HSS vs Carbide Inserts for the HSM - Great post at HSM

HMF

Site Founder
Administrator
Joined
Sep 22, 2010
Messages
7,223
There is a very informative post over at Home Shop Machinist. I am posting the first post below as an excerpt.
The URL is:

http://bbs.homeshopmachinist.net/showthread.php?t=44874
Enjoy!

Nelson


-----------------------------------------------------------


HSS vs Carbide Inserts for the HSM ?

--------------------------------------------------------------------------------

This topic has been done to death but recently a sale advertised in the Enco catalog prompted a few experiments performed on HSS tooling and carbide inserts. The post is intended for HSM interest, focused on what works well in small lathes found in the home shop. Here are the materials that were evaluated and their cost:

1/4 M2 HSS blank, hand-ground @ $1.10 each (Enco 383-5316 , mfr: Interstate) with shop-made toolholder in 4-way toolpost, the tool is always centered (no shims)

5/8 toolholders for inserts, 5 piece set $35.00 (Enco TG250-1402) with 3/8 I.C. carbide inserts, 10 @ $3.00 each (Enco TG340-1014, mfr: Interstate)

Here are the five various tool holders; there is a RH and LH tool for facing to a shoulder, a RH and LH angled "roughing" tool (identical to the first two except that the cutting face is oriented at 30 degrees) and a "sort-of" threading tool (doesn't have enough side relief to cut coarse threads).

My first impression was that there may be no need for several of these toolholders, depending on the lathe toolpost. By rotating the toolpost slightly, the two right-angle turning tools ("right angle" in that they are normally positioned at ninety degrees to the turning axis) can perform the same operations that the two angled toolholders are intended to perform. The "threading" toolholder can also be replaced by others in the set - the same insert is used for all holders (sixty degree included angle). The only reason for preferring any of the tools (other than RH or LH) is convenience, such as proximity/interference to/with chuck or tailstock.

Toolholders intended for "shouldering" may not be useful for the HSM unless the depth of cut is small. Most small lathes don't like to take a cut like this (cutting edge parallel to the shoulder) without complaint: excessive chattering and tool marks are typical. To make the shouldering operation without chatter, the spindle RPM must be slowed WAY down which sometimes defeats the purpose of the carbide tool for reducing turning time.

There's an alternative, however, for small lathes. One can turn at the desired speed/feed nearly up to the shoulder then switch off the motor, leaving the feed engaged (this is also the way that I approach single-point threading in a small lathe). After the spindle has wound down, the workpiece can be rotated by hand until the shoulder dimension is reached - at this point, disengage the feed and continue to rotate the spindle by hand until a clean, chatter-free face is obtained. Generally this takes only a turn or two.

A more conventional technique - probably the one that most of us use - is to turn the diameter to the shoulder with a cutter that is angled (so that the face is slightly concave) disregarding the chatter. Then withdraw the cutting tool (with the cross-slide), facing the shoulder to dimension with no chatter marks and accurately perpendicular to the lathe axis.

I don't own a QC toolpost, they limit rigidity on small lathes, IMO. The toolholders that I ordered needed to be modified to fit my 4-way so that the inserts are exactly on the spindle centerline (I don't use shims). I purchased a larger set intentionally so that I could custom fit them to my lathe. To keep the cutting edge close to the edge of the four-way toolpost instead of hanging out excessively, I needed to remove 1.50 inches from the shank length. To position the cutting tool on the spindle centerline, about .060 needed to be removed from the bottom of the toolholders.

Neither of these modifications would present a problem if the toolholders were made from a (typical) semi-hard material, like 4140. But to my surprise (and delight) the toolholders were HARDENED - I had not expected this from a modestly-priced set. Normally, one would whack off the excess length with an abrasive cutoff wheel in a die grinder but my brother-in-law borrowed my compressor a month or so back for an auto restoration project (note to self: remove wheels from compressor and discard when compressor is returned). I installed an abrasive cut-off wheel in a trim router and cut down the shank of the holder. The toolholder was placed in the mill vise (upside-down, on parallels) and the thickness reduced to dimension in the vertical mill. (This operation could have been done three times more quickly and with better results in the horizontal mill but at that time I didn't have the correct carbide tooling for the machine.)

The mill spindle speed was 3200 RPM and a 1/4 solid carbide 4-flute end mill was installed in the collet (the largest carbide end mill in the drawer). With toolholder firmly clamped in the mill vise, movement from all slides was minimized by tightening/snugging up the gib locks. Half of the excess material was removed with three longitudinal passes at about two inches/minute. A finer feed removed the remainder of the material (four passes at about one inch/minute). Fine feed was necessary to avoid stressing the tip of the brittle end mill. As we know, a vertical mill is not a rigid machine so care must be exercised … even when backing the cutter out of a cut there could still be sufficient flexing to chip a cutting edge if rapid feed is employed. Here's the cut-down holder with insert installed in a four-way toolpost:

We know that there are two reasons for choosing carbide cutting tools over high-speed steel:

1. Take advantage of higher spindle speeds (three times faster than HSS).

2. Ability to turn material that is too hard for HSS tooling (as in the example above where the hardened shanks of the toolholders could only be sized with either a solid carbide cutter, a grinder or wire EDM).

