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Lynn Standish
01-08-2009, 01:19 PM
I have been told the rule of thumb for the maximum depth a boring bar should be used for is 5 times the diameter of the bar for HSS and 10 times for a solid carbide bar.

Some of the brazed bars have a piece of carbide brazed on the end of the bar to do the cutting. In many cases, the part of the bar that is held in the tool holder is 1/2", then immediately is reduced to 1/4" or less in diameter for 2 or more inches to the tip where the brazed carbide is located. The material of which the bar is made is uncertain, but let's assume it's HSS. At .250" diameter, 5 times diameter would be 1.25" maximum bore depth, yet the 1/4" portion of the shank is over 2" as mentioned.

My question is this: Even if this boring bar is only used to a depth of 1.25", there is over 2" of the shank unsupported. How does the bar know the difference between the overall length of unsupported shaft and the actual depth of the bore?

It seems obvious that that type of bar should be avoided unless there is no other way, and a LOT of spring cuts are taken.

Comments?

Carld
01-08-2009, 02:13 PM
There are a lot of "rule of thumb" sayings but that don't mean something won't work. I never used a rule of thumb for boring bars, I just set the job up with the bigest bar I could use and started the cut. If it chattered I changed the angle of the cutter and the height of the cutter untill it worked.

On the other hand, you can stick the bar out to far. The trick is to keep using a bigger bar untill it finally works. If that fails then you have to devise plan "B".

I haven't used the bars that reduce in diameter and don't plan to as I don't like the looks of them.

jkilroy
01-08-2009, 02:14 PM
Always use the biggest bar with the least overhang as possible. Solid carbide bars do work quite a bit better.

DR
01-08-2009, 04:29 PM
..........................................

Some of the brazed bars have a piece of carbide brazed on the end of the bar to do the cutting. In many cases, the part of the bar that is held in the tool holder is 1/2", then immediately is reduced to 1/4" or less in diameter for 2 or more inches to the tip where the brazed carbide is located. The material of which the bar is made is uncertain, but let's assume it's HSS. At .250" diameter, 5 times diameter would be 1.25" maximum bore depth, yet the 1/4" portion of the shank is over 2" as mentioned.

.................................................. ............

It seems obvious that that type of bar should be avoided unless there is no other way, and a LOT of spring cuts are taken.

Comments?

Your last sentence pretty much sums up the situation.

Another thing, some of the type bars you mentioned are used in boring heads. In my experience using the bars in boring heads, they don't seem to have as severe drawbacks as when used in a lathe.

SGW
01-08-2009, 04:53 PM
James summed it up; use the biggest and shortest that will do the job. Things being what they are, sometimes you're stuck using a skinny bar with a lot of overhang. For those times, holding your tongue right sometimes helps. :D

darryl
01-08-2009, 05:37 PM
What if you don't have thumbs? :)

Generally, the best cuts are made when the total of the system is more rigid. In the case of the boring bar this means using the shortest and fattest one that will still work, or compromise and coax what you have to do the job. It still has to be mounted, so the compound, crosslide, even the bed of the lathe all contribute to flex, so wherever you can eliminate some flex, the better. It's a total system with resonances, so it can still be possible for say a longer boring bar to work better than a shorter one- all I'm saying is that the quality of the work is not going to be strictly or predominently affected by the length of the overhang. I find in many cases that the predominent factor is the preparation of the cutting edge, which includes all the angles and the height of the cutting edge with respect to the spindle axis.

My results are generally better if I bypass the compound and mount the boring bar directly to the crosslide in a suitably made holder. I bought a set of boring bars of which most break that rule of thumb with regard to overhang. Some of the shorter ones have the carbide piece way too high, so without a lot of grinding on the top of the carbide, I can't even get close to a zero degree rake angle, let alone any positive rake.

As far as 'springy' boring bars and the tool continuously taking spring cuts with every new pass- the geometry around the cutting edge can be made to pull the cutter into the work, in which case a spring pass will not be able to further remove material, or it can push the cutter away from the work, and in either case you might never be able to get to the desired dimension, or it can be such that the cutter neither flexes away or towards the work under cutting forces. You might dial in say 1 thou, and be able to find that indeed only 1 thou of material came off.

