Disc Sander Question

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I just bought a Harbor Freight 10" Disc Sander.
If I place a straight edge across the face of the disc, it's not flat. It is "higher" in the center of the disc, i.e. the straight edge rocks from side to side. With the straight edge held flat against the one side of the face, there's about a 1/16"gap at the opposite edge of the disc.
I'm new to disc sanders, so I'm not sure if this is an issue or not.
Thanks for your comments on this situation.
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DerbyDad03 wrote:

Its an issue. But then its Harbor Freight, I'd return it and bring your straight edge with you if your planning on buying another from them.
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"DerbyDad03" wrote:

----------------------------------- The answer is "Yes" and "No".
You can find disc sanders that are tapered just as you describe and there are disc sanders that are totally flat.
You can even find a 10" sanding disc that is designed to fit a table saw that is tapered on one face and flat on the opposite face.
Supposedly the tapered face is designed to produce a jointed edge.
I wasn't to successful when I tried it, but I'd chalk that up to operator error as much as anything.
Personally I'd take it back to H/F and buy my power tools from somebody else.
Lew
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On Sat, 26 Mar 2011 20:15:51 -0700, Lew Hodgett wrote:

But that only works if you can tilt the disc slightly. I doubt the HF sander has that capability.
I have a tapered disc for my tablesaw. Tilting it to remove the taper so that only a small arc touches the wood, and using the rip fence, I can get a pretty smooth edge.
But unless I've got several pieces to do it's more trouble to set it up than it's worth.
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: On Sat, 26 Mar 2011 20:15:51 -0700, Lew Hodgett wrote:
:> You can even find a 10" sanding disc that is designed to fit a table saw :> that is tapered on one face and flat on the opposite face. :> :> Supposedly the tapered face is designed to produce a jointed edge.
: But that only works if you can tilt the disc slightly. I doubt the HF : sander has that capability.
: I have a tapered disc for my tablesaw. Tilting it to remove the taper so : that only a small arc touches the wood, and using the rip fence, I can : get a pretty smooth edge.
I'm confused. Why is this better than a plain flat disc?
And if the disk is flat, you get a 90-degree edge when the TS blade is set at 90 degrees, etc., but the tapered disc is going to require some adding/subtracting.
-- Andy Barss
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Andrew Barss wrote:

Two reasons...
1. When the disc is tilted so the taper is vertical at the center line of the disc, the outside edge of the disc is farther away from the fence. That allows you to feed a board along the fence without hitting the edge of the disc. If the disc is flat and you want to remove, say, 1/32 along the board edge you would not be able to feed it without hitting the edge of the disc. Get a kitchen plate and play with it, you'll see what I mean.
2. If the disc is flat it is mostly sanding across the grain; when tapered, it is only contacting the wood at one point and is moving *with* the grain. Cutting with the grain with anything always gives smoother results.
--

dadiOH
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For removing 1/32" from the edge of a board, my first choice would be a hand plane -- but before that, I'd ask myself if it's really necessary to remove such a trifling amount.

You're going to have to explain that one, I'm afraid.
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Doug Miller wrote:

You mean about cutting with the grain? What's to explain? If you use, say a router, and cut a rabbet both with and cross grain, which is smoother? Ditto any machine I'm familiar with. Moreover, any tool marks less obvious with the grain and are easier to remove since they can be removed in the same direction as the natural wood grain.
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No, about the role the taper plays in that.
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     snipped-for-privacy@milmac.com (Doug Miller) writes:

I'll try. I'll start where I was confused and see if that helps. Because I was imaging the wrong tilt to stuff.
Take this tapered/cone disc mounted on a table saw (figure that's the bast setup for visualizing -- familiar to all here).
With the arbor horizontal, you have a cone pointing at the fence.
Now, tilt the arbor until the disc directly above the arbor is vertical. You have a cone pointing somewhere below the fence. Everywhere *except* that vertical line is sloping away from fence. That line is parallel to the fence.
Turn on the motor, and the grit is all going in (slanted) circles. But on that vertical line, as each grain passes for that mathematically short time, they are moving horizontal. By the time they are moving down, they've moved away from the wood and are no longer in contact.
That's the ideal, where the contact is infinitely narrow. With real materials, you have a narrow contact (1/8 or 1/4 inch) and a nearly flat arc.
Now, I was puzzled because I was picturing sanding along the "down meets table" part of the disc with the taper meaning that the outfeed (where the disc is going up) isn't in touch. That could have a benefit (I imagine), but it isn't what was being talked about.
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Doug Miller wrote:

