Plunge Line Problems

[Hugh Buchanan]

I have been making knives for only a short time. I am having a problem creating symmetrical plunge lines.

• I use a 4x36 grinder with the side rails cut out, creating a 4" platen.
• I use a jig made of steel angle to hold the work at a constant angle.
• I built a large table and base, with a steel top on which to manipulate the jig. The grinder is bolted into this table and base.
• I am using aluminum oxide belts from Combat Abrasives.

My problem is that I am unable to control the shape of the plunge line from one side of the blade to the other. One side will have a vertical and square plunge line, while the other side will develop a curved, sweeping plunge line. I have tried to control the shape by hanging the belt past the edge of the platen. According to Walter Sorrels, this should produce a curved, rather than a straight and vertical line. This doesn't seem to provide consistent results. The shape of my plunge lines seems to depend as much on the phase of the moon as it does on the overhang of the belt.

Any suggestions will be appreciated. I am reluctant to grind one more piece of metal until I know how to control the shape of my lines.

 

[Randy3000]

What do the edges of your platten look like? Are they worn out and lumpy? You said you cut some sides off, is it a smooth cut?

 

[navman]

After grinding in the bevels, I use a filing guide and a chainsaw file to even up the plunge, then, with the file guide still on the blade, a piece of 120 grip paper glued to a surface block with a radiuses edge to clean up the plunge.

Other than that, practice and checking often is key. Start with your weak side grinding, but do not grind to the plunge.

 

[AVigil]

Break in the edge of the belt with a piece of steel , run the grinder with the belt overhanging and take some steel and grind the steel while getting the belt to bend over the edge of the platen.

Do it for both sides.

Also do not PLUNGE at the plunge line. Use a slight sweeping motion almost as if you are using a hair brush on tangled hair.

Sweep little by little and take it easy.

If you grind to fast, to hard and to deep you can not see where you start to make the mistake,

 

[HSC ///]

how is the tracking on your grinder?
consistent and true? or does the belt bounce around?

another option is a carbide file guide and a 9" disc grinder which gives you a sharp plunge but even and consistent
or file guide and file in straight bevels.

 

[Hugh Buchanan]

What do the edges of your platten look like? Are they worn out and lumpy? You said you cut some sides off, is it a smooth cut?

I wasn't very handy with an angle grinder when I cut these, but the cuts are smooth and are parallel to the line of the belt. I will take another look at them to make sure that the cuts are perpendicular to the surface of the platen.

I spent the day (subsequent to the initial post) practicing on some cheap pieces of mild steel bar from Home Depot. I discovered that the jig and the belt are not going to shape the metal automatically. The angle to which the jig is set is not the angle that is impressed on the blade. There is something funky happening between the blade and the belt that I can't see. For example, if the jig makes an angle of 20° with the belt, the bevel angle the belt grinds on the blade will be some larger number, say 30°. In order to make the bevel climb toward the spine, I have to exert considerable pressure near the spine. I found that using pressure in this way will combine with the belt overhang to shape the plunge line. With my set-up, I use pressure and belt position to shape the plunge line from the edge to the highest point near the spine. When I have the shape and depth of the plunge line I want, I blend that line down the blade to the point.

I was successful in grinding three blades this evening using that method. The three blades were not identical in shape and depth of their bevels, but each one was symmetrical in shape and bevel size from one side of the blade to the other.

Many thanks to those who responded to my request for help. I will incorporate these ideas as I progress.

 

[Hugh Buchanan]

how is the tracking on your grinder?
consistent and true? or does the belt bounce around?

another option is a carbide file guide and a 9" disc grinder which gives you a sharp plunge but even and consistent
or file guide and file in straight bevels.

The tracking leaves something to be desired. It is a Harbor Freight grinder. The tracking roller probably needs to be machined (I saw someone do this on YouTube). The belt oscillates about 1/32 of an inch at the grinding point, so I need to be careful near the plunge.

