Laurie Tedcastle
Member
- Messages
- 119
- Location
- NW Northants (UK)
Part 1
IF I remember correctly, the question of correct blade tension has been raised
on here before, but never answered. I have never examined or worked a large
industrial class metal cutting bandsaw, but presume these have an inbuilt
mechanism for setting the tension correctly. Surely someone will soon correct
me on this.
I understand that some wood cutting vertical bandsaws have a rather inaccurate
spring/pointer mechanism. Certainly, the hobby type small metal saws (like my
Clarke 6"x4") have no system at all and probably no guidance in the manual.
Bandsaw blades require tension to:
- provide friction on the blade driving wheel to move the blade
- for beam strength. The tighter the blade is stretched, the more rigid it
becomes and the less tendency it will have to deflect in the cut especially
in thick stock.
But excessive blade tension can lead not only to premature blade breakage
(usually at the weld joint area) or even break parts of the machine.
Blade manufacturers give "ideal" values for the tension stress for each blade
material type usually:
Carbon steel blades 14500 - 21750psi, Bimetal blades 24650 - 34800psi. Note,
this is independent of blade size. I believe the metric equivalents are:
100-150N/mm2, 170-240N/mm2.
The best source of information, I have found, on bandsaw blades is on the USA
blade manufacturer Lennox Tools website (Lennoxtools.com). Search their site
for the excellent(pdf) "Guide to Bandsawing". They also make a Blade Tension
Measuring Meter (but sit down before looking at the price!). Starret also do
Tension meters.
The stress in the blade can be easily determined by measuring the elongation of
the blade over a known distance as it is tightened. This can be done with a
simply made gauge. A pic of one I made in 2016 installed on my 6x4 bandsaw:
My thanks go to Jaques Maurel for the gauge design in an article in
Engineering in Minature (June 2016). Basically a simple frame clamped to the
blade. The lever arm at the right hand end is pivoted in the horizontal member
thus driving the dti plunger as the blade elongates. The frame is clamped to
the blade with pointed grub screws either side of the blade. Dimensions are
not critical. A search of the web will even turn up a similar device made from
wood.
When the blade bends around the blade wheels, the stress increases somewhat in
that area. Therefore I would suggest sticking to about 18,000psi on the small
hobby type bandsaws.
Now for some maths to find the elongation for your chosen stress level:
Tension strain = elongation / distance between gauge clamping screws
Hooke's law: tension stress = Modulus of elasticity x tension strain
The modulus of elasticity for steel is about 29,000,000 psi at room
temperature. It varies insignificantly, for our purposes, with different steel
alloys.
Thus elongation = (tension strain x distance between clamping screws)
---------------------------------
modulus of elasticity
For my gauge the distance between clamping screws is 3.150"
so: elongation = (18000 x 3.150)/29000000 = 0.001955"
For convenience, round up to 0.002"
This equates to 1.3/4 turns of the tension handwheel, on my saw, with it's
16tpi tensioning screw. I normally don't bother with the gauge and just use the
1.3/4 turns. But I do use the gauge after fitting a new blade
For my gauge I use a 0.0001" div dti to get a reasonable needle movement.
continued - part2
IF I remember correctly, the question of correct blade tension has been raised
on here before, but never answered. I have never examined or worked a large
industrial class metal cutting bandsaw, but presume these have an inbuilt
mechanism for setting the tension correctly. Surely someone will soon correct
me on this.
I understand that some wood cutting vertical bandsaws have a rather inaccurate
spring/pointer mechanism. Certainly, the hobby type small metal saws (like my
Clarke 6"x4") have no system at all and probably no guidance in the manual.
Bandsaw blades require tension to:
- provide friction on the blade driving wheel to move the blade
- for beam strength. The tighter the blade is stretched, the more rigid it
becomes and the less tendency it will have to deflect in the cut especially
in thick stock.
But excessive blade tension can lead not only to premature blade breakage
(usually at the weld joint area) or even break parts of the machine.
Blade manufacturers give "ideal" values for the tension stress for each blade
material type usually:
Carbon steel blades 14500 - 21750psi, Bimetal blades 24650 - 34800psi. Note,
this is independent of blade size. I believe the metric equivalents are:
100-150N/mm2, 170-240N/mm2.
The best source of information, I have found, on bandsaw blades is on the USA
blade manufacturer Lennox Tools website (Lennoxtools.com). Search their site
for the excellent(pdf) "Guide to Bandsawing". They also make a Blade Tension
Measuring Meter (but sit down before looking at the price!). Starret also do
Tension meters.
The stress in the blade can be easily determined by measuring the elongation of
the blade over a known distance as it is tightened. This can be done with a
simply made gauge. A pic of one I made in 2016 installed on my 6x4 bandsaw:
My thanks go to Jaques Maurel for the gauge design in an article in
Engineering in Minature (June 2016). Basically a simple frame clamped to the
blade. The lever arm at the right hand end is pivoted in the horizontal member
thus driving the dti plunger as the blade elongates. The frame is clamped to
the blade with pointed grub screws either side of the blade. Dimensions are
not critical. A search of the web will even turn up a similar device made from
wood.
When the blade bends around the blade wheels, the stress increases somewhat in
that area. Therefore I would suggest sticking to about 18,000psi on the small
hobby type bandsaws.
Now for some maths to find the elongation for your chosen stress level:
Tension strain = elongation / distance between gauge clamping screws
Hooke's law: tension stress = Modulus of elasticity x tension strain
The modulus of elasticity for steel is about 29,000,000 psi at room
temperature. It varies insignificantly, for our purposes, with different steel
alloys.
Thus elongation = (tension strain x distance between clamping screws)
---------------------------------
modulus of elasticity
For my gauge the distance between clamping screws is 3.150"
so: elongation = (18000 x 3.150)/29000000 = 0.001955"
For convenience, round up to 0.002"
This equates to 1.3/4 turns of the tension handwheel, on my saw, with it's
16tpi tensioning screw. I normally don't bother with the gauge and just use the
1.3/4 turns. But I do use the gauge after fitting a new blade
For my gauge I use a 0.0001" div dti to get a reasonable needle movement.
continued - part2