I had a question about the X rail placement. Is there an exact measurement for this distance or is it the same distance as the thickness of the rack? How much clearance should there be between the vertical section on the main X beam and the rack?

JohnAttachment 829

There is clearance between the rack and the beam, about 5mm [3/16"] very roughly speaking. The metric rack is 15mm wide, while the inch rack is 1/2". The rack is flush with the outside of the rail.

The clearance is left there for the guys who don't have perfect beams after welding. Right now I can't remember the exact amount, but it can not be one specific number - depends on other tolerances and rack width.

Thanks Gerald.

My table measured out to plan. I have 23/32" space - evenly on both sides.

Gerald,
What is the best way to quickly and accurately check for straightness of your beams when you get them? :confused: And how much off of perfect is acceptable?

Sighting along them with the eye is good enough. Final straightness in the rails is by shims, you just want to keep shimming to a minimum.

Gerald,

I have this question that I have kicked around with JR on multiple occasions.

Do you see any disadvantage to turning the main beams the other way? That way, sawdust falls right down and the hollow outside could store bits, clamps, shuttle pro and the like. Not that the storage is that important but it would me "nice".

I didn't want to do it when the time comes without asking you if it was stupid for some reason I didn't consider.

Thoughts?

Pointing the beam out, and used for storage, could become messy. It can't be considered as clean storage, because sawdust does collect there. Table vibration could cause the goods to move and foul a passing X-motor. The table will always look cleaner with the dust collecting ledge pointing inwards.

The only reason for which I would turn the beams out, would be to run the x-motors inside the beam, inboard of belt-drives, reducing the overall width of the machine. That's one for JR to think about :), but it won't work for the other axes :(

Other advantages of pointing the beams in:
- oversized/odd-sized boards/signs can protrude into that space.
- wide, hinging-open dust feet can protrude into that space
- clamping the rails with C/G-clamps during alignment is a lot easier

New to CNC and I have a question about the steel
 

Hello all,
First I would like to say that I am new to this forum and I am very excited about getting started. I have been reading and reading and reading ( and subsequently getting more excited). I talked with Joe from Alabama and will probably start by getting the laser cut parts from him. However after looking through the parts I started making a shopping list of the other parts that I would need for the frame. And so I called around to some steel mills to get quotes. The problem that I am having is with the x main beam. After converting from 21kg/m (plan 10 10 322) I came up with something roughly 31 lbs/ft, and the steel mills had nothing even close about 9-11 lbs/ft. Am I looking at this wrong? I am sorry if this is a real stupid question I am a complete amateur when it comes to steel, so this is going to be a real challenge but one I am excited to tackle.

Let me clarify
 

I want to clarify that I am NOT trying to go cheap, I want my build to be as close to Gerald's (structurally) as possible. I just want to make sure that my calculations are correct when going from kg/m to lbs/ft. I used and online calculator and that is where I got the roughly 31lbs/ft. I also noticed in another thread that someone mentioned that it was roughly 15lbs/ft (about half of what my calc) which is why I am confused. Also the steel the I was inquiring about was 7" x 2" which fell in the range listed in the drawings, I might have to go taller and wider to get the appropriate gauge which is fine.
Again thank you for looking.

1 kg = 2.2 lbs
1 meter = 3.208 ft
1 kg/m = 2.2/3.2808 = 0.670 lb/ft
21 kg/m = 21 x 0.67 = 14 lb/ft

A 7" x 2" beam at 11 lbs/ft will be fine.

And of course; Welcome Derek! :)

My Apologies
 

I am sorry for repeating something that was already stated (and for posting it in the wrong section). The sad thing is I remember reading all this early on and must have forgotten it or had it pushed out with all the other reading. Hopefully I will not do it again. I will have many posts as I get closer to ordering my parts. :D And thank you very much Gerald for all the hard work that you have put into this.

Derek,

I know I drove Sean crazy with all the questions I asked him. We want to be sure we get the right stuff because we want our MM to be the best we can build.

