Sanding revs ?

[OldWood]

First time turning yew and was impressed by the quality of finish I was getting, but found that it takes such a high polish from the fine grades of paper that sanding lines are rather apparant.

I was doing this at the club and was surprised to be told that I should sand no faster than 500rpm as any faster will lead to burning. Hmm - well I've been turning for 3 years and have always sanded at the speed I did the final cuts and only once on a very narrow edge did I get a burn and that was a nice feature, so was left on. I can understand that burning is a hazard if the sanding is done too vigorously, but is the carte blanche instruction from the 'more experienced' turner correct ?

Rob

 

[Blister]

500 rpm or slower for me

and normally use a power sander on a variable speed drill running at a slower speed

very frequent sandpaper changes and never use the same bit of paper twice

Don't skip any grits

I normally sand from 120 to 400 sometimes 600

 

[CHJ]

Slower speeds are best, need to let the sanding medium have chance to cut rather than flash over the top and also to reduce the friction heat generated which wrecks the abrasive adhesive.
Yew needs extra care regarding friction when sanding due to the fact that it very easily succumbs to micro cracking of the surface due to excessive heat, slowing the lathe down helps in this respect.

 

[cornucopia]

Kind off Rob- to a certain extent the sanding speed is realative to the size of the piece being sanded. for example i wouldint sand a yew pen at 500 rpm as it would be too slow
As a genral rule sand your work at roughly half of the speed you were turning the piece at.
Yew will only have micro cracks on end grain if you gets too hot- side grain isnt such a problem.

 

[OldWood]

500 rpm or slower for me and normally use a power sander on a variable speed drill running at a slower speed very frequent sandpaper changes and never use the same bit of paper twice

Interesting as I've clearly been sanding at too high a surface speed. Most of the wood I've been using isn't that hard - spalted maple, spalted beech, sycamore, gean, ash and elm - and I'm not aware of having any problems, though perhaps someone with a keen eye would be critical. Certainly one observation that I can think of is that the maple I was using was showing signs of going a bit out of shape in sanding and that would no doubt be the heat accelerating the removal of the last vestiges of moisture in the wood.

Why not use the same bit twice ? I use Abranet so the dust passes through it and doesn't collect in lumps and build up hard spots. There is also the second advantage of re-use in that grit gets finer as the paper wears. Please don't think I'm challenging the wisdom - it just makes it easier to tell all the wood turners I know why they are doing it wrong.

The yew is probably the hardest wood I've turned - cuts beautifully and is taking a nice shine, but it does rather look as if I should go back to 180 and run back up through the grits, and certainly to take advantage of the finish that it will take I need to get some 600.

Rob

 

[jumps]

Why not use the same bit twice ? ..................... There is also the second advantage of re-use in that grit gets finer as the paper wears.

Rob

I think you may have answered your own question here. One of the keys to good sanding is controlled use of precise grits.

Heat is a byproduct of friction, fresh paper cuts and produces less heat.

Another reason is contamination, you might not see the dust on the surface but it will be there - paduak dust on ash gets in the grain and is a pain to remove! Obviously I have picked an extreme.

I use small pieces once (well twice if it's a piece that's getting reversed), and feel I probably use the same overall quantity and without fear of contamination etc

 

[myturn]

There is also the second advantage of re-use in that grit gets finer as the paper wears. Please don't think I'm challenging the wisdom - it just makes it easier to tell all the wood turners I know why they are doing it wrong.
Rob

Grit doesn't get finer as the paper wears, it just stops cutting. 80 grit is 80 grit and just becomes blunt 80 grit as it wears down. The grit rating refers to the density of the particles and you don't get more as the grit wears down.

 

[jpt]

Another cause of the problem could be too much pressure of the paper against the wood, you should only just use enough pressure to hold the paper against the surface and let the grit do the work. Any more pressure than this and all you are doing is producing heat and putting deeper marks in the surface which you have to take out with the next grit down.

john

 

[monkeybiter]

Thanks OldWood, I'm learning a lot from these answers

 

[OldWood]

Thanks OldWood, I'm learning a lot from these answers

Aren't we all - and it's interesting that two oft-quoted reference authors on woodturning - Rowley and Darlow - don't make any mention of this, and I haven't seen anything in the Wood Turner either. The principles are pretty basic when one steps back and looks at them, but are somewhat counter-intuitive to the novice.

Rob

 

[hughie]

Oh well I'll jump in the deep end

I just about always power sand, rarely do it by hand. With power sanding I use either or both a 240v pistol drill and inertia sanders.

When using inertia sanders its best if the revs are kept high as this drives the sander and I tend to have the revs up a bit also for the power sanding as well.

Heat yes it does generate heat and you can deal with this by a couple of methods, both of which involve compressed air.

The simplest is just use compressed air blowing onto your work piece Ok you have to direct it some where you want it ie dust collector. It pays to have some sort of flow control as full blast can be a pain and it also saves compressor run time, not to mention the pennies as well

Then there is the more technical method of using a Ranque Hilsch Vortex. They are expensive to buy but quite cheap to make. They have the capacity to produce compressed air at temps well below zero if required.

The beauty of this sort of sanding is that you can quickly and effectively remove all sanding marks and produce a fine finish, generally finer than hand sanding for the same grit grade.

 

[jumps]

hughie

firstly welcome

second, I believe that compressed air has the added advantage of keeping the cutting edges clear on the paper, which also keeps the heat build up down. certainly a virtuous circle!

 

[nev]

never use the same bit of paper twice

:shock: :!:

are you made of money? or sandpaper :?: :mrgreen: