Drilling and tapping in solid brass.

[#1]

One of my morse keys needs the pivot threads to be re-drilled and tapped to the next size out..

At present, they are 6mm, so the next size up would be 8mm .

Drilling through about 10mm of solid brass. As already drilled and threaded, should not be too difficult to follow through with a bigger drill bit ,

What size drill bit(s) should I use to ensure that the 8mm tap will give a good cut , and not be "slack"

[#2]

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@shutter wrote:

What size drill bit(s) should I use to ensure that the 8mm tap will give a good cut , and not be "slack"

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What pitch on the 8mm 

1.25 Pitch   is a 6.9mm drill

1.00 Pitch  is a 7.1mm drill

Are you going to start with a taper tap then a plug  or can you drill straight through

I would try on a scrap piece of metal first and use plenty of lubricant and keep it square 

If you worried about it being slack use some Loctite or similar 

[#3]

@shutter 

Start with a taper, then use a plug. Use plenty of lubricant with brass and reverse/clear the tap of swarf regularly. It's very easy to clog the tap and end up over cutting the thread.

[#4]

@Baldrick1  Thanks for the reply... haven`t got a taper,  only standard HSS drill bits.. and a set of taps and dies.... I was thinking of starting witha 6.5 thne 7 then 7.5 but not sure if that last would be too much, and  not leave enough metal for the thread to be strong. 

[#5]

@shutter 

Not a taper drill, a taper tap. If you start with a small hole you will jam a plug tap.

[#6]

@Baldrick1 None of my taps are tapered... only what I think you call plugs... ( parallel tapped thread )

I have a 7mm tap, and an 8mm tap, but can only get 8mm pointed grub screws that I need for the job.

[#7]

Haven't you got the little yellow book? Probably still have mine from my apprentice days.

[#8]

@billnotben  Huh ?        Only little yellow book I know about is/was yellow pages ! ! ..

[#9]

It was a must have book back then that has been well used.

[#10]

Hmm.   Will copy that and then enlarge it, to be able to read it ! ! ! ! ... cheers 

Done ! ... but I still don`t know which size drill bit to use ! ! ! ...

[#11]

Larger.

Which drill? Of course if you're doing it with a hand drill it will depend how steady your hand is. So at the end of the day it will still be a bit of trial and error.

[#12]

@billnotben  I have a choice...  a hand cranked drill, or a 12 v portable drill, or a 240v drill I can use in a pillar drill stand...  but it is "slightly off" true vertical to the base. 

As the hole and thread are already through the brass shoulders, it`s more a case of just widening the diameter of the hole, leaving sufficient to tap the new 8mm thread into it..

Originally it would have been a British Thread,... being made during the 1930`s,  but over the years of abuse by hundreds of different "fists" ( wireless telegraphists ) ... the threads have worn somewhat, and are not fit for purpose... i.e. the "pointy screws" that act as the pivot point can "wobble", slightly, which upsets the way the key works, causing errors in transmission.

[#13]

Just to explain the "Pointy pivot screw"

being replaced with this.... https://www.ebay.co.uk/itm/401859514894

M8 A2 Stainless Cone Point Grub Screws Hex Socket Set Screw DIN 914

Note.... Drill Bit Sizes available....6.5mm   7mm   7.5 mm 8mm 

[#14]

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@shutter wrote:

... causing errors in transmission.

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Random extra dots? 

[#15]

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@shutter wrote:

only standard HSS drill bits.. and a set of taps and dies.... I was thinking of starting witha 6.5 thne 7 then 7.5 but not sure if that last would be too much, and  not leave enough metal for the thread to be strong. 

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In my book the drill for a 8x1.25 pitch is 6.9 and Baldrick1 chart is slight smaller at 6.8

A easy way to remember is to take the pitch from the size 8-1.25 = 6.75

Therefore if you use your 6.5 drill that's approx 11.8 thousands of an inch or 5.9 each side smaller and brass being quite soft it's negligible not like taping stainless steel

Like I said before try in a spare piece of brass use plenty of lubricant and back off  every quarter of a turn and remove the swarf so the tap does not jam

Good luck