Need help with boring.

I've never been very good at boring. Well, lets face it, I really don't have much skill with any of this yet. But boring has always been a challenge. I have a SB9A and an alloris AXA tool holder. What are the proper tools for boring, and facing blind holes?

I'm a hobbist, making tools and parts for my motorcycles. I'm now making a set of SS replacement brake pistons for an old bike.

I got the front ones done, working on the back ones now but this is really starting to tick me off. I can't get a good finish on the bore or the face of the blind hole.

This is what I have now. Lots of chatter and not too good for blind hole facing. Though the finish of the hole isn't important for function, I want to do it right.

A good, stiff boring bar with a sharp insert is step one, step two is selecting a proper speed and feed.

I don't like the bar you have pictured, with its reduced neck you're unable to choke up on it to minimize deflection. The feed will come from trial and error.

For boring into a blind hole, you really need to move the carriage to a firm stop on the lathe bed. If you don't have a firm stop for the carriage, it's very easy to jam the boring tool into the bottom of the hole which makes ugly marks and ugly noises. If you're using a boring bar like the one pictured, you need to make sure that it has a relief angle in the plan view; that is, the whole forward cutting face of the tool cannot be allowed to touch the bottom of the hole or it will chatter. Either grind the tool so that there is relief, or position the tool holder so that only the point of the tool can touch the bottom of the hole.

Flushed with that success, the rest of the story is to take very light cuts. From the pictures you sent and the tool shown, I think you'd be best off if you took no more than a .005" deep cut on each pass (.010 on diameter) If you use a carriage stop and don't let the whole tool face hit the bottom of the hole, you can end up with a decent finish at the bottom. However, for a really good bottom finish you'll need to take one more final light cut along the face of the bottom. For that, move the tool to that bottom corner, advance slightly and take the final cut inward toward the center.

Also remember that most boring bars are much more springy than a square tool bit used for OD turning so if you're finishing the bore to a diameter, you need to take a few repeated cuts at the same crossfeed position to get to the final diameter.

You'll be there all day taking 5 thou passes. Here's a few simple guidelines for successful boring.

1. Drill as close to final size as possible while still leaving enough stock for a rough and finish pass.
2. Get as big a bar in there as possible, with minimal overhang (don't leave a lot more than you need hanging out of the holder.)
3. Set your cutting edge on or slightly above center.
4. leave a smallish amount in the back of the bore so you can go back and clean up the back face.

Vandis did have some good points though, such as relief angles on your cutting edges. You'll probably need a green wheel for grinding a carbide tool like the one you pictured, but I'd chuck that thing and get a good indexable boring bar.

Those brazed insert boring bars are junk.

Even an old round lathe boring bar, with square holes at ends for HSS tool bit, will work MUCH better. You can grind the HSS bit to suit stainless just fine...

Carbide tooling of that style will work poorly with very fine cuts... Most Carbide tooling is ground to bulldoze off material, instead of a more shearing like action with HSS.

The blunt bulldozer blade takes a lot of shock, while a sharp thin edge in carbide, simply breaks off. The extra pressure needed to start the bulldozing... springs the tiny boring bar shaft...

Cobalt boring bars are available in the exact same style as pictured. They can simple be sharpened by grinding the face.

They are primarily used in a boring head on a mill. Always use the largest size shank/bar you can squeeze in hole for ridigity...

Proper tooling, feed and speeds are all very very important parts of getting good results boring. Having said that, it has been my experience that the single most important factor in boring is rigidity. Without having your work as well as your machine and cutter in it's most rigid state the whole job becomes an exercise in frustration and often defeat. Granted a 9A is not akin to heavy boring or the most rigid of setups, with a bit of time and patience the desired parts can come to fruition. Remember light DOC and many spring cuts. Back cuts often work well to remove some of the deflection, but keep in mind that the tool may cut deeper in reverse direction than forward. Not to be used any were near the finish end of things.

Many times on relatively shallow wide holes like that I don't use a boring bar at all remove as much as possible with a drill bit then grind a longish piece of HSS and hang it out the holder as far as necessary. This works really well with the right holder combination and a lamp post holder
Chuck those brazed carbide bars all they do is chatter and chip only use them in the boring head on the mill and don't like them all that much there either.
The index style is a little better the bigger and stiffer the better, if you haven't guessed rigidity it the key.
And with a SB9 HSS is usually a better choice on stainless or any kind of steel.

