Spiral Router Bits vs. Straight Router Bits

WHY SPIRAL BITS ARE BETTER FOR PLUNGE CUTS.

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Because a spiral bit is designed much like a drill bit, it makes plunge cuts easily. The cutters of a straight bit do not overlap, so if you plunge straight down deeper than 3/32 in., you might burn away the wood in the middle, but you won’t cut it.

The increasingly popular spiral router bits borrow technology from the metalworking industry. Spiral bits look like drill bits and are most often made of solid carbide, so they are super sharp and leave a superior cut on wood. Two flutes ground around the body of a spiral bit smooth vibration by spreading the cutting action over a longer edge. With their drill-like point, spiral bits are also better for plunge-cutting. All of these advantages also mean less wear and tear on the router, but don’t throw out all of your old straight bits just yet. 

The new solid-carbide spiral bits come with some disadvantages. The first is that the cutters are expensive. A typical solid-carbide spiral bit is likely to cost at least $50. A similarly sized straight bit with carbide-tipped cutters will run somewhere in the range of $7 to $23.With a cost differential that large, you will want to know what you are going to do with this bit and that you will use it often enough to get your money’s worth. To highlight other differences, let’s compare the qualities of spiral bits and straight bits. 

Both Spiral and Straight Bits Have “Plunge-Ability”

You can plunge with both types of bits, so they’ll both work for, say cutting mortises. But because most spiral bits are ground on the tip end of the flute, somewhat like a drill, you can plunge straight down as far as you like, without stopping.You can’t really plunge any deeper than about 1/8 in. with a typical straight bit. Inspect the end, and you’ll see why (see the photo at left). On most straight bits there is a space above the web, between the cutters, where no cutting takes place during a straight plunge because there is no cutter overlap. Chuck a straight bit into your drill press and plunge it into a piece of wood. After about 3/32 in., the middle of the bit bottoms out. To go any farther, the bit has to abrade the wood away in this middle area.

This doesn’t mean you can’t cut mortises or plunge with a straight bit. You just have to sweep the router while you are plunging. You should probably cut mortises in passes not much deeper than 1/8 in. anyway, but with a straight bit, such shallow passes are just about a must. 

Spiral Bits      

ADVANTAGES

•        Very clean cut 

•        More cutter in wood means less vibration

•        Better plunge-cutting

•        Less wear and tear on router

•        Direct chips up or down 

DISADVANTAGES      

•        Limited lengths and diameters

•        Can be somewhat risky to use

Straight Bits

ADVANTAGES

•        Wide variety of lengths and diameters

•        Guide bearings on tip or shank mean better template routing

•        Less expensive 

•        Greater ability to resharpen 

DISADVANTAGE

•        Plunge-cutting requires more effort and skill 

THERE’S NO CLEAR WINNER.    Solid-carbide spiral bits are becoming increasingly popular because they make a very clean cut. But they have many limitations compared to the straight bits that have been popular for years. 

This article is excerpted from The New Best of Fine Woodworking Working With Routers. 

All spiral bits make clean cuts. This veneered plywood shows the effects of the three types of spiral bits on the top and bottom edges.

Spiral Bits Leave a Clean Edge

The three basic cutter configurations for spiral bits are up-cut, down-cut, and a combination of the two, known as a compression bit. ... A down-cut bit sends the chips downward; an up-cut bit sends them up to ward the shank. (On a router table, all direc-tions are reversed.)

Besides directing the chips, the advantages of these configurations are best illustrated by the quality of cut, especially on veneered plywood (see the photos above).A down-cut bit will leave a clean edge on top but a ragged edge on the bottom; an up-cut bit will accomplish the opposite. This is great until you want to cut a dado with no tearout on the face. A down-cut bit will leave a clean top edge, but it sends the chips downward, into the dado where they have no place to go.You can make this cut, but you have to take it slower than usual to give the chips a chance to clear.

For woodworkers who work with A-grade veneers on both sides of the stock and must have a clean edge top, bottom, and middle, the compression bit is a good choice. It has an up-cut configuration on the tip of the bit and a down-cut spiral ground on the shank. By lining up the bit just right, you can get a superior edge across the entire thickness of the wood.This virtuosity comes at a hefty price:A typical compression bit will cost about $90. 

