Introduction: Improving an Otherwise Low Precision Woodworking Drill Jig

Even though this improvement was made a year ago, I thought I'd still present it here. The drill guide shown,, was purchased to drill accurate holes. Was I ever surprised. This drill jig is inadequate for drilling such holes.There is so much clearance between the housing and the bushings that location was poor and perpendicular holes were difficult, if not impossible, to achieve.

After some thought, I set upon the task of improving the precision the jig. The results, in my opinion, were very good. No data was collected prior to the modification so the comparison with my mod is only a subjective one. However, I am sure that an objective comparison would bear out my belief.

I had gone so far with the improvement as to start a process to market my revision. The cost of said endeavor as well as my age made me unsure of being able to obtain a payback on the investment. I halted the process after spending the initial cost of preliminary design.

I even submitted my services to modify the jig of others via eBay, had no hits there. I realized that, again, the payback would not even cover shipping costs.

With this background out of the way, I'd like to proceed with the purpose of this Instructable: to guide those that have this jig in improving its precision.

I have presented here a picture of the jig in question. It is from an advertisement. Consequently, the picture is rather poor.

Step 1: This Is Where We Are Headed

The pictures show where we are headed by describing it pictorially.

But first, I should cover how this jig is utilized and the procedure used to locate the jig and to drill the hole. This will help in understanding the process and the inaccuracies we will be trying to correct.

This jig consists of a housing and drill bushings, There are 6 bushings: 3/16", 1/4", 5/16", 3/8", 7/16" and 1/2". The first step involves the 3/16 locating rod and the 3/16 bushing. First, the 3/16" locating rod is inserted into the bushing from the bottom side, blunt end first. A knurl on the rod precludes inserting the rod from the top side.The bushing with rod is then inserted into the housing. Push the rod so that just enough remains for a secure grip. Now, place the point of the rod at the location of the hole and then slide the drill jig and bushing down the rod, resting the housing on the surface to the surface. Next, the guide rod is removed and either this bushing, or one of the others, is inserted. One note is worth inserting here. The knurl I mentioned earlier precludes removing the rod without removing the bushing. If one is drilling a 3/16" hole, this only increases the chance of moving the housing, because the rod must be removed from the bushing with one that he bushing can be returned to the housing. Consequently, I found the knurl cumbersome, the cons greatly outweighing the pros..I carefully removed the knurl with a small file.

Now to the hole misalignment, we are attempting to reduce via this Instructable, The first possible misalignment occurs when locating the drill jig over the target. The rotation of the bushing in the housing allows for movement of the housing relative to the target. Then, when starting the drilling operation, the rotation of the bushing in the housing allows for further movement from the target. If a through hole is being created, then the hole location on the opposite side of the workpiece is misaligned further because of the non perpendicular hole. Two other sources for error. is the wobble of the locating rod in the bushing and the drill bit wobble in the bushing. I haven't mentioned further hole locating errors. The error created by these sources could be reduced by plating the bushing ID's. This is beyond the scope of this Instructable, mainly due to my not attempting this "fix".

Step 2: The Housing Mod

When I modified mine, I began with the housing mod and then modified
the bushings. I felt that this would be better. The housing would help support the bushing during the many fits to follow. Let us begin.

We are going to prepare the housing to accept a 21/32" diameter brass tubing, with a 5/8" ID, and 3/4" in length (which we have carefully prepared). The housing has a very small rib on the bore near the bottom of the hole that is a stop for the bushings The brass tubing will be the stop after the mod. I like to removed a little of it it to allow the telescoping tubing to be placed a little closer to the work surface, thus reducing further error.

Go slowly and check frequently, the fit the bushing in the housing. Just shoot for a "slight" press fit of the bushing. With this task completed the tubing can be bonded in place. I would suggest a slow curing epoxy, say 30 minute. It is excellent for large gaps. Even though there are gap filling (high viscosity) cyanoacrylates, I still prefer epoxies. They are available in several curing times. Also, they, CA's, may still cure to quickly, for you'll need some time to insert the tubing.and make sure it is positioned properly and vertical. A 15 minute epoxy should be fine. I used a 5 minute, the only one I had on hand. It worked fine. Now is the time to apply the epoxy, and insert the tubing. Mix up a small quantity of equal parts A and B (most commercially available epoxies are mixed one to one). Take care in using equal parts. That is very important. I have used visual checking in the past without detrimental effects.This method is obviously not error free. Adding to this, the resin slumps differently than the hardener. making volume comparison eve more difficult. I have just recently utilized a gram scale. Also, mix them together thoroughly, also very important. I use a toothpick for small amounts. I spin the toothpick while mixing. The toothpick is also excellent for applying it.

Coat the inside of the housing. Try not to apply any epoxy to the housing bore directly above the brass tubing.The large diameter of the best fitting bushing will reside almost entirely in the housing. and will help in the registration of the tubing. Applying a very thin coat of vaseline, or even a light oil, on the brass tubing ID and the bushing OD. We definitely do not want to bond the bushing to the tubing or the housing ID.

