I have recently been upgrading my lathe tooling to carbide. I have a Dorian #16 tool holder for my BXA toolpost that takes TPG 322 inserts. The #16 holder integral TPG inserts on both ends for both facing and surfacing. My issue is dealing with the razor wire I get when turning. Hot, long stringy shrapnel is less than pleasant to deal with while the chuck is running and by no means pleasant to clean up afterwards.
I purchased some chipbreakers that clamp to the top of the insert. Looks like a TPG insert with one side beveled back 45 degrees or so. What is the best way to set these up? My attemps basically alter the direction the chip comes off the cutting edge. I have adjusted them closer and farther away from the cutting edge. I have yet to get a chip to break up. Help is greatly appreciated.
In general, the heavier the feed rate, the farther away from the nose the chipbreaker should be located. You didn't say what type of lathe you had but you should increase your feed until either the chip breaks, or the insert breaks. This may turn out to be pretty (scary) fast. That's normal.
I've never heard of a "clamp-on" chipbreaker. I've heard of inserts that have chip-breakers built in (virtually every insert available today). I've also heard of people trying to cut with the carbide anvil that the cutting insert is supposed to sit on... Do you have a link?
If you intend to produce a thick (not depth) chip (high fede rate) about .060". Thin chip (light feed), around .030".
Its been my observation that getting a chip to break is directly related to cutting speed, cutting feed, and depth of cut also. Decreasing cutting speed will help break the chip, but slow the operation. Increasing the feed will help, but only to the point where rigidity becomes an issue, or the finish is unacceptable (non issue for roughing). Increasing the depth of cut will help, but only to the point of creating chatter (see rigidity), or occasionally, the ribbon will begin to form again.
This all assuming tool survival.
I usually select a feed that will give a desirable finish, select a depth of cut as deep as possible, that I'm comfortable the machine and setup can handle, then adjust the RPMs to get the chip to break (usually lower). Use the RPMs to control the plume of chips coming off the tool also, so a lower setting will make the process less painful, and less of a mess.
There probably is a mathematical formula for determining ratios to make a perfect number 9 chip, but that's way beyond my desires at the moment.
I gather you're using the positive rake inserts for finishing, not roughing. If not, it's a mistake. The clamp on type chip breakers tend to be troublesome. If you use them to break chips when roughing, they tend to erode at the joint and soon trap light chips under the breaker. Using positive rake inserts for serious metal removal isn't a good idea, anyway, because you tend to experience premature tool failure, especially as compared to negative rake inserts.
Stick with negative rake (assuming you have the horsepower and rigidity to run it) and use the positive rake for finishing exclusively. The sad thing is that unless you have a cut both deep enough and feed fast enough, the chips are unlikely to break well. That tends to go against what your objective may be if you're finishing.
If you're using the inserts and breaker for finishing only, try setting the breaker very close to the cutting edge. How close depends on the variables, depth of cut and feed rate. In my experience, you have to learn to live with some sharp strings when finishing unless you pay for inserts that are ground. Even then it's no guarantee. Finishing cuts tend to produce strings.
Those chipbreakers are available with 3 different backsets. i.e. there are three different distances that they can be from the cutting edge. Actually your problem is probably one of feeds and speeds. A chipbreaker, on ductile materials, is not going to work below .005"/rev feed and you will need some spindle speed too. Lots of the smaller lighter SBL's, etc. won't pull this kind of horsepower so you are then back at the beginning. If you plan on using a chipbreaker you better have two HP on small shafts. I have a part I run from 4340 that I take .150" off in a pass at .008"/ feed and I have trouble getting the chip to break. I use TPG's and they are the only insert I have ever had work, despite Kennametal's tech visiting and offering various solutions. Leigh @MarMahine
They used to be fairly common......IRC ~ .02 /.03 in. setback from the insert cutting edge would be a good starting point, final setting was by trail and error.
Another common method was to grind a groove or step into a flat top insert or brazed tool.
Basically they both became obsolete with the advent of the molded inserts.
Today a chipbreaker can be molded into a negative rake tool, making the "effective cutting angle" positive while also providing economy as twice as many indexes are now available before the insert is thrown away.
I use 1" shank tool holders and I use the TPU 432 inserts. I use the roughing chip breaker during heavy cutting. .017 feed rate and up to
5/8" off the diameter in one pass. I get the desired 69's everyone likes to see. When it comes time to finish I put in the finishing chip breaker. When the chipbreaker doesn't extend out far enough I put various diameters of steel wire behind it. I use paper clips a lot for this cut into short lengths. Small paper clip = small wire, large paper clip = larger wire. I don't take the time usually to play with the chipbraker unless I am doing more than one part. I also use CNMG 432 inserts, but I usually prefer the TPU inserts because of cost. Also the TPU 432 inserts are thicker and will take more abuse than the TPU 322 inserts. For heavy interupted cuts the negative inserts work well.
