On 27 September 2026 at 18:35 DC31k Said:

What is your, or do you have some, plan for the initial hole?

A 10mm diameter twist drill followed by a 12mm centre cutting mill might be a good place to start as then you can buy a boring bar that fits into a 12mm diameter hole and the bar itself only needs to worry about an extra 3mm of flat bottom. That bar might have a 10mm shank.

If the finished hole is 15mm diameter and the flat bottom is to be cut purely by the boring bar, the maximum bar diameter would be 7.5mm, which is quite small.

It might be good to start/practice with the same diameter but a flat bottom at 3mm deep so you can see what you are doing.

What is the corner detail at the base of the hole? With an insert bar, there will be a corner radius. If a square corner is needed, a slight undercut may be required.

On 27 September 2026 at 17:52 Andy Brocklehurst Said:

I’ve never done any boring on a lathe before but a friend has asked if I would make him a part out of aluminium so I thought it would be a good thing to start on! It’s a simple part just

A straight 25mm od round with a 15mm id bore going 50mm deep but with a flat bottom at the end, it’s basically a cap. Dimensions aren’t critical but I thought it would be a good practice to treat the part as if they were.

There’s a vast array of boring bars so what boring bars would I need to get to make this, especially the flat bottom part? I’ll be using a wm180 lathe

Many thanks

On 27 September 2026 at 18:33 JasonB Said:

The flat bottom will be your biggest problem as the tip of the boring bar bar will need to go from ctr to outside of the hole which means it can only be 7.5mm maximum thickness which i snot ideal at 50mm depth.

If he can get away with a hole drilled say 12mm or 1/2″ with the resulting conical shape that is then bored out to 15mm you stand a better chance

My weapon of choice after drilling 12mm dia would be an insert boring bar of 8 or 10mm shank with a CCGT 060202 insert, paraffin to lubricate the cut.

If it does have to be flat make a D bit from 15mm silver steel to finish the bottom after drilling and boring.

Thanks Jason,

Ill get that bar and inserts, and drill out as you suggest.