Choosing Your Drill Guide: Egg Cup, Mini and Dog Bone Guides for SF and Threaded Bushings

A drill guide holds the drill bit square to the aircraft skin so every hole comes out round and where you marked it, with a slip fit or threaded bushing doing the guiding. Drilling a hole at the wrong angle causes problems. Two issues come up often, and both are easy to run into even with careful work:

• An off-axis hole is not perpendicular, so it comes out elongated instead of round. This weakens the fastener's grip and can force the part to be scrapped, an expensive mistake in aerostructure work.
• Even with steady hands, curved or slippery surfaces are hard to control. The drill bit can wander before it bites into the material, starting the hole in the wrong spot.

A drill guide solves both problems. It holds the bit at a true 90-degree angle to the surface and keeps it from wandering, so every hole comes out straight, round, and in the right place. It serves a simple, yet critical function in the workshop.

How to Choose Your Drill Guide

A few factors matter when choosing a drill guide. Some come down to preference; others affect how well the tool performs for your job.

Step 1: Choose your bushing type

Decide which bushing type you will use, or check what your workplace already standardizes on. The two main options are Slip/Fixed (SF) renewable bushings, sold in the aircraft trade as slip fit bushings, and threaded bushings. This choice is mostly a matter of preference, since both types are industry standards.

Step 2: Choose your housing type

Your housing choice depends on where you are working. For easy-to-access parts, the egg cup drill guide, also called a drill stand or drill cup, is a good fit. Its shape improves ergonomics and gives a stable base. For tight spaces, the mini drill guide is the better choice.

Wescon manufactures both SF bushing and threaded bushing housings, so either option is covered.

Step 3: Choose your footpiece material

Your work surface determines which footpiece to use. Both egg cup and mini drill guides come with two standard options:

Heavy duty plastic

• Characteristics: surface hardness lower than most aerospace alloys, non-abrasive
• Advantage: gives a no-mar contact patch that prevents circular scoring or "halos" on finished surfaces
• Application examples: certain aluminum alloys, painted surfaces

Anodized aluminum

• Characteristics: high surface hardness, strong structural rigidity, excellent thermal conductivity
• Advantage: gives maximum wear resistance, and holds up in high-friction, high-heat applications without the degradation or deformation seen in plastic
• Application examples: titanium alloy work surfaces, carbon fiber

Special Use Cases and Accessories

Special application 1: Improving mini drill guide handling

The WES5620 mini drill guide series is built for tight-space applications, which means its compact form factor can make it hard to hold comfortably in some jobs.

To fix this, we offer the WES5630 series handle that attaches to compatible drill guides for better grip and control: WES5631 for the 3/8"-16 guide and WES5632 for the 1/2"-13 guide. Here is an example of the complete assembly, drill guide with the handle and bushing installed:

Special application 2: Drill hole resizing

Some precision drilling jobs need a two-stage process. First, you drill a precision hole using a bushing with a smaller inner diameter. Then you ream the hole to its final size. This requires two SF bushings for the job.

The dog bone guide is built for this. It holds two SF bushings in a single drill block, so you do not need two separate guides to complete the job.

Special application 3: Curved or contoured aerostructure work surfaces

Standard drill guides work well on flat or lightly curved surfaces. But on aerostructures with extreme curvature, a standard unit can struggle to sit stable without shifting.

For this, use the WES5703-2-T drill guide. Its tripod base gives added stability on heavily curved surfaces.

Drill Guide Selection Tables by Bushing Size

With the information in this guide, you now have what you need to make the right choice. To make this easier, check which bushing type you use, then go to the matching table below. Go to Table 1 if you use an SF bushing. Go to Table 2 if you use a threaded bushing.

Table 1: SF bushing guides

View the ANSI code of your SF bushing (in the format SF-XX-XX) and find the matching guide type below.

These links take you directly to the series configurator, where you can review the tool and choose your footpiece.

Table 2: Threaded bushing guides

Check the thread size on your drill guide and find the matching style below.

These links take you directly to the series configurator, where you can review the tool and choose your footpiece.

Choosing the right drill guide comes down to three things: the bushing type, the housing, and the footpiece. Get these right and you get straight, accurate holes every time, no matter the surface or the job. Every Wescon drill guide, bushing and kit in this article is made in USA.

Ready to configure your next drill guide? Browse our drill guides catalog | Contact our engineering team

Read next: drill bars, the snatch block for hole repair

A drill guide starts a new hole square. When the hole already exists and is crooked or mismatched, the tool is a drill bar, the one mechanics call a snatch block or steam boat. It clamps to the part with clecos through a slot and holds the same SF or threaded bushing over the old hole so you can ream it round and on center. Wescon makes them for 1/2 and 3/4 in SF bushings and for 3/8-16 and 1/2-13 threaded bushings, single ended, double ended and with swivel heads. Read the drill bar guide.