasm and wish you the best of luck! Sounds like you know what you're doing.

Gary

[garagon127]

Hawkmoon77,
Knowing that it is early in the design stage, I thought you might like this video of a pinion roller drive system that I thought would be great for a CNC machine. Here is a link to a video from Nexen group ( I have no affiliation to them) that I stumbled on one day.
[ame=http://www.youtube.com/watch?v=vPPhwoe2QMU&list=PLF7B7E3CDC98AED5B&index=21]‪Roller Pinion System‬‏ - YouTube[/ame]
This is so smooth and precise...someday I hope will be in MY future!

Gary

[hawkmoon77]

Thanks for the info Garagon,

I've seen those systems before, but haven't seen a video that demonstrated it so clearly. I understand that those systems are pretty good. When used with a servo, they are very fast and precise.

[asuratman]

Hello,
Goodday, anybody here use this nexen RPS system? Do you know about the cost approximately for specific size? I think its expensive.

[Eddymeister]

Thats interesting reading there hawkmoon...

I can understand your thinking as I'm in the initial phase of designing... Building a machine to a set of specs is definitely more cost effective in the long run rather than selecting a component bolting it together and hoping for the best...

What program are you using to give you such results... Is that Solidworks by chance...

I'm no engineer but have the patience and passion to learn about this...
My preliminary design is based around large PFC or Post formed channels to create the gantry and X Axis substrate...
Would you know of any simple programs that would help me test my design as you have...?

I'm an aussie but 300 pounds of gantry sounds incredibly heavy...? Isnt that around 120Kgs? Or is this a figure that include max cutting loads as well with a built in margin for error...

Good post mate...
Keep it up..

[hawkmoon77]

Eddymeister:

Thats interesting reading there hawkmoon...

I can understand your thinking as I'm in the initial phase of designing... Building a machine to a set of specs is definitely more cost effective in the long run rather than selecting a component bolting it together and hoping for the best...

Agreed. There is certainly some liberties that can be taken regarding specs as cost and ease of building is a factor as well. Over time, I will be able to paint a more complete picture of how these early stage decisions influence the outcome of the design.

What program are you using to give you such results... Is that Solidworks by chance...

Yes. I am rather comfortable with it as a CAD program. For modeling, I set up some simple real life load tests (extending a plank of steel, applying weight, and measuring deflection with a dial indicator) and compared those real world numbers to Solidworks simulation suite. I was rather pleased with the results. Well, except when MDF was involved. That took some tweaking before I got reliable results.

I'm no engineer but have the patience and passion to learn about this...
My preliminary design is based around large PFC or Post formed channels to create the gantry and X Axis substrate...
Would you know of any simple programs that would help me test my design as you have...?

I don't know of any "simple" programs that can do advanced simulation and load analysis.

I'm an aussie but 300 pounds of gantry sounds incredibly heavy...? Isnt that around 120Kgs? Or is this a figure that include max cutting loads as well with a built in margin for error...

Actual loads will consider weight, velocity, inertia, and additional Z-axis applied forces. For these static tests, I have to pick the relative weight that will be seen at any given point (I chose the maximum along the modeled curve, and rounded up). It adds up fast. A gantry of 175 pounds, with added Z-axis load, and quick changes in direction could cause momentary loads nearing 300 pounds.

Good post mate...
Keep it up..

There are literally volumes worth of more information I plan on sharing. Stay tuned.

[yackback]

I just wanted to say that this is EXACTLY the type of reading I was hoping to find on this forum today. I hope that you will continue to share your findings as this could easily shape up to be the best tutorial on CNC building I've ever come across.

Thank you for sharing!

[hawkmoon77]

Chapter 2: Xaxis Support Design, cont.

Let's look at what happens with the addition of a center leg. Picture #1 shows the updated configuration. Load is centered at the point of maximum displacement. Results are as follows:

Picture #2: Third leg centered to Xaxis Rail
Weight = 11.5 pounds per side
Length = 26 in. per side
Displacement = .137 mm.

This configuration was quite successful. Compared to the first trial, this trial cut the displacement in half, while using the same length of steel as the smallest braces I modeled. As compared to other trials, this trial resulted in less displacement with half the amount of material.

Although the load bearing ability has been increased, and displacement is measuring well (a bit high, but within the range I want at this stage as it will improve slightly as the remainder of the table is designed), a complete lack of leg braces could cause trouble with any side forces applied to the table that would seek to move it out of square. Simulation demonstrates this is indeed a problem (see Picture #3).

Some bracing will be required, the challange will be determining the minimum amount needed. I have modeled several configurations, and small braces are more than adequate. The basic premise will be that shown in Picture #4, but I highly suspect that what is shown is already over engineered. Also, because I am avoiding welding as much as possible, attachment of the braces to the legs would be accomplished with a triangular cut of steel plate bolted to both leg and brace (the brace could be bolted directly to the back of the Xaxis rail, so no special accomidation is needed at that contact point). If steel plate were added (in front, in back, or on both sides), even more unnecessary rigidity would be achieved. Further investigation into how minimal the steel profile can be will have to wait until after the table bed is incorporated into the structure.

[hawkmoon77]

Chapter 3: Bed Design

The bed will have substantially less support than the Xaxis rails. While it will not bear any of the weight of the gantry, it will still need to be able to handle a large amount of weight with little displacement. (Remember, a precise Zaxis will do no good if the bed itself shifts every time the cutter presses against the workpiece). Fortunately, much of the weight on the bed is static, and is spread over a wide area.

Assuming 75 to 150 pounds of MDF (depending on how many layers will be required given MDF's flexibility) and 50 pounds or so for larger work pieces, we are looking at 200 pounds spread across the table top. Add to that the isolated weight that will result below the cutter. Then, multiply everything by a safety factor.

