Warps during grinding

D

Don Nguyen

Joined
Oct 4, 2011
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There have been a couple times where I encountered warping during grinding. It's always with a coarse belt (24 or 36 grit), by the way.

Using a strong magnet to hold the blade will induce a warp. Trying to remove large amounts of material using a wheel also induces a warp. I haven't figured out in what way each will affect the warping exactly, I just know to be careful when going those routes.

Anyone else ever encounter such issues? What's the culprit here - heat? Are there tons of stresses taking place with heavy grinding?
 
I have ground a lot with stone wheels is a tool room and here is what I have learned
Heat, depth of cut, dull wheels, Heavy cross feed, lack or low coolant, stress releaved or not, type of material being ground, total amount being ground off and other variables come into play.
Here is a real basic tip.
Put the billets on the table on a angle. Increasing the angle, lighter cooler cuts and finer feeds is a good place to start.
When you are grinding listen to the grinding sound. How does it change from using a sharp belt to a dull one.
Are you getting burn marks on the billet? If so you Are getting it too hot.
PM me or ask some questions if I am unclear.
Dave
 
Thanks for the reply David.

I am only doing this on a standard belt grinder, not surface grinder, so there isn't much precision with my methods. The steel is completely annealed 1084 from Aldo. I was temporarily using some cheaper belts, maybe that could be the reason? They don't cut that well and started burnishing fairly soon, even for a coarse grit.
 
Hi Don,
My Goof,
I read "using a strong magnet", Put together with the Idea of the belt sander I came up with the modified surface grinder people use.
Humbly,
Dave
 
Yes there is a bunch of stress with heavy grinding.When you remove a large amount of metal from one side and include the heat from grinding the steel will want to curl into the ground side,
Swap sides more often during grinding especially on thick stock, try to remove metal equally from both sides.Also when finished grinding it is good to do stress reliveng cycles before H/T.

Stan
 
Yeah, problem is most likely heat. Even thicker metal is pretty bad for it. Alot of welders have this problem when they weld their plate without quenching/cooling it in the trough. Heavy grinding would do it pretty easily seeing as alot of heat is being generated. Try stopping every couple of minutes to dip the metal you are working on, even if the water isnt at an extremely cool temperature it will still help to fight the warping in the metal.
 
I dip pretty often, but today at the grinder I had to get on the anvil to straighten out the blades a couple of times. I'm not getting them hot enough to even show any oxidation colors. I do use a fair amount of pressure and a relatively high speed though.

I haven't heard of other people having this problem as often as me though.
 
As said, a lot of stress and heat build up in rough shaping. On some steels, I get the blade red when grinding. If using a magnet, it pulls the softened steel toward the magnet when hot. then the steel cools quickly as you remove it from the belt ( and maybe dip it in the slack tub). This "sets" the bend caused by the magnet.
Most of the time, I just straighten the blade regularly.
However, these stresses are sort of locked in the blades metal structure, and can come back to haunt you in HT.....unless you deal with them first.................. A normalizing cycle and a little straightening take care of it permanently by resetting the structure.
 
I grind each side down without taking too much off of one side at any one time and work the plunge lines up each side a bit at a time. By doing this two things happen 1) is not creating stress risers that may cause warpage during HT, and 2) it allows me to check my work often avoiding the mistake of grinding off too much. Work with a fresh belt to reduce heating and the tendency to push the work against the belt.
 
Just a thought, could the culprit simply (or in addition to the other issues) be the pressure you're putting on the metal? What if you took a knife you're working on and push it up against your bench as hard as you'd push it on the grinder to see if it bends? If that is the case then you can use a push stick to get the pressure you want without having to force the blade in a bending motion.
 
What steel Don?

I've seen some amazing warp on powdered steels that still have the can metal on. It usually goes back the other way when you switch sides but it can make for a challenging task.
 
Stan is pretty right on about this. You should try to grind in a balanced fashion, back and forth between sides to reach finished size/shape. I've had this problem for sure, a LOT more when I went to grinding chef knives- 2" tall by 10" long and .100" max thickness warps badly if you try to aggressively rough it into a blade from a forged blank, one entire side at a time.

The worst I've had it happen was grinding some AEB-L at a friend's house. Kind of shocking, the extent of it. I've seen it happen a lot to a milder extent in 52100 and W2. The biggest thing, I think, is removing material and applying heat to the steel unevenly. If you keep an eye on it, when the steel starts to cup towrd your grind, switch to the opposite side of the blade in the same spot to pull the warp back out of it. That is, if you still have the option of removing material there. That's why flipping the blade fairly often helps.

Another thing I noticed, which I'm sure does not help, is I will take heavy grinding passes the length of the blade, while keeping the push stick in the same place. Even if the push stick is in the middle of the blade, between heel and tip, it runs out into space by a couple inches on either side during a full grinding pass. That is to say, much of the time it is applying more of a bending force than the directly supporting force that I need when grinding hard. So, I tried to take shorter grinding passes and move the push stick more often, so as to directly apply the blade to the platen when grinding. It seems to help.

Also, Stacy is right on about normalizing after grinding. I do it anyway, whether there is grind warp or not. If there is grind warp, so much the better- you can straighten the blade hot and it will "set" nice and flat.
 
OTK said:
What steel Don?

I've seen some amazing warp on powdered steels that still have the can metal on. It usually goes back the other way when you switch sides but it can make for a challenging task.
Click to expand...

1084 from Aldo. It was completely annealed, but I've seen similar effects with hardened steel too.

I don't get much heat at all from grinding, even pre-HT. I have been using a lot of pressure and grinding off more on one side before switching though. It's just so tempting to do that and try to get off as much steel as quick as possible.
 
I recently experienced the same thing with the same steel and a brand new sharp 36 grit ceramic belt, but I was doing a thin chisel grind. That probably had a lot to do with it.
 
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