I am building my cnc router and am just wondering if what I have will do the job that I want. I have the xylotek 3 axis kit, and have a table that is roughly 60" x 30". I will be using the table almost exclusively to cut acrylic in the .25" to .375" range. The table uses a belt system on the X axis (60") that does not use any sort of reduction in the belt system. The rails are 20mm hard steel with dual pillow block on each rail. the y and x axis are both 12mm hard steel with flanged bearings for the guides. The tables frame is all 2x4 wood. All is driven by 1/2 x 10 acme screw. The spindle will be a porter cable trim router. Will this table have the strength to handle a full size router spindle? IPM is not a concern as long as it does the job. I am more concerned about accuracy rather than speed but what do you think a reasonable IPM will be?

Thanks

.001 resolution would be fine , lots of guys are going to jump all over me saying this but you need to concider what you are cutting , and the machine that you are cutting with , with a higher resolution you are probably going to get some noise when arcing or what have you but its only noise , look at the shopbot which is quite costly , they have the speed by sacrificing resolutions and repetability , my last setup had a resolution of .0008 at full steps and ran quite well ,accuracy wasn t an issue ,it did what i wanted it to

post some pics it sounds like your on the right track

Right now it is just a big pile of parts but I have everything that is needed to get most of the way done with it. I lack a few small things that I will need later in the build and am just waiting to see what else I need before I make my last order.

Well....unsupported shafts over 60 inches.....accuracy will not be that great.....calculate the sag of the shaft.....you may have the resoulution.....but the rest of the system will be a challenge....

Just an idea but how about switching the gantry movement to the 30" to reduce the sag and the 60" to the Y axis of the gantry top, that way the Z axis will not have much weight to make it sag over 60".

Joe

The 30" 12mm shafts will flex like a noodle. Another member made a similar machine and posted a video showing it. I just shecked, and yhe video is gone, but it moved a LOT with only light finger pressure.

The 30" 12mm shafts will flex like a noodle. Another member made a similar machine and posted a video showing it. I just shecked, and yhe video is gone, but it moved a LOT with only light finger pressure.

well if i switched it i would not use 12mm shafts on the 30" i would use large on both axis. 30mm or larger.

this is not tubing it is solid hard steel shafts. the 60" do not sag any when supported from the ends. I can change the x and y axis to 16mm if I need to. I would like to see that video if anyone can find it.

the 60" do not sag any when supported from the ends
That is not entirely true. Any shaft supported only at the ends, no matter how large it is, will sag some just from its own weight. It is just a matter of how much. In the case of a 60 inch long 20mm shaft, the sag would be a little over .04 inches just from its own weight. Even if you could live with this much sag, that is not the major problem. The main problem is the dynamic cutting loads. That shaft would deflect nearly 1/10 of an inch for each 12 lbs of load. Like Gerry said, "it would flex like a noodle". Any cuts would be extremely jagged from all the vibration.

SWAG's work SOMETIMES but hardcore math/engineering ultimately rules.

Take shaft flexing or "droop" resistance.

Essentially the stiffness of the shaft is a function of the 4th power of the diameter. Thus a 1.5" diamter shaft will be 81 TIMES stiffer than a 0.5" dia shaft:

(1.5*1.5*1.5*15)/(0.5*0.5*0.5*0.5)

Thus if I had a choice of tubing versus shafting, tubing is the clear winner for STIFFNESS as in "droop resistance" - the larger the diameter the better.

Packaging is a more esthetic issue that may or may not be as calcuateable (sic) but that's a personal choice.

Granted, this is an over simplification as wall thickness comes into play too but, regardless, the OD is clearly the more critical variable that needs to be considered when it comes to STIFFNESS.

Figure your loads, do some math (check out beam section of Machinery Handbook) and you should be able to figure out something that is rigid, affordable and one that passes the "reasonableness" test.

I am pretty sure that I can find an anwser to the issue here cheaply. The pillow blocks that I have are open eneded for a supported shaft. However the shafts that I have are not drilled to accept supports. I think that if I can add at least one support in the middle that this will be enough to reduce the flex down to a reasonable ammount. 20mm rails that only have to stretch a 30" span between supports sounds fairly strong. This would require me to drill and tap the rails at least in one spot. Does anyone know of a support kit for rails like this that are not tapped? or a DIY on how to do it?

google for "beamboy" and down load it. A freeware beam analysis tool that will tell you how much a beam of a given size, shape, material,... will deflect for a given load.

It's easy to use and the price is right. I find that it is NOT intuitive how much deflection you will get for a given beam. Beamboy solves that problem very nicely.

Ken

you are right, I setup the rails today and put a little pressure on them and they do bow a bit. I am going to have to stiffen up the 60" rails for sure with at least 1 or 2 braces between the ends. I think that I can drill and tap the rails fairly easily. and then just run a bolt with a spacer between the rail and the mounting surface, that should help support it and help lower deflection some. Enough to make me satisfied with it anyways..

I am going to have this mount made and have it welded to the center of the rail. This combined with the end supports should keep it pretty steady. 20mm rails with a 26" unsupported span should be fine and keep the accuracy up there.

Do you have drawings or layout of the machine now?

Joe

I do not, I could draw some up in the cad but that would take forever... I may start to work on it tomorrow but dont look for a finished drawing for about a week. I can not spend too much time in one sitting working on it.

I am going to have this mount made and have it welded to the center of the rail.

I would definately not weld a support to precision shafting. After welding, there is no way it will be precision any more. There is virtually no way to weld it without warping it. Much better to drill and tap it.

that is what I was afraid of.... warping that is. Do you think that a 20mm rail would warp much even with small welds (about 1" long) in 4 places on the bracket? This rail will never be used for anything else besides this machine so I am not worried about rendering it useless for anyting else.

Re: welding distortion

It is not a question of IF it will distort, but rather a question of HOW MUCH it will distort when/if you weld it.

It can distort 0.0009" or 0.0090" per weld. 4 welds could turn it into a pretzel or a rippled mess - all depends.

You can NOT stop weld distortion but you can at best "manage" it with proper pre heating, weld procedure and post weld stress relieving.

Using the proper procedures is the only way to "keep the accuracy up there", a rather vague and undefined spec but perhaps adequate for the application.

I do not, I could draw some up in the cad but that would take forever... I may start to work on it tomorrow but dont look for a finished drawing for about a week. I can not spend too much time in one sitting working on it.
No problem, just thought if you had some drawings people can look at it and see how you will construct it and possibly sugest how to suport the rails for ideas.

thanks for the comments. I think that drilling and tapping would be a better way to go after I read what you guys have to say. The main rails are 60" long with supports on each end and there is exactly 21mm of gap between the rail and the piece below it. I am really needing to strengthen this up a bit and am having trouble figuring out how to do it.