I spent the day (subsequent to the initial post) practicing on some cheap pieces of mild steel bar from Home Depot. I discovered that the jig and the belt are not going to shape the metal automatically. The angle to which the jig is set is not the angle that is impressed on the blade. There is something funky happening between the blade and the belt that I can't see. For example, if the jig makes an angle of 20° with the belt, the bevel angle the belt grinds on the blade will be some larger number, say 30°. In order to make the bevel climb toward the spine, I have to exert considerable pressure near the spine. I found that using pressure in this way will combine with the belt overhang to shape the plunge line. With my set-up, I use pressure and belt position to shape the plunge line from the edge to the highest point near the spine. When I have the shape and depth of the plunge line I want, I blend that line down the blade to the point.

I was successful in grinding three blades this evening using that method. The three blades were not identical in shape and depth of their bevels, but each one was symmetrical in shape and bevel size from one side of the blade to the other.

Thank you for taking the time to help.

​

 

[Hugh Buchanan]

Break in the edge of the belt with a piece of steel , run the grinder with the belt overhanging and take some steel and grind the steel while getting the belt to bend over the edge of the platen.

Do it for both sides.

Also do not PLUNGE at the plunge line. Use a slight sweeping motion almost as if you are using a hair brush on tangled hair.

Sweep little by little and take it easy.

If you grind to fast, to hard and to deep you can not see where you start to make the mistake,

Thanks, Adam. I will try to incorporate the techniques you offered. It sounds like good advice.

See my reply to the other suggestions. I think I worked out some of the issues and was able to put respectable bevels on three blades this evening.

 

[Hugh Buchanan]

Navman:

Thanks for taking the time to help. Since I'm not grinding freehand, it will be difficult to incorporate a guide. I spent the day practicing (as you suggested) on some cheap steel bar stock and figured it out. I solved the problem by applying hard finger pressure near the spine. Using this technique in conjunction with belt overhang, I was able to shape the bevels consistently and grind three acceptable blades this evening.

Thanks again!.

After grinding in the bevels, I use a filing guide and a chainsaw file to even up the plunge, then, with the file guide still on the blade, a piece of 120 grip paper glued to a surface block with a radiuses edge to clean up the plunge.

Other than that, practice and checking often is key. Start with your weak side grinding, but do not grind to the plunge.

 

[Hugh Buchanan]

Adam:
I looked at your knives. They are beautifully made!

 

[Rhinoknives1]

You have been given some good tips for using that HF Grinder you have, My 2 cents is to not dump a lot more money into that HF machine. They work well for certain tasks, but they aren’t a persision machine and you best save for a quality 2 X 72” machine if you wish to go to the next level of knife making with proper tracking and the availability of the best belts are in that size.—————I wish you the best no matter what you do!

 

[Steve Beckwith]

My solution to a consistent plunge grind is to turn the belt around so that I grind both plunges with the same edge of the belt.

 

[Hugh Buchanan]

You have been given some good tips for using that HF Grinder you have, My 2 cents is to not dump a lot more money into that HF machine. They work well for certain tasks, but they aren’t a persision machine and you best save for a quality 2 X 72” machine if you wish to go to the next level of knife making with proper tracking and the availability of the best belts are in that size.—————I wish you the best no matter what you do!

Good advice. Thanks Lawrence.

 

[Hugh Buchanan]

Steve:

I like that idea.
Thank you.

 

[Josh Rider]

You could file the plunges in before hand with a chainsaw file. Then just grind the bevels to blend in.

 

[skillgannon]

A carbide file guide can help.

 

[Laredian]

I personally have never had success tracking the belt off the platen to grind my plunges. I always felt I had way to little control to shape them that way, but that is probably only the cause because I don't have the skills for that specific method.

What works for me is starting your grind not where you want your plunges to be in the end, but about half an inch or so down the blade from where you want your plunges. That way you can concentrate on working your grind up and getting nice, sharp and consistent grind lines. Then you can take a file guide to the spot where you want your plunges and very slowly carve away the steel until you blend in with your previously done main bevel grind. It's important not to hog into the edge of your platen at this point because if you do, you won't be able to get a nice sweeping curve in your plunges.

