Spline cutting jig modified to fit Delta Unifence and question for you physics pros out there

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Project by jfk4032 posted 01-12-2014 03:34 PM 5407 views 8 times favorited 20 comments Add to Favorites Watch

After researching LJ and Youtube for spline jigs, I found a simple model from The Homestead Craftsman on Youtube and modified that version to fit the Delta Unifence weird profile.

I added some knobs, threaded inserts and threaded rods for functionality. Nothing too complicated or fancy about this jig, but it does the trick and works great. But something strange was happening…

I bought a new Freud flat top blade and set that up to the desired height into my jig. When I cut the splines for the cutting board in the first picture, this brand new blade began to spark as if I was cutting into some metal or a nail. I double checked all around the cut to make sure I didn’t have a bonehead design with a nail or screw in the path of the blade, and that was not an issue. I rotated the cutting board and made the next cut and the same sparks happened for that cut and the next two. I inspected the blade afterwards and it look fine to me, just some discoloring on the carbon tips, which you would expect from being used.

I called up Freud and explained the situation and they never heard of such a complaint. They sent me a new blade by returning the “problem” blade.

After the second new blade arrived it occurred to me what the issue possibly could be and was hoping that perhaps any of you Lumberjockers out there have had such an issue, or are versed well enough into physics to concur with my idea.

The little triangle block clamping wedge is loose in my design, so like many of my jigs, I countersunk a rare earth magnet into it and a corresponding rare earth magnet onto the face of the jig so I don’t lose it. As seen in the second to last picture above. Perhaps the proximity of the magnet when the clamping block is in use to the rapidly rotating saw blade (as seen in the last picture) when passing each other within an inch or less created some kind of eletro-magno thingy (you like my scientific terms?).

Am I delusional or could this be possible? Or could this be just a freak bad blade? I’d love to get some feedback from all of you MacGyvers or physics teachers out there.

I have since cut a different triangle clamping block out of some scrap particle board and didn’t attach any magnets and when testing that, the second blade didn’t spark. I had to send back the original blade that did spark in order to get the new one with no additional cost to me. So there is still a slight doubt in my mind as I couldn’t test one clamping block with magnet versus the other without all other things being the same.

-- ---Joel; Central MD...rookie empter nester and getting back into woodworking!

20 comments so far

View maplerock's profile


529 posts in 3291 days

#1 posted 01-12-2014 03:53 PM

Are you sure the blade is not nicking the insert?

-- Jerry... making sawdust in the Knobs of Southern Indiana

View jfk4032's profile


384 posts in 4018 days

#2 posted 01-12-2014 04:03 PM

No, it’s not, but even if so, the jig 45 degree supports and the insert were just plywood, with no metal anywhere in the vicinity of the blade path through the jig.

-- ---Joel; Central MD...rookie empter nester and getting back into woodworking!

View longgone's profile


5688 posts in 4800 days

#3 posted 01-12-2014 04:37 PM

Try the new blade with your magnet setup…and if it also sparks the problem might just be in the Twilight Zone effect. I personally think that to have sparks the blade must contact metal and not just be in proximity of it.

View jfk4032's profile


384 posts in 4018 days

#4 posted 01-12-2014 04:42 PM

Hey Greg,

You and me both…but after double and triple checking, there was no metal in contact with the blade or even in close proximity to the blade…only the magnet which was almost an inch away at the closest point of intersection…one thing I didn’t mention above is that the sparks only occurred when the blade passed under the jig (which was close to the magnet and no where near any other metal)

-- ---Joel; Central MD...rookie empter nester and getting back into woodworking!

View lew's profile


13546 posts in 5247 days

#5 posted 01-12-2014 04:56 PM

Anytime you pass metal through a magnetic field, you create a current flow in that metal object (Faraday’s Law- I think). The faster the movement and/or the stronger the magnetic field- the greater the current flow. So it very possible you have created the correct conditions for this to happen- moving metal blade and strong rare earth magnets. As for the sparks, current flow creates heat and maybe enough heat to ignite the very fine wooden dust particles.

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View Dark_Lightning's profile


5113 posts in 4600 days

#6 posted 01-12-2014 05:19 PM

Yes. You made a generator. This is a classic demonstration I used to show when I taught physics. Find a tube (copper, aluminum) that has an inside diameter close to the outside diameter of the magnet. Drop the magnet into the tube and observe how long it takes to fall. The magnet moving past the metal generates currents, which make a counter electromagnetic force (EMF) that opposes the motion. I’ve seen setups that were suitably made so that the magnet could take as long as 30 seconds to fall one foot. And, yes, even at that slow speed, sometimes sparks are observed. So, given the speed of the perimeter of your saw blade, that was probably pretty spectacular. This is also how magnetic brakes work. Did your blade stop faster with this setup (if the magnets were still near the blade)?

