Question re: homemade jointer knife grinding jig

[mjl5007]

Quick question for those that have used/are familiar with the homemade jointer knife grinding jig described in PTWFE and in the Jointer manual -- the one made from a 6" scrap of 2x4 with a saw kerf slot cut into one long edge at a 45° angle.

Are the wood screws intended to screw down onto the "top" of the knife while it is in the jig in order to clamp it in place?

The drawings for the jig show that the holes for the wood screws used to clamp the knife into the slot should be placed 3/8" back from the front face of the jig (the face with the slot cut into it), and spaced 4-1/4" apart. from each other.

My jointer knives (both sets -- the ones installed on the jointer and the extra set in the "box o' goodies" that came with the machine) seem to be shorter than 4-1/4", which means that the knife will fit between the two screws, rather than the screws actually coming down and clamping directly into the knives.

Is that correct, or am I missing something?

Also, while I'm at it -- how crucial is it that the slot be exactly 1/8" wide, so that the knife fits snugly in the slot without any play? I'm using the ShopSmith 50T combo blade, the kerf of which is slightly less than 1/8" (very close to 1/10", I believe), so I had to make the slot in two passes, adjusting the blade position with the quill ever so slightly.

[ERLover]

I am doing some guessing and surmising here. If using wood screws and set wider then the blades, they are there to pinch the wood tight, not hold the blades like a set screw pressing against the blade.
I am sure if you did the second cut with the saw blade to the correct width you are fine, the end result is the same.

[mjl5007]

I had the same thought -- but then I re-read this page, and noticed the part where it mentions to "be sure the point of your screw will engage your Jointer Blade, when inserted into the slot." That would seem to indicate that the screws *are* supposed to act like set screws and clamp the knife directly, rather than indirectly by pinching the wood together.

Of course, the article also says to put the screws 3/4" from each end of the 6" long block... which means the screws will be 4-1/2" apart, which is even wider than the 4-1/4" mentioned in the Jointer manual and PTWFE. Given that the Jointer blades are 4" long, 4-1/2" spacing would seem to not work at all.

That said, this afternoon I re-ground the spare set of jointer blades that came with my SS -- they had several large nicks in them, so I figured no big deal if the grinding doesn't go well as they're otherwise useless anyway. I used the jig with the screws at 4-1/4" spacing; tightening the screws did indeed pinch the kerf slot enough that I couldn't get the knife back out, but it did not hold it entirely immobile, because the knives are wedge-shaped; they still were able to slide back and forth in the kerf between the screws (only a small amount, maybe 1/16-18"), and I could also pull them about halfway out. Still, I went for it, and the results weren't terrible. Took quite a few passes and the addition of several thicknesses of paper between the jig and rip fence to get the nicks out, but the knives certainly look much better. However, I discovered that at least one of the knives was not ground straight -- it is slightly convex, tapered off at one end, but only very slightly. I used the jointer's outfeed table as the reference when checking for straightness. Not sure how that happened, as I made sure the knives were fully seated in the kerf slot. And I don't really fancy grinding them again trying to straighten them up, lest I remove too much material.

If only we had a decent sharpening service in town... or any sharpening service, for that matter.

[reible]

I don't own nor have I ever owned a shopsmith jointer but I'm wondering if the design has changed over time and if the change might not be reflected in the old PTWFE design verses how things are now.

I think the jointer head and wedges changed at some point but again I don't have any personal experience.

Ed

[ERLover]

I had the same thought -- but then I re-read this page, and noticed the part where it mentions to "be sure the point of your screw will engage your Jointer Blade, when inserted into the slot." That would seem to indicate that the screws *are* supposed to act like set screws and clamp the knife directly, rather than indirectly by pinching the wood together.

Of course, the article also says to put the screws 3/4" from each end of the 6" long block... which means the screws will be 4-1/2" apart, which is even wider than the 4-1/4" mentioned in the Jointer manual and PTWFE. Given that the Jointer blades are 4" long, 4-1/2" spacing would seem to not work at all.

That said, this afternoon I re-ground the spare set of jointer blades that came with my SS -- they had several large nicks in them, so I figured no big deal if the grinding doesn't go well as they're otherwise useless anyway. I used the jig with the screws at 4-1/4" spacing; tightening the screws did indeed pinch the kerf slot enough that I couldn't get the knife back out, but it did not hold it entirely immobile, because the knives are wedge-shaped; they still were able to slide back and forth in the kerf between the screws (only a small amount, maybe 1/16-18"), and I could also pull them about halfway out. Still, I went for it, and the results weren't terrible. Took quite a few passes and the addition of several thicknesses of paper between the jig and rip fence to get the nicks out, but the knives certainly look much better. However, I discovered that at least one of the knives was not ground straight -- it is slightly convex, tapered off at one end, but only very slightly. I used the jointer's outfeed table as the reference when checking for straightness. Not sure how that happened, as I made sure the knives were fully seated in the kerf slot. And I don't really fancy grinding them again trying to straighten them up, lest I remove too much material.

If only we had a decent sharpening service in town... or any sharpening service, for that matter.

Maybe typos, call the MS on Monday and ask for a clarification!!!! Couse, Cuzz, then we are just guessing and surmising here. Get it from the source in real time and voice to voice. It isnt that hard.
Cordially,
Anne Sullivan

[mjl5007]

Maybe typos, call the MS on Monday and ask for a clarification!!!! Couse, Cuzz, then we are just guessing and surmising here. Get it from the source in real time and voice to voice. It isnt that hard.

Sure, I can definitely call and ask the MS -- but it was the weekend, so nobody to answer the phones there, and I figured *somebody* here has to have made and used this jig in the decades since it was first published and might be able to clarify things for me, so I thought it was worth posting the question.

As for changes to the Jointer over the years that might explain things, you may be right -- I'm not familiar with all the details of the Jointer's history -- but either way, the jig design and dimensions I'm talking about are right out of the Jointer Owner's Manual that came with the Jointer I have in my possession, so I think it's fair to assume that the jig described in the manual should match up with the actual piece of equipment that the manual came with.

I'll give the MS a call and post back with what they say.

[JPG]

Where are the jig plans in PTWFE/version?

[mjl5007]

Revised Edition (1984), Ch 20 (Care and Maintenance), p. 337.

[dusty]

I was going to read what PTWFE said about this but my edition offers no plans for a jointer blade sharpening jig. But if I may, I will offer 'the obvious' to this discussion. The mechanism (be it a slot or otherwise) must be such that you achieve repeat ability every time you insert a blade and prep it for sharpening. The three blades need to be very close to identical.

If I was to build such a jig, I would make it so that all three blades could be inserted simultaneously. My thinking being that this is a more reliable way to get identical results.

Do not over look the fact that the blades are tapered 2°. The tapered face is the surface that will become the reference surface when the blades are installed in the jig.

You may well not be interested in that deviation but I mention it just in case. It will have an effect on the resulting bevel angle.

[JPG]

The two screws squeeze the jig to securely hold the knife.