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Old 25th Jun 2021, 4:05 pm   #1
SiriusHardware
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Default Tapping a hole in metal: What size hole?

One for the metalwork Gurus: I have an instance where I am going to need to drill a hole for a large machine screw in a thickish bit of steel plate, and tap it so that the screw can be freely screwed in and out as often as needed. I don't want to use a self-tapping screw because in my experience they can only be screwed in and out a few times before they become a slack fit.

Excuse my ignorance, but I really don't know whether I should drill the hole using the same size drill as the tap and bolt, or whether I should drill it slightly smaller and rely on the tap to expand the hole to the right diameter as it cuts the threads.

I've used taps to re-cut damaged threads in an already existing threaded hole and in those instances I have always used a tap of equal size to the screw / bolt which will be used in the hole.
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Old 25th Jun 2021, 4:11 pm   #2
eaglework
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Default Re: Tapping a hole in metal: What size hole?

Hopefully this link should give you an answer - https://www.avontapdie.co.uk/tapping-drill-sizes

No connection but have used as a data source in the past.
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Old 25th Jun 2021, 4:12 pm   #3
GrimJosef
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Default Re: Tapping a hole in metal: What size hole?

There is a correct tapping drill size for every tap (thread, diameter). If you know the thread type (metric, BA, Whitworth, BSF etc, etc) then Google will find you the relevant table of tapping drill sizes for the different tap diameters.

Cheers,

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Old 25th Jun 2021, 4:23 pm   #4
SiriusHardware
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Default Re: Tapping a hole in metal: What size hole?

Thanks gents, it seems that the general answer is that you do NOT use a drill which equals the 'M' size of the tap / screw to be used, but rather one which is of slightly smaller diameter. That's what I instinctively expected.
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Old 25th Jun 2021, 4:24 pm   #5
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Default Re: Tapping a hole in metal: What size hole?

This book has been my standard reference for tapping data for the last 45 years and it's always handy on the bench.

https://www.amazon.co.uk/workshop-de...31232096&psc=1
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Old 25th Jun 2021, 4:38 pm   #6
GrimJosef
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Default Re: Tapping a hole in metal: What size hole?

Quote:
Originally Posted by SiriusHardware View Post
... it seems that the general answer is that you do NOT use a drill which equals the 'M' size of the tap / screw to be used ...
Yes. The tapping drill is smaller by an amount that depends on the thread profile. If you use a drill larger than this then the screw will be loose. If you use one smaller then there is an increased risk that the tap will snap as you try to cut the excess metal away. Of course for this to work properly you need to drill the tapping hole carefully. A drill press will ensure that the bit runs true. If you have to use a hand-held drill (either powered or hand-turned) then there's a real risk of wobbling and the final hole turning out larger than it should.

Cheers,

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Old 25th Jun 2021, 4:52 pm   #7
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Default Re: Tapping a hole in metal: What size hole?

You will need to use a lubricant when tapping the correct size hole, 3D oil or similar will do.Les.
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Old 25th Jun 2021, 4:55 pm   #8
SiriusHardware
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Default Re: Tapping a hole in metal: What size hole?

I will bear that in mind as well, thank you.
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Old 25th Jun 2021, 5:06 pm   #9
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Default Re: Tapping a hole in metal: What size hole?

As a guide, a tapping drill is the diameter less the pitch plus a bit to the nearest you have, that is for 60 degree threads, some are 55 degree near enough though. For example M3 x 0.5mm, a 2.5mm drill is perfect, M8 x 1.25, 6.75mm, who has one of those, go for 6.8mm.
Believe it or not spit is a great lubricant for tapping aluminium. Any old oil will work but once you try a proper tapping compound you will never look back.
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Old 25th Jun 2021, 8:44 pm   #10
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Default Re: Tapping a hole in metal: What size hole?

Quote:
Originally Posted by merlinmaxwell View Post
M8 x 1.25, 6.75mm, who has one of those
I bet you do - 17/64"
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Old 25th Jun 2021, 9:08 pm   #11
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Default Re: Tapping a hole in metal: What size hole?

The size also depends on the material being tapped and the type of tap. There are taps that do not remove metal but displace it. They require a larger hole than ordinary taps as metal flows from the troughs to form the peaks. Some taps are marked with the appropriate hole size. My copy of Machinery's Screw Thread Book lists a range of hole sizes for each thread size, with the corresponding %age depth of thread.
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Old 25th Jun 2021, 9:43 pm   #12
David G4EBT
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Default Re: Tapping a hole in metal: What size hole?

Quote:
Originally Posted by mark_in_manc View Post
Quote:
Originally Posted by merlinmaxwell View Post
M8 x 1.25, 6.75mm, who has one of those
I bet you do - 17/64"
It's a commonplace Imperial size.

