My guess is that if the insert was level the minimum diameter possible would INCREASE otherwise the heel of the tip would rub.

Dave

Nothing wrong with the boring bar. It is how they are intended to be to be. Note that the cutting point of the insert is at the half height of the bar.

Andrew.

On smaller boring bars the tips are angled to create clearance between the front of the tip and the bore of the hole being machined.

Steve p

Or on larger diameters, crashing the boring-bar tip into the chuck or spindle!

That bar in the middle of my photo is 6mm on the round shank, bit less across the flats. 7MM is an odd size is it actually 8mm dia and 7mm flat.

I can happily take 25thou cuts with that in steel with it. Make sure you have the corner of the insert on ctr height and also think about using the **GT inserts which are the bright coloured ones intended for non ferrous metals. They are sharper and will deflect less than the comparatively “blunt” **MT that most of these bars are supplied with at hobby level.

Make sure you have no more bar sticking out than you need to get to the bottom of your hole as increased length increases the chance of chatter.

Add driving the cutter into the bottom of a blind hole as another way you can damage the insert.

I fully agree with Jason about using the sharp polished GT type inserts for smaller cuts on steel. They are sharp like honed HSS and intended for aluminium, but very good with steel. They can stand a certain ammount of interrupted cutting, I like them when boring soft jaws, and thats about as interrupted as you can get. The blunt inserts work best when taking deep cuts, but need rigid support that larger machines have. An intermediate grade would be for stainless steel,which is quite fine edged. Carbide shanked boring bars are better than steel ones because of their stiffness and density, but unforgiving if mistakes are made. I am lucky to have a supply of Densimet at the museum in ground bar form, 1/2″ and 12mm which has both the stiffness and density of carbide bars, but is not brittle and it can be machined. I have made some boring bars for lathes and also for mill boring heads. Here is a pic of one of the boring head tools.

I also take advantage of the boring and threading bars having the tiip height at half the bar size like DG, I was surprised that he was informed that you need a 20mm threading bar for er16 type inserts, I think I have smaller diameters as well as 20mm that take them, I will check tomorror when I get back to the museum, if I remember.

At last years Midland show, I purchased from JB Cutting Tools, an 8mm boring bar that takes the 5mm round GT inserts, I find this gives a perfect shiny finish to a bore,with fine cut and slow feed. In fact ,on the last two model Diesel engines I have made, I found I did not need to use a reamer on the bore of the cylinder.

Keith

The reason for the hole in the insert not being coaxial with the threaded hole is to pull the insert tightly into the seating of the holder. I have made many insert holders for the simpler shapes and also 16 and 22 lay down threading inserts and displace the hole for the thread about 0.004″, 0.1mm in the direction of the inside corner. Sometimes there might be a hint of light showing under the insert after tightening, and I usually remove it and the screw and make sure the socket is clean, and there are no burrs. When tightening the insert screws, use the proper tool, normally a Torx with the plastic handle which is purposely made short to limit the torque. Some years ago, I snapped a DCMT07 in half tightening up even the proper ammount. Chinese inserts have improved since then.