Author Topic: Back bevel / cutting bevel -should I only touch front when I have a good nick?  (Read 8033 times)

Offline Boston_turner

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I may be putting too much together, but I am about to try this new jig for the first time *and* use it on my 20" planer blades off a Powermatic 209.  The blades are laminated HSS

I was trying to be careful, but still managed to put a board through with a nail buried in it.  Nice 200 year old American Chestnut, but with a nail...

I have a pretty good sized nick in the edge of all 4 knives, though one is worse than the others.  The cutting bevel cannot be more than 1mm wide.  The back bevel is about 5mm (1/8" thick, 40 degree back bevel).  The cutting bevel is supposed to be 35 degrees according to Powermatic.  If I work out the nick I will likely enlarge the primary bevel, perhaps doubling it.  This will eventually double any resharpening time down the road.  On the other hand tackling the back bevel would be very time consuming I imagine. 

I'm already dreading a bit the 8 hours I will probably need, based on other people's observations on this forum.  People have talked about taking 2hr per blade and on shorter blades.  My hope is that this primary bevel is narrower than the others.  That could speed it up. 

What do you all think I should do?  Just focus on the front cutting bevel and not worry about it getting a lot wider? 

Thank you for your help.

Offline Jeff Farris

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Typically on double bevel knives, you would take approximately equal amounts off both sides. How deep is the nick in the worst blade?
Jeff Farris

Offline Boston_turner

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As near as I can measure it is the full thickness of the front bevel.  I took a look with a digital caliper and that seems to be about 0.5mm.  The back bevel is around 4.6mm

I can't see how to upload a picture, that might clarify.  Is there a way to do this?

Offline Herman Trivilino

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You have to upload your photo to someplace like photobucket, and then post the link to the image here.
Origin: Big Bang

Offline Boston_turner

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Thanks. 

I hope this proves useful, or at least interesting to others who might cringe and be glad it is not your blade!

https://docs.google.com/file/d/0B5KBTk26mpinblFFOGtiTnMxMjQ/edit?usp=sharing

Offline Jeff Farris

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That's not too brutal. I would take an equal (or thereabouts) amount off both sides, keeping the bevel size close to original.
Jeff Farris

Offline Boston_turner

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Thanks for the advice.  I'll do that.  I plan to tackle the front bevel first, get that in good shape and then go after the back.  I will plan to take it down by as much but I may call it a day if it takes more than 8hrs / blade  :-\

Offline Jeff Farris

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It won't. Keep your stone clean. That's the disconnect.
Jeff Farris

Offline Boston_turner

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I thought anyone reading might like to know how this worked out. 

The process was complicated by a problem with the jig, which I'll get to. 

First, as the instructions say, true the stone.  I confirmed that it was 90o to the side with a try square.  I then setup the jig but found that getting it parallel to the stone was very difficult indeed.  I was only able to do it by first locking down the left post and then cranking the right post as hard as I could.  I was just able to get it parallel as judged by paper or shining a light through but I almost needed a pipe wrench to do it.

This meant that I could not just evenly drop the posts by turning both nuts the same amount.  I had to overshoot the drop on the left and then come back up by forcing the right side.  It was finicky and took many tries but in the end it was ok. 

I did the back bevel first but had a decision to make.  The Powermatic 209 manual says the cutting angle should be 35o.  The knives that came from Powermatic with the planer were clearly not set that way.  The back bevel was 40o and the secondary bevel was about 5o more.  I chose to reproduce the knives as shipped since they were producing amazingly good results on a range of hardwoods. 

I was surprised by how quickly the process went.  After setting the initial contact I dropped it 0.3 and that removed the secondary bevel completely, but just.  It actually only took about 10 min to do that, maybe less. 

After doing all the knives on the back bevel I reset for the front bevel.  That was quite fussy as well.  I had to raise the posts by around five full turns or more and gauged the angle by sighting along the blade behind the stone. 

When that was parallel and set right I dropped 0.3 again.  In about 5 minutes or less it set the front bevel.  In hind sight I should have only dropped about 0.1, that would have been sufficient.  The front bevel is at least 2x fatter than it needs to be. 

I could tell that the surface was getting finer gradually over the course of doing the back bevel on all four knives just by looking at the grind pattern.  Only once the front bevel was done on all knives did I reset the stone to fine and took them through again.  Lastly I spent a little while on honing and got pretty good results overall. 

I checked the edge against my machinist steel (true to less than 0.0005" over 35").  I had a slight dip at the knife ends.  I think I put too much pressure on the ends as I was grinding. The worst had a dip of 0.011", not too bad overall. 

After mounting then with the Powermatic jig I checked them with a dial indicator.  Most were about 0.019 but one had a slightly higher middle of around 0.035".  For what I need I think that will do for now, at least I didn't want to start over. 

I think I can get them all to less than 0.010 the next time but I plan to send that jig back first.  It should not be that hard to get it parallel.  The setup took about 2+ hrs.  The actual grinding of all 4 knives, including back and front bevels and honing took maybe 1.5 hrs.  If the jig had worked as described the setup would have been 15-20 minutes.

Based on my experience it is hard to understand the time needed as reported by some.  If I had to guess though I would suspect that your jig might not be truly parallel.  If so, you would be cutting with a much smaller surface area and it would clog up much faster. 

I have read numerous reports of the jig not getting parallel.  I suspect this is a bigger problem that Tormek might realize.  It suggests to me that a redesign would be helpful.  Perhaps you could make one of the posts have an internal acme thread to do the initial setting for the center of parallel and the fix that with a screw.  Then you could continue with the base adjustments as you have them.

The current design demands a lot from the weld on the jig posts.  Any imperfection and you have a problem like mine. 

Offline Herman Trivilino

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First, as the instructions say, true the stone.  I confirmed that it was 90o to the side with a try square. 

I don't think that's what Tormek calls true.

Quote
I then setup the jig but found that getting it parallel to the stone was very difficult indeed.

Use the truing tool to make the surface of the stone parallel to the universal support rod.

Origin: Big Bang

Offline Boston_turner

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I guess I should have been explicit.  I did use the truing tool and then confirmed that the surface was flat and perpendicular.