Making A Boring Bar

[rwm]

I would like to make a new boring bar rather than buy one. I have a nice piece of .780 stock. My plan is to turn that between centers to get it down to .750. I would like to use a TCMT 321 insert since I run those on other holders.
I am inviting suggestions and design help. I have a small mill that I can use to cut the insert pocket. Should it be horizontal or tilted at all? This will be predominantly used for hogging out aluminum. Not yet sure how to cut the pocket and get the screw hole in the correct place. I guess that's just math.
Pics to follow.
Robert

 

[eeler1]

Aren't boring bars usually hardened? Might affect what material you use.

 

[rwm]

This was discussed in another thread. Hardness does not equate to stiffness. Apparently mild steel is sufficient. .750 is the thickest my holder will hold.
OK not exactly between centers:

The TIR of this chuck is only 0.0015" so this will be close enough.
Another question: What is best practice for preventing the tiny grooves that occur when machining down the diameter of steel barstock? I assume fast speed and light finishing cut? Tool type? Angle?
Robert

 

[mikey]

I haven't made an inserted tip boring bar but if I was going to try it, I would try for the SCLCR-type that takes CCMT and CCGT inserts. The CCGT inserts for aluminum are quite good. They mount with the insert flat, not at an angle. I looked but couldn't find the pocket dimensions for this type of insert.

In my experience, more lead angle (turn the tool more clockwise on the post) and a larger nose radius will give better finishes. Cutting oil, higher speed, slower feed and light finishing depths of cut also help. I much prefer HSS for this kind of thing.

 

[rwm]

Good thoughts Mikey. I should probably have gone to HSS.
I have no good way to machine a pocket for a diamond shaped insert. In fact my plan for the T is to just have a single wall along the back of the triangle contact the insert. I am thinking that if I use a 3/8" end mill and the insert has a 3/8" IC I can easily find and drill the center. I'm sure the diamond shape would be more durable since it is 80 deg vs 60 deg. I suppose I could cut a pocket with a rotary table to line up the cuts maybe?
Bar:


Found out my lathe cuts a taper of .001 over 6". I assume the tailstock is off .0005? I don't think that is worth chasing. If it's really just a tailstock issue it will also be .001 taper over 30".
Robert

 

[rwm]

Next question: Screw size for the insert and a source. I see plenty of metric screws on ebay from China with a delivery date way in the future. I have a set of tools that uses 6-32 screws and others that are metric. Is there a preferred standard? I can get 6-32 torx from McMaster pretty quick. I can pull a screw off another tool but I would like to use something very common.
Robert

 

[mikey]

The taper inside an insert's hole is standardized. You can find screws to fit these tapers on ebay from Kennametal and others. A standard screw will not hold the insert solidly enough.

 

[T Bredehoft]

What is best practice for preventing the tiny grooves

Use a tool with a larger radius, ie., if you are feeding .003 and your tool radius is .003 there will be a ridge between the grooves. if your tool radius is .012 there won't be a radius. Just an example, .003 is a pretty small radius for a tool.

 

[rwm]

Tom- My tool was a TCMT 321- hence a radius of 0.016. For the final cuts I advanced the cross slide 0.005 and then 0.002. I still get ridges. I am not disputing what you are saying. Next time I will try a 322 insert (radius=.032) or a HSS tool with a radius and see how that goes.
I am thinking about putting a gearhead motor on the far end of my lead screw for long cuts such as this so I can advance the tool even more slowly with the spindle at high speed. I assume a toolpost grinder would be the ultimate way to do this?

Mikey- good info- I decided to go with a M3.5 x .6 screw since I have another tool that uses that and metric seems to be more common. Also a US seller so I will have it soon.


Robert

 

[rwm]

Bad news...maybe. I just realized the mystery metal I started with is not mild steel. It is pretty hard. Can be filed with difficulty. Not sure about tapping this.
Robert