Drilling a large bolt circle.

We received a request today to quote drilling 144 holes in a 8'-3 1/2" bolt circle. 108 of the holes are 3/8" dia., and 36 are 13/16". We have not been told what the tolerances are yet. When our estimator asked me about it, I told him to find out what the customer needed. The material is a rolled 5" x 3" x 3/8" angle ring. I know good and well that we're not going to quote this thing. We're a fab shop and not a machine shop equiped to handle this, and maintain any kind of a close tolerance. I'm just curious how some of you might do this, if a half way close tolerance needs to be held.

Thank,
Scotty

1-Has a drawing been provided, that shows the hole location tolerances
and relationships to the rolled angle?

2-The customer needs to define-exactly 'how' the hole features will be inspected.
The vendor inspects the part the same way the customer will.

When our estimator brought it to me it was just a hand sketch with only the hole sizes and the bolt circle. I'll know more tomorrow. I'm more than a little leary because I'm fairly sure that the shop that asked us to quote it is equally equiped as we are. I suspect they have an alligator by the tail, and they're trying to find a new home for it.
I don't want to own it.

Scotty

I'd do it in a horizontal boring mill with a digital readout.

Bill

If this is just a fab shop ring for a transition or ductwork of some kind, these are usually just laid out with a tape and soapstone. Being it is rolled angle, I'm betting that's what it is. Lay out your bolt circle on the angle. Figure the circumference (99.5") and divide by 143 (number of holes after 1)... put a mark on the circle every .6958" and knock it out with a radial drill or mag base. Every one I have seen leave a fab shop was built this way.

If it's a 747 turbofan inlet ring.... pass, lol.

Nothing to it.....tack weld a couple of say 5" wide x 1/8 thick hunks of sheet metal in a + across the ring. Measure across, find the center, drill a center hole. Rig up a center pivot 49.5" from the center of the drill bit in your drill press. Drill the holes 2.17" apart and you are good to go. If you are worried about additive error measure to find 4 hole locations after drilling the first hole. This will let you reset 4 times and will make any error smaller.

Mike C. said:

If this is just a fab shop ring for a transition or ductwork of some kind, these are usually just laid out with a tape and soapstone. Being it is rolled angle, I'm betting that's what it is. Lay out your bolt circle on the angle. Figure the circumference (99.5") and divide by 143 (number of holes after 1)... put a mark on the circle every .6958" and knock it out with a radial drill or mag base. Every one I have seen leave a fab shop was built this way.

If it's a 747 turbofan inlet ring.... pass, lol.

Click to expand...

Mabe I'm screwed up but .6958 apart on a 312.599" circumference gives you 449.26 holes.......

Correct. The last hole should only go 26% of the way through the material, per the drawing.

Gordon B. Clarke said:

As this thread is about bolt circles some may find this helpful or useful.

http://www.f-m-s.dk/CDH.pdf

I even made something that made measuring hole distance easy. Accuracy? About ±0.02mm (±0.001").

Click to expand...

So what is the constant for 7 and 9 holes? What did you make? photos please.....

I wouldn't touch that limber dick rolled angle ring unless I had both halves of the assembly and only when the fabrication is finished and complete. As fabbed it wont be round within an inch. The only way you get a 120 hole circle to line up in limber fabs is to clamp mating parts together and match drill them.

It would be just like a fab shop to send a flange ring off to a machine shop for facng and drilling then apply a 16 pass full penetration weld to the shell it was intended for. Naturally, the weld shinks the pitch circle and turn the flat machined flange concave and curly like a coffee filter. And who get blamed? "Aint my fault. The damn machinists down the street drilled it."

Not by the hair of my chinney chin chin. Them fab guys gotta coordinate better than that.

Forrest Addy said:

I wouldn't touch that limber dick rolled angle ring unless I had both halves of the assembly and only when the fabrication is finished and complete. As fabbed it wont be round within an inch. The only way you get a 120 hole circle to line up in limber fabs is to clamp mating parts together and match drill them.

It would be just like a fab shop to send a flange ring off to a machine shop for facng and drilling then apply a 16 pass full penetration weld to the shell it was intended for. Naturally, the weld shinks the pitch circle and turn the flat machined flange concave and curly like a coffee filter. And who get blamed? "Aint my fault. The damn machinists down the street drilled it."

Not by the hair of my chinney chin chin. Them fab guys gotta coordinate better than that.

Click to expand...

X 2 - 10ft barge pole

sjm512 said:

We received a request today to quote drilling 144 holes in a 8'-3 1/2" bolt circle. 108 of the holes are 3/8" dia., and 36 are 13/16". We have not been told what the tolerances are yet. When our estimator asked me about it, I told him to find out what the customer needed. The material is a rolled 5" x 3" x 3/8" angle ring. I know good and well that we're not going to quote this thing. We're a fab shop and not a machine shop equiped to handle this, and maintain any kind of a close tolerance. I'm just curious how some of you might do this, if a half way close tolerance needs to be held.

