# Thread: Fixed Gantry - Trying again

1. If it is not too much bother, would it be possible to run one more calc for me?
- 6 x 6 x 1/4 in Al tube
- Unsupported distance of 5 ft (so 2 ft shorter)

- An extra 1/4 in Al plate running the length of it on the router side (will be used to make a dead flat surface for the rail mount)
- One version of this is to have this Al plate on the bottom of the gantry
- The the other version is to have it on the side of the gantry

- 100 lb load vertical and horizontal load (just for internal consistency if nothing else, and 100 lbs is easy to scale from)

My hand calculation (without the extra 1/4 in plate) is less than 0.001 inches, so I am not sure if I messed up.

I even tried a 4 x 4 x 1/4 inch, and at a 5 ft length, the deflection still looks very low - less than 0.001 inch.

Here are some more numbers for the box section without the plate.
(see attached)
The 4x4x0.25 section is showing a 0.006" deflection.
A 1/4" plate will probably conform to any irregularities in the box beam. It's probably not a solution for achieving flatness.

Chris

2. Hello Chris,

Thank you very much for running those calcs. I realized my math error this morning - when I was calculating the area moment of inertia for the square tubes, I forgot to divide by 12.

You are absolutely right about the 1/4 in Al would just conform to the existing shape, and I didn't explain the purpose or idea very well. The challenges for that part in my mind are:
- How to make a very flat surface for mounting the linear rail (type TBD) ?
- How to make it all adjustable to deal with errors / mismeasurements in being straight / parallel after the build steps

I only have hand tools really, so anything needing serious precision has to be hired out. Some ideas so far:
- Having a shop machine a very flat surface on one face of the square tube or whatever it is all made from. This makes for a flat surface for mounting, but I have not figured out a way to adjust for alignment afterward.

- Pour a layer of self leveling epoxy on one face of the tube to make it very flat. Have a 1/4 in thick piece of Al bar stock twin milled on the faces to make them parallel and a constant thickness. Add some slots for an adjustable position to the gantry beam, and mount the rails to this flat stock.

I will admit that I am worried about the budget for this project, so some of the final approach will come from what I can find a deal on.

3. The x-rails (Glacern supported 20mm) of my machine are bolted to the outer 80/20 extrusions of the table and it was a concern having a flat and aligned mounting surface (even more for the y and z, with a wood surface).

Therefore I set the rails into a thin film of viscous epoxy (actually I used J-B Weld). I had the rail contact surface first covered with silicone grease so that I can break them out in the future if I ever need to. Since the J-B Weld cures quite slowly this left me a few hours to get the rails aligned better 2/1000" parallel and perpendicular by selective clamping or shimming. After curing I drilled and tapped the mounting holes into the substrate (using the rail holes as pilot) so that the bolts don't deform the rails when tightened.

4. Jerry - Thanks for that suggestion. I have seen your build, and I am concerned that the workmanship and precision build approach you have taken is well beyond my current abilities and equipment. It is an interesting approach to consider.

I was watching a movie and doodling on graph paper last night. These images are definitely not engineering drawings, but instead just a roughed out concept needing more thought.

The main difference between it and other "fixed" gantry designs is that the entire Y axis moves up / down for the Z motion. The reason for this, is that I would like to be able to make mortise and tennon joints. Since the entire router will be sitting on the floor, this means that I will be flipping the router on its side (pointing toward the x feed end) to make these shapes.

It is still a concept, so we will see what actually happens as the X axis portion develops.

Edit - The images are too large, so I will scale them down and try to post them later.

• Trying again with the images.

• I could use some advice on bonding square tubes to each other.

I am on a path of using square tube to make the main frame work. The size will either be 3 x 3 or 4 x 4 inch x 1/4 in thick, depending on how the load calculations come out.

I had planned to use Al to minimize weight vs steel, but the total weight is getting high enough that it is less and less likely that I could lift this thing into the back of my mini van anyway.

I don't have welding capability, so I was thinking of using epoxy at the joints. The main question - how do you make a strong L or T joint using square tubing and epoxy ?

I had hoped that the joint itself would be strong enough, but I have doubts, and of course there is the alignment question.

- Bond a square piece of wood to the long beam at the joint the size of the tube ID
- Slip the second pipe onto this wood piece
- Apply epoxy, and screw it together using the wood as a common point.

I guess a block of Al could behave similarly.

Is this a common approach ?

Thanks

Harry

• I would be more inclined to bolt the square tubing together with a combination of 1/4" steel angle and flat bar stock. It's a lot of work to make the parts and drill and tap the holes. It probably wouldn't look very good either. That's why I weld my steel projects.

Epoxy will only hold until it gets a shock load that makes the joint pop apart. A wood insert will be no stiffer than the wood.

CarveOne

• Thanks for the comments. I would have to hire out the welding, but that is viable. I have followed a number of build threads over the past few years, and of course welding has the risk of warping the frame, so that is a consideration of course.

