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darryl
12-18-2010, 11:53 PM
Ok, it's pretty much time for me to make a flat workbench. I don't mean flat as in look, it's horizontal- nor flat as in within .0001 everywhere corner to corner. I mean flat within about 15-20 thou corner to corner. I made one of these up a while back, but it's been reused for something else. I'd like to do a better job of it this time.

The previous project was an x-y table to carry a router, that outlived its usefulness. When built, the table (about 30 inches by 60 inches) was flat within 10 thou over most of the surface, and down 20 thou on one corner. Built from mdf and plywood, mounted on a castored base which allowed it to tilt side to side, but not end to end. A pair of clamps secured each end of the stand to the underneath of the table. This let me level the table side to side, and at the same time keep all four castors on the floor while keeping twist out of the table.

If I moved the table, I'd loosen one of the clamps and let the base twist as it may, then tighten the clamp again. The table could be off level by a small amount, but at least it wasn't twisted. If I loosened both clamps, I could relevel the table side to side and keep twist out of the table. That system worked and I'd use it again, but I know there are other methods.

Now I'd like to build another flat table. This time I'm considering making a frame from steel tubing, with lots of cross pieces. To keep the frame from twisting, I'm thinking of having some triangles emanating from a central vertical tubular post, four of them heading towards the four corners of the frame, and two shorter ones going directly to the sides. The frame would be about the same size as the original table, about 30 x 60 inches. Theoretically, if the triangles had a true 90 on one corner, and the central post was straight and even in diameter, the top edges of the attached triangles would all sit in one plane. A look across the top edges would see them being 'flat'. No drop in the middle, no sag on the outer limbs- The rectangular frame sits on top of that, and it's all welded up to be a unit.

The triangles might be 10 inches wide at one end, where they attach to the central post, and the outer end lopped off where the triangle becomes maybe 2 inches wide. Maybe there would be some material removed from the central parts of them, leaving webbing that keeps the rigidity- just thinking of ways to keep the weight down.

At any rate, this time I might make two legs on one end adjustable separately, then the other pair of legs adjustable for height as a unit, and tilt also as a unit- same as the previous stand. To adjust, once the table is in position, I'd first set the tiltable end to level, then go to the other end and loosen both legs, allow any twist to even out, at which point that end of the table is also level, then go back and set the height of the paired legs for a longitudinal table level. That should be it. Now I have a level framework, well supported on sturdy legs- hopefully a little better than my last one.

The mdf surface could be directly laying on the frame, or it could be festooned with little threaded rods on flanges, each adjustable for height and mating with holes in the framework. I would start by adjusting every one of them to the same height, then fine tune the table flatness after that.

As I said earlier, I'm not looking for surface plate accuracy. I know I can achieve flatness within twenty thou overall fairly easily with a simple wooden structure, with a lot of care taken in cutting and assembling of materials, and the design of internal bracing. This dooms the table to whatever shape it is in after assembly, since there then becomes no way to alter it short of surfacing on a giant surface grinder. Not going to happen. If I add the myriad of adjustable supports with this new idea, I'm free to get the whole thing within 10 thou fairly easily, probably better.

Just looking for other ideas, shooting this one down or otherwise- maybe adding something that would enhance the project. Anyone have any thoughts?

Astronowanabe
12-19-2010, 12:23 AM
I have wondered about pouring a layer of thin slow curing epoxy to let it float to its level, I had been thinking of a it for a level bed for a lathe to sit on but it may be good for the final step of your table top

darryl
12-19-2010, 12:32 AM
Yes- I like that idea. I might just keep the myriad adjustables at the same time. Start with a good frame, level the table, add an edge, then pour a fairly slow setting compound to a depth of at least 3/8 inch. Enough depth at least so that surface tension plays no part in the final level.

I've been continuing to think on this project. A second idea on the base is to make a triangular box the full length of the table, and about two thirds the width. The leg structures mount on the ends of the box, and the frame overhangs the sides. One of the three sides of the box is horizontal, and the frame mounts to that. This could be modular then, for easier transport.

RussZHC
12-19-2010, 01:12 AM
First thought was a welding table or similar and Ebay yielded this:

http://cgi.ebay.com/CLAUSING-VINTAGE-LAYOUT-STEEL-WORKBENCH-WELDING-TABLE-/310280536395?pt=LH_DefaultDomain_0&hash=item483e29414b

but then I started thinking about general table and, perhaps more importantly, the structure supporting a table top and one of the other auctions shows this:

http://cgi.ebay.com/HEAVY-DUTY-36-x-36-x-1-1-2-STEEL-WELDING-WORK-TABLE-/360329337005?pt=LH_DefaultDomain_0&hash=item53e54d58ad

which for me is more about the supporting structure than the table as a whole.

What is the planned use for your table...i.e. how "multi-function" is it going to be? How heavy could items be that get placed on top...impacts...welding...assembly ?

I like the idea of multiple jacking screws/bolts...using your OP, how does one go about attaching all those supports together and keeping where the top will be relatively in plane...I mean to be able to get that flatness the supporting structure has to be somewhat close...even with adjusting screws.

Another general question I have is do some of the parameters or difficulties change depending on the size of said table e.g. long and not very wide (say a table for a HSM size lathe) versus many welding type tables that are often nearly square. I maybe overthinking this but when using wood there are often things one does structurally that either don't need to be done using steel or that can be done differently with the same end result(s).

Bill736
12-19-2010, 01:18 AM
I was going to use MDF for the top of my assembly table in my shop. I poured a bit of water onto the MDF, and also onto some oak veneer 3/4 inch plywood , let the water sit for a couple of minutes, and then wiped it off. I found that the oak veneer did not noticably absorb any water or swell. The MDF, however, did absorb water, and swelled up slightly as a result. The swelled area shrunk down after a day nor so, but retained a fuzzy and rough finish. Since the plywood is also much stronger than the MDF, I used a double layer of plywood for my table top. I don't need the flatness that you are seeking, but I was disappointed by the water absorption of the MDF.
Perhaps if the MDF was coated with a finish of some sort, it would solve the swelling problem, but it would still be weak and prone to sagging if not supported extremely well. It remains to be seen how stable my 1 1/2 inch thick plywood table top will be, but so far it makes a solid top that I can also screw vises to even through the " end" grain.

Paul Alciatore
12-19-2010, 01:20 AM
MDF will react to solvents. It will distort. I know from an experience painting a turntable I made for TV work. It was nice and flat and then I painted it. It was warped for a week or more before all the solvent finally evaporated. And I am talking about easily seen fractions of an inch, not just thousandths. It finally returned to a fairly flat state, but I still think it was left somewhat warped from the experience.

If you pour epoxy on it, the epoxy may dry nice and flat on a warped top caused by the epoxy itself and then become warped when the underlying top flattens out again.

I wouldn't be surprised if the adhesives used in making plywood do the same thing - probably to an even greater degree.

You may want to consider some more inert material for the top. Perhaps one of the countertop materials sold in the home supply stores. Rough it up and then pour the epoxy on top of that. At the very least, seal the MDF or plywood and let it dry for a couple of weeks before pouring the epoxy layer.

The Artful Bodger
12-19-2010, 01:34 AM
I have been thinking about a concrete bench top to support all my machines on a walk around structure. I have not decided or even figured out how to do it yet.

macona
12-19-2010, 01:40 AM
There are water resistant versions of MDF available.

If you do go with plywood go with baltic birch. Really nice stuff.

darryl
12-19-2010, 02:44 AM
A few more ideas- my use of the mdf top was for its relatively flat surface and hardness compared to other sheet materials. In this app, I'd definitely coat it with something, top and bottom, before the final levelling process. Laminate has been good to me so far, so that would probably be my choice. Whether I would do that before applying an epoxy pool to the top- probably yes. One of the things I've had happen is air leaking up from the substrate, creating bubbles that either float to the top, or remain stuck below. I would not want either situation to mar the epoxy work.

I know from past experience that laminate sheets are pretty even in thickness from edge to edge- not necessarily from sheet to sheet, especially sheets pulled from stock. You don't know from where in the production run any two sheets might be pulled- In any event, the thickness variation in a laminated top has more to do with the glue application, and the subsequent applying of the laminate. Even with an epoxy pour, I'd probably want a laminated top over it to protect, and to take the wear and tear. It would be a major concern to me as to how it's applied. For my table, I think I'd forgo the epoxy pool and try to get my laminate application as smooth as possible directly to the mdf, both sides. It makes sense to me also to use the highest density board- it wouldn't have to be mdf, it could and maybe should be hdf.