Thank,
Scotty

Click to expand...

Your instincts are well calibrated: no quote. Big floppy ring, lots of small holes to unspecified limits, suspect the brown end of the stick is extended your way without further information.

If your are just curious there have been a great number of similar size and configuration (and much larger) parts in aerospace that have been been produced in different facilities and yet assemble upon arrival. The effort required to accomplish the technology was epic, beyond expensive and required many fine minds to crack the nut and was accomplished before computers.

Just another day for all of those involved who learned by starting with a question such as yours and beginning a path to finding out how to do it.

If it really bugs you, hope it does, find the answer.

I find it hard to exchange ideas with fab .folks,some think the bigger the hammer the better.
I could go on,but i shant,the above may well be to much,oh hell,that deer in the headlite look when you try an splain why it might cause promblems is so common.
Gw

I would need more info on tolerance..but I would do that job on our CNC floor mill. 125'' Y and 17' X

I would talk to the requester to define what is wanted and what is needed and how the PAYMENT for work is to be confirmed.

This could be a simple job with no tolerances in that the mating part will be match drilled in the field (unlikely, but it could happen, no?)

If it turns out the job is indeed possible and not an alligator's tail due to poorly defined specifications and expectations. I would lay the ring on the floor. make it round by prods and pushes, find the diameter, then use the OD of the ring to set the hole circle (i.e. measure in) then layout the holes with a wing divider and pencil.

All work is done to meet the needs of the requirement and the customer. Just make sure you know what they are. And then charge for your time!.

Cheers

I "liked" the posts I agreed with, and that's the issue of a flimsy piece that may not have any QA along the various hand-offs from one vendor to another.

The vendor rolling the ring can't or won't drill the holes?

Depending on the leg, seems like a magnetic base drill should stick.

The vendor rolling the ring doesn't have a magnetic base drill?

If one shop is significantly warmer or colder than the final assembly point, seems that "Hilarity Ensues" may be as accurate as the bolt pattern.

As far as the six basic customer service needs of friendliness, empathy, options, control, information and fairness, perhaps offer to drill 4 "starter" holes as accurately as possible, then show up on site to finish up the rest?


Additional charges would apply.

Back in my days on heavy layout we "chorded off" pretty large pitch circle (33ft once) but first we had to establish the pitch circle The usual way was to determine the out of rouns and fit a strut across the diameter to round it up. We had some pretty talented fabricators so flimsy pieces were close to ideal geometry by the time we got them.

In this strut we found the center of the ring's critical feature (ID, pilot or ID) then trammeled the pitch circle radius. If the bolt circle had to straddle or fall in orientation to a part feature a diameter was scribed.

Then a pair of dividers was set to the chord and strided off counting carefully. Errors accumulate and small errors of chord on a large hole count can add or drop a full hole chord unbeknownst to the layout man. The plan says 120 holes on a 120" hole circle and you lay out 119 or 121? Hard to sweep that under the rug. It might take 2 or 3 paint outs to get the PC right.

Anyway the chord formula is:

c = chord length.

N = # holes

PC = hole pitch circle

c = PC x sin (180 / n)

Hm my blood sugar is low (44). I can't really think Would someone please confirm my formula?

Some practical points about laying out large, high hole count flanges: No matter how carefully you set your dividers to the calculated chord you never get them quite right on the first go around. A tiny error error will accumulate; 0.005" x 120 holes = 0.600". I always established scribed quadrants and watched for how the locations fell on the quadrants as well as on the entire circle. It's a good check on the hole count too.

BTW, the forgoing is called "polar" layout.

If you're a calculating fool you can figure the hole locations by rectangular coordinates but don't swap any digits when you write down the table or enter them in the DRO or or or.

These days I figure hole circles with a spreadsheet then cut and paste the offsets on the drawing. Lordy, I'm lazy.

"Mabe I'm screwed up but .6958 apart on a 312.599" circumference gives you 449.26 holes......."

Yup, about to go to bed and half awake. Forgot to multiply diameter times Pi to get the circumference. I got 2.17 with that correction.

Want to measure hole center distances? Got a digital caliper with inside jaws? Are the holes equal sized?

Caliper a hole set the display to zero. Caliper between the two holes. The display read the center distance directly.

If the holes are not equal sized, measure and do the math.

Gordon B. Clarke said:

Sure, but what has that to do with a pitch circle?

Click to expand...

That's why God made self-retracting pocket tapes. But you gotta figure in the height of the chord the tape hook spans on tne hole radius.