-- Steven.......Random Orbital Nailer

View jfk4032's profile


384 posts in 4018 days

#7 posted 01-12-2014 05:24 PM

Thanks Dark Lightning and lew for the physics lessons, I thought I might be on the right thought path. I didn’t notice the magnetic break effect because I pushed the jig all the way through and off the end of the table saw before knocking off the power. Do you think the carbon tips in the first blade were sparking due to this effect or as lew mentioned perhaps the fine dust particles in the area were igniting?

-- ---Joel; Central MD...rookie empter nester and getting back into woodworking!

View Gianni's profile


232 posts in 3465 days

#8 posted 01-12-2014 07:40 PM

My guess would be the sparks were basically plasma effect from the magnet/current generation and blade friction building up additional static. I don’t think there are any concerns about damage to either the blade or the wood.

View ChuckV's profile


3517 posts in 5018 days

#9 posted 01-12-2014 09:14 PM

This is very interesting.

I really do not mean to derail this conversation. But, I am curious what the physics-minded people think would happen if you did this with a SawStop. I know that the blade hitting a piece of metal can trigger the brake. How about in this situation where the blade itself is part of a generator?

I think I remember something from physics about Maxwell unifying the force of magnetism and the force of hotdogs.

-- "Join the chorus if you can. It'll make of you an honest man." - I. Anderson

View Woodstock's profile


266 posts in 4779 days

#10 posted 01-12-2014 11:54 PM

I may have an alternate theory.

For my profession I operate a 2,500 watt laser CNC table to cut up sheets of up to 3/4” thick 4’x8’ and bigger of mostly cherry veneer covered MDF and ply. You would be amazed how often I see actual sparks, (not embers) every few minutes being blown around on the cutting area and walls. As if you were grinding steel.

The sparks are from very small metal chips in the man-made wooden sheets embedded during manufacturing from the maker’s knives that get burned up as the laser beam strikes them (inside a nitrogen jet coming out of the laser head to keep oxygen out of the beam’s path). I say this as one specific suppler we use is known for metal chips in their MDF, and more than the normal “Fourth of July fireworks”.

I am suggesting there might be a small metal fragment imbedded in the wood, and part of the blade is striking it creating the sparks. But I would think it would wear away quickly as each tooth stuck it.

If that is not plausible it may just be some sort of electromechanical spark being generated as was originally suggested.

Now I’ve done it. This is going to haunt me for several days thinking about it…


-- I'm not old. Just "well seasoned".

View TopamaxSurvivor's profile


24921 posts in 5167 days

#11 posted 01-13-2014 12:40 AM

ChuckV, My guess it is certainly a possibility. Any change in the characteristics of the sensing circuit could tigger it.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View jumbojack's profile


1691 posts in 4115 days

#12 posted 01-13-2014 04:58 AM

perhaps a high concentration of silica in one of the layer of ply or the material you are splining?

-- Made in America, with American made tools....Shopsmith

View exelectrician's profile


2339 posts in 3919 days

#13 posted 01-13-2014 06:52 AM

Yep! you have magnetic induction going on, the weld material could be heating to melt point and sparks fly when the circuit breaks. Or it could be induced current sparks are jumping across the tiny lazer gaps in the blade.

-- Love thy neighbour as thyself

View jfk4032's profile


384 posts in 4018 days

#14 posted 01-13-2014 01:56 PM

Thanks for the physics theories and lessons everyone…Without the magnet in place there are no sparks now, so problem solved. At least from my end…for you guys still hypothesizing about the real reason, go for it!

-- ---Joel; Central MD...rookie empter nester and getting back into woodworking!

View Robsshop's profile


923 posts in 4466 days

#15 posted 01-13-2014 03:18 PM

Very nice spline jig design and construction. I have plans to build one in the near future and will most likely build something similar to yours, only I will probably use mine in conjunction with my TS sled.

As to your mystery fireworks show I guess it is somewhat possible for the mag theory but I am going to have to agree with Woodstock’ s explanation on this one ! I use to work with my Dads construction buisiness and have cut my fair share of plys over the years and can tell you that there is a lot of misc. junk mixed inbetween the layers. I have seen the sparks fly as well and have found that the lower grade plys are usually the worst !

Once again nice jig and sharp looking cutting board, hope you figure out the mystery and thanks for sharing !

-- Rob,Gaithersburg,MD,One mans trash is another mans wood shop treasure ! ;-)

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