My uses for the table include laying out mold parts and fiberglassing. I have an idea in mind which requires good sized parts to be made, and they must fit well. It's just common sense that you'd start with a good working surface. A part of this project will require some vacuum bagging, so there needs to be enough warp-resistance in the table to handle that. There will be a lot of messiness- overflowing of resins, etc, and that will also dictate the surface. I can't detail that project right now, but if this gets off the ground it will be a good product.

There has been a lot of discussion regarding lathe beds and stands. I believe also that often the stand is one of the more important things. I used to think that concrete was the way to go, but then other combinations of materials have been shown to have better properties in some of these uses. You may get good use out of concrete as part of a stand, but you probably wouldn't use it for a lathe bed. That's where epoxy-granite comes in. E/g is interesting to me in that you can have a basically stress-free and dimensionally stable product within a few days, a week at most. Once you get past the cost of the epoxy and the details of fabricating with it, including making leakproof molds, etc, the rest is design work, a properly working and scheduled molding plan, and seeing your finished product at the end.

Just today I was thinking about round rod ways, and how they could be supported fully, made perfectly parallel, and totally planar. My thoughts have ranged from using a surface plate to start with, epoxying the rods directly to it, creating the lathe bed, to using the surface plate as the starting point for the mold- orienting the rods, then filling the mold cavity with e/g to the point where the rods are partially submerged. This comes off the surface plate, and becomes the machine base. Lots of molded-in fasteners and reinforcing materials would be included in the structure, and some of these would be for caps to secure the ends of the rods so they don't lift off the epoxy-granite casting. Some of these would be for securing the headstock to the ways, so that would also suffice to keep the rods in place at that end. I for one am not interested in drilling and tapping mounting holes in the rods, or in surface plates, so this becomes a way to get a solid structure that won't come apart, and is not subject to unsupported length of rod flexing and allowing chatter. Being able to set up and cast the structure on top of a surface plate is a way to achieve an accuracy that would be difficult by other means. Keeping the surface plate as part of the base appeals to me, but without drilling holes in it to help secure parts, you're relying on the epoxy bond alone to keep it integrated with the rest of the structure.

I'm not saying it won't work, and work well by just being glued- it probably would. This wouldn't be a machine to do heavy high speed metal removing. It would instead be a precision tool-room type of machine. I could go on, but this is a topic that has been extensively covered in another thread- polymer concrete, which had morphed into epoxy-granite. I don't recall exactly- hasn't that one gone over 300 pages now-

This is another project which I feel I will be doing some time from now, using a surface plate as a machine basis- more on that when or if I undertake that challenge. In the meantime, I have another lathe project on the go, and I need to have more time in my day to work on it.

Oh, I almost forgot to mention- from having sawn up probably a hundred sheets of material in the last few months, I've seen a lot of flat warping going on- usually the cut closes up on you while cutting a 4x8 sheet the long way. Mdf is the least offensive this way, at least so far. Plywood and melamine faced particle board are both bad for this, and it seems that it's gotten worse over the last few years. When I built my first 'flat' table, I purposely cut my edging and interior bracing pieces wide, passing them through the saw another couple of times to get them straight on the edges. I had to build a long fence for the saw to help with that, but it worked out that I could cut lengths from the 8 ft pieces, and have both ends square to both edges. That is almost an art in itself. My dad, who had his own construction company for many years, still says that you can use a piece of plywood to check if something is square- he's just trying to be helpful, but doesn't realize that you can't rely on any of that anymore. I don't know if I've ever shown him how CRS warps when cut- but I'm deviating. I need a break to make my dinner. Later-

Duffy
12-19-2010, 11:38 AM
Darryl, without trying to preach:- MDF stands for "medium density fiberboard." There is also HDF or "high density fiberboard." The commonest example is "tempered hardboard." Masonite used to be a brand. Anyway, this product is waterproof, flat and parallel. Whether you can find it in greater thicknesses than 1/4" may be a problem. I do know that it WAS provided up to 3/4," as it was used for control panels for Chlorinators made by Wallace and Teirnan.
ANYWAY, if you bonded even a 1/4" layer, using a non-waterbased adhesive to a sheet of 1" mdf, it would make a pretty sturdy surface. Living in Chilliwack, if you cant find a couple of pieces of OLD GROWTH Douglas fir timber, (probably salvaged from a trestle,) you are not trying! Use it to build a table frame. The old, used timber is remarkably stable, an absolute PIG to work with, but will make a "proper" table frame. As an after thought, if you ran up a bunch of threaded inserts, say 5/8-11 OD and your favorite ID, and crazy-glued them into the table top, you would have a useful hold-down grid system.
I suggest this because it seems to me that an assembly of weldments is really an assembly of little "heat distortions."

