What is the Advantage and Disadvantage of Parting and Grooving Inserts

Cemented-carbide cutting tools have been in use since the late s and are now ubiquitous in the metalcutting world. Around , tools that accept cemented-carbide inserts became common in metalworking and are still considered a basic necessity.

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Indexable cutters are typically constructed of a steel body that has machined features, allowing for the accurate positioning of inserts. Steel tool bodies also provide a method for clamping inserts along with a method for attaching the assembled body and inserts into the machine tool. 

Benefits Over Solid Tools

Before the proliferation of carbide inserts, it was always necessary to remove solid cutting tools from machines to sharpen them. Tool sharpening was such a significant amount of work that large manufacturers often had grinding departments dedicated to tool sharpening. Therefore, the single most significant benefit offered by indexable tools is the ability to renew the cutting edge without having to remove the cutting tool from production. Renewal of the cutting edge is typically done by unclamping the insert and turning or flipping it (indexing) to a fresh cutting edge or replacing a completely worn insert with a new one.

Solid drills, taps and other types of cutting tools are still used, depending on the application, but the indexable turning tool is the predominant tool of modern turning operations. Indexable tools have effectively displaced HSS, brazed-carbide and custom form tools, which were common prior to the proliferation of CNC machines. An indexable turning tool paired with the multiple-axis motion of a CNC machine tool simplifies the creation of complex shapes, virtually eliminating the need for dedicated turning tools. 

In addition, applying indexable drills on a lathe provides some distinct advantages over traditional solid HSS or carbide twist drills. It is common for a programmer to make a hole with an indexable drill and then step off center and cut the hole to a larger diameter, which can eliminate the need for a boring bar. CNC lathe turrets frequently suffer from misalignment because of wear and tear, and misalignment of the machine can damage standard drills. On the other hand, indexable drills are more forgiving than standard drills when used on machines with questionable alignment. 

Groove-turn inserts, which have made grooving tools multifunctional, like indexable drills, present a significant development in turning tools. Old grooving technology allowed cutting in only one direction; groove-turn inserts allow the user to make radial as well as axial cuts. 

The Common Touch

There is a tremendous amount of commonality among indexable turning tools. Turning inserts are manufactured in common shapes, such as diamond, square and round. The shapes and sizes of these inserts are made to standards governed by bodies such as ANSI and ISO. Their commonality gives the programmer and machinist an almost unlimited selection of insert grades and cutting-edge geometries. All of the major cutting tool manufacturers produce turning inserts to these standards, so finding an insert that gives the best performance is relatively easy.

Unlike turning tools, indexable milling tool bodies tend to require insert shapes and geometries that are not common among manufacturers, forcing users to purchase inserts made specifically for the brand of mill they use. However, toolmakers have been developing and offering families of milling tools that utilize their proprietary shapes across a range of cutter bodies. Having a family of cutter bodies that accept the same insert reduces tool inventory while providing some flexibility to programmers and machinists. 

Most of the major toolmakers manufacture custom indexable form tools. Unlike form tools for turning, form tools for milling are still a viable way to complete a complex geometry, especially when a shop is trying to reduce cycle time and cutting tool inventory. Parts with features such as multiple steps, radii and chamfers require the use of multiple tools to create them. The advantages of custom, combination indexable form tools are reduced cycle times, reduced tool inventory and improved part quality.

Unfortunately, custom indexables are expensive and often require modified inserts, making the inserts expensive as well. For these reasons, custom indexable tools are usually reserved for high-production environments or for manufacturing very expensive parts. 

A significant indexable milling innovation was the development of plunge milling for roughing large cavities like the ones machined in molds. Tools used to rough deep pockets or shoulders can be quite long. In traditional roughing operations, radial loading of long cutting tools from side-to-side cutting motion induces chatter. Plunge-roughing is done by feeding the tool axially, which directs cutting forces into the spindle taper, where the machine is most rigid. Plunge-roughing results in much higher metal-removal rates when long tools are needed. 

On the Other Hand …

Indexable tools do have some drawbacks. Inserts are usually made by pressing carbide powder and binding materials into a die under high pressure. After forming, the inserts are heated to high temperatures and sintered, binding the powder and other materials together and giving the insert its strength. This process results in a cutting edge that is stronger than the edge of a ground cutting tool. Although stronger, the formed cutting edge is also less keen, or sharp, which can limit the ability of the tool to effectively take a shallow DOC, which can make finishing difficult. 

