Table: Adjustable/Lift Design
 

Structural Question:

I'm in the process of designing an adjustable table that would raise the spoil board portion of the table up and down 12". This would incorporate building a free floating table surface inside the outer base. It would have vertical movement via 3/4" Acme thread using chain & sprockets via an electric motor to rotate the threads. This would then cause the table to rise/lower from the attached welded 3/4" hex nuts.

Below are some preliminary images with some sections hidden for clarity. Please let me know if more details are needed to explain my idea better.

In the design, there are four corner points with the acme threads. The table/cutting dimensions are 100"x53". The perimeter of the steel assembly will be C4x5.4 c-channel and the interior 4 cross members will be C3x4.1 c-channel. The thickness of the spoil board will be 1.25" (3/4"+1/2") MDF.

My question (this is where I loose it), should I put an additional riser support pair in the center of the longer C4x5.4 (see red question mark in image)? My concern is the weight issue of the spoil board and any addtional material on top and whether the longer c-channel will deflect somewhat to affect the quality of the work above.

Adding another support pair in the center will somewhat complicate the chain and sproket design at those points. I'm wondering if I can get away with just the four outer point and having the chain wrap around all four at 90°.

Thank you for any comments, suggestions or ideas - David

http://i585.photobucket.com/albums/s...ABLELIFT01.jpg
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I don't see an issue with your floating table per se, but I think you've greatly compromised the rigidity of the base frame which will destroy the performance of the machine. Remember that the cross supports and the table itself are structural elements of the MM design, and you'll need to compensate for their removal.

Brad- The images don't show it and I haven't yet drawn it out, but the front and back base sections will be include into the design. These two elements will be bolted directly to the side bases. The top member will be 1.5"x3" steel tube, the bottom and diagnals will be 1.5" square steel tubes. Similar in design and construction as the two longer side bases.

Basically the base will be 4 sided with the appropriate diagnals. All sides will be welded and then bolted to each other at the corners. The adjustable table will then float insided this box to freely move up and down. Thanks-David

Dave I was wondering why you would need such a deep cutting area? Do you plan on extending the Z axis? Would it be feasible to machine what ever you are making in layers?

As mentioned I think you will loose a good amount of rigidity. It also looks like a lot of critical elements to make this work. Do you have the tools to machine and measure. I think 6 screws would be a starting point. You might want to consider running the screws all the way up to the top of the x rails and install 2 nuts on each screw. Install one towards the surface and one where you have it located now. This should help out with any swaying on the table. You could even mount a brace in the center to take out any bowing of the screw under pressure.

I also notice that you didnt abandon the ideal of X rails from the other topic. Should also mention that the racks get installed under the V rails and you will not need the extra pieces of angle iron. As drawn the x motors will not reach. Good luck with you adventure.

Here's a better image showing the base only without the adjustable table within.
http://i585.photobucket.com/albums/s...ABLELIFT05.jpg

Right now you have nothing keeping your table square. A small hit from loading your work can knock it out of aligment. With the orignal design all the cross bracing and spoil board keep the table square. I can't remember the number off the top of my head but I think it's around 100 bolts that hold it square. Your ideal will keep the X rials at equal distance apart but will not keep it square. Needs a lot of cross bracing.

Russel- First, yes I have a 22" z travel and there are two purposes for the adjustable table:
a) By bringing the table up closer to the gantry, at that point it will limit the z travel to 12" max. This will help with a better finish in the project for smaller 3d objects. Basically less play in the z direction.
b) I plan to add an indexer for turning. By lowering the table, I'll get 22-24" of clearance below the bottom of the gantry for larger diameter projects.

Second,my original idea was to extend the threads near the top closer to the x beam. However with the table near the bottom and the bolts exposed above the z-plate and router would hit the threads, unless I shrink the width of the table. I need to further study this idea to get better stability in the base. Post#5 actually show the base with all the proper components and diagnals.

Please disregard the details on the x beam, those are other ideas I had but are not concrete.

Thanks for the input.-David

Russel- I can't believe I overlooked the horizontal cross/diagnal bracing. After a while you're worried about the small details that sometimes you overlook the obvious.:o

Also, there are dead spaces (non cutting areas) at the front and rear of the x axis. I can install bracings/gussets at each top corner. Also as you can see in red I can add futher longer diagnal bracing at the bottom horizontal cross members. Hopefully this should further stabilize any movement diagnally.

