This is usually called a keyhole bit mainly used for hanging pictures but have been adapted for other uses never seen them any bigger

 

The few 'keyhole bits' that I looked at didn't cut a slot that would properly constrain a 7/16" bolt head. I was just reading an old thread here on the subject and they suggested a 1/2" dovetail bit, which also sounds promising.

What I did was measure the dimensions of the bolt head of a cheap 1/4 20 bolt, and rotated the bolt in the caliper to find the point where the bolt was able to spin in the slot (1/2"). I used these constraints to search several sites (Amazon, Rockler, Woodcraft) for a slot cutter that would meet these constraints and this is was what came up. It's pretty cheap too!

 

Jeff............this is a very timely post for me. I'm planning some jigs, and a few other things that will need t slots. But at the present, the cost of t-track and t-bolts is pretty cost prohibitive. For example, the drill press table I want to build will require about 38 inches of track, and a few t-bolts. Since none of these will really really require the strength of a steel t-track, your idea may be a real money saver for me. Thanks. Jim

 

I used Incra track on my DP table Jim. For drilling I find that I need more clamping force than I'm comfortable with using a milled track. The 1/4 20 bolts work great in the Incra extrusions, and I'm sure that they would fit other brands. Just have to do a test fit before committing to any brand of extrusion. I'm sure not buying any more 'toilet bolts' for a buck a piece anymore.

Good luck on your drill table.

~jeff

 

Nice find. Might have to add that to my bit collection. A lot of T-Tracks out there work with 1/4 bolts. It's not the cost that bothers me, but rather the time it takes to use one casually - ie for one-off jigs. It makes it less likely I'd make a jig with it since I do a fair number of one-offs in the course of a project. This makes it easy to whip out a task specific jig. I probably would not use a routed T-Slot like that in places where a traditional metal T-Track is used because of wear and tear but can see uses for it. It might be ok in maple or other hard wood but not MDF or plywood.

By the way, I don't think it qualifies as a key-hole bit because it doesn't seem to be "plungeable".

 

One tip for making that cut easier is to make a couple of cuts with a saw blade first. It gives the chips an escape route and let's air in to cool the bit. The bits will last up to 3x longer if you do it that way.

 

Nice find bob - I've been using a 3/8" keyhole bit and modifying the heads of 1/4-20 carriage bolts on the bench grinder. Peachtree and Hartville tool carry a "mini" t-track that works fine with the 7/16" bolt head.
Phil is right about that bit not plunging, at least not unless the groove is routed first. There is no cutter completely across the bottom. It will work fine for through slots though. I usually route the groove first anyway to reduce the stress on the bit. The weak spot on the t-slot bit, and the dovetails as well, is between the head and the shank. Could be one reason you are having trouble finding one in 1/2" shank, the larger shank wouldn't bring much to the party as the neck is smaller than the shank anyway.
I use a t-slot for sacrificial fences. The fence on my router table push block has an MDF face so when it gets boogered up, I just slide it off and knock the end off on the miter saw, reinstall it and it is good to go.

 

... The fence on my router table push block ...

You have a fence on your push block? I'd love to see a pic!

 

This got me thinking about making a jig for half lap joints. I'm thinking of making theater flats with half lap joints. Flats have what are really short rails and long stiles. Seems to me that I want to cut two joints at the same time when talking about lap joining ends, but for a 10 foot canvas covered flat, I think it should have two cross braces (rails), which would be end joints on the cross braces (rabbit cuts), that fit into two "T" dados on the long sides of the flat. Because flats have to be perfectly square to fit tight together, the jig would have to allow for routing both end and center pieces and it would have to allow cutting two at a time for perfect alignment.. The width of material is 3.5 x 3/4 inches, so the jig would have to be a minimum of 9-10 inches or so to allow for a sacrificial piece on each side.

Just thinking this through, I also think the jig would have to have a cross piece perfectly 90 degrees to the long jig pieces that you'd use to line up the workpiece. Since wood varies in thickness, the side guides would need to be slightly proud of 3/4 inch thickness, which means the fixed cross piece would need to either be adjustable in height or have some sort of guide so it could be tightened to hold down the workpieces. I guess you could have that adjustable piece butt up against a permanently glued and screwed index piece. Maybe some gripping material on the bottom of the movable hold down cross piece to press the workpieces down flat in the jig.

Assembly would be on 3 folding tables about 60 inches long, so to avoid sagging, i'd have to have a couple of support pieces the same thickness of the jig's base that would sit on the other tables. The jig itself would slide up and down the the 10 foot length of the rails. The movable hold downs top and bottom act as router guides.