Variation in insert pockets and insert size can cause the cutting edges of multiple-insert milling tools to lie in slightly different planes. The result is often less-than-desirable surface finishes or noticeable steps in shoulders. In these instances, the user may be forced to use another type of tool for finishing. 

As industry continues to demand modularity, lower inventory levels, higher productivity, increased tool life and universal tooling solutions, indexable cutting tool technology will continue to make advances and replace obsolete cutting tools.

Out with the Cam-out Effect

When tightening or loosening screws with conventional conical profiles, such as the cross-recessed Phillips head, the cam-out effect is a common problem. In accordance with the triangle of forces, some of the energy cams out of the screw, causing the screwdriver or bit to slip out of the screw head. The user has to counteract cam-out by applying additional force.

Profiles with straight sidewalls, like hex, Torx and Torx Plus drive systems, eliminate the cam-out effect. The basic difference between the Torx and the Torx Plus designs is that the former incorporates a 15° drive angle and the latter has an almost 0° angle, which ensures optimal torque load is transmitted to the screw. Although the straight sidewalls of the Torx drive prevent cam-out, its 15° drive angle prevents full engagement of the drive bit and fastener and still permits a small amount of radial stress, which can reduce bit life.

With the hex design, the points of contact cause stress risers to develop. In addition, damage to the screw head is almost inevitable, and the transferable torque is relatively low in comparison with the level of force that has to be applied.

The danger when tightening and loosening screws is the high stress concentration that occurs when turning the fastener. High stress occurs mainly in hex screw heads, which can cause stress in the tool and screw. The elliptically based geometry of the Torx profile and the associated lack of angular edges minimize stress concentration. This, in turn, protects and extends the life of the screw and tool. The elliptically based geometry also provides a small tolerance range between the bit and screw, improving force closure.

The Torx Plus profile further improves the force closure via its working angle of almost 0° and its larger core diameter. This allows a higher torque transfer and prevents the screw and tool from suffering damage.

Producers of carbide inserts use the Torx or Torx Plus system to fasten inserts to tool bodies, but the trend is toward Torx Plus because it more efficiently transmits the torque generated by the driver to the screw. With the Torx Plus drive, little or no end load is required to keep the driver engaged in the recess, reducing operator fatigue and muscular stress during manual assembly. As a rule, carbide insert manufacturers prefer hand tightening over power-driven systems. 

—Alan Richter, based on information provided by torque-screwdriver supplier Sloky USA, El Monte, Calif.

The blade type parting tool has one major disadvantage. The holder is mounted in the toolpost. The additional overhang caused by the holder increases the bending moment on the toolpost, topslide and cross slide. That increases the deflection of the tool in the cut compared with the parting tool held directly in the toolpost. The deflection includes downward movement and twisting. The twist causes binding which increases the load on the tool which further increases the deflection.

The ideal position for the parting tool would be directly over the centerline of the cross slide. That would eliminate twisting of the tool in the cut and resultant binding. Unfortunately that us not really possible with a QCTP.

If you want to use a blade type parting tool and have an Aloris style toolpost there are blades available that will replace the HSS cutoff blade in the Aloris parting tool holder.

The rear mounted parting tool eliminates many of the problems with a parting tool in the toolpost. With a parting tool mounted in the toolpost cutting forces deflect the tool into the work. With a rear mounted parting blade the deflection is away from the work. That eliminates the tool digging into the work with resulting disastrous results.

On the top one, the tip is just a tight slide fit in the angled slot, and the stiffness of the tool is determined by the depth of the blade. Protrusion is selectable by to the user.

The lower one clamps the tip with the capscrew, but has a stiffness mainly determined by the shank depth in the toolpost slot. The shoulders force a minimum protrusion.

My choice would usually be the top type for stiffness and versatility – I can't readily think of a reason I might choose the lower one. Perhaps if you need all positions of a 4-way to be available it might offer less occlusion at the rear?

glad it's not just me then………

got fed up with tipped tool version above (no1) so made a sturdy over the top parting tool holder that's mounted central to the saddle /cross slide……..all very well for making washers but it's quite incoveinient when needing other cutters for different jobs on the same turnig….getting it on/off…….

Contact us to discuss your requirements of Parting and Grooving Inserts. Our experienced sales team can help you identify the options that best suit your needs.

I went back to the HSS type of blade holder but this time using a HSS cobalt blade…….I also with a Dremel grinder created a groove on the top cutting surface, so now have a semi hard blade that doesn't blunt qite so redilly with the conveinence of using the 4 side quick change tool holder…..

have just bought a h/d rear mounted tool holder and just waiting for the HSS/co blade…….so lets see how it works….

on the big stuff 50mm plus I just gave up with parting off and just use the band saw, inconveinient but makes my nerves a lot happier…….

also when parting off I up the chuck speed to the next highest setting, seem s to work for me…..and have the luxury of flood coolant……

Ive always thought it a bit daft how most people condemn the 4 way tool post and fit quick change ones, when the 4 way is the more solid platform. Not so convenient though.

