Reaming

A reamer finishes a hole that has already been drilled. The drill leaves the hole a little undersize; the reamer, a tool with many straight or helical flutes, takes off the last few tenths of a millimetre and leaves the hole at its final size with a better wall. The cut is light and the tool is guided by the hole itself, which is the source of both its accuracy and its main limit.

How much better than a drill? Craig Ewing of ISCAR put it this way in Cutting Tool Engineering in 2021: drills today typically hold IT8 to IT9, and “if you want to go tighter than IT8, you need to go to reaming”, which easily reaches IT7 and in some cases IT5. Catalogue machine reamers are commonly made for H7 holes: Hoffmann’s are “finish ground for fit H7”, and ISCAR describes its BAYO T-REAM heads as “suitable for reaming applications of H7 hole tolerance range”. In inch terms, Kennametal says a properly applied reamer holds size “within a few ten-thousandths of an inch”. None of our sources gives a general roughness range for reaming; one maker’s figure is Gühring’s, below Ra 1.6 µm in grey cast iron and below Ra 1.2 µm in nodular iron for its HR 500 CAST reamer.

What reaming does not correct

“Reamers follow the hole”, in Kennametal’s phrase, so “any runout or drift during the drilling operation will remain after reaming.” Bysterbusch of Komet said the same in Cutting Tool Engineering: “The reamer follows the hole”, adding that a reamer can slightly improve straightness if it also cuts at its front. MAPAL’s advice in the same article was to bore when the hole’s shape or location needs correcting. Kennametal’s remedies come before the reamer: spot drill deep enough to chamfer the hole slightly, so the drill does not walk; some machinists, it notes, also true the hole a diameter or two deep with a boring bar or an end mill. ISCAR asks for a pre-hole that is “smooth and straight, without deep scratches on it.”

So reaming fixes size and finish. Position and straightness come from the operations before it.

Reamer types

The German standards sort reamers by how they are driven and how they are held. DIN Media lists, among others:

A hand reamer is turned by hand, a machine reamer by the machine. A Turkish lecture note from Ankara University also lists taper reamers, reamers with inserted blades, shell reamers mounted on an arbor, and adjustable reamers. Gühring sells its carbide machine reamers “similar to” DIN 8050 and 8093 (parallel shank) and DIN 8051 and 8094 (Morse taper); up to Ø9.5 mm they are solid carbide, above that they carry a carbide head.

Beyond the solid tool, the makers describe several constructions:

• Single blade with guide pads. MAPAL launched its single-bladed reamer in 1954, with one adjustable blade clamped in a slot and two strip-shaped guides; today, in MAPAL’s words, “the tool is supported by the guide pads in the bore” while the blade cuts, and the diameter can be set “to an accuracy in the μ range”.
• Exchangeable heads. ISCAR’s BAYO T-REAM puts a carbide head with 6 to 8 edges on a bayonet holder, with “Low runout (maximum 3µm)” and coolant directed at each edge, for Ø11.5 to 32 mm. MAPAL’s CPR has a replaceable head; Kennametal names its modular RHM line.
• Cermet. In one case reported by Cutting Tool Engineering, a Komet cermet-tipped reamer ran at 4,385 rpm against about 1,800 rpm for the reamers it replaced.

Flutes: where the chips go

On a reamer that cuts right-hand, a left-hand helix sends the chips the way the tool is going. ISCAR: “The left-hand spiral is designed especially for through hole reaming. It directs the just-formed chips forward after formation.” Kennametal: reamers with a left-hand helix “tend to push the chips forward” and “should be used on through-holes or where generous chip space exists.” Hoffmann’s left-hand reamer is for through holes, and from Ø3.9 mm also for blind holes because of its short lead.

For everything else, Kennametal says, choose straight flutes for short-chipping materials such as brass and cast iron, or a right-hand helix for stringy materials, “to avoid chip packing in the bottom of the hole.” With straight flutes, ISCAR adds, the chips pass along the flutes and the coolant helps push them out.

The sources do not agree on brass and cast iron. Newman Tools recommends “a left-hand spiral-fluted reamer with negative shear action” for brass, cast iron and some plastics, where Kennametal suggests straight flutes.

Interrupted holes. A hole crossed by a keyway or a cross-hole catches straight flutes. “If you have a straight flute, every time the tooth comes around it catches and pounds like nobody’s business”, said Morrison of Alvord-Polk in Cutting Tool Engineering; he recommends spiral flutes there.

