Knife Grinding Jig

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About: I am a young engineer who enjoys making and restoring whatever I can in my free time. I like the challenge of making new things with the tools I have available.

I was recently looking for a knife grinding jig so that I didn't have to free hand it if I didn't want to but they were all in the $70 to $150 range and there was no way I was going to pay that when I knew I could make one myself. I then started searching for DIY options and came across this: Adjustable Angle Knife Grinding Jig. I liked how it used a spring hinge which made this simpler and more cost effective. I have access to a decent amount of scrap steel so I modified the design to be more solid and include a couple additional features.

Step 1: Materials

Below is a list of materials and tools that I used to make this. This could be easily modified depending on what you have available.

Materials:

  • Adjustable Spring Door Hinge (I used the largest 4.5 inch size for stability and strength but smaller sizes should also work)
  • Bolt (Must be fully threaded and recommended to be at least 4.5 inches long. I used a 5 inch long 5/16 bolt.)
  • Cap Nut/Acorn Nut that fits your bolt
  • Steel Plate (I used a 7.5 x 3 inch and 6 x 2 inch piece of 1/4 inch thick steel. I originally used a 6 x 3 inch plate for the front plate but changed it to a 2 inch wide plate later)
  • Steel Block (Around 4 x 1.5 x 0.75 inches)
  • Plastic sheet to reduce friction on the bottom (must be the size of the bottom plate, 7.5 x 3 inches)
  • Assortment of bolts (These can vary depending on what you have available. Future steps will highlight what bolts I used)

Tools:

  • Angle grinder or hack saw
  • Saw to cut Plexiglas
  • Drill press and drill bits
  • Tab and die set
  • Sander/grinder/file to clean up edges
  • (optional) 3D printer for knob

Step 2: Attach the Hinge

The hinge needs to be attached to both plates. I used the hinge as a template for where to drill the holes and then drilled and tapped the holes to accommodate the bolts that came with the hinge. There should be a little gap so that the plates do not rub against the round part of the hinge when it rotates.

Step 3: Bolt Post

To be able to adjust the angle, a bolt is used that pushes against the hinge. I used a small chunk of steel, not sure the exact dimensions but probably close to 4 x 1.5 x 0.75 inches, that I bolted down to the base plate and threaded a through hole to allow the bolt to thread into. I set the block at the back of the hinge so that I would have a large angle I could adjust to in either direction. You will need a bolt, preferably two, to be able to attach the post to the plate. I used two 1 inch long 1/4 inch bolts.

Step 4: Reducing Friction

I have seen other designs use a plastic plate on the bottom and I though it was probably there to reduce friction but I wanted to look up the friction amounts myself just to double check. For those who are curious, the following are the coefficients of friction for steel on these various materials: Hint, lower is better.

  • Steel on Steel: 0.7
  • Polycarbonate (Lexan) on Steel: 0.45
  • Acrylic (Plexiglas) on Steel: 0.2
  • Nylon on Steel: 0.1

As you can see, adding any of the above plastics on the bottom will allow it to slide easier. I only had a scrap piece of Lexan that was large enough but it still reduces the friction some. This layer also provides clearance for the bolts that hold on the bolt post.

I used the bottom plate as a template to drill clearance holes for the bolts for the hinge and the bolt post. I ended up switching from hex head bolts to button bolts for the bolt post because there was not enough clearance to turn them with the Lexan plate on. You could of course just put on the bolts and then the Lexan if you didn't want to switch.

I then drilled a hole in each corner and countersunk it so that I could use a small flat head bolt to attach the Lexan to the bottom plate.

Step 5: Getting It Ready

Now that it was mostly complete it was time to tighten the hinge and start cleaning it up.

To set the hinge, you need to use an hex key wrench to twist the side of the hinge and then insert the small pin when you have twisted as much as possible. The tighter the better.

Because I used the bolts that came with the hinge, the bolt end stuck out of the front of the plate. I just adjusted the front plate to be parallel with my grinder and sanded them down to be flush with the plate.

Step 6: Adjustable Angles

Because the bolt post is pretty far back and I used a long bolt, I have a large degree of adjustment. I can pretty much go 45 degrees in either direction of vertical.

Step 7: Smaller Front Plate

As I stated before, I didn't like how big the 3 inch tall front plate was so I replaced it with a 2 inch tall plate. If I were to lower my tool rest on my grinder then the 3 inch would work. Depending on your need, any sized front plate would work.

Step 8: Test It Out

If you want, this is as far as you need to go to have a functional grinding jig. I made some more additions that I will show in the next steps

But, I wanted to test out the jig at this point to just see how well it worked. I just did a quick grind and was really happy with how much faster it was to get a good even grind.

Step 9: Addition 1 (Set Screw)

During my test grind, I found that it was easy to accidentally adjust the angle when grinding. I originally wanted to put in a set screw but held off because I wasn't sure it needed it but after the test grind I think it would be very helpful. Definitely recommend adding this.

Step 10: Addition 2 (Knob for Bolt)

It was difficult to adjust the bolt by just turning the end of it so I decided to 3D print a simple knob that I glued to the bolt to make it easier to turn the bolt. If you don't have a 3D printer then you can just use a wrench , socket set or buy a knob like This One to turn it. I have included the Tinkercad link to the knob I made for my 5/16 bolt below.

