Cockeyed boring bar

[MikeKParticipant @mikek40713]

I just got a set of boring bars.  They seem to be well made and all.  But I’m not understanding why the shank flats are about 17 degrees off from the plane of the insert.

Is there something special about this type of bar?  Should I still strive to get the cutting edge at lathe center height?

I’m thinking that this angle is going to restrict the minimum bore since I have to raise the tool height so much to get the cutting edge at lathe center.  I’m tempted to fix them so the insert is level.

Mike

[David SeniorParticipant @davidsenior29320]

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

Dave

[MikeKParticipant @mikek40713]

Good point.

[Andrew TinsleyParticipant @andrewtinsley63637]

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.

[MikeKParticipant @mikek40713]

Okay, sorry for the amateur question.  I checked the end against a drawing circle template and it does fit through a much smaller hole than if the insert were level.

Mike

 

[StevePParticipant @stevepye68246]

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

[JasonBModerator @jasonb]

Some of mine, see how the angle gets steeper the smaller the bar is so the edge of teh insert does not rub

[MikeKParticipant @mikek40713]

Yeah, that makes sense now.  Thanks.

Is this type limited in depth of cut?  My boring bar experience is limited to homemade ones, a brazed carbide set I’ve used a couple times, but usually using a 2-flute HSS end mill as a boring bar (which works great).

After a few 10-thou-ish cuts with the 7mm bar the insert tip chipped away.  And it was screaming no matter the depth of cut.

Mike

 

[old martParticipant @oldmart]

10 thou with a 7mm bar is quite a heavy cut, especially in steel, the bar probably deflected and that might have been why the tip crumbled. Some inserts are better made than others, and there are many grades for the many types of metal being machined.

You should build up a selection of bar diameters, if possible, sharing the same insert size, and always use the biggest that will start in the hole and also have the shortest length of bar unsupported that will reach without crashing the toolpost into the chuck. This is where a stop for the saddle (carriage ) is useful.

[Nigel Graham 2Participant @nigelgraham2]

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

[MikeKParticipant @mikek40713]

Yeah, 10 thou with a 7mm bar was probably too much, and would explain the screeching.  Hitting the chuck may have been the reason for the insert chipping.  I only just realized that, after pulling the part out.  The part was against the chuck, I was boring all the way through, and the diameter was very close to what the chuck will allow.

[JasonBModerator @jasonb]

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.

[David George 1Participant @davidgeorge1]

I recently wanted a larger boring bar than would fit into my toolpost. I wanted to screw cut a large nut with a full form thread insert and that shape comes in a 16mm size insert and that fitts into a 20mm boring bar. I also found from the supplier of the boring bar that the center height of boring and screw cutting bars have the cutting edge on the center of the boring bar shank. so it was easly to make a new fixed toolpost with a hole on the center height of my lathe and at the same time add an extra hole for smaller boring bars at the same time.

 

David

[MikeKParticipant @mikek40713]

The bar is indeed 7mm.  The set came with 7, 8, 10, and 12mm.  6mm would have been nice, if they insert would fit.

This was my setup.  I haven’t made a toolholder specific for boring bars yet, so I used one of my standard ones.

I got these bars because I didn’t want to keep using my 2-flute end mill, even though it does a great job.  Have yet to find any enjoyment from using carbide…I can grind my own HSS bits and they cut so much better.  In my experience, carbide complains and leaves a poor surface finish.

Mike

[JasonBModerator @jasonb]

Out of interest what diam hole were you cutting, most bars have a minimum diameter and if yours is 7mm then I would expect that to need a 10 or maybe even 11mm hole at the minimum otherwise there is a risk of the holder or insert rubbing which won’t do anything for the finish.

You also mention the carbide “complaining” this can be a result of several things. Excessive stick out is one of them, 5 x dia is the usual suggested amount for this type of bar. Too light a cut can also be a reason as the tool can get pushed out of the cut.

This is my 6mm bar with a CCGT 060202 insert, approx 25mm stick out. First cut is 0.050″ off dia so 25thou DOC and no complaints. Second cut is 0.010″ off dia so 0.005″ DOC and you can hear the high pitch though the finish is still acceptable. EN1A steel, approx 16mm dia hole.

[MikeKParticipant @mikek40713]

Thanks for making that video, Jason.  I have another part to make coming up so I’ll take in the suggestions and maybe also make a video.  You got some nice chips there.  I’ve only ever gotten nice curlies when using HSS tooling.

[MikeKParticipant @mikek40713]

Oh, and the bore’s finished dimension was 0.625″.  I think I drilled to 21/64″ and then switched to the boring bar.

[old martParticipant @oldmart]

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.

[old martParticipant @oldmart]

I checked the threading bars at the museum and found the 16mm shank one for the size 16 threading inserts. It can manage a smaller diameter than a 20mm and I have made some specials for this size insert which are smaller than the 16mm.

 

 

 

[KEITH BEAUMONTParticipant @keithbeaumont45476]

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

[MikeKParticipant @mikek40713]

Remaking my bushing, I drilled to 1/2″ and used the 10mm boring bar instead of working up with the 7mm bar.  I was surprised to get some nice curlies like Jason shows in the second half of his video.  Good finish with 0.010″ DOC (20 thou off dia), not as good with 0.015 DOC.  Could have pushed it further I’m sure.  This is with the CCMT inserts.  I’ll definitely try the GT inserts.

Since I’ve already have this thread, I might as well ask my other concern…

The boring bar’s threaded hole and the insert’s mounting hole aren’t coaxial.  All four bars are like this.  There’s no debris hanging up the insert, but I have to hold down the back of the insert with my thumb while I crank down the screw pretty hard to get it to seat well.  I’m worried I’m going to break the inserts.

[old martParticipant @oldmart]

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.

[MikeKParticipant @mikek40713]

Thank you, old mart.

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