# Drill size for M5 tapping?



## student123

I need to drill & thread into macine grade ali for a M5 bolt
web search says use 4.2mm hole. My drill bits run 4mm then 4.5mm
cant see a straightforward imperial equivalent to 4.2mm.

better to go with the 4 or the 4.5mm bit to drill the hole?

Mike


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## JimM

I normally use a 4.5mm and haven't had any problems to date - in fact I think 4.5mm is the size you would use for a fine pitch M5

HTH

Jim


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## tel

You will get away with a 4.5mm for M5, but you really should get the proper 4.2mm. In fact, the best investment any workshop that deals with metric threads can make is a set of 1 - 5.9 x 0.1 steps drills.


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## bob ward

You're not making parts for the Hadron Collider so a 4.5mm drill will work as has been said. 

Personally I would procure a 4.2mm drill bit, seeing as you are in the UK you shouldn't have any trouble finding one.


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## Tin Falcon

print a copy of this
http://littlemachineshop.com/Reference/TapDrillSizes.pdf
Tin


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## Bluechip

student123  said:
			
		

> I need to drill & thread into macine grade ali for a M5 bolt
> web search says use 4.2mm hole. My drill bits run 4mm then 4.5mm
> cant see a straightforward imperial equivalent to 4.2mm.
> 
> better to go with the 4 or the 4.5mm bit to drill the hole?
> 
> Mike



Mike

This is a source in UK, I've found them to be very good value for the price. You can just get the individual sizes for tapping small Metric. Although it would be better to get a set as Tel says. You will find some materials tap better with a tenth or so larger than the nominal size.
http://www.axminster.co.uk/axminster-packs-of-5-ground-hss-drill-bits-prod20559/
Dave BC


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## Ned Ludd

Hi Mike,
The Imperial equivalent to 4.2mm is a number 19 drill. Personally I would use a 4.3 or number 18 (80% thread engagement), but anything up to 4.5mm(60%) will do, depending on material and application.
If you aim for 75-80% engagement, especially on small size threads you will break fewer taps, not that anybody on this site is clumsy enough to break any of course.
Ned


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## Lew_Merrick_PE

Hi Mike,

If you go to http://www.scribd.com/Lew Merrick, you will find a document _Pilot Hole Considerations_ that gives the equations for calculating different fits of pilot (tap drill) holes and their impact on the strength of the joint. The "standard" tap drill is supposed to create a 75% of full-thread engagement threaded hole. This is a convention based on providing maximum tap life in most instances along with generally acceptable axial load carrying capacity in the joint. If an actual 75% of full thread engagement is used, then the joint is 90% as strong as a 100% full thread engagement joint.

The standard M5 thread is more formally defined as M5 X 0.8mm pitch thread. A ø4.5 mm pilot hole in this instance would create a 48% of full thread engagement joint. This would be joint that is 53% as strong as a 100% full thread engagement joint in axial loading. That would be the *best* you could get from the joint as other factors (quality of the tap, quality of the tapping set-up, etc.) would degrade the value from there.

Yes, 90+% of the time using an oversized (by the chart) tap drill will work just fine. The problem is that a *real* answer requires an understanding of the application and loading of the joint. The thing working in your favor is that most designs use screw/bolt sizes significantly greater than actually required. A typical 1100 MPa yield strength M5 screw will not yield until 13.8 kN of force is applied to it. Designers often use "eyeball sizing" rather than looking at the actual loads. This leads to conditions of (1) using a much larger thread than necessary; or (2) failing the thread (usually when the screw or bolt goes into bending mode).


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## Jasonb

11/64ths would be a reasonable fractional size to use if you don't have letter/number drills.

Jason


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## arnoldb

Mike, you've been given a lot of good advise.

As a side note, I also use metric most of the time. I learned early on that the places where I can buy taps also sells the correct tap drills, and I make it a point to buy the correctly sized tap drill (or a couple for <4mm taps) when I buy the taps. I then store the taps and their drills together, separate from my "normal" drill bits. This saves a lot of frustration when it comes to tapping.

As another aside, this works well for drills for reamers as well; for my reamers up to 8mm, I buy high quality drills that are 0.1mm undersize, and test them to drill to size (not over size), and then keep them with the reamers. Above 8mm, I use drills 0.2mm under size e.g. a 9.8mm drill for the 10mm reamer. This also saves a lot of frustration later on.

Regards, Arnold


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## Lew_Merrick_PE

arnoldb  said:
			
		

> I then store the taps and their drills together, separate from my "normal" drill bits. This saves a lot of frustration when it comes to tapping.
> 
> As another aside, this works well for drills for reamers as well; for my reamers up to 8mm, I buy high quality drills that are 0.1mm undersize, and test them to drill to size (not over size), and then keep them with the reamers. Above 8mm, I use drills 0.2mm under size e.g. a 9.8mm drill for the 10mm reamer. This also saves a lot of frustration later on.


Although I live and work in an almost exclusively inch-based world, this is very good advice. Most all of my taps live in wooden trays with polycarbonate (Lexan -- acrylic would be better, but I have a customer for whom I make polycarbonate templates all the time and have lots of small pieces left-over) slide lid. Each tray is routed out for the standard tap drill, plug, bottoming, and spiral point taps (at a minimum) with a "slide-top side" routed so that the clear lid fits on nicely. All four sides of the tray have the tap information branded (woodburned) into them so I can identify them when stacked on a shelf.

I also keep my "extended-undercut" taps in a similar tray set. Anyone who has not used these will be in for a joyous surprise when it comes to tapping deep holes -- you can go deeper than rational whenever you need to do so. As these taps are usually 6 or 8 inches (150 or 200 mm) long, keeping them with the "standard" taps does not work well.

I have thought about doing the same thing with my reamers, but it is impractical for me. I was reaming (7075-T6) aluminum earlier this morning and I will be reaming 303 stainless steel this afternoon. You need different allowances for the different materials. Were I working in a narrower range of materials, it would probably work.


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## student123

Gents,

Thanks for the many great responses.
I went with the 4.5mm: first centre drilled then 2mm pilot (maybe overkill? ) then 4.5mm then tapped. Worked fine.

Mike


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## ruzzie

Hi all
As a general rule to find the drilling size for metric taps, you subtract the pitch from the tap diameter.
eg M5x.7 = 4.3 drill

cheers 
Paul


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## mklotz

ruzzie  said:
			
		

> Hi all
> As a general rule to find the drilling size for metric taps, you subtract the pitch from the tap diameter.
> eg M5x.7 = 4.3 drill



Works equally well for Imperial...

1/4-20

0.25 - 1/20 = 0.2 => #7 drill

(Now, if only the Imperial system used a rational, size-based nomenclature for drills.)


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## Deanofid

mklotz  said:
			
		

> (Now, if only the Imperial system used a rational, size-based nomenclature for drills.)



Stoppit!!


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