Tinkercad Link

Step 11: Complete

This is as far as I have taken this design at this time. If you are interested, below are some ideas of potential additions.

  • Put a grid of threaded holes on the front plate so I can use bolts to hold knives in place,
  • Add knob or handles to the back of the bottom plate to give you better handles to move it around.

If you have any recommendations on improvements or additions please let me know in the comments.

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    25 Discussions

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    Thank you. It's been really handy for grinding all sorts of things when it needs to be at a specific angle.

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    yourbadneighbor

    2 months ago on Step 11

    I would put a nut on each side of your long bolt to lock the angle you set. I would think the set screw would mess up the threads after a short time. The 2 nuts would keep it much more stable.
    The rest of the design seems perfect! Great work!

    4 replies

    Because the set screw only needs to be tight enough to stop the bolt from moving, it didn't appear to mess up the threads in my testing but there is that possibility. I like the ease of the set screw over having to mess with two nuts every time I want to adjust it. I may just have to replace that screw every so often. But if you would rather, using two nuts is definitely a great option.

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    ArthurJ5IJustLikeMakingThings

    Reply 2 months ago

    I have two ideas here: 1. Use a wingnut for one of the nuts maybe with a split washer. Then you only have to tighten finger tight. 2. Use a small bit of brass rod the diameter of your set screw hole. Make the brass rod length twice the diameter of the hole to keep it from turning around in the hole.

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    IJustLikeMakingThingsArthurJ5

    Reply 2 months ago

    I like the brass rod idea. I think I have some brass rod that I can use between the set screw and bolt. I'll try it out and update this if it works well.

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    mikeczyourbadneighbor

    Reply 2 months ago

    I was thinking the same thing about how the set screw would slowly ding up the threads on the adjusting bolt - and thought using extra nuts, as you suggest, would solve the problem!

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    ArleyB1

    2 months ago

    Very Cool and a great tutorial . Got my vote

    1 reply
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    DonS89

    Question 2 months ago

    Do you just move the fixture parallel to the belt or is there some movement required to sharpen the curved portion? If just parallel then wouldn't a track help?

    4 answers
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    IJustLikeMakingThingsDonS89

    Answer 2 months ago

    Sort of, you move it parallel to the belt but you also have to move it closer or further into the belt as you grind and based on the shape of the knife. As you get to the curved part, you need to move it closer to the belt so that the grind follows the blade profile. If you didn't then you would have a sharpened edge that would taper off to a dull tip.

    It seems to me you need one more tweak to this device to make it repeatable. After all, that is the whole point of what you are doing by keeping the MAIN angle the same along the length of the blade. If you had a flat, non-curved blade, you could just run the blade across the grind platform left to right and have a repeatable angle along the length.

    Addressing this problem DonS89 describes I would think requires a contoured template of some sort such that as you move the blade across the belt the blade point of contact is kept normal(@ 90°) to the direction of belt travel. For example as the flat edge is moved right( across the belt) and the curve enters the belt grind, the right side of the blade needs to dip downward in the same curve pattern as the left end of the blade. Doing so continually moves the blade edge so it remains normal to the cut line. I envision a curved template projecting from the right side of the grind platform and angling down with the particular blades curve pattern. The template likely needs some reasonable width to allow one to hold your "device" at the same angle as set. If made from Ferrous metal if you incorporated a strong magnet into your device's base it would help hold it against a template that may be only, say 1.5" wide. Hopefully, my description makes sense!

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    desbromilow

    2 months ago

    to reduce the thread turning easily, simply drill a cross hole to make whipper snipper cord (strimmer line) and insert a length in so it rubs on the side of the thread - if needed you can then put a smaller screew/ bolt in to increased the force of the strimmer line against the thread to further reduce moevemnt, but without marking up your thread.

    2 replies

    I've seen it used in a number of model engineering groups - it has the advantage/ principle of a "nylock nut", but built in where you want it. Some people place the strimmer line at right angles to the thread, other people drill a hole parallel to the axis which intersects the thread profile and do it that way (this second method is not adjustable for pressure)

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    dave5201

    2 months ago

    Nice job.
    With regard to the knob you made, you can also get some very nice (and inexpensive) knobs from Amazon and other places. Most of the smaller plastic knobs I used to replace old ones on my milling machine had a threaded metal insert. The larger ones had a set screw. A wise old machinist once told me "Don't make hardware that you can buy for cheap."
    OTOH, I have been known to make knobs myself when I could not find something commercially available that I liked. Last year the plastic knobs on my auto air conditioner gauge manifold fell apart, I could not find suitable replacements anywhere incl the mfg. So, I made some of my own. They are much better than the OEM's.

    2 replies
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    IJustLikeMakingThingsdave5201

    Reply 2 months ago

    Thank you. I had looked at buying knobs but they were about $10 for a pack of ten and with my 3d printer I could make and print one in about 2 hours for just a couple cents worth of material. I didn't need ten of them and this knob wasn't going to see a lot of load so I just went with the quicker and cheaper option for me and printed one. But definitely a great option for those who just want to buy a knob. I'll make a note of this on the instructable.

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    dave5201IJustLikeMakingThings

    Reply 2 months ago

    I hear you. I don't have a 3D printer, but have a metal lathe and Bridgeport mill. We work with what we have...good point.