Thread: Surface Grinding Thin Parts

If the blades are oriented perpendicular to table travel, is it ok to grind a bunch of them at once?

Wondering if any heat is retained in the wheel or if it all goes into the workpiece and swarf?

I've always used the shim-stock method to mitigate warped parts from being pulled flat by the magnet. You can also use a sheet of copier paper between the part and the magnet to give a slight amount of cushion without sacrificing magnetic draw. When you have two solid surfaces being pressed together, one little .0005 particle or burr can translate into .0005 of warp. Paper helps alleviate that.

A lot it can just be helped by pre and post op inspection. If you know your chuck is flat to the necessary tolerance, take your parts over to the granite and sped some time with a test indicator and sharpie marker. After you know which area's are out by how much, you can counteract them with shim stock when you load up the chuck. Grind one side with the shim stock, then the other side without, then re-check the parts on the granite and see how close they came.

With that, I've found that thinner wheels are better (less contact means less heat, they just take longer), minimal depth of cut per pass with lots of time sparking out lets the part cool and relax while the cutting is still happening.

Lots of coolant..... I'm in the party that says always use coolant unless the material can't have it (which has been ultra rare for me). I'm surprised how many guys refuse to mess with coolant because "it makes a mess" and "you don't HAVE to have it." IMO it boils down to having as little heat as possible on ANYTHING you grind, regardless of how thin it is, but thinner means it's more sensitive to heat. An added bonus is that while you might think that coolant contributes to mess, in the long run your grinder and it's surrounding area will be much much cleaner, because the coolant is keeping all the metal and abrasive dust inside the machine instead of just letting it fly out into the shop.

Ultimately: 1. counteract any and all movement that happens when the part is pulled down to the chuck, and 2. keep heat to a minimum, both in when it is generated during the cut, and in how quickly it is pulled away from the work.

Originally Posted by AD Design

First off I’ll have to state that I’ve never made a knife, or anything else, from M390. My reading up on it tell to be an excellent material, but my experience with the properties are zero so use your own experience/judgment with any of the following comments.

Grinding has many variables and all relate to the time/money ratio. Time spent can mean more in operating cost per piece IF that’s an important consideration. Reducing time per piece may/may not be important depending upon how much trouble gone through for how much time saved.

Money spent on better tools/equipment can save time but will enough time be saved to make it worth the money spent? Only you can answer this question. Having said all that consider the following:

Orientation of work- Yes it’s a best practice to orient the work perpendicular to wheel/table travel to reduce time-in-grind for a given pass and reduce the generated heat. If not much heat is generated and warping or burning marks aren’t present the work can be set parallel for less overall grind time.

Not much perceived heat- That’s likely because you’re only taking .00025-.0005 depth of cut (DOC). If you’re ok with how long it takes then no change is needed. If you want to reduce the time then a deeper DOC will be required. This may generate more heat and call for measures to counter the heat.

Coolant- No perceptible difference? If you increase the DOC you may very well want to use it

Double sided tape- Only used it for rubber or other non-ferrous work, never trusted it due to generated heat loosening the adhesive

Wheel Grit- Using an 80 is most often used for finer detail dressed on the wheel or very minimal grinding. I’ve very rarely used an 80-100-120 grit wheel and only when a form had to be closely matched or I was “picking out” the geometry of a detail. Flat grinding like you’re doing doesn’t require a fine grit. Yes you may get a better surface finish with the 80 but you’re using less than .001 DOC. Use of 46 wheel is for greater DOC (less time grinding) with less heat.

Surface Finish- Just based upon looking at the photos (I may be wrong here) you should be getting a better finish with the 80 @ DOC you’re using. You need to use a sharp diamond for dressing the wheel and there are techniques for dressing that affect the final surface finish. Dressing the wheel at .0015-.002 per pass is good (variables here too). How fast you traverse the wheel while dressing affects the surface finish too. Are you verifying the entire periphery of the wheel is being dressed? Photos look like either vibration, skipping, or partially dressed wheel showing “waves” in surface. How are finishing the surfaces after heat treating and grinding?

All of the above are to reduce the amount of grinding time. If you’re ok with what you’re doing then no change is needed and also may not be worth the money spent. That’s all up to you to decide.

