Is cost, ease of build, routable area, etc. most important?

Design of structural and load bearing framework.

Which objective is most important?

Is as inexpensive as possible to construct.
3
21%
Can be constructed using only common woodworking power tools, such as a circular saw / table saw, and drill / drill press.
3
21%
Will minimize or eliminate the need for obscure, uncommon tools or bits, that likely must be purchased simply to complete the build.
0
No votes
Has finite dimensions, allowing the machine's structural components to be cut ahead of time or offsite, and packaged as a "kit".
0
No votes
Has the capability to cut most, or all, of the pieces needed to duplicate the machine ("Self-Replication").
2
14%
Has clear, and concise documentation, allowing novices to successfully construct it with ease.
4
29%
Includes information about quality low-cost sources for parts and materials, as well as an accurate estimate for the cost to build it.
1
7%
Is supported by the community, who can provide experience, wisdom, and advice for new DIY CNC'ers.
1
7%
 
Total votes: 14

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Awesomeness
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Re: Is cost, ease of build, routable area, etc. most importa

Postby Awesomeness » Thu Dec 09, 2010 7:28 pm

Here's how I understand these terms...
Precision: How fine of movements the machine can make. A machine that can move 0.001" is more precise than a machine that can only move in 0.005" increments.
Accuracy: How true to real measurements the machine's cut in theoretical measurements is. Does a 5" CAD drawing cut out a piece of material exactly 5.000" long?
Repeatability: The ability to go to a known point, go somewhere else, and return to the exact same point. When the machine cuts multiple passes do they all line up well?

My first machine, using skate bearings and threaded rod was extremely precise, probably to just a couple thousandths of an inch. I have heard claims that threaded rod is not as accurate as some other things like ball screw, in that it may be off a little in turns per inch. You should be able to easily correct for that, if it's off by a consistent amount (e.g. 13.001 TPI instead of 13.000 TPI). My machine was also very repeatable, since despite the possibility of inaccuracy, the inaccuracy was always in the same places in the mechanisms, so it always cut "similarly wrong".

I honestly don't feel like I got any useful increase in accuracy out of my bigger machine, with chain drive and v-groove bearings. With my small threaded-rod machine, I was able to do 3D carvings, and cut out circles and gears that were only out-of-round by a couple thousandths of an inch.

What has your experience been?

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Awesomeness
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Re: Is cost, ease of build, routable area, etc. most importa

Postby Awesomeness » Thu Dec 09, 2010 7:35 pm


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airnocker
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Re: Is cost, ease of build, routable area, etc. most importa

Postby airnocker » Thu Dec 09, 2010 9:15 pm

I only tried the 3/8" x 18 threaded rod on the Z-axis using back-to-back brass nuts for an anti-backlash solution. Part of the problem was me, and the way I wanted my implementation done which resulted in the nuts being inaccessible after the Z-box was mounted on the Y-gantry. This made tweaking the nuts a major hassle since it involved disassembly of stuff. It also required upper and lower bearings for the threaded rod and very good alignment of same, (and here is an example where .01" accuracy of bearing locations in wood is "good enough" just doesn't hold much water for me, nice spreading wings not with standing.)

It would have been a lot easier if I offset the threaded rod to the side, but I wanted in running down a hole in the back wall of the Z box, which then would have reduced my overall Y-axis travel and I didn't want that. But the $19 it cost me for the ACME lead screw and the $14 for the nut was easy to digest as a tall-end expense.
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Re: Is cost, ease of build, routable area, etc. most importa

Postby beermkr » Thu Dec 09, 2010 9:35 pm

Mike Pensinger
Chief Brewer, The River Company Brewery, Radford, VA

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Awesomeness
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Re: Is cost, ease of build, routable area, etc. most importa

Postby Awesomeness » Thu Dec 09, 2010 10:07 pm


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airnocker
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Re: Is cost, ease of build, routable area, etc. most importa

Postby airnocker » Thu Dec 09, 2010 10:47 pm

Lead screw antibacklash nuts are specific to lead screws and provide two purposes, one as the lead screw nut and the other to eliminate antibacklash as a factor during forward and reverse screw rotation. For threaded rod one must do implement a similar method work-around. One might use a threaded rod lock nut with a nylon insert, or use two back-to-back nuts tightened against each other to a point where there is no end play during forward or reverse rotation of the rod. Mach3 has settings for addressing backlash but it again is something that has to be tweaked and rechecked along with the threaded rod nuts periodically since they are going to wear due to friction serving this function. The reason I used brass nuts, 1) I had them and 2) less friction.
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servant74
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Re: Is cost, ease of build, routable area, etc. most importa

Postby servant74 » Fri Dec 10, 2010 5:09 am

As airnocker pointed out that I did use some generalities, and some ambiguous statements. Things like wood to only .01" ... I was considering that most wood workers can't deal with measurements that fine, and even at that things like sanding, glue, and finish cover up such fine detail even if it is done. Much furniture has built in 'wiggle room' for wood to move since it is expands and contracts with humidity and temperature. One of the claims to fame of MDF is that it 'doesn't move'. Well it does, but it is small, and it is does it in all dimensions. Normal 'grained wood' contracts and expands differently if you are measuring cross grain vs. with the grain. And if you are making items of say the size of a kitchen cabinet door, the note of .01" is much finer than the wood may move. ... Right now we have a door that has moved a quarter inch over a span of under 2 feet (ok, I haven't maintained it well, but that is another story... the point is, it CAN and will move given the chance).