Exactly, which is why I've stuck with my 4 way toolpost, even though I've got a genuine Dickson toolpost sitting on the shelf. It's odd that many people are happy wasting time taking lots of tiddly weeny cuts, but are obsessed with saving a few seconds with a QCTP.

In the OP the top tool is for parting only, with inserts for same. They do not like taking small cuts. In my experience anything less than 4 thou per rev and they chatter.They can be used to turn grooves, but it's not a nice experience, only done it once. If I need to cut an accurate groove I use a HSS toolbit as they don't chatter with fine cuts. However, if you get the conditions right the insert parting tools work well:

The steel part bottom slighly left (~2-3/4" diameter) was parted off with an insert parting tool with no fuss and an excellent finish; better than I get for normal turning.

The lower tool in the OP can take a multitude of inserts and has a different, more positive, clamping arrangement, rounded inserts are particularly suited to profiling, like this:

The profiling cut isn't pussyfooting about, note the blue swarf! If I remember correctly DOC was 20 thou and feed 8 thou per rev and a spindle speed of 800rpm. And yes, I know it's a Dickson toolpost, but that's what the copy unit came with.

My modus operandi for parting off with an insert tool is have no more sticking out than needed. My toolpost automatically sets perpendicular to the cut. In the above picture you can just see the radial "gear"on the left that aligns the normal toolpost. Lock the topslide and tighten the gibs and lock the saddle. Part off under power with a minimum of 4 thou per rev and always coolant except for brass and cast iron. Spindle speeds are normally in the mid hundreds of rpm.

Andrew

I don't understand why the first type is advertised as a parting tool only, but the second as a parting and grooving tool. Can anyone enlighten me about the functional differences of these two designs of bit holder?

Robin

Michael, Neil and Andrew,that holder that Michael shows is not what the OP shows. The tip are different and have better side cutting geometry and can therefor cut sideways which is why the MGEH is also described as suitable for profiling, even more so with the round nose inserts.

The QA-GTN that the OP posted just uses a standard GTN parting insert but due to the holder being stiffer (sideways) will be better able to cut sideways but at the expense of not parting as deep as the blade type will allow.

Edited By JasonB on 27/08/ 10:16:47

Hi Jason

Must have misread the post. Robin went on to mention the second type which I presumed was the profiling one.

Cheers Michael.

Michael, Neil and Andrew,that holder that Michael shows is not what the OP shows. The tip are different and have better side cutting geometry and can therefor cut sideways which is why the MGEH is also described as suitable for profiling, even more so with the round nose inserts.

The QA-GTN that the OP posted just uses a standard GTN parting insert but due to the holder being stiffer (sideways) will be better able to cut sideways but at the expense of not parting as deep as the blade type will allow.

Edited By JasonB on 27/08/ 10:16:47

I've got both the first Arc parting tool the OP shows and the MGEH. The MGEH I have actually has squarer ended inserts. It's more prone to the insert slipping backwards when parting hard. I've also tried the other style and my impression is that all the ones with screw clamps are better for sideways cuts although the MGEH may well have the advantage for this.

In practice I tend to make multiple straight in cuts and then take a fairly light side to side cut to finish, although the other day I shaped a domed end on a steel bar with the MGEH by eye which was all side cutting, just too lazy to change tools

Parting under power with TCT inserts is an eye opener, the only issue I have come across is that when cutting alloys you get build up unless you use cutting fluid. Uncoated parting inserts seem to be like hen's teeth.

Neil

Not sure if the Korloy Sawman inserts are 100% compatible with the industry standard "GTN" pattern but either way, Cutwel do the uncoated "H01" grade for cutting loominum. Flood coolant seems to help with parting loominum if you don't have the right insert.

For profile, sideways cutting, grooving and shallow parting off, I use the MGT and KGT grooving systems. As pointed out, these inserts have cutting edges on the sides as well as the end. And I see APT stock the polished, uncoated version at £4.

One way to get yourself a serious crash during parting is to allow the work to slip in the chuck. That doesn't end well if you are parting off with power feed, which is the best way to make consistent but decent progress. It seems to bugger the holder as well as the insert…

Murray

Edited By Muzzer on 27/08/ 14:57:11

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