Vise- I didn’t elaborate on a vise because I needed to see what you’re making. It appears that a custom set of jaws would need to be made to contact enough points/surfaces. This will only serve to better hold the knife blank when grinding. Increasing DOC also increases the force that’s pushing on the blank. Might/might not be worth the trouble and expense for custom jaws dedicated to ONE model. Blocking in is cheaper but doesn’t grip like a vise/fixture will. This is especially true when grinding the bevels.

You state that M390 is magnetic (good) but how magnetic is it? When on the mag-chuck how much force is needed to move it? This is where the balance of DOC, heat, and blocking all come into play.

My apologies for the long-winded post. I hope some of this helped. Others may have additional thoughts. I’m not the last word in grinding, especially on an unfamiliar material like M390

Thanks for your reply. I imagine my surface finish isn't great because of some minor vibration, or lack of skill (maybe both). I am very new to this and Ill be honest its much more complex than I assumed. I will try orienting the work perpendicular and I may try a 46 grit wheel. From what I can tell I'm not introducing any additional warp but I'm not removing it much either. I think I will try to shim using a dial indicator before and after and see how that goes.

Originally Posted by M.B. Naegle

I've always used the shim-stock method to mitigate warped parts from being pulled flat by the magnet. You can also use a sheet of copier paper between the part and the magnet to give a slight amount of cushion without sacrificing magnetic draw. When you have two solid surfaces being pressed together, one little .0005 particle or burr can translate into .0005 of warp. Paper helps alleviate that.

A lot it can just be helped by pre and post op inspection. If you know your chuck is flat to the necessary tolerance, take your parts over to the granite and sped some time with a test indicator and sharpie marker. After you know which area's are out by how much, you can counteract them with shim stock when you load up the chuck. Grind one side with the shim stock, then the other side without, then re-check the parts on the granite and see how close they came.

With that, I've found that thinner wheels are better (less contact means less heat, they just take longer), minimal depth of cut per pass with lots of time sparking out lets the part cool and relax while the cutting is still happening.

Lots of coolant..... I'm in the party that says always use coolant unless the material can't have it (which has been ultra rare for me). I'm surprised how many guys refuse to mess with coolant because "it makes a mess" and "you don't HAVE to have it." IMO it boils down to having as little heat as possible on ANYTHING you grind, regardless of how thin it is, but thinner means it's more sensitive to heat. An added bonus is that while you might think that coolant contributes to mess, in the long run your grinder and it's surrounding area will be much much cleaner, because the coolant is keeping all the metal and abrasive dust inside the machine instead of just letting it fly out into the shop.

Ultimately: 1. counteract any and all movement that happens when the part is pulled down to the chuck, and 2. keep heat to a minimum, both in when it is generated during the cut, and in how quickly it is pulled away from the work.

Thanks,
I will try the shim method and see how it goes. I will post my findings. Thanks for all the tips.

I have done a lot of grinding as a tool and die maker with over 40 years experience.
1. Find out if you can buy precision ground flat stock close to the size you need.
2. If you have to grind use a soft coarse wheel like a A36 (46} H - Vitrified.
How to Read a Grinding Wheel Spec
3. Raise the bolted back stop on your magnet to just below the your finish size.
4. Find two pieces of material below your finish size to be used for blocking.
5.Put one piece against the back stop, than your work piece sliding it up against the blocking material and slide the second blocking piece up against your work.
6. Make sure you put the concave side of your part down on the magnet.
Some times it helps if after positioning your work you remove it and turn on the magnet.
Than turn off the magnet. Do NOT de-magnetize! This will help to hold your blocking material in place.
7. Put your work piece back in between your two blocking pieces and lightly tap the one blocking piece against your work. Grind using low feed and lots of cooling.
Another way of grinding real thin pieces is by using double sided carpet tape. Your parts are still good thickness.
8. After cleaning up one side flip your part and grind with magnet on.

I was taught to put wet newspaper under the thin work.

We had cluster diamonds as roughing dressers. We would notch the center of a 1/2 inch wheel with a single point diamond and then touch up dress with the cluster while leaving the groove there. It was like having a rougher and cleanup cut in one pass.

One grinder had a flood nozzle and we had misters on the others.

