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Post Questions about the "book" CNC Plans here

Official support for products purchased from the BuildYourTools.com and BuildYourCNC.com sites, and the book "Build Your Own CNC Machine (Technology in Action)" by Patrick Hood-Daniel and James Floyd Kelly.

Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Mon Jan 13, 2014 2:11 am

The book machine's design won't work for a machine that big. You really can't make it much/any bigger than it already is. There are a number of serious issues that appear when it's scaled up.
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Re: Post Questions about the "book" CNC Plans here

Postby tpthatsme » Sun Feb 23, 2014 2:47 am

I have been looking at this book and plans for a few years now and I think I am close to pulling the trigger on this project, but I am afraid that this plan is a little dated. Even the website suggests to buy a kit instead of follow the book plans. Is this still a solid machine for the size/cost/technology?
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Sun Feb 23, 2014 7:34 am

tpthatsme wrote:I have been looking at this book and plans for a few years now and I think I am close to pulling the trigger on this project, but I am afraid that this plan is a little dated. Even the website suggests to buy a kit instead of follow the book plans. Is this still a solid machine for the size/cost/technology?


The mechanical build of the machine is still perfectly viable. The electronics listed can all still be had and used, I think, or similar updated variants, if the exact part numbers have changed. Or you could go with some newer electronics, but they don't really affect the mechanical design any.
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Re: Post Questions about the "book" CNC Plans here

Postby Arbiter » Wed Aug 13, 2014 12:20 am

Awesomeness wrote:The book machine's design won't work for a machine that big. You really can't make it much/any bigger than it already is. There are a number of serious issues that appear when it's scaled up.


Hi Awesomeness :). Thank you very much for all the great posts and advice! I am new to CNC builds,and I am a mechanical engineer by training. I bought the book a few years ago and I am finally ready to start building in the next few months. I wanted to ask if you could elaborate on the reasons we can't make this current design bigger?

I'm not an expert on CNC design, so my arguments below may be totally invalid, which is fine by me, as I am trying to learn about the design variables. It seems to me that your biggest issues would be stiffness and inertial mass of the enlarged gantry system. The book design limits you on the stiffness of the table, but I saw someone suggesting to put some angle iron below the table where there is still some room under the X-axis gantry, which would seem reasonable. I'd have to run the numbers, but it should be possible to stiffen the frame in such a way that would allow some expansion of the bed size...?

Additionally, if you were to attempt to widen the frame, I'd imagine you'd need to increase the height of the cross-beam to compensate for the added span. This would add weight, but I imagine some of this could be mitigated by removing some un-necessary material in the stiffeners through weight reduction pockets, similar to what we do on jet engine and rocket casing designs. I'm not saying these design changes are easy, or that they are even worth it, but at first pass, if you have access to a good CAD package to model the stretched dimensions and the weight reduction of the stiffeners, I am failing to see how the book design couldn't be scaled somewhat. I tried searching the site numerous times for a more thorough discussion of the design variables, and if I haven't found the post that describes the serious issues in detail I apologize! Anywho, I am excited to get started! Thanks for listening!

-Chris
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Wed Aug 13, 2014 12:51 am

Basically, as you try to make the design bigger, you create problems, and in trying to add components to solve those problems, you introduce more problems or make other problems worse.

The book build is driven by a single lead screw in the center, so as you widen the gantry it becomes even more prone to racking than it already is [because the ends of the gantry are free to move on the rails]. Bigger lever, more force, bigger moment. That's not what you want when you're trying to keep tight tolerances.

Similarly, as you add supports to the table to keep it from bowing, you make it thicker. This again moves the driving forces (screw) further away from the cutting bit, and introduces more leverage onto the bearings.

Then there is lead screw whipping. With 5' and 11' lead screws (the table sizes generally required to get 4'x8' real working area) you're going to have massive screw whip problems, unless you add supports or make them some absurd diameter (e.g. 1.5"?). Those additions introduce their own problems, like where to put the supports and how to move them out of the way, or needing bigger drive electronics to turn 150lb screws.

This just goes on and on, but it's just bad news in every direction. It's clearly just not the right design for a large machine. Why try to fix a poor design to [potentially] make it viable, when the solution has already been laid out by the other designs out there that don't start with these issues? So you can follow a $20 book? Far better to use a design that drives the gantry from both sides of the x-axis (e.g. two motors, a single motor with an axle across X, etc.), and doesn't have a support that has to loop around under the table (allowing you to support the table properly, when it's not there).

I can't prove this, because it hasn't been attempted, but I would also caution, after seeing how big a money pit this hobby can be, that it would cost you more to try to engineer solutions to the flaws in the design, than just to start with one of the more expensive designs that doesn't have those problems in the first place. That's what it's all coming drown to right? People see the "$800" price tag in the book build, which as an aside I've personally documented is nonsense anyways and is more around $1500+, and want to hold out hope that they can do it inexpensively. I'm probably like $30k into this hobby, several machines later, and while it was nice to spread that out over a handful of years, I honestly would have been better off just getting financing on a ShopBot probably. That doesn't apply to everyone, obviously, but it's a point that shouldn't be dismissed. That "little change" you think you can add, like adding a torsion cable system to prevent racking, turns into hundreds of dollars after you go through a bunch of parts to implement and perfect it. I'm sure you know how that "budget creep" happens in your professional engineering projects.

