Surface Finish

[woodchucker]

I had very little expectation, but it works. Does not make chips, as much it makes little steel feathers. The finish is a lot smoother, but not like glass. Maybe one more pass and we'll call it good.

Thanks for the new trick.

I actually did watch one of Mr>Pete's videos on the tool. I like him. He may ramble, but at least you can hear him and understand what he is saying.

what was the trick? I can't tell from that pic...

 

[C-Bag]

This is my version of that. I got this in a bunch of shop ground bits in a garage sale. It’s become my goto. I set it to around straight on for roughing/hogging and then get it close to flat against the work for finish. Had to come up with jig to do that scoop.

 

[bug_hunter]

what was the trick? I can't tell from that pic...

The trick was to grind the vertical shear bit as suggested. Probably can't see the bit in the photo as i am a horrible photographer, but you could see the fluffy, feather like ships.

 

[bug_hunter]

This is my version of that. I got this in a bunch of shop ground bits in a garage sale. It’s become my goto. I set it to around straight on for roughing/hogging and then get it close to flat against the work for finish. Had to come up with jig to do that scoop.

C-bag, does your bit shear with the leading edge, or cut from the top? Is the scoop to provide a breaker or increase the cutting angle?

 

[C-Bag]

When you say leading edge are you talking about the lefthand edge? I get kinda bogged down in trying describe but if the left point is set like a typical tool it cuts great with the scoop being a chip breaker. I typically take .050 DOC and can take up to .080 on steel and .100 on aluminum which is crazy on my Chinese 9x20. Then rotate around to the long edge for the shear tool. It’s all about keeping it sharp and I leave my tool grinder to those relief angles to touch up. Keep thinking I’ll make me a slo speed hone but so far this has worked out great. The tricky part is the scoop because as I reface it the scoop goes away and the whole thing doesn’t work like it did. Especially the chip breaking.

 

[projectnut]

It looks to me like the cutting edge of the tool comes to a sharp point. It also has signs of discoloring from heat. It's hard to know if the discoloration was done when the tool was ground, or it's heat buildup when turning. Personally, I would radius the edge a bit more, and use some cutting oil. Rule of thumb is the radius of the cutting edge should be a minimum of 1.5 times the distance the tool moves with each revolution of the spindle. An example might be the spindle turning at 700 rpm with a feed rate of 2.00" per minute. In this case the tool would move appx. .003" per revolution. The cutting-edge radius should be a minimum of .0045". A .010" radius would give a better finish but would leave a more radiused shoulder at the end of the cut.

 

[mikey]

bug_hunter, can you show us your original tool and the angle you used to do the original work? I ask because Benmychree is right; A36 doesn't usually finish well. However, it can finish acceptably with the right tip geometry and if you use the right part of the tool to do the finish cutting. If you show us a close up of the tool from the top, side and front then we can maybe give you some help. A HSS tool depends on its tip geometry to work well and you need to use the right lead angle so the desired cutting area comes into contact with the work.

 

[bug_hunter]

bug_hunter, can you show us your original tool and the angle you used to do the original work?

OK, first off, the shear bit did a pretty good job, but it would be nice to be able to achieve a smooth finish with a single tool.

Top:



Side:



Front:



The bit was presented to the work at about a 5 degree angle to avoid rubbing. and to try to achieve a leading shoulder.

Let me know what else is needed for an assessment.

Thanks!

 

[mikey]

Okay, I think the tool is a big part of your problem but that's a good thing because we can fix it.

The overall shape of the tool is fine. The shape is typical for a roughing tool.

You have side rake and that's good. You have no back rake and that's not good.

The biggest problem is that you have no side or end relief so your tool is rubbing and not cutting.

The nose radius is too big so it is contributing to the deflection.

At a 5 degree lead angle, you are cutting with the nose radius and you cannot get a good finish that way. The best finish is produced by a shearing edge. On a turning tool, this edge is typically the end cutting edge.

Okay, so what to do. I suggest you re-grind your tool. Go here and read up on how to get the angles right and what those angles should look like. Show us the tool after you regrind it and we can tell you how to angle the tool so that it cuts with the area it should cut with. Then we'll see how your lathe works with A36.

 

[bug_hunter]

Show us the tool after you regrind it and we can tell you how to angle the tool so that it cuts with the area it should cut with. Then we'll see how your lathe works with A36.

Looks to me that I still have too much nose radius, and need to drop the top a bit more. I stopped because the top edge started getting red while grinding.