Any Issues
 

Gerald, I've just about completed the table assembly and am getting ready to mount the X rails and rack. I'm going the optional route by using aluminum angle with purchased V track. My question is this: since my overall table size is 5x12, is there a possibility that the weight of the gantry will cause the aluminum angle to deform? My concern is that since the entire weight of the gantry rests on the 4 V rollers, and the V track( and associated vertical leg of the aluminum X rail) overhang the main beams with no support under the vertical leg of the aluminum angle, that it might deform.
I am not so worried about the Y rails, since the weight of the car isn't as much. Am I missing something in the drawings that provide support, or am I just worrying too much.....

Steve

The overhang is only about 5/8" to the center of the V and the alu is 1/4" thick. That is actually massively stiff (the ratio, length to thickness, is 2.5)

If you are loosing sleep over it, put the vertical screws (that hold the rail down on the beam) closer to each other, to prevent twisting between the screws.

For more peace of mind, space those screws so that both gantry wheels are not in the middle of "spaces" at the same time.

Main Beams are bowed out.
 

Next problem to overcome. As I sight down the main X beams(14 feet) they are bowed outwards. haven't strung out a sight string yet, so I'm not sure exactly how much of a bow I've got, but by my seasoned eye, it looks like it's probably close to an inch of bow. I've read posts where welding was the preferred solution to pull the bow back in line. Can anyone elaborate more on this? I've never tried this technique. My welder is a Hobart handler 210, hopefully that will get hot enough to do this. Oh yea, I haven't welded on the legs or cross beams yet (actually I plan to bolt on the cross beams). Don't know if this will make a difference or not.

SteveE...

Steve, correcting 1" of bow is not going to be easy. Are you sure it is that much?

Bow
 

I'll rig up a sight line and snap a picture of it later this afternoon. Let's hope my eyesight is bad....

If the beams are bowing out, that means you have to shrink (shorten) the flanges pointing to the right side of the sketches below:

Here's a snapshot of the bow. Sorry that the focus isn't better


I should mention that the string is on the opened side of the C channel.

The bow looks to be about 1/2 to 5/8 at the middle.


SteveE

Okay, that bow is opposite to what I first thought. 5/8" over 14ft sounds reasonable. Will make another sketch when I get home from the commute that is starting in 5 minutes.

(That beam is not supposed to be drilled & tapped yet - we can't promise to get all the holes in a straight line ;))

Double drat!!
 

I have a tendency to jump in before I check the water a lot of times!!!!

One of these days, I'm going to learn to write down all the steps needed in project in the order they need to be performed in, and then follow it!

SteveE.

This will make it curve the other way:



Don't worry about the holes at this stage. You could enlarge them later and put washers & nuts below. (If the oversized holes in the rails don't give enough tolerance)

The bolted/welded cross-supports will also help tremendously to hold the beams straight.

Bow
 

Gerald, last couple of questions before I fire up the welder.

1) Is it better to start in the middle of the beam and work my way out to the ends, or does it not make a difference?

2) After all is straight, true and cooled down, can I go back and grind the welds down smooth. Or will that just let the beam bow again?


Thanks for the input and help!!!

SteveE

It doesn't matter where you start, but you see results quicker if you start in the middle. Grinding the welds will relax them very slightly and re-introduce a little of the original bow.

Double drat!!!
 

Just a note for those that are in the planning stages and have not yet purchased steel. When you find a supplier of the large C channel, be sure to check for the squareness of the channel before purchasing. I purchased 2 20' channels to get the 2 14' rails I needed. After all welding and drilling I discovered that one of the channels was not even close to square(flanges were not parallel to each other nor were the flanges perpendicular to the face of the channel), while the other was perfect. I contacted my supplier and found out that they normally purchase from American Steel, but just prior to my delivery they got in a shippment from an import supplier. So my order was mixed. So let my woe's be a word of warning........ By the way, it was the import steel that was way out of square...... On a good note though, my supplier is replacing the bad channel for free, just my time and effort lost....

SteveE

I was researching steel channel online. I ran across this one site that listed the channel size like this: (7" x 12.25#). I'm assuming that 7" is the height. What is the 12.25#?

it refers to the weight of the beams. 12.25lb (#=pound) per foot. I think the drawings state ~21kg/m which is ~ 14.1lb/ft. I don't think the couple of pound variation will make much difference...

