Machine finish grit size

[Hengelo_77]

I practised some free hand hollow grinding on an old blank that was subpar.
Now I'm actualy quite happy with the result, and I may even get it HT'ed.
This is grit 240.

If you do a machine finish to what grit size do you take it?

 

[Joshua Fisher]

I practised some free hand hollow grinding on an old blank that was subpar.
Now I'm actualy quite happy with the result, and I may even get it HT'ed.
This is grit 240.

If you do a machine finish to what grit size do you take it?

180-220 followed by either a medium scotchbrite or a cork belt loaded with some cutting compound works great for a belt satin finish. A scotchbrite will wash out lines a bit but I’ve found that to be less of an issue with hollow grinds than other types of grinds, cork won’t wash out as much, other structures abrasive belts work well also. Trizact/compact grain belts/norax id suggest trying a bunch and see what you like best. My belt progression currently is 36-50 in ceramic (I like the vsm 880 and vsm actirox) 120 in either the same ceramic or zirconium belt (been trying the zirconium belts recently cause they are cheap and cut sharp so the cost balances out with a ceramic but is a bit faster and you change belts sooner) from there my favorite pre polish belt is felt backed aluminum oxide belts like the Deerfos line, I use a 180 in that belt then go to my scotchbrite if doing a satin finish. For higher grit finishes I’ll either go up in the deer belts to 320-600 then hand sand or trizacts in that range. I really only go up higher in belts if I want to hand sand. I’ve also seen some phenomenal finishes just going up to 1200 in the deer belt then buffed with black compound and skipping all hand sanding.

 

[tim37a]

Hi Hengelo,
I am going to start hollow grinding my kitchen knives. Yours looks pretty good - what size wheel did you use?
Tim

 

[Joshua Fisher]

Hi Hengelo,
I am going to start hollow grinding my kitchen knives. Yours looks pretty good - what size wheel did you use?
Tim

I know the question wasn’t directed at me but I make my living mostly from kitchen knives I make and I do hollow grinds 99% of the time, I’d recommend either a 14”-20” wheel or get a 24-36” radius platen, the small the wheel the shorter your grind will be which isn’t as desirable. I do all of my hollow grinds on a 36” radius platens now and it lets me bring the grind as high as I want. Here’s and example of a 36” radius hollow on a 7” Bunka.

 

[Hengelo_77]

Hi Hengelo,
I am going to start hollow grinding my kitchen knives. Yours looks pretty good - what size wheel did you use?
Tim

Thnx I have a 11,5" - 29cm wheel.

And Joshua, thnx for the replies

 

[tim37a]

I have a 14" wheel which I haven't had a chance to try yet. I would like to get a 36" radius platen but I don't know anywhere to buy one. Any suggestions about who makes them?

 

[Joshua Fisher]

I have a 14" wheel which I haven't had a chance to try yet. I would like to get a 36" radius platen but I don't know anywhere to buy one. Any suggestions about who makes them?

Brodbeck ironworks just started making them and I believe they are setup for multiple machines. I have mine setup here with a mister for rough grinding. I have another bracket for this platen assembly so I can quickly change between this and my flat platen.

 

[Scaniaman]

220 grit on hard wear resistant steel, sometimes red or blue Scotchbrite after.

On other steels 400 grit. I can see no reason to go higher than that on a knife that actually will see use.

 

[_Dan_]

320 personally.

 

[fitzo]

I have a 14" wheel which I haven't had a chance to try yet. I would like to get a 36" radius platen but I don't know anywhere to buy one. Any suggestions about who makes them?

Contenderworks .com sells a number of sizes of radius platens and a magnetic water chiller to slap on the back. Darren is very easy to work with.

 

[tim37a]

Thanks Fitzo and Joshua. I am going to buy a 36" platen and cooler. Watch out, world !!!

 

[Joshua Fisher]

Thanks Fitzo and Joshua. I am going to buy a 36" platen and cooler. Watch out, world !!!

Get a mister over a cooler, a cooler only helps the platen the mister help the platen and grinding blades.

 

[_Dan_]

Get a mister over a cooler, a cooler only helps the platen the mister help the platen and grinding blades.

I use flood water from my house plumbing - IMO it's necessary to have coolant on the contact area. The temperature you feel in your hand is not close to what it is at the abrasive contact. Iron is a relatively poor conductor of heat. It also helps with belt loading.

