A Question on Kerf

[pops92]

Not having used a table saw a lot I am after advice on Kerf. Cutting a finger slot 8mm deep with 2 passes on a blade with 2.5mm kerf should the cut be 5mm all the way down and if yes why is mine narrower at the top of the cut.
All the fingers are marginally narrow at the top than the bottom
Could this be a cheap blade problem :?:

 

[YorkshireMartin]

My guess, and it is only a guess, that the blade might be slightly dull, so theres a bit of vibration before it engages the material fully. Are the joints perfect in terms of alignment?

 

[MattRoberts]

I'd guess it's more to do with fence alignment. If it's slightly out, it will be wider at the bottom.

Although you contradict yourself saying it's 'narrower at the top' and then 'marginally wider at the top' so it's hard to tell

 

[pops92]

The joint are perfect as far as pitching.
You've lost me Matt as to fence alignment please enlighten.

 

[MattRoberts]

If the blade is not exactly 90 degrees to the table & fence, it could be cutting at a very slight angle. Because you're pushing the wood through this angle in a straight line via your jig, it might account for the kerf being slightly larger at the bottom than the top after you've done your second cut

 

[pops92]

Ah now I'm with you Matt have not thought about that.Will check that in the morning.
Let's say that OK for the moment would the cheap supplied blade be a possibility do you think?

 

[MattRoberts]

I don't see how really. Cheap or not, a blade will always create a slot the width of its widest point, and that width should be consistent throughout. I can only think that it's because you're making two cuts per finger that there's a problem. Have you made a single cut and measured top and bottom width?

 

[pops92]

Yes have done that with single cut and wider at the top than bottom just the same. However the slot is wider than than the kurf of the measured with a vernier.

 

[RobinBHM]

Is this an end joint, like a bridle joint?

If do my guess is that when the slot is cut, some tension is released and the finger is bowing away from the slot.

To prove it, you could check the tenon thickness not the slot gap. A possible solution would be to start with a saw cut in the middle allowing any trnsion to be released then do your 2 cheek cuts.

 

[pops92]

These are finger joints that I am doing on a jig I have just finished. The joint shown is just a single pass which still has the error at the top very slight but it's still there.
Will check the blade is at 90° tomorrow and may get a new blade and go from there.

 

[Steve Maskery]

I think Matt is probably right. Your mitre slots and blade are not aligned with each other. You are pushing your workpiece very slightly sideways as it goes over the blade, so the teeth cut the RH side as they come down but they also cut the LH side on the way up. The TDC is the only place where the cut is correct.
You need to fettle your TS.

 

[pops92]

Sorted.
Once I'd put a bit of heat in the garage it was time get to the bottom of this. What I should have done first inspect the blade. :roll:
Cleaned all the crap off that had built up in the gullies and the sides. Happy with the clean up,time for another run at 5mm fingers.
Yes I know you are saying should of done that at the start. Only ever used it for rough cutting. This is all learning curves for me.
Any way, low and behold the fingers are now the same all over.
Thanks to everyone for there input on this self enduced problem.
Basic maintenance I here you say.
Correct :wink: