May I be Nosey?

[Art Mann]

Keith,

I agree with your assessment that the stiffness of welded steel is greater than bolted together aluminum extrusions of the same size. On the other hand, I have purchased custom made precision robots that were constructed mostly of aluminum extrusions. The positional accuracy of these robots was similar to CNC router specs. I know that aluminum can be engineered to work well. I just don't know if kit CNC routers are engineered that way. I don't have a good way to find out without building one. My expertise is electronics, not mechanics.

[Gary Campbell]

Jerry...
Like Keith says, both truck frames and large building are made to flex. They have both bolted and welded connections that are engineered to provide the flex that will keep those connections from failing as the trucks go down rough roads and the buildings sway from wind and ground vibration. Neither condition applies to a CNC platform. To keep cutting consistent reducing frame flex to the minimum is desirable. Making the frame heavier reduces the amount of vibration that is transferred to the part. These engineering principals have remained unchanged for a long time.
I am sure that you really don't wish to hear this from me. ShopBot has built their machines by design to be light, flexible and easy to move. Nimble in other words. Let me quote Ted Hall from the SB webpage as he explains the ShopBot design decisions.
"From a machining point of view, it is always better to be heavier. Weight provides substance, mass and rigidity. These factors lead to smoother cutting, but these factors also lead to higher costs, because all aspects of the motion system must be beefier. "
"For our full-size tools, the table on which the gantry runs is a bolt-together assembly. It is shipped unassembled to reduce shipping expenses and to allow convenient set-up anywhere in your shop. It would be slightly stiffer if welded as a single component; however, experience has convinced us that it is more practical to have a tool that can be easily readjusted with standard tools if ever knocked out of alignment and whose parts can be readily replaced if ever damaged."
These quoted excerpts are from the ShopBot Tools website that can be read in its entirety here: http://www.shopbottools.com/mProducts/ShopBotdesign.htm
So as you can see, and you should read the entire article if you have not already, a heavier and stiffer, welded frame will yield in better cuts. I disagree with his word "slightly". I would use "substantially". The lighter duty bolted frames lowers machine and shipping costs for their end-users. Surely with the number of units that ShopBot sells and the reputation they have they are not doing it wrong. You should also note that in most paragraphs those decisions are driven by lowering the cost. Those lower costs may have made it possible for most of us to own a CNC machine.
With that said, there are a number of users that desire or require higher quality of cuts from their machines. They purchase costlier heavier welded frames that are milled and motion is provided on linear rails and bearings. I am one of those.
My 3rd ShopBot (sold but in use til the end of the month) was built with a welded frame, 3/4" billet aluminum parts, linear rails, etc. All of the higher cost items that the machines don't come with out of the box. This machine yields extremely higher quality cuts that either of the other 2 I have owned, but it was built for that purpose. I call it the "mythbuster" machine as it dispels all the "bit deflection" and "accumulated deflection" myths that have been passed around by many unknowing users. My bits don't deflect when taking .100 deep passes in 6061 aluminum at 1.5 to 2 ips. With a ShopBot. Welded frame, linear rails, 4 by 4 weighs almost double what a 4 by 8 PRS does. It's simple, a heavier, welded frame yields higher quality cuts at faster speeds. I can prove it and it not brand specific. Buy heavier, higher grade, more expensive parts and the machine cuts better.
"From a machining point of view, it is always better to be heavier" (Ted Hall) I rest my case.

[bobby milam]

Keith,

I agree with your assessment that the stiffness of welded steel is greater than bolted together aluminum extrusions of the same size. On the other hand, I have purchased custom made precision robots that were constructed mostly of aluminum extrusions. The positional accuracy of these robots was similar to CNC router specs. I know that aluminum can be engineered to work well. I just don't know if kit CNC routers are engineered that way. I don't have a good way to find out without building one. My expertise is electronics, not mechanics.

Have you decided what you are going to be doing with it? It seems that this thread has made some think that the aluminum isn't a good machine. I believe that it is. My shark does a good job as long as I use it appropriately. I want a machine to put into heavy use as more of a production line type of work. If it is going to make me money then I want to buy the best that I can possibly get within my budget. Maybe you might be a candidate to go with a smaller machine to start with, see if you are satisfied and even need a larger table. If so then either build onto it or sell it and upgrade once it has made you the money necessary to get one of your other machines.

That is what I am doing right now. when I upgrade, it is going to be because my shark has made the money to purchase the new machine. If I can't make enough money off of it to do it, I probably don't need to invest more money into this venture.

[Art Mann]

Maybe you are right about buying a "learning" machine. I honestly don't know where this is going to lead. I can say I am not looking for a full time job using it. If I do buy a learning machine, I think the kit built alternative is a good one. For the price of a Shark Pro Plus HD 2, or a Shopbot desktop, I can build a machine with much greater capacity, beefier construction and faster advertised cutting speeds. My head is just spinning from all the good (but sometimes conflicting) information! I can say that, due to the sale of some unused recreational vehicles, I now have sufficient funds and plan to buy something soon. I gotta see Camaster first, even if I decide it is too expensive as a start, because I am so close.

[bobby milam]

I think you are on the right track. I'd buy a kit before the shark also. Can get more for the money and accessories like the touch pad can be made by yourself instead of $90. I wish I had taken time in investigating options instead of jumping in blindly.

[Roy Nielsen]

It seems that this thread has made some think that the aluminum isn't a good machine.