"Doing it right" is knowing when finish matters and when it doesn't.

allan

Good point allan the inside of caliper pistons don't need a great finish

No it doesn't need a good finish. My point is that "I" want to do it right. I need the practice, I want the ability to make a nice smooth bore when the time comes. This seems like a good time to practice, when the results won't make scrap.


I was shopping for a new boring bar and remembered that I had bought one a while ago but never used it! I found it in the back of my cabinet still in the box it came in. It did work better, but I really need to figure out the speeds and feeds. I hate working with SS.

Hi Jeff K,
Yes, stainless steel is a PITA to turn on a light duty machine as you have discovered. If using a carbide insert I pick an insert that has a chip breaker edge. That edge is less inclined to "plow" the metal and more likely to slice it as mentioned above with the HSS cutter. "Stainless steel" can be one of many alloys. They all tend to have a high "toughness" rather than hardness but that toughness is still an issue especially for tool wear.

In a pinch I use an endmill cutter as a boring bar. As long as the endmill cutter can fit the pilot hole you can use one tooth positioned properly to do the job. That cutting tip can even be on the opposite side of the bore where it may be more visable. Keep in mind that the tip end of an endmill cutter is a bit fragile but they do cut well.

Gary in AZ

While that new boring bar is a LOT better than a milling bar like you were using before...

It is a basic TPG insert.. Kind of generic, master of none...

Now if the bore is ~1.1 and you are using a 5/8ths boring bar.. You would be better off running a 7/8ths bar..

Why are you stuck on carbide???

The hole is shallow, You are not doing production with full pressure coolant...

HSS will get the job done, with good finish... You can even get a HSS insert for that bar from Warner...

I am not "stuck on carbide" I was merely showing what tools I had available in house already.
My original post explained that I am looking for guidance on proper tool selection for this type of work. I have little to no experience with this, and I have to learn on my own. I assumed that the indexable tools would be the best choice for me. I didn't want to spend a lot of time figuring out if the poor results of my work were due to my speeds/feeds or my crappy tool grinding skills (or lack of them) so I opted for pre ground tools. I have been making some HSS tools but I spend a lot of time grinding and regrinding.

I recommend splurging on a small solid carbide indexable boring bar from Maritool. They use sharp ccmt inserts and are super rigid. They're also inexpensive. http://www.maritool.com/Indexable-Tooling-Boring-Bars-Carbide-Boring-Bars/c50_142_108/index.html

You'll never be sorry.
Jordy

Which SS alloy is this?

allan

303 Stainless Steel.

Yeah that boring bar still flexes way to much funny thing found they cut as good or better coming out. Goes back to bigger stiffer.
I always used low or medium compound on steel and stainless with the SB9, you will learn slow and steady.
Learning to grind your tools right is a large part of the learning, you just don't have the brute strength to wast on inefficient tools
If you notice a common theme of HSS you are right it works better for many things.

re ..fine finish
FWIW ....fine finish w/ lite lathes requires finish cuts under thou.( 3 or less better yet w/ HS stoned cutters w/ a FLAT spot on the cutting edge facing you, large enuf to take out the feed marks.) 1/32 or larger , if no chatter...must be used w/ under thou doc .....carbide requires to much force ...HS can be made sharper ...
.external , the very best finishes in my shop are from a shear grind HS bit which traverses the feed lines at circa 60 degree angle .....regularly used on my crftsmn 12x36 when i am serious ...i can also get a smooth finish w/ no feed iines at .007 feed ( the minimum on my 1895 14 in) uaing shear grind bit @ 3 thou doc....i have not tried that grind internally
used externally , the easies way to visualiz/grind is to grind a radius of 3-4in inches ( a VERY slight full face curve )on a round bar brokend mill works ) , w proper front & side clearance & mld back rake ..put this in a boring bar holder ,BUT mount it so the bit is at rt angles to the lathe center line ...rotate the bit so th eleading is down & runniong at 65 deg up towards the tailstock ...the bit cutting face shud then run form low front at southwest to high rear at north east....when you see the result of a 3 thou SECOND finish cut ,you can grind a square bit to address the work at that angle in a regular tool holder...
best wishes
doc
now that i am tinking abt it , i will grind a shear bit for internal finishing & run a test!