just need an honest opinion.

[SatanKlawz]

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

[dertsap]

.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

[SatanKlawz]

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.

[ViperTX]

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....

[joecnc2006]

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

[ger21]

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.

[joecnc2006]

Originally Posted by ger21 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.

[SatanKlawz]

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.

[jeffs555]

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.

[NC Cams]

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.

[SatanKlawz]

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?

[lerman]

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