Deep bore with mini boring bar

[gstprecision]

I have to bore and rethread some brakes for Tikka M18x1. After reboring and threading I wanted to re-bore the holes to proper caliber at the same time to keep everything concentric.

My problem is that my boring bar is extending so far out of the tool post it flexes and does not cut at all. Any tricks or other boring bar design? I have a mini 1/8 boring bar for minimum bores of 0.200"

I was thinking of placing the boring bar in a round stock to compensate for that, leaving just enough to go through the brake.

Thanks
GST

 

[Rflshootr]

You can do that if you have the room. If it will be tight space wise, set your round stock up in a V block in the mill and drill the hole off center. put a couple of set screw holes in the thick side and put the whole thing in the tool block on the lathe oriented with the bar to the side where you need the extra clearance.

 

[Woodhunter]

Select the bar required and place it in the square stock holder. One is for 3/8" bars, the other is 1/2"

Holes in the square stock are bored such that the tool bit is on center when the assembly is clamped in the 4 way toop post.

 

[shortgrass]

I have to bore and rethread some brakes for Tikka M18x1. After reboring and threading I wanted to re-bore the holes to proper caliber at the same time to keep everything concentric.

My problem is that my boring bar is extending so far out of the tool post it flexes and does not cut at all. Any tricks or other boring bar design? I have a mini 1/8 boring bar for minimum bores of 0.200"

I was thinking of placing the boring bar in a round stock to compensate for that, leaving just enough to go through the brake.

Thanks
GST

Is your bar made of steel or is it carbide? Are you using inserts? Are the inserts positive rake or negative rake? For those small bars carbide wins hands down, every time. Whether your bar is steel or carbide it should never be extended from the tool holder any further than what is necessary. Be sure the tool tip is on center and sharp! Take cuts of reasonable depth. Feeds & speeds. The basics..... I've got a Kennametal 3/16" carbide bar that I use the same as you are using your bar (will fit inside of a .240" hole) and have NO problems with deflection.

 

[mtngun]

My problem is that my boring bar is extending so far out of the tool post it flexes and does not cut at all. Any tricks or other boring bar design? I have a mini 1/8 boring bar for minimum bores of 0.200"

A pic of the boring bar would help.

Is this one of the Micro100 solid carbide bars? If so which length? Generally a 0.200" I would not go much longer than 1.00" length. They make them longer, but in my experience the longer bars flex too much.

 

[Woodhunter]

A pic of the boring bar would help.

Is this one of the Micro100 solid carbide bars? If so which length? Generally a 0.200" I would not go much longer than 1.00" length. They make them longer, but in my experience the longer bars flex too much.

Yep.

The rule with boring bars is to extend as little as possible.

I have a small boring bar ground from a 1/2" square HSS tool bit, the shank portion is about 3/16" in diameter. It does not flex! I will take a photo and post tomorrow.

 

[gstprecision]

The bar is an Ultradex Scldr 1.2 with a diam of 0.187 using carbide inserts.

I have been using it overhanged to about 1.5" and I knew I was pushing it. I might be grinding a boring tool out of square hss bit.

As I said the flex is out and down hanging out that far of the toolpost so it does not cut at all just drags. I might be getting a slightly larger bar after the holidays when my supplier re-opens

GST

 

[Rflshootr]

In the meantime, try taking a piece of 3/8" or 1/2" round stock about the length of the spacing of 2 of the set screws on your tool block plus 1 1/4" and drilling a hole for the bar to side neatly into. Split the stock lengthwise on 1 side with a hacksaw so that when you clamp it the sleeve collapses around the bar. Hang the bar out of the sleeve the length you need to bore the small hole plus about 1/8".

 

[rputzbach]

solid carbide

I use solid carbide with a 1/4" shank. Indicate barrel true in the lathe, thread barrel, screw on brake. Then pre bore with drill and then take the last .030 with boring bar with .005 cuts. It won't flex. I go .020 over bullet dia.

 

[alinwa]

I use solid carbide with a 1/4" shank. Indicate barrel true in the lathe, thread barrel, screw on brake. Then pre bore with drill and then take the last .030 with boring bar with .005 cuts. It won't flex. I go .020 over bullet dia.

It looks as though those are ground to cut on the in-feed....have you tried cutting your way back out with them?

Howsabout the rest of you'se???? Cut on the IN-stroke or the OUT-feed? Or are there rules for when to do each way?

I try to bore my way back out whenever possible. If nothing else, when fitting brakes on rifles it eliminates the chance of swarf scoring the crown.

That said, I pretty much bore all my brakes using a fixture, not the actual rifle barrel.

 

[Pete Wass]

How about a chucking reamer?

If you have access to one, that is.

Pete

 

[mtngun]

It looks as though those are ground to cut on the in-feed....have you tried cutting your way back out with them?

Howsabout the rest of you'se???? Cut on the IN-stroke or the OUT-feed?

Yes, cut on the in feed. The cutting angles are such that in feed pushes the bar slightly into the cut, while out feed pushes the bar away from the cut.

I sometimes do what amounts to a spring pass on the out feed, just to smooth up the surface a bit.

GST, I didn't catch whether your bar (not the insert) was carbide or steel. Carbide is stiffer. But even carbide flexes, and I'll stick with my recommendation of 1.00" max protrusion on that diameter carbide bar.