Unequal spacing and odd numbers. Many reamers have their teeth spaced unevenly around the body: Hoffmann’s H7 machine reamer has six teeth at unequal spacing, and Gühring’s 1408–1429 carbide machine reamers have “extreme unequal flute spacing” from Ø3 mm. A patent by W. C. Arnold for Rockwell International (filed 1979, granted 1987) gives the reason: reamers tend “to chatter, rifle, cut out-of-round”, and with unequal spacing “resonances are damped out eliminating chatter.” John Kiiffner of GWS Tool Group makes a related point for flute counts: “With an odd number of flutes, you get a more precise round hole than if you use an even number.” An odd count, the article adds, reduces a reamer’s tendency to lobe a hole.

How many flutes. Kennametal: “More flutes mean a faster feedrate can be used but with less room for chips. If packing is a concern, select a reamer with a lower flute count.”

Lead and back taper

The lead, or entry chamfer, at the front of the reamer starts the cut. ISCAR: “it is important to select a lead geometry that covers the reaming allowance.” Its H-REAM tables give each lead its range of stock:

A blind hole needs a short lead. Hoffmann’s left-hand reamer is suitable for blind holes “due to short chamfer lead”, and Gühring’s HR 500 Active for blind holes has a chamfer 0.4 to 0.5 mm long.

Behind the lead, the diameter falls off slightly toward the shank. ISCAR: “The back taper prevents the reamer from jamming, as well as lowering reaming forces and improving surface quality”, and “Incorrect back taper may cause unstable reaming, accelerated wear and rough surface finish.” On ISCAR’s adjustable reamers the front diameter is set larger than the rear by about 0.015 mm (its setting procedure gives front and rear values 0.010 mm apart). MAPAL’s EasyAdjust tools keep the back taper fixed while the diameter is set.

Stock allowance

The reamer needs enough material to cut. Kennametal: “A common misconception is to leave too little material for the reamer to remove”, and “Too little material will cause the reamer to rub or burnish and results in accelerated wear and poor surface finish.” Too much leads to “chip packing and possible tool breakage.” ISCAR lists too little allowance as a cause of a hole that comes out too small.

Reamer size

16

mmGühring rowAll materialsHardened, ≤ 48 HRCHardened, ≤ 63 HRC

Gühring

0.20–0.30 stock
15.70–15.80 pre-drill

Newman / ICS

0.30–0.46 stock
15.54–15.70 pre-drill

Kennametal

0.64 stock
15.37 pre-drill

Kennametal 3 %

0.48 stock
15.52 pre-drill

Gühring’s table, in millimetres on diameter:

Kennametal gives three steps in inches, by drill size: .010 in below ¼ in, .015 in from ¼ to ½ in, .025 in from ½ to 1½ in; its blog gives the shorter rule of drilling “approximately 3% smaller than the finished hole”. Newman Tools’ table runs from .003–.005 in for the smallest reamers to .012–.018 in at ½ to ¾ in, and Productivity’s varies the stock by work material as well, from .001–.002 in at the smallest size (.015 in) to .022 in for aluminium at 1 to 2 in. Hoffmann prints the allowance on each tool: 0.2 mm for its Ø13 mm H7 reamer, 0.2–0.3 mm at Ø14 and 0.3–0.5 mm at Ø28 for its HSS-E reamers, 0.1–0.2 mm for a Ø11.5 solid carbide reamer.

The tables agree in shape but not in size. At Ø10 to 12.7 mm, Gühring’s 0.2–0.3 mm sits below Kennametal’s .015 in (0.38 mm), with Newman’s .010–.015 in (0.25–0.38 mm) overlapping both; from 12.7 mm Kennametal steps up to .025 in (0.64 mm) and Newman to .012–.018 in (0.30–0.46 mm). Newman also says that stock on roughing should not, in most cases, exceed 2 to 4 % of the tool diameter, and that finishing stock is “.002 to .004” in; the sheet does not explain how that second figure fits its own table. In steel, cast iron, titanium and nickel-base superalloys, where a thick chip takes too much heat, Bysterbusch of Komet advises: “You’re better off removing as little as you can.”

