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# Custom made 3/4" boring bar up-scaled from a 3/8" boring bar that can't hold it's pants up.

When I was having boring troubles a few years back I took some time out on the CAD system to make some simple drawings to show the actual clearances for various bore sizes, angles and tool shank sizes.

Which was illuminating. Especially so on smaller sizes.

I found my tool grinding instincts and simply following the books were generally not good. Either vastly inadequate clearance or just plain wrong. It was all too often pure luck that I didn't run out of clearances in unexpected places. Since that exercise things have gotten much less stressful. The usefulness of CAD in quickly doing accurate assessment diagrams rather than hand waving estimates based on look alone is vastly underestimated. My life has gotten much easier since I picked up the habit of such checks. About 10 years too late!

If my insert bars are anything like typical the minimum hole size specified tends to be more than a little optimistic.

Clive

@rons What diameter and depth are you trying to bore?

@rons What diameter and depth are you trying to bore?
Diameter = 1.2500 - 1.2505
Through hole depth = 4.385

Diameter = 1.2500 - 1.2505
Through hole depth = 4.385
I guess you are trying to use a 3/4 bar because that is as big as you can hold? Rule of thumb says 4:1 or 3" max on a 3/4 steel bar. You'd be money ahead to buy a 10:1 3/4 bar or a 1.25 steel bar and a make/buy a way to hold it.

I guess you are trying to use a 3/4 bar because that is as big as you can hold? Rule of thumb says 4:1 or 3" max on a 3/4 steel bar. You'd be money ahead to buy a 10:1 3/4 bar or a 1.25 steel bar and a make/buy a way to hold it.
The current setup is hitting the hole from both sides with a .375 bar extended out 5".
I tried to gauge the accuracy with bluing the inside but the chatter was too much.

I am figuring out how to dimension the insert pocket for this new .750 bar.
When I get that done it will be easy to upscale to a 1" bar or more.

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I'm doing a job thru alum right now where the Hougen annular cutter makes an awesome finish cut on the mill. Not sure how that would work in a lathe.

Diameter = 1.2500 - 1.2505
Through hole depth = 4.385
And you said it's Alu, - a plain bit of 3/4 bar with a small piece of CORRECTLY SHARPENED** HSS in the end will do with no problem at all.

** Means adequate clearance, high top rake and most importantly - small nose radius, and when I say sharp I mean 8X or 10X glass sharp.

This or similar:

A TMX boring bar I have has a flat on the top. The flat adjusts the bar to tilt down 10 degrees.
The insert is a CCMT-32.52 with side relief angle equal to 5 degrees.

The boring bar I made is using TCMT-32.51 with a 11 degree side relief angle.
Anybody have an opinion whether that 10 degree angle should be used?

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Along the lines of the suggestion for Mr Mari's products. this will probably sort you out, and your work and lathe will thank you for it.

Replacement for post #29:

The black TMX boring has a flat on top to adjust the bar's rake for -10 degrees.
The insert is a CCMT-32.51 with side relief angle equal to 7 degrees.
The boring bar I made is using TCMT-32.51 with a 11 degree side relief angle.

-Isn't the rake angle of the insert more important than this -10 degree tilt?
The tilt with some undercut underneath the insert will allow a greater depth of cut.
My bar does not have that tilt and undercut (yet).
-You think that having a bar rake of 0 degrees with a restricted shallow depth of cut is going to cut AL better?

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View attachment 418428View attachment 418429
A TMX boring bar I have has a flat on the top. The flat adjusts the bar to tilt down 10 degrees.
The insert is a CCMT-32.52 with side relief angle equal to 5 degrees.

The boring bar I made is using TCMT-32.51 with a 11 degree side relief angle.
Anybody have an opinion whether that 10 degree angle should be used?
Why not try your boring bar at varying degrees of rotation? You have it mounted in a collet holder that will give infinite adjustment..

Why not try your boring bar at varying degrees of rotation? You have it mounted in a collet holder that will give infinite adjustment..
The collet in the holder is just for reference when cutting up the bar. The bar will taken out of that and stuck into the black L23 holder.

Some of the bars angle the tool for a little more clearance. This is especially important at smaller diameters.

And I don't recall after reading through all this who disagreed with Limy Sami but I fully agree with him. I occasionally use a CNMG for heavy roughing and both the boring bar and the work often pay the price due to chips getting hung up. I don't like them for boring either, unless it's for very heavy roughing where a hangup of chips doesn't matter much or for finishing with a chipbreaker that has very tight control of the chips. Even still, I watch like a hawk over a finish cut with a chip hook at the ready, and often need to use it. All it takes is one stray chip hanging up and the result is a big nasty scratch or gouge.

Why not try your boring bar at varying degrees of rotation? You have it mounted in a collet holder that will give infinite adjustment..

IDK if the clearance angle is all THAT important. Consider that - as EK just said - the relationship at small D and large D will be different anyhow. Mostly it is about making sure that the heel of the (negative) insert clears the bore, while also trying to keep the least amount of acute angle for the chip to flow.

This is one case where the glass needs to be half empty. A little extra negativity is lots better than even just a wee bit too much positivity. Nothing wrong with chucking it up in a collet and eyeballing that tip undoubtedly below C/L.

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I am Ox and I approve this post!

Thank you for helping with answers. Got a sliding fit that feels like a linear bearing.
The face plate in the back is what the blocks mounted on.
The nice thing was I could dial in the fit from too tight to just right.

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