granite slipper plate

[darryl]

Not sure if I've asked about this before, but at one time I had been considering building my own belt sander and wanted to use a granite tile as the backer for the belt. Now I'm getting close to building it, and wondering if the tile will be able to withstand the slipping of the belt across it.

Seems to me it will be fine, and the way I intend to build it, the tile will be replaceable anyway.

Normally you buy the machine, which would have a cast iron platen (backer plate, slipper plate- ) I intend to build my machine using mdf laminated both sides with countertop laminate. There will be steel where appropriate (bearing seats, etc) but other than that the whole structure will be this laminated material. The granite tile will be placed behind the belt in the 'work zone'.

I could of course line it with graphite impregnated fabric, but instead I'm thinking to make up a 'lubricator' of this fabric and have that in light contact with the back of the belt.

I'm also thinking that because the belt I intend to use is 12 inches wide, a crowned roller would be less ideal than some other method. I could copy the method used in the larger thickness sanders, where the angle of the upper roller is varied to keep the belt tracked within certain limits. I do intend to look into this in depth before I commit to any design.

At this point I'm just wondering what you all might think about using a granite tile as the platen-

[Toolguy]

Can't hurt to try. If you find you are replacing tiles too frequently, you can always use a steel plate tile next.

[mattthemuppet]

I read a build where someone used a piece of furnace glass as a platen, might be worth looking into

[chipmaker4130]

The stone will be more sensitive to abrasion wear than steel, but more importantly it will not transfer heat away to any significant degree. This will shorten the life of your belts.

[darryl]

Hmm- I didn't consider the heat issue. I wonder how that would compare to the difference between using a graphite cloth backer or bare cast iron-

There are times when I want the softer backer, but not generally for this machine. Since I'll be able to exchange the backer anyway, I suppose I'm free to experiment. Just need to make sure the thickness of it- whatever material it is- is enough to sit slightly proud of the belt path.

In reviewing my requirements for this machine, having the ability to easily and quickly change the backer would be an asset. I've just been considering whether I could get away with leaning the structure backwards to some degree, which would let the backer plate simply lay against the structure without fastening. It would of course have a lip at the bottom to support the backer, and there may need to be something to prevent the bottom from kicking out- a magnet perhaps, with a steel plate epoxied to the tile and a slight recess in the structure to accommodate that.


The table, or fence, would have to be angled to suit of course. What I have in mind is something more substantial than what's on my existing sander, which is just a piece of cast being held with one bolt from one side- looking like it's ready to break off- . I want to create a solid 90 degree point, plus have the ability to set a few angles. I have no use for the disc part of the typical floor model belt sanding machine, so that won't be incorporated. What I'd like to do is have the bottom roller shaft extend out both sides of the machine so I can mount some other thing, like a cutoff disc for instance, on the same stand. Maybe the drum with its soft backer, perhaps the option to mount sanding sleeves. I'm heavily leaning towards having all these functions on one machine, mainly for compactness since I'm virtually out of room for separates.

It will be interesting to see how I can accommodate changing the belt with relative ease.

[PT Doc]

Pyroceram would work well.

[boslab]

The automatic sample linishers in work had what i think was carbide, where you get a 6 x 4 x 1/4 chunk of carbide is a mystery but they were glass hard, mounted to a copper plate with a serpentine channel cut in, the two were fixed together with allen screws after a liberal squirt of heat transfer gunk, the white stuff, water was fed through the cooling block from mains via a pressure reducing valve to the drain.
This method helped belt life tremendously, it was easy to see when a belt was hot, there was a standing wave in front of the sample so the edge was radiused and belts would die very quickly, if i was building a linisher i think i would use a cooled slipper plate myself, saves money!
Mark

[darryl]

Interesting. I'll have to consider the degree of heat build-up I get in normal use. This will be a hobby machine after all, but there are times when I want to remove material fairly quickly- at least at something like the rate of which the machine is capable.

I have a question regarding the shape of the rollers. The bottom, powered roller would be straight, but the top one is often crowned. I've read that it can be crowned only in the center third, and also it can be turned down at the ends instead- leaving the center two thirds flat but slightly larger in diameter than the ends. Seems I would prefer this, as it's both easier to achieve and would have less tendency to bulge the center of the belt. If it was a narrow belt I'd just turn a crown and be done with it, but this would be a foot wide belt. It would be easier to just roll off the diameter for about 3 inches at each end of the roller.

Another question I had is about the length of the rollers. I plan to make the bottom one the full 12 inches, but is there any reason why I can't make the top one an inch shorter?

[strokersix]

Be careful you don't ignite your belt sander... Heavy ferrous stock removal generates a lot of heat, sparks, piles of swarf burning, etc.

[ironmonger]

Maybe a steel plate with small holes fed with compressed air, this would cool the belt and reduce the friction betweet the belt and the plate.

paul

[darryl]

Perhaps we're over-thinking this. I've been using this elcrapo belt sander for a few years now, and I haven't set anything on fire- there's no slippery plate behind the belt, just the bare cast iron surface. I do like the compressed air idea though-

I have the option to use a graphite applicator pad if I choose to, just to add a bit of lube. At any rate, I'm not going to worry about that part right now. I'm going to stick with the laminated mdf idea for the basic structure, but now I think what I'll do is laminate the inside with sheet steel and stay with plastic laminate for the outside. The mechanical assembly that carries the rollers, etc will be made from steel.

The structure will be in the form of a tapered box for the inherent shape-holding ability. Part of one side will hinge away for belt access, and the front will also hinge out for the same reason. I've decided to combine the cut-off disc and the sanding drum into this one machine, so there will be a one-piece table as part of the front. Because the top of this table will be just below the center line of the drive roller shaft, there will be an auxiliary table that will fasten to it to bring the height up for the belt sander portion. This will also give a way to have either an adjustable table, or separate fixed-angle tables for use on the belt. With this separate table out of the way, there will be lots of support for anything I'd be cutting on the cut-off disc. I'm thinking I'll have a guard that can be placed in front of the belt so I don't have to worry about 'accidental abrasion' when I'm not looking at the belt.

The cutoff disc and the sanding drum will mount to the ends of the roller shaft, so they will more or less be hanging out in space by about six inches. Just to maintain a good amount of rigidity, I'll use a 1 inch diameter shaft. It would be a stretch for me to turn a portion down to 5/8 for the cutoff disc, or to bore the end for a 5/8 insert, so I'll make up a mounting adapter instead. If I could insert the shaft through my lathe spindle, I'd bore it to 5/8 and use a turned insert to hold the cutoff disc, but alas- this is one limitation of my lathe. I've seen those flanges that have a wedge type mechanism that tightens down onto a shaft when you pull the wedge in- I'll look at something like that to mount these outboard parts.

Got some materials today- got one or two parts cut. I'll be using the surface plate in a bid to help me get the roller shafts and the platen support in the same plane. The mounting of the upper roller will be interesting-

One of the benefits of having this combo machine- I'll gain a whopping 3 sq ft of floor space-