 

[fitzo]

Thanks Fitzo and Joshua. I am going to buy a 36" platen and cooler. Watch out, world !!!

You're welcome, sir. Enjoy!

BTW, Darren's magnetic chiller can be had with a tapped hole and a spray attachment. It works quite well, and does keep the blade cooler even without the mist, but the caveats mentioned above are reasonable.

Water spray or flood is ideal if you are set up for the mess. I'm not; I still keep the dunk bucket close. But, then, I'm old, set up the basement shop 30 years before all these fancy chillers and sprays, don't wear gloves, and grind real slow anyhow, making knives that aren't for sale. I don't much fit into the new paradigm, a dinosaur from an earlier time. I just happened to know where you could get radiused platens.

Good luck, and enjoy !

 

[Sando]

Funny you mentioned the radiused platen. I just made one recently. I didn't want to spend the $$ until I tried it out. Works pretty well. I'll see how long it lasts, but it was free.

I used 3/8" mild steel - 8" x 2". Then I scribed the arc using 36" of welding wire taped to the floor and the steel in a clamp. Like this:


(OK That shows string, which failed because it stretches. Welding wire worked great.)

I milled along the arc to save time. Then I finished it on the grinder until it matched the line (more or less).




It works:




(And no I haven't sharped it yet.)

 

[mooniesdl3]

Contenderworks .com sells a number of sizes of radius platens and a magnetic water chiller to slap on the back. Darren is very easy to work with.

I've got a 72" from there, with the magnetic water chiller, works very well. I've been thinking about getting some other sizes.

 

[Joshua Fisher]

You're welcome, sir. Enjoy!

BTW, Darren's magnetic chiller can be had with a tapped hole and a spray attachment. It works quite well, and does keep the blade cooler even without the mist, but the caveats mentioned above are reasonable.

Water spray or flood is ideal if you are set up for the mess. I'm not; I still keep the dunk bucket close. But, then, I'm old, set up the basement shop 30 years before all these fancy chillers and sprays, don't wear gloves, and grind real slow anyhow, making knives that aren't for sale. I don't much fit into the new paradigm, a dinosaur from an earlier time. I just happened to know where you could get radiused platens.

Good luck, and enjoy !

My high tech solution to minimize mess, actually keeps 90% of the water off of me haha

 

[fitzo]

View attachment 2099825
My high tech solution to minimize mess, actually keeps 90% of the water off of me haha

That’s a nice idea. I wasn’t thinking about myself, actually; an apron would take care of that albeit not as cool as your idea. I have experience with 4x108 diamond belt wet sump grinder in a glassblowers shop and remember that mess clearly, LOL.
No, I had in my mind the area around my grinders, and the two all-steel belt driven early KMGs. I would have a rusted out mess in no time. I have tried misting, and it is unfortunately not for my setup. Wish it was. I’d steal your board idea in a moment and say thank you, sir.

 

[Joshua Fisher]

That’s a nice idea. I wasn’t thinking about myself, actually; an apron would take care of that albeit not as cool as your idea.
No, I had in my mind the area around my grinders, and the two all-steel belt driven early KMGs. I would have a rusted out mess in no time. I have tried misting, and it is unfortunately not for my setup. Wish it was. I’d steel your board idea in a moment and say thank you, sir.

Haha it beats a rain coat, my trick for preventing rust on all surrounding surfaces is 3-4” thick layer of dust to protect them. I actually don’t have much near my grinder so it works but I definitely see how it doesn’t work for every shop.

 

[Taz]

I got my 36" radius platen from AmeriBrade. I made a platen backer using a regular platen and then attaching rare earth magnets to the platen, so I just need to swap platen faces. I have several platen faces (mild steel, radius platen, glass, leather, different hardness of felt, etc). Doing something like this, you can use any steel radius platen on your grinder. Makes things simple and not having to order from a certain brand constantly! I covered my radius platen with leather as well, so I can go up pretty high grit wise with a good finish.

For a machine satin finish, I go up to A30 belts, Trizact Gators or Norax, or a 320 Compact Grain AO belt or something similar and then go to ScotchBrite or Cork belt loaded with compound.

Magnacut at 63 HRC with a satin belt finish:

CPM 20CV with a high polish satin belt finish. I went up to SunMax Structured abrasive M5 belt and then the loaded cork belt after that. I use the felt and leather platens to allow me to get smooth belt finishes: