ADvise on grinding on belt sander

[Rasmus Kristens]

Hi guys

Until now i have ground the primary bevel on all my knifes with a bastard file and a grinding jig.
This weekend i modified my 4x36" to accomondate knife grinding in a better way than out of the box.

The result is this:



My question is: Is it more dificult to grind with a belt sander and grinding jig than i expected or am I just useless? ;-)

This is the firs blade where the primary bevel is made on the belt sander:



As you can see, the grind line is a bit wobbly, and i'm not sure if im doing anything wrong or if my setup on the belt sander is doing something it shouldn't?

The grinding jig is a angle iron withe a bolt and a locknut to controll the belvel angle.

I already see quite some wear on the steel table. Its just from a piece of 0.4x4.7" mild steel. Is this to soft to use?

[jimmontg]

First don't worry about wear on your table. Put brass nuts on the bottom of your angle iron to set your grind bevel.
Now is the belt in any way bouncing as it goes over your spacing plate?
Or does the belt tend to wobble slightly side to side, even a tiny bit? I'd bet it does and that is the belt riding right on top of the the bow in the top wheel. On that type of a sander that is an idler wheel, first check and see if it is loose a little bit. If it is a little bit try to bend the arms in that are holding the roller by putting a wedge on the outside arm after putting the belt on. Also you can take a file and take a little bit of the center of the bow on the wheel. Not much, and experiment until you see the belt stays steady and does not move side to side.

Also when you go to grind your knife do not push the belt sideways with too much pressure, let the belt do the work.
Try this last sentence first if there isn't any wobble to speak of unless it wobbles when you start grinding which just means you'll have to take some of the crown off the top wheel, but not too much, it needs some crown, but its made for a wider belt than you are using.

Hope that helps Rasmus.

[Rasmus Kristens]

Hi Jim.

Thanks a lot, that gives me a lot to work with. The belt does tend to slide across the platen.

When you say file off some og the crown of the idle wheel, do you meen just in the center of the wheel or in the total width of the sanding belt 2"?

[Rasmus Kristens]

Okay i went straight to the task.

I turned out the idle wheel had about 1 mm sideways play within the ball bearings, so I modified a spacer to fit and now there is zero play within in wheel! Great success! :-)

Then i bend the holding arms in, that also gave significantly more stability.
Finally i sanded off some of the bow on the idle wheel.

Now im just exited to put the kids to bed tonight so I can go to "My room" and test it out.

As always thanks for the support.

[jimmontg]

Just a little in the middle of the wheel, I noticed it has a line in the middle. Those 4x36 belt grinders usually do have a wobble issue. They are made for sanding wood and not so much for metal.

[Rasmus Kristens]

Yes i know they are for wood, but with my skill and the amount of knives i create i cant defend spending the money on a "real" knife belt sander.

So this will have to do for now :-)

But your tips sure helped a lot!
I still need to work on my technique, when i make contact with the belt or leave it again, i sometimes hit it at an angle which is one of the reasons i have uneven grindlines, i'm working on improving that.

[jimmontg]

Just a matter of lining up your plunge line and being very close then move straight in and move to the side evenly/gently, don't press too hard that's what heavier grits are for, do not try to make 80 grit do the job of 60 grit. You'll get a feel for it, just a matter of practice.
Do you have a belt cleaner?, Helps the belts last longer.

[Rasmus Kristens]

No i don't, they are quite expensive here.
I can get 5 belts (which i divide in to 10x2") for the same price.

Is it worth to buy one then?

How should i progress through grids?
I have 40, 80 and 120 grid for belts so far. Can i jump to a 240 or shoud i get a 180 before?
I grind the primary bevel before HT, should i take the finish to a higher grid than 120 before HT?

[jimmontg]

Seriously, use a 240 from 120 is fine, I do it and have done it. On your last grinds with 120 go light. Where the belt cleaner really works well is on the higher grits that clog up easier.

Now for the last few years I use a tiny bit of cutting wax. I mean tiny bit. I will rub it on the belt by hand and get it across the whole belt and lightly grind. It will spread itself. Whatever you do don't put it on while the belt is running straight from the tube or you'll have a big mess. A little goes along ways or is cutting wax too expensive there too?
\
A tube lasts for a long time, I've had mine for 8 years and use in on my bandsaw for cutting metals too. I use it to tap and thread screws with a little bit of Tap Magic added. It is called Stick Kut, but there's dozens of types and some are made for grinding like this stuff is. By the way, don't mix your metal belts with lube on them with your wood belts if you can help it as it tends to leave little pieces of metal mud in the wood.

[M&J]

Nice that you are able to adapt a tool to improve your efficiency! Good job!

No worries on the uneven grind, that can be cleaned up after heat treat when the steel is harder. Getting it as consistent does help save some time in finishing though. Having an "oops" moment when soft is one of those jump up down sessions. My process is to go back in following HT to clean and straighten the cut if it is close enough. It blends when rubbing the blade out.

An option for your grinding jig is to use a phenolic base on the fixture. May also cut out a new work surface top to put in place over the machine table to preserve it. I do that with the bandsaw since it cuts steel and wood.

Thanks for the tip on grinding wax Jim! Reminds me that I've been using the same stick of Tap Ease for the past 20+ years. Down to the last inch now.

My process is to end at 120 grit though I do some light clean-up with 180 or 220/240G. That depends what belts are on hand for that. In the coarse stuff I run 80-120-180/220G progression typically. When they come back from HT I'll use a structured abrasive starting at A100.