Advice on hole placement for modular style table top

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I found a steel plate at the metal store this weekend that is 1" thick 26" x 48" and 279lbs for my new welding table. It is flat compared to a straight edge and has surface rust from sitting outside. There is a skin on it that may be rust all though I believe it is hot rolled. I am planning on building the framework out of 2.5x2.5x3/16 steel tube and wanting to put a grid pattern of holes in the top but am stuck on what diameter and center distance to place them. I do not want to weld to the plate so the plan is to attach some 1/2" thick angle to the frame and use 3/4" bolts to attach the top from underneath.Thanks for the help,ChrisTOO MANY TOOLS & NO MORE SPACE
Reply:If I was going to use the table for jigging, I'd be tempted to choose a hole size that matches readily available milling clamp setups. That way you can use the bolts that come in those rigs for the step clamps and so on. I think my set is either in 1/2-13 or 5/8-11 off the top of my head..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I agree on mill clamps like step clamps. http://www.use-enco.com/CGI/INLMK3?PMK0NO=1444560There are a number of different sizes available in both standard and metric. I would make sure that I chose a bolt diameter that worked with one of them. Smaller holes will keep more strength in the plate and will be less of an inconvenience. Bigger holes will have more holding power and be better for bigger objects. 5/16 18 would be good for small to medium size projects with moderate clamping force. You can put quite a lot of force on something using pretty small screws. 3/8 16 would be good for medium size projects and pretty good clamping force. 1/2 13 is a very common size when it comes to mills and CNCs. It is good for some serious clamping force and if used right could twist your welding table into the shape of a potato chip. 5/8 and 3/4 I would only consider if you were trying to fixture tank plating and then you would need a much thicker table to keep things from moving. Personally I couldn't do holes in my primary welding table. I don't know what kind of welding you are doing but if you are doing stick or mig sooner or later you will find yourself grinding spatter off the table. The holes will will focus a grinders force around them and you will start getting low spots around each hole. The holes will snag corners of parts moving them on and off the table, stuff will get in the bolt holes and you will have to tap them to keep the threads healthy etc. If I was doing a lot of fixturing I would have a separate table for it. Clamping also puts a lot of twisting forces on the table and thicker is much better. Two half inch pieces of steel sandwiching some 2x4 square tubing would probably have 50 times more rigidity and strength in the directions it would be being pulled than your 1" piece. Granted we are talking about some pretty serious clamping to seriously let alone permanently distort that top. Aside from step clamps you might want to see what other kind of fixturing you want and what common hole patterns are. From my days involved in designing fixture plates for machining I seem to remember unit's of .75 to be common so .75, 1.5, 3, or 6. Even if you find a common hole pattern you will still find many things that are different so I would try to find as many of the tools and clamps you want to bolt to the table first then plan your bolt pattern around what you find.Lincoln precision TIG 275Millermatic 140 MIG
Reply:At 2'x4' I think you may be over thinking the need for holes in this table. If you are really insistent in drilling a bunch of holes in it, I would just put them in the middle 12" or so, as you are going to be able to clamp down darn near everything with C-clamps from the edge. As far as welding it to your frame, I say, weld away. It is only going to take 6 1" welds around the perimeter to secure it. Just build the frame, weld it on and be done with it. I have a couple of 4x6 tables, and can clamp just about everything to them as well. On the rare occasion that I can't I just tack it. I didn't used to like that, and still don't, but have found it just doesn't matter that much, and the time savings in throwing on a quick tack makes sense, as the few seconds it takes helps me to make more money. Eventually I will find an acorn table, but until then, I will just tack in the center of the table.
Reply:Good point on the c clamps. I used to do most of my work on a 4'x4' table and very rarely needed anything more than c clamps. You can also do things like span the table with a length of square tubing with a clamp at each end. Those can be hard to put in place on your own but once you get it they work well for a lot of situations.Lincoln precision TIG 275Millermatic 140 MIG
Reply:Originally Posted by DSWIf I was going to use the table for jigging, I'd be tempted to choose a hole size that matches readily available milling clamp setups. That way you can use the bolts that come in those rigs for the step clamps and so on. I think my set is either in 1/2-13 or 5/8-11 off the top of my head.