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hughie
19th August 2011, 10:35 AM
:) This to all the wood theorists, rocket scientists, and deep thinkers out there and for that matter anybody who has an opinion on the matter.

Now we have all heard that we must sand down through the grits and never miss one :no: tis a cardinal sin of wood turners everywhere to miss one.

Now when I started out a few years back I could not afford to buy all the grits within the brand I was using, namely Noritake Astra dot. So I took a punt and bought a range at the same time got a inertia sander and a few power sanding heads.
Love technology :2tsup: so couldnt help myself. :U

This sort of sanding produces very fine results a few grades down the range than the grit being used and so jumping a few grits works, well for me any way. :)

So my question is, Why? :? Why does this sort of sanding produce the result it does.

I have my thoughts on this, but dont want colour the discussion with them at this point.

bowl-basher
19th August 2011, 10:54 AM
Can't answer your question but you are dead right.......
I use an inertia sander but some times dont have the discs so will sand by hand and the diffrence in finish is very marked not to mention the time factor ... By hand I will finish to 600 but with the inertia 400 will produce a better finish
Hope someone can give us the answer
Bowl Basher

wheelinround
19th August 2011, 11:01 AM
I see this thread is going to become clogged just like the grits do Hughie.:;

I was taught working through the grits also, yet at the same time by same teacher I was taught in industry there is no time to work that way customers demand speed and cost saving so jumping a few is paramount.

Having sanded both wood and steel using such as a 9" sander on steel bumper bars and other such for chroming, down to now pens and finer works wood, fibreglass, resin/epoxy. Sanding for a high gloss paint finish on vehicles.

Simple if you start with even 40grit it wares as does 1200 grit, they also clog therefore it gives you a grade difference, pressure applied also makes a difference.

If you have say 600 and are using DO it can bring 600 down to 1400 due to the oil.

Difference between inertia and hand is rotation of inertia in comparison to your hand will not do the twist :p Edited as I saw BB's post

Ed Reiss
19th August 2011, 11:02 AM
hmmmm...perhaps the "sand" in sandpaper is friable and breaks down much quicker under power than by hand ~~~ so it might just drop from say, 400 grit to 600 under use :fisch::whatonearth:

Ed Reiss
19th August 2011, 11:03 AM
Ray...ya' beat me to it :2tsup:

dr4g0nfly
20th August 2011, 08:34 AM
I've noticed this and always assumed (nothing else to base it on) that static paper when hand sanding (the old way) enabled the grit to cause the scratches that you need to remove with the next grit size down..

Now with the the rotational speed of the inertial sander over the turning wood combined with the lighter touch of the grit (and possibly even improved abrasive material) that a better finish was achieved reducing the requirement to sand every grit.

But that is just my way of viewing the effect!

rsser
20th August 2011, 03:25 PM
Hmm, nice conundrum.

Recall one R. Raffan saying at a show that an inertia sander will give a finish a grit or more 'above its rating'.

I don't hand sand with faceplate turning, just powered and sometimes inertia.

Inertia is gentler and is driven by the spinning wood, not by 240v and an impatient user of the drill.

The other variable is the Astra where IIRC the abrasive dots are well spaced out.

I wait with 'baited' breath Hughie for you to colour in the discussion.

hughie
20th August 2011, 07:25 PM
I wait with 'baited' breath Hughie for you to colour in the discussion



:U you got me, here's what I suspect is going on with this.

I agree that the inertia has a gentler action and this in part must effect the out come.

Also I suspect that the action of spinning keeps the paper free of dust build up and so presents clean grit each time. Thus the cutting action of the grit is somewhere near its maximum cutting efficency.
That the random nature of the cut and the crossing of the cuts with a random action is a factor. An overlay of random fine cuts not unlike lapping.
That the rotary action cuts across the grain tangentially as opposed to the stationary hand held action.
That the paper is on a foam backed pad that allows a measure of conformance to the work surface. With the pressure to the sanding area dropping towards the outer edges, providing a feathering action.
That the pad is driven by a greater diameter and therefore its speed is expotentially increased. This increases cutting capability similarily.
I suspect that its a combination of the conditions coming together; with the pressure by the user, the displacement of the pressure across the pad by the foam backing, including the feathering action as well. Coupled with the high speed random action of the pad and the constant presentation of 'clean grit' to the work piece.


Happy to be contradicted and corrected, bring on the rocket scientists :U

robo hippy
21st August 2011, 04:17 AM
Well, abrasives have come a long way in the last 10 or so years. One of the new things is film/plastic backing instead of paper. Another is electrostatic charges being used to stick the particles to the backing. This gives you a much closer grit size, and lines them up so they are closer to the same height, and you don't get as many off sized pieces as you did when the grits were sifted, and then dropped onto the adhesive and backing.

As to power vs inertial sanding, I have never had an inertial sander. Just went straight to power. There seem to be two schools of thought in power sanding, one is very high speed (flex shaft, or high speed drill rpm of 3600 to 10,000), and a feather light touch, and the other is slow speed, and just enough pressure to get the abrasives to cut, which is the method I use. I would guess that this is similar to what an inertial sander does. For me, my lathe is running at 15 or so rpm (no electronic read out), and my drill at 600 or less. My bowls are warped, and I either have to have the lathe running that slow, or use the spindle lock and sand while it is stationary, then rotate to another lock point. If you don't push hard, and run at slower speeds, it is pretty gentle. If you push harder, the disc wants to jump around a lot, and you skip over spots. You can be too heavy handed with your sanding drills.

Abrasives are cutting tools. They are sharp for a bit, the eraser sticks can clean them up a little, then they get dull, and then you toss them. The coarse grit ranges (80 to 120) will remove stock, tool marks, and smooth out the form a bit. Medium range (150 to 220, or 260) will clean up the marks and 'ease' edges. The fine grits (320 and above) are polishing out scratches from the coarser grits. Stepping up in grits gets you cutting and sanding away the scratches from the previous grit. If you take too big of a jump, you have to spend a lot more time sanding. I guess it is like going from roughing cuts with your turning tools to shaping cuts to finish cuts. I used to start all my bowls at 80 grit, then 100, 120, 150, 180, 220, 320, and 400. Now, I can start at 120 most of the time, then 180, 220, 320, and 400. I tried some of the 260 grit and found that to be too big of a jump. Seems like there would always be a few 120 scratches that weren't totally gone, and 220 would get them out, but with the 260, I had to spend way more time. There are some that skip from 120 to 220, but again for me, that is too big of a jump.

I don't know about the grit being able to drop from 400 to 600 or whatever. The abrasive particles are at 400, and will chip down a bit to expose fresh cutting edges, kind of like grinder wheels, but there will still be 400 grit particles, and if you want a 600 grit finish, you have to use 600 grit abrasives. By the time it wears down to a finer finish look, you are no longer cutting, but burnishing/polishing, and it is time to toss the disc.

I have noticed with the spinning disc, there is little or no tendency for the fiber to lay down like it does when you hand sand.

I prefer firm pads for grits up to 220 or so, then the softer pads. 80 grit on a soft pad doesn't cut as well as 120 on a firm pad.

robo hippy

rsser
21st August 2011, 08:14 AM
There's no good reason by and large to trust the grit ratings with common abrasives anyway. There's a deal of variation in particle size. Maybe when you skipped a grit and it worked, it was a product of this.

As RH mentioned, there's the possibility that as the alox particles fracture their effective cutting edges get smaller.

wheelinround
21st August 2011, 11:07 AM
No matter what grit you use the material also clogs & fills no matter which method.

How often do you stop and wipe clean the item being sanded?

If wet sanding this happens even more unless its flushed clean often.

