With my introduction of the DTR-3, I have received many questions recently about what Dovetail Rule (DTR) is best for squares you may already own. I have written this blog post to help people check their existing square to find the DTR that is the best fit for their existing square. You can always purchase a Dovetail Square (DTSQ-2 ) which is a complete Square with a DTR installed. You Can purchase a complete Dovetail Square here. Many woodworkers go this route so it’s always setup and they don’t have to go through changing their graduated rulers to the DTR in their square.
However, we also understand that many already have some squares and would rather use one they have. In some cases, we could already have more than one square and can leave a DTR setup as a Dovetail Square, which is ideal.
Starrett makes a quality square that is very common in the marketplace either as a used item or brand new. NOTE that Starrett 6” double squares and 6” Combination squares use thicker rulers than their 4” squares (both combination and double).
Combination squares have a complex head, one side at 45 Degrees and the other at 90 degrees to the ruler. A double square has both sides of its head (casting) at 90 degrees to the ruler. The picture below is a Starrett 6” Combination Square (part number 11H-6-4R).
I will now show how to measure your square by using popular Starrett 6” and 4”squares. The 6” Rulers measure about .076” in thickness as shown here.
This is about the thickness of our Dovetail Ruler DTR-1 which is nominally .075”. Note that the slot in the cast head on the Starrett 6” Combination Square measures at .0845” That’s wider than the ruler by about 10 thousandths of an inch. You can purchase a DTR-1 here that will fit 6” Starrett squares with rulers and slots this size as well as many other brands 4” Squares; such as Lee Valley, PEC, Pinnacle. Please post Comments to this blog as a community we will continue to list brands that are compatible with the DTR-1.
The other measurement we are concerned about is the “height” of the ruler groove. Measuring the Starrett Ruler, the edge to the slot is around .342”. The groove to the edge distance is the “throw” of the clamp shoe. The clamp shoe tightens down with the spinning of the thumbscrew drawing the ruler tight against the groove in the square head casting. DTR-1 units are nominally .330” tall which is .012” less than the groove height. (It could be helpful to think that the DTR is half a ruler tall) Most squares have no issue with this small difference in height as the threads on the clamp shoe make up this difference with a very small extra turn.
DTR-1 units work VERY well in 6” Starrett squares as they are .075” thick and .330” Tall. DTR-3 units can also work well since they are about .080” thick and .335-.340 Tall. DTR-3 Units can be purchased here.
Which Dovetail Ruler (DTR) is best for you? You will need a Vernier caliper and measure your square ruler and slot to make the best decision. NOTE: A digital caliper is helpful, like the one in the picture – inexpensive ones can be about $10-25. Machinists will spend $100-200 for higher quality calipers. (If you know how to use and read an “analog” dial caliper you probably are not reading this blog.)
Note that you need some clearance in the width of the slot to the thickness of the ruler, it’s not as forgiving as the height.
Below is a Starrett 4” Double Square, this unit I purchased used and cleaned up in my shop. This unit is still in production in 2023 (Starrett 13A).
This unit has a ruler that is about .064” Thick (.0645 in the picture) and The height is about .300” (.2995” in the picture).
The Slot in the cast head is slotted at .0715”, for this particular square the clearance is .007” Our DTR-2 is .064” thick and .300” tall. The DTR-2 was made specifically for 4” Starrett Squares. I do not know of any other 4” squares that use rulers this thin.
I hope you all find this blog helpful to choose the best Dovetail Ruler for your existing square.
Thank you all for your support!
-Chris
I’m not sure my Starrett combination squares fit any of these “normal” measurements. Please let me know which Dovetail Rulers will work for me.
4″ Starrett Combination Square: .065″ thick rule, “height” of the ruler groove (from edge to the slot is .253”, Slot in cast head is .071″.
6″ Starrett Combination Square: .081″ thick rule, “height” of the ruler groove (from edge to the slot is .470”, Slot in cast head is .086″.
Hello Michael,
Thank you for posting so that others can learn as well! I would love to see some pictures of your squares to get an idea of their vintage. In your case with the measurements for the two squares that you have shared above, your best solution is the DTR-2 for your 4″ Combination square. The clamp shoe will allow to be higher for the .300 height of the DTR-2 vs the .253″ of your ruler slot. It’s when there isn’t enough threads left to pull the clamp show further down where there is an unusable condition; you can’t clamp enough to keep the DTR “locked” down. In this case there should be plenty of threads to still engage your clamp shoe with the thumbs screw. If you can see the top of the clamp shoe in your tapped hole of the thumbscrew you should have no problem! I hope this helps you and others!
Best wishes,
Chris