What bit do i need to do this?

[Annette Elmore]

I'm an aeromodeller and have a small lathe.

I've made some venturis like the one in the photo.

The hole on the side of the venturi is threaded 4mm fine pitch.

What I need is a bit that will create a round flat recess around the 4mm hole so that the needle valve assembly will seat properly and create a good seal.

I ordered a flat 8mm mill bit but it was not what was needed.

Sorry to be a bit wordy but I didnt know another way of describing what I'm after.

Annette

 

[Annette Elmore]

The venturi is 18mm Delrin.

 

[Lazurus]

Would a nitro proof o ring do it|?

 

[Annette Elmore]

Already got a fibre washer on which is ok but i would rather have a flat.

 

[novocaine]

I'd normally grind a custom drill bit from a standard fluted bit for this.
although I have to admit I'm a bit confused by what you want to do though, could you draw a picture for clarity?

 

[Annette Elmore]

Here is a commercial venturi with the flat I mean!

 

[galleywood]

What was the problem with the 8mm mill bit that you ordered?
Did you test it on anything?
What equipment do you have to perform this machining task?

 

[porker]

Could you mount the assembly on the cross slide of your lathe and use a small milling cutter in the chuck of your lathe? This will give you a flat surface.

Just re-read your post and maybe I have not understood what you are trying to do. Why did the milling cutter not work?

 

[novocaine]

You're right that a milling bit wont do exactly that. You can see from it how its done though. Its stepped bit. You are unlikely to be able to grind a 1 stage step so you'd be better drilling the 3.75mm hole for the thread form then use a stepped bit with a flat ground to drill out the flat ( this way you can undersize the nose but still use it as a guide for the 8mm cut). Then tap to suit.

 

[ED65]

A custom flat-ground HSS twist bit is the first thing that occurred to me too as I just discovered the technique a couple of weeks ago.

If you have a grinder and you're interested in giving it a try Annette there are vids on YouTube showing how. Here's one, How to Grind and Use a Flat Bottom Drill.

 

[novocaine]

fresh light of day.
that looks to be dished in, similar to a normal 8mm bit, you could grind the tip a bit flatter and it would still self centre.

the actual tool to do is is called a pilot mill, it's similar to your 8mm end mill but has a nip stuck out the end that centres it in your 3.75mm hole. it's not worth buying unless you plan to make hundreds of them, you'd be better of making your own.

something like this might do it too.

https://www.google.com/search?safe=off& ... 3OI2_jInqM:

 

[novocaine]

fresh light of day.
that looks to be dished in, similar to a normal 8mm bit, you could grind the tip a bit flatter and it would still self centre.

the actual tool to do is is called a pilot counter bore cutter, it's similar to your 8mm end mill but has a nip stuck out the end that centres it in your 3.75mm hole. it's not worth buying unless you plan to make hundreds of them, you'd be better of making your own.

something like this might do it too.

https://www.google.com/search?safe=off& ... 3OI2_jInqM:

heres the real tool. for refrence

http://imsbolt.com/alfa-tools-m4-3-flut ... nterbores/

 

[galleywood]

I am assuming that the M4 thread is already there - so a pilot toll will damage it.
Waiting for a reply as to why the 8mm mill tool was not suitable.

 

[novocaine]

you can't (or perhaps that should say you shouldn't) plunge a flat nosed bit straight down without having a massive about of rigidity, something that most hobby mills simple don't have. add a pilot and that issue is reduced considerable.

if the thread has already been cut then the order of operations is wrong and you need to make a new one or simply file a flat on it for this occasion and know for next time that you do the counterbore first then tap it.

 

[galleywood]

It is unclear if the OP has made the body that has the M4 hole - or if she is making a component that fits into the body and wants to add a spotface to the body to help seal a needle valve assembly.

 

[J-G]

... that centres it in your 3.75mm hole. ...

Most of what Novocaine says is good advice but I do hope that Annette did not drill a 3.75mm Ø hole for the M4 fine thread!

Even for the sloppiest 7H fit that is 3% too large. The correct core dia. is 3.46 - 3.57mm (5H fit) and that difference will be significant in this application where the specific issue is one of being gas tight.

 

[novocaine]

my bad, I'm claiming tiredness. my head jumped to a 3.5mm hole, then I checked it on a table on the phone and didn't read the right value. should have got the tables out properly.

J-G you are 100% correct on that one.

 

[Gerry]

If I was making a venturi out of delrin or even soft Aluminium I would make a jig to hold it out of a piece of MDF/Ply then just use a router and a trimming bit to flatten it off around the hole. It's a tiny area of material that you need to remove to seat the needle valve and that doesn't provide an airtight seal you will need something on there to stop a vacuum leak.
I would imagine at this point your venturi is worth more than the Magnum GP :twisted:

Gerry

 

[J-G]

J-G you are 100% correct on that one.

That's a first
I normally make a silly mistake when I pontificate! I did check this a number of times since my first reaction was based upon M4 Course rather than Fine

For about 8 years, when I worked for Herbert Small Tools, my life was 'Threads' - specifically, Coventry, Namco, Fette etc. Dies and Thread Rolls - so whenever I see a mention of threads, errors show up like a red mist