The frames built from T slot / 8020 type materials are commonly build with butt joints and similar T connections, so I was just hoping to find a way to mimick this without the expense of actually using T Slot material.

Perhaps I can have some Al blocks milled that match the inside of a square tubing, and screw and glue those to the frame tubing. Those would act sort of like a socketed joint similar to how bicycle frames are sometimes made, except the socket is on the inside instead of the outside.

I wonder what it would cost to have 20 or so 2 x 3.5 x 3.5 inch Al blocks made to match the inside of 4x4x 1/4 tubing ? Maybe even 1 in thick might do it ?

• Originally Posted by harryn
I wonder what it would cost to have 20 or so 2 x 3.5 x 3.5 inch Al blocks made to match the inside of 4x4x 1/4 tubing ? Maybe even 1 in thick might do it ?
It may still be cheaper to have it welded. The only way to know is to get quotes on both welding and making that many aluminum plugs.

A knowledgeable weldor can build accurate weldments. It's novice weldors or careless weldors that build crooked and warped assemblies. Precision welding takes time and skill, but it's done everyday.

CarveOne

• Last night I started adding up just how many feet of square tube are going to be needed for this project, an approx. cost, and worse yet, the weight.

In rough terms, it will take 10 each 6 ft pieces of 4x4x1/4 square tube, plus rails, motors, drives, etc. Plus the 8x8x1/4 tube.

In steel, this puts the build in the 800 - 1 000 lb range, and in Al, about 1/2 of that. Either one is far to heavy to actually tilt up on its side, or to lift into a minivan without serious effort. The only way this thing is going to be portable, is if I mount wheels on it, and turn it into a trailer.

I was pretty sold on building it with Al, except for the price, then last night I found Al and steel tube for about the same price. ( \$100 - 150 each for 6 ft using onlinemetals.) Even worse, the specs on twist and straightness of hot rolled structural tube are fairly - loose. Unless I can find hand picked pieces, the welding warp might not matter anyway.

My son really wants me to build it for heavy duty use so he can at least try some things with steel, so that might drive it in that direction a bit.

I guess it is time to start thinking about the plywood prototype of this thing.

Harry

• Originally Posted by harryn
Last night I started adding up just how many feet of square tube are going to be needed for this project, an approx. cost, and worse yet, the weight.

In rough terms, it will take 10 each 6 ft pieces of 4x4x1/4 square tube, plus rails, motors, drives, etc. Plus the 8x8x1/4 tube.

In steel, this puts the build in the 800 - 1 000 lb range, and in Al, about 1/2 of that. Either one is far to heavy to actually tilt up on its side, or to lift into a minivan without serious effort. The only way this thing is going to be portable, is if I mount wheels on it, and turn it into a trailer.

I was pretty sold on building it with Al, except for the price, then last night I found Al and steel tube for about the same price. ( \$100 - 150 each for 6 ft using onlinemetals.) Even worse, the specs on twist and straightness of hot rolled structural tube are fairly - loose. Unless I can find hand picked pieces, the welding warp might not matter anyway.

My son really wants me to build it for heavy duty use so he can at least try some things with steel, so that might drive it in that direction a bit.

I guess it is time to start thinking about the plywood prototype of this thing.

Harry
Harry, I wonder if this is getting complicated because you are trying to construct a welded frame to high specs. I am working towards a summer rebuild of my 24x48 machine, and am trying to eliminate all wood for the usual reasons. My 4'x6' table design has two 3x3x0.25" tube runners extending the full length of the table, held up by 2.5x2.5x0.25 tube legs welded in place. Diagonal braces are welded from the legs to the middle of the 3x3. These two sides are then connected by bolt-on frames which are cross-braced for racking; my issue of portability is getting the bits home from the shop and thinking of selling the house in the future. This metal work essentially supports the working table, and I expect I can make it flat to within 0.125", easily skimmed off the top spoil sheet. Legs are on adjustable feet to make sure they all reach the floor.
For X motion rails I am going with V-bearings on angle iron, bolted onto 2x6x0.3" channel steel which is bolted to the side of the 3x3 runners. ACME or racks on the inside of the channel. I am planning 0.25" of vertical adjustment available on the channel/rail, so the two channel rails can be tweeked to be parallel in the vertical planes. The angle-iron rails for the V-bearings can move parallel to the plane of the table to make them parallel. If the top of the channel is not flat enough to shim I will use self-leveling epoxy.
Long story short : welded steel table for strength and convenience, bolt-on critical elements for adjustment and transport. It weighs in at ~500 lbs for the table+X rails

• Look up the local steel fabricator suppliers and ask them for a quote. In this economy they are more competitive and many of them will sell to individuals when they were more selective previously. Tell them what you are building for personal use and they may give you good prices and free local delivery.

Sometimes they have cut-offs that they let go at lower prices. Ask.

CarveOne

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