@ Steve - Your problem totally answered my question about purchasing really long beams like this. I was assuming that the longer the beam the greater the chance it would be out of whack. How do you check out he material before buying?!?! I was going to purchase what I need online or maybe call them up over the phone and explain to them the importance of straightness with my order.

I found a place discountsteel.com where I will probably purchase my material. I can buy pretty much exactely what I need size wise. I can buy the main x beams in several sizes. In my situation, two 7" x 12.25# x 11' beams for 268 without shipping and tax.

I actually found out about the 12.25# thing. My cad software gave me the specs for it. I just dont know how all that was derived from 12.25#. About the x rails,... I was thinking about using .375" unequal angle for my rails. The v-groove bearings would have more of a surface to roll on maye lengthening the life of the rails?!?! What do you think?

Oh ok,... thats fine. I'll get the 14.74 if thats what the plans call for. I need this router to be rock solid.

Gerald,
I found one thread where someone used bent up c channel for the table cross bearers and main beam. You didnt seem to have any objections to that at the time. Do you see any downside to doing this? I am pricing some steel for my table and my supplier, who also does my bending for the laser cut parts, said he could bend me up any size I needed cheaper. They are a little slow in work right now so it would be cheap. :)

If the bent channel is ok do you have any recommendations for plate thickness etc?

The main reason I am entertaining this is I am thinking of having a larger height than stock for 3D work and the larger beams add up in weight rather quickly. :)

Heath,
Knowing what I know about cutting 3D stuff. I would recommend using the 8" channel and then building a spacer or step down frame for the extra depth. This would put your cross bearers lower in the z direction.
This has already been done on one MM recently.

For routine work, you can have a "boxed out wooden or mdf" riser in place that if done well, could double as a vacuum plenum.

Just my 2 cents.

Sean

Heath, are you thinking in terms of bending up one wide C-channel instead of bolting together two narrower ones? How tall do you want to go?

(PS. it would help if you could point me that other discussion to refresh my head)

Gerald,
I was thinking of going in the 10 to 12 inch range for the main beam. My bender, who is probably going to supply my steel as well, suggested he could bend me some beams more economically than buying the standard channel. Also I was looking in some tables and for the 12 inch beams they get pretty heavy real fast. The smallest lbs/ft was 20.5.

The thread was Ries in Equador.
http://www.mechmate.com/forums/showt...4&postcount=22

. . . . his beams were a fait accompli, so I decided rather to just keep quiet.

As an absolute minimum, I would pick 6mm thick, but 5/16" or 3/8" would be better.

Few bending companies will bend that accurately over 3m. Their tools are typically worn in the middle and the frame of their bender will flex at the huge tonnage required. Typically, the ends of your channel will be bent 93 degrees while the center part is only bent 87 degrees.

And then you must realise that plate of that thickness will have an outer bend radius of more than 1/2". That means your angle rail will have at least 1/2" less support.

Gerald,
So, I will ask the question. Would it be more accurate in this situation to use a 5/16 or 3/8 HRS flat bar and control your stitch welding and setup?
It's a lot more work, but feasible if the Section modulus/centroid/bending numbers worked out for the new made up section.
You still have the issue of unsupported rail in my picture below, albeit only 3/8" The angle section could be moved inboard, but that brings up the challenge of structural members and angle of the flanges, etc.

Picture below show with 3x3x5/16 angle and 3/8 x 12" HRS flat bar. Overall height as shown is 12-3/4" face to face

Attachment 5308

I would rather start investigating rectangular box tubing (maybe a stacked pair) before trying to weld an angle iron in a straight line.

(The flanges of a channel beam are always thicker than the web)

Also if one goes to a wide channel (tall) then it should start getting vertical ribs.

Welding on angle iron longitudinally is the best recipy to make it cure (violently) What about using I-Beam or H-section? They are nrmally heavier and more stable in shape.
Trim off excess pertrusion under the rail angle using the Skate. Do you think it will work?

Gerald,

I like the idea of rectangular tubing. Since I have problems with my C Channel, I thought it would be another way to go.