Measuring threaded holes

Can any of you suggest a good way of accurately measuring the distance between the centres of two threaded holes? In the past I’ve put fasteners in both holes and then measured the outside distance and inside distance with callipers and done the maths. Sometimes though the bolts can be a bit wobbly so it’s not easy to get a consistent result. I’m talking about say M5 bolts as a distance of 30 or 40mm. TIA

Put a nut on each bolt and tighten down to lock the bolts into the holes . The thread shape will tend to centre the bolts. I measure the outside distance and the diameters of the bolts to do the math. I find inside measurements harder to get accurate.

Do you have the equipment to make pins that fit the holes with 'zero' clearance? If so, measurement and maths is straightforward (as you clearly know).

For a more versatile method - again assuming suitable equipment is to hand - produce a couple of pins of smaller diameter than hole ID. Fix these, truly vertically, into a pair of blocks or plates, so that their upper end is accessible. The idea is to stop the pins from wobbling, to keep their axes parallel to those of the holes (assuming the holes are made normal to the surface). Then measure the 'outside' distance across the pins, as the pins are forced into contact with the 'inner' walls of the holes; measure the 'inside' distance between the pins, as the pins are forced into contact with the 'outer' walls of the holes; measure the pin diameters; do the maths.

To be pedantic, these measurement methods don't directly measure the distance between the holes' central axes, as the threads may not be truly coaxial. If you are asking about that, I can't think of a method at present...

1. Insert transfer screws into the holes, rest a piece of brass or aluminium on the protruding points and tap down lightly with a hammer without letting the piece of metal move. Measure the distance between the punch marks.

2. In the absence of transfer screws, using a caliper first measure the distance between the near edges of the holes and then measure the distance between the furthest edges. Calculate the centres of the screw holes accordingly.

3. Find a drill bit exactly the diameter of the holes, place it in the drill chuck of a mill and find your way into the holes using the quill. Having a DRO will let you work out the distance very quickly.

With calipers measure first the distance between the closest edges of the two holes then measure the diameter of one hole. Add the two together and you have the centre distance.

Or measure the daimeter of one hole with the calipers and press the zero button without moving the jaws. Then measure the distance across the two far sides of the holes and that is your centre distance.

One has to do the maths if using a vernier caliper, but like Hopper’s method, the same can be be done with the two bolt/pin method if using a digital caliper. Vic presumaby has a vernier device?🙂

A comprehensive set of numbered drills (or 0.1mm metric drills), as pins, would likely not introduce a large error in an application such as this.

+ 1 for this method but sometimes if using an electronic caliper the body may foul the part.

Emgee

Thanks for your thoughts folks, very helpful. I concede it was a bit stupid of me not to think of using a couple of nuts! That’s why I don’t hesitate to ask on here!

There are threaded plugs with a cylindrical top made specifically for measuring the position of threaded holes. They are very expensive. I usually use shoulder screws, sometimes called stripper bolts assuming that the ground diameter is concentric with the threads.

Alternatives to using a nut: use a cheese-head or socket-cap screw wound down till the head meets the surface. Or a bolt rather than screw: tighten it to the thread run-out then measure across the un-threaded shank.

In either case, measure both inside and outside distances and find their mean.

Using the mill as Bill suggests is fine but you don't need a DRO to be able to do it. Use the hand-wheel dial but make the table's last movement to register in both holes, in the same direction as that for moving to the second hole.

Similarly with the caliper: it does not matter if that is of vernier or digital type. The method is the same.

'

Occasionally I find myself having to measure across two holes too far apart for any high-precision instrument I own, though this is usually where reasonably wide tolerances are acceptable. Forced to use a rule, I do not try to estimate placing the rule level with the centres by eye, but measure to the edges of the holes. Right-hand on both, left-hand on both; calculate the mean.

For measuring some parts of my steam-wagon, the firm-joint calipers and rule are all I have.

I used to see one or two of my superiors at work trying to measure by rule between centres estimated by eye. I tried to persuade them that measuring across like edges is easier and more accurate. Unfortunately I lacked the right Ologies in Computerese, Physics and Ropey English, so had no more success there than teaching them that for a row of matched items, "at xx mm pitch" is both correct and shorter than "at xx mm centre-to-centre distances" ! Luckily the nearest mm was appropriate, and often the best obtainable .

Would be better as "What Didn't You Do Today?" for me.

First Covid jab yesterday, and knew I'd quite likely have side-effects.

That was in the morning. I drove home (about 43) miles fine, had a meal, pottered about, moved my EW lathe on its trolley to a new location in the house. So far fine; just occasional, slight aches in the jabbed arm.

Then started to feel extremely tired so went to bed at about 10pm. Could not sleep though - it always takes me a long time to drop off but I think I managed barely a couple of hours of broken sleep in the next 12 hours. At one time I went through a long phase of bouts of violent shivering despite not feeling cold, but that stopped abruptly and rather eerily. These don't seem on the side-effects list I was given (Oxford / Astra-Zeneca).

So today I'm ambling round like a wooden man made of smoke, feeling washed-out, get-up-&-go gorn, no appetite. I am not in a state to risk using machine-tools and no desire anyway; but I'll try to make myself do some engine designing for my steam-lorry - I keep having to revise it.

Ah well, I'm sure I'll be right, if not tomorrow than pretty soon. My second dose is in May.

Exactly my symptoms - but after two days I was as right as - well, rain, I suppose.

Cheers, Tim