For Imperial and metric drills of every type, and for taps and dies, I invariably use UK Drills. (Based in Wigan).

Why? Keen prices, everything I've bought from them has been excellent quality, no minimum order value, service is fast and postage is free.

https://www.ukdrills.com/?gclid=Cj0K...IaAumNEALw_wcB

A 17/64" HSS drill bit from UK Drills is 3/48 inc VAT:

https://www.ukdrills.com/hss-drills/...rial-drill-bit

Taps usually come in sets of three - taper, (AKA '1st cut') plug (AKA '2nd cut') and 'bottoming' (AKA '3rd cut').

Taper is to start the thread, plug is to complete the thread right through the plate, and 'bottoming' is only needed for blind holes. You'll struggle to get an M8 thread started with just a plug tap, but as long as you have clearance for a taper tap to run it's full length through the plate would probably manage with just that, especially if you're unlikely to need to use it any more.

Having said that, an M8 HSS three tap set from UK drills is only 4.86 inc VAT post free.

https://www.ukdrills.com/taps-and-di...taps-m8-1-25mm

(A single taper tap is 2.16)

There was a forum thread a little while ago about the merits of 'step drills' (sometimes mistaken called 'cone drills' which look similar, but aren't stepped). UK Drills titanium coated step drill sets are excellent, well priced and worth a look:

https://www.ukdrills.com/hss-drills/hss-step-drills

Hope that's of interest.

Every success in your drilling and tapping project.

(No connection with UK Drills except as a satisfied customer).
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Old 25th Jun 2021, 10:25 pm   #13
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Default Re: Tapping a hole in metal: What size hole?

Very satisfying to drill and tap holes, makes a nice job. A single tap usually comes in a little plastic tube thing so I keep the correct drill in there too. My old boss had a full set of metric taps and he made a little stand out of hardwood and aluminium to hold the tap and its drill side by side, everything labelled with original Dymo tape.

For small tapping jobs I now use a taper tap that fits into the hex end of my ratchet screwdriver.

https://fr.batzo.net/en/products/wer...hread-p1907566
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Old 25th Jun 2021, 10:44 pm   #14
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Default Re: Tapping a hole in metal: What size hole?

An old mechanical engineering friend gave me this tip many years ago. I have used it many times so I can vouch that it works.

Find a nut that matches the thread you want to tap. Then find a drill that only just passes through the nut. Quite logical really.

One advantage about using this method is you do not have to worry about using an imperial drill when tapping a metric thread and visa versa.

Al
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Old 25th Jun 2021, 11:03 pm   #15
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Default Re: Tapping a hole in metal: What size hole?

That's a top tap tip
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Old 25th Jun 2021, 11:55 pm   #16
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Default Re: Tapping a hole in metal: What size hole?

The drill size and (%) depths for ISO M8 1.25 given in the Machinery book are as follows:

6.5mm (98), 6.6mm (91), 6.7mm (85), 6.8mm (78), 6.9mm (72), 7mm (65).

17/64" is about 6.75mm , representing about 80% depth.

Something to be borne in mind if using data from old books is that the various pre-ISO national Metric threads had slightly different forms from the present international standard. The book lists the tapping sizes for the older metric standards, some of which differ significantly from those specified for ISO metric threads.

Last edited by emeritus; 26th Jun 2021 at 12:01 am.
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Old 26th Jun 2021, 12:22 am   #17
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Default Re: Tapping a hole in metal: What size hole?

Define 'thickish piece of plate'? Unless the plate is at least as thick, (assuming the plate is steel, much thicker if any softer material is used), as the diameter of the bolt being screwed into it you will not have a strong fixing as there will be too few threads in the plate, see HERE for minimum thicknesses for different diameter bolts. A sheet metal 'Nutsert' or 'Rivnut' might be a better option if the plate is not thick enough...
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Old 26th Jun 2021, 7:17 am   #18
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Default Re: Tapping a hole in metal: What size hole?

^^^
This is what I was wondering. How thick is 'thick' and how often is 'often'? And how much load is it going to be subjected to?

The alternative would be to fit a nut instead. Have a look at clinch nut, blind clinch nut, rivet nut and press nut etc..
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Old 26th Jun 2021, 3:40 pm   #19
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Default Re: Tapping a hole in metal: What size hole?

Yet another option.

Standard hex nut tack welded over the hole. Hole would be clearance for the screw. It sounds a bit of a bodge but I have seen this on commercial machinery. All depends on the space available I suppose.


Bye
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Old 26th Jun 2021, 6:36 pm   #20
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Default Re: Tapping a hole in metal: What size hole?

In addition to the obvious metric and imperial sizes, there are also 'number' drills, some of which hit useful sizes between the familiar values.

If you come across a set of number drills, they can be worth grabbing if the price is good.

David
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