EDIT: I bolded the "long story short" answer.
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Re: Post Questions about the "book" CNC Plans here

Postby Arbiter » Wed Aug 13, 2014 2:43 am

Hi Awesomeness! Thanks again for the great reply! You laid out in clear terms what I was not able to think of. I realized my profile mentioned I was interested in building a 4 X 8 machine, which is NOT what I was thinking about doing by making the book machine bigger :). That's definitely not a good idea! My initial thoughts were that it might be nice to make it maybe 1 foot longer and maybe 6 inches wider, just to get a sniglet more room :). Again, if the design is truly marginal to begin with, then a stretch is definitely not in the cards! I don't have a problem tackling the normal book machine, but part of the reason we get into this is to tinker :)... Otherwise I would just get a shop bot :), or join the local maker-space (Which I may do anyway :) )... The financial aspect of it is not a big problem, $800 Vs. $1500, as long as I can get a working machine to tinker with :). I got the book because it was the first step by step set of instructions I had seen. Thanks again for laying out the bottom line though!

-Chris
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Wed Aug 13, 2014 3:52 am

I just want to reply on your use of the term "marginal". Please don't get the wrong idea about the book machine. It does work, and can do a lot of great things. But imagine it like calling a bicycle with a weedwacker engine on it a "motorcycle". Sure, a motorized bicycle is cool, but it's not in the same class as a "motorcycle". And if somebody came around talking about trying to put a full sized motorcycle engine on a bicycle frame, you'd tell them "Whoa cowboy, there is a reason a motorcycle is a big, heavy machine, with a thick frame and large tires."

So a good way to talk about the book machine is that it is a balanced design, without a lot of major, glaring bottlenecks [depending on your use/expectations]. It is a design that meets its requirements well (e.g. easy construction, good accuracy, simple components from hardware stores, etc.). A design based on different constraints, like faster speed or larger size, would result in a significantly different solution.

People that would call themselves "power users" will probably ultimately find themselves upgrading from the book machine, most likely due to speed. They will however have learned a great deal in the process.
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Re: Post Questions about the "book" CNC Plans here

Postby Arbiter » Sat Aug 16, 2014 2:45 pm

Hi Guys!!!! The day is here! I am buying the electronics! I am going to be building the book machine, and potentially use the electronics for a bigger machine eventually. Does anyone have a feel if the stepper kit I am posting would be good at least for the book machine? Thanks!

http://probotix.com/index.php?view=product&path=22&product_id=48

Kit Includes:

3x ProboStep VX Uni-Polar Stepper Motor Driver
2x HT23-400-8 400ozin Stepper Motor (X- & Y-Axis)
1x HT23-280-8 280ozin Stepper Motor (Z-Axis)
PBX-2 Parallel Port Breakout Board
24Volt 6.5Amp Switching Power Supply
6ft. DB25 Male-to-Male Cable
3x 4" IDC Cable 10-pin
New improved 8-wire motors allow for the use of the more powerful ProboStep motor drivers and higher voltage power supplies. Lower inductance uni-polar configuration and higher voltage power supplies translate to faster top speeds.

This Hybrid kit features powerful 400ozin motors for your X- & Y-axis and a 280ozin motor whose weight is more practical for your Z-axis.
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Sat Aug 16, 2014 3:27 pm

If you're planning to make your book build with 1/2"-13TPI threaded rod, you're going to have some issues using those same motors later for a chain drive. The reason is that the threaded rod turns 13 times to move only 1" (13:1), while the chain sprocket on a BlackToe moves 2.5" for each turn (1:2.5). That's a 32.5x difference!

So for the book build, you would want motors that like to go fast, because 50 inches per minute (ipm), which is a slow cutting speed, is 650 RPM. On the chain drive, you only need motors that can handle 20 RPM.

More voltage will give your motors more power at higher RPMs. So if this kit is available with much higher voltage power supplies (e.g. 48V, 72V), I'd select that over the 24V.
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Re: Post Questions about the "book" CNC Plans here

Postby Arbiter » Sat Aug 16, 2014 6:46 pm

Awesomeness, thanks for the reply! These motors would be fine for the book build then? But, if i upgraded to a bigger machine then i'd have to get a new motor set. Is there a kit like the probotix one that would suit both machines well? Based on your post i'd really want 2 different type of motors anyway? Thanks again!

Chris

Ps i re read your post... sorry was on my smartphone! So i think the impression is that i probably need to get a beeffier power supply so i can get a faster rpm.. thanks!
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