Bandsaw - blade tension interlock

[Malcolm McLeod]

In case anyone else wants to create an interlock....

I have a pre-color change Rikon 18" bandsaw (model 10-345, 2.5Hp, 11 FLA), and it has the handy blade tension release lever. It uses a typical dbl-pole/sngl-throw (DPST) mechanical switch rated at 20A - but this has burned out on me once already (and factored into decision). I discovered that if you forget to move the lever and start the saw with no tension on the blade it makes a mess:
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This is a close-up of the top edge of the lower door where the blade has taken a bite. Hard on blades; hard on the door; hard on egos.

As you can see in the photo, I have committed this cerebral flatulence once and my father has done the same. That was enough for me, so I decided to build an interlock to the start circuit, and considered 2 alternatives:

Interrupt the power to the existing Start/Stop switch across a new DPST limit switch (L/S) rated for full amps/volts of the motor. Position of the blade tension carrier triggers the L/S. This has the advantage of simplicity and minimal new parts; downside is that the cheapest L/S I could find rated for >11A was $350.
Replace the OEM Start/Stop with a magnetic starter - with 240VAC coil - and a SPST L/S. The L/S now only needs to carry the amp draw of the starter coil, and has the added plus of not allowing the saw to re-start after a power failure (i.e. grid fail or CB trip).


In both cases, I considered trying to locate the L/S so that a blade 'break' would also cause the starter to drop out, but finally tossed it as too complicated and probably prone to damage from either blade teeth strikes or similar disasters. I just don't need the added maintenance hassle.

I sourced all the parts for option 2 online for ~$125 and I liked the anti-restart benefit, so option 2 it is - -

[Malcolm McLeod]

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^ L/S mounted and cable run thru cord grip into the saw's spine. This after clamping in place and testing that L/S is properly triggered by tension carrier's up/down motion.


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^ With upper wheel back in place. Cable and L/S clear wheel by 2.5" at closest approach. Used an old Omron L/S with integral cable that has been following me for years.
Adder: To clarify, this switch trips if the lever (with cam) has moved the tension carrier to its upper position. It does not in any way sense the actual tension on the blade. If I engage the lever but leave the spring tension completely 'off', I could still have issues, but has not been the case to-date.


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^ Made a new switch cover plate, punched holes for 2 switches (N.O. 'Start'; N.C. 'Stop'), aluminum bracket to mount the starter on (cantilevered off the front plate). Grey cable at right is the L/S.


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^ The 'package' slides into the original switch opening, so no mods to the saw frame (other than cord grip hole).

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^ And finished trim (couldn't match paint exactly, but close...:o). All of changes are hidden inside the saw. Works like a champ.

I can post parts and/or electrical schematic if anybody is interested, but parts may vary from saw to saw (amps) - and schematic is just a pencil sketch.

(And sorry, the evil photo-rotation gremlins have attacked.)

[Alex Zeller]

My Grizzly has a key switch (I'm assuming is part of the mag switch circuit). I remove the key and hook it on the tension lever to remind me.

[Malcolm McLeod]

I like the key switch feature, but I tried reminder ‘flags’ - and still managed to wreck 2 blades - 1 of them $200+ carbide:mad:. Finally decided $125 in parts and an afternoon’s work was cheaper than one more blade.

[Mike Kees]

Malcolm I like what you did there. I used to own a Steel City 18'' with a tension lever and once switched it on without tension as well.

[Malcolm McLeod]

Thanks Mike.

In case it’s not apparent, if a saw already has a magnetic starter, it should be a simple matter to install a $15-20 L/S. Once mechanical installation is set, just wire the switch in series with O/L or ‘ahead’ of the Start/Stop switches (to break power to them).

[Randall J Cox]

I started my 18" minimax untensioned once. Sure woke me up!!! To keep from doing that again, I took the advice of someone on here and when I detension (don't have a lever, I just unscrew tension) I put a clothespin on the blade. That reminds me to tension before starting. Has worked for a number of years now. Simple but not idiot proof. Randy

[John K Jordan]

That's a fancy idea. I did that once on my 18" Rikon but still had enough residual tension that I made some cuts before I noticed.

Now I use a method I stole from someone. I set this on the table when I detension.
Low tech but foolproof if you use it. (Hang it on the tension lever.)

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JKJ

[Rod Sheridan]

Creative solution.

I just leave mine with the tension on, that way I don't forget.............Rod.

[Bert McMahan]

I think your second solution is more elegant, but I don't think you need as big of a limit switch as you said, though I'd love to learn if I'm wrong here. This switch:

https://cdn.automationdirect.com/static/specs/limitabp.pdf

is like $30. It has a "Break" spec of only 3.1A at 230 VAC, BUT that only happens if you try to detension the saw while it's on. I assume no matter how tired you are, you likely won't be doing that :)

Its Make spec is 30A at 240 VAC, which means it should be able to carry your current. In fact, it should be even better, as your limit swich won't ever actually make or brake the load- your main power switch will do that.

Am I wrong in my analysis here? I feel like the limit switch failing during an online blade detensioning is probably the least of your worries, after buying a new pair of underwear :)

[Malcolm McLeod]

I think your second solution is more elegant, but I don't think you need as big of a limit switch as you said, though I'd love to learn if I'm wrong here. This switch:

https://cdn.automationdirect.com/static/specs/limitabp.pdf

is like $30. It has a "Break" spec of only 3.1A at 230 VAC, BUT that only happens if you try to detension the saw while it's on. I assume no matter how tired you are, you likely won't be doing that :)

Its Make spec is 30A at 240 VAC, which means it should be able to carry your current. In fact, it should be even better, as your limit swich won't ever actually make or brake the load- your main power switch will do that.

Am I wrong in my analysis here? I feel like the limit switch failing during an online blade detensioning is probably the least of your worries, after buying a new pair of underwear :)

Bert you are likely correct.

However, I like to build things, but not so much RE-build my tools. I mentioned in OP that the original mechanical starter (20A) has burned up once. I opened it and found one contact pad had literally been cut off by arcing - on a gently used saw! As I alluded to earlier, this influenced both my planning and eventual decision. If I had pursued option 1, I would have erred on the side of caution and used a DPST L/S rated for >11Amps (my FLA), and would really look hard for a minimum rating of 20A. Bandsaw has the slowest wind-up of all my tools, and so the longest dim-the-lights-current-draw. ....Call me sissy.:eek:

As it is now, the L/S carries only ~0.5A AND should never see a power-on 'make' or 'break' cycle. ....Call me satisfied.:D