Small Boring Bar Can Only be Used on Big Holes?

[SteveHGraham]

I wonder if anyone else has noticed this issue. I have a small boring bar which, unless I am mistaken, can't be used on small bores.

I bored a hole about 0.875" in diameter in a piece of aluminum. The width of the bar I used is around 0.560", at most, from the edge of the insert to the other side. I couldn't get a good finish, and when I started twiddling around with it, I noticed that the relief on the insert is so slight, the hole has to be really big before the lower edge of the insert stops dragging on the cut work.

The insert edge contacts the bore at its widest point, pretty much. Below the cutting edge, the work curves back toward the bar. The insert is about 3/16" thick, and the relief angle on the side is not big. In order to keep the lower edge off the bore surface, you apparently have to have a big hole. So the bar is less than 5/8" wide, but the hole has to be a lot bigger.

If I had a thinner insert, the problem would be reduced. I am wondering if I bought the wrong inserts at some time in the past, but I think I would have noticed if I replaced a thick insert with a thick one.

[BadDog]

Different inserts have different relief clearance. You just need a small bar that takes inserts with enough relief (as presented to the surface) to allow the bore desired. As you noted, the size of the bar is only one of the limiting factors. But even negative inserts can be used in smaller bores by changing how it's presented, assuming that the profile can still provide properties that are still within the allowable range for the process and material. So assuming a clockwise tool spindle (reverse for work mount spindle), rotating the bar counter-clockwise will increase clearance allowing a smaller bore. The rake angle is on the end, so that's not directly affected by rotation, but the presentation is now tilted and increases the force pushing the bar away, and may also crowd the chip in the bore.

I like my carbide (insert and brazed) tooling for many things, but boring small holes to me is much more approachable with ground HSS steel where I can control each property to my needs. For this specific purpose I acquired some round HSS so that with a start somewhat like a D-reamer, small(ish) hole boring bars become quite a bit easier to make. However, I've not spent enough time to be sure how well this will work. Really small hole boring bars are another thing to themselves, but generally seem made along the same approach.

[SteveHGraham]

I got out my old man specs and a flashlight, and it looks like there was a tiny bit of aluminum melted onto the bottom of the insert, so that's probably 90% of the issue. I guess I can bore out a piece of scrap and see. Now I know to check for aluminum and knock it off the insert while I'm trying to adjust the height.

RE HSS, I'll tell you why I don't use it all the time. It's a pain cutting the blanks into short pieces for boring bits. It just occurred to me to check Enco to see if they sell tiny blanks. It looks like 2 1/2" is their shortest size.

Am I missing something here? I don't know why it never occurred to me to check for short blanks before. It now seems like an obvious thing to sell, but Enco doesn't have them.

[BadDog]

They cut easily with a cut-off abrasive blade. In fact, nick it good on both sides (or around) and break it if you don't feel like going through it.

And be careful knocking build up off inserts, it can take a chunk of carbide with it. Not that you can do much that I've seen to avoid it other than keeping it wet so it doesn't form, but if it takes the cutting edge with it, or pits the flow surface, you'll want to know.

[SteveHGraham]

I don't have an abrasive chop saw because I opted for a carbide job. I do have a hand grinder with cut-off wheels. I have used a Dremel. That was no fun. I guess the angle grinder will do it.

[f350ca]

I have a Kennametal boring bar that takes TNMG inserts, the insert sits tilted down to give clearance. This bar has a flat to orientate it, setting the insert at the correct angle. Could yours need to be turned in the holder to get the clearance needed then the height set to put the cutting edge on or just above centre.

Greg

[BadDog]

I use a 4.5" angle grinder with cut-off wheels. I use it for lots of stuff, so grabbing it to cut HSS is a non-issue, and it makes VERY short work of them. I wouldn't even consider dragging out my abrasive chop saw for this.

I have a Kennametal bar just as you describe. It's a 1", and I think the minimum hole is something like 1.5" or so. I think the smallest insert bar I have (solid carbide, I hate to using anything else if I can avoid it) is around 1/4" with a 3/8" minimum hole. It uses a tiny little insert with a lot of clearance that doesn't hold up particularly well. I've found that the 3/8 solid carbide bar (min hole just under 1/2" I think?) is the smallest I prefer to use. The bar is rigid, and the inserts are readily available and hold up very well. If it's not solid carbide, that cut off would be at about 1/2" bar, unless it's a relatively shallow hole. Inserts don't like flex and vibration, so when I get below those limits (depending on depth), I switch to HSS. But I believe that all my insert bars have flats to orient. Only the brazed carbide and HSS have fully round shanks.

[revrnd]

I have a SI-SCLC R boring bar that has a 1/2" shank dia'. The minimum bore is .560" It uses CCMT inserts. I haven't had any problems using it.

[revrnd]

BTW, it has flats that allow it to be used in a typical tool holder.

I wonder in the case of your boring bar, the use of a holder like this would help you out:

boring bar holder.jpg (11.18 KiB) Viewed 2765 times

You can rotate the boring bar to increase the relief.

[SteveHGraham]

Sounds like the bar I have, except for the part about not having problems. But it's starting to look like operator error is involved here.

[SteveM]

One trick you can use is to mark the front of the bit with sharpie.

If there is any rubbing, you will see exactly where because the sharpie will be marked off.

Steve

[revrnd]

This is what mine looks like:

boring bar.JPG (9.17 KiB) Viewed 2734 times

What is the OD of your bar?

This chart from the KBC catalog shows the shank dia' & min' bore size:

http://www.kbctools.ca/products/CARBIDE ... /8575.aspx