Routing Dovetails and Changing Bits

[Glen Blanchard]

Regarding cutting dovetails on a router table with an Incra Jig or Jointech system – If I have determined the correct height for the bit (let’s say a 14 degree, ½” bit for this discussion) and (for whatever reason) change to another bit (same specs, same manufacturer as the first) what is the likelihood that my joint will not fit and I will have to change bit height? How would this change if the replacement bit, although having the same specs, was made by a different manufacturer?

I would imagine that tolerances for U.S. bits, such as Whiteside, are tighter than many of the Taiwanese imports, but are the variances from one bit to the next great enough that it will show up in a dovetail joint?

[Tom Jones III]

Personally, I would not be able to take a bit out of a router halfway through cutting a drawer, put the same bit back into the same router and make it look like I never stopped.

You might be able to make it look "good enough", but that depends on what "good enough" really looks like.

[Glen Blanchard]

Personally, I would not be able to take a bit out of a router halfway through cutting a drawer, put the same bit back into the same router and make it look like I never stopped.

You might be able to make it look "good enough", but that depends on what "good enough" really looks like.
Good point. I guess I should have mentioned that I have a digital router lift, and should be able to reproduce any given bit height to within .001" or so. However, for the sake of argument, let's just say that you were able to EXACTLY reset the bit height each time. How would you answer the question then?

Edit: I was not really referring to changing bits half way through a series of cuts, but rather merely tossing an old bit out after a project has been completed, and begin using a new bit on the next project.

[Jim Dunn]

Assuming that the bits are the same diameter, height and angle you should be able to re-zero the bit and raise it to the same height as before and cut the same dovetail. That said, I doubt it would work 100%, as each bit will cut differently depending on the relief angle and sharpness of the bit.

[glenn bradley]

This kinda bumps up against that thing where woodworking, although it involves machining material, is not metalworking. I haven't tried it but knowing me, if I even stopped halfway and changed bit height and then tried to return it to the starting point to continue, I would probably see the difference. Generally when I'm doing DT the router, once set, doesn't change till I'm done. That's just me protecting myself from myself.

[Glen Blanchard]

This kinda bumps up against that thing where woodworking, although it involves machining material, is not metalworking. I haven't tried it but knowing me, if I even stopped halfway and changed bit height and then tried to return it to the starting point to continue, I would probably see the difference. Generally when I'm doing DT the router, once set, doesn't change till I'm done. That's just me protecting myself from myself.
Thanks Glenn - but I did not mean this to be a discussion about changing bits in the middle of cutting a joint - but rather in more general terms.

I guess to word it differently, if you could accurately measure bit height down to .001" would you see a difference in optimal bit height from one bit to another?

[Glen Blanchard]

Let me try again. Let's say (somehow) you are able to set your router bit at EXACTLY the same height each time. Furthermore, let's say you have it it set perfectly for a Whiteside 1/2" 14 degree dovetail bit and you complete your project with perfectly fitting joints. Now you want to begin another project using a DIFFERENT Whiteside 1/2" 14 degree dovetail bit and you put it back at the EXACT same height as the first Whiteside bit. Would your joint come out fitting well, or will you have to play around with the height of the bit?

Now, what if you use a CMT 1/2" 14 degree dovetail bit?

[Jim Becker]

Chances are that you may be fine with another bit of the same model and manufacturer although specs could change over time, but from a different manufacturer, I wouldn't trust it one bit. There are too many metrics involved and with dovetails (particularly half-blind) a "proverbial hair" will affect fit.