Introduction to Indexable Tooling for the Metal Lathe

[Karl A]

You are correct that the specific tool recommendations in the book are for medium sized (10-14") lathes. There are indeed smaller 3/8" and maybe even some 1/4" shank tools that are equivalent to those recommended in the book.

I just looked at the Sherline indexable tool offerings, and the specs are all for positive inserts CPMT, DPMT, etc. which have an 11° relief angle and a 0.250" I.C. and appear to be industry standard. Their turning/facing toolholders are not sufficiently specified to know precisely if they are neutral or positive rake, and the lack of industry standard nomenclature suggests to me that the Sherline toolholders are proprietary.

For the Rhombus (80°) CPMT21.5 type inserts Sherline sells, the industry standard turning/facing toolholders would be designated SCLP such as the 3/8" square shank Kennametal 1095594 (right hand) and 1095686 (left hand), both of which mount the insert at a positive 5° rake angle. So it's likely the Sherline toolholders mount the CPMT inserts at a positive rake also. Below is the drawing for the Kennametal equivalent toolholder - you can clearly see the positive rake mounting. The positive rake puts less demand on the required HP of the machine and should be freer cutting than a neutral rake holder which is probably why Sherline chose this geometry and positive rake insert type.

View attachment 392407
That said, quality of finish will have a lot to do with the specific insert, nose radius, etc. The Sherline inserts are CPMT21.51 with a 0.016" nose radius which is why they spec a minimum depth of cut of 0.004". Sherline says this about their inserts:

"Carbide cutters give good finishes on hard-to-machine materials such as cold-rolled steel. They will also work on aluminum, brass, or leaded steels, and will last practically forever. However, the best finishes on those materials are still achieved with a good, sharp high-speed steel tool."​

For the sharpest insert cutting edge you'd want a CPGT insert rather than a CPMT which Sherline doesn't offer, hence their admonition about using HSS on softer materials. The difference between the two types is covered on page 23 in the book and relates to the tolerance. The "G" tolerance is the most constrained and the inserts are typically ground to meet that tolerance spec, and thus will have a sharp (rather than blunt) cutting edge. The Iscar 5510088 would be an excellent CPGT21.50 (0.004" nose radius) insert for semi- and fine-finishing turning with a wide variety of materials including aluminum. And because of the smaller nose radius, minimum depth of cut would also be less - perhaps as low as 0.0015".

Hope this helps.

David Best

Yes, it helps.

I have the Sherline 55° diamond-style right-hand and left-hand toolholders. I have not read any statement by Sherline that the inserts mount with a positive rake. From using squares to look for an angle, I concluded that the inserts do NOT mount with a positive rake angle.

I looked at the cutting edges of the of my DPMT 21.51 inserts from Sherline, using a 7x loupe. The edges appeared surprisingly rough.

Thank you for the 80° rhombus-style toolholders and insert recommendations for Sherline lathes.

 

[mikey]

Sherline seems to favor the 55 degree inserts over the 80 degree rhombus for most of their tools to enhance access to shoulders. I'm not sure this is a valid choice because clearance is clearance and both 55 and 80 degree inserts give you that clearance, not to mention the increased strength of the 80 degree insert.

I think Sherline is relying on the positive rake geometry of the inserts they sell instead of machining in a rake in the insert holder pocket. They used to sell Valenite inserts but I'm not sure what they're using now. Ever since Joe Martin died, I have been less impressed with Sherline.

I own 3/8" SCLCR tool holders from Iscar and Seco and have used them on the Sherline lathe. They work but require you to use finishing inserts and smaller nose radii so pay attention to the inserts you buy. Finishing inserts have a much smaller gap between the cutting edge and the contour of the chip breaker; they are more fragile than roughing inserts but require less horsepower to cut. The insert I use most on the Sherline have the AK chipbreaker for aluminum; these work well on aluminum but also work on steel and stainless, although the edge doesn't hold up as well.

Personally, I think inserts on a Sherline don't make a lot of sense. If your only lathe is a Sherline and you have to cut higher carbon steels or have a work-hardened piece of 304 SS then okay, you gotta do what you gotta do, but for the vast majority of the work a Sherline lathe is called upon to do HSS is a far better choice. You have to remember that the Sherline motor only has 0.08 HP so it cannot take heavy cuts with an insert; it just doesn't have the power or rigidity an insert requires. Do inserts work on this lathe? Yes, they do, but your choice of inserts is critical. In particular, pay attention to the nose radius as David points out. Sherline's inserts have a 0.0015" nose radius which is fine on a larger lathe; for the Sherline, a 0.004" NR is a better choice for accuracy.

