Most of the work at JTR Specialties is honing, and buyers and engineers tend to ask us the same questions about it. These are the ones we hear most often.
What parts need honing?
Any bore that has to seal, slide, or fit a mating part to a tight clearance is a candidate for honing.
Engine cylinders and liners are the classic example. Piston rings only seal properly against a bore that is round, straight, and finished with the right texture. We hone aircraft cylinder liners, including finned liners that are plateau honed for ring seating and checked for roundness at three heights.
Hydraulic parts are the other big group. Actuator bores, landing gear cylinders, valve bodies, and fuel control valves all depend on a tight, consistent clearance between the bore and a piston or spool. A tighter clearance means less leakage and a seal that lasts longer. Honing is how that clearance is held from one end of the bore to the other.
Bushings, sleeves, gear bores, compressor parts, and gun barrels before rifling are honed for the same reasons. In each case the bore has to be the right size along its full length and finished well enough for whatever runs inside it.
What bore problems can honing fix?
Honing corrects most of the shape problems that drilling, boring, and reaming leave behind. These include undersize bores, taper, bellmouth, barrel shape, out-of-round and lobed bores, waviness, reamer chatter, and boring marks.
It works because the stones contact a long section of the bore at once and take material off the high spots first. The tool does not need to be aligned to the part, and bore length is not much of a limit.
Honing also runs cool. Grinding and boring can damage the surface layer of the metal, and worn tools can work-harden a bore. Honing generates very little heat, so it is unlikely to distort the bore or change the material at the surface.
What can’t honing fix?
Honing will not move a bore. The tool follows the bore’s existing axis, so it cannot correct position, perpendicularity to a face, or concentricity to an outside diameter. If a bore is drilled in the wrong place, honing gives you a round, straight, on-size bore in the wrong place.
Honing also cannot make an oversize bore smaller. If the bore is already past the top of the tolerance, the options are sleeving, plating, or a new part.
When a print has positional callouts, the location has to be set by boring or ID grinding before the bore comes to us for honing. Our article on choosing between honing and ID grinding covers that decision in more detail.
How much stock should I leave for honing?
Leave enough to clean up the worst error in the bore, and not much more.
Our rule of thumb is to leave at least twice the geometry error you need us to correct. If a bore is .001 in. out of round, leave at least .002 in. on the diameter. If there is .002 in. of taper, leave at least .004 in. Keep in mind that these numbers are on the diameter, not per side. Mixing those two up is one of the most common reasons a bore comes in without enough stock.
For a reamed or finish-bored hole that only needs final size and finish, a few thousandths on the diameter is usually plenty. Bores with heat-treat distortion, rough finishes, or worn surfaces on repair parts need more.
Too little stock and the bore will not clean up, leaving low spots from the previous operation. Too much stock and the job takes longer, the stones wear faster, and the price goes up. If you are not sure, send us the incoming size and how the bore was made, and we will tell you what to leave.
What surface finish can honing produce?
Honing can produce anything from a fairly coarse crosshatch for heavy oil retention to a near-mirror finish. Grit size controls how rough the finish is. Spindle speed and stroke speed control the crosshatch angle.
The crosshatch is the reason honing is used on so many sliding bores. The valleys hold oil, so a ring or seal always has a film to ride on. A bore that is too smooth or glazed does not hold oil well and can wear out a ring or seal early.
Plateau honing is a second, finer step after the base hone. It knocks the peaks down to a flat bearing surface and leaves the valleys alone. Most cylinder liners are plateau honed today because the rings seat faster and the peaks that would have broken off during run-in are already gone.
If your bore runs against rings or seals, specify more than Ra. Ra is an average, and two very different surfaces can have the same Ra. Engine and ring builders use Rpk for peak height, Rk for core roughness, and Rvk for valley depth to describe a plateau finish, as Engine Builder explains. If your print calls out those values, we hone and measure to them. If it only calls out Ra, we will ask what the bore does before we pick a process.
Which honing stone does my material need?
Stone choice depends on the material, its hardness, how much stock has to come out, and the finish required. There are four main abrasives, and Sunnen’s guide to honing abrasives covers them in depth.
Aluminum oxide is the standard choice for steels and other tough materials that resist shearing. Silicon carbide works better on brittle and low-shear materials such as cast iron, aluminum, and bronze, and it is often used for fine finishing in almost any metal. CBN is the go-to for hardened steels and wear-resistant ferrous alloys, and it lasts much longer than conventional stones. Diamond is reserved for materials the others will not cut well, such as carbide, ceramics, and hard chrome.
Grit size and bond hardness matter as much as the abrasive. A stone that is too hard glazes and stops cutting. A stone that is too soft wears fast and makes it harder to hold size.
Can you hone hardened steel, plated bores, and exotic alloys?
Yes. Hardened steel is routine honing work. Hardness changes the stone we use and the cycle time, but it does not stop honing from working.
We also hone bores in Inconel, titanium, 17-4 PH and 15-5 PH stainless, A286, and aerospace aluminum. Each needs the right abrasive and oil. Titanium and some stainless grades can gall or tear if the stone and oil are wrong, so we set those jobs up carefully.
Hard chrome and other plated bores can be honed, usually with diamond. The plating has to be planned with honing in mind. The bore should be machined oversize, plated slightly under the finished size, and then honed to size. If the bore is plated at finished size and then honed, most of the chrome comes off or what is left is too thin. Tell us the plating thickness and whether the bore is honed before plating, after, or both.
Why does honing oil matter?
Honing oil washes chips out of the bore, keeps the stones cutting, and controls heat. A good oil keeps the stone surface clean so fresh abrasive keeps reaching the work, which keeps the finish consistent from part to part.
Oil also prevents the stone from welding to or tearing the bore surface. That matters most in gummy and galling-prone materials like aluminum, stainless, and titanium. The wrong oil, or a dirty one, shows up as torn surfaces, inconsistent finish, and stones that load up.
Blind bores need extra attention. Chips collect at the bottom, so we make sure oil reaches the closed end and flushes it out.
How do you measure a honed bore?
We check honed bores on an air gauge inspection station from Western Gage that measures to 10 millionths of an inch, and we verify size and roundness to 20 millionths.
Air gauging is a good fit for honed bores. It does not touch the surface, so it cannot scratch a fine finish. It reads quickly enough to check the bore during the job, not only at the end. We measure at several depths to find taper, barrel shape, and bellmouth, and at several angles at each depth to find out-of-round.
Lobing is easy to miss. A three-lobed bore can read the same diameter at every angle on a two-point gauge and still be out of round. When we suspect lobing, we use tooling that is set up to catch it.
Surface finish is checked separately with a profilometer, which reports Ra and the Rk parameters when the print calls for them.
What should I send to get a honing quote?
Send the drawing, the material and hardness, and the size the bore will be when it arrives. Tell us how the bore was made, whether it is through or blind, and whether it has cross holes, ports, or grooves. Tell us what runs in the bore and whether the bore has to be sized to a specific mating part. Include the quantity and any inspection or certification requirements.
If the bore is blind, check whether the design allows a relief groove at the bottom. A short undercut lets the stones run past the end of the finished surface and makes a straight blind bore much easier to hold.
Talk to us about your bore
JTR Specialties has done honing, lapping, grinding, and testing in South El Monte, California, since 1995. We hone, ID grind, and lap in-house, in soft and hardened materials, for aerospace, automotive, and medical customers, and every bore is inspected before it ships.
Send us your drawing and we will tell you what the bore needs. Call (626) 444-1124 or email jtrspecialties [at] gmail [dot] com.