What size drill

what kind of drill bit are you using to drill into brass

in the past visiting a workshop I have seen slow spiral bits used for brass or

standard jobber bits with the first mm or so reground so the cutting edge had a zero degree rake

John

PS

for brass

used bits with a slightly dulled  cutting edge is better than a new bit

Edited By john swift 1 on 08/09/2016 20:23:18

A tap will cut on the way out if the threaded portion goes below the start of the hole. If you look at the Emuge catalogue, a good nights reading, they give maximum thread depths for their taps. As has been said befor in other threads full depth threads don't need to be more than one to two times the thread diameter. Softer metals require deeper threads ( depth refers to length of thread in hole not major diameter of thread).

Howard

Thats me Bu**ered then

So whats one to do if you need a thread longer than the threaded part of a tap. The 1to 2 D is OK for fixings but what if teh thread is for something else like this one that was 90mm deep

Question the design! Usually you would counterbore to reduce the thread length to keep it within design limits of the cutting tools.

Howard

You can always pass the tap through on a through hole.

Howard

Just looked at your photo again.

Design needs more than one part in the blind threaded hole, if it were counterbored only the one at the end of the hole would be engaged in the thread.

Two bronze seats for a pump

This is the part that is threaded, its not a through hole but you can't see all of teh part in teh earlier photo

Edited By JasonB on 08/09/2016 20:48:39

Its all right questioning the design but what happens after questioning? If the item to be produced has to have a long internal thread to perform its function, then some method of achieving it has to be found. We would not get very far if tools could only be used within their limits.

I fail to see how a tap can cut in the reverse direction. I've cut threads to twice the tap length in soft materials and never experienced any cutting action when removing the tap.

Ian P

Do both parts in the hole have to held by the thread? Without seeing the deisign it is difficult to comment.

Howard

Yes

What I can't see is that if I stop the tap 1/2 a turn before teh cutting part enters the hole and wind it back out then it will be OK but you say that if I were to go 1/2 a turn beyond where the cutting part stopps it will overcut teh thread? Just don't see how 1 turn will make all that difference, have you got a link to the catalogue?

We can all exceed design criteria, and we all do at times, if we get away with it then we are fortunate. But when things fail, as in the primary question we can only offer correct suggestions.

Howard

Jobber drills are tapered towards the shank, I seem to remember a figure of about a thou per inch of length. The web also thickens towards the shank so a much sharpened or resharpened broken drill will have a large chisel point and require thinning. All my drills (mostly Dormer) measure undersized the same as Andrews. I find that if the finished size is important then double drilling will give a more accurate fit and can even be a bit tight. If freehand sharpening is used then it is very easy to cut oversize unless great care is used to keep the cutting edges the same length and the chisel point the correct angle.

Mike

**LINK**

Just like files, tools used on steel are never the same again when used on brass. Brass likes a sharp edge.

Sharp yes but with zero rake.

Its not to bad drilling a hole from scratch but if increasing the size of an existing hole in brass make sure the cutting edges of the drill have been stoned flat to remove the rake (as if the drill had no twist).

I drill quite a lot of brass and have now gathered quite an assortment of drills that I keep for the purpose.

Ian P

You stop being so darned theoretical, do what you've done (and the rest of us do) and never notice the difference.

Thanks for the link HowardT but I could not see it mentioned on that page is there a bit I'm missing or should I go to a particular section? I did look last night defopre you posted the link but could not see anything.

Also they all seem to be machine taps not hand taps that 99% of us use.

I have a lot of Emuge Franken taps and have had to thread deep holes on many an occasion never encountered a problem [ yet ] Probably the best taps available.

Jason,I think you are correct in that all Emuge Franken taps are " machine taps " I've never seen a hand one. I have the current paper catalogue here and can't find any hand taps in it.

cheers

Just managed to find this pdf on taps, it seems their thread depths are based on clearing the swarf on high performance taps as far as I can see. Can't see the bit about going below the surface.

Looking at some of the details there I can see an M10 tap that has a 22mm length of teeth but can be used to cut upto 3xd which is 30mm so seems it is OK to run them below the surface. Also another M10 that only has 10mm length of cutting surface but can go to 2.5D which puts it 15mm below the surface!

As Bandersnatch says us hobby engineers tend to tap the hole first as we don't have much control of the diameter thread the tap will produce and then cut the male part which is easily adjusted to fit the female as required by adjusting the die or depth of cut when screw cutting on the lathe. We don't need it to fit another part from the other side of the world.

Those bronze seats being threadcut M18 x 1.0 with a test gauge cut using the same tap.

Edited By JasonB on 09/09/2016 08:21:23

Edited By JasonB on 09/09/2016 08:25:57

Edited By JasonB on 09/09/2016 08:35:45

Back in the real world, an often helpful habit when drilling then tapping is to place the taper tap into the drilled hole and look at how far it goes in before you start rotating it to cut the thread.

If the hole is the right size to give you 75 per cent thread depth, the taper tap should slide into the hole to the point where the teeth are 1/4 of full depth, as judge by eye.

If the tap will not slide very far in, say to the point where the root valleys are only just visible as tiny indents at the very beginning of the tap, drill out a size bigger until it goes in until the root valleys are a good 1/4 of the full thread depth at the point where the hole begins.

I can categorically confirm that the shank size is often smaller than the twist bit OD. Not by more than a thou or two.

I regularly use drill shanks for certain precision purposes, and they often mike up smaller than the other end.

PS most of my stuff is Dormer.

Cheers,

Julian