A CNC operator opening the tool magazine at the start of a heavy-duty milling cycle reaches for a tool holder that is invisible to the part drawing but decides whether the cycle survives the cut. The two holders on the bench at the start of the cycle are usually the side-lock end-mill holder and the boring-bar insert holder, and the choice between them is the first procurement-side decision the operator has already committed to by the time the cutter enters the spindle. The DerekMetal SLN side-lock holder product page and the DerekMetal SFC/SLC/SSC roughing holder product page are the two reference documents for the engineering comparison below.
The two holders are not interchangeable. The SLN side-lock holder is the right answer for a cylindrical-shank cutter under heavy axial and radial load — a face mill with a 25 mm straight shank taking a 6 mm depth of cut in alloy steel at 80 m/min cutting speed. The SFC/SLC/SSC roughing holder is the right answer for internal rough turning of a pre-existing hole — a 40 mm minimum bore diameter in a cast iron housing with a CC**09T3 insert taking a 1.5 mm depth of cut per side at 220 m/min cutting speed. Putting the wrong holder on the wrong cutter is the most common cause of a holder failure on the first part of a heavy-duty cycle.
The clamping mechanics
The clamping mechanics of the two holders are different in three ways that matter for heavy-duty milling.
**Surface area in contact with the cutter.** The SLN side-lock holder screw bears on the flat ground on the cutter shank, which gives a full-width flat-to-flat contact between the screw face and the shank flat. The screw is sized to the shank diameter — M6 for a 6 mm shank, M8 for an 8 mm shank, M10 for a 10 mm shank, M12 for a 12 mm and 16 mm shank, M16 for a 20 mm and 25 mm shank. The shank is a solid steel cylinder of the same diameter as the cutter, so the screw bears on a thick steel substrate that does not deflect under the screw clamping force.
The SFC/SLC/SSC roughing holder screw bears on the carbide insert pocket, which is a thin steel pocket designed to locate the insert and resist the cutting load. The screw contact area is the insert screw thread (typically M2.5 to M4 depending on the insert size) and the screw bears on the insert clamping area, not on a flat ground on the cutter. The shank of the boring bar is a thick steel cylinder, but the insert pocket is a localised clamping surface.
**Clamping force per unit shank area.** The side-lock screw generates a clamping force that is approximately proportional to the screw size and the torque applied at installation. A correctly torqued M10 side-lock screw on a 10 mm shank generates a clamping force in the order of several kilonewtons per square centimetre of shank area, which is the highest clamping force per unit area of any standard end-mill holder geometry (including ER spring collet, hydraulic chuck, and shrink-fit holders).
The insert screw generates a clamping force that is approximately proportional to the screw size and the torque applied at installation, but the insert pocket geometry and the insert thickness limit the maximum clamping force. A correctly torqued insert screw generates a clamping force that holds the insert in the pocket under the cutting load, but the per-unit-area clamping force on the insert is lower than the side-lock clamping force on a cylindrical shank, because the insert pocket is a localised clamping surface rather than a full-width flat-to-flat contact.
**Anti-rotation friction.** The SLN side-lock holder's screw-on-shank-flat geometry creates a high anti-rotation friction between the shank flat and the screw face. The screw bearing on the flat prevents the shank from rotating under the cutting load, which is the engineering reason the side-lock holder survives heavy radial load on a face mill or a heavy-duty end mill at high feed rates. The Sandvik Coromant tool holder reference page walks the same screw-on-shank-flat anti-rotation principle on its side-lock holders that compete with the SLN in the heavy-duty end-mill holder tier.
The boring-bar insert pocket geometry creates the insert anti-rotation friction through the screw bearing on the insert clamping surface, but the anti-rotation margin is lower than the side-lock holder because the insert pocket is a localised surface and the insert pocket walls can deflect under the cutting load. The boring-bar insert pocket is engineered to hold the insert under the typical rough-turning load profile, but the rough-turning load profile is different from the heavy-duty milling load profile that the side-lock holder is designed for.
The runout tolerance
The runout tolerance of the two holders is specified by the manufacturer at a defined test condition, and the test condition is different for the two holders. The procurement-side reading of the runout specification requires understanding the test condition.
**SLN side-lock holder runout specification.** The side-lock holder runout is specified at the cutter shoulder, at a distance of 4×D from the spindle face, where D is the cutter shank diameter. The runout tolerance is typically specified in the order of a few microns at the test condition. The runout tolerance is achieved when the cutter shank flat is clean, the screw is torqued to specification, and the cutter shank is fully seated against the holder face.
