Re tapping holes

[chuckk]

Hello all,

I have a 1/4-20 UNC hole that I like to change to a 5/16-18 UNC. Hole location is critical and I don't know how to make sure it doesn't move. Thanks! :-]

 

[L Vanice]

Can the hole (and its surrounding material) be mounted on the table of a vertical milling machine with the hole directly centered under the spindle? Can you then drill and tap for the larger thread using the mill?

If the job has to be done with a battery drill, it will be difficult to assure precise location and squareness. So much depends upon details not yet supplied.

Larry

 

[technocrat]

For general drilling, the larger drill will follow the the existing hole. You are only drilling out the 1/4 threads. So just drill it with the new tapping drill. For a bit more precision, set up the part in a drill press or vertical mill and make sure the existing hole is in line with the drilling axis. Use a pin in the existing hole. It is actually quite hard to make a new drill hole without following the existing (pilot) hole, indeed this is why pilot holes are drilled. A bad drill grind can make a drill wander, so use a new one if you want to be sure. To correct an out of position hole, you would usually use an end mill or plug the hole and start again.

 

[BajaFire]

Can you please clarify "make sure it doesn't move" - are we talking don't move by .060", or don't move by .010"

Could set it up in a VMC and interpolate the hole with a 3/16 endmill. I'm not confident the drill will follow the existing hole with the uneven threads pushing it around

 

[memphisjed]

Plunge out with an end mill. Even if just well clamped in a drill press. A 1/4 inch mill yields a sliver (machinist sliver, most of say dead on) over 1/4 hole. You can push a 5/16 tap thru that if just a couple holes.

If you have nc machine, do what Mr fire says.

 

[Booze Daily]

Put a .201 drill in your drill press.
Run the quill down and thru the 1/4-20 tapped hole in your part.
Clamp the part to the table.
Plunge a 1/4" endmill thru.
Tap.

 

[chuckk]

Thank you all for your help. I ended up putting the 1/4-20 tap in the hole and used it to line up the mill spindle. Clamped, drilled and tapped. Was shooting for 0.010". Came out ok.

 

[???]

Was shooting for 0.010"

The proverbial machinists sliver. Good thing you weren’t aiming for a bees dick that’s .0001” and even worse is the gnats cock at .00001”. I have heard talk of the poofteenth. Sort of like Bigfoot talked about but never seen. Those that claim to have worked to it are generally untrustworthy the type of guy who adjusts his tolerance based on feel, need a tighter tolerance squeeze that mic a bit harder.

 

[rjwalker1973]

I would probably just screw a tap into the 1/4-20 hole and sweep the tap body to zero with an indicator. Should put you well within tolerance. F drill it out and tap. Or you could use a .250 endmill first.

 

[Doozer]

Thank you all for your help. I ended up putting the 1/4-20 tap in the hole and used it to line up the mill spindle. Clamped, drilled and tapped. Was shooting for 0.010". Came out ok.

It would have benefitted your audience if you would have put that tolerance in your first post, and clarified if that is .010" total indicated runout or bilateral plus or minus .010". It respects the time and effort of who might be responding. That means for you, getting a better answer.

-D

 

[Superbowl]

It would have benefitted your audience if you would have put that tolerance in your first post, and clarified if that is .010" total indicated runout or bilateral plus or minus .010". It respects the time and effort of who might be responding. That means for you, getting a better answer.

-D

I could do better than .010 with a hand drill and a set of Harbor Fright bits.