flange layout

How do I layout 16 equally spaced holes on an open flange that is 36 inches in diameter?
This flange is on a pressure vessel.

What level of precision do you require? Are these to be spaced around the flange, a certain distance in from the edge? Cheap and dirty, use a portion of a bandsaw blade around the perimiter, counting the number of teeth between hole centers. Figure your circumference, and use a blade with the proper amount of teeth to divide into the required number of holes. Epoxy it to the flange, clamping the ends until the epoxy dries. Heat and remove when done. This is tedious, but depending on the amount of effort you put into it quite accurate. Mike

The Machinist Handbook has a section called "lengths of chords for spacing off the circumference of circles" (pg 962 23rd edition). It works great for doing large bolt patterns. All you have to do is figure out how to scribe the circle for the pattern. You could tack something into the center so you could center punch and use dividers to scribe the circle. The circle HAS to be ROUND. The chart is set up for a circle with a diameter of 1. Multiply the value given for the appropriate number of spaces times the diameter of your circle (dia X 0.195090 for 16 spaces)to get the distance to set your dividers. You just step off the appropriate distance with dividers for each hole. Center punch the first hole on the circle then walk around it ith the dividers. I start going CW then go CCW to double check the hole locations. Try it out on chunk of plate first to get the feel for how it works.

Steve

[This message has been edited by MarshSt (edited 02-23-2002).]

36 inch diameter = circumference of 113.097. If you divide that by 16 you get 7.068 inches. If you can find or make a roller with that circumference you can scribe a line on the o.d. of the roller and roll it around the circumference of your flange. Every time the scribe line goes by, mark off one hole. Looks to me like you need a roller of exactly 2.25 inches . This should should put your holes right on the money.

TheMetalDoc

it depends on how many you have to do and the accuracy required, but you could make yourself a jig to attatch to a precision dividing head/rotary indexer . Set it upon a bridgeport and do it just like you'd do a smaller plate. If you put in a pair of setup dowels in the jig you could keep the indexer off the side of the column and still nail the radius dead on. you'd have to be picky on the degree spacing as 1Mintue Of Angle is 0.005" at 36" diamteter.

Sorry I didn't look at this post sooner.

How many and how close, I presume you need to make holes in plate for bolts, and that there is ample clearance allowed on bolt holes.

Down and dirty for a single part, do the layout on a large sheet of butcher paper. Get it layed flat on flange, centered up good, might include some centering marks on this paper. Tape down good, use center punch and punch centers thru paper.

If you are doing several of these you might make up a better template out of linen board, fiber board, etc, and use a transfer punch, clamping this templait into position.

If it has to be dead nuts, find a large rotab and a large mill, or jig borer.

Good luck

Thanks to every one for your suggestions. However, more questions arise; how do you set a divider to a four or five place decimal MARSHSt? To TheMetalDoc: This is an excellent idea; I'm surprised no one markets a tool that will accomplish what you suggest.And Halfnut, the only problem with your suggestion is that the template is not going to set flat on the flange because of a raised gasket face. I suppose it would work if one were careful and used a template made from a stiffer material than butcher paper.

Ahhh... WHY don't you just buy one? If this is a pressure vessel the flanges are to some spec. (ANSI or other) and the time and expense of you fabricating one would far out weight the cost of a "store bought" one. OH... I looked up standard flanges, I can't find ANY 36" dia. with 16 bolts! Better look again. GOOD LUCK!

I was pulled off that job to attend to a hotter one before making any measurements. I pulled the 36" diameter out of thin air while sitting at the computer. Buying a flange would appear to be the simplest solution but it is welded to a neck on a 30,000 lb pressure vessel.

Sorry I made it sound so complicated. I could explain this a lot better if I wasn't such a lame typist, I'm more the hands-on type. You don't have to set up to 4 decimal places, that is just how the math works out. Any error you have in the divider setting will stack up as you go from hole to hole. I set the dividers with a pair of calipers and walk around the circle scribing the marks lightly the first time. When you get back to the start point you will be either long or short, divide the error you get by about 16, adjust the dividers accordingly and walk around again. On that big of a circle you can probably eyeball the adjustment. If you lay it out on a piece of sheet metal you can do it on the bench where you can work on it easily and you can use the sheet for a template. It really is a lot easier than I make it sound. Try it on a small circle with only 4 holes to get the hang of it. Works good for making gaskets too.

Steve

Thank you MarshSt. Dividing the error by the number of holes is the "missing link" I was looking for. The solutions I've read in this forum have been very ingenious and I wonder why some gifted machinist hasn't written a book with tips and tricks such as these. Some set-ups are unique, but there are others that are standard in craft. I've yet to see anything written on the subject. By the way, I have over thirty years experience repairing everything from torpedos to locomotives. But I'm new to maintenance machining and still learning.