The DerekMetal SLN side-lock holder product page lists the side-lock screw sizes for the BT-SLN, SK-SLN, HSK-A-SLN, and HSK-A-SLNB holder series, which covers the BT-SK, CAT, and HSK spindle interface families with cutter shank diameters from 6 mm to 25 mm. The runout tolerance is the same across the four holder series because the side-lock geometry is the same — the spindle interface determines the holder-to-spindle fit, not the holder-to-cutter fit.
**SFC/SLC/SSC boring-bar runout specification.** The boring-bar runout is specified at the insert cutting edge, at the insert centre line, where the insert reference surface is in contact with the insert pocket. The runout tolerance is typically specified in the order of a few microns at the test condition. The runout tolerance is achieved when the insert pocket is clean, the insert is seated against the pocket reference surfaces (the bottom and the side wall), and the insert screw is torqued to specification.
The DerekMetal SFC/SLC/SSC holder for roughing product page lists the SCFCR, STFCR, and SCLCR insert pockets for the DEREK, MITSUBISHI, and HITACHI insert families, with minimum bore diameters from 25 mm to 40 mm. The runout tolerance is the same across the three insert pocket styles because the insert pocket reference geometry is the same — the insert style determines the pocket-to-insert fit, not the boring-bar-to-holder fit.
**Runout sensitivity to installation discipline.** The side-lock holder runout is sensitive to three installation variables: the cutter shank flat cleanliness, the screw torque value, and the cutter shank full seating. The boring-bar insert pocket runout is sensitive to three installation variables: the insert pocket cleanliness, the insert seating against the reference surfaces, and the insert screw torque value. The boring-bar insert pocket is more sensitive to installation discipline because the insert pocket reference surfaces are smaller than the side-lock flat and a localised contamination on the insert pocket reference surface shows up directly on the insert cutting edge runout. The Modern Machine Shop trade reference on CNC tool holders walks the runout tolerance methodology across the standard end-mill holder families.
The balance grade
The balance grade of the two holders is specified by the manufacturer at a defined test condition, and the test condition is the same for both holders. The procurement-side reading of the balance specification requires understanding the test condition.
**SLN side-lock holder balance specification.** The side-lock holder balance is specified at the G2.5 balance grade at the rated spindle speed, where the balance grade is a measure of the residual unbalance in the holder-tool combination expressed in gram-millimetres. A G2.5 balance grade at a 25,000 RPM rated spindle speed corresponds to a residual unbalance of approximately 1 gram-millimetre per kilogram of holder-tool mass, which is the standard balance grade for general-purpose milling. The Sandvik Coromant engineering reference on rotating unbalance walks the underlying balance grade methodology the G2.5 specification references.
The side-lock holder balance grade is achieved when the cutter is balanced to specification by the cutter manufacturer and the holder is balanced to specification by the holder manufacturer. The side-lock holder geometry does not introduce significant additional unbalance because the holder is a symmetrical steel cylinder with a single side-lock screw.
**SFC/SLC/SSC boring-bar balance specification.** The boring-bar balance grade is typically specified at the G2.5 balance grade at the rated spindle speed, with the same test condition as the side-lock holder. The boring-bar balance grade is achieved when the boring bar is balanced to specification by the holder manufacturer and the insert is balanced to specification by the insert manufacturer.
The boring-bar balance grade is sensitive to insert selection — a heavier insert shifts the boring-bar centre of gravity off the spindle axis, and a lighter insert shifts it the other way. The procurement-side specification of the boring-bar balance grade therefore includes the insert reference, not just the boring-bar reference.
The tool-life economics
The tool-life economics of the two holders are different in three ways that matter for the per-part cost calculation.
**Cutter replacement cost.** The SLN side-lock holder accepts a cylindrical-shank cutter, which is replaced when the cutter wears. The cutter replacement cost is the cost of a new cylindrical-shank cutter, which is typically lower than the cost of a new boring-bar insert holder with the inserts fitted. The cylindrical-shank cutter is a consumable item in the same way as a brazed-tip end mill.
The SFC/SLC/SSC boring-bar holder accepts a carbide insert, which is replaced when the insert wears. The insert replacement cost is the cost of a new insert plus the cost of the index time on the machine, which is typically lower than the cost of a new cylindrical-shank cutter. The carbide insert is a consumable item in the same way as a brazed-tip end mill, but the index time is shorter.
**Holder replacement cost.** The SLN side-lock holder replacement cost is the cost of a new side-lock holder. The side-lock holder is a consumable item in the sense that the screw thread wears over time and the holder eventually needs to be replaced, but the holder typically lasts for the life of the spindle interface. The MachiningCloud digital tool catalog tracks the holder replacement cost across the standard end-mill holder families in real time.