Good thing nobody has thus far brought up how well carefully built wood frame Solsylva machines can do, to include light aluminum work. If interested, watch Alex Crane's Youtube videos cutting guitar body from wood and an aluminum part, plus others, as examples.

Art,

I still think you have some research to do. Again, I highly recommend spending some days on the CNCZone DIY build forums to get a better feel for the technology. It is good that at this point you realize you don't know everything and are asking the right questions and weighing against what you realistically think you'd like to accomplish. I've seen a lot of build logs started by people who think they know way more than they do; usually those never get beyond the design stage.

I firmly believe that the CNCRP 24x48 is an excellent choice and one that you could inexpensively and easily grow to a "full-size" machine yourself. You may want to investigate the recommendation for NEMA34 stepper motors versus much less expensive NEMA23 setup--looks like ~$1K difference. If you do determine you truly need and want the NEMA34s, then you may want to also investigate using servo motors rather than steppers for improved reliability. I've seen a lot of discussions about 34s being slower than well chosen 23s at the same gearing/resolution, something you would better understand than I given your background.

My machine's gantry is probably about 1/2 to 1/3 the weight of the CNCRP's, but I can reliably rapid at 840 IPM while temporarily using lower capacity resistors in line with the steppers (I did hit 1200 IPM while tuning my settings, but it was just too much for a mid-size machine and it was getting some errors/locks--the limiting factor, I believe, was the controlling computer, a dual-core Atom CPU). For reference, I'm using dual CNCRP rack and pinions for my X direction with 381 oz-in steppers, 48V power supply, essentially the same stepper drivers as used in the Gecko G540 box, and 2.5K Ohm resistors (versus 3.5K optimal--trying to keep heat down until I can properly heat sink the drivers). Rapids speeds are nice to position the router (and brag about), but probably won't make a whole lot of difference on artistic/intricate/small jobs where the head doesn't move far "in the air" during a particular job; different story for large jobs where the head has to reposition long distances over large material. The cutting speeds I choose are much slower, I'm not sure I've ever set a job to run at over 150-200 IPM for "generic" straight cuts mainly due to the size of my jobs, experience level, and not wanting to break bits. Acceleration is usually much more a contributing factor for smaller/artistic/intricate jobs than IPM; for instance, when cutting lithophanes, I've never seen the IPMs go over 100 and usually are under 50 during the job due to the constant up/down Z movement that causes the X or Y to need to slow down.

Roy

[bobby milam]

I almost mentioned wooden machines in my post but the way this thread has gotten I felt that it would be like throwing gasoline on the fire so I left well enough alone. Now you are responsible for the next 20 pages arguing the merit of wood. LOL

[Roy Nielsen]

I also like to stir up bee nests from time to time. And one time I tugged on Superman's cape.

[Keith Outten]

If you stay within the operating parameters of your machine as it was designed your aluminum, wood or plastic frame will be just fine. The problem is that after awhile many people will try to machine a project that is way outside of their machine specifications.

Although you can haul two tons on a half ton pickup truck it is unwise to do so, the same type of situation happens all to often with CNC machines. If you know upfront that heavy machining jobs are in your future buy a machine to handle big jobs. Shopping for a machine based on price doesn't make sense whether you are a professional or a hobby woodworker.

[Gene Crain]

shopbot pr

[Jerome Stanek]

shopbot pr

And who says a bolt together unit can't do the job. Nice work Gene. Wish I didn't sell my PR after the upgrade it was so accurate and the resolution was incredible.

[james mcgrew]

Hummm Let me see,, heavy stable frame, High resale value, Massive forum and owner support, Stable long term controller, non proprietary parts, yea they got that,, oh, about the frame thingy !! well being as happy as I am with my Harley Davidsons and my John Deere tractor I am stickin with the IRON !! and as always Good Job Gene !! IMG_4777.jpgIMG_7898.jpgIMG_4230.jpgDSC02261.jpgDSC02258.jpg this X3 sold at 5 years old for 85% of original value !!DSC00768.jpg I guess I should mention the 24 or so owners who came to camaster after owning the "Other Machine" yet 0 (Zero) going the other way !!

[Jerome Stanek]

What model John Deere Tractor do you have I also have one

[Ross Moshinsky]

Hummm Let me see,, heavy stable frame, High resale value, Massive forum and owner support, Stable long term controller, non proprietary parts, yea they got that,, oh, about the frame thingy !! well being as happy as I am with my Harley Davidsons and my John Deere tractor I am stickin with the IRON !! and as always Good Job Gene !! IMG_4777.jpgIMG_7898.jpgIMG_4230.jpgDSC02261.jpgDSC02258.jpg this X3 sold at 5 years old for 85% of original value !!DSC00768.jpg I guess I should mention the 24 or so owners who came to camaster after owning the "Other Machine" yet 0 (Zero) going the other way !!

That's all lovely but I couldn't get that machine through my door and I don't think I could make much money with a machine in the crate sitting in a parking space, could I?

People have different wants/needs. They have different budgets. They are able to take different levels of financial risk.

[james mcgrew]

Well aware of different levels of need and risk,, this Stinger we purchased to make sink holes for the Amazon dot com distribution facilities, each had 12 RR with 12 sinks each plus each had a trash hole. 288 precision cuts,, this machine took the "Sting" out of cost and wel paid for it self on the 2 facilities (578 holes at 30.00 a pop !) and it will fit thru a door !