Insert the tubing so that the bottom end is set slightly above the "vee". The drill jig must be able to straddle a 90 degree (100 grad) corner. But it should be as low as possible to further reduce alignment error.

As to epoxy on the skin I am not concerned with epoxy on my fingers from a health standpoint. That is me. I do not recommend this. A plus is that I haven't used, what I consider a detrimental amount of epoxy, over my lifetime so as to be concerned.

Here is a page that discusses epoxy and its cleanup:

Even with a 5 minute epoxy I recommend letting this bond to cure overnight.

After the epoxy has cured, remove the bushing and feel free to cut the plastic housing at the end of the tubing. With this modification that registration is will not be required.

Step 3: The Drill Bushing Mod

The bushing tn my housing measured 0.630. I reduced this by 0.001, 0.6290. That will be my target. Mine now range from 0.6265 to 0.6285. There is a little wobble in the housing on a couple. A diameter of 0.028 would have been optimum. I wish I had had a lathe. This variation could have been eliminated. One help here is that I used the axiom: "measure twice and cut once". I expanded on this axiom. I measured many times and many places as I filed the diameters to size. More importantly I checked them in the bushed housing many times. That is the true test. If you have a lathe then roundness and zero taper will be achieved.

My "lathe" was a drill press and a file. I tied to maintain an light and even pressure. I placed my finger, applying the machining load, as close as possible to the center of the file to hopefully reduce tapering the surface. Below, I show the bushings with the mandrels installed. I have described the mandrel components for each. The 3/8" bushing, as you notice, does not have a mandrel. You will understand shortly when comparing my description of the 3/8" drill bushing and the 1/2" drill bushing. A 3/8 SHCS was used for both. The 1/2" utilizes a 3/8 ID X 1/2 OD sleeve by 1-1/4" long.

A 1" micrometer (or a dial or digital caliper that can measure to 0.0005", is mandatory. Remember we have a fairly tight tolerance to achieve. Proceed carefully, we will not be removing much material. I also want to suggest that you carefully hold the file in one hand and apply slight pressure with the index finger of the other right in the middle of the files width. Please be careful using safety equipment, goggles and maybe a pair of heavy gloves. Yes, you will loose some sensitivity with the gloves, so be cautious..

Remember the axiom: Check, check and check again, with the measuring tool of choice. Do not forget to check the fit in the bushed housing, that is the ultimate check.

The mandrels are as follows:

3/16" bushing: 1 each 10-24 X 2-1/4 screw 2 each 10-24 nuts Install the 3/16" bushing onto the screw and tighten with one nut, use the 2nd nut as a jamb nut. This mandrel needs to be secure and the jamb nut properly applied. The forces on the bushing will tend to loosen using only 1 nut. Not sure if a nylock nut will be adequate.

1/4" bushing: 1 each 1/4-20 X 2-1/4" bolt 1 each 1/4-20 nut (sufficient bolt loading can be achieved to prevent the bushing from spinning) 3/16" washer Note: This was needed because the thread length of the bolt was just a little to short. Also, I prefer using the next smaller washer if at all possible. I disagree with the industry standard for washers. Helps reduce washer deformation, especially with wood applications. Note: I will also use a washer two sizes larger with the smaller washer in wood applications, soldering the two together. This helps in distributing the load to the wood over a larger area thus reducing wood stress under the washer. If one observes properly bolted wood construction this is taken further (much larger, and thicker washer).

5/16 bushing: 5/16-18 X 2-1/2" bolt 5/18-18 nut 1/4" washer (quantity as required) Note: the thread length of the bolt I used dictated 3 washers. Washer size as previously defined.

I could have used 4 washers, but I liked the idea of the protrusion of the un-threaded portion acting as a locking feature for the nut.

3/8" bushing: 3/8-16 X 2-1/2" SHCS (socket head cap screw) (this was style of bolt was used because the head diameter was just under 5/8" diameter. The head of a hex bolt would protrude beyond 5/8" diameter) 3/8-16 nut washers (if required).

7/16" bushing: 7/16-14 X 2-1/2" hex bolt (I milled the head down to just under 5/8") (you may not think it necessary, but I liked to make things professional looking) 7/16-14 nut washers (if required)

1/2" bushing: 3/8-16 X 2-1/2" SHCS (the one used for the 3/8" bushing), a 3/8" ID X 1/2" OD by 1-1/4" long bushing. 3/8-16 nut (the one used for the 3/8" bushing) washers (if required).

Step 4: We're Done!

Well, that's it.

As you can see I made a nice box, from 2 inch lumber. If look real good you will see that I included a rubber keeper for the locating rod. I even added rubber keepers for the bushings and the housing.

My cross slide vise in a bench drill press allowed me to precisely mill (rout) the various cavities. I measured every item so as to produce an accurate CAD model for documenting the geometry.

I hope to utilize what I have learned here at Instructables to applying labeling for the bushings. Just a nice touch. CAD will again be used to eliminate my inability to precisely locate the lettering.