I wanted to consider how useful carbide can be for improving productivity (fast metal removal) in my home shop environment. I limited consideration to mild steel - HSS cutters are quite adequate for non-ferrous materials with a few notable exceptions that most of us will not encounter. I'm not that familiar with modern carbide tooling but the rule of thumb that I've used for setting speeds is around 300 SFPM - compared to 90 SFPM for HSS. This corresponds roughly to a spindle speed (in RPM) of 1200/diameter, where diameter is the workpiece diameter (for lathes) or the cutter diameter (for mills and all perimeter-cutting tools) in inches.

We might estimate the practicality of using carbide by considering the AVERAGE diameter workpiece (the one that accommodates 75% of our needs). In my case, that would probably be around 0.5 inch diameter. Using my rule of thumb, that translates to a spindle speed of 2400 RPM. My small lathe is old and the maximum spindle speed is around 2200 RPM. That is high enough so that carbide is obviously practical for improving productivity in my case.

To those of you routinely turning smaller workpieces, your lathe may not have high enough spindle RPM to make carbide a good choice - you'd have to be able to spin up to 4800 RPM to take full advantage of carbide tooling to turn 0.25 diameter workpieces. Here's a sanity estimate to determine the average steel workpiece diameter that can be efficiently used with carbide based on the maximum RPM of your lathe:

If your maximum spindle RPM is 1200 RPM, the smallest diameter that can efficiently utilize carbide is about one inch

If your maximum spindle RPM is 2400 RPM, the smallest diameter that can efficiently utilize carbide is about 0.5 inch

If your maximum spindle RPM is 3600 RPM, the smallest diameter that can efficiently utilize carbide is about 0.3 inch

It's easy to improve performance with carbide tooling when the workpiece is LARGE (without interrupted cuts). But if you need to turn SMALL diameters, most lathes don't have enough speed so there is little/no advantage using carbide unless hard material must be turned.

While carbide cutting tools are very hard they are not very sharp. Because of the brittleness of the material, it's pointless trying to achieve a razor edge (even if possible) that will chip at the first encounter with a workpiece. This means that it's difficult to achieve close tolerances and fine finishes on slender workpieces because of deflection of the work (due to tool pressure) and the "whip" caused by increased spindle RPM. That may not have to be a problem, however, if one isn't timid about depth of cut.

A rigid small lathe can easily take a depth of cut (DOC) of 0.250 in mild steel with a sharp tool. The inference: take the material off in a single pass, if possible, right? The trade-off is deflection (the workpiece ending up tapered). This may not be a problem for many applications - depending on the taper of course.

And that is what we are going to explore in this very brief evaluation. All experiments will be performed on my small "Compact Eight". This machine is very rigid because the ratio of the length of the ways to the width of the ways is small (about 5:1), it's fairly heavy and the 4-way toolpost is well-supported. The machine is probably equal in rigidity to an average 12 inch lathe with a QC toolpost. I bought the used lathe over thirty years ago and it seemed pricey at the time but it produces accurate parts in a reasonable length of time. The lathe routinely removes a cubic inch per minute of mild steel with a sharp HSS tool and we'll soon find out how it performs with carbide inserts.
....

http://bbs.homeshopmachinist.net/showthread.php?t=44874
 
Not being a member there, I'll comment here. I use HSS, carbide and Stellite in my home shop, along with various homemade boring bars made from oil hardening rod stock. The idea that you can't hone carbide to an absolute razor edge is complete nonsense. It can be as sharp as any other material. Coated carbide is a different animal and I avoid it for home use, but if you have the right toys, regular uncoated carbide can be as sharp as you want. The easy way is with an Accu-Finish grinder and the fine diamond wheels. The cheap and not-much-harder way is to get some diamond lapping film. Make a little jig to hold the tool at the right angles and lap by hand. I relieve the area around the cutting edge slightly with a green wheel so I don't have to lap much and the job goes way faster. That's good practice even with the Accu-Finish. My main tools, a left cutting lathe tool and a facing lathe tool, are both brazed carbide tools. I tune them up every couple months as needed, but the big benefit of carbide, for me, is that they don't get dull quickly. HSS needs to be sharpened often if you want to do good quality work. The carbide is almost always ready to go. I use more positive rake on it for home than you'd use commercially and my typical cutting speed is about 700 RPM for 1" size work- pretty darn slow for carbide, but it works fine. Only if I really need to remove material in a hurry will I move the jackshaft belt over and go to 1400 RPM or so. Note that some coated carbides require high speed to function properly, but not uncoated. Now, if all you have is a green wheel, I hesitate to recommend carbide. It will certainly work, but you'll do better work with HSS.

Conrad
 
Conrad:

Here is a copy of the post attached for you to peruse.

Best,

Nelson
 

Attachments

  • PRIMER ON CUTTING TOOLS By Randy C.pdf
    695 KB · Views: 41
Last edited by a moderator:
Thanks, that's really an excellent write-up overall, though I think the HSM can certainly put carbide tooling to good use even on a slow light lathe. OTOH, I strongly agree that everyone needs to learn to grind decent geometry HSS tools. I use HSS about 50% of the time. In either case the quality of the edge is all important. An edge that's been properly lapped or stoned after grinding will cut better to begin with, and should hold up longer. It's very difficult not to slightly round a cutting edge when sharpening by hand. The top surface also needs to be lapped so that grinding scratches aren't part of the cutting edge.

Best,
Conrad
 
Back
Top