drilling micro holes

[Dalboy]

I am in the progress of drilling a part for the fuel mixer on the farm boy I am making. This requires a small cross hole of a #76 bit through a 1/" brass rod which has a centre drilled out and threaded.
What is the best way to do this. The first attempt failed and broke the bit after drilling through the first side.
So far I have lost one bit and broke two (I dropped the first and being the size it is never to be seen again damn shop gremlins )

[simplyloco]

A photo would help, but drilling very small holes can be difficult unless you have the following:

A high enough speed - for example my Proxxon runs at 6000 rpm, adequate for most jobs down to 0.4mm if used gently!
The ability to 'peck' at the hole to clear the swarf regularly to prevent the drill jamming and breaking
A suitable jig/fixture to provide adequate drill guidance. For small diameters I will usually use a piece of square bar drilled to take the work and a precisely located drill sized hole in the top.
Hope this helps.
JB

[Chipswitheverything]

Like many other modellers on the forum, I have made the Geo. H Thomas Pillar tool and the mini-drill, designed for small hole sizes.  The drill was designed to run at speeds up to around 2.5 to 3 K rpm, which is well below the theoretical speeds for very small drills, but I have used it to successfully drill many holes at .25 mm, half the diameter of the No 76 drill.  But admittedly, not doing the deep drilling that your job is requiring, though many holes have been cross holes through stainless steel hypodermic tubing, not the easiest material.  The sensitive feel and control of the mini-drill has been the helpful factor, and the easy ability to "peck" as mentioned before.

You can obtain ( or make) a sensitive drill accessory to use with a bigger drill or milling spindle.  They have a very small chuck mounted on a spring-loaded, sliding spindle, keyed to a larger dia. tube held in the standard chuck or collet. The sliding element has a ball-race pressed or Loctited on, just above the little chuck.  The feed is then done by finger-tip control, holding the outer of the ball race, quite independent of the main quill drilling.   Dave

[steamer]

Something to pay attention to with brass, standard twist drills have a bit of rake, and when they break through, they tend to grab, which you found out first hand.    Taking some of the rake off the drill with a stone will help drilling brass.   Of course, it then wont cut steel so well.

Here's a sketch of what I mean

Dave

[steamer]

Also, if you hunt around, you can find single straight flute drills  ( D bits) commercially on the market that will give you zero rake and be less troublesome in brass, though you need to clear this type of drill more often.

Dave

[Jasonb]

If you can then drill the small hole first as doing it after will mean as you start the opposite side the drill bit will be deflected by the angle of th ethread.

You could come at it from both sides but even then have an uneven exit.

If you have  alot of work in teh part then maybe screw a piece of brass into the threaded hole to support th edrill on exit and entry.

No need for a jig, tiny spot drill to start then just drill it.

[Dalboy]

Thank you everyone, I think that I have solved the problem by reading the answers here and on another forum.

Just clear up a couple of points which I forgot to mention’

1 I am using a jig to keep the drill bit in line

2 yes, I have been pecking at the work to clear the swarf from the bit

3 I am drilling at a fast speed (as fast as I can on the milling machine.

Now for the reason I think that I have been having which is so simple, but until someone mentioned it I missed.

And that was, I am trying to drill right through with a thread in the hole I am trying to cross drill and the thread is causing the bit to wander off course and breaking the bit.

I will try again with the remaining two #76 bits wish me luck

And thank you again for all the help gents

[Vixen]

I am in the progress of drilling a part for the fuel mixer on the farm boy I am making. This requires a small cross hole of a #76 bit through a 1/" brass rod which has a centre drilled out and threaded.
.........snip..

Are the drills breaking when you reach the threaded section. It could be the drill is snatching just when it reaches the V of the thread. You could try putting a brass screw into the centre hole first, to remove the discontinuity

Mike

Edit    We both posted at the same time.

[Krypto]

Oooh, oooh, I get to post my Dumore video! 

<a href="https://www.youtube.com/watch?v=tCnfdj-l_yM" target="_blank">http://www.youtube.com/watch?v=tCnfdj-l_yM</a>

If you haven't already, check out the tiny carbide 1/8" shank drill bits designed for drilling holes in circuit boards. They are much sharper than the HSS bits I have and easier to handle. Cheap too, or at least they used to be.

As shown in the video, you want lots of speed, a fixture to hold the work and a sensitive lift if possible. To be quite honest, on such a small hole as shown, I couldn't feel much even with this setup.

[Jo]

I'd drill the small diameter cross hole in the bar and then add the threaded larger one after. Any hole or gap in the piece you are drilling into is going to snag such a small drill and break it.

If you have to drill a cross hole and then a through hole of very small diameters: Drill the cross hole first and "Bung" the hole up by push a thin diameter piece of rod of the same material as the parent which is a snug fit in the hole into the hole you just drilled and leave it there, then drill from the end through the entre of the piece. You should find the "bung" will fall out in two pieces. 

Running the drill faster will help but with care you can get away with slower speeds.

Jo