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View Full Version : Centering on existing holes in "adverse" conditions?



jmarkwolf
11-18-2015, 10:56 AM
I'm frequently asked to "stand-in" as a machinist here at work. We don't have any machinists, but we do have a small shop with band saws, a disc sander and a big mill/drill, with which we do drilling and light machining of mostly aluminum and plastic assemblies for prototypes. Anybody in the office that knows how to pull a quill handle is allowed to use the equipment (but that's another story).

I have a small shop with a Bridgeport at home and, as an electrical engineer, I can pull a quill handle better than some. :)

A 700mm wide x 900mm long x 75mm deep assembly (open top box) CNC milled from billet, we commissioned from our satellite office in Germany, failed some 6mm dia x 12mm deep threaded blind holes at the bottom surface of the box, at the customer in-coming quality. Don't ask me how.

I was asked to fix the problem with Helicoil inserts.

This, of course, entailed centering on the existing hole, drilling, tapping and inserting the Helicoil inserts.

The existing holes were approx 15mm from the interior walls of the "box", too close to the wall to swing a dial indicator, or a T-handled tap wrench for that matter (needed a tap extension for clearance).

I ended up clamping the work to the table, chucking up a drill bit sized to drop in the original threaded hole, and CAREFULLY moving the table so the drill bit would drop through the existing hole without "much" drag as I pulled the quill handle. I regarded this my 0,0 for each of the half dozen holes needing repair.

It appears that I got away with it as all the holes passed a go/no go gauge test and had the required thread engagement measurement.

I'm wondering, however, what the preferred approach might've been to center on the original holes, given the tight constraints.

RichR
11-18-2015, 11:12 AM
I think I might have used the blueprint. The holes X/Y coordinates should be called out relative to a reference point. Mount the piece
square on the table. Indicate the reference point. Crank the table to the X/Y coordinates.

darryl
11-18-2015, 11:13 AM
A hole that's already threaded is more difficult to center on. One way is to turn some threads off a piece of threaded rod (short section of a bolt probably) to leave a smooth and concentric stub. Then insert this into the threaded hole and indicate off the stub.

Something I've done in the past is to turn a piece of pvc such that it just barely will thread into the hole by hand without wobbling. Then indicate from that.

Toolguy
11-18-2015, 11:19 AM
In that situation I would have put the drill bit in the hole, then clamped the box to the table. Easier to line up on the hole. When centering on a threaded hole with open access, you can use the drill bit trick or if you want to indicate, turn the head of a correct size bolt to make sure it's concentric to the threads, screw it in the hole and indicate the head.

Fasttrack
11-18-2015, 12:14 PM
It all depends on how accurate it needs to be. If available, RichR has the easiest solution but we don't always have that luxury. In which case, a stub works well. (See link below for gauges made for this specific problem). Also, depending on the thread pitch, you can use a coax with a long arm to get in there and indicate. The ball has to be large enough relative to the pitch that the pitch doesn't seriously impact your ability to find the center.

http://www.ring-plug-thread-gages.com/ti-hole-location-overview.htm

Black Forest
11-18-2015, 12:23 PM
Take what I say for what you paid for it!

I had to do that recently as far as line up on an already threaded hole. I went to the lathe and single pointed a stub to screw into the hole. I wanted the threads to be tight that is why I turned the threads and didn't use a bolt. I turned the threads so I had a real hard time screwing the stub in by hand. Then I used my 3d Taster to find the center.

Mike Amick
11-18-2015, 01:22 PM
Why not just cut the head off of a bolt, screw it into the hole. Then lower the chuck over the bolt and tighten it,
then clamp the work down.

Mcgyver
11-18-2015, 01:24 PM
don't make it more difficult than it needs to be. You don't give tolerances, but normal practices is big clearance and tolerances for the clearance hole for the mating part.