Picture #1 shows an outer bed frame (in gray). Obviously, the table will need cross supports for the center. Placement is critical, as gantry movement below the bed must be unobstructed. Note also that the bed's longitudinal beam is attached on the inside face of each leg, which is opposite the Xaxis rail (which is mounted on the outside face of each leg). This was done to avoid interference between the bed frame and the vertical supports of the gantry that will be placed atop the Xaxis rail. Had I positioned the bed's longitudinal beam on the outside face of the legs, it would have been directly above the Xaxis linear motion slides, requiring me to offset the vertical gantry supports from the Xaxis rail. I see no reason to do so.

[hawkmoon77]

Chapter 3: Bed Design, cont.

I want to re-emphasize that the exact dimensions of the structural members of the design are only estimated (guess work) at this point. The ultimate goal of these initial tests is to determine the best orientation of the parts, and not to determine the exact profile and gauge of steel that will be used.

With that said, there are only several ways the bed cross supports could be aligned to provide needed support at the bed's center (i.e. parallel to the Xaxis rail, perpendicular to it, or in a grid pattern). I looked into each alignment, and below are some of my observations. Note that for each simulation, two forces have been applied: one spread evenly across the bed, and a second isolated force under the cutter at the center of the bed.

Picture #1: Three cross supports perpendicular to the Xaxis rail
Combined Weight = 76 pounds
Combined Length = 159 in.
Displacement = .244 mm.

Picture #2: One cross support perpendicular to the Xaxis rail, two cross supports parallel with the Xaxis rail
Combined Weight = 63 pounds
Combined Length = 131 in.
Displacement = .137 mm.

Well this is interesting indeed! In the second configuration, we've managed to reduce the amount of required materials and cut the displacement nearly in half. But it turns out, we can do even better than that. Recall how in Picture #2 one cross support runs perpendicular to the Xaxis and two cross supports run parallel to it. Picture #3 reverses the configuration, such that one cross supports runs parallel to the Xaxis and two cross supports run perpendicular to it (look closely at the center intersection of the cross supports in Picture #3 to see the difference). It turns out that this configuration uses the same amount of material, but results in 12% less displacement!

Additional cross supports may be required - not to make the table frame more rigid, but to add integrity to the MDF bed top. This additional support will likely be smaller gauge steel with an "L" profile (rather than the rectangular tubes used up to this point). When the bed frame is fully designed, I will then test the displacement of the MDF at it's weakest points to determine what additional support, if any, is required.

At this point, I have a decent bed from a load perspective, even though there is still a bit more displacement than I want (but within tolerance considering no side braces have been accounted for yet). It also does not consider additional forces that could cause it to twist out of square. These issues will be addressed next.

[zool]

This response is a suggestion concerning the x-axis support design and the bed design.

What is contained in this response is based on the design of this build: http://www.cnczone.com/forums/cnc_wo..._sized%5D.html

and second moment of inertia calculations done in the design phase. http://en.wikipedia.org/wiki/Second_moment_of_area

[This will also free you from having to add a chapter to your encyclopedia of errors!!]

To minimize the deflection [the degree to which a structural element is displaced under a load; both compression and twisting] of both the x-axis support beams, as well as the bed support, AND minimizing the cost of construction of the structure of the proposed machine, it would seem prudent to combine the x-axis and bed support.

By integrating the x-axis and bed support the deflection of the bed beam support as shown in Picture 1 of Message 17 would be vastly diminished as the span would be reduced by one-half as the middle leg would support the x-axis as well as the bed support.

In addition the cross beams as shown in Picture 1 of Message 17 would now minimize any twisting of either the x-axis or the bed support. Further, when a bed is attached to the cross rails, this would now be one-half of a torsion box. If you put a couple rails length-wise perpendicular to the cross rails, this would complete the torsion box and provide an extremely stable platform for both the x-axis and bed support.

[Lapin]

I saw that the Nexen RPS system was discussed. I use em in my Router which u can find here. I can just tell that its a great linear motion system, quiet and fast. I am really happy with it.

Keep up the good work Hawk!

[hawkmoon77]

This response is a suggestion concerning the x-axis support design and the bed design.

I appreciate all suggestions, and corrections are even more welcome. That being said, I'm not sure how to interpret your post as it seems somewhat unfriendly. Maybe it's just me, in which case I apologize - no harm intended.

What is contained in this response is based on the design of this build: http://www.cnczone.com/forums/cnc_wo..._sized%5D.html

I was not familiar with that particular build, but I did thoroughly examine the mechmate, which uses a similiar configuration. I will not speak negatively of anyone's machine, as I'm sure they are more than happy with the results, and a successful build is a big achievement.

and second moment of inertia calculations done in the design phase. Second moment of area - Wikipedia, the free encyclopedia

My software simplifies much of this math for me. For example, I can add a linear motor that pushes the virtual gantry across the table, add the friction as calculated by the LM slide vendor, and it will not only tell me the magnitude of the relevant forces, but show the effects on the build. It's pretty neat and saves a lot of math and chance of error.

[This will also free you from having to add a chapter to your encyclopedia of errors!!]

Yeah, I don't really know what you mean by that.

To minimize the deflection...

I appreciate your response on this. I'll carefully consider it as an option. I think it is important to say that I do not believe that I can design the best CNC machien ever. It will have limitations, and disadvantages over other types. So please don't take what I say as the bible on this topic by any means. My posts are meant as nothing more than a means to document the process I'm going through, right or wrong... with the understanding that I'm not only happy to acknowledge and correct the wrong, but hope that it serves as a learning experience for others.

I also considered mounting the linear rails to the side of the bed support itself, no need for two seperate rails. We'll see...

By the way, do you have any thoughts about ballscrew placement in the design of the machine you linked too?