A chucking reamer is certainly faster and more accurate if you are content to follow the existing hole, but I got the impression GST wanted to true the existing hole?

I prefer the solid carbide Micro100 boring bars for small, deep holes. They are sharper than insert tools so they will deflect less in the cut. As I type I'm boring with a 0.145" stem (0.180" minimum hole) Micro100 bar with 1.00" protrusion. Rules of thumb for speeds and feeds go out the window when boring small holes. Take light cuts at very slow feeds with a razor sharp, zero radius tool. Withdraw frequently to clean chips. Spring passes are often helpful. ALL boring bars flex to some degree, it's just a question of whether it is manageable.

 

[CMaier]

i was under the impression the carbide wants high speed not slow speed.
i use a micro100 long small bar ...i try to turn the speed up when using it.
???

Take light cuts at very slow feeds with a razor sharp, zero radius tool.

 

[mtngun]

i was under the impression the carbide wants high speed not slow speed.
i use a micro100 long small bar ...i try to turn the speed up when using it.
???

Today I'm running 600 rpm on a 30 caliber brass hole, which works out to less than 50 sfm -- off the charts slow, but it works. Feeds are also ridiculously slow. I'm getting a mirror finish. I'm not saying that faster RPM's won't work -- in my case RPM's are limited by the imbalance of the rotating mass more than anything.

The important thing is a slow feed. The flexible bars do not like to be pushed hard. If you push them too hard they'll let you know by squealing. Push them harder still and you'll hear a "snap" followed by silence.

 

[CMaier]

i do not consider 600 rpm slow on bbl dia work.

 

[gstprecision]

Lots of good info.

The bar is Steel and the inserts are carbide.

I do not have access to a brake reamer, and I prefer to bore them with a bar. Most brakes the hole is not concentric so I use boring bar to ensure they are.

I normally bore on the barrel when I install them but I also rebore a bunch for resale to customers as I get my blanks with 0.220 or 0.250" bore. I make a stub and thread then bore all the brake without removing the stub. When it is removed I scrap it and make a new one everytime.

Alinwa what is your fixture for boring brakes?
GST

 

[shortgrass]

carbide speed

i was under the impression the carbide wants high speed not slow speed.
i use a micro100 long small bar ...i try to turn the speed up when using it.
???

Your 'impression' is correct, C. Running too slow with too light a feed will give a poor finish, where as higher speeds and feeds will leave a much better finish. When you're left with a poor finish the tool isn't working efficiently. I've been boring small holes (1/2" to 1/4") for many years on a production basis. Proper speeds and feeds have everything to do with proper finished size and surface finish. There are several on-line calculators that can be useful. Remember, when figuring speeds and feeds, the rigidity of the set-up and the machine should be taken into consideration. At slower speeds a HS cutting edge would probably work better (especially since HS inserts have come into being - I'd still prefer a bar [the tool holder] be made of carbide) Many doing barrel work use carbide inserts because of the convenience, even though it may not be the 'best' for the slow speed application that barrel work is on a smaller sized lathe.

 

[Woodhunter]

Brake for 338 Lapua. Cartridge on the right is a 308 Win.

I bore the exit hole of the brake after dialing the barrel in true to the lathe centerline. Thread the barrel, crown the barrel, screw on the brake, bore the exit hole. Then crown the brake in the same setup.

 

[alinwa]

.....
Alinwa what is your fixture for boring brakes?
GST

My fixturing is pretty simple but does take a little time to make... I've got 15-20ft of 1" diameter 12L14 bandsawed into 2ft sections and stocked near my lathe. When I need a fixture I throw a hunk into the 4-jaw, spin it around a little and center it up leaving about a 1" overhang. I drill and bore a reference hole thru the center to about 4.5" depth. Once I get a perfectly smooth and true inner race generated I then true up a short referencing surface the outside and turn/thread a stub for the brake. Key is to leave the inner bore race large enough that all future tooling will pass thru, and to true it and bore it to a polished finish. After working on the brake I drag the hunk over to the bandsaw and whack the tool off at about 4" long, spin the raggedy end on the belt sander and set it in the box for future use. I useta' just make a sacrificial each time but I've learned to reuse them....


Next time I use the stub I chuck it up using a dial on the short outer surface then indicate the inner surface for final linearity. I've found that with a short 2-3 inch gripping surface it's quite easy to dial in a fixture within just minutes. Worst case a couple light taps with a brass hammer.

I fixture whenever my brake doesn't have a large "expansion chamber" or isn't set up to leave the muzzle accessible. In other words, when there's any danger of the tooling touching the crown.

Also (here's where it gets weird ) I've been convinced for a while that unless one has a nice meaty shoulder to work with it's hard to keep brakes from flopping around on the threads......so I often double-shoulder them or get them to bear at the front as well as the shoulder, or fit a recess inside the brake etc. Ain't gonna argue the flopping about part but IME muzzle brakes experience a HUGE pulling shock in use, like harder than you could swing a baseball bat, in reverse. They must be TIGHT to achieve repeatable accuracy.... and sometimes it's hard for me to achieve this when drilling/boring in place.

 

[CMaier]

Al,
'splain why ?
why can't you torque in place and then true ?
i am a big believer in torque wrenches for repeatability.

. They must be TIGHT to achieve repeatable accuracy.... and sometimes it's hard for me to achieve this when drilling/boring in place.