Straight Bits Come in Many Sizes and Bearing Configurations

Router-bit manufacturers have difficulty making solid-carbide spiral bits with cutting diameters larger than their shanks. So for small-shop hand routers you won’t find many bits with a cutting diameter larger than 1/2 in., the size of the largest bit shank. Spiral bits also come pretty much in a few standard fractional sizes up to 1/2 in. Straight bits, on the other hand, go through dozens of fractional sizes, all the way up to 2-in. dia. cutters. Depending on the job you have in mind for your router bit, straight bits also come in a variety of cutter lengths. So you can buy close to exactly the length of cutter you need.

Straight bits also have a huge advantage over spiral bits when it comes to template routing, because you can buy them with guide bearings. And those bearings can be mounted on the tip of the cutter or on the shank of the cutter, depending on your needs and your template. The bearings are made for a variety of cutter diameters and lengths. It is really too bad that solid-carbide spiral bits can’t accommodate bearings a little more readily. With their superior edge cut, spirals make great template cutters when used with collar guides. But when it comes to bearing-guided bits, spirals seem to be available only with bearings mounted on the end of the bit. There are some problems with this: The cost is high (about $80); it precludes cutting only partway through the work, which means full-thickness cuts only; and the template has to be under the work, an inconvenience. Shank-shod, bearing-guided, solid-carbide bits (spiral bits with the bearings on the shaft end of the bit), which would permit template routing with the template on top of the work and trim cutting through only part of the work face, are not available. For this type of routing, you’ll have to stick with straight bits.

This article is excerpted from The New Best of Fine Woodworking Working With Routers. 

Sharpening Spiral Bits Is Difficult, if Not Impossible

Some woodworkers like to sharpen their straight bits, although I find it difficult to get it right and always send out my bits for sharpening. Carbide-tipped straight bits usually have enough carbide thickness to be reground four or five times, and the tech- nology to do so is common.

A few services claim to be able to sharpen spiral bits. But I haven’t found anyone who can sharpen spiral carbide to factory standards. To me, this translates into a substantial loss. Here’s why: The spiral bit costs twice as much—or more—as a straight bit, and the straight bit can be reground up to five times. A sharpening service charges about $4 to regrind a straight bit, and the cutter of ten comes back sharper than it was from the factory. So even if I pay $23 for a straight bit and sharpen it five times, I still pay only $43. Spiral bits might stay sharp longer than straight bits, but even so, the cost of using spiral bits will always be higher. 

Spiral Bits Can Be Risky to Use

Spiral bits work incredibly well in the production environment and especially in CNC (computer numerically controlled) router industrial applications. But in a hand router, their use sometimes imposes unusual risks not associated with the equivalent or bigger straight bits.The down-cut spiral bit's screw-driven forces are sufficient enough to pick the router up and twist it out of your hands—with no warning. I know, because it has happened to me. On end grain the spiral bit is getting even more traction, so the risk is even greater—a pity, too, because a sweet end-grain finish is attractive.

The up-cut spiral bit can have the opposite effect. It wants to pick up the work. So you must secure the work in some kind of fixture or hold it by a clamp. (I never rout anything that is not secured or clamped, but some people do.) The up-cut bit’s tendency to pick up the work also happens quickly and without warning.

My teaching and woodworking are centered on routing, so I have a cabinet filled with more than the weekend woodworker’s supply of router bits. I do keep a few solid-carbide spiral bits because, when I want a beautiful face cut or I am cutting narrow mortises, and I have the money, there is just nothing better. But my cabinet is mostly full of a wide variety of straight bits. For general-purpose work, for template and pattern routing, and for those times when need a large-diameter bit, I still reach for one of my straight bits.

This article is excerpted from The New Best of Fine Woodworking Working With Routers. ©2005 by The Taunton Press

PAT WARNER is a woodworker and college instructor who lives in Escondido, California.

Gun Drilling & BTA Drilling:  These are the right tools for the absolute deepest of Deep Hole Drilling.  “Deep Hole” is defined by drill diameters, and specifically the depth to diameter ratio.  When used properly, gun drilling can produce straight holes that are much deeper than conventional style drills are capable of.

Here’s some quick information on various deep hole drilling solutions from our Deep Hole Drilling Guide:

The depth to diameter ratio chooses the right process to produce straight deep holes…

While we can use special g-code such as the Conversational Programming of G-Wizard Editor to extend the reach on deep holes beyond Parabolic Flute Drills and Peck Drilling, to go really deep takes even more specialized tooling.  Premium solid carbide twist drills are all done by 20 diameters, to maybe a bit more with hand tuned custom cycles.