If you try this you probably still won't be perfectly even and symmetrical on both sides, which is absolutely normal and no problem at all. If you have established somewhat decent plunges with a file guide you can then clean everything up with a small needle file. It is just a matter of how much time you are prepared to invest and quite a bit of elbow grease.

Also gator belts work very well to blend your main grind into your plunges, because the abrasives on those belts are so evenly distributed and shaped. The main advantage in my opinion is that those belts are more forgiving if you can't hold your workpiece flat on your platen, which most beginners struggle with. If you apply uneven pressure or lift one side of your workpiece up from the platen, you "dig" in and create an ugly high spot on your grind line. This does not happen as quickly with gator belts, also they leave a very nice surface finish.

Edit: Basically the best advise I can give is try multiple things and try to work out what works best for you. There are some amazing smiths and knifemakers on here and I dare say probably none of them use exactly the same methods when grinding plunges, forging etc... They may be very similar, but everyone has their own little deviations. So learn as many methods as possibly, but don't stick to them religiously...adapt them to you and your needs.

 

[Hugh Buchanan]

I personally have never had success tracking the belt off the platen to grind my plunges. I always felt I had way to little control to shape them that way, but that is probably only the cause because I don't have the skills for that specific method.

What works for me is starting your grind not where you want your plunges to be in the end, but about half an inch or so down the blade from where you want your plunges. That way you can concentrate on working your grind up and getting nice, sharp and consistent grind lines. Then you can take a file guide to the spot where you want your plunges and very slowly carve away the steel until you blend in with your previously done main bevel grind. It's important not to hog into the edge of your platen at this point because if you do, you won't be able to get a nice sweeping curve in your plunges.

If you try this you probably still won't be perfectly even and symmetrical on both sides, which is absolutely normal and no problem at all. If you have established somewhat decent plunges with a file guide you can then clean everything up with a small needle file. It is just a matter of how much time you are prepared to invest and quite a bit of elbow grease.

Also gator belts work very well to blend your main grind into your plunges, because the abrasives on those belts are so evenly distributed and shaped. The main advantage in my opinion is that those belts are more forgiving if you can't hold your workpiece flat on your platen, which most beginners struggle with. If you apply uneven pressure or lift one side of your workpiece up from the platen, you "dig" in and create an ugly high spot on your grind line. This does not happen as quickly with gator belts, also they leave a very nice surface finish.

Edit: Basically the best advise I can give is try multiple things and try to work out what works best for you. There are some amazing smiths and knifemakers on here and I dare say probably none of them use exactly the same methods when grinding plunges, forging etc... They may be very similar, but everyone has their own little deviations. So learn as many methods as possibly, but don't stick to them religiously...adapt them to you and your needs.

Laradian:

Thanks for taking the time to reply. I appreciate your comments and having the benefit of your experience.

Break in the edge of the belt with a piece of steel , run the grinder with the belt overhanging and take some steel and grind the steel while getting the belt to bend over the edge of the platen.

Do it for both sides.

Also do not PLUNGE at the plunge line. Use a slight sweeping motion almost as if you are using a hair brush on tangled hair.

Sweep little by little and take it easy.

If you grind to fast, to hard and to deep you can not see where you start to make the mistake,

Adam:

I had to think about your suggestion before I understood it properly. You are suggesting that the belt backing, not the abrasive should be broken-in. You're saying that the belt has to be more flexible to produce a good plunge line.

 

[Hugh Buchanan]

Adam:

I had to think about your suggestion before I understood it properly. You are suggesting that the belt backing, not the abrasive should be broken-in. You're saying that the belt has to be more flexible to produce a good plunge line.

 

[AVigil]

Adam:

I had to think about your suggestion before I understood it properly. You are suggesting that the belt backing, not the abrasive should be broken-in. You're saying that the belt has to be more flexible to produce a good plunge line.

Right, 50 and 100 ceramic belts are stiff and do not like to flex. This can give you a right angle plunge line if you are not careful.

I over hang, lets say a 50 grit Blaze belt" 1/4" over one side of the platen and take a bar of steel and while the belt is running use the steel and bend the belt over the edge of the platen. Repeat on other side.

This will allow for that stiff belt to bend when working the plunge as the belt hangs over a wee bit