Originally Posted by Stang Bladeworks

Thanks for your reply. I imagine my surface finish isn't great because of some minor vibration, or lack of skill (maybe both). I am very new to this and Ill be honest its much more complex than I assumed. I will try orienting the work perpendicular and I may try a 46 grit wheel. From what I can tell I'm not introducing any additional warp but I'm not removing it much either. I think I will try to shim using a dial indicator before and after and see how that goes.

I occasionally grind thin blades for injection molds, and after heat treat they are more likely than not to have some bow. I always inspect them on a surface plate to find the concave side, which I want down, then measure to determine what size shim in the middle will will keep the work supported. Keep in mind when shimming work on the magnet there will be very little holding it, so block it in well.

I once had the misfortune to have some S-7 blades come back from heat treat bowed more that the grind allowance I had left. These were approximately 4" x .75 x .125 and had some slots and holes, and I figured it was worth some time to try to straighten them, since they were scrap as they were. I used my 6" mill vise and three dowels; two at the ends on one side and one opposite in the middle. It's surprising how much they had to be over-bent to have any effect. I took the first one and squished it in the vice, making note of the handle position, then checked it on the surface plate; back to the vise and swung the handle a bit further, repeated until it was within my grinding allowance, then did the rest the same. Worked just fine, and didn't lose any.

One thing that hasn't been mentioned... If this question would have been put to the Abrasive Machining forum, the old grinder hand Michiganbuck would have chimed in by now that you should only grind in one direction, table left to right so the wheel is pushing into the work, then come back without advancing any feed. This does two things; first, it gives the part some cooling time, and second, it gives an indication if the part is bowing up off the vise. The climb cutting back stroke should throw minimal sparks; nothing has advanced, so the wheel is barely touching it. If you get sparks on the back stroke like you are grinding, you are, because the part is "sucking up", and it's time to let things cool down, re-dress the wheel, or what ever it takes, before you lose the part.

Dennis

Thanks again for all your replies everyone,

I tried the shimming method on one blade after it was heat treated and did get a better result. The blade isn't perfect but its within .0015" at the worst and the pivot area seems much better. I only had some feeler gauges so I was somewhat limited. I plan to buy some proper shim stock maybe .001" and try again on the next blades. I think I will be able to get better results with practice. The feeler gauges are somewhat narrow so I don't think they are the best solution for this. I will also give the newspaper trick a go and experiment with some of the more intricate methods suggested as I do more blades.

So far I have been laying the blade on the chuck with the magnet off and mounting my dial indicator on the head. I measure the blade and set the lowest point to zero. I then move the table and note the high spots. I shim the blade and turn the magnet on and retake the measurements ensuring they are the same as when the magnet was off.

The heat treat finish on the blade makes it easy to see where material is removed. I then flip the blade and grind the other side flat. All of this grinding was done with .0005" or less DOC and spray coolant. I couldn't feel any heat in the part with my bare hand. I used an 80 grit wheel but I will try a 46 as well and see if there is a noticeable difference.

Unfortunately my heat treat takes about 14 hours so it may be a while until I have more blades ready to surface grind. I will be sure to continually post what I tried and how it worked for me.

feeling the blade after the pass may not tell you how hot it got at the contact point as the grind is happening. I think you are heating it up more that you think. if you had a lot of warpage as you described you are either heating it or its got a lot of residual stress from HT. gotta figure that out.

Good to know,
I don't think its warping any more its just a residual warp from the Blanchard grinding. That being said I think I'll just run coolant to be safe. I re tested the hardness after surface grinding and it was 62.75 HRC which is exactly the same as before I started grinding. It definitely warped a small amount from the quench but nothing that wouldn't grind out. I am trying to take light passes to keep the heat down as much as possible.

Originally Posted by cyanidekid

if you had a lot of warpage as you described you are either heating it or its got a lot of residual stress from HT. gotta figure that out.

Good point ... ask the heat treater if he is quenching end-on or not. Doing shafts, having the heat treater suspend them from an end for quench makes a big difference to how straight they come out. It's a bit more work, so they tend to just lay your parts in the basket and drop it into the oil, sideways. One side cools down much quicker than the other = warped. You could make a little rack they could sit in during heat treat, so they didn't do that.