Just keep in mind that a sharp, well-ground HSS tool with a thoughtfully designed tip geometry will vastly outperform a carbide tool on this lathe. It will rough far deeper, size more accurately, finish finer and will cut what you dial in. Of this, I am absolutely sure. HSS tools are also much cheaper. A good tool will last for over a decade without requiring a re-grind and will remain accurate over that time. If you cannot yet grind a good tool then I suggest that you learn to do it. It is, by far, the best investment you can make for your lathe.

 

[davidpbest]

Yes, it helps.

I have the Sherline 55° diamond-style right-hand and left-hand toolholders. I have not read any statement by Sherline that the inserts mount with a positive rake. From using squares to look for an angle, I concluded that the inserts do NOT mount with a positive rake angle.

I looked at the cutting edges of the of my DPMT 21.51 inserts from Sherline, using a 7x loupe. The edges appeared surprisingly rough.

Thank you for the 80° rhombus-style toolholders and insert recommendations for Sherline lathes.

The inserts Sherline lists for your setup are for positive rake toolholders. DPMT. Of course it is entirely possible that their toolholders (being proprietary to Sherline) have some bastardized rake configuration - Sherline does not publish the rake spec for their turning/facing toolholders. DPMT type (positive) inserts are typically used in boring bars rather than turning/facing tools because they provide substantially more relief angle (see page 76 in the book). Most turning/facing toolholders for the 55° inserts are for DCMT inserts which have a 7° relief angle and are intended to be used with a neutral rake toolholders. So it could be that Sherline decided to "standardize" on the DPMT insert primarily for boring bar reasons, but their turning/facing toolholders for that insert are neutral rake.



The "M" designation in the insert specification is a loose tolerance and I'm certain this insert is right out of the mold and has not been ground to a sharp cutting edge. So I'm not at all surprised by your comment about the rough cutting edge. If you want an insert for your Sherline toolholders with a ground cutting edge that is sharp, you'd be looking for a DPGT21.5 type. Several manufacturers make them. With and 11° relief angle, even if the insert is mounted in a neutral toolholder, it should perform well - assuming your Sherline toolholder pockets are indeed compatible with industry standard DPMT21.5 inserts.

Here are some alternative DPGT21.5 inserts to consider with 0.004" nose radius:

https://www.mscdirect.com/browse/tnpla/01046127?affiliate_id=141648&click_id=3842462382&cid=af_q218&clickId=3842462382

This version is specifically graded for softer materials like aluminum:

https://www.mscdirect.com/product/details/15592983

Here is one example on eBay, although these have a larger (0.016") nose radius - they look genuine Kennametal to me, but who knows?:

8 pcs. DPGT21.51HP K313 KENNAMETAL INSERT (loc I16) | eBay

Find many great new & used options and get the best deals for 8 pcs. DPGT21.51HP K313 KENNAMETAL INSERT (loc I16) at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Alternatively, you could try a DCGT insert on the assumption that your Sherline toolholders are indeed neutral rake. This would be an inexpensive way to find out:

DCGT 21.5VFR-J10 J740 | Tungaloy | Insert | Sold Individually | DCGT070200FR | eBay

Sold and Priced Individually. • The cutting edge sharpness and “J-Coat” realizing long tool life contribute to the outstanding cutting performance in the surface finish and wear resistance. • All-rounder for general steels to stainless steels and free-cutting steels.

www.ebay.com

You could of course outfit yourself with the 80° style toolholders referenced in my prior post here, or use HSS.

David

 

[davidpbest]

Keep in mind that I have zero experience with the Sherline. @mikey will have more "field tested" thoughts about what works best with that machine.

 

[Karl A]

I appreciate the detailed replies and recommendations.

I purchased the Sherline 55°-rhombus-style carbide-insert toolholders two months after I bought the lathe. Two months after that, I wrote down two statements that Mikey made in 2015 in a thread here on Hobby Machinist:
"... buy a QCTP and use 3/8" HSS tooling instead of 1/4". You will rough more deeply, size more accurately, finish finer, chatter less, and save money with HSS. The Sherline lathe responds very well to tool geometry intended to reduce cutting forces."
"If you insist on carbide, I would go with SCLCR/L toolholders that take CCMT/CCGT inserts. ..."
These statements are mirrored above.

