Make a quick drill jig from a piece of square bar held in a vice securely bolted to your drill press table. Drill your 3.5mm hole through the bar. Then rotate the bar 90 degrees , but keeping the same side against the fixed jaw of the vice. Now drill your 1.5mm hole through the square bar.

Now you can stick your round bar in the 3.5 hole and drill your 1.5mm hole through it.

Maybe it’s the sizes I’m used to working with as I don’t give 1.5mm a second thought.

If into the end of round bar use the lathe & face the bar, spot drill the end and then in with the 1.5mm drill.

If it’s flat bar then I drill using my mill but clamped to a drill press table or in a fixed vice I would again spot drill and then use the 1.5mm

Cross drilling round bar, again hold in fixed vice, locate centre of 3.5mm rod, spot and 1.5mm drill.

If the hole is deep then retract the drill often to clear swarf and use some cutting fluid

Cross drilling jig in early MEW around 1992/3. Made mine then and still using it. set up time about 30 secs!!!!

If it is not too important, and is cross-drilling, I securely fix the rod in my vise, and lower a milling cutter until the faintest of mark appears.  It is then dot punched and drilled.  Might not be sufficiently accurate for a small diameter rod, but usually close enough for most things I machine.🙂

Made a mess of doing as the OP wants to do. Broke a carbide drill to boot (1.2mm) Broken bit stuck and in quite deep.

Finally spark eroded the both the drill and the hole. Am I glad I made a spark erosion machine!!

ANDREW.

The best method rather depends on what equipment you have and what accuracy is required.

Cross drilling jig may be OK if you have the lathe to use it on but if you are having problems drilling a hole what is the likelyhood of making the jig with the vee perfectly central and that also assumes you have something to machine the vee with.

If you only have a bench drill then holding the work in a vice and bringing the point of a spotting drill down to trap a steel rule will get you quite close. Clamp the vice when the rule is horizontal and then spot before drilling. If you have a batch to do then clamp something against the end of the rod and subsequent ones will go back into th evice in the same position.

As for me if I need it accurate I will hold in the mill vice and use an edge finder and DRO to get th ehole position, both across the rod and in from the end. Probably more accurate for the end position than marking out and lining up in a cross drilling jig. Also works if the piece of 3.5mm bar is very short.

A Vee-block for a limited range of diameters does not need be of Vee-profile.

It can be a rectangular channel, which may be simpler to make especially if for a cross-drilling pad held in the lathe tailstock.

I centralise a Vee-block on a bench-drill simply by gently holding the block, in vice, down with a piece of faced bar in the chuck, or the chuck nose itself, while clamping the vice to the table.

I once had to make a coupler where the hole was offset from the centre axis:

Did it in the mill by bringing an end-cutter down to make a flat, centre-drilling a starting hole, and following through with a twist drill.

Had to go in the lathe for knurling and grooving.  The exciting part was the rod being 1″ diameter and 4′ long.  Had to take special precautions to stop it whipping!

Dave

Ian, maybe you can tell us what machines you have available.

For three or any number in line I would do as I mentioned and just feed th emill along the required amount to get the spacing, as the rod remains held in a vice they will all be in line. But if you don’t have a mill its not worth suggesting such methods.

On 19 November 2025 at 17:38 Ian R Said:

I need to drill three holes in line in my 3.5 mm bar How would you suggest I ensure the 3.5 mm bar does not move inside the square bar

Thank you

What’s the square bar doing, Ian, and how are you holding the work for drilling?

On the face of it, if you have a mill, the job should be very straightforward, but without further explanation from you it’s impossible to know exactly what you’re attempting.

The GHT / Cedric method is brilliant but may not have been understood.
The guide piece drilled centrally in the lathe when subsequently held in the vice turns that vice into a cross-drilling jig by placing the guide hole equidistant from each jaw with the jaws 3.5mm apart. Then a square or round bar can be inserted between the jaws underneath the guide for cross-drilling.
A clamp on the target bar can be used to prevent rotation/provide a reference plane. The distance from the end can be measured relative to the guide with callipers allowing for half the guide diameter.
After a few years one could build up a collection of such guides. Bother, should have started this years ago.

On 19 November 2025 at 19:12 Bazyle Said:

The GHT / Cedric method is brilliant but may not have been understood.
The guide piece drilled centrally in the lathe when subsequently held in the vice turns that vice into a cross-drilling jig by placing the guide hole equidistant from each jaw with the jaws 3.5mm apart. Then a square or round bar can be inserted between the jaws underneath the guide for cross-drilling.
A clamp on the target bar can be used to prevent rotation/provide a reference plane. The distance from the end can be measured relative to the guide with callipers allowing for half the guide diameter.
After a few years one could build up a collection of such guides. Bother, should have started this years ago.

Thanks for clarifying. Here is a picture of a mock up showing the basic principle.