Help! Hollow Grinding!

M

Mike Jones

Google Master
My grinder is finally finished (mostly... few things left) so I thought I'd give a wing at hollow grinding. First off, 60 grit belt, at 3500 sfm, it goes it goes pretty fast!

I took this up to 400 grit, but I'm having trouble holding the same exact position for the length of the blade, and then repeating it. I've uploaded a few pics of what I've done. Any suggestions on how to blend the lines at least?

hpim1841q.jpg

hpim1842u.jpg

hpim1843.jpg



Thanks for any advice.
 
Best thing I could tell you is to find a knifemaker close and go watch them hollow grind. it is hard to tell someone without actually showing them. it looks like you need to lock your elbows in to your body to hold the blade steady. how are you supporting the blade on the point end to keep it from rocking?
 
Mike PM me and you can come over and see and use my set up. It's really easy to use and you might get an idea or 3 to modify your grinder with. It looks like your not holding the knife steady while drawing it across the wheel.

Jason
 
My take on grinding

Mike,
I'm no pro but I have hollow ground a few. It would help to diagnose any major technique errors like the things Mr. Miller is talking about if you described your process of grinding in as much detail as possible.

From my experience it should go something like this.
1. Scribe centerline of the blade edge.
I actually scribe 3 lines on the edge. center and one .010 on each side of center.

2. Begin grinding edge facing up at approx. 45 degree angle down to the outside lines on both sides.
Now you have a tiny hollow on each side and from now on every time you approach the belt you'll be trying to land right back in the hollow. It's going to be difficult to land perfectly in the existing grind while it is still a very short and shallow grind but will get easier as the grind grows in height and depth.

3. Use a very light touch to get back on the hollow then apply light pressure toward the spine so there is a small gap between the edge and the belt and grind in smooth passes the length of the blade.
Remember to keep switching sides every few passes to walk both grinds up the blade together. Also keep the tiny gap between edge and belt until the last few passes. No gap means your thinning the edge instead of removing steel from the top of the grind.

Other thought from one newbie to another-

Listen to the sound the belt makes while grinding. You'll notice a difference between being perfectly flat on the hollow and grinding toward the spine.

Are you using a push stick to support the blade with your off hand ? I use a piece of maple with a shallow notch cut in the end that creates a little shelf for the spine of the blade to sit on.

Keep in mind that small mistakes early in the grinding process are usually ugly but meaningless. If you ride out of the hollow toward the spine it'll be ok until you start getting close to final dimensions.

I may also consider grinding a few blades with an 80 grit or even 120 to start. It sounds counter productive to grinding and it is. However, it is much better to make mistakes slow and learn from them. One of the first things to learn is how to come back to the belt in the same place each time without messing up. It's IMHO a matter of practice and feel so having to do it alot in the begining is OK.

I find when approaching the belt to keep a light touch and err towards the edge side NOT the spine side and things will be going fine in no time. A small divot right on the edge is usually erased anyway when thinning the edge down to final thickness but taking a bite outta the spine can be a much bigger problem.

Hope this helps ya out Mike
-Josh
 
Mike
First of all congrats on the grinder! I wish I had gotten in to this when I was your age, hang with it buddy.

I agree it looks like you are rocking your blade. It is hard to explain how to hollow grind without showing you. The thing that helped me the most is lock your arms, put your elbows to your side and dont move them. Move your whole body back and forth to move the blade across the belt. I stand in front of the grinder some stand to the side.

Also once you put the blade on the wheel make a complete pass, resist the urge to take it off and look at it. I hold the knife with my thumb on the ricasso and use it to apply pressure toward the edge or spine depending on where I want to work the grind, with my off hand a I let it the spine of the blade ride on my index finger and my other thumb on the blade (makes a good heat gauge:eek:) Your off hand is just there to act as a guide.

Most of all dont rush it! It will come to you.

I hope this helps some, I know it is as clear as mud but it is much easier to show than explain.

Good luck

Sean
 
I can't really add anything to whats been said. I can just say again, Set that groove first. Then just go back to it next pass. It is easy to adjust to the edge or the spine later in the process.

Paul
 
Thanks everyone for your answers/suggestions. Especially Josh Dabney for such a detailed explanation of what is supposed to be done. I also watched Jarod Todd's video on it and it made even more sense. I went back and REALLY locked in my elbows, and went back to 60 grit, and I think it turned out pretty nice.
hpim1845.jpg


I hope the dip in the grind height won't be too bad once I start finishing it.
 