I know the movable cross piece would be a guide for the router, so the actual cut would be made starting the distance from the edge of the base to the edge of the cutter. This is a job for a plunge base. Once the adjustable hold down is positioned, a light cut would indicate the offset for the base to cutter distance for the first edge of the joint. The second edge has to be measured from the other side and account for the distance from the top of the base to the joint.

If someone has any suggestions or would like to make a drawing, I'd appreciate it. Attached is a rough pencil sketch about 1 inch per square scale.

 

When I get a break, I will whup it out for ya. :smile:

 

Something like this?

Lemme know of any additions/corrections on dimensions. I can add the knobs last.

 

Wow, that was fast! Thanks MT.

I don't want to make it quite that wide because it will be used on no wider than 4 inch wide stock and I have to move the jig, not the stock. To do 4 pieces at once I would have to allow 16-18 inches in width (including sacrificial stock) and have some filler pieces for the router to ride on.

I found a video on flat making on Youtube https://www.youtube.com/results?search_query=Making+theater+flats , and the guy was using a fine hand saw and near freehand router to do the width lap cuts, then the router to cut all the rest. That would make me nervous and I'm not sure I could get the repeatability of a jig.

But as I look at your drawing, I can see that really, I don't need to have a movable bottom rail at all, just the top rail needs to move, and really, only half an inch of adjustment at most since nearly all stock will be withing 1/8th of 3.5 inches wide. I will use pre-cut spacers to set the upper, adjustable hold down/guide.

This drawing really clarifies the project.

Because I'm going to be cutting on the ends of long stiles, I want to restrain the tendency I anticipate for the ends to lift off the base. I think a longer base on the bottom edge will help with that, and of course, I'll make some supports to go under the stock further down so it comes into the jig straight on. Because the stiles are 10 feet long, they will be supported by 3, 60 inch wide folding tables. The cross pieces, the rails, will only be (exactly) 4 feet wide.

I know where the knobs go, so no need to do that unless you want to.

I suspect this project will go on for awhile. The stage involved is small, but it still takes about 10 flats to span it, and there needs to be at least two extras with window openings and at least 1 with a door. All will be covered with heavy muslin. I hope to make two set of flats so the directors can do two set plays. This is going to cost some money to build and it will all be contributed to our local theater arts guild. Maybe I can write it off somehow.

 

Thanks Tom. Glad I could help out.

 

Thanks MT Stringer



MT: It really helps to write the thing out. Makes you picture it and think about what you really need.

BTW, you have a serious camera in you mug shot, and Stringer in journalism means a free lancer who works pretty regularly for one outlet. Is that what you're doing? I was a journalist for a decade and a pretty good photojournalist and feature guy. A great life except for the endless deadline. That was back in the film and darkroom era, in fact I'm about to give away my old lab gear and the last 35mm camera.

 

I have this other idea that I'm hoping to find an application for, so I thought I'd share.

I bought a 6' extruded aluminum rod in 3/8" on Amazon Prime for ~$14 thinking that it would be handy for making . . . something. Can you say "addict"?

So I'm thinking about taking a couple of flat boards, routing 3/8" grooves in both boards with a round nose bit, and sandwiching the aluminum rod between the two boards, something like this:

I can then slide . . . whatever . . . keeping it parallel with the reverence plane.

So help me think of a good application.

 

How about a ski jig for a trim router? This is one Harry built.

 

A really long edge guide.

 

I finally got a chance to try this bit today, along with the 2" hole in the table for DC while routing slots. It was time to update my drill press fence and I had a perfect sized scrap of 3/4" MDF. Due to the geometry I tried making the cut in one pass which took a pretty slow feed, even with my 3 1/4 HP Triton. I know, I know, next time I'll cut a dado in stages then run the bit through for the 'wings'. After the 1st pass the channel cut was too snug for a 1/4 20 bolt, but another swipe with the fence advanced 1/32" did the trick.

The hole in the table worked like magic. I could watch the bit throw out a river of dust and it just disappeared when it went over the hole. I highly recommend this mod. My original thought was to drill a 2 1/2 inch hole all the way through the table and plug my ShopVac hose in the bottom. My biggest drill bit is a 2 1/8" Forstner so plan B was to buy a plastic dust port and mount it under the 2 1/8" hole. All in all about 20 minutes to modify my table, and there was damn little dust visible after the cut (25" in slot).