Making T square on CNC router?

[Hoang N Nguyen]

I was kicking around the idea of making a larger T square out of aluminum and having it cut on our CNC router here at work. It would likely be about 1/4" thick, 12" T at each side of the body and the body itself be about 36" long. After that I can joint and rip down a piece of hardwood and attach it to the T along the edge as a reference point. Only reason why I'm even thinking about it is because I can get it done for free. Anyone have any thoughts about why this might be a bad idea? The router is accurate to within .002-.003" but that's dimensional, it should be able to hit a 90 degree turn dead on.

I'm wanting a larger T square like this for when I get my track saw and using it to square up cross cuts on sheet goods.

[glenn bradley]

I've often wished for the same thing and will be interested in the responses you get.

[Jamie Buxton]

For use with a track saw, I find a giant speed square is better than a T square. I made mine from wood. I snug the speed square up to the edge of the plywood, put the track next to it, and cut. The square stays in place while I'm cutting, ensuring that the track doesn't move. If you make your square a T square, you'd have to use it to write a line on the plywood, then move the square out of the way, then align the track to the line, and then cut. That more operations, and more opportunity for something to slip.
My speed square is about 3 feet by 3 feet.

[Hoang N Nguyen]

I'm more concern with accuracy then I'm with speed. I have a few projects coming up that involves a lot of ply for cabinets. I wanted to be able to lay a square down and draw a line that's perfect 90 degree and then lay my track saw on that line. I'm sure a good quality speed square will do the job just fine but at what cost? Remember I can get this done for free at work. I've tried a drywall T square once and it was a disaster.

[Jamie Buxton]

I'm more concern with accuracy then I'm with speed. I have a few projects coming up that involves a lot of ply for cabinets. I wanted to be able to lay a square down and draw a line that's perfect 90 degree and then lay my track saw on that line. I'm sure a good quality speed square will do the job just fine but at what cost? Remember I can get this done for free at work. I've tried a drywall T square once and it was a disaster.

If you use a T square to draw a line, then align the track to the line, you have the possibility of misalignment. If you leave the speed square in place while you cut, you eliminate that possibility. You also get speed. Also, the speed square automatically aligns with the edge of the plywood; with a T square, you have to manually do that alignment, again offering the possibility of misalignment.

You can make a speed square which is just as accurate as a T square.

[Hoang N Nguyen]

321883
Something like this would work right? The whole thing will be routed from a solid piece of ACM (Aluminum composite material) then 2 pieces of jointed hard wood will be attached to use as a reference guide.

[Mike Wilkins]

Not sure what track saw you are using, but the Festool system has a square that connects to the track/rail for just these type situations. I don't have a catalog in front of me right now, but it is available. I also don't have one just yet.
I have also wanted a much larger square for layout purposes; may as well make my own out of wood, like Chris Swartz has done.

[pat warner]

I question the precision of a tee square.
A lousy 1/10 of a degree error in laying the Tee will result in a 1/32" error/ft in parallelism and squareness. 36" is a long way. Precision squares of this length are prohibitively expensive.
I'd be thinking of another way to set the saw/router or whatever set up. Squares are indicators not measuring tools.

[Keith Outten]

I have been making custom Triangles on my CNC Router for some time. Mine have all been machined from Dupont Corian.
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[Greg R Bradley]

Some good points here, let me add a few more.

You can't use a T-Square and leave it in place unless you want to saw into/though your square.

I've used my big Woodpeckers T-Square (TS-xxx) for this application but it doesn't work very well for the reason explained by Pat Warner.

My big Woodpeckers square (PSxxx) works well as it references on a long surface on the plywood.

Your big CNC square would do that also but pay attention to Pat's info and double check it from the other side. Make sure you build it as a triangle or put a step into it like Woodpeckers does or it will be a pain. I'd build in extra pieces so you can mount it to the rail. Make it the same thickness as the channel and the add pieces on top with screw holes and captive nuts in the channel.

I have the Festool square. It is designed for matching angles and is far from the precision you need for doing this over long distances. I've setup one at each end as you can lock it to the rail.

It starts to make sense why people buy the parallel guides or Seneca's version that may be better - I haven't tried them yet.

[Art Mann]

I question the precision of a tee square.
A lousy 1/10 of a degree error in laying the Tee will result in a 1/32" error/ft in parallelism and squareness. 36" is a long way. Precision squares of this length are prohibitively expensive.
I'd be thinking of another way to set the saw/router or whatever set up. Squares are indicators not measuring tools.

If you are building cabinets, 0.063" is not that bad of an error over 36". It is doubtful you could achieve that level of accuracy just due to equipment variation alone. Just measuring a 0.18% error with any level of confidence with an ordinary cabinet ruler is not that easy and it is impossible with a tape measure. I have built quite a few cabinets and I can't think of a single situation where that level of error would have made any difference.

[Cary Falk]

I agree that a speed square would be better in this situation.

[Dan Hintz]

Expect the errors listed (and a lot more!) if you're making this out of aluminum and don't take the proper precautions. You'll want to cut it in multiple steps, removing the bulk material, then more, then a final pass (at a minimum) to relieve the stress in the aluminum blank as you go. If you don't release the stress as you go and try to do it in one pass, I think you'll be surprised at how much that sucker has bowed by the time it comes off of the table.

[ian maybury]

Hi H. A dead interesting project, but with subtleties that are perhaps not at first sight obvious. It could involve some trial and error. Not enough direct experience here to be certain, but the following could be considerations.

Triangles do seem to stay put through changes in temperature etc, and are probably the best hope for an accurate square. As Pat it requires scarily accurate angular settings if quite large errors over a distance like 36in are to avoided.

I wouldn't want to mix wood and aluminium, as there's likely to be relative movement - meaning at minimum that your wooden block would need to be attached in a way that allowed for this. Which isn't conducive to accuracy.

It might be an option to attach two strips of aluminium flat bar in the same grade of material, but getting them on dead straight and accurately positioned isn't easy either. Dowel pins pressed into blind holes/ through the triangle giving a light interference fit with a retaining adhesive might be advisable for location, but they can be hard to get to stay put in aluminium. Drilling a slightly below nominal hole to give a fit like this requires a very accurate drill press and a reamer or maybe a doctored end mill. Then countersunk screws to hold the assembly together. Then (see below) finish machine to ensure alignment.

Aluminium may mark wood, and scratches/stains easily - so anodising is worth thinking of. Aluminium is soft, so the square wouldn't be great for use with a knife or anything likely to remove material from the edge. Brian Lamb (Lamb Tool Works) on FOG offers a smaller version from plate (with no bars) for machine set up - it's normally only a dial gauge that runs against the edge.

1/4 in thick could possibly be a little flimsy at the size you have in mind - maybe check out a trial piece before deciding. It'd in one way (general stiffness) be better from a high strength heat treated grade such as 6061 T6. Which shouldn't be heated or it'll move quite a lot (residual stresses) as Dan says. It'll probably move anyway as a result of being machined. MIC 6 cast plate is much more stable but harder to machine, soft/easily bent and doesn't anodise well.

Not sure if it would work, but one way around movement in a high strength material might be as Dan to treat it a bit like wood. Mill the square slightly oversize, attach the flat bars, and let it lie for some time. Then very lightly finish cut/skim all of the reference edges to ensure it's square. (including the registration edge of the flat bars - the end of the tool could be let cut very slightly into the face of the triangle/plate) Definitely not heavily enough to put heat in and cause more movement. Then anodise.

Might a modern high accuracy CNC mill be more accurate than a CNC router?

Might another option be to build a square from flat aluminium bar with pin joints at the corners - maybe building a fine adjuster into one end of the long leg to permit setting to precisely 90 deg?

[Bob Falk]

Why not weld together an aluminum t-square from flat stock (one leg could be two pieces to provide the reference edge) and then square on the CNC or router table?

[Walter Plummer]

Where I used to work they cut us large (4' +) drafting squares from left over acrylic. They drilled 3/8" thru holes parallel to the edges while on the CNC. We could use them flat for drawing and lay out or drop in a couple of pull out steel dowels to use a "fence". http://www.mcmaster.com/#standard-dowel-pins/=z21kb8 . The dowels have a threaded hole in them so you can add a screw and washer to make sure they don't drop out at the wrong moment. I think I like the two point contact better than a straight fence. Hope this makes sense.