What Boring Bar Diameter

[Texasbowhunter]

Plz forgive me with the stupid questions I'm asking while I'm learning some of the fundamentals for the craft...
So I was curious the other day and got to thinking about boring bars and the diameter of the bars as to what be better suited for general all around needs, and to what diameter hole/or depth of hole they should/would best suited for...Since bars are expensive and I am just a novice and can't afford multiple bars I was thinking I may just get a couple to cover most of the bases I may come into contact with from time to time...
Is there a General consensus/understanding on Diameter of the bar in relation to the depth of the hole your cutting?
My thoughts were and they are only thoughts but would a 3/8" dia. bar be used for say up to 1" depth
1/2" be used up to 2"-3" depth
3/4" be used up to 3"-4" depth
1" be used up to 4"-5" depth
being all equal with the diameter of the hole and material
I understand a 1" diameter bar would not go into a 3/4" hole
However if that hole was say 4" deep and had to be within a given tolerance then what size of bar should be used?
Thanks
Paul

[nickel-city-fab]

Yeah, that looks good. My rule of the thumb is, "Use the fattest bar you can get in the hole, and still have room for the chips."
Also, "Try to have it choked up in the tool holder as far as possible without smacking into the part'" (real short stick out)
I wish I had a better selection of boring bars, but so far they all work pretty much the way you said. Some of them take inserts, some use HSS (home-made). My biggest one is 3/4" dia x 10" long and it does just as well as the carbide-tipped smaller bars.

The 3/8 one is handy for starting out smaller holes, but it does flex -- takes a few spring passes.

[Rich Carlstedt]

Paul you are not far off, but as NCF said, use the biggest bar you can .
In general, you look at the hole size first , not depth as the criteria ,
Then as the hole deepens you have two choices ,, lighter cuts or a stiffer bar . ( like all carbide)
and for really deep holes you must consider Gun drilling, OR line boring the work

There is another way that you don't see anymore ....that is a "balanced " boring bar , that can do really deep holes
This is a bar with 2 cutting edges on the front of it and they are 180 degrees apart and predetermine the bore size as the cutting edges offset the load from the other edge

Rich

[sarge41]

Texasbowhunter: If you are learning the trade, why not make some boring bars, It would be good experience and not cost much. I would make the first ones to use high speed bits. later, you can make some to use inserts. The first one do not need to be exotic steels, just use what ever you have. If you happen to have a piece of drill rod, that is good. Good luck.
Sarge41

[mattthemuppet]

the general rule of thumb is 4-6xD where D= diameter of the bar for a steel bar. So a 1" boring bar can go 4-6" deep. You can go deeper than that with care and you can get carbide shank boring bars too. Generally though you use the biggest bar you have that will fit.

Check out the SCLCR boring bar bundles, you can get a set of 4 or 5 different sizes with inserts for not much money. That'll get you started for sure.

[754]

The chrome rod from strut shocks seemed to work good for me..

[nickel-city-fab]

Texasbowhunter: If you are learning the trade, why not make some boring bars, It would be good experience and not cost much. I would make the first ones to use high speed bits. later, you can make some to use inserts. The first one do not need to be exotic steels, just use what ever you have. If you happen to have a piece of drill rod, that is good. Good luck.
Sarge41

That is exactly how I got some of my boring bars, made out of surplus Jeep axle with a hole drilled cross-wise for a HSS bit, and another hole for a set screw. Works just fine.

[old mart]

Common commercial practice says 4X the bar diameter for depth with steel bars. You can exceed this if you take small cuts and have the patience.

[deltap]

I read somewhere that all steel has about the same modulus of elasticity and it doesn't matter what type of steel the bar is made of because the same cross section will have about the same amount of flex. Maybe someone else with more knowledge will chime in here. Carbide does not sharpen to as fine an edge as high speed and seems to leave more of a tapered hole. Boring bar should honed very sharp at least for finish pass.

[Texasbowhunter]

Texasbowhunter: If you are learning the trade, why not make some boring bars, It would be good experience and not cost much. I would make the first ones to use high speed bits. later, you can make some to use inserts. The first one do not need to be exotic steels, just use what ever you have. If you happen to have a piece of drill rod, that is good. Good luck.
Sarge41

I had never given any though to this to be honest...
However I'm game...Got any pics I could see to get an idea on how to fab one?
Paul
Updated:
Never mind I did a DIY search and found several videos on how to build one...I was not aware that one could fab one without the use of a mill...
Thanks for a direction

[nickel-city-fab]

I had never given any though to this to be honest...
However I'm game...Got any pics I could see to get an idea on how to fab one?
Paul

Here's a pic of a couple of mine. The set screw is just a regular 10-32 set screw you can get at any hardware store.
You can drill both ends of the bar, so that one end the tool points out at 45 and the other end is at 90 degrees.
These were made out of an old jeep axle.
By the way, if you size the drill holes right, you can use square tool bits too.
I just shoved some scrap ones in there for the photo so you can get the idea.



