Beall Tap drill size

[strat]

Hi

I have just purchased a Beall tap, M33x3.5. The drill size is 29.5mm. Now forstner bits I can get are 28 or 30mm. Is it better to go under or oversize? would the species of wood used make any difference?

Many thanks for your help.

strat

 

[jpt]

I use a 30mm bit and it works perfectly.

john

 

[Keithie]

^ same

 

[SVB]

As above, 28 mm will bind I would think.

That being said, perhaps drill a test hole with both - wouldn't be the first time a wood drill gave oversize hole.

S

 

[J-G]

In engineering, it is standard practice to cut an internal thread with only 5/8 (62.5%) Height for a number of reasons - one being that cutting a full depth thread - even in brass - is difficult and takes more power than a reduced height thread.

The bore dia. for a 33 x 3.5 thread under those circumstances would be 29.21mm.

In timber, the demands are not quite as high so you might get away with a smaller bore. The full depth of a 3.5mm pitch 60º Metric thread would be 3.024mm so the bore to achieve that would be 26.95mm.

A 28mm bore would in fact give an 82.6% thread depth and might well be a better option in wood - with all the caveats regarding grain direction of course. It will need more power than a 30mm bore but I would certainly prefer the deeper thread depth.

 

[strat]

Thank you for the sage advice. I have a 30mm bit but not a 28mm. I think on the strength of the advice from you I will buy and try a 28. thanks again.

 

[Random Orbital Bob]

In engineering, it is standard practice to cut an internal thread with only 5/8 (62.5%) Height for a number of reasons - one being that cutting a full depth thread - even in brass - is difficult and takes more power than a reduced height thread.

The bore dia. for a 33 x 3.5 thread under those circumstances would be 29.21mm.

In timber, the demands are not quite as high so you might get away with a smaller bore. The full depth of a 3.5mm pitch 60º Metric thread would be 3.024mm so the bore to achieve that would be 26.95mm.

A 28mm bore would in fact give a 82.6% thread depth and might well be a better option in wood - with all the caveats regarding grain direction of course. It will need more power than a 30mm bore but I would certainly prefer the deeper thread depth.

What a fabulously detailed and relevant response. That's going in my personal reference library for future use. Many thanks.

 

[Sheptonphil]

Cant argue the post from J-G, but I certainly have always used a 30mm Forstner bit in hard and soft woods.

The only difference, on soft wood I harden the thread with thin CA glue.

I have made 20+ jam chucks and plates this way.

Phil

 

[Keithie]

I had a bit more of a think about this.

Obviously what J-G said applies to metal and, as far as I know, is correct.

I dont know much about wood or metal but I know a little about both (hobbyist with both) and my experience is that they dont behave the same way in all situations.

My guess is either that 28mm or 30mm might be appropriate hole size for an m33 3.5mm tap for wood and that it might indeed depend on the wood.

I looked at the actual thread depth on my spindle ...to see ...and it didnt look to be much more than 3mm (though I dont have a precise way of measuring) andcertainly not 4-5mm. Maybe 33-3.5=29.5 (possibly adjusting for angle of side of thread?) but not 28.

Anyway, I thought the easiest way to tell would be to sink a 28mm hole in a pieceof beech (a 30mm hole worked well for tapping in a similar piece). Borrowed a near neighbour's 28mm saw tooth bit. Tried to thread with my Beall m33 3.5mm. Used up my week's profanity quota before succeeding. So it might be me or depend on the wood or who knows what ...but I found that a 28mm bore hole for an m33 3.5mm tap in beech isnt as easy as a 30mm bore. Maybe it would have been easier in softer wood? .. but then maybe softer wood might not be as good a chuck! ..who knows!

Def doesnt mean 28mm wont work in some cases.

 

[J-G]

First an apology - I was WRONG - Many years ago my specialty was Threads and Threading but I haven't needed to use that knowledge for some time and I am guilty of not fully researching my response - just working from fading memory.


Kieth's post made me look again, initially to better explain how much more power would be needed when using a smaller bore. Once I started to draw the thread form (to ascertain the amount of material to be removed) I quickly realized my error - the 5/8 thread depth is quite correct, BUT is of the Triangular Height. This makes a massive difference, since the real FULL depth of thread (DoT) is actually only 3/4 of the Triangular Height.

The attached drawing shows a cross section of one side of the internal thread (Mauve) along with the amount of material to be removed when the bore is 30, 29.2, 28.45 & 28 mm. The 28.45 is the root dia. of the Tap (assuming the Beall's manufacture to standard dimensions). The 29.2 is the bore for a 5/8 DoT.

I've calculated the cross-sectional areas under the different bores to try to indicate how much more difficult it is to cut the thread with smaller bores. They are shown in Yellow (1.9565 mm2), Orange (2.917 mm2, Brown (3.993 mm2) and Blue (4.772 mm2).

I think it reasonable to assume that the amount of power needed will be relative to this area (though it may well not be linear) so taking the 30mm bore as the base, I estimate that a 29.2mm bore will need 49% more power, 28.45 - just over twice and 28mm nearly 2.5 times the power. Hence Keith using up his week's profanity quota

 

[Inspector]

The Beall Tool Company have 2 You Tube videos in the Instructions section on using their spindle taps. I haven't watched them as they are 7 minutes long each and I'll watch when I need to, but I'm fairly sure they discuss the tap drill hole sizes. Have you viewed them to see what they recommend?

http://www.bealltool.com

Pete

 

[woodpig]

The problem is the material used. It may well be better to use a slightly larger hole than otherwise indicated to prevent tear out. In this case you can't beat doing a trial run in some scrap wood of the same type if at all possible.

 

[strat]

Thanks again for the comprehensive responses and links. i have a 30mm bit so i think I will pick up a 28mm then i am covered depending on material.

Thanks again for taking the time and effort to pass on your knowledge.