Typically, Gun and BTA Drilling are done on special deep hole drilling machines that are designed for the purpose.  Such machines are capable of contra-rotating both the cutting tool (Gun or BTA Drill) and the workpiece, which gives the best results on large depth to diameter ratio holes.

Gun drilling can also be done on a lathe, but the disadvantage is that the lathe can only rotate the workpiece and not the drill at the same time.

Straight deep holes are not easy!

What is Gun Drilling?

Gundrilling developed to help make straighter more accurate gun barrels.  As you can imagine, deep straight holes are critical to accurate gun barrels.

It uses a fairly simple geometry solid carbide tip on a long, thin cutting tool that pumps fluid through the core of the tool to flush chips from the tip back out through generous straight flutes:

Gun Drills by Sterling

The tips are similar in geometry to solid carbide twist drills except there is no twist–the flutes are straight.  Coolant under pressure forces the chips back out along the length of the grooved shaft.  These tips are either solid carbide or brazed carbide–it takes a tough cutting tool to deal with these depth to diameter ratios.

Gundrilling equipment for deep hole drilling consists of

• A thin cutting tool with solid carbide cutting edge(s), the gun drill itself.
• Toolholding components to pump the coolant and evacuate chips.
• A machine–either a lathe or a dedicated gundrilling machine.

Why does gundrilling work better for straight deep holes?  It’s all about the chips.  In a deep hole, it gets harder and harder to remove the chips as depth to diameter ratio increases.  The deeper the hole the more difficult chip removal becomes.  Pumped coolant at a pressure that guarantees chips are blown back out of the hole together with an avenue the chips can move through are key.  

Add a solid carbide tip and you’ve got a gundrill for virtually any material.

When to Consider Gun Drilling

Among deep hole drilling solutions, choose gun drilling when:

• Hole depths are greater than 20x.
• Deep Holes would require reaming or honing to meet tolerances.  Gun drilling requires no reaming or honing because the surface finish of the drilled hole is already very good.
• With drilled diameters of less than 20mm.  Larger diameter deep holes can be done more efficiently with BTA Drililng.

Deep hole drilling can be done on virtually any material when these conditions are met.

Sometimes even relatively shallow holes (by gun drilling standards) benefit from gun drilling when straightness, diameter, and surface finish requirements are high.  The hole size, finish, and straightness of a gun drilled hole are all higher than with twist drilling and other deep hole drilling solutions, so eliminating a second reaming or honing op via gun drilling may save time.

Contact us to discuss your requirements of Single straight hole carbide rod. Our experienced sales team can help you identify the options that best suit your needs.

Suggested reading:
A Guide To Pigments, Micas, Coloured Clays and ...

Solid carbide twist drills are not always the answer.

Gun Drilling on a Lathe

While gun drilling can be done on a lathe, it is better done on a special-purpose deep hole drilling machine.  The advantages of the deep hole drilling machine include the ability to rotate both workpiece and gun drill (in opposite directions), whip guides to support the long drill and workpice, plus high pressure coolant for better chip evacuation.

Contra-rotating tool and workpiece contributes to straighter drilled holes.  Dedicated deep hole drilling machines also have more sophisticating monitoring of loads and coolant pressures that correctly identify when the tool is dulling and needs to be changed.

Anthony Fettig, CEO of UNISIG, says the dividing line is at about a 40:1 depth to diameter ratio.  That’s the point where purpose-built deep hole drilling machines start to have a significant performance advantage over using gun drills on conventional lathes and milling centers.

Even so, gun drilling on an ordinary lathe can still be effective.  Here’s a gun drill in operation on a manual engine lathe with just a mist coolant unit:

Image via CTE Magazine.

Such a simple rig will take holes to much greater depths than ordinary twist drills can reach.

Given that whipping can create a real safety hazard as well as completely destroying the gun drill, some important procedures must be followed when gun drilling on lathes without whipping guards:

1. Create a pilot hole at least 1 to 2 diameters deep and 0.0005 – 0.001″ larger than the gun drill.
2. Position the gun drill in the hole before starting rotation.
3. Always stop rotation before removing the gun drill from the hole.
4. Consider a steady rest on the lathe to help stabilize gun drills.  If you don’t have a steady rest, you may have to use a series of progressively longer gun drills.  Try to keep less than 40 diameters length of gun drill unsupported.
5. When using mist instead of high pressure coolant, reduce feedrates by 60%.  The smaller the hole, the more important higher pressure coolant becomes.

