Drill size for tap

[fixedfixer]

Novice question sorry. I want to tap a 3/8 th BSF in a brass part. Will an 8mm drill be okay or should I really use 8.3mm bearing in mind I’m only tapping a few threads for a DIY project.

 

[Seadog]

If you try to use too small a pilot you may find problems with starting the thread. Try it in some scrap first to see how it pans out.

 

[Agroshield]

If you try to use too small a pilot you may find problems with starting the thread.

You can cheat a bit and countersink the undersized hole a little, which will help the taper tap to start. If it won't show, give it a light counterbore with an 8.5mm drill.

 

[Dr.Al]

Generally I'd say err on the side of bigger rather than smaller. 8 mm works out as 90% thread engagement which is going to be hard on the tap, although you'll be okay if you go carefully. 8.5 mm (a useful drill to have as it's tapping size for M10) would give 60% engagement which would be plenty for most applications. For reference, 8.3 mm gives 75% thread engagement.

 

[Pete.]

Novice question sorry. I want to tap a 3/8 th BSF in a brass part. Will an 8mm drill be okay or should I really use 8.3mm bearing in mind I’m only tapping a few threads for a DIY project.

8mm is too small, the minor diameter of that thread is only 7.9mm. You can't go too much bigger either, for instance if you went for 8.5mm you're only 0.21mm short of the pitch diameter.

If you have no choice but to use the 8mm drill then grind it a bit lopsided so it cuts over-size.

 

[The_Yellow_Ardvark]

http://www.moonshiners.org.uk/MZ/zeus.pdf

This may help.

 

[fixedfixer]

Thanks for all the replies, information and suggestions. I’ll order an appropriate drill and do job properly.

 

[MattF]

Try it & see. Brass is somewhat forgiving of minor discrepancies, & unless everything on your drilling setup is cock-on, there's no guarantee the final hole won't be a smidge over 8mm anyhow.

 

[James1979]

http://www.moonshiners.org.uk/MZ/zeus.pdf

This may help.

That’s handy cheers.

 

[fixedfixer]

Try it & see. Brass is somewhat forgiving of minor discrepancies, & unless everything on your drilling setup is cock-on, there's no guarantee the final hole won't be a smidge over 8mm anyhow.

I don’t expect my Aldi Parkside pillar drill is in anyway ‘cock-on’ and 8mm drill could end up 8.3mm you never know

 

[voipio]

Do you have a 21/64" drill?

 

[fixedfixer]

Nope. My diy tools and things are a bit limited.

 

[slim_boy_fat]

Nope. My diy tools and things are a bit limited.

Don't despair, we all start out somewhere.

 

[The_Yellow_Ardvark]

Nope. My diy tools and things are a bit limited.

Only for now.
We will wait untill you catch up.

 

[Dr.Al]

If you want to read a bit more about thread engagement, I'd recommend having a read of Harold Hall's excellent description: http://www.homews.co.uk/page621.html - a quote:

With these factors in mind a starting point is to attempt a thread depth of between 60 and 70 %, actual values depending on the drill sizes available. If these figures come as a surprise then note that Tubal Cain in his book "Model Engineer's Handbook" quotes an even lower figure of 50 to 60% for most materials. If you would like more detail on the subject then Tubal Cain goes into greater depth in his book.

If you want to work out thread engagement for a given tapping drill size, then there are a couple of handy references on my website (and I'm sure other options are available). The way I worked it out for 3/8" BSF was to look that one up in my thread size table: this link goes directly to the relevant row. That gives you the (metric) diameter and pitch: in this case 9.53 mm diameter and 1.27 mm pitch.

You can then go to the tap drill size calculator and put those figures into the "Custom ..." boxes and slide the Thread Engagement slider around to see the different drill sizes for whatever thread engagement you want.

 

[bricol]

http://www.moonshiners.org.uk/MZ/zeus.pdf

This may help.

I had to pay for my "metric revision" 30 yrs ago!

 

[Morrisman]

I don’t expect my Aldi Parkside pillar drill is in anyway ‘cock-on’ and 8mm drill could end up 8.3mm you never know

It'll always drill 8.2 or 8.3 until the day you need an oversize hole, then it will come out a reamer type fit at 8.000" It's a general rule of machining.

 

[The_Yellow_Ardvark]

I had to pay for my "metric revision" 30 yrs ago!

I liberated mine from a dockyard.

 

[brightspark]

http://www.moonshiners.org.uk/MZ/zeus.pdf

This may help.

65 pence look at the price of em now i scored a couple for 3 quid from autojumble and thats cheap

 

[bourbon]

https://www.ukdrills.com/ This place is good. I use them often