Need to still think about your previous suggestion to stiffen the table in relation to the all-threads.

http://i585.photobucket.com/albums/s...ABLELIFT06.jpg

I think I would consider a sub base that you could remove for the extra Z space that would give you the best of both worlds
or just extend out the front travel and put index on adjustable z

To stiffen the table you could move the vertical adjustment screws further toward the centre of the table and iclude angle bracing from the nut up to the bottom of the C4x5.4, i.e. the four jacking points dont have to be at the corners, by having a bit of cantilever you reduce the chance of the centre section deflecting (move in no more than 1/4 of the total from each end) That may also give you more room to cross brace the main outer frame.

Alan-Great point, thanks. I can move those assemblies to the next interior C3x4.1 cross member, which are 20" on center. This would be them within the 25% range you recommended to only 20% from each corner.

There would then be only a span of 60" between points. Do you think I can still get away with four points instead of six? Like I said before having four instead of six points would greatly simplify the chain and sproket design to drive the threads. Thanks-David

60" is a long span for your material choice. since there are unused void under the board, Should you consider the bed a lattice space frame made out of think wall square hollow section or pipes & you can span a lot longer then that.

A CNC table must be be able to carry rapidy changing horizontal cutter forces. It is the x-rails that impart these forces, and thus a stiff connection between the x-rails and table surface is desired.

Table: Adjustable/Lift Design-Version 2
 

Here's some more work done to better improve rigidity and strength, not to mention additional weight.

Russel- To answer one of your original questions regarding if I have the know-how and machines to take this task: the answer is yes. Twenty years in carpentry and building, both using wood and metal. I have most of the tools necassary to cut, weld and measure.

Some of the changes and improvements are as follow:
1-Added the missing horizontal diagnal bracing at the bottom and top of the base, image 3 &5.
2-On the floating table, moved the corner thread assemblies 20" closer to the center. This reduces the longer C4 channel (X axis) span from 100" to 60", image 4 & 5. As per Alan_c recomendation.
3-Added addional diagnal bracing to the thread assemblies for better rigidity, image 4 & 5.
4-At the thread assembly where the two nuts are in contact with the thread, I increase the contact distance from 1.75" to 5.5", image 5. I was never happy with the original contact distance and think that this will reduce any flexing of the thread as per Russel's recomendation.

The last image shows the inner table in the top most raised position. I know that there may still be room for improvement, so please comment. Thanks-David.

http://i585.photobucket.com/albums/s...ABLELIFT07.jpg
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My first original idea was to do such a thing. However, I'm having an issue trying to fit this machine in my small shop without an additional 100"x53"x12" sub base taking up more space.I'm trying to utilize the space under the table to serve this purpose. Thanks-David.

Thats looking better, I would add more bracing as indicated below, and some method of locking the table to the X-rails while working as Gerald indicated.

Attachment 9145

Ken- I see, I know get where you were going with your original post. I like it. Basically you are refering to a triangular truss structure using lighter components similar to the image below.
http://i585.photobucket.com/albums/s...russDesign.png
I understand this concept very well, as I use it many times in the construction field. Correct me if I'm wrong, but bydoing it this way I can change to steel component in the floating table:

a) The current four pairs of 1.5"sq tube diagnals can be replaced with a smaller .75" or 1" sq. tube, since a broader design with more triangular points will be attached to the c-channel platform.
b) The perimeter C4 c-channel can be reduced to the same as the interior C3 c-channel cross members. Since there are more tringular points welded to it, it no longer has to be that deep.

After absorbing this idea, I think this may further help stiffen the rigidity in relationship to the side base-sides. Not noly is the floating table rigid at the horizontal area where the top c-channel structure is, but will also be as rigid at the bottom section of this assembly where the nuts are that ride on the thread.

Alan- In additon, with this new concept I still agree with your point and trying to "lock/clamp" the top of the floating table to the xbeams and base-sides. I've been thinking of a linear guide that would always be in contact between the top of the floating table and the base-sides. Once the desired height has been achive then a clamping or bolting system can then lock both the floating table and base to itself.

Any ideas? Thank-David.

There's a saying I've been using for decades: "I see said the blind man to the deaf man as he picked up his hammer and saw":)

I believe the idea of a lower table with riser blocks was rejected due to storage space. Here's another take on that:

If it fits your usage, why not build 1/2 the table at normal height, and 1/2 at the lower height. Build your riser blocks as storage containers for various shop items, and design the system so that the riser blocks can go underneath the high side of the table when they aren't in use.

2 Comments:
- if you can achieve a locks/clamps, then you can discard 50% of the steel you have shown above.
- whatever clamp system you employ, it must not disturb the alignment of the x-rails on tightening the clamps.

linear guide system could be a little pricey... cam followers up and down a channel would be a nice guide or a simple set of rails made out of U.H.M.W. plastic would also work.