Bolster
12-19-2010, 01:17 PM
If you use wood of any type, be sure to finish both sides of the table. Otherwise moisture will enter from one direction and not the other = warp.

My work tables are made of solid-core doors, then covered in formica.

darryl
12-19-2010, 03:27 PM
I did mention that I considered hdf to be the choice over mdf. I'll have to see if it's still available. It used to be- we made speaker boxes out of it years ago. I also mentioned that I would laminate both sides of it- I see the results of one side laminate construction everyday. Not that countertops warp all the time, but they do warp if not held flat. Of course when installed, they are held flat and that's not a technical application anyway.

There is a product that is two sides laminated, and it's used in elevator walls. I scrounged some of it when a local apartment was refurbing one of their elevators. It does seem to have a harder core than normal, but it is also a particle board core, and not very thick- it's only 3/8 total thickness. I will check to see what's available in 3/4 or 1 inch. I would laminate two sheets of 1/2 thick myself if I can be assured of getting it flat enough to start with.

Duffy, I did have a considerable amount of old growth fir planks, they were the full sized versions, 2 inches by 8 and 10 inches. I built my mill stand from them, and my previous flat table stand, amongst other things. It's gone now- but I do appreciate the sturdiness and the stability of that material.

I have also considered the warpage factor with a welded structure. It's certainly possible for such a framework to be welded up flat, and have it remain flat. It takes longer for the welder to do since he has to hop around the project a bit, tacking here and there, and using some symmetry techniques. I've had similar things done in the past, and asked for such techniques to be used in order to minimize distortions. I found that in general, as long as I explain what I'm after, and don't come across as being smarter than the welder, he will follow the sequence that I outline. I will usually bolt or rivet the framework together to begin with, then if necessary I'll provide the jig to hold it in alignment. When they see that I know what I'm doing, they're happy enough to add their part to it (the welding). When I see them stop and think, and ask a lot of questions, then I know they're wiling to do it right. I pay what they ask for when the job is done. The various shops are pretty good to me overall.

I have a chance to pick up some steel tubing made for a rack system. I probably will go with a welded steel frame.

Robin R
12-19-2010, 05:16 PM
I made an assembly table top by laminating 2 pieces of 3/4" MDF together, then gluing 2" wide strips of MDF on a 12" grid to this, with another sheet of MDF to close off the bottom. To make the top more durable I covered it with Arborite, edged with maple. That was about 8 years ago and it has held up well in daily use. It's so stiff I think it could be mounted on a pedestal and suffer very little sag.

RussZHC
12-19-2010, 08:50 PM
Would you consider honeycomb laminate structure...imagine cost is very high, have always been fascinated by those structures.

http://www.woodweb.com/knowledge_base/Polyethylene_Honeycomb_Cores_and_Veneer.html

and this is kinda neat...

http://stonesheets.com/StoneSheets_Materials.html

part of the reason I asked about use was "portability"...some of the early items from Ebay there was an almost universal refusal to ship due to weights involved.

For what its worth, I like the thought of using a surface plate for machinery base esp given the relatively low cost.

Paul Alciatore
12-19-2010, 09:27 PM
There are water resistant versions of MDF available.

If you do go with plywood go with baltic birch. Really nice stuff.

When I said "solvents", I didn't mean just water.

Bill736
12-20-2010, 12:54 AM
I ran across a material that's tempting to use for a bench top layer, but there's a problem. Last year I bought three sheets of a brown, Masonite looking material, that was sold as underlayment for carpet, or as a smooth top flooring layer. It's around 3/16 inch thick, and has a very smooth surface, and is rather dense. I was going to cut it up and use it to line parts drawer bottoms in my workshop. In the big box store, I didn't notice the strong odor from the material. When I got it into my workshop, however, the odor was overwhelming, and I had to take it outside. Months later, it was still too smelly to use . I cannot imagine anyone putting sheets of that material under their carpets, since the odor would drive them out of their house. What is that stuff ? ( I wound up cutting up some old sheets of Masonite to use for the drawer bottoms. It had almost no odor compared to the above material.)