Do the sketches here help? http://woodworker.com/fullpres.asp?PARTNUM -430&LARGEVIEW=ON
If not, grab a dinner plate and hold the bottom against a wall; now tilt it so the bevel is touching the wall. See?
--

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: Andrew Barss wrote: :> :> I'm confused. Why is this better than a plain flat disc?
: Two reasons...
: 1. When the disc is tilted so the taper is vertical at the center line of : the disc, the outside edge of the disc is farther away from the fence. That : allows you to feed a board along the fence without hitting the edge of the : disc. If the disc is flat and you want to remove, say, 1/32 along the board : edge you would not be able to feed it without hitting the edge of the disc. : Get a kitchen plate and play with it, you'll see what I mean.
Got it, and thanks for the explanation. The plate analogy was excellent. (And thanks to Lobby downthread).
-- Andy Barss
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Not blaming you for wanting to save some money but you might not be able to find a place that sells lower quality tools. For a sander that size expect to pay in the $150~$200 range for something tollerable.
With a surface like what you described you might be lucky to be able to keep sand paper stuck to the surface.
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DerbyDad03 wrote:

It's called a beveled disk.
If you think on it, you'll see that the center of a rotating disk moves much less abrasive against the work than the outer edges. The beveled edge evens out the contact. If you push a piece of work against the entire surface, the parts of the work in contact with the outer edges of the disk will have more material removed than the center.
Also a beveled disk allows you to use a disk sander as a joiner.
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HB:
Brilliant observation. Any point on the disc travels and cuts faster as it is more distant from the center, which the taper adjusts for.
An inventory of the varying utility of the disc configurations Lew mentioned would be illuminating.
Regards,
Edward Hennessey
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On Sun, 27 Mar 2011 12:47:12 -0700, Edward Hennessey wrote:

With a rigid disc, only the high part ever contacts the wood unless you run it through at an angle corresponding to the taper.
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LB:
Agreed. Wouldn't that be the procedure? If not--and you are using a flat disc and flat piece of wood--what would the effect of the differential speed of the disc at points progressively further from the center be on the piece sanded?
I have a tribe of different sanding machines but since no fixed-disc, stationary unit is among them, it would be worthwhile to a have the benefit of experienced understanding which relates to my earlier inquiry about the use of discs of varying profiles.
Regards,
Edward Hennessey

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On Sun, 27 Mar 2011 16:39:34 -0700, "Edward Hennessey"

In my experience, (12" disk on shopsmith), you have 2 methods of sanding, puching between fence nd disk and moving the disk towards the fence for squaring or getting uniform lengh... If you're running the stock between the disk and fence, as you would through a table saw, I can't see where spped at any point of the disk is relevent because you're pushing the stock past the entire disk, right??
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message wrote:

MD:
The thread somewhere took a subtle jog from the OP's (and my) interest in a separate machine driving a sanding disc where contact across the total face may be desired to a table saw mounting a disc.
Apart, we agree on your point. I hope Baja is being good to you.
Regards,
Edward Hennessey
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I am not buying in to the statement that the disk is beveled on a disk sander to compensate for the rate of stock removal. It would be a night mare trying to keep the surface flat.
Either way I don't use my 12" disk sander for sanding a straight edge unless it is a short edge, < a couple of inches . I the disk sander more for rounding a straight edge. Use an edge sander If you want to sand the edge of a board straight. Basically a disk sander sands in the wrong direction, against the grain of the wood, if you use it to sand the edge of a board. It really is more of a shaping tool.
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