On the issue of chatter and resonances, you can find that a longer bar works better than a shorter one, but everything is interdependent. Spindle rpm, type of material, diameter of the cut, feed rate and depth, overhang of the work out of the chuck- the list goes on. Generally, use a shorter bar, but in practice use what gives the best results. By the way, damping can make a big differenct to the results. Even a strip of duct tape applied to a boring bar can make or break the result.

tattoomike68
01-08-2009, 06:50 PM
I dont know about little lathes but at work we grab a few extra thousands and take a cut. measure the test cut, dial in a few more and let it ride.

most the time a big blue coil goes out the headstock so all is fine.

The Fixer
01-08-2009, 08:08 PM
Biggest bar u can fit in the hole is always step 1, from there you will (as already stated) start to play with angles and feeds and tool height. I find if the depth of cut is not big enuf you don't get proper chip loading at the cutting edge which causes no end to grief too. All types of tooling HSS or carbides all have a recommended chip load, it's no different than the chatter ou get off the drill press if you don't keep enuf feed pressure to give the right chip load'
just my .02c.....

Evan
01-08-2009, 09:04 PM
Solid carbide is MUCH better than steel. You can stick it way out without chatter. I use solid carbide tools almost exclusively these days and I can hang them out over an inch or more and they cut as if just the nose is out.

lazlo
01-08-2009, 09:26 PM
I have been told the rule of thumb for the maximum depth a boring bar should be used for is 5 times the diameter of the bar for HSS and 10 times for a solid carbide bar.

Modulus of Elasticity (rigidity) for tungsten carbide is 2.3 - 3 times that of steel, so a 2.3 - 3x stickout for carbide (over steel) boring bar is about right.

The equation for flex is:


D = F * L^3
_________
3 * E * I


Where F is the cutting force, L is the length, E is the modulus of elasticity (the rigidity of the material) and I is the moment of inertia.

The moment of inertia for a solid round bar is Pi*r^4 / 4.

So a 0.81" carbide boring bar has the same stiffness as a 1" steel boring bar.

BadDog
01-08-2009, 09:27 PM
Likewise I'll throw my hat in on Carbide.

Couldn't seem to "justify" the cost for a long time. "I can get it done without that, just not as fast, but I can get it done..." Then I got a largish bar that was broken off bit still plenty long enough for my needs, and it was CHEAP along with a box of inserts no less. Then I got a few chances to use it and compare it against normal steel bars of similar size. WOW! I've now picked up some more sizes including a recent 3/8" because I got tired of "suffering" on the smaller holes, AND I had several bored hubs that needed a pretty accurate 1/2" hole 3" long. Try that with steel! Even carbide doesn't like that, but it'll do the job if you don't hog in and break it. I even bought that one NEW (though "wholesale"), and if you know me, you would know that speaks volumes. Compared to me, Alistair spends money like a drunken sailor on leave... :p

Lynn Standish
01-09-2009, 10:16 AM
I understand about the carbide. Circle brand is usually running some sort of promotion where you buy the solid cabide bar and they throw in a box of 10 inserts, so that is what I do.

The rules of thumb are apparently worthless, because everyone seems to say the same thing. i.e. "use the shortest, biggest diameter bar you can, but here are some tricks to try when the bar has to be too long and smaller in diameter".

lazlo
01-09-2009, 10:43 AM
The rules of thumb are apparently worthless, because everyone seems to say the same thing. i.e. "use the shortest, biggest diameter bar you can

I wouldn't say the rules of thumb are useless. If you look at the deflection formula I posted, it confirms the 2 - 3x stickout rule of thumb for a carbide boring bar over steel.

Two other key things about that formula: notice the L^3 term. Boring bar deflection increases with the cube of the length. So you do want to keep the boring bar as short as possible, but the 2 - 3x carbide versus steel rule still applies.

The other key thing that formula shows is that diameter is king: notice the r^4 term in the denominator. A small increase in boring bar diameter gives an enormous increase in rigidity. Much more than carbide over steel.

So a .8" carbide boring bar has the same rigidity as a 1" steel boring bar, and a carbide boring bar costs 3 times as much. If you have the clearance, just going up in boring bar diameter makes more difference than the stickout.