Jim- I was only using the words 'linear guide' as a term to describe the idea and function. I realize they are pricey and I cannot afford such a concept. One idea I do have is to create and make a linear guide using a 3/4" dia. rod that would go up and down inside a 3/4" ID piece of pipe. There would be a hole on the side of the pipe with a welded nut, then a screw that would tighten onto the rod and press against the opposite wall of the pipe. Very simple, effective and strong wothout any complicated parts. Thanks-David.

any guide might be a problem if you are trying to run those acme screws independently,
is it possible to lift in the center and just bolt in the correct position

4931T116
Steel, 2" Square, 6'Length, Perforated Tubing for Heavy Duty Telescoping-Tube Framing $34.80 Each.

This tubing looks like a good linear slide. Perforations provide safety locking pins. Still need a bolt like you mentioned to tighten the gap between the tubes.

Ever thought about using small air pistons to do the lifting. You could lift evenly without binding on low cost linear slides.

Dave, I am still wondering why you need the table to rise and lower.

With as much work that would go into a rising and lowering the one table you might want to consider building 2 purposeful tables.

Russel- In post#7 I answer that question you posted originally.Post #15 explains why I cannot have any additional components outside the envelope of this machine. I have a TINY shop and will have to consider moving some of the larger woodworking machines into the adjacent utility room. Thanks-David

Table: Adjustable/Lift Design-Version 3
 

Ok guys, here another sketchup based on some of the recomendations here. First, thanks for the advice and help.

1)The Base has remaind the same, however the adjustable table has changed somewhat. I decided to go with a truss system for the structure that supports the c-channel sub-structure which eventually holds the spoil board. I lighten this lower truss structure to 1" sq. tube, since the truss design will allow for better rigidity and strength. This also allows to reduce the original C4 c-channel permiter to a C3 c-channel like the rest of the interior cross members.

2) As per Gerald's recommendation, a verical linear guide system was added to the four outside corners of the table. A guide/clamp block will help in two ways: prevent any sway or flex from the 3/4" riser threads and most importantly lock the table to the side base in the X axis. This will hopefully stiffen the base for any horizontal and diagnal forces applied.

The following images show some details of the changes. Some sections were hidden for clarity.

Gerald-in a previous post you mentioned "if you can achieve a locks/clamps, then you can discard 50% of the steel you have shown above." I added the locks/clamps but I'm a little unsure as to what components can be discarded. Please explain or draw on one of the images, and thank you.

Again as always, comments are welcomed and appreciated. Thanks-David

http://i585.photobucket.com/albums/s...ABLELIFT13.jpg
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Do you really think you will need the infinite range of the threaded rod? I would think you would set the table to a reasonable working height and would only very rarely change it for the odd large piece and then put it back for a long time again at the regular table height. Why not just have a few pre-drilled locations to which you could set the table to. Already leveled, etc.... Just a thought.

Jim- Post #1 explains how the table will raise and lower. I plan to use sprokets attached at the bottom ends of the 3/4" riser threads and then employ a continuos chain to drive all four threads at the same time. I haven't yet figured out the details for this, I just want to get the structure above finallize before I tackle that new design issue.

I have seen this method used in woodworking workbenches, where the operator cranks a handle and raises and lowers the surface top of the workbench. This allows for various work heights for different projects.

At this point I may just use the same principle and use a hand-wheel to operate the table. The table will maybe be only moved once per project then locked in place while the router does its job.

Dave,
Now that you have a rigid bed, the screw won't work, I'll go with jim's idea on using jack for the job, even easier construction. I would suggest a scissor lift for the job.
http://www.lift-equipment-parts.com/...lift-parts.jpg
Your lock & latch don't kook good to my eye. Why don't you look into scaffolding joints for inspiration?
http://www.alps-scaffolding.com/imag...ing%20home.jpg

Then put a cheap bottle jack under each corner and raise and lower the table with those..

Yes you're correct, I won't be using the threads for precise lifting. I can always Zero the router bit to the general height of the table. The threads just make it easy to raise and lower the table and weight.

The previous post and #1 explain how the system will work. This is another reason I'm just using "over-the-counter" standard thread or all-thread as we call it around here, I'm not looking for precision, just a comfortable working height. Thanks-David

No matter which choice of lifting mechanism, you still need fix holes + pin/bolt to locate the bed position before lock/latching the bed.
Dave,
The design is getting more complicated then necessary, you need to consolidate...