Gun Drills are happier with cutting oil than typical water soluble coolant used with CNC.  Keep the coolant temps below 80 degrees F so you don’t lose viscosity on the oil.  All that is telling you is that where coolant is concerned, lubrication and keeping chips evacuated  is more important than cooling.

Check with application support for specific coolant recommendations based on the  depths of your hole and the tool in use.

Coolant guidelines for gun drilling:

Diameter Ideal PSI Min PSI GPM @ Ideal PSI         0.125 1500 500 1         0.187 1150 400 1.6         0.250 925 350 2.5         0.375 675 300 4.5         0.500 525 250 7         0.625 450 200 10         0.750 400 175 14         1.000 300 150 20         1.250 250 125 28         1.500 200 100 36

Another area to fine tune for your particular application is the nose grind or geometry.  If not specified, most gun drills have a N-8 nose grind and R1 relief.  This is fine for steel or inconel.  But, in the case of aluminum or brass, you’re better off with an N-4 nose grind and R4 relief.  Consult your gun drill manufacturer for best results as they’re the experts.

Gun Drilling Feeds and Speeds

Our G-Wizard Feeds and Speeds Calculator will figure Gun Drill feeds and speeds for various depths:

In fact, G-Wizard can produce feeds and speeds for nearly all deep hole drilling solutions needed to manufacture your parts.

You can download and be using G-Wizard today!

Gundrill Types

Choosing the right tooling will maximize your performance.

Single Flute Gundrill

A single flute gundrill is the most common design.  A single flute with straight, not-helical flutes will produce shorter chips which are easier to evacuate than long ones.

Double Crimp Gundrills

The term Double Crimp essentially means cutting with 2 flutes.  Double crimp gundrills have 2 chip evacuation flutes, but they’re smaller than  a single flute gundrill, so they must be used on a metal that produces smaller chips like cast iron and aluminum.  Because double crimp gun drills have 2 flutes, they can operate a higher feedrates than single flute gundrills.

If you’re in doubt about whether a double crimp will work in your application, share your specific requirements with applicaton support at your vendor.

Related Tooling

High Precision Rifle Buttons

High precision rifle buttons (or rifling buttons) are precision carbide tools for cutting the rifling grooves on a gun barrel.  A rifle button can be pushed or pulled through the barrel, but this is done without rotation (so more like a broaching operation).

Push and Pull Reamers

A push or pull reamer may be used to remove 0.003″ to 0.004″ of material before using a rifle button.  This ensures accurate ID and good surface finish of the barrel.

Gundrill Wear

It is important to remember that the most critical interface where the carbide tip is subject to the most traumas is the entry and exit of the part whenever the boring cycle is interrupted.  Tool life should improve if you minimize interruption of the boring cycle to avoid that critical interface.

What is BTA Drilling?

There are several differences that separate BTA Drilling from Gun Drilling:

• BTA Drilling involves a drill bit mounted on a long tube as tool.
• Chips are evacuated through the center of the tube and coolant flows around the outside of the tube, which is a smaller diameter than the hole.
• BTA cutters are either brazed or inserted carbide, not solid carbide.

BTA stands for Boring and Trepanning Association.  BTA Drilling can achieve feedrates that are 5-7 times that of Gun Drilling, hence it can deliver a lot faster boring cycle.  The downside is the tooling is bulkier, so it can only be used on larger diameter holes.

Here’s a simple animated diagram showing how BTA Drilling works:

In-House Deep Hole Drilling Solutions?

Many shops outsource deep hole drilling solutions for metal, particularly when there is a need for BTA Drilling or the kind of very deep holes where dedicated deep hole drilling machinery really shines.  However, if your manufacturing needs can be handled with a gun drill on a lathe competitively, it’s not that hard to bring the process in-house.

Doing so can enhance your shop’s manufacturing profits or at least reduce manufacture turnaround times. 

Before purchasing any gundrill or BTA tooling, phone your vendor’s application support to be sure you are getting the best cutting tool for your manufacturing application.  Give their experts your specific requirements.  Straight deep holes need all the expertise you can get to ensure a quality end result when boring stock.  When it’s as easy to come by as a phone call to application support to discuss your specific requirements, why miss out?

Conclusion

We given you some quick information about how to succeed with these deep hole drilling solutions and an idea of what tooling is available.  You should understand the basic information needed after going through this content. 

A little work with your vendors on specific requirements and you’ll no longer be challenged by hole depth.  Having experience in this area will be helpful to most any shop making the types of parts or products that require deep holes.

If you want to learn more, please visit our website Carbide Rod With Coolant Hole.