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Post Questions about the "book" CNC Plans here

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

Postby Awesomeness » Sat Aug 16, 2014 8:45 pm

Well, I think part of what I'm trying to illustrate is that the drive electronics that are "right" for your machine are VERY particular to YOUR MACHINE. And for that reason, the right answer for one machine is almost certainly not the right answer for another machine.

One thing you can do is use real leadscrews on your book machine. A 1/2" 10-TPI 5-Start screw is 2 turns per inch. 2:1" is way closer to 1:2.25" than 13:1" is.

Or you could buy bigger motors and put a 1:2 gearing on your book machine, to get the RPM.

There are a bunch of ways to solve the problem. Which you choose is based soley on your constraints (e.g. budget, what you plan to use the motors on in the future, how much driving force you need, etc.). You can wing it, and just pick some stuff that "looks close", but you run the risk of having poor performance. You really need to figure it all out.
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Re: Post Questions about the "book" CNC Plans here

Postby hexus179 » Mon Aug 18, 2014 3:09 am

I can't seem to find this information anywhere, so please forgive me if it's obvious and I've missed it. I plan on building the "book machine" for my first build. From what I've gathered the actual cutting area is 20" X 40", but what is the Z travel? I've looked in the book several times, and around here and I just can not find it anywhere.
For what I'll be using the machine for I'm sure I'll need a longer Z, but what is the actual cutting area of all 3 axis's of the book machine?
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Mon Aug 18, 2014 3:22 am

I forget exactly, as it's been a few years since I retired my book machine, but I think it was about 5" of bare travel. That means you can actually cut about 1.5" or so, after you take into account bit length, spoil board, material clamps, etc.

Be forewarned, the z-axis is the weakest link of all these machines (even a $250k Haas CNC, really). It may not be a trivial task to add more z movement.
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Re: Post Questions about the "book" CNC Plans here

Postby hexus179 » Mon Aug 18, 2014 4:32 am

Hmm I may need to reconsider my build then. I'm a woodturner and turn several bowls. My plan was to use the machine for the traditional flat stuff like signs and what have you, and add a 4th axis for rotation to engrave things on the side of a bowl. I already have the electronics up and running just haven't built the actual thing since I didn't know what to do about the Z height.
I was just going to extend everything so a bowl would fit under it sideways, but I may not be able to. I can turn up to a 16" bowl on my lathe so I'd need a lot of z axis if I were engrave on the side of one that big.
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Mon Aug 18, 2014 5:15 am

There are a lot of issues people often don't know about the z depth. Another thing is the length of mill bits that you can get. For example, the longest cutting flutes you can get on a 1/4" diameter bit is like 2" (because any longer and they can't grind it accurately). So unless the projects you want to do can be done well with huge 1/2" bits, things like cutting 6" deep are really out of the question [for 3-4 axis machines]. (Let alone the problems that if you have a 6" long bit, to cut 6" deep, that means you have to be able to pick that bit up 6" above 6" deep material, for a total of 12" of travel.)

You can do thing like cutting a hole in the table, and putting the rotary axis down inside the table, to try to gain z depth too. Or on machines like the BlackFoot design, you can put the rails up on risers. Basically, there are other creative solutions.
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Re: Post Questions about the "book" CNC Plans here

Postby hexus179 » Mon Aug 18, 2014 12:59 pm

I'd only need the depth to clear the bowl diameter. I'd have it sideways so I could do a decorative cut around the rim, or something on the side. The actual cut wouldn't be very deep.
That's what I was thinking though too. Cut a hole in the table and drop the 4th rotation axis down, rather than build and raise the gantry up. Then I'd have a problem with the X axis lead screw though so I'll probably have to use 2 lead screws on the left and right and sync two steppers for it.
Anyway thanks for the help. Time to brainstorm and build.
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Mon Aug 18, 2014 3:16 pm

hexus179 wrote:I'd only need the depth to clear the bowl diameter. I'd have it sideways so I could do a decorative cut around the rim, or something on the side. The actual cut wouldn't be very deep.


I don't think you're thinking far enough into the problem. If you were doing a true cylinder, like a coffee mug, you're right, you wouldn't need much depth. But a simple hemispherical cereal bowl is probably 3" around at the base, and 7" around at the mouth? Since the bottom mount of the spindle/router is probably 6" wide, you're not going to fit it around that curve, so you have to do all that reaching with the bit alone (e.g. (7"OD - 3"ID)/2 = 2" deep).
Bowl4thAxis.png
Bowl4thAxis.png (9.78 KiB) Viewed 3838 times


The bigger the bowl gets, or the steeper the side gets (e.g. flatter soup bowl vs. hemispherical bowl), the bigger the challenge becomes. You get in a tug-of-war between how long you need to reach, and how small your bit can be. So even at the 2" deep here, you're already looking at a minimum of a 1/4" diameter bit. Yes, you can use a bit with a sharp point, but then you are probably talking about 4-5 hours to complete carving on a cereal bowl sized object.

On a steeper bowl, you'd probably even want to figure out how to build an angled jig for the 4th axis, so that your bit stays more perpendicular to the machined surface.

As you can see, this rapidly turns into an extremely specialized use-case. For best results, you'd want to specialize your machinery for this... it's not something you just "use your CNC for too". Either way, it's going to end up requiring lots of time, money, and resources to make the modifications necessary.
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Re: Post Questions about the "book" CNC Plans here

Postby hexus179 » Tue Aug 19, 2014 12:27 am

Ah I see your point and you're absolutely right. To get around that I'd need yet another axis to tilt the spindle, right?
I'm determined though. Thank you for all the help!
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Re: Post Questions about the "book" CNC Plans here

Postby Awesomeness » Tue Aug 19, 2014 1:26 am

Well, "maybe". Certainly a 5-axis machine would let you make sure that the cutting bit is always perpendicular to the material. However, 5 and 6 axis machines are very expensive, and they add enough complication that it could probably be said they are realistically impossible when built from plywood.

Since the area you want to carve is probably mostly located in a band of the top 1/2 of the bowl (toward the lip), you could probably do well by building a jig that would elevate the 4th axis at like a 45 degree angle, or something. I'm not sure how you'd compensate for that in your software... you certainly aren't going to do it with cheap $150 CAMBAM-like software. You're almost certainly going to have to move up to "pro grade" stuff (e.g. $400 BobCAD, $3000 MasterCAM, etc.).
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Re: Post Questions about the "book" CNC Plans here

Postby maineshops » Tue Aug 19, 2014 10:03 pm

Hexus.... have you considered using a rose engine to do your decorative work on your bowls? they are easy to build and more versatile for what you are doing. i'm a wood turner too and between the two machines i won't live long enough to try all the possible combinations between the two. i tend to use the cnc to do art work on platters and the rose engine to do the round stuff. as Awsom says increasing the shut height on the cnc creates too many problems. i'm a tool and die maker by trade and i built my machine all out of steel and al. i've got close to 4 inches shut height on the z and it is not as ridged as i would like. my winter project will be to try to correct that and other errors that i made. just have fun. Dan
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