Thread: size hole to tap

how do I find the size hole to drill to tap a certain thread or thickness to thread a certain thread,? have always wondered and guessed it only to be ruined because i drill the hole too big etc.thanks in advance.bob

There are charts for tapping drill size (engineer's black book, machinery's handbook),but a quick and dirty method for metric threads is the nominal diameter less the pitch (for example, M6x1mm thread; 6-1=5, so a 5mm tapping drill). Male threads are usually a smidge below their nominal size (say 5.95mm for that M6)

Michael

There are tables to refer to for tapping drill sizes.
You could buy a book, or search online under the thread type, for example Metric.
You will need to know the diameter and pitch/threads per inch of your desired thread.

Hi Bob, if it's metric that you're wanting to do, the general rule of thumb is diameter minus pitch. If you're wanting to tap 8X1.25 for example, 8 minus 1.25 being the pitch = 6.75. If it's imperial threads you'll need a thread drilling chart, as there is no rule of thumb as far as I know.
Kryn

Hope this helps Bob.

Apart from the tapping drill charts you can find the length of thread is generally 1 1/2 times the diameter of thread.

so is cutting taps external and forming internal? thankyou.bob

Hi Bob, taps cut an internal thread, and dies cut an external thread.
Found this on Google which'll explain the differences https://www.natool.com/blog/2016/07/...s-forming-taps
Forming taps create thread by displacement of material within the hole.
Kryn

Originally Posted by jack620

Hope this helps Bob.

Might be worthwhile to point out to people that this chart does not follow the normal recommended thread engagement. Running 65-70% thread engagement may be fine for applications that you are doing but the normal recommended is 75%.

Robyn, next time you are in at one of the engineering supply shops / bolt shops / bearing shops / ect ask them if they have a free drill & tap chart. From several trade events I have a couple of SuttonTools Tapping Drill Size Charts, on one side they have all the common thread types sizes and drill sizes. The other side they have a Metric / Imperial fraction / inch decimal / gauge chart. It's about 3/4 the size of an A4 sheet and is meant to fold up to put in your pocket. Have had a few of the poster ones and that but much prefer the Sutton one as it's small and has 99% of the info you will need.

A really useful reference I have seen on this topic with heaps of appropriate charts in the back is "Workshop Practice Series 12" by Tubal Cain, "Drills, Taps and Dies".

thankyou all for your help and information, greatly appreciated........bob

Originally Posted by KBs PensNmore

Hi Bob, if it's metric that you're wanting to do, the general rule of thumb is diameter minus pitch. If you're wanting to tap 8X1.25 for example, 8 minus 1.25 being the pitch = 6.75. If it's imperial threads you'll need a thread drilling chart, as there is no rule of thumb as far as I know.
Kryn

Oh yes there is Kryn! Exactly the same formula as metric is used for imperial UNC/F etc as well as BSW/F etc. For UNC you get same % depth contact as metric as it has the same thread form. Slightly less % for Whitworth form.

Rusty

Originally Posted by MasterSpoon

Might be worthwhile to point out to people that this chart does not follow the normal recommended thread engagement. Running 65-70% thread engagement may be fine for applications that you are doing but the normal recommended is 75%.

The recommended percent thread engagement varies with the material being tapped. That's why I made a table with various percent engagements. Try tapping titanium at 75% engagement and see how far you get! The recommended figures in my table comes from various tap manufacturers. Here's an excerpt from one manufacturer's website:

"There are a couple ways to calculate the hole size for a corresponding thread. The basic method is to subtract the pitch of the thread from the major diameter. For example, a 1-inch-8 UNC thread calculates to a 0.875-in. hole size, and a M10x1 would be 9 mm. This calculation method is most commonly found in manufacturers’ catalogs and on threading charts. For a long time this was acceptable as the drills most likely would oversize the holes compared to the nominal diameters, and the resulting percentages of thread engagement were adequate for strength and low torque.
Modern drills achieve close tolerances to nominal drill size, which means that the percentage of thread engagement is increasing, and so is the torque on the taps. Assuming the drill creates a 0.875-in. hole diameter for a 1-inch-8 UNC thread, that equates to a 77 percent thread engagement, which is too high for most applications. The optimal percentage of thread engagement for cutting taps lies between 65 and 70 percent. This yields a thread that will not fail under normal loads while keeping tapping forces low for good tool life."

If you have an hour or so to spare.

https://www.practicalmachinist.com/v...ead+engagement

A TAFE metalwork teacher told the class I was in that it's not as critical as you might imagine, and that 50 per cent engagement gives 70 per cent strength.