Hollow grinding

Sean Yaw

Sean Yaw

Gold Member
Joined
Feb 26, 2019
Messages
417
When you are walking the top of the bevel towards the spine on a hollow grind, how do you ensure you don't grind a trough out of the middle of the blade? I.e., How do you make sure the thinnest part of the ground blade is at the edge, and not somewhere behind the edge? This is not an issue with flat grinding, but on a wheel, I am having a hard time seeing how to prevent this. Are there any references or videos you recommend for learning to hollow grind. Everything I have found on youtube just does it without discussing angles or blade placement on the wheel, or anything. Thank you.
 
I have been trying some hollow grinds recently and looked up some old threads to make sure I'd seen all the tips.
I have had the same worry, but haven't had it happen yet.
I'm doing some straight razors with a 2.5" wheel. Those are nearly straight so I set up the rest so that the edge just reaches the height of the axis. Then I can hold the spine flat on the rest and push it into the wheel. Those are relatively easy.
The trickier one is with a 10" wheel and a curve towards the tip. I ground holding perpendicular to the edge so I had to finesse the curve with no rest. I found it useful to increase belt tension and adjust the speed until it doesn't grab during a light touch. I wasn't happy with it one day (the tip was a series of poor passes), but came back the next and really got in the groove and cleaned it up nicely. I think you just have to use a light touch and keep checking frequently to be sure you are on the right track. I keep calipers right beside the grinder.
 
I dont have much experience in hollow grinding, but for me it was more about where you put the pressure and not about where you are positioned on the wheel or what angle you take. Do your 45s and focus your pressure towards the edge. The hollow grind calculator is a good point to start so you rougly know what grind height you should have for a given stock thickness and wheel size.

Another lesson learnt by mistake, if starting with a low grit belt, leave more on the edge then you planned for to account for the difference of belt thickness in comparison to higher grit belts.
 
I ruined a blade I had spent a long time on this afternoon and was quite frustrated, which led me to my initial post. I've been having a heck of a time taming this new 2x72 + wheel I got. I went out to do some practice tonight and realized that at least part of my problem is that I am having ice accumulate on the belts which was making it really hard to keep the blade in contact. I guess grinding in the garage at 5 below has some drawbacks...
 
Just don't use too small of a wheel and you will be fine. There are calculators online that can tell you what wheel to use for a given stock thickness and grind height...
 
Right wheel size for the job is the answer. If you get to where a 16" wheel won't do it, use a radiused platen.
 
Your more than perfectly fine using that new 12" wheel doing slip joint blades. It's definitely a feel thing. I went from flat grinding with a jig. To hollow grinding with a jig. To freehand. I had a hell of a time completing a freehand grind on my 12" wheel. I had read somewhere that going to a smaller wheel had helped a few guys to quickly get a feel for freehand hollow grinding. I was so frustrated I said screw it and bought an 8" wheel and I'll be damned if it didn't help me 100%. As you know, I don't use my 8" at all anymore but it really helped me get a feel for what was going on and where and how to apply pressure. It gave me confidence. Once you step up to a big wheel you gotta approach it with confidence and own it or it'll get away from ya. I also started using a cheap Wells Lamont Neoprene palm coated glove on my guide hand. They don't last long but you'll stay focused on the work and not on grinding your knuckles off. Some guys like to use magnet or sticks but I like to feel the blade personally. Next, grinder speed. Maybe I'm doing it wrong but through an entire knife build with exception to surface grinding my vfd never goes above 30. I grind my 60 and 120 belt at like 19 - 20 setting. When I go to the higher grits I'm running at like 12ish. Bottom line, slow = more control. Lastly, as you already know this but practice! And by that I mean don't waste an opportunity. Your going to screw up. Hell I'm so new I still screw up at least once at some point in almost every blade I grind. What has helped me to be successful almost everytime though is I didn't quit on those early blades too soon. If I screw a grind up I keep going until I fix it or until the blade is whittled down to nothing. Even if I hit the point I k ow a blade is scrap I keep grinding on it with the intent of learning something. You will learn a lot in failure but if you give up too quick and toss a blade in the scrap bucket at the first mistake and don't take the time to try and learn to fix it your not maximizing your time or really learning anything. Anyway, sorry for the long post. Admittedly I still have a lot to learn when it comes to grinding a blade but I figured I'd share some of what has helped me get the point I'm at now. Good luck! Your a smart and talented maker Sean. You'll pick it up quick.
 
A bit odd maybe, but I went from flat platen grinding straight to hollow grinding on a 500 mm / 24" wheel. And I found it much easier to master the grind on the wheel 🤔 Easier to keep the transitions clean and crisp, and since there is less heat building up that keeps you focused too.
 
Do what John said.

I will add you need to make a groove and then progress up the blade. Do not just grind pass after pass without establishing a groove.

When you have the groove the wheel will ride in it and you then apply pressure to move it up the blade.
 
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