I would chuck up a pin gauge that just fit the minor diameter. don't have a pin gauge, turn a stub. don't have a lathe, use a tap drill sized drill. tap the work piece into position until you can plunge the pin into the hole smoothly. Mount the drill and drill away. The bit will mostly follow the existing hole, the set up will be accurate to a few thou (more than good enough for fasteners) and there is no need to clamp work this large for a 6mm hole

Carm
11-18-2015, 01:33 PM
You did well, considering the unknowns involved:

Was the presumably drilled hole on location to start with given the depth~3"
Was it perpendicular to the reference

Couple things I always keep in mind:

Engineers or draftsmen working under them hate oddball numbers, Americans like fractions or their decimal equivalents and Europeans the corresponding metric. If a quick scale check looks like 3&3/4" (3.750 ) it won't be 3.742 or such unless there is an absolutely compelling reason.
No sensible designer calls out line on line for a bolt or screw mating hole, so on work of the nature you just did there is a bit of wiggle room.
Were I faced with doing a number of individual locations similar to your job I would make a headless shoulder bolt to reef down to the surface perpendicular to the thread (assuming such condition) of sufficient length to either indicate or feel with a wiggler.

Paul Alciatore
11-18-2015, 02:07 PM
Why not? Because bolts are notoriously loose toleranced ? Not only are the threads going to be under the size of the female threads in the hole, but there is no spec on how concentric the shank of the bolts are to the threads.

Almost any other method would be more accurate. The most accurate feature that you have in this situation is the minor diameter of those existing threads.

I would find or make a pin that is as close of a fit in the holes as possible. Put it in a hole and tighten the drill chuck on it. Then clamp the work down and drill.

You can argue about the concentricity of the drill chuck. If you want a better method, turn a concentric point on the pin and make a rod with a similar point with drill rod. Spin the drill rod in the chuck and align the point of the pin with the center of the blur of the point in the drill rod. Then clamp down and drill.




Why not just cut the head off of a bolt, screw it into the hole. Then lower the chuck over the bolt and tighten it,
then clamp the work down.

dave5605
11-18-2015, 02:41 PM
They wouldn't let you Tig weld closed the bottom ends of the tapped holes? Anyway, if I had to use helicoils I would have done it the way you (the OP) did it.

Mike Amick
11-18-2015, 02:51 PM
Why not? Because bolts are notoriously loose toleranced ? Not only are the threads going to be under the size of the female threads in the hole, but there is no spec on how concentric the shank of the bolts are to the threads.

Almost any other method would be more accurate. The most accurate feature that you have in this situation is the minor diameter of those existing threads.


Well ... there ya go then. Thanks for answering my "why not" question Paul. That's why I'm here, to learn.

old mart
11-18-2015, 03:07 PM
I would have screwed the tap down the hole and aligned with that.

sarge41
11-18-2015, 03:20 PM
Damn: Old mart beat me to it! Curses!!!

Sarge

jmarkwolf
11-18-2015, 03:39 PM
Excellent input everybody, thanks!

I think I like old mart's idea the best.

Screw a tap in, tighten the chuck on it, then clamp the work down, and reset the X & Y!

Brilliant.

boslab
11-18-2015, 06:23 PM
Regrind a drill to a piloted counterbore, the pilot part slips into the hole and guides the cutting edges, it's easy to do, bingo perfect centre, sometimes your lucky and find the size is an actual counterbore!( found that bit out the hard way after regrinding a drill to do the job, looked in toolbox and there's one staring at me)
Mark

TN Pat
11-18-2015, 06:56 PM
To center over a threaded hole, find a gage pin that fits snugly in the hole. Center over that with an indicator. If the shop lacks gage pins (which is blasphemy), you could turn one right quick.

boslab
11-18-2015, 07:17 PM
You could also use (even though I prefer the aforementioned gauge pin suggestion by TN) one of these, I've used them both ways, chucking the shank and using the ball to centre, and by using them the right way!
http://www.stronghandtools.com/valtra/products/tcb.php

Mark

vpt
11-18-2015, 07:18 PM
Or if it isn't super critical just plunge the drill bit in the threaded hole, let it figure out its own center on the way down, thread, insert, let the fitter worry about if they are in the right spots or not.

If they failed though I would think they would want to up the size of the hole/thread/fastener if room allows?

Paul Alciatore
11-19-2015, 05:18 AM
Yea, I like that tap idea too.