Last night I was trying to tap a thread in some round stock steel. I drilled the starting hole with a 13/64" bit (which i believe is correct). I started the tap and was less than 1/4 inch in and started backing out and 2 of the 4 flute tips chipped (best i could tell).
I’ve ordered an authentic Morse replacement that should be here by Thursday. I should be able to get it to the cabinet by Fri.
I apologize for any inconvenience to other members. Please let me know if we think the one i’ve ordered isn’t the correct one. I’ve left the chipped bit in the case.
For a 1/4-20 hole, you should have used a 7/32 drill.
13/64: 0.203125"
7/32: 0.21875
Basically, your hole was too small for the tap to cut through it, ie, too much load on the tap. Reference the below link when doing tapping operations in the future. Drill and Tap Chart
Also, in the future, the proper use of a hand tap means you thread at most a full turn, then reverse a half turn. This will ‘break’ the chips and prevent them from placing too much stress on the tap. You were lucky, you only had a broken tap. In most cases, the tap breaks in you piece, effectively ruining it.
Also, if the required torque starts to rise, you should unscrew the tap, and clean it and the hole of chips.
Anyway, good on you for making good the broken tap!
Thanks for the reference @riceball2015. I’m sure this topic has been covered before but i found a couple of references that seem to differ from the one you’ve referenced. Maybe it’s that the ones below don’t differentiate between harder metals (Steel) and softer metals (Aluminum and Brass). Thanks again.
Agree @wandrson. Thanks for the reminder. 2 other things that occured to me… Though I didn’t notice any burs on the inner diameter after drilling, maybe I should have checked for sure and even run the debur tool around the edge. And, in hind sight, I don’t think I actually looked at the marking on the drill bit or measured it. So it may have been in the wrong bin… Lesson learned: Always confirm with measuring bit or at least with markings…
I’m neurotic and will check the dimension of my endmills/drills/reamers even if the number is printed/stamped on the damn thing. Never trust that parts are put back where they belong.
I’ve heard so many rules of thumb for hand tapping, ranging for 1-4 full turns then a half turn back.
The alternative: get spiral taps, proper cutting fluid, and power tap that sucker with the bridgeport.
The charts you provide are for 75% engagement, while Paul’s is for 50% engagement. When hand tapping you want to use the lowest engagement that will still provide the needed strength. Generally 50% will do for most people. It will certainly be less likely to break the tap in harder materials.
But proper tapping procedure will usually allow you to successfully achieve a 75% engagement in most mild steels.
Proper cutting fluid is essential for cutting threads in steel. Which is something we no longer stock in the machine shop.
Not really, drilling the larger hole for %50 removes the material needed to obtain a 75% engagement. Lots of proper cutting fluid, a secure vertical alignment, and a cautious tapping approach is the best bet.
BTW, I recommend RapidTap as the best tapping fluid.
I’m pretty sure there is a 1 gallon bottle of fluid that says ‘cutting’ in the name. It was in the yellow cabinet. Was on top shelf. Black bottle… I didn’t use that fluid but did use whatever is in the small tub w/brush…
The fluid I have seen is either wd40, which is great for aluminum, but not for steel. And a thick viscous fluid of unknown origin in the cup with a brush. It was way too viscous to be a good tapping fluid.
You can pick up a nice small can of Rapidtap from CDC in Richardson.
I used a number 7 drill a couple weeks ago, my chart & the chart in the tap box shows it. It’s all kosher, taps break sometimes.
Here is the one from the app I use.
So it may have been in the wrong bin… Lesson learned: Always confirm with measuring bit or at least with markings…