how to square up a hole that is not square?

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dansawyer

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The project is to drill a square, straight hole perpendicular to a flat surface. I currently have a hole that is not square in the correct location.
I am trying to drill mounting alignment holes in a South Bend 9a cross feed. The final hole will be 3/8. The template is a right angle milling attachment. I drilled a mounting hole in the milling attachment and used it to create the center of the alignment hole in the cross slide.. I mounted the cross slide and placed it in a drill press vise. I thought I did a good job aligning and squaring the surface before drilling. But apparently I did not or there was drill wonder while drilling. I placed a drill in the hole shank first and viewed the alignment to a square, It is about 15 thousands off in 2 inches. The current hole is 1/4 inch. I would like to correct the error and end up with a square 3/8 hole.
Can this error be corrected? Would an end mill do a better job of creating a square hole? Assuming the error was wondering rather than alignment would an end mill wonder less? If the error was alignment and that is corrected would an end mill do a better job? Should this be done with a reamer?
 
The wander can only be corrected with an end mill at this point . A reamer will just follow the existing hole.
Before I had a mill, I would break out the rat-tail file(s) and carefully adjust the hole by hand to a few thou of the final hole, then drop a drill of the final size through to finish. Lots of work to say the least but sometimes the only way to save the part when the hole goes crooked
Mark
 
I wish crooked politicians were as easy to fix.
 
Good idea! Which one of us will hold him down?
 
@dansawyer

Yeah a reamer will just follow the drill.

If I understand correctly, you put the drill bit in the hole to use as a gauge and checked that with a square, right?
0.015" in two inches doesn't sound that bad.....Is this a bolted interface?
If so, most would have a clearance hole drilled in one piece anyway.

You could considering filling the first hole. Cut a short piece of similar material rod that is a tight fit in the hole. Knock it in with a small hammer, leaving it proud of the surface, and then grind and file to get it back flush to the finished surface. The diameter of this pins interference fit will depend on the material and thickness. Try one in some scrap material first.

That would give you a fresh flat surface to try again.....and also a new centre location to re-drill.

other thoughts.........
Sorry, I do NOT understand the size of the work piece. Could it be mounted to the lathe and turned (in a chuck or bolted to face plate)?
Boring is commonly done to make drilled holes better (on centre, true size, square).

If not, then using an end mill on a mill is the best next step. Ensure the end mill is "centre cutting".
Use a dial indicator mounted in the spindle to set it up square to the mill axis.
You should be able to get the hole squared-up to the surface.

Then you still need to make the round hole square.

Files would work.

Using a small (1/8") end mill could also kinda square the corners of the hole, but the final corner radius will be the same as you end mill radius.

Here's a link to Clickspring making a square broach:



Best of luck!
Please let us know what option you choose and how it turns out.

-brino
 
When you say square I assume you are meaning perpendicular to the surface. I would use a drill bushing to correct this issue.
 
Having no idea what the part looks like or if it can be mounted in a mill, the center of a hole can easily be moved to the right spot with a boring head.
 
I would use a 5/16" end mill to correct the hole but it is imperative that this be done on a milling machine with the part securely clamped in place. If it is attempted on a drill press, you won't have enough rigidity to make a new cut and the end mill will tend to follow the old hole. Once the hole is cleaned up. you can drill or use a 3/8" end mill to open the hole to the final 3/8" diameter. The reason for the two step approach is that the end ill will experience some flex and tend to deflect in the direction of the old hole. The first operation will fairly well true the hole and help to ensure that the final cut is as intended.

If you don't have access to a milling machine, I would follow Brino's suggestion and plug the old hole. Use a material with similar hardness to the part. Turn the plug for a tight fit and slighrly longer than the hole depth. Peen the end of the plug to upset the material for a tight fit and file or grind it flush with the part surface. I would drill a pilot hole first which should be entirely within your plug then open the hole to slightly larger than your old hole. The hole walls should now be entirely within the part. Drill to final size.
 
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