Need advice on working accurately with parts that are longer than a mill's travel

I need to drill bolt holes on the ends of 25 foot long I-Beams, and I need the spacing between the holes to be fairly accurate. The spacing between the sets of holes will be around 24 feet apart.

I'm planning to do the drilling on a Bridgeport knee mill (with a DRO), additional supports on each side of the mill to support the overhanging beam.

The mill only has an X-axis travel of 30" or so, which is a far cry from 23 feet.

So I have a few ideas for how to handle this:
A) I could use a tape measure to mark out and establish the origin for the second set of holes. However, I don't know how much I can trust the accuracy of a tape measure over a long distance.
B) I could use a laser measuring device (the type they sell in Home Depot) to establish the origin for the second set of holes. I think I can trust this to be accurate within around 0.1".
C) I could use the mill's travel to "inchworm" the part along, in which I would:

If I use the inchworm technique and move it 30" each time, I will have to do 10 of the repositioning cycles to make it to the other side.

If I use techniques A or B (which rely on a tape measure or laser measure), I suppose could technically just manually mark out the hole locations and use a magnetic-base drill to make the holes, and it will probably be close to as accurate as using the bridgeport to make the holes.

What do you think will be the most accurate way to get the spacing correct?

Do you have any different suggestions for how to work with parts that are far too long for a mill's travel?

Assuming you are to drill into the flanges and not the web, make a plate up with a block on each end just wide enough to fit over the flange. Fit one block with a screw so it will clamp the block to the flange. Mark the block out and fit drill bushes for the holes.
Make a second block and fit bushes etc as well, now drill a pair of parallel holes in the two blocks to suit bars with threads on them fit tubes over the rods to set the space between the blocks.
You now have a drill jig
Clamp it to the beam, drill one end, reverse the beam and drill again

How accurate do you want them? If your going to use bolts there will be clearance. The Bridgeport idea would be a costly one plus you will probably scratch the table up. If you use Peters Idea, use a tape measure and Mag drill.

Richard King said:

How accurate do you want them? If your going to use bolts there will be clearance...

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Yeah, application makes a big difference in this case, If these parts are going to be anywhere structural in nature then a tape measure and
a mag drill are all you need. In structural work holes are always oversize--1/16" - 1/8" depending on size of bolt. You may be overthinking this...

With the exception of the hook on the end, decent quality tape measures are quite accurate. Back in the day, I would use a 2 ft. scale to check the tape measures at the store, picking the most accurate to minimize bending the hook. I could layout and cut stock to better than .015. How close do the holes have to be (size and spacing)?

pMetal said:

1) Start with the machine table at one extreme of its X travel
2) Move the table (with the workpiece) over 30" and drill a small pilot hole
3) Move the table back to its starting position
4) Slide the part over so that the pilot hole is in range of the spindle
5) Pick up the hole location with a center finder
6) Repeat steps 2-5 until I get within range of the far end of the workpiece and can use the table's normal travel to continue normally.

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I used to do all the long jobs at my last machine shop because I had the mill on the left end of the line.

I used a procedure similar to yours, but with one major difference.

I would drill several holes since they were usually spaced a foot or so apart.
When I got to the last one, I would leave the drill bottomed out in the hole.
Releasing the clamps on the work, I would move the table back to the other end.
The DRO on the machine gave precise movement, and the spindle did not move.
I then tighten the clamps for the next run.

- Leigh

I find it very hard to believe that your bolt holes would require the accuracy of DRO.
I'm all for trying to make things as accurately as possible with the tools I have, but in cases like this it's just a waste of time to make it more accurate than it needs to be.
Unless these "bolt holes" are for mirror positioning bolts on a 24 foot telescope mirror or something like that, a tape measure and mag drill is your best bet.
If a DRO is the most accurate thing you have and you are determined to use it for this, and you don't trust a tape measure, why not make your own "yardstick" using a piece of flat stock and the 30" of travel you have. Then use it with clamps & stop blocks to measure out the 24 feet.
I don't know why I'm even suggesting this. Just use the fkking tape measure.

The beam will change by .0017" per degree F. So 10 degrees you will be out over 1/64 inch. Mark them with a tape measure and drill them with a mag drill.

If you want to be really, really fussy I have a set of inside mics for sale. I think they go over 400".

APD said:

I find it very hard to believe that your bolt holes would require the accuracy of DRO.
I'm all for trying to make things as accurately as possible with the tools I have, but in cases like this it's just a waste of time to make it more accurate than it needs to be.

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You've obviously never made your living as a machinist.

The DRO is already mounted on the machine.

Why should I kludge up some alternate less-accurate positioning method?
THAT would be a waste of time.

