Boring holes with nice finishes

[jmarkwolf]

I'm not a rank beginner in machining, but I'm firmly categorized as completely unable to successfully bore a clean hole with a nice finish, either on the lathe or the mill, without severe striations and screeching and expending lots of oil attempting to keep the screeching to a minimum. Pics below.

Recently I needed to bore a 1.686in dia hole, 2in deep, in the end of some 2.25in dia cold rolled round stock. The largest diameter drill bit I own is 1in, so using progressively larger drill bits, I achieved the desired 1in dia hole in my 12x36 Grizzly G4003G lathe, without difficulty. Then I moved the work to my Acer Emill (full size Bridgeport clone) and successfully opened the hole to 1.25in dia with a couple of progressively larger endmills, without difficulty.

After which I proceeded to bore the hole to the required 1.686in diameter, using economical soldered carbide 1/2in shank boring bars on an equally economical boring head.

As mentioned, the effort was extremely noisy, trying different boring bars, different feed rates, different depth of cuts ranging from 0.010in to 0.030in, and different spindle speeds, making a spring pass after each cutting pass, all with copious amounts of oil.

The resulting hole (as usual) had severe striations, and I could even feel a "hump" in diameter dimension, approx midway down the depth, by running my finger down in the hole. I was ultimately able to clean up the bore somewhat with additional spring passes, at the risk of oversizing the hole dimension, and moving the work back to the lathe and running shop roll and scotchbrite in there.

My boring head make/model is unknown (probably Asian), taking 1/2in shank bars, purchased at a used tool store, but it seems to be "tight enough". I checked the diameter after each cut with a telescoping gauge. The bars are the most suspect, at least in my mind. They are (also Asian in origin), choked up enough to only reach the bottom of the hole, but I'm wondering if they are resonating under load, hence the noise.

I thought maybe it's time to graduate to better tools, but after looking at the prices of Criterion boring heads, I thought better of that idea. Don't bore holes often enough to warrant the expense.

So, aside from just trying a better grade of boring bars next time, can anyone identify fault(s) in my process. I've never been able to achieve a nice finish while boring.

Advice and suggestions gratefully received.

 

[C-Bag]

I’m in no way an expert and I have much the same tooling and way smaller machines. All my brazed carbide boring bars came dull. I sharpened them on my carbide tool grinder and put a small radius on the tip. Then I am careful to mount them in either the lathe or my no name boring head with a back rake relative angle. If I start getting probs with finish a basic hone refinish on the grinder and good to go. I’ve done 2.5“ in steel, 3.25” cast iron and 3” aluminum. Unfortunately I only took a pic of the aluminum, but work for me. I hope I’ve not now jinxed myself or opened myself for flaming…..

 

[Jim F]

Brazed carbide tools are not sent sharp, they need work to cut correctly.
Also, that is a lot of stick-out on the boring bar.
try choking up on it.

 

[Badabinski]

I have no facility for grinding carbide (or anything, really! No room for my bench grinder right now), so I've taken to cutting down cheap 1/2" indexable boring bars and then loading them with HSS inserts. It's absolutely not the best (or cheapest) way to go about things, but it was the only way I was able to do any useful work with a boring head on my little mill.

 

[NC Rick]

Rigidity is very important. HSS seems the best option for lower rigidity situations. I also am no expert but always get something between pretty good to excellent surface finishes. If you hear it singing, stop because it isn’t working. You want you cutting edge above or + from the center line and a touch with enough relief so you don’t get rubbing.

 

[Lo-Fi]

Brazed carbide tools of the economical variety are almost always of a geometry that gives negative rake in a boring head. They cut appallingly. It's a similar story as the Chinese 123 blocks that have the "clearance" hole smaller than the threaded ones: They've listened but not quite understood.

You'll have a much nicer time with insert boring bars, or the type that take 1/4" HSS. The brazed ones can be made to work, but they often need a lot to make them usable in a boring head.

The explanation goes thusly: when mounted in a lathe, the center of the bar can be set below work centre height to bring the top of the carbide to, at very least, neutral rake. In a boring head you don't have this option, so end up with negative rake and horrid cutting action because it's rubbing more than cutting.

Hope that helps! Been there myself with brazed boring tools.

 

[f350ca]

Sounds like your cutter is rubbing. Try rotating it to where the bottom of the cutter and boring bar are clearing the bore. May end up with negative rake at that point.

Greg

 

[mikey]

Back when I was a baby hobby guy, I used brazed carbide boring bars all the time. (Hey, I wasn't cheap; I was poor, and there is a difference!) These tools can actually bore a pretty decent hole but you have to prep the bar first and then use it properly.

I suggest you take a diamond stone and sharpen the side and end edges, then get the top flat. Make sure there is adequate relief on the side and ends. You do NOT need a nose radius on these tools, although a very tiny, tiny, tiny one won't hurt. You want to put a nice smooth finish on the insert.

To go 2" deep, you need to use the biggest bar you can fit in your pilot hole but it will work just fine. Set the bar in your boring bar holder so that the very tip, the corner of the bar, points up a few degrees, just enough to see it pointing up; then lock it in the holder. Put the holder on your tool post and set the height of that tip at center height PLUS 0.010". You want the tip to be 0.010" higher than center, okay? Next, align the center of the boring bar with the center of the lathe and then nudge the tip just a few degrees to the left. What you have done with these maneuvers - raising the tip above center and turning the tip towards the wall of the bore - is to reduce tangential and radial cutting forces so that the bar will cut with greater ease and accuracy. Once you set the bar like this, you should be able to do the entire bore without touching the set up.

Brazed carbide needs more speed than HSS to cut well. I normally use a cutting speed that is double that for HSS so about 200 SFM will do. I would make roughing passes at about 0.020" deep and finish at about 0.005". Try to feed so that your chip looks like a coiled spring. Use coolant and definitely use a carriage stop so you don't crash into the bottom of the bore.

It might help to read this before you get too far along.

 

[MrWhoopee]

Sounds like your cutter is rubbing. Try rotating it to where the bottom of the cutter and boring bar are clearing the bore. May end up with negative rake at that point.

Greg

This is the problem. The cutting edge of those brazed carbide bars is above the center line of the shank. When the bar is rotated to put the cutting edge on center, the top surface now has a negative rake. I takes a large amount of grinding to correct this condition. In this picture, the bar on the left is as received, the one on the right has been ground to achieve NEUTRAL rake, not even positive.


Note that the BOTTOM of the carbide is roughly at the centerline of the bar.

These bars can be used in a lathe because they can be lowered without rotating. In a boring head they're just not worth the trouble.

 

[Parlo]

Get a set of these 1/2" indexable mills and grind some clearance on the side of the insert. You get 3 milling cutters that can also be used as boring bars. Win win. £57.00. Sharp Aluminium grade inserts are great for finish cuts. I get great quality machined finishes on slower rpm to reduce chatter and use the fastest feed on the quill.