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hwingo
05-15-2009, 11:26 AM
I have a question regarding boring on a lathe, tool flex, and surface finish.

I am using a 3/4" boring bar with carbide insert (triangular w/60 degree angles). Depth or length of the hole is 3.6". The material is SS. When boring a hole (~1.250 ID), the entrance hole is finalized to the desired size but the exit hole is easily several thou smaller. I find it necessary to make several final passes before the exit hole *comes close* to being the size of the entrance. I assume this is the direct result of tool flex?? If this is the case, and since the boring bar's length is *fixed* and length doesn't change, why would it not flex at the entrance as it does at the exit?

Regarding surface finish, when a cut is made toward the head stock, the cut is bright and very smooth. When boring through to the opposite end and the carriage travel is reversed, more material is removed but the finish is quite poor in the opposite direction. Why? What should I do to get the same finish coming back as I do going forward? The insert is *new*.

Harold

lakeside53
05-15-2009, 11:38 AM
How's the insert mounted verse the work? What insert? You insert may well be wearing before it gets to the far end, or, the SS is work hardening. What type of stainless? What happens if you increase the depth of cut?

Assuming your triangular insert is not oriented to a 60 degree point right at the work, you'd need to change the lead angle on your insert to bore both ways with the same finish. Hauling it back without increasing the doc on stainless might contribuate to work hardening of the material and lousy surface finish.

Carld
05-15-2009, 11:52 AM
The bottom of the cutting tip may be rubbing on the bore if the insert is straight sided. Cutter clearance is very important when boring. Also the height and angle of the cutter in the bore is important. The flex on the bar should be constant from one end of the bore to the other end. Perhaps your spindle bearings may be loose or the gibs on the compound, cross slide or carriage.

hwingo
05-15-2009, 01:51 PM
Hi Guys,

I think I'm OK with the position of the point against the work piece. If standing at the tail-stock and looking at the tool relative to the bore, the tip is slightly below center and the point is turned to about the 8 o'clock position as compared to perpendicular.

Carld, I never thought about checking the gibs on the carriage although I did tighten the gibs on the compound.

Harold

moldmonkey
05-15-2009, 02:31 PM
"Spring" cuts can be necessary on less than ideal set-ups & machines. With less tool pressure they can cut oversize though, so I like to rough out, take a spring cut, measure, divide the stock left into a couple of uniform DOC finish passes. I measure in between the finish passes to make sure there aren't any surprises from having less tool pressure.

On finish passes, I would bring the tool off the work before retracting out. A indicator and mag base set to show cross-slide travel will get you back to where you were. I typically use indicators rather than dials just to avoid issues with backlash, operator error (forgetting to count turns:o ), etc.

When I'm having trouble, I find it helpful to run back through and check the basics of the setup: insert/tool sharp, on-center, not rubbing, etc. It's so easy to get to working and forget about those things.

hwingo
05-15-2009, 02:46 PM
Having a DRO on my lathe helps me get back to zero (I hope:eek: ). I gather that flex is considered normal under these conditions and repeat passes are in fact necessary.

Harold

Glenn Wegman
05-15-2009, 03:25 PM
Hi Guys,
I think I'm OK with the position of the point against the work piece. If standing at the tail-stock and looking at the tool relative to the bore, the tip is slightly below center and the point is turned to about the 8 o'clock position as compared to perpendicular.
Harold

If I understand you correctly, and depending on the borong bar configuration, you may have created a negative rake condition at the interface of the cutting tip and the part.

I believe you may have better results if the cutting tip is on center and horizontal. Another issue with the cutting tip being rotated down as you describe is that the dial graduations are no longer accurate in relation to bore size. Fortunately, the bore size would increase less than the dial indicates

Carld
05-15-2009, 04:01 PM
I agree, and it may help to raise the bar above center and keep it level. The bottom of the cutting tip rubbing against the side of the bore is a real issue not to be over looked.

Actually, if the tool is level and below center it's not cutting negative, it's cutting positive and may be contributing to flex and chatter.

hwingo
05-15-2009, 04:27 PM
Hi Guys,

If I'm not badly mistaken, when the cutter was above center and at 90 degrees, the heel of the cutter was dragging. That's the main reason I dropped it below center and slightly turned the point down so the point was engaging the work and not the heel. When I get home this evening I will raise the cutter but I am nearly certain that I will still need to rotate the bar so the heel doesn't drag.