The big bar is 3/4 x 8", the smaller bar is 5/8 x 10"
the tool bit holes are drilled for 1/4" round bits.

Hope this helps. That big bar is one of my best performers.

[nickel-city-fab]

Here's a better shot of the smaller bar, with a 3/16 square tool bit in it,
this setup works great for both boring and threading.

[tom_d]

Plz forgive me with the stupid questions I'm asking while I'm learning some of the fundamentals for the craft...
So I was curious the other day and got to thinking about boring bars and the diameter of the bars as to what be better suited for general all around needs, and to what diameter hole/or depth of hole they should/would best suited for...Since bars are expensive and I am just a novice and can't afford multiple bars I was thinking I may just get a couple to cover most of the bases I may come into contact with from time to time...
Is there a General consensus/understanding on Diameter of the bar in relation to the depth of the hole your cutting?
My thoughts were and they are only thoughts but would a 3/8" dia. bar be used for say up to 1" depth
1/2" be used up to 2"-3" depth
3/4" be used up to 3"-4" depth
1" be used up to 4"-5" depth
being all equal with the diameter of the hole and material
I understand a 1" diameter bar would not go into a 3/4" hole
However if that hole was say 4" deep and had to be within a given tolerance then what size of bar should be used?
Thanks
Paul

Looks like you're new around these parts. Welcome to the forum. Don't worry about asking questions. It's only stupid to not ask. There's an incredible depth of knowledge among the members that's just waiting to be shared. Only way to do that is by asking questions. Even the members that have been machining for decades, often as a profession, learn new things from this exchange.

You don't need to fill the tool crib on day one. Buy your tooling as you need it. It spreads the costs out, and you're only buying what you need. Before focusing on any particular bar diameter it's best to think about what you will be using for a cutting tool. Will these bars have carbide brazed to the ends? If you go with a replaceable insert design, what inserts will you be using? Some bars have a hole for a piece of hand ground HSS that's locked in place with a set screw. What materials are you most often working with; mild steel, aluminum, plastics? This will influence both the cutter and its geometry.

the general rule of thumb is 4-6xD where D= diameter of the bar for a steel bar. So a 1" boring bar can go 4-6" deep. You can go deeper than that with care and you can get carbide shank boring bars too. Generally though you use the biggest bar you have that will fit.

Check out the SCLCR boring bar bundles, you can get a set of 4 or 5 different sizes with inserts for not much money. That'll get you started for sure.

Yup. What I've grown up with as a general rule of thumb for boring bars is five diameters. Whatever the boring bar diameter x5 is what I can expect to have for bar extension from the holder without having to resort to any sorcery for dealing with tool chatter and poor surface finishes. And yes, that's with a generic steel bar.

[Texasbowhunter]

Thank You for the pics and tips...
So I have a couple boring bars at the present...1/2 and 5/8
I have a holder for my BXA mount which allow a 3/4" bar or I can remove the sleeve and have a 1" hole which to my uneducated mind would allow for a 1" bar to be inserted and held securely in place...So the thought was perhaps a 3/4" bar would allow for me to bore a hole say 3"- 4" deep? But if I'm going to buy one I might just look for a 1" that seems would do the job...
Now I think carbide bar might be out of the question due to cost vs how often I would use it...so a steel bar would be the choice...I saw a 3/4" bar 10" long in a reasonable price range...Thoughts? or just hold out for the 1" bar...One thing I would like to find is having the inserts that I presently use to interchange with the boring bar....I haven't looked real hard yet at the 1" bars but hope that the inserts I use would fit providing the consensus is the holder will secure the 1" bar properly...
Thanks
Paul

[rickyb]

I read somewhere that all steel has about the same modulus of elasticity and it doesn't matter what type of steel the bar is made of because the same cross section will have about the same amount of flex. Maybe someone else with more knowledge will chime in here. Carbide does not sharpen to as fine an edge as high speed and seems to leave more of a tapered hole. Boring bar should honed very sharp at least for finish pass.

All steels have the same modulus of elasticity so all deflect the same amount. Hardness of steel comes into play by increasing the load at which permanent deformation occurs. Since a boring bar won’t get near to yield loading, all steels will behave the same, soft or hard. Localized stresses in the holes of old school bars can reach yield and loosen the inserted bit.

lower deflections and less chatter in a bar of specific size can only be had by using materials with a higher modulus like carbide.