Mike,
Your already making great progress !
Another tip. Wrap a piece of painters tape around the blade where you want you plunge to stop. This gives a visual reference that is easy to see while grinding your bevels.

This will asist in getting nice even pluges. Stay away from the tape a little with your roughest belts and refine the plunges with a sharp 220.

Another thing from my experience is this. Dont use XY weight belts to get a nice plunge. I haven't been able to get anything remotely nice with a stiff belt. I think it is possible but I cant do it. A 220 J-Flex or norton Norax will curl around beautifully if run 1/8" over the edge of the wheel/platen and all but elliminate the belt riding over the edge of the plunge and chewing up the ricasso.

Just another observation from my experience

-Josh
 
Josh, those were great explanations. I have nothing much to add to that.

Mike,

Steel is cheap and most of the time in making a knife comes after this stage. That is a big improvement over what you started with. But once the grind lines get that washed out it is hard to bring it back. Personally, I'd write that one off and try again with your improved skills. I'll bet the next one is even better.
 
Here's another tip that has saved a blade for me already.

Profile your blade with a little extra steel on the edge than you want your final profile to be. Getting the edge too this is a common newbie error so after you get to thin you can grind a little off the edge and VOILA... sweet !

Guess who taught me that trick.... Yup Nathan the Machinist. Who incedentally I met at the ginding demo at blade show and is as cool a guy in person as on the forums. Thanks for that one Nathan !

-Josh
 
Well then I guess I can't go wrong with what Nathan says. I decided to scrap the blade, but it HAS taught me a lot about working with this grinder, and hollow grinding. Maybe tomorrow I can work on a new one.
 
Josh Dabney said:
Here's another tip that has saved a blade for me already.

Profile your blade with a little extra steel on the edge than you want your final profile to be. Getting the edge too this is a common newbie error so after you get to thin you can grind a little off the edge and VOILA... sweet !

Guess who taught me that trick.... Yup Nathan the Machinist. Who incedentally I met at the ginding demo at blade show and is as cool a guy in person as on the forums. Thanks for that one Nathan !

-Josh
Click to expand...

Cool man. And the knife you showed me was excellent.

Please let me clarify a little, my grinding "tip" was mostly about grinding tips. I tend to lose a little control when grinding a pointy profile - so I have found it helps to leave a little meat behind the tip until you're done, and finish the profile last.
 
Nathan The Machinist said:
I'll dust off this example

Here is an extreme example of what I'm talking about

knife_before_and_after.jpg


This was almost completely ground before the profile was finished.
Click to expand...

Nathan,
You know late at night when you are watching that infomercial about the super magical spaghetti cooker, and you say to yourself "why didnt I think of that?"

Yep thats how I felt when I saw your post, great "tip". Ill put this one to use sir.

Sean

Just noticed new smiliesShort Bus
 
Nathan The Machinist said:
I'll dust off this example

Here is an extreme example of what I'm talking about

knife_before_and_after.jpg


This was almost completely ground before the profile was finished.
Click to expand...

Do you suggest that I shape the edge, and then grind it, and then come back and finish the rest of the knife? It seems a little weird to me, but I could try it.
 
Congrats on finishing the new Grinder.
You are wearing safety equipment when you grind I hope.
I a have only been Hollow Grinding about 6 months but I will share a couple tips.
Grind both sides to the spine together. Dont try to get one side almost finished before starting the other.oops
If you are freehanding the grind you can look at your grind line on the side thats facing you to get the lines to match.
Know when to step away from the grinder. If your not "feeling it" then take a break and come back later.
Practice.
 
Go Mike said:
Do you suggest that I shape the edge, and then grind it, and then come back and finish the rest of the knife? It seems a little weird to me, but I could try it.
Click to expand...

No, not really. I think you should follow Josh's suggestions and get your "groove" working. I was simply clarifying something he had said.

You will find that once you have a nice wide well established hollow you will learn how to follow that hollow and make adjustments. You will also find that the tip can be challenging (some more than others) because there is no room to form a nice wide well established hollow. In that case you can leave some meat in the profile that you can remove once your grind is finished. The example I posted is an extreme example.
 
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