I appreciate the agreement between Mikey and David on recommending 0.004-inch nose radius on carbide inserts for Sherline lathes.

I am nearly ready to begin grinding steel keystock, before grinding HSS blanks.

Having the right-hand carbide-insert toolholder has helped me get to this point.

To some degree, carbide inserts have been a distraction. I would be further along on grinding HSS tools if I had spent less time learning about carbide inserts. Knowing about them will probably be good in the long run.

I ordered one DCGT 21.5VFR-J10 J740 insert. I am curious to see how well it fits and works on a Sherline toolholder.

I took photos of the left-hand Sherline 55° toolholder and its insert -- to show that the insert appears to not be mounted with a positive rake angle, and to show the roughness and dullness of the cutting edge of the insert.



Karl

 

[davidpbest]

That tool holder is clearly neutral rake. A DCGT or DCMT insert should work if the pocket is industry standard. IMO the insert in your photo is inappropriate for a lathe like a Sherline - it’s straight out of the mold, and the nose radius is excessive. But what do I know - I’ve never used a Sherline. What I can say is that I wouldn’t attempt to use that insert on my 1340 with aluminum - it would tear rather than cut. It’s probably swell on cast iron.

The DCGT 21.5VFR-J10 J740 you ordered should work with your left handed tool holder, but not the right handed version since there are only two cutting edges. But at least you’ll know if it fits and whether a 4-edged version of a DCGT will work with your right handed tool holder. Keep us posted.

 

[mikey]

I am nearly ready to begin grinding steel keystock, before grinding HSS blanks. Having the right-hand carbide-insert toolholder has helped me get to this point. To some degree, carbide inserts have been a distraction. I would be further along on grinding HSS tools if I had spent less time learning about carbide inserts. Knowing about them will probably be good in the long run.

A lot of us start out with carbide just so we can learn to use the lathe. Nothing wrong with that, and the knowledge you gain from your experience with inserts is always valuable. One day, you may step up to a larger lathe and everything you learn on a Sherline lathe will transfer, including your knowledge about carbide insert geometry. I honestly believe that a hobby guy should know how to use all kinds of tooling so he can use the right tool for the job at hand. Elitism in tools is foolish; competence is not.

With that said, if you approach HSS tool grinding and appreciation for tool geometry in the same way you did for inserts then you will realize the potential of your lathe very quickly because the key to unlocking this lathe is tool geometry.

My apologies to David for derailing his thread. I do not wish to detract from the comments on his book and will be writing my own review of it soon.

 

[bigolbeast]

Just ordered mine

 

[Karl A]

That tool holder is clearly neutral rake. A DCGT or DCMT insert should work if the pocket is industry standard. IMO the insert in your photo is inappropriate for a lathe like a Sherline - it’s straight out of the mold, and the nose radius is excessive. But what do I know - I’ve never used a Sherline. What I can say is that I wouldn’t attempt to use that insert on my 1340 with aluminum - it would tear rather than cut. It’s probably swell on cast iron.

The DCGT 21.5VFR-J10 J740 you ordered should work with your left handed tool holder, but not the right handed version since there are only two cutting edges. But at least you’ll know if it fits and whether a 4-edged version of a DCGT will work with your right handed tool holder. Keep us posted.

The DCGT 21.5VFR-J10 J740 insert arrived. I installed it on the right handed tool holder, and used it to turn down 1144 steel and a bronze-aluminum bushing. Here are photos that compare the DCGT insert on the right-handed tool holder and the Sherline DPMT insert on the left-handed tool holder. In the photos, the left-handed tool holder is on the left.



Note: I damaged the tip on the trial insert. The nose radius was 0.001 inch. The finish was poor during the finishing pass (2500 rpm spindle, 0.0003 inch / spindle revolution, 0.0005 depth of cut.) on two 1 1/8-inch diameter bronze-aluminum bushings.

 

[Karl A]

Here is another photo. This shows the holders and inserts from the underside. The experimental insert is on the right side.

The experimental insert DCGT overhangs the tool holder more than the Sherline insert DPMT. I think that the extra overhang is due to experimental insert having a smaller clearance angle: 7° versus 11°. The inserts are the same size (0.25-inch inscribed circle) on top and have the same thickness (3/32-inch). The smaller clearance angle gives the experimental insert a larger underside. I conclude that the experimental insert does not fit the tool holder well.