Yes, precision, accuracy, and repeatability are all important. But we aren't talking that accurate if we are making the tools out of wood products. I wouldn't use it to make gears for a fine watch and expect the same results as making it on a 'professional quality' mill in a climate controlled atmosphere. Most of us will use our rig in a garage, some are lucky enough to have a heated and cooled shop, but that is the minority of hobbyists.

I'm still working (slowly) on putting together a book machine in my 'garage', and trying to figure out how to get a workshop set up too. But that is what happens when there isn't enough space to do what you 'want'.

One of the points I tried to make is that to be very useful not every machines has to be 'the ultimate', as much as most of us would want that, to be very useful. Making tools that work within the tolerances needed for the application is more important to most than making 'the ultimate'.

One of the quotes I think I remember from buildyourcnc.com was 'build ugly to build pretty'. I would think if you can get the .01" tolerance if cutting MDF on an initial machine, could make it pretty easy to make a machine that is more precise in its dimensions than at least I can, cutting things 'by hand'. So one of my desires is to get 'something working' so I can re-make it to whatever the next stage of 'right' is :)

On lead screws, I have seen video's of them whipping under load, so making sure they are held properly over much of their length is unsupported, like when the gantry may be near one end of the length, and it being held with just a bearing at the other. That has to do with the load, step frequency, diameter of the lead screw, etc. I wish I understood all the variables, but this is just by observation. That is why I think that for at least 'longer' runs, that either chains or rack and pinion would be better.

I have found some comparatively inexpensive ACME threaded rod and nuts in a catalog from Enco ( http://www.use-enco.com/CGI/INSRAR?PMSECT=0000000451 ). I would suggest to use larger diameter threaded rod for longer runs, just to keep down the 'whipping' issue.

At one time I saw a anti-backlash nut made using two nuts and a spring. Both nuts were mounted on the all-thread with a spring between them. One nut was fastened to the frame to be moved, and the other was allowed to 'float', but not turn. This way the spring took up slack, and kept the nuts pressed against the threads. The point is, even anti-backlash nuts can be DIY with some time, patience, and ingenuity.

Using ball lead screws on is interesting. It does reduce the friction on the screws and lets it move more freely, while keeping backlash down. If we are focusing on the .01" of error or so, the expense isn't really needed but it wouldn't make your rig work smoother and need less torque from your motors, IMHO.

I too, like beermkr, am planning on using EMC2. Mach3 is a great package, but since I am comfortable with Linux and want to keep my costs down. There are quite a few users that are not 'linux geeks' that use EMC2, and the open source community help a lot and supports it well. EMC2 is not just a hobbyist package, it is used in serious production work on large machines daily. There is a downloadable 'live cd' version of EMC2 available at LinuxCNC.org if someone is interested in just trying it out. Documentation is available there too if someone has the interest.

In many ways I think I see LOTS of potential in having another public design. I was saddened when MechMate.com went 'closed' with their plans. (Plans cost $100 currently, they can change in the future again, but I doubt it...), so I am excited about seeing another robust design (more than one?) available.

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beermkr
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Re: Is cost, ease of build, routable area, etc. most importa

Postby beermkr » Fri Dec 10, 2010 1:41 pm

Okay, The thread is becoming a little muddled but I have to qualify my "Airnocker is a genius" statement:

Some tools:

Transfer Punch set (Harbor Freight - Cheap)
IMG00283.jpg


Dowel Guide
IMG00284.jpg


Angle clamped in using a bock of wood to support it:
IMG00285.jpg


Resulting punch mark using the correct transfer punch to mark the aluminum:
IMG00287.jpg


Bearing set on the angle and centered on the punch mark
IMG00289.jpg


Now the dowel guide is a multi-tasker and makes the cost a little more justified. The only issue is that the HF version will not work for this as it is an auto centering and we need the mark to be off center in this application. The only other issue is that you need to put an extra inch or so into the jig to hold it correctly. This means you will have to measure and trim the ends off after you get the holes all drilled and tapped.
Mike Pensinger
Chief Brewer, The River Company Brewery, Radford, VA

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airnocker
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Re: Is cost, ease of build, routable area, etc. most importa

Postby airnocker » Fri Dec 10, 2010 5:44 pm

Now that's what I'm talkin' about. Nice Mike.

Doing an adaptation to the dowel centering jig so that the drill hole is always referenced to the end of the angle, say .5" from the end and .25" from the inside corner of the angle is what I'm envisioning. The .25" dimension assumes skate bearing diameter of .8655".

The drilled holes are then threaded for the 5/16"-18 bolt. The jig could also be used to keep the thread tap perpendicular during threading. With this configuration you could take any length of angle and accurately drill the four holes on each end with no hassle and no drill press.

I'm working on some drawings for the centering jig now. Will post when done.
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Re: Is cost, ease of build, routable area, etc. most importa

Postby servant74 » Sat Dec 11, 2010 2:29 am



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