Tapping issues

[natewi]

Hello all,

I am having some trouble finding the right combo for tapping 3/4" blind holes into mild steel. I have a pneumatic tapping machine. My main tap size is 8-32, and it seems everyone recommends a #29 drill, however I'm getting stuck a lot, breaking taps, and overall seems like it's a very tight fit. I'm tapping drawer handles at 3/4" deep.

1. Can I size up to a #28 drill to allow more space?
2. I don't know as much as I should, but have been researching pitch diameter and other factors that would play into this issue. Can you recommend exact tap/drill specs for this application? (Flute style/#, pitch diameter/brand etc)

 

[dgehricke]

You may be tapping holes for 75 % thread if you increase the size of the drill it will give you a tap size that's about 60% thread and another step up will reduce the thread from 60% to what ever relates to the drill size you use.This will make tapping a lot easier. Check the internet for thread sizes and drill relationship that may help.
regards
dgehricke

 

[pontiac428]

You can definitely upsize your tap drill in order to gain more room in the minor diameter of the tapped hole. A 3/4" deep hole is 5x your tap diameter, so I would imagine that it would need a little opening up, especially if you are using hardware store fasteners instead of precision screws.

Typically in industry, oversize taps are specified when room is needed for coatings or for application requirements. For the home shop, our best choice for altering the fit of a tapped thread is to change the minor diameter with a larger or smaller drill.

There is a lot of metrology science that goes into measuring threads, so it is likely that this discussion will open up and "go there". The nuts and bolts are good to know, but common sense is what gets you there. Bigger hole, easier fit.

 

[Illinoyance]

Your existing setup will give about 69% depth of thread. For that length of engagement I see no problem jumping one drill size. Are you using a spiral fluted tap? Are the chips exiting the hole? Have you considered a thread forming tap?

 

[mikey]

I agree - jump up one drill size, use spiral fluted taps to improve chip evacuation and be sure not to bottom the tap to avoid breaking it. Also use a sulfur-based cutting oil or a wax stick lubricant.

 

[natewi]

Your existing setup will give about 69% depth of thread. For that length of engagement I see no problem jumping one drill size. Are you using a spiral fluted tap? Are the chips exiting the hole? Have you considered a thread forming tap?

Yes I am using a spiral fluted tap. I just recently became aware of thread forming taps, but not sure I understand the difference fully.

Your existing setup will give about 69% depth of thread. For that length of engagement I see no problem jumping one drill size. Are you using a spiral fluted tap? Are the chips exiting the hole? Have you considered a thread forming tap?

I am using a chuck collet that has a clutch drive, which has eliminated most breakage. Would I be able to use a clutch collet with thread forming taps? I'd think that more torque would be required to form vs cut?

 

[mikey]

Roll taps, what you're calling forming taps, use a larger drill than a cutting tap. You can look up the proper drill size if you use them.

Roll taps work best in softer materials like aluminum and brass and are not usually used in steel. They do produce better, stronger threads and there is no chip production at all.