Thread: Thickness llimits

Originally Posted by labr@

Have just broken another blade on a bandsaw box and although I think there were a few contributing factors it started me wondering what the practical limits are for stock thickness with small blades.
What thickness of timber would you be confident cutting with say a 3mm 13TPI blade or a 4mm 10tpi blade?.

For the most efficient cutting of timber there should be between 3 and 6 teeth in the workpiece.
eg using a 1 TPI blade theoretically limits the workpiece thicknesses to between 3 and 6"!

A 13 TPI blade is in theory limited in thickness to between 6 and 12mm. A 10TPI is 7.5 and 15 mm.
There's nothing preventing cuts in thicker materials but don't expect to cut anywhere near as fast.
I don't have direct experience with 3-4mm blades as the highest TPI blades I have for my 19" BS are 2 6TPI, a 6mm and a 19mm.
Theoretical limit for these is 12.5 and 25mm but I will cut up to ~50mm thick (even thicker for softer woods) at normal band speeds with the these blades, albeit slowly.

The main issue with thicker wood is sawdust clearance. One way to get around this is to run the blades faster.
Conventional/upright WW bandsaws typically run at about 3000 fpm whereas the most efficient wood cutting is at about 5500 fpm which is what bandsaw mills use.

The BS mill we have at the tree loppers yard runs at 5000 fpm and uses a 1TPI blade and cuts up to 36" wide - that's 36 teeth in the cut but because it runs so fast it has no problem with this.
Unfortunately the blade gets really hot really quickly so it needs cooling (a fine spray of water) but that is not practical in a ww workshop.

If I forget to turn on the cooling water, when I do turn it on steam will rise from the blade indicating its hotter than 100ºC
When the water spray is running the temperature difference of the band going into the cut compared to going out of the cut is <10ºC

My home BS has variable speed and I can run the 6TPI blades faster for a short while to more easily tackle thicker pieces.
The blades do indeed get hot and expand, which reduces the band tension so I add a touch more expansion but just remember to back it off when you finish.
The same applies to blades that get hot from poor sawdust clearance and may be a contributor to your breakages.
FWIW a 3600 mm long blade that is heated by 100ºC will expand about 4mm in length which is not good especially for smaller bands.

Thanks Bob, all of that makes sense but the main things for me with bandsaw boxes are accuracy and smoothness of cut.
I suspect that the sawdust clearance is more concerned with speed and that by going slower it is possible to have more teeth in the cut. Of course more teeth cutting at one time requires more push overall to make them bite so there is going to be a limit somewhere.
Also 15 mm is not thick enough for a box - even at 6 tpi the formula does not allow enough thickness - and I'm sure there are people doing thicker boxes than that with fine blades. I just don't know what thickness they are getting up to with those blades.

Originally Posted by labr@

Of course more teeth cutting at one time requires more push overall to make them bite so there is going to be a limit somewhere.

I'll disagree with this. If you have to push to get any blade to bite... it's blunt.

You should always push just hard enough to maintain the feed rate at which it wants to cut. Any "harder" and, especially with thin blades, you're forcing the cut and probably bowing the blade backwards, reducing accuracy and efficiency while running hot and shortening blade life.

Too slow is just as bad, for different reasons.

The rate at which any given blade will cut best depends greatly on the choice of timber; different timbers shave differently and some turn to powder while others make shavings. This affects how well the sawdust is scavenged , which in turn effects the feed rate as Bob mentioned.

With thicker boards the feed rate drops off dramatically.

Let's say you're using the 10tpi blade. It'd probably cut fastest through 4-6mm material thickness. It'll still cut an 8-12mm piece (ie. twice as thick) quite happily provided you remember that it'll also have twice the amount of sawdust to remove and so will need twice as long to make the same length of cut. ie. half the feed rate.

In theory you could cut a 16-24mm thick piece at 1/4 the base feed rate, but in practise it'd be even slower as the extra sawdust and blade length in the cut generates extra heat so everything runs hotter. This is what determines the upper limit.

I often cut 1" thick softwoods with a 10tpi 10mm blade but I wouldn't push my luck with hardwoods if I want accuracy & clean cuts.

Being only a 4mm blade, well... that makes feed speeds even more critical.

Originally Posted by labr@

Thanks Bob, all of that makes sense but the main things for me with bandsaw boxes are accuracy and smoothness of cut.
I suspect that the sawdust clearance is more concerned with speed and that by going slower it is possible to have more teeth in the cut. Of course more teeth cutting at one time requires more push overall to make them bite so there is going to be a limit somewhere.
Also 15 mm is not thick enough for a box - even at 6 tpi the formula does not allow enough thickness - and I'm sure there are people doing thicker boxes than that with fine blades. I just don't know what thickness they are getting up to with those blades.

My examples were for "efficient cutting" which means fastest feed rate. Pushing harder is a no-no. Once the gullets are full of sawdust pushing is a complete waste of time and just adds to friction causing blade expansion and greater likelihood of breakage.

Like I said, I will cut 50 mm thick with my 6 TPI blades (even 75 in softwoods) but its not fast cutting.
The thickest curved things I cut were the half dozen or so 50 mm thick jarrah Veranda post brackets for the neighbour.
The main radii of curvature was about 300 mm but they had 100 mm diameter rounded ends which was tricky, and I did those that with the 6mm wide 6TPI blade.
Not something I'd like to be doing every day but it did work out albeit slowly and no blades broken.

Hi all,
I have successfully cut many boxes ranging between 70- 90mm using 3mm 14 tpi blades never had any trouble with sharp blade in a variety of timbers. The tension on the blade needs to be correct and obviously set up square to get parallel sections, I have been surprised how easily these thin blades cut and handle thicker boxes. this box is unfinished but shows the thickest box I have done it is Dilenia 90mm thick.
Regards Rod.

Thanks Rod, that's the kind of info I was looking for. I have done some experimenting since starting this thread and worked out that about 75mm is the limit.

That box you showed is a very nice design .

I have made lots of bandsaw boxes and the only limit is what can be sanded on spindle sander usually about 100 mm. If you have the right sharp blade and take your time you will have more success.