Need help with statrting a new build

[Bob Cole]

This is my first attempt at starting a build log: any help in getting this thing up and running would be appreciated.

I am about ready to start on a CNC router project. i have drawings that I have started for the main frame assembly, and a rough draft of a gantry system. I have NO cnc experience, and NO experience with CAD drawing. I have been a sign designer and mechanic most of my adult life. I have access to several high end drawing programs,{ie} Illustrator CS2, and FlexiSign PRO. I am very proficient with Illustrator.
I have drawn some full scale plans in Illustrator for my table, and gantry. I am planning on building the majority of the frame work from 1.5" x 1.5" 8020 aluminum. My main frame and the gantry will be truss construction. I have rails and pillow blocks for the "X", and "Y" systems. Can some one here look at my plans, and tell me if I will have any deflection problems with the system as I have it currenty drawn? I am about ready to job out the cutting and milling work to get the componants ready for assembly. I would greatly appreciate if someone could review my ideas, and point out anything that I may be over looking. Most of the dimentions are on the drawings, but Since I drew them full size I don't know how to post them on this site, or if they need to be reduced for viewing here?

Thanks. Bob

[Halfnutz]

I cant see how your going to run the main lead screw, usually the sides are unsupported so that the gantry bottom truss can move the whole length, and the lead screw is attached to the center of it. If you have the sides sitting on the base you cant run that cross member underneath as most gantry routers do. They support only the two ends of the router. Look atr K2's website for examples of classic gantry router design. When I built mine I basically copied their mid size frame and it worked great.

[Bob Cole]

I have considered what you are talking about. "usually the sides are unsupported so that the gantry bottom truss can move the whole length," But I am not sure the "X" main frame will be strong enough to support its' self without center legs. The Gantry I am planning will be run with rack and pinion on both sides of the frame with two matching stepper motors.

Do you have any imput regarding the "strenghth" of my proposed main rails. I am estimating that the finished gantry with router, motors, and rails will not be over 250lbs.

[ger21]

You might want to try using something like Beamboy to calculate deflection.
http://www.geocities.com/richgetze/

Just a guess, but I don't think the 1-1/2" extrusion will be nearly stiff enough for a machine that big.

[Bob Cole]

Gerry:
Thanks for the web site.
Did you note that the side frame rails will be a truss design? 1.5" +1.5"+4" spreader,
all will be checked for trueness, and square, then screwed and epoxied together to make what I hope will be a Very strong member.

[jeffs555]

I dont think those vertical 1.5x1.5 spreaders will do much to increase the stiffness. Angled spreaders to make triangles between the two rails would be better. Much better would be a solid web to make an I-beam. A lot depends on how much deflection is acceptable to you. Even with a 4"x0.5" aluminum web to make a 7 inch I-beam, deflection in the center could be 1/10 inch or more.

8020 has a deflection calculator, but I haven't used it. Don't know if it just handles the extrusions, or if it will calculate compound structures built from the extrusions. http://www.8020.net/Design-Tools-26.asp

[Bob Cole]

Well guys,
I have been in touch with the folks from 8020, and they did a deflection projection for me. According to their initial calculations if I follow their logic by placing my 4" risers at 10" O/C between my top & bottom stringers. I should end up with less than .004 deflection with a 250# load centered on my 128" main rail. That looks good enough for me. No center leg supports required, 122" free span side rails WOOHOO, I'm a happy camper.

Now on with the build. Tonight I will draw up my "Y" gantry system.

If any of you find a problem as I go along I would GREATLY appreciate you input. I don't want to get into this and find out I overlooked something 5 or 6 steps back. I definately DO NOT want to reverse engineer this build.

[Bob Cole]

Well Folks here is the drawing for my gantry. I still have to add the "trucks" for moving this thing up and down the "X" rails, but they are on the shelf, ready to go.

I am also posting an updated drawing of the main table {less legs} for your review.


Thanks,
Bob C.

[Halfnutz]

Sounds like youve got it pretty well figured out. Toothed belt drive is a good idea. Make sure you calculate speeds and consider whipping on that 64 inch ballscrew. It needs to be pretty large to handle any really fast spinning. Large diameter screws cost a lot at that length!

[Bob Cole]

Halfnutz;
The ball screw is 3/4" Bosch Rexroth w/matching drive nut. I picked it up brand new in the original packaging at a surplus outlet.

Do you think that using a toothed belt drive that I could drive the "X" axis from just one side?

[Halfnutz]

Bob, the 3/4" ballscrew should be fine unless you are really trying to move very fast, it depends exactly on what the pitch is, but 3/4" is fine if youve allready got it.

Unfortunately I've been told not to try driving just one side of the gantry. Its tempting I know, I thought about it myself, but there is a chance of racking, especially under high loads, so the bottom line is avoid it if you can.

Check out this thread, it has a lot of good info in it:
http://www.cnczone.com/forums/showth...ight=ballscrew

[jeffs555]

I have been in touch with the folks from 8020, and they did a deflection projection for me. According to their initial calculations if I follow their logic by placing my 4" risers at 10" O/C between my top & bottom stringers. I should end up with less than .004 deflection with a 250# load centered on my 128" main rail. That looks good enough for me. No center leg supports required, 122" free span side rails WOOHOO, I'm a happy camper.

I would seriously question their calculations. According to beamboy, a solid aluminum bar 128"x7"x1.5" will deflect 0.01 inches with just a 100 lb load in the center. A structure made of aluminum extrusions will deflect more than a solid aluminum bar, and probably a lot more.