Thread: Dovetail saw TPI

Franklin, good luck with 20 tpi!! Wear extra strong lenses! the highest I have done to date is 18 tpi.

The higher tooth count are best for thin boards, say around 1/4" and under. A plate with a high tooth count will be smoother in the cut, but also slower. You can "mimic" the 20 tpi by filing some fleam on the teeth, or increasing the rake angle (most dovetail saws are around 5 degrees. Make it 10 degrees and you will notice the difference).

If you have a dovetail saw that is 14 tpi, I bet it is the Veritas. This has 14 degree rake, which is very gentle. Stick with it.

I do not like progressive filing. It just makes more to keep track of. A light touch is all you need.

Regards from Perth

Derek

Hi Derek,

The saw I'm looking at sharpening is actually a vintage steel backed Disston with a closed handle. I have had it for a while but not used it because it tends to bind in the cut. I have another vintage 10" x 14TPI saw, open handle pistol grip that cuts well but has no maker identification I can see, it would also benefit from a touch up, but I wouldn't try and change its geometry.

The teeth on the Disston are definitely not sharp and presumably not enough set. I'll sharpen first to see if that fixes the cut before trying to enhance the set, however that will be another challenge since my saw set only dials down to 12TPI.

Veritas say their 20TPI is intended for 1/2" and under, which would cover drawer sides I think. I reckon 20TPI would feel just like scratching at the cut. Rather than the Cosman progressive TPI filing I think I might try a progressive change of rake as Paul Sellers suggests.

Agreed, filing 14TPI is going to be enough challenge for my eyes, not to mention I don't think I have a file suitable for 20TPI.

Cheers,

While the teeth very likely need to be sharpened, binding is usually caused by too little set.

Regards from Perth

Derek

I, for one, consider 20tpi to be a "specialty" pitch on a backsaw.

I had a Veritas 20tpi saw and I got rid of it. It's not a pitch that I care for in my personal work. I found that I experienced binding with it right out of the box.

Perhaps my sawing skills have improved since then, and it was user error. I'm sure Lee Valley would not sell a saw which bound in cuts just by virtue of its factory setup. Nonetheless, the physics are certainly there to explain why it would be more likely to bind. Smaller tooth = shorter tooth = less possible set = smaller kerf to plate thickness ratio.

It's also damn hard to sharpen 20tpi. Hell, it's hard to even see it, and I'm in my early 30's.

I also found it very slow.

I eventually switched to a 15tpi saw, which, I think, is the pitch which Lie Nielsen includes on their dovetail saw (although mine isn't a LN). I think this is a more traditional pitch, and I find it cuts very quickly (3-5 strokes per dovetail cut typically), and is easy (or at least easier) to sharpen and set. I don't think I've ever had it bind on me either.

That's just been my personal experience. If I was making a lot of teensy boxes in 3-6mm thick stock, then I may be singing a different tune. I have a tiny, 5-6" dovetail saw and I'm actually not certain of its pitch. I THINK it's 18tpi. I use that one for thin stock joinery, which I cut very rarely.

I guess my final thought would be that if it's an extra saw, and you think you may need to cut tails in thin stock soon, then have at it at 20tpi, but if it's just a general use saw, then I'd consider a more coarse pitch.

My US$0.02.

Cheers,
Luke

Well I sharpened it this morning with the progressive rake tooth shape (14TPI), first inch 30, 2nd inch more aggressive and the body of the saw zero. I like the result. It starts easily and cuts smoothly without any additional set.

The saw had a noticeably concave belly and needed a fairly aggressive jointing before sharpening, I wonder if this had anything to do with it tending to bind?

When I tested the sharpened saw I just grabbed a thinnish scrap from the bin and had no problem. Yesterday I started dovetailing a couple of drawers. Cutting tails first, I clamp both sides in the vise to cut two sets of dovetails at the same time. I note starting the saw on the effectively much thicker double board was a bit different. The attack angle on the board made a big difference to starting the cut. The negative rake at the tip made it feel more difficult to start a flat cut straight across the boards, the cut started best at an angle on the edge, moving to horizontal once reaching the aggressive rake teeth.

I'm thinking the progressive change doesn't need to be so long, perhaps just the 1st inch at a medium rake would be OK.

Originally Posted by Luke Maddux

I, for one, consider 20tpi to be a "specialty" pitch on a backsaw.

I had a Veritas 20tpi saw and I got rid of it. It's not a pitch that I care for in my personal work. I found that I experienced binding with it right out of the box.

Perhaps my sawing skills have improved since then, and it was user error. I'm sure Lee Valley would not sell a saw which bound in cuts just by virtue of its factory setup. Nonetheless, the physics are certainly there to explain why it would be more likely to bind. Smaller tooth = shorter tooth = less possible set = smaller kerf to plate thickness ratio.
Luke

I have used a near new Veritas during this week that jammed in the cut.

Originally Posted by Chris Parks

I have used a near new Veritas during this week that jammed in the cut.

That is indicating that it is the set that is too little rather than the teeth not being not sharp enough.

Regards from Perth

Derek