Thin Parting Blade

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ctwo
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Thin Parting Blade

Post by ctwo » Tue Oct 09, 2018 2:13 pm

I created an ~45 thou thick parting blade from 3/8" HSS, a little longer than 1/2". Is it unreasonable to expect this blade to part 1" Delrin without wandering around? If not, what flaw should I look for? I can see the blade bending either in or out and it makes concave or convex faces. Later, I will try aluminum.

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spro
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Re: Thin Parting Blade

Post by spro » Tue Oct 09, 2018 3:50 pm

There are remarkable tool holders which support the extended blade. Your blade may be too thin for that now but worth exploring. Iskar and Valenite sell them. The "foot" or support must be slightly narrower than the blade of course. The bottom of the blade would have precise groove to fit the shape or vise versa. Anyway, it is tough steel which doesn't bend and is twice the support of the blade alone.

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Re: Thin Parting Blade

Post by Harold_V » Tue Oct 09, 2018 3:51 pm

It isn't beyond reason to expect the tool to cut properly in Delrin, but aluminum will be more demanding.
I hand grind ALL of my parting tools, although I usually use ½" tools to enhance strength. I'd use larger, but my tool holder won't accept them.

In order for the tool to cut without wandering, it's important that it have balanced features. It should also not touch anywhere but at the point of cut. Any contact with the sides of the tool, or even an uneven cutting edge will give cause to wander. Once it starts, it feeds upon itself, a condition made worse by materials that are more difficult to machine.

When you grind such a tool, it's important that it have relief in two directions. One of them, which you likely understand, is top to bottom. Both sides should display an equal amount of relief, and it need not be excessive. The second direction is towards the shank. It, too, need not be excessive, but it should be identical, side to side, as should the top to bottom relief.

If you hope to part without the tool wandering, it's a good idea to not grind a lead on the cutting edge. The lead hoped for, which would have the intention of parting without leaving a tit on the part, is generally the cause of a tool wandering. Not necessarily true for wide cutting tools, but for narrow, long ones, it's pretty much a death knell.

Lastly, you can create great chip flow and a little positive rake by grinding the top of the tool, using the periphery of the wheel with the tool applied parallel to the wheel. Care must be employed, so you don't grind an angle (side to side), but if your wheel runs true and you don't have a work rest in the way (I don't use a rest on my tool grinder), you can easily control the grind. It does take a little skill, but that comes quickly as you grind tools.

H
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spro
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Re: Thin Parting Blade

Post by spro » Tue Oct 09, 2018 4:10 pm

Hi Harold. I believe in an earlier tutorial, you mentioned grinding a groove dead center atop the blade. The parting forces allowed the chips to flow out that groove. That was remarkable knowledge and itself prevented excessive wander. Really thin bits should work the same at microscopic level but that would require some super precise grinding.

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Re: Thin Parting Blade

Post by ctwo » Tue Oct 09, 2018 5:02 pm

I roughed this using my angle grinder, then brought it in closer on the lathe, mounting the tool like a trepanning tool. From there, I used a 1" diameter stone in the lathe chuck, to create the top edges first, then raising the tool in steps to finish the sides. I added a few thou relief on each side with the cross dial, then I finished it freehand on the bench grinder. It ended up looking like a knife profile. There is about 5 thou relieve from tip to shank, and more under the edge as the bottom is almost a knife. I'm not that good freehand.

And, I started with a square tip, but did add a bit of lead both ways to try and correct the problem. I'll straighten it up and try it again, and try power feed, but I think I've thinned the lower part of the tool too much. Worst case is I think I'll get a 1/16" parting blade setup.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...

spro
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Re: Thin Parting Blade

Post by spro » Tue Oct 09, 2018 7:31 pm

Well, you have a lot of work invested in this and I hope it works fine. There will be some adjustment at the beginning because the full length will wander. It will not protrude as far as when the deep cut allows guidance.

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ctwo
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Re: Thin Parting Blade

Post by ctwo » Wed Oct 10, 2018 8:49 am

I blackened the sides and do not see any rubbing, but it looks to have a slight bulge on the right side. I'll try to address that carefully. This is with a slow power feed, I do not recall if 19 or 20 tenths.
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parting-blade-20181009_171631.jpg
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...

spro
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Re: Thin Parting Blade

Post by spro » Wed Oct 10, 2018 1:46 pm

I'm impressed.

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Re: Thin Parting Blade

Post by Harold_V » Wed Oct 10, 2018 4:33 pm

It appears the tool is cutting a concave surface (wandering). Look closely for the outside edge (the side you see in the picture) causing the tool to deflect. It doesn't take much on such a thin blade. The tool may even be held at a slight (side) angle, causing the tool to rub ever so slightly. You may be able to see that by stopping the tool while it's deep in the cut. See if it's touching the material anywhere.

Delrin, like Teflon, cuts almost without resistance. When conditions are right, you will get a straight cut.

H
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ctwo
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Re: Thin Parting Blade

Post by ctwo » Thu Oct 11, 2018 11:14 pm

Indeed. The tool rocked when tightened in the holder, like it is moving away from a radiused corner. I ground it again to straighten the outside edge, remounted with an indicator, and that works better.
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parting-blade-20181011_205458.jpg
parting-blade-20181011_204332.jpg
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...

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Re: Thin Parting Blade

Post by Rich_Carlstedt » Tue Oct 16, 2018 11:25 am

late to the party here, but wanted to mention a few things.

Harold's response is exactly what should be looked at in making a good parting tool.
let me add that when doing plastic, you have another concern. Cutting generates heat and unlike metals, the heat is not carried off in the chip necessarily. and since plastic is a poor conductor of heat, there is lots of thermal expansion at the point of cutting. Plastic generally has 4 times the expansion rate of steel, so keeping heat down with good tool clearances is paramount to success. Cut, do not rub !
As a side note, I have used hacksaw blades for very thin parting tools. Grind off the teeth and the wavy set and you can make a very effective parting tool from .010" to .025" wide and best of all, it's very cheap and will sharpen up your hand skills as Harold mentioned.
Rich

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Re: Thin Parting Blade

Post by Harold_V » Tue Oct 16, 2018 2:59 pm

Rich,
The use of a hacksaw blade for narrow parting (or grooving) is an interesting idea. Would you mind sharing with us how you hold the piece of blade?

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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