The reamer’s own tolerance

A reamer for an H7 hole is not itself made to H7. It has to stay below the hole’s maximum, because, as Kennametal puts it, “reamers tend to cut slightly larger than nominal”; Kennametal’s advice is to choose one “at the bottom of the hole’s tolerance band”. ISO 522 sets the reamer’s limits from the top of the hole’s tolerance:

• high limit of the reamer = maximum diameter of the hole − 0.15 IT
• low limit of the reamer = high limit − 0.35 IT

where IT is the tolerance of the hole, and both values are “rounded upwards to a whole multiple of 0,001 mm”. The standard’s own example is a 12 H7 hole: IT7 is 18 µm, 0.15 × 18 = 2.7, rounded to 3 µm. Carried through, the reamer is 12.008 to 12.015 mm. Hoffmann’s Ø13 mm reamer for H7 holes is made to +0.008/+0.015 mm, which is the same rule in the same size band.

Nominal size

13

mm

Hole class

H6H7H8H9H10H11G7F7JS7K7N7P7

Reamer max

13.015 mm

Reamer min

13.008 mm

Hole tolerance

IT7 = 18 µm

Unless another grade is ordered, ISO 522 says, reamers are made to m6, “the standard stock grade”; the rule above is for special tolerances. For a 12 mm reamer, m6 is +0.007/+0.018 mm. The German counterpart, DIN 1420, “Reamers; manufacturing tolerances and designation”, is still listed as current. Makers also sell reamers to their own tolerances: Gühring’s 1427 NC reamers are 0/+0.004 mm up to Ø5.5 and 0/+0.005 mm above.

For the hole classes themselves, see the ISO 286 fits calculator.

Cutting data

Kennametal’s rule against drilling: “Cutting speeds should be about two-thirds that of drilling SFM for similar material”, and “feeds should be two to three times higher.” The feed per revolution is shared by all the teeth, so a reamer with many teeth takes only a thin chip on each.

Hoffmann HSS-E, DIN 208 BProductivity HSS (converted from SFM)

Hoffmann’s table for its HSS-E machine reamers to DIN 208 B:

Hoffmann gives the feed only for steel below 750 N/mm²: 0.15 mm/rev at Ø14 mm and 0.25 mm/rev at Ø28 mm.

Productivity’s table, converted from SFM, puts carbide well above HSS for every material except titanium:

Productivity calls its figures estimates that “will vary by application”. Newman Tools sorts steel by hardness instead: 60 to 90 SFM (18 to 27 m/min) below HRC 30, down to 8 to 12 SFM (2.4 to 3.7 m/min) at HRC 60 and above, with no split by tool material.

Holding the reamer, coolant, blind holes

Holders. Kennametal recommends “a high-quality collet chuck, shrink-fit, or hydraulic toolholder for minimal runout”, and says: “Don’t use a drill chuck or side-lock holder.” Where misalignment is present, it recommends a floating holder instead. ISCAR’s remedy for a hole that comes out oversize or conical because the reamer and the pre-hole are not on the same centre is the same: “Use a floating reamer chuck or correct pilot hole.” Gühring’s finishing holders can be adjusted to correct “concentricity and alignment errors”.

Coolant. Newman: “In all reaming operations, use constant-flow coolants”; light oil for steel above HRC 50, soluble oil and water for cast iron and non-ferrous metals. Nestor of MAPAL told Cutting Tool Engineering that “soluble oils and semisynthetics work better”. Kennametal asks for clean, well-maintained coolant, through the tool where the machine has it, and warns: “in no case should you peck the reamer as you would a non-coolant fed drill.”

Blind holes. “Bottoming out in blind holes” is on Kennametal’s list of causes of reamer breakage. In a blind hole the chips have to come back up: on ISCAR’s INDEXH-REAM, the coolant “reaches the bottom of the blind hole and evacuates the formed chips”, which then travel back through the holder’s long flute.

Faults

Kennametal’s Tech Tip #105 lists fourteen causes of reamer wear and breakage, among them misalignment that “creates a bell-mouth condition”, lack of chip space in the flutes, dirt or burrs in the collet or spindle, wrong speed or feed, poor lubrication, too little or too much stock, poor hole quality, improper regrinding, chatter and a set-up that lacks rigidity. ISCAR’s troubleshooting table, from its inch catalogue:

For chatter, Newman’s advice is to “slow cutting speed and increase feed appropriately”, and it warns that “Improper feed rate can cause excessive tool wear as well as an inadequate hole finish.” A drill is not a substitute for a reamer: “Drilling to open up a hole usually causes drill failure due to unequal and uneven stress along the drill’s cutting edge and is not recommended”, said Monster Tool’s Lynberg in Cutting Tool Engineering; he suggests reaming twice instead.

For the hole before the reamer, see drilling; for threading the hole instead, tapping.