Harold

Glenn Wegman
05-15-2009, 05:15 PM
Grind the heel for proper relief!

lane
05-15-2009, 05:45 PM
One thing to remember. Some boring bars do not work. I does not matter how much you paid for it , they are Not all equal. Some work every time some some of the time and some not at all. Been their done that.

doc-zeus
05-15-2009, 09:15 PM
Check to be sure your lathe is level - in all directions. This problem sounds like you have a slight twist in the bed.

(a carpenter's level is not nearly accurate enough! You'll need to use a very good machinist's level!)

Level that thing out, then see if it still turns a taper!

Doc-Zeus

philbur
05-15-2009, 09:58 PM
I have read, been told and confiirmed on a number of ocassions that a static boring tool (as in a lathe) has a natural tendency to produce a tapered hole, whereas a rotating boring tool (as in a mill) does not. I have yet to identify why.

Phil

j king
05-16-2009, 10:20 AM
I have found that a tool will wear after several feet of boring. : )


http://img.photobucket.com/albums/v24/ikimjing/jims%20pics/100_1889.jpg


Really you get tool deflection and loose your size quickly.The farther you bore, it builds up.This is a small amount but it does accumulate.Thus the need for a spring or preferably a light finish pass.

rws
05-16-2009, 10:50 AM
While I agree and have experienced the same effect that a bored hole tends to get smaller toward the farthest end, the question is still why?

The boring bar support never changes, so the amount od deflection should be the same at the start as in the end. So where/why does it get smaller?

I threaded a barrel for a tuner attachment not long ago. The tuner was made on CNC machines, the jam nut and main body all made that way. I know my threads on the OD of the barrel were the same start to finish. But, the jam nut went on fine all the way, then trying the main body (about 1-1/4" long), it went on about 3/4 the way and tightened up. Same scenario as being discussed here.

Carld
05-16-2009, 11:05 AM
hwingo, it would be imposible for the tool to rub on the heel when raised if the radius of the front of the cutting tip is smaller than the bore.

When you lower the tool you are bringing the heel of the cutter tip closer to the bore the lower you go. You really can't lower the cutter much below the center line unless you have a lot of relief on the heel.

If you have a QC tool post you can easily raise and lower the tool without changing the angle of the cutter and see what I am talking about. You get better results with the cutter set above center line than below for most jobs.

RPM
05-16-2009, 11:52 AM
I do a fair bit of boring, mostly with smaller bars than your 3/4", and have the same problem. Why, nobody seems to have answered, but still it seems to happen to everybody.And having a light lathe like my Atlas doesn't help :-(

And it's not just carbide inserts -reading Spary's 'Amateur Lathe', (1948) he refers to this, using HSS. He suggests several intermediate passes at the same depth setting, so that the taper can't build up as you go, and then when getting close to final size, several passes at the same depth setting, which removes nothing from the entrance, but only removes tiny amounts of material from deeper in the bore .

Works for me...

Richard in Los Angeles

lakeside53
05-16-2009, 12:28 PM
That works on most metals, but on some stainless it won't as the surface will work-harden with the fine cuts (i.e. - same setting passes). I use a solid carbide boring bar (got several sizes cheap at a garge sale!) and a very sharp insert to minimize defecltion problems while keeping the DOC constant for the final passes

Paul Alciatore
05-16-2009, 01:38 PM
While I agree and have experienced the same effect that a bored hole tends to get smaller toward the farthest end, the question is still why?

The boring bar support never changes, so the amount od deflection should be the same at the start as in the end. So where/why does it get smaller?

.....

Another possibility here is headstock alignment. If the headstock is not aligned to the ways, you will get a taper. But a second pass will not correct it.

The most likely cause here is something is rubbing as the cut proceeds. If cutting a harder material, wear on the insert could do it, but a quick exam with a magnifier should tell you if this is the case.

Note: A good magnifier is an indispensable shop tool. I have two high quality Hastings Triplets in my pockets at all times for at least the past 30 years: 11X in the right and 20 X in the left. And a lesser quality 2X as well.

Quetico Bob
05-16-2009, 02:26 PM
Quote
“While I agree and have experienced the same effect that a bored hole tends to get smaller toward the farthest end, the question is still why?

