shaper knife design

I have a job that I have been running for awhile on the shaper with a set of custom ground knives and I am wanting to achieve better finish and longevity. I am cutting plastic, specifically Nylatron, it starts out as round bar and I cut it into a "+" shape. I think the shape of the knife is causing some reweld issues once I have ran enough to heat up the knife and dragging. The grinder made them to my drawing and does have relief included, I am just wondering if I can do better as I need to redo the knives now. The inside corner of the part is sharp and the leg of the + is parallel. I am getting good finish on the vertical faces, but on the horizontal, it is rewelding like it is dragging. On those sharp inside corners it is app 7/16" doc. I have a single speed on the shaper and I am running the slowest feedrate on the feeder at 10 ft per min. It gives a chip that is 0.008" thick.

So, I am thinking if I can change the shape to give that inside corner a small radius like 0.030-0.040" it might help that edge durability and I am also thinking about changing the horizontal leg from being parallel to having some taper, say 3-5 degrees, thinking it may reduce or eliminate any dragging.

I ran these on my cnc router the first time and had a bit of the same issues, the vertical face would be nice but the bottom cut on the bit would not leave a nice finish.

Hopefully the pics are good enough to help explain what I am doing. The pics show the bar after the first op, they get turned around to do the other side.

Jason

You might also consider increasing the relief angles.

beside cutter geometry, you might look into compressed air cooling. If heat is the problem, try making the tool cooler.

Have you tried increasing the feedrate? I have not cut delrin but have cut UHMW and polycarbonate. I have found too slow a feed can cause rewelding issues.

I did not try any air to cool it. I am only guessing that it is producing some heat as it looks like it. It was cold enough in the shop, app 40 deg F, that I did not feel any heat when I handled it, but by the time it gets through my guides to where I pick it up is app 20" after the cut.

The next step up on the feeder is quite the jump, I don't recall what it is at the moment, but I remember when I first set the thing up a couple years ago it was way too fast for the machine to handle. This is the only nylatron I have ever cut. When I did it on the cnc, I experimented with all sorts of different feedrates and faster didn't help on the finish and actually tried to chunk/chip the parts vs cutting. I wish it was delrin, but then someone else would probably be willing to do it.....

I wish I had a knife grinder in house so I could experiment.

Make a cutter profile that is more like a W than a T and run the stock at 45* to what you are now running. Of course being round stock that is only an abstract.

Relieve the insides of the form tool so it is not making contact along the sides of the rib. Give it more of a “C” shape rather than a “U” shape.

The ideas you mentioned in the first post, plus a bit more relief as others mentioned, is probably about all that can be done with that profile.

I thought of using a inverted V knife, or full W form as another posted, but then you double the number of passes, and risk losing the registration ("timimg") of the features without a more complex set up.

Did you do any experiments before starting the project by stacking 3 cutters: a topping knife with the beveled corners held between 2 rebate cutters in a stack separated the desired distance? If the taper you mention is beneficial, it should run easier. If parallel sides are actually better for whatever purpose these serve, maybe try the rebate heads separated by a spacer to see what the result is. You can make several tries on one piece by gradually reducing the spacers, since the experimental part is just to prove (disprove) the cutting finish, no final dimension. If that seems productive, add a cutter with the topping feature.

smt

robinr said:

Make a cutter profile that is more like a W than a T and run the stock at 45* to what you are now running. Of course being round stock that is only an abstract.

Click to expand...

I would try this approach as well. Eliminates all drag clearance issues. Your existing problem may also be caused by re cutting chips and chip extraction needs to be addressed too. Sometimes an air blast can be directed to not only cool the cutter, also to direct chips away and into the extractor.

And learn to make your own knives for crikies sake!

sfriedberg said:

You might also consider increasing the relief angles.

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Hard to tell if there is any in the pic.

It appears that you only have 4-5 degrees of axial clearance on the vertical portion of the knives. On a 90 degree angle you will need 10 degrees. On a Weinig knife grinder, this is achieved by tipping the grinding wheel 10 degrees sideways after your finish pass. If set up correctly you should see a .010-.020 line at the cutting edge. Ask your guy to install more relief. It is very simple.

Thanks guys, I do know how to grind knives on a knife grinder, but I don't have one and to be honest this is the only job I have had in the last 5 years where I needed a knife ground so I can't quite justify buying one unless it is cheap. I don't have much interest at the moment to grind them by hand like Smt does.

The profile does have relief, the pics are bad as they were taken for other reasons not to show the grind.

I had not thought to run the profile at 45, I am not sure that I want to redo the whole setup to do it. This job while recurring is not all that often nor high qty so I do not want to keep investing time into it when I don't think I'd see the return. Usual order is 500 ft of rods, so 1000 ft of cut total every 9-12 months. The nice thing with the setup I have now is that I run one side and then install an indexer plate and just run the other side without any other adjustments. It makes it pretty simple.

Spruewell said:

Relieve the insides of the form tool so it is not making contact along the sides of the rib. Give it more of a “C” shape rather than a “U” shape.

Click to expand...

This seems like a good idea, if in fact those horizontal edges are parallel, and not opening up towards the spindle. The knives are only going to cut at the outer part, the rest is just rubbing. And it does appear that there is very little relief on those edges, seems like more would help.

You can modify those knives with a small wheel in a dremel or die grinder to test out ideas. I use Mizzy heatless wheels to modify and/or kiss hss knives. Easy to put a slight hollow in a bevel to allow honing back to a keen edge without re-grinding.

Well, I got to run the latest batch on Friday and it went well. I had a different company make some knives for me with the profile updated to a 4 degree taper on both top and bottom of the tongue and also had them put a 0.030" rad on the corners. I also had him regrind the relief on the tips from the default angle the material comes with to one that leaves more material there backing up the tips. They also increased the relief on the top/bottom to 10-12 degrees.

They made them out of M3 with a diamond coating of some sort. I ran 1,200 ft of cut and it looks really good without any degradation of the edge that I could see with the naked eye. It still shaves off my fingernail as well. The surfaces of the tongue have a good finish on them, every once in a while there would be a short section where the top would have a tiny amount of reweld but I noticed it because I was looking for it, bottom never had any reweld. I have another 1,000 ft to run in the next 2 weeks, so will see how well it holds up.

So far I am happy with the results, if something bad happens to these knives, I am out $100 vs $300 for the carbide, so I think it was a good decision.

I did find a grinder that if cheap enough I might try and bring home...