Tapping/Reaming Blind Holes

[eliot15]

Just looking for "rules of thumb" for determining depths when drilling/tapping/reaming blind holes.

Lets say I want to cut two holes in mild steel.

Hole One: Tap Depth 1.5" +.060/-.000

- 1/2-13 NC Tap, 3 spiral flute, semi-bottoming (I think, what does that mean by the way?)
- .421 Drill

How deep should I take the tap drill? How deep should I take the tap?

Hole Two: Reamer Depth 0.375"

- 5/16 over (+.001) chucking reamer (whats a "chucking" reamer?)
- .302 drill

How deep should I take the drill? How deep should I take the reamer?


Thanks very much for your thoughts. Also be grateful to hear any other thoughts about the tappping scenario. Someone suggested, for example, I add a compulsory stop to the program and blow out the holes before tapping them. Is that necessary? etc. etc.

[christinandavid]

Do a few test holes to determine the actual depth of full thread at a nominal tap depth using a bolt or preferably a thread gauge.

This will tell you how much deeper the tap needs to go to produce the required thread depth and you can then drill your hole full diameter to the corresponding depth. Allow a little clearance if you can, .05" or so.

Use same method with a nice fitting dowel to set your drill and reamer depth.


Do not run tap or reamer at excessive speed if you need them to last. Yes, stray swarf can overload the tap.

DP

[matty82]

The rule of thumb I generally use for tapping and to keep my taps in good condition is to go 2x diameter on the depth. (ie- 1/2" tap can go 1" deep.) Then you can hand tap the rest of the hole if needed. If you're machine tapping using a spiral tap in a blind hole, you can generally go within .060 of the depth of the drilled hole. So I would check how far you need to tap to get the correct thread length, then drill it .060 to .080 deeper. I also usually add in a stop before the tapping so I can blow out the hole and use a neat cutting oil, as opposed to machine coolant.

As for the reaming, if you're going 0.375 deep, Id be drilling at least 0.500 deep. If you dont have that much room to drill that deep, Id use two reaming cycles and go 0.25 deep the first time, then go down to 0.375. Swarf is the biggest killer when reaming blind holes. You're hole will come out oversize if the swarf builds up in the bottom of the hole, and rubs on the reamer.

Hope this helps,

Matt

[eliot15]

Thanks very much DP and Matt for both of your replys. I appreciate you taking the time.

Placing bolts and dowels in a couple of test holes definitely makes crystal clear sense, and you seem to be in agreement on the .05 to .06 or so clearance. If anything, I was allowing more clearance than I need to.

Thanks again.