Paul39
21st August 2011, 11:16 AM
I labor under the delusion that if I skip a grade or two of the series that the finer grit will eventually get down to the smooth part of the timber, but take longer.

I think the theory of 80, 120, 180, 220, 260, 320, 400; is that each finer grade quickly removes the scratches of the previous.

I have not timed whether going from 80, 120, to the end of 220 takes any less time than going from 80 directly to 220 for a longer time on 220.

I have only recently gotten some 180. Also not all of my paper is of the same manufacture, so I'm sure there are some variations in grit.

I would think that one could start with 220, and eventually get the piece smooth, but would take forever to sand out the tool marks and torn grain.

Edit, Robo Hippy covered it.

NCPaladin
21st August 2011, 12:20 PM
I don’t know if it is right or wrong but I was taught to go up 50% and it works for me. Such as 120, 180, 280, 400, 600. If it is an open grain wood I seldom go to the 400 and 600 but may go to 320 as a final step.
In agreeing with Robo; I was also taught 120 grit will not become 180 when as it gets worn, it will just be dull 120. Get new paper to cut properly and keep heat down.

NeilS
21st August 2011, 12:39 PM
Here in Australia many of our outback roads are unsealed (no bitumen) and depending on their use and weather conditions they can become very corrugated before the grader next comes through. If you drive slowly over the corrugations they can shake you and your car to bits. You get a better ride if you go faster so that you glide across the tops of the corrugations. You are in effect shifting the grit size from coarse to fine(r) by changing the relative speed. I think the same principle applies to sanding.

If you could move your hand (axially) as quickly as a rotary (inertia or powered) sander I expect you would get similar results for the two methods.

A light touch, particularly with the coarser grits, is required to avoid the deep scratches that persist through the subsequent finer grits. If a heavier hand is needed to remove a deep tool finish (no matter how good your tool work some woods are prone to deep bruising) then a doubly long period of light sanding is required to complete that grit before moving to the next grit.

On the finer grits I stick to the 1 => x1.5 rule. That is, don't jump more than 1.5 times the previous grit size.

It's worth noting that the US and European P grit sizes (http://sharpeningmadeeasy.com/grits.htm) don't precisely correspond, especially above #180.

The more random abrasive pattern created by an inertia sander may help to crosscut and breakup any higher ridges created by earlier deep scratches. I think those that reverse direction with each new grit are getting some of this same effect.

rsser
21st August 2011, 02:57 PM
Nice analogy Neil. Not only not sealed, not gravelled ... esp. over your way!

Seems wood density and fibre compression from the tool then springback are further variables.

And if toolmarks or tear-out have to be removed, it's not sanding as finishing, it's stock removal.

All I can add is that if the wood is soft or v. hard, I have to take particular care with power sanding and that includes stepping up through each grit. Wiping out the bowl or platter between them. Inspecting and backtracking if nec. I sand both with the spin and against it.

Rookie
22nd August 2011, 11:26 AM
I don't know about the technicalities of particle size and rotation and non rotation on sanding, but ...
Sandpaper traditionally comes in, say, 40, 80, 120, 180, 220, 360, 400 etc etc, and we have been taught to go through those without missing a grit.
Let's theoretically suppose sandpaper only came in 80, 180, 240, 400 etc etc. I would say that we would have still been taught the same thing over the years.

Randir
22nd August 2011, 02:10 PM
I would think that one could start with 220, and eventually get the piece smooth, but would take forever to sand out the tool marks and torn grain.

This is something I've always thought, but I know that on metal it isn't the case. I've done some work with making jewellery, and if you skip a grade when sanding/polishing sterling silver you won't ever get out the previous marks (or not in the limit of my patience, anyway). Wood is more forgiving, so perhaps it works, but I doubt it would be more time/effort efficient.

On the main topic, my thought would mainly be on the cross-cuts, particularly if you're a lazy hand sander and don't move the paper around a lot.