Yet another parting tool question...

After a spot of parting off unpleasantness:

I had to buy a new tool. The one I went for came from Arc Euro Trade - pretty much a direct replacement for the Glanze:

This works very well, but I wondered about the other design of holder Arc offer:

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

Marketing. Parting is simply a groove taken all the way to the centre. Any parting tool can be used to cut a groove. But any grooving tool can only part off to the depth the tool can be plunged into the job.

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.......

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......

A chap in the US, I think his forum name was Forrest Addy, suggested a packing piece under the PT holder itself, to stop the sag, if possible. Seemed like a good idea to me. 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.

ARC MGEH one piece clamp type Parting & Grooving Tool Holders

Suitable for external grooving, parting, turning, relieving, profiling.

Hi Robin

Can you see the left arrow above the grooving arrow?

This tool can take small cuts side to side.

Cheers Michael.

I am sure that if you asked the horse, you would get the information ‘straight from the horse’s mouth’.

Not calling Ketan a horse, but you should understand the analogy. Ask ARC. They obviously know, or would not have described each of them as such.

What Michael says, inserts held in place by a screw can take light side-to side cuts as well as fore-aft ones.

Neil

The average home workshop lathe is not ridged enough for some parting ops. I have made a rear mounted parting tool holder with an ARC supplied blade like the OP. Works well on things like EN1A I can part 40mm diameter at 1100 RPM with power feed of 0.05mm/rev. EN24T and EN16T are fed by hand at around 800RPM and not so easy.

I must confess I've never had a great deal of success with parting off until I invested in a Riley style rear tool post. Since then life has been pretty much stress free.

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

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 27/08/2018 10:16:47

So what's a Riley style rear tool post look like. I've tried Google but it just throws up lots of GHT type

Edited By duncan webster on 27/08/2018 11:16:20

Hi Jason

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

Cheers Michael.

Riley tool post mentioned here:

http://www.hemingwaykits.com/acatalog/Rear_Tool_Post.html

Bear..

Thanks, it's a GHT mk2

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

I found all my parting woes were down to my tool alignment, speeds, feeds and lube (or lack thereof).
​All the parting tools I bought in the hope of finding "The Right One" now work perfectly well, including one like the item in the OP, which sits in a home made holder with massive overhang and stick-out, mounted in a QCTP holder on an RDG pattern Dickson QCTP on the standard Myford 7 top slide.

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 27/08/2018 14:57:11