holding a boring bar in a collet?

RoboMiller · Jun 24, 2014

someone in our shop came and asked me for a collet for a boring bar. the boring bar is 3/16 dia. it has a flat along its entire length for set screws to put in the block on a lathe turret as most of you probably know what im talking about.

i said, i wouldnt hold that in a collet. because it has a flat, and although small, i feel like it wont be on center to the collet and doesnt seem to be the correct way to do it.

what say you pros about this?

Mechanola · Jun 24, 2014

Why not?

I don’t find it a bad idea. The flat won’t displace the bar anywhere.
Question is rather: does someone want to put the bar into a spindle or into a tailstock or what?

DMF_TomB · Jun 24, 2014

if tool has a flat and put in collet stress in tool holder is uneven. i have had many thin walled tool holders crack i suspect from holding tools with a flat.
.

Dupa3872 · Jun 24, 2014

I do it all the time. Make parts

greggv · Jun 24, 2014

We do it all the time with no problems.

upnorth5axis · Jun 24, 2014

it depends on flat size small ok big maybe not

PBMW · Jun 24, 2014

I do it all thetime

dksoba · Jun 24, 2014

RoboMiller said:
someone in our shop came and asked me for a collet for a boring bar. the boring bar is 3/16 dia. it has a flat along its entire length for set screws to put in the block on a lathe turret as most of you probably know what im talking about.

i said, i wouldnt hold that in a collet. because it has a flat, and although small, i feel like it wont be on center to the collet and doesnt seem to be the correct way to do it.

what say you pros about this?

That's a great idea. I'll start doing it ASAP!

Captdave · Jun 24, 2014

We do it on 3/8" or less, not sure I would want to try it on anything larger.

BR3 · Jun 24, 2014

Why not just get/make a bushing for a boring bar. It's just a round plug to fit in the tool block with a hole sized for the boring bar and tapped holes for set screws down the side. You could make one almost as fast as I'm typing this. Just don't use set screws bigger than the flat or you might deform the bar enough to make it difficult to remove. These things have been around forever.

Like these:

Boring Bar Sleeve - Boring Bar Sleeve Manufacturer & Supplier, Bhosari, India

Dave K · Jun 24, 2014

You guys who do it all the time, how do you rotate the tool properly so the insert is on center?

Terrible Juan · Jun 24, 2014

Dave K said:
You guys who do it all the time, how do you rotate the tool properly so the insert is on center?

I would like to know the answer to this as well. Those boring bars have flats on them for a reason: to align the insert equal to the slant of the axis.

Please don't tell me you "eyeball" it on center? Please don't.

alphonso · Jun 24, 2014

Terrible Juan said:
I would like to know the answer to this as well. Those boring bars have flats on them for a reason: to align the insert equal to the slant of the axis.

Please don't tell me you "eyeball" it on center? Please don't.

Eyeball it, test cuts on face until tit goes away, hold a square against flat and holder until they feel right, on flat bed machines pinch a 6 inch scale between work and tool adjust tool until scale is vertical. Use collets all the time for bars up to 3/4.

NHBBman · Jun 24, 2014

We do it often at my shop. They can be aligned by using a square or a pair of calipers (locating off the front face and back of the shaft like a square). Rest the square against the front face of the turret and align the bore bar flat so that it is flush to the square. Some of us can eyeball pretty well though after 25+ years of machining. I do it when I get lazy. We have .625" dia. hardinge bushings that are slotted like a collet and they are only used when we can't find the proper sized sleeve. Typically 3/8" and smaller bars.

Dave K · Jun 24, 2014

alphonso said:
Eyeball it, test cuts on face until tit goes away, hold a square against flat and holder until they feel right, on flat bed machines pinch a 6 inch scale between work and tool adjust tool until scale is vertical. Use collets all the time for bars up to 3/4.

I realize your answer to this will be "we done it for years..." but it seems like you're asking for trouble. Take a heavy enough cut, and that bar could spin in the collet. Instant meltdown.

Dave K · Jun 24, 2014

NHBBman said:
We do it often at my shop. They can be aligned by using a square or a pair of calipers (locating off the front face and back of the shaft like a square). Rest the square against the front face of the turret and align the bore bar flat so that it is flush to the square. Some of us can eyeball pretty well though after 25+ years of machining. I do it when I get lazy. We have .625" dia. hardinge bushings that are slotted like a collet and they are only used when we can't find the proper sized sleeve. Typically 3/8" and smaller bars.

Even if you've gotten good at it, why would you do it? It's so easy and quick to tighten a screw or two against the flat. No eyeballing, no squaring up, no pairs of calipers etc. Just clamp and it's done.

M.B. Naegle · Jun 24, 2014

Sounds to me like it's likely to work if you need it to, but for better reliability, use it the way it was designed to use IMO.
In a pinch, I can make a slot in thin sheet metal using a drill bit.... It'd have to be a big pinch though.

alphonso · Jun 24, 2014

Dave K said:
I realize your answer to this will be "we done it for years..." but it seems like you're asking for trouble. Take a heavy enough cut, and that bar could spin in the collet. Instant meltdown.

How heavy a cut can you take with 1/4 or 3/8 bar? I only use ER and TG collets and never have a spinning problem. And, yes, I've only been doing this for 55+ years, so I still have a lot to learn.

Dave K · Jun 24, 2014

alphonso said:
How heavy a cut can you take with 1/4 or 3/8 bar? I only use ER and TG collets and never have a spinning problem. And, yes, I've only been doing this for 55+ years, so I still have a lot to learn.

Not much but you said you use this method up to 3/4 bar. I'm not saying you need to learn anything, just wondering why take that risk when a screw is so easy?

NHBBman · Jun 24, 2014

Like I said Dave, I would only do it if the proper sleeve is not available. The slotted bushings/collets we use are designed for this and have never had any spin. They are actually fairly rigid.

holding a boring bar in a collet?

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