An overdressed fine wheel will make all kinds of heat. You want about a 46 with a coarse fast dress. If it's balanced, the finish will still be very good.

Originally Posted by EmanuelGoldstein

Good point ... ask the heat treater if he is quenching end-on or not. Doing shafts, having the heat treater suspend them from an end for quench makes a big difference to how straight they come out. It's a bit more work, so they tend to just lay your parts in the basket and drop it into the oil, sideways. One side cools down much quicker than the other = warped. You could make a little rack they could sit in during heat treat, so they didn't do that.

I am heat treating these myself. I quench them in aluminum plates that I surfaced on my manual mill. The plates are installed in a wood workers vise and clamped down onto the blade when its removed from the kiln. The aluminum plates provide the proper rate of cooling and serve to help keep everything straight. I also use compressed air to aid in cooling. The blades are kept in a 300 series stainless pouch to ensure all the carbon is not burned up at the high austenitzing temps required for higher hardness. The blades are then stored in liquid nitrogen to ensure full conversion. Then the blades are triple tempered. There is a fair amount of thermal stress applied during this process but I have never had a substantial warp. As far as I know this is the best practice for this type of heat treating.

Thanks, I will try switching to a 46 and see how it goes on the next one.

I gotta do something about the brain, it's going downhill fast

I would seriously look into Cubitron 2 wheels. Doing gear teeth they were MUCH faster, cut colder, accuracy better, in one job saved 8 hours. Yes, hours. Those things work super and I bet they'd be wonderful in a job like this. And you have a small wheel, so the cost is nothing to be worried about.

Originally Posted by EmanuelGoldstein

I gotta do something about the brain, it's going downhill fast

I would seriously look into Cubitron 2 wheels. Doing gear teeth they were MUCH faster, cut colder, accuracy better, in one job saved 8 hours. Yes, hours. Those things work super and I bet they'd be wonderful in a job like this. And you have a small wheel, so the cost is nothing to be worried about.

Thanks, that's great to know. I will make some calls and see if I can order one. I appreciate everyone sharing their tips. I had no idea surface grinding was so complex. I bought an old one on kijiji years ago assuming it was fool proof... Wrong again.

Would you recommend a 46 as well? It seems to be the general consensus so far.

Much appreciated.

Originally Posted by Stang Bladeworks

Would you recommend a 46 as well? It seems to be the general consensus so far.

Don't listen to me, cuz I'm more of an id, cylindrical and form grinder, but I've always gone for about a 60. But with those kinds of grinding you are absolutely flooding the interface with coolant.

If you are going to try the cubitron, maybe talk to a sales guy who knows his stuff ?

The other thing you might consider is, don't worry too much about the beauty of the ground finish, just get it flat, then microburnish ? All that is, is tumbling with small steel balls instead of ceramic media, but it gives a really really shiny finish.

If I read your post correctly, you are liquid nitrogen cooling the blades after quench and before temper? If so, that would be a no-no for most steels.

Back to grinding. The OP mentioned grinding .0005" passes through the heat treat scale. I remember, when I was grinding with Aluminum Oxide wheels, that that pretty golden scale will glaze a wheel right quick, with all the heating issues that brings. I haven't tried the 3M Cubitron wheels mentioned, but my go-to is Norton SG5 "seeded gel" wheels. Now, I find the high spot, drop the wheel one and a half or two thousandths, and rip all that crap off in one pass, and can often continue without redressing.

I've standardized on 46 grit wheels in the G hardness for my mold work; it gives a finish suitable for the purpose. For comparison, a 46 grit finish is still better looking than the finish on typical pre-ground flat stock, but the point is to get the parts flat and to size, leaving a small allowance for a finish pass with a finer wheel for appearance.

Dennis

?
Using some form of ferrobend could maybe give you a perfect soft-jaws type custom mount for each blade, in 30 secs.
Dam it, with pins to contain the blade blank, put ferrobend down in the dam, pour hot water (90C) or olive oil (200C) on it, place blade on it.

Pour colder coolant of choice, and the blade blank is perfectly fixed with zero possibility of bend.

Theres many choices of metal mixes, 50-200C, and theyre supposed to be endlessly reusable.