The boring bar support never changes, so the amount od deflection should be the same at the start as in the end. So where/why does it get smaller?”

You are right about deflection, unless the tool is repositioned part way through cut more like cutting tip wear and possible work hardening especially if you are not using flood coolant.

All parameters must be maintained (or as close as possible) from start of cut to end including temperature if you want an accurate bore. Personally I do not think it is humanly possible depending on how far you want to measure it.

I use flood coolant and a fresh insert (if critical) when I’m within .010 and have reasonable success.
Cheers, Bob

4GSR
05-16-2009, 02:34 PM
I have found that a tool will wear after several feet of boring. : )


http://img.photobucket.com/albums/v24/ikimjing/jims%20pics/100_1889.jpg


Really you get tool deflection and loose your size quickly.The farther you bore, it builds up.This is a small amount but it does accumulate.Thus the need for a spring or preferably a light finish pass.

John,

You need a bigger boring bar, that one dosen't look like it is big enough for the job!!!:D :D :D

I ran a boring operation many years ago using a 6" OD bar sticking out of the chuck about three feet. I was line boring compoents to a trepanning machine. I would have a heck of a time with that 6" bar vibrating trying to make a 8" ID cut. Boring bar was around 8 foot long.

Ken

4GSR
05-16-2009, 02:47 PM
hwingo,

If you have a older model lathe, there is very good chance you have excessive wear in the carriage to the bed. I've seen this to add to the problem of back cutting when reversing out of the bore.

With the right cutting tool, I get my best results by cutting on the reverse out of the bore movement. This also works very good on ID threading, start at the back of the bore and come out.

Ken

Quetico Bob
05-16-2009, 03:04 PM
Forgot to mention, if you do use a fresh insert at .010 or there about,make sure you reset your depth of cut. If you don't you will more than likley end up over size.

.RC.
05-16-2009, 07:33 PM
I have found that a tool will wear after several feet of boring. : )


http://img.photobucket.com/albums/v24/ikimjing/jims%20pics/100_1889.jpg




If you used a solid carbide bar you probably would not need the stiffener bar on top :D

hwingo
05-17-2009, 04:37 AM
Hey Guys,

You've given me much to think about. Attached is an image of my current position of the insert to the work. This position seems to have worked the best. It has provided the smoothest cut, with less chatter, and the best finish.

I tried to raise the cutter above center but chatter became unbearable. I even tried to "grind the heel" to prevent dragging but there was insufficient support and the tip easily broke. Strangely, temperature had a great bearing on how well the boring bar cut. After allowing the work and insert to cool for at least an hour, and without changing cross feed, I reentered the bore and more material was easily removed whereas prior to cooling, no new material was removed with additional passes. So for some strange reason allowing the work to cool reduced the amount of taper I was experiencing.

Harold

http://i234.photobucket.com/albums/ee38/hwingo_2007/ToThePoint.jpg

Glenn Wegman
05-17-2009, 09:11 AM
That's looks to be the incorrect insert for that bar. I believe that is a positive bar (note position of flat on top of shank) and the insert appears to be for a negative bar as it has no relief, or the insert is for a different application, sucg as a milling cutter, requiring very little relief. Is the hole in the insert countersunk on both sides?

What is the designation on the bar? (a bunch of letters)

Is the insert a TNxx? Or a TPxx?

knedvecki
05-17-2009, 09:27 AM
Hwingo,
Might I suggest, if letting the part cool down before making additonal passes helps, How about using a cold air gun or better yet, water soluable coolant on the tool / part?

hwingo
05-17-2009, 12:12 PM
That's looks to be the incorrect insert for that bar. I believe that is a positive bar (note position of flat on top of shank) and the insert appears to be for a negative bar as it has no relief, or the insert is for a different application, sucg as a milling cutter, requiring very little relief. Is the hole in the insert countersunk on both sides?

What is the designation on the bar? (a bunch of letters)

Is the insert a TNxx? Or a TPxx?

Hi Glenn,

I know very little about carbide inserts much less positive or negative boring bars. I needed a boring bar that was at least 10 inches in length and .750" in dia (to lessen spring) so I saw this bar in Rutland Tool's big book , which came with inserts, and I purchased it.

