Why do my 4" saw blades keep cracking?

[Philipintexas]

I'm using 4" Dia., 1/16" thick circular saw blades (Metal cutting) with a 1" center hole in a home-made mandril. I made the central hold-down bolt/plug a very snug fit in the 1"center-hole. In use, they "seem" to get hot and crack radially from the corner of the key cutout of the center hole. I'm speculating that as they get hot the center-hole wants to contract and my snug center plug doesn't allow that? So they crack.? Should the center hole "plug" be somewhat undersize? I want the blade to run true so I made mine snug. I'd appreciate help as I'm running out of blades.

 

[Blogwitch]

Make your mandrel about 0.002" smaller.
It won't make any difference to the cutting depth if the saw isn't mounted exactly central as you should set up with the saw running and you will hear the high tooth catching on the job, that is your setup point.
I have yet to come across a normal HSS saw to be perfectly round due to the way they are ground, the first tooth will be cut to perfect size whilst the last teeth will usually be cut slightly less deep as the grinding wheel has worn slightly. The only ones I have come across that cut perfectly round are solid carbide ones, and I suspect they were ground or made to much tighter tolerances.

You might be cutting or feeding too fast and they are overheating. Flood coolant or air blowing help, plus of course, correct lubrication for the material being cut if you are not flood cooling.

John

 

[django]

It won't be the bore of the blade getting smaller, because as the blade gets hotter the bore will get bigger (see co-efficient of linear expansion) but it would a good idea to keep the blade as cool as possible (slow speed and plenty of coolant) Also thin blades tend to wander especially on deep cuts which can lead to breakages. hope this helps

Paul

 

[10K Pete]

Double check that you don't have radii that are pinching the sharp edges
of the hole when the blade is clamped. That could induce stresses and/or
uneven stresses that could cause the cracking. It's important that the only
clamping force is evenly around the flat part of the blade and none are
radial in nature.

Pete

 

[mcostello]

Slight radius on key might help.

 

[tornitore45]

If the blade get hot it takes the shape of a potato chip because the hotter strip near the cutting edge has a longer circumference than the cooler strip just more central.
The stress and possible binding cracks the blade radially and allow the geometry to be a flat disk again, with tiny wedge missing.
A metal saw should not get more than warm, slow down and use lubricant/cooling.

 

[Hopper]

What rpm are you using, and what metal are you cutting? It should be max of 100 rpm for steel.

 

[Philipintexas]

Thanks for the input, I was sawing a length of 3/4" thick 6061 using cutting oil but running at too high a speed. Now that I've priced replacement blades I'll be sure to take my time in the future...

 

[tornitore45]

I have cut 1/2" aluminum plate on my 10" table saw, an unnerving operation but safe enough with precautions, full mask and ear protection.
Parts that can fit on the mill, I cut with a Thin kerf 7-1/4" carbide tipped blade designed for wood. The wide gullets really keep the cut clean. Really any toll that cut wood will cut Al just fine by simply reducing the speed. I have used carbide tipped router bits too.

 

[Charles Lamont]

Thanks for the input, I was sawing a length of 3/4" thick 6061 using cutting oil but running at too high a speed. Now that I've priced replacement blades I'll be sure to take my time in the future...

For sawing aluminium alloy with an HSS saw I would want to run at something like 200 ft/min. Your 4" saw has a circumference of about a foot, so that would be 200 rpm. Slower is allowed! Ideally you would flood with coolant to wash the chips away but a regular squirt or constant dribble from a washing-up liquid bottle of paraffin (kerosene) will do.

 

[Blogwitch]

If you can get away from such a thin blade, then 1/16th to 3/32" wide tungsten blades from small circular saws can be obtained from as small as 3" diameter. Not only will they cut through almost anything, because of the larger teeth, they don't clog or overheat so much. I use them dry most times.
If you go down a few posts on here, you will see 'rage' blades, and are very thin and really stand up to a lot of abuse. You would need to make your own holder though.

http://www.homemodelenginemachinist.com/showthread.php?t=25816

John