I have another approach, build a honeycomb-ish sandwiched bed that is very rigid out of MDF, place a scissor lift in the middle fot the lifting, attach 3 or 4 pipe poles to the corners of the bed with position holes & use the scaffolding joints to lock the position on to the fixed table.

Ken- How won't it work? All four points will be moving at the same time. If you can image a machinisht vise employing the intergrated dovetail ways and using the linear screw to move the table left to right, now just take this concept and rotate it 90°. The only difference is that my guide/clamp block will not be so tight to the linear vertical rail.

I also thought about the scissor lift, as in the one I use for my ATV. However, cost and rigidity was a factor in not using this idea.

In Post#22 I explain the same concept that you mention using scaffoling joints. But the way I saw it was that the rod riding inside the joint would only be attached to the structure at the top and bottom. So if the table were half way lifted the rod may (or may not) have some flex. By using the C-channel as a guide or rail the channel is technically attached to the structure from top to bottom. Thanks-David

Your x-rails beams are weak in the y-direction when the gantry sits in the middle of a board . . . . .those beams will wobble a lot.

Dave,
On paper, running 4 screws to move thing in unison is perfect, but my ex-day jobs told me it isn't as simple as they seem. A simple illustration is 2 man 3 leg contest. A single point push or pull works with less hassle both in building & in the long run.
My design philosophy is let each mechanism handle their purpose well. This had helped me to come up with things that are so fundamental that many think its out-of-the-box. On that, the lifting mechanism should only do lifting & the lock & latch should just do the lock & latch & do it extremely well. once you get into position, you lock the bed & the lifting mechanism can go take a cuppa in the pantry ;)

Having a few more lock & latching point securely fasten to the bed onto the table also make the two as one and hence improve the rigidity of the table when its most needed during production. In this aspect, the space frame is ideal as it can span a lot further & still has very good rigidity.

BTW, you can make a scisor lift cheaply, if cost is a constrain, replace the hydraulic ramp with a screw & nut, that will also allow your hand crank adjustment.

Gerald- That thought crossed my mind. These are just prelimanary conceptual ideas. I will take it into consideration and adjust the design accordingly. I'm still curious to your previous comment regarding "discarding 50% of the abpve material". Thank you

Once you clamp/tie the table top to the x-rails, then you suddenly get a lot of strength, as per basic plans, which need very much less steel.

Ken- I unerstand your opinions and thoughts. Like I say to many around me, "I smell you".:D

However after over 23 in carpentry and building, I've had my share of erecting and using scafoldings and I don't beleive that these types of joints will be sufficient to reduce flex or movement like the more rigid and continiuos connection of the c-channel or for that matter any type of linear rail system.

I also agree with you and Gerald that the xbeam will be weak in the center with no locking point to the table. Like I said before that thought crossed my mind, I just haven't gotten around to all the details, just the general drawings of how the different components of the structure work with each other.

Before I conpleted the last revision, my first thought was to punch holes on the c-channel rail and just bolt the table to the fixed based at the four points(now should be at least six points). I wasn't too thrilled with the clamp system. It could lead to slipage. I agree, it should be a pin or bolt like system once the general height has been established.

As far as your first comment, the four threads working on paper, I'm not to sure if I agree with this. I have personally seen this concept work on a workbench. Also having worked on some high-end mult-million dollar homes, I have had my share of taking an idea from a piece of paper and making it work in the end. That was my role, job superviser, making sure what was on paper and spoken about got built.

Again thank you-David

Ken- Cuppa? Had to look that one up in the dictionary.:)

If we simplify to using a bolt system to lock between the table and base, what would be your opinion on the following? As you saw in the last revison, I added four vertical linear rails at the outside corners of the table. Originally, I wanted to put them right next to the riser thread assembly but didn't have the room. Now that we have established that we don't need a guide rail system, but just have two plates riding by each other with a series of holes that would align and bolts placed through them. This can be as simple as two smaller width L steel sliding by each other.

a) Should I keep the four outside corner points at their current location and add another one in the center. Three points on each side.
b) Still keep the points at the outside corners and add another two in between for a total of four contact points on each side.
c) Move them next to the riser thread assembly (approximatly 20" from ends) and still add one in between.

I personally prefer choice 'a' because this would spread out better the contact points to the x-beam side base and not concentrate it so much near the center. However go with 'b' for more bolted points.

Also, by incorporating choice a or b, would the horizontal diagnals at the bottom of fixed based be required any more? I believe this is where Gerald is going with his last post.