The boring bar has the following nomenclature:

NBS_12 3/4 x 10" INTRNL TRNNG TL/HLD

and the insert has the following nomenclature:

TPGC-321 TIN TRN/BOR INSRT USA


The insert has a hole only on one side. The hole's minor ID is .120" and uses a 4-40 screw. Years ago I used a totally different style boring bar with HSS cutters that could be easily shaped and sharpened. Now they have a two billion different cutting tools (inserts) for a billion different applications that seem to fit only one or two holders. I'm learning quickly that I had better be certain for "what I wish".

Now, help me out here. If it looks like the wrong tool and insert for the job, what is the appropriate bar and insert that I should be using? Any information is helpful.

Also, you mentioned the "flat on the top of the bar". Why is that important and what is the significance? Your replies will be helpful in more ways than you can imagine.

Harold

j king
05-17-2009, 01:18 PM
The insert looks to be a negative insert. I have used positive inserts in neg bars but you cant do the opposite.

There are many things that could cause a tapered bore. One that isnt posted is a tool that heats up as you cut.If it is dull at all or rubbing it will get hot.As it heats up it will change the size of the cutter.It will grow and take a little more stock.Not much but it will happen.

One of the main things in machining is observing EVERYTHING that is happening.You will find in time that when you look at something all the pieces will come together and you will be able to solve almost all your problems.

Jim

hwingo
05-17-2009, 01:28 PM
You guys are assuming that I know more about this than what I actually know. Negative & Positive rakes???? Negative & Positive boring bars???

Fellows, you are dealing with an ignorant individual desiring to learn more so I can "work smarter and not harder". So tell me about positive and negative rakes and negarive and positive boring bars.

I needed (past tense) to enlarge a hole that was 3.60" deep to an ID of 1.70". Which bar and which insert should I have been using ..... positive or negative ..... and just as importantly, why?

Harold

Paul Alciatore
05-17-2009, 02:03 PM
He is getting a smaller hole at the end of the bore. If the problem was the tool heating up, it would be larger.

If the work was heating up, the hole would be smaller when it cooled and contracted.

1.25" ID, 3.6" long, the hole is several thousanths smaller at the end, AND COOLING reduced the taper. Sounds like an expansion problem in the work piece.

1. Tool is slightly dull producing excessive heat while cutting. DO CHECK IT.

2. Tool is rubbing, again producing excessive heat.

3. Tool is being used at incorrect rake angle, again producing excessive heat.

4. Cut is too agressive. You could try a slower feed or shallower depth of cut.

5. No or too little coolant. You could try more coolant.

I have seen videos of the fancy CNC equipment and they seem to use a lot of flood coolant. I suspect this is mostly to control the temperature of the work piece to avoid expansion and the errors associated with it.

j king
05-17-2009, 02:33 PM
Paul.
I DO realize that. I was just pointing out things that most dont think of when trying to bore to a size and have problems. Same thing will happen with a boring bar on a mill. Jim

Ps. Alot of people are using these inserted bars but arent running them anywhere close to the required speed for the carbide to perform correctly.

Glenn Wegman
05-17-2009, 04:55 PM
The boring bar has the following nomenclature:

NBS_12 3/4 x 10" INTRNL TRNNG TL/HLD

and the insert has the following nomenclature:

TPGC-321 TIN TRN/BOR INSRT USA

Harold

Well, I'm confused!

The bar nomenclature above seems to be more of a manufacturers part number or designation sot it does not tell me much.

As for the flat on the top of the bar, it usually indicates the index angle of the bar as if it were mounted in a boring head or turning tool holder, the screw securing the bar would press against the flat, so in this case the flat would (should) be horizontal.

As for the insert being a TPGC, the C designates that the mounting hole is countersunk on both sides. I'm not sure that there is such an animal as a TPGC since a TPG should have 11° relief angle so there is no reason why you would want to be able to install the insert upside down, so there is no need for the hole to be countersunk on both sides. The insert in the picture shows what appears to be 0° relief, so the insert in the pic looks to me more like a TNGC,(just guessing) which would make sense, as you would be able to use all six points on it since it is a negative insert. I'm guessing that the correct insert for that bar is a TPGB 321 which would be 11° relief, with a hole, and countersunk on one side. The bar would be rotated so the insert and flat on the bar would be horizontal and the cutting edge on center of the bore, and the insert would have the necessary relief so it would not drag or heel.

May be perfectly wrong since I'm not an insert wizzard, but that's my take on it anyway!

Glenn