How to Indicate An Existing Bolt Hole Pattern?

[ChandlerJPerry]

Hey all, this is actually a work related question but I figured the folks on here would have some good suggestions. At the end of the day today while disassembling some components at work I snapped off two 2-56 fasteners inside locking helicoils. One is above flush so there's a chance of getting it out, but the other is flush with the part, I'm thinking I will have to drill it out on our Bridgeport. The holes in question are part of a 12 hole round bolt pattern. I have access to the blueprint for the part, which provides me with the bolt circle diameter and degrees, meaning I can figure out the X and Y coordinates of the holes. The problem that's occurred to me is how to properly indicate the bolt pattern so that everything lines up? The mill does have a DRO, but to my knowledge bolt hole functions on a DRO can only be used to make a new pattern, not indicate off an existing one. I had the thought of sticking gauge pins in two holes across from each other on the pattern, holding a parallel to them and sweeping the parallel with a dial indicator to get things dialed in. Will definitely work, may just be tedious/time consuming. Anyone else know any DRO tricks or any other methods that might get me there?

 

[twhite]

If the screws are not bottomed out. You most likely will be able to remove them. The one that is proud should unscrew. When I have had to do this in the past. I like using a pair of wire cutters or small plastic cutters. They tend to get a better bite. Mind you I have crappy ones just for this.

As for the flush one. Try using something sharp such as an exacto and spin it out. Sometimes I can even use my finger tip and press and turn it out.

If all else fails then setup in your mill as you suggested. Try to put the hole that needs drilling 90 degrees from your parallel. That way you have an easy calculation half the diameter. Then make the same setup for the second.

Slow and steady.


Cutting oil is my blood.

 

[ChandlerJPerry]

I actually did try doing just that on the exposed screw with a small pair of side cutters, however I couldn't get it to turn. I unfortunately suspect the excess friction from the locking helicoils galled/welded the screws in, as I was removing them with a power tool and spinning them quite fast as a result. This is the first time I've had any such trouble with locking helicoils, but I guess it's all part of the learning curve right?

 

[twhite]

Looks like you are going to drill them out. The good thing is you have a bit of leeway. The screws will be softer than the Heli-coil. So the drill will self guide to an extent. Drill out then replace the Heli-coils. I actually prefer using a ball mill to clean the top of the broken screws. Then go to a spot drill. Hopefully you have a 1/16 ball or max 3/32. Just to get a nice spot to spot drill.

Let us know how you end up fixing it.


Cutting oil is my blood.

 

[woodchucker]

I would try a lefty drill bit. I have a set just for that. If it's not locked in there too badly, the left will start to drill and then grab and twist it out.

Now being 2-56 all bets are off. I would see if you have a left drill bit small enough to remove it or can order one quickly. Because heli coils can be tough to drill out as well.

 

[RJSakowski]

The bolt circle function on my DRO permits starting at any angle. I would indicate three holes,preferably 120º apart. Three points will define a circle. Once you have the coordinates of the three holes, you can draw it out in CAD and determine the coordinates of the remaining holes.

 

[Just for fun]

I busted a 2x56 a while back in a tri pod head. They had used lock tight when they assembled it. I used a small mill to flatten the broken screw off then drilled a hole in it, tapped in a small easy out, heated it with a heat gun, and it came out with ease.