The SFC/SLC/SSC boring-bar holder replacement cost is the cost of a new boring bar. The boring bar is a consumable item in the same way as the side-lock holder, but the boring bar wear out faster than the side-lock holder because the boring bar pocket is a localised clamping surface.
**Per-part cost calculation.** The per-part cost calculation on a heavy-duty milling cycle is the sum of the holder cost amortised over the holder life, the cutter cost amortised over the cutter life, the insert cost amortised over the insert life, and the machine time cost. The side-lock holder typically wins on the cutter cost amortisation because the cylindrical-shank cutter is cheaper than the boring-bar insert holder with the inserts fitted. The boring-bar insert holder typically wins on the index time amortisation because the insert index time is shorter than the cylindrical-shank cutter replacement time.
The procurement-side decision matrix
The procurement-side decision matrix for the side-lock holder versus the boring-bar roughing holder has three branches.
**Branch 1: Cylindrical-shank cutter under heavy load.** The SLN side-lock holder is the right answer for a face mill, an end mill, a drill, a reamer, or a tap with a cylindrical shank under heavy axial and radial load. The side-lock holder screw bearing on the shank flat generates the highest clamping force per unit shank area of any standard end-mill holder, and the screw-on-shank-flat geometry creates the high anti-rotation friction that survives heavy radial load on a face mill or a heavy-duty end mill at high feed rates.
The DerekMetal tool holders catalog lists the side-lock holder as one of the standard end-mill holder families alongside the spring collet chuck, the face mill holder, the side cutter holder, the Morse taper adapter, the tapping chuck, and the hydraulic chuck. The tier in which the side-lock holder sits is the heavy-duty end-mill holder tier.
**Branch 2: Internal rough turning of a pre-existing hole.** The SFC/SLC/SSC roughing holder is the right answer for internal rough turning of a pre-existing hole in a cast iron, steel, or aluminium housing. The boring-bar insert pocket is engineered to hold the insert under the typical rough-turning load profile, and the insert index time is shorter than the cylindrical-shank cutter replacement time.
The boring-bar insert pocket minimum bore diameter (Dmin) is the geometric constraint on the boring-bar selection. The SFC pocket starts at 25 mm Dmin, the SLC pocket starts at 25 mm Dmin, and the SSC pocket starts at 25 mm Dmin, with the larger pocket sizes (SFC10CA-09, SLC10CA-09, SSC10CA-11) extending the Dmin range to 40 mm. The procurement-side specification of the boring-bar includes the insert pocket reference, not just the boring-bar reference.
**Branch 3: Hybrid high-speed or repeatable-runout environment.** The shrink-fit holder, the hydraulic chuck, or the high-precision collet chuck is the right answer when the production environment demands repeatable high-speed runout without the side-lock screw's anti-rotation friction. The side-lock holder is a heavy-duty holder, but the side-lock screw is a manual friction and load that the operator applies at every cutter change. The shrink-fit holder, the hydraulic chuck, and the high-precision collet chuck are designed for high-speed repeatable runout.
The procurement-side specification of the hybrid environment includes the spindle speed rating, the runout tolerance, and the balance grade. The shrink-fit holder is rated for 25,000 RPM and above; the high-precision collet chuck is rated for 25,000 RPM and above; the hydraulic chuck is rated for 20,000 RPM and below. The side-lock holder is rated for 15,000 RPM and below.
The four procurement-side questions
The procurement-side decision between the SLN side-lock holder and the SFC/SLC/SSC roughing holder is answered by four questions that the procurement engineer works through before issuing the RFQ.
**Question 1: Cutter shank or insert pocket?** The SLN side-lock holder accepts a cylindrical-shank cutter. The SFC/SLC/SSC roughing holder accepts a carbide insert on a boring bar. The first question determines which holder family is the right answer.
**Question 2: Cylindrical-shank or pre-existing hole?** The SLN side-lock holder is the right answer for external milling, drilling, reaming, or tapping on a cylindrical-shank cutter. The SFC/SLC/SSC roughing holder is the right answer for internal rough turning of a pre-existing hole. The second question determines the application.
**Question 3: Heavy radial load vs cooling radial load?** The SLN side-lock holder survives heavy radial load on a face mill or a heavy-duty end mill at high feed rates. The SFC/SLC/SSC roughing holder survives the typical rough-turning load profile on a boring bar. The third question determines the load profile.
**Question 4: Spindle speed rating?** The SLN side-lock holder is rated for 15,000 RPM and below. The SFC/SLC/SSC roughing holder is rated for the boring bar application, which is typically 8,000 RPM and below. The fourth question determines the spindle speed compatibility.