- Leigh

Used a method more or less as described by Leigh to put holes at around 10" intervals in approximately 9 ft and 14 ft long steel box sections to hold heavy duty continuous hinge supplied pre-drilled with 8.5 mm Ø or thereabouts holes. Box was 2 x 2 and 2 x 4 nominal, 1/8" wall.

Holes in box had 1/2" Ø rods drilled and threaded M8 welded in place for the hinge fixing bolts. All the bolts lined up well enough to screw in but some were about as tight up against the sides of the holes in the hinge as could be got away with.

Designed and laid out the workshop with a strategically placed door for over length jobs so only had to shift the Bridgeport about a foot to make space. Still ended up swinging the 14 ft bars end for end to get all the holes in.

I'd say that its essential to drill right through with a reasonable size drill to provide proper location when you move the table. I have a matched pair of vices which I set the right distance apart so the holes could be drilled. Leigh was braver than I in using a drill in the last hole for location during the move. I swopped a steel rod in. Breaking or bending a drill would have been the last straw.

Don't reckon your chances on picking up a small hole at each shaft. Tried that on a shorter job. Got away with it. Just. But things didn't go well.

Don't overlook tape measures. Class 1 steel tape has a specified error of around ± 50 thou maximum over 30 ft. Almost invariably much better if a good make.

Don't know if the digital display tapes with holes in the tape and a count sprocket are still around in a suitable length. Used the holes directly rather than the calibrations in an early example over about 12 ft and the thing was scary accurate. Maybe ± 10 thou.

Clive

The real Leigh said:

You've obviously never made your living as a machinist.

The DRO is already mounted on the machine.

Why should I kludge up some alternate less-accurate positioning method?
THAT would be a waste of time.

- Leigh

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You obviously didnt read (or understand) my post.
I suggested that he NOT do it that way.

What method was used to measure the mating part? (Or are you working to a print?)


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ripperj said:

What method was used to measure the mating part? (Or are you working to a print?)

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Not sure to whom your question is addressed.

I was usually working to prints for prototypes.

That's important because I normally do prototypes to zero tolerance, or pretty close thereto.

Print dimensions define the function of the part. Print tolerances define the error in dimension due to tool wear and machine calibration within which the part will still be usable. .

- Leigh

Lufkin (and others) make very precise tape measures that have temperature compensation scales. You must also apply the correct tension, also.

The real Leigh said:

Not sure to whom your question is addressed.

I was usually working to prints for prototypes.

That's important because I normally do prototypes to zero tolerance, or pretty close thereto.

Print dimensions define the function of the part. Print tolerances define the error in dimension due to tool wear and machine calibration within which the part will still be usable. .

- Leigh

Click to expand...

I meant the OP, sorry


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Mr. Colman is correct,
Never rely on the accuracy of the full stroke of a Bridgeport. Use the Bridgeport to make your jig. It's now a drill press job with an apprentice at the helm.

Leigh's method is compounding error, if you were to use this method a welder with a blow torch would produce better results

Use a 24" vernier caliper to check your tape measure. Some offshore tape measures are no good. Using the DRO is asking for trouble. IMHO

basalt said:

Use a 24" vernier caliper to check your tape measure. Some offshore tape measures are no good. Using the DRO is asking for trouble. IMHO

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Is a Dial Caliper okay? What about a Digital?

otrlt said:

Leigh's method is compounding error, if you were to use this method a welder with a blow torch would produce better results

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Nope! Bin there dunnitt.

See my post #10 above.

Theoretically a small compounded error can build up but with a DRO, care and appropriate technique it won't be an issue in practice. Few thou per table move maximum with pretty much ± random distribution given good technique at ordinary machine shop level. If you want to be uber picky intrinsic error of the move can be held to inside the DRO error. But now we are talking lab techniques and mega frustration. Bin there dunnitt too, not enough money in the world to pay me enough to do that again tho'. Mr Boss was very lucky I let him live!

Tool wander when drilling the hole is almost certainly going to be the major error source. We are talking Bridgeports after all.

Picking up on my earlier comment about using the count wheel drive holes on an early digital display retracting tape measure maybe a length of steel drive tape could be used as an accurate ruler. For example :- Metal Drive Tapes for Robotics and Automation Systems . Dunno how accurately spaced the holes are and what cumulative error is like but a length could be strapped onto the beam and the holes used as location points. Tape holes unlikely to be in the right place to do your holes direct but the DRO makes handling a small offset easy. One of those times where a centring scope in the spindle is handy. Is there a cheap modern electronic centering scope replacement based on a CMOS / CCD camera feeding a smartphone / tablet display?

Clive