Drilling (over-size) when tapping

S

slow-poke

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Yesterday, I was tapping a 5/8" thread in steel > 1" deep.

With the hole drilled to 0.531" , tapping was a slow progress and the risk of breaking the tap was not negligible. I was using cutting fluid and I think my technique is okay. I decided to open it up an additional 0.003" with the boring bar and it really helped.

Obviously every application is unique so over-sizing the hole in many applications would not be appropriate. I'm curious to hear what is acceptable when it's a non critical fastener and You want to err on the side of not breaking the tap an inch into the piece that you just spent an hour making?
 
Selecting a fit class is as important as selecting a thread pitch. In production, you leave the tap and drill chart behind, instead using four-digit decimal drills for thread engagement that takes into account the strength, type of use, and coatings applied to a finished part. Less engagement is easier to thread in and out, but wobbles and isn't as strong in tension as a tight thread. Tight threads aren't tolerant of frequent removal and use.

Basically, you alter you minor (internal threads) or major (external threads) diameter to alter thread engagement. Check the Machinery's Handbook for a full chart (not the handy reference chart held to your calculator with a rubber band) with fit class info.
 
I use the really handy chart from Littlemachineshop.com for selecting tap drills. It has the sizes for both 50% (steel/ cast) and 70% (alu, plastic) threads. I follow it pretty closely, though I sometimes go inbetween for steel if I have a limited number of threads. Works great and I haven't snapped a tap (or had a thread pull out) since using it.
 
mattthemuppet2 said:
I use the really handy chart from Littlemachineshop.com for selecting tap drills. It has the sizes for both 50% (steel/ cast) and 70% (alu, plastic) threads. I follow it pretty closely, though I sometimes go inbetween for steel if I have a limited number of threads. Works great and I haven't snapped a tap (or had a thread pull out) since using it.
Click to expand...
matthemuppet,

Thanks printing.........
 
I wing it... But, if the going is tough: are you using a high quality tap? Is it coated? Is your cutting fluid specified for the material you're tapping (even then, there's sometimes a better fluid that will make things remarkably easier), are you fixtured to start your tap absolutely straight? are you / can you use a spiral-pointed tap? - more taper equals easier cutting, but they're often not too useful in blind holes.

I haven't broken a tap larger than #2-56 in many years (unfortunately I broke the #2-56 tap just a couple week ago, so sometimes stuff still happens...)

GsT
 
The machinerys handbook has a tap drill chart and the drill gets bigger as the material gets thicker. I got my first copy of that book 6 months ago and now I know enough to be dangerous to myself and others near me.
 
mattthemuppet2 said:
I use the really handy chart from Littlemachineshop.com for selecting tap drills. It has the sizes for both 50% (steel/ cast) and 70% (alu, plastic) threads. I follow it pretty closely, though I sometimes go inbetween for steel if I have a limited number of threads. Works great and I haven't snapped a tap (or had a thread pull out) since using it.
Click to expand...
I keep a copy of the same chart from Little Machine Shop to find recommended drill sizes. I just tried tapping my first threads and broke the tap - it was a #1-72 tap. I looked at the chart and I realized I used the drill size for 70% instead of 50%. That was my first mistake. My second mistake was only buying one tap (from McMaster-Carr) instead of two. I should have figured I might break it being my first attempt. Grrr.
Now I have to buy another tap, but this time I'll buy an extra for a backup. Ah well.
I envy you guys who don't even need charts.
 
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