The four procurement-side questions are the operational translation of the engineering differences between the two holder families into a procurement-side decision framework. The four questions run end-to-end before the RFQ is issued.
The DerekMetal SLN side-lock holder product page, the DerekMetal SFC/SLC/SSC roughing holder product page, and the DerekMetal tool holders catalog cover the engineering reference data, the procurement-side specification data, and the operational data for both holder families. The DerekMetal contact page is the procurement entry point for holder sample requests, holder specification datasheets, and RFQs for heavy-duty CNC machining centre tooling.
Frequently Asked Questions
What is the difference between the SLN side-lock holder and the SFC/SLC/SSC roughing holder?
The SLN side-lock holder holds a cylindrical-shank end mill, drill, reamer, or tap by a side-lock screw bearing on a flat ground on the cutter shank. The SFC/SLC/SSC roughing holder holds a carbide insert on a boring bar by an insert screw bearing on the insert pocket. The two holders are not interchangeable; the SLN is for cylindrical-shank cutters under heavy load, and the SFC/SLC/SSC is for internal rough turning of pre-existing holes.
Which holder generates the higher clamping force per unit shank area?
The SLN side-lock holder generates the higher clamping force per unit shank area because the side-lock screw bears on the full-width flat ground on the cutter shank. The SFC/SLC/SSC insert screw bears on the insert pocket, which is a localised clamping surface on a thin carbide insert edge rather than a thick steel shank flat. The side-lock holder clamping force is approximately several kilonewtons per square centimetre of shank area at the correctly torqued screw value.
Which holder is more sensitive to installation discipline?
The SFC/SLC/SSC boring-bar holder is more sensitive to installation discipline because the insert pocket reference surfaces are smaller than the side-lock flat and a localised contamination on the insert pocket reference surface shows up directly on the insert cutting edge runout. The SLN side-lock holder runout is sensitive to the cutter shank flat cleanliness, the screw torque value, and the cutter shank full seating, but the tolerance band on each variable is wider than the tolerance band on the boring-bar insert pocket reference surfaces.
Which holder should I use for a face mill taking a 6 mm depth of cut in alloy steel at 80 m/min?
The SLN side-lock holder is the right answer for that load profile. The side-lock screw on the face mill shank flat generates the highest clamping force per unit shank area of any standard end-mill holder, and the screw-on-shank-flat geometry creates the high anti-rotation friction that survives heavy radial load on a face mill at high feed rates. The SFC/SLC/SSC boring-bar insert holder is the wrong answer for that load profile because the boring-bar insert pocket is engineered to hold the insert under the typical rough-turning load profile, not the heavy-duty milling load profile that the face mill generates.
Which holder should I use for internal rough turning of a 40 mm pre-existing hole in cast iron?
The SFC/SLC/SSC boring-bar holder is the right answer for that load profile. The boring-bar insert pocket is engineered to hold the insert under the typical rough-turning load profile, and the insert index time is shorter than the cylindrical-shank cutter replacement time. The minimum bore diameter (Dmin) on the SFC10CA-09, SLC10CA-09, and SSC10CA-11 pockets is 40 mm, which matches the pre-existing hole diameter.
What is the spindle speed rating on each holder?
The SLN side-lock holder is rated for 15,000 RPM and below, which is the standard rating for heavy-duty end-mill holder geometries. The SFC/SLC/SSC boring-bar roughing holder is rated for the boring-bar application, which is typically 8,000 RPM and below. The shrink-fit holder, the hydraulic chuck, and the high-precision collet chuck are the right answers when the production environment demands repeatable high-speed runout above 15,000 RPM without the side-lock screw's anti-rotation friction.
How is the procurement-side decision between the two holders run?
The procurement-side decision runs through four questions: (1) cutter shank or insert pocket? (2) cylindrical-shank or pre-existing hole? (3) heavy radial load vs cooling radial load? (4) spindle speed rating? The four questions determine which holder family is the right answer for the application before the RFQ is issued.
This article is published by the DerekMetal engineering team. Ningbo Derek Metal Co., Ltd. (DerekMetal) specialises in tool holders, boring tools, anti-vibration holders, angle heads, milling cutters, turning tools, carbide end mills, carbide inserts, and the SFW-24P water-cooling shrink-fit machine. The product line covers BT-SLN, SK-SLN, HSK-A-SLN, and HSK-A-SLNB side-lock holder series with cutter shank diameters from 6 mm to 25 mm, and the SFC/SLC/SSC roughing holder series with minimum bore diameters from 25 mm to 40 mm for DEREK, MITSUBISHI, and HITACHI insert families. Derek tools are used in more than 70 countries by experienced technicians with high technical manufacturing and inspection equipment.