Thanks all for some very good background. Since this is a hobby and I'm just getting up to speed with insert tooling, The TPG I am using are for both roughing and finishing. I'm trying to assemble a low cost but versatile selection of cutting tools. I also have milling cutters that take the same TPG inserts which is a plus to keeping things simple. Having seperate negative rake tooling just for roughing is a bit specialed for me at this point. I also realize that I can get inserts with chip breakers but again that is a bit specialized and involves $$$. I realise that negative rake tooling has twice as many cutting edges but their ability to do finer work with delicate parts is a drawback for me at this time.
The machine I'm using is an 8000 series Clausing Colchester powered by a 5 hp motor. It's a very capable machine for a home shop. Feed rates I recall using are about .010 to .020 per rev. Someone mentioned using slower cutting speeds. The coated insterts I am using don't like anything below 1000 rpm or the finish goes to hell. Depth of cuts I was dealing with were in the .010 to .030 range. I would like to stick with TPG inserts and find a way to make them work. Is negative rake tooling the only way to go on this???
The machine I'm using is an 8000 series Clausing Colchester powered by a 5 hp motor. It's a very capable machine for a home shop. Feed rates I recall using are about .010 to .020 per rev. Someone mentioned using slower cutting speeds. The coated insterts I am using don't like anything below 1000 rpm or the finish goes to hell. Depth of cuts I was dealing with were in the .010 to .030 range. I would like to stick with TPG inserts and find a way to make them work.
For finishing work, the feed rates of which you speak tend to be too high. When you're doing fine work, you'll find that a much finer feed rate and often a much shallower depth of cut will be needed. A good example is if you're trying for a bearing fit. Taking a depth of cut of .030" pretty much guarantees you'll miss the desired diameter. You have to sneak up on precision fits with equal depth multiple passes and fine feed so you take into account tool loading and surface finish. It's a different world from machining rough and loose. When you machine that way, almost all carbide tooling goes to hell, especially where finish is concerned. In truth, unless you have a precision grinder available, you get pretty good at polishing.
Getting that nice, shiny finish you desire is a function of more than speed, it revolves around feed rates and depth of cut. That works against you when you're doing fine work.
No. Negative rake should be used for roughing only, unless you are turning diameters that have very loose tolerance. You appear to be aware of the problems with negative rake already. You can't take shallow cuts with light feeds and get good results with negative rake. A well equipped shop should have both type tools. The cutting principles are very different with negative rake as opposed to positive rake. Negative rake actually makes no contact at the cutting tip when applied correctly, which would be a serious depth of cut and high feed rate. When you use a negative rake insert this way, you can see that the cutting action takes place well back of the tip of the tool. So much so that in the old days, we were told to hone a small 45 degree flat on the tip of the tool (which, in theory, doesn't even make contact with the piece being machined), which actually increased tool life. It kept the tip from chipping away, leading to premature tool failure. Makes little sense when you talk about it, but worked great on the machine.
With a machine of your quality, you'd be well served to work towards negative rake for use when roughing. You have the horsepower needed, and the cost will be paid back quickly because of the increased tool life. You'll also enjoy the benefit of better performance, assuming you use them correctly. If you select something like a TNMG 322 insert and buy them in two grades, C2 and C5, you'll have a decent selection to handle the vast majority of materials you'll encounter. You gain, not only by using both sides of the inserts, but by a much longer edge life with each insert. Positive rake inserts simply do not last very long when roughing, especially when compared to negative rake. The TNMG insert is likely the cheapest insert on the market and are available at bargain prices on ebay, well under $1 each, if you're so inclined.
Once again, to emphasise, for most coated carbide inserts, the rake will be negative for small cut depths, even if it is a nominally postive rake insert. This is a result of the honed edge that coated carbide typically has.
Thanks for the pointers Harold. The depth of cut range from .010 to .030 was the roughing cuts I've been taking. When I approach the desired dia I back down to about .005. On my last part I did a finish pass at .003 and the TPG surface finish was a combination of smooth and clean intermixed with rough spots. I assume the cutting edge was transitioning from pulling off a chip to smearing metal across the surface of the part. My previous pass at .005 provided a better surface finish.
I was a bit surprised that I used up the cutting edges as quickly as I did for my first turning job since going to inserts. Economies of even my small scale would suggest negative rake tooling if indeed they last longer. I imagine as I get more experience with them, I will get a better feel for what hey like and don't like. I'm doing to keep an eye out for negative rake tooling at the next auction I go to.
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