Disc/Belt sander speed control?

[MetalMuncher]

I have one of these Belt/Disc sanders from Harbor Freight.

http://www.harborfreight.com/4-inch-x-36-inch-belt-6-inch-disc-sander-97181.html

Have any of you found a safe way to vary the speed on one of these machines? I read some articles on AC motor speed controllers. (I know a Router Speed Control, made for brushed motors, probably won't work. I don't see any brushes on this motor.) The articles provided enough information to confuse me, as the motor data plate isn't very informative regarding the "type" of this motor (shaded pole, PSC, etc.?). It lists 3/4HP capacity, 3.5A.

My plan is to try lapping the gib strips for my Sieg X2 mill using the sander. I found a web store named "Econaway Abrasives" that sells custom made sanding belts in various sizes (they have a form to input the dimensions you want) all the way up to 2000 grit, so I have some ordered spanning the full range of what they offer. But I am thinking slower speed on the belt might make things easier for me, as the shape of these strips makes them hard for me to hang onto.

I bought a replacement set of gib strips from LMS, to try this project. BTW, if any of you are still using the original gib strips like I am, which have the surface consistency of 000 sandpaper, these replacement strips are WAYYYYY nicer! They actually look machined, and in fact might not need to be lapped at all. But I wanted to try this out to see how well it would work. I had tried manually lapping the OEM strips years ago, but their odd shape, coupled with my fingers, made doing so very difficult, so I gave up. Since this sander has a stop on the belt for abutting the work piece, it may make things much easier for me, and surely require far less elbow grease.

--- I was just examining the sander and noticed the drive pulley on the motor is a bit larger in diameter than I expected. So perhaps I can try a smaller drive pulley. I don't aim to slow it fantastically, just somewhat, to allow better control while holding the strip. As it is, I'll likely have to make some kind of holder. Maybe mill a slot into a block of UHMW about half the thickness of the strip.

[Fergus OMore]

Without access to something like a surface grinder or a tool and cutter grinder to accurately grind FLAT gibs, I'd say that your chances of using a belt sander from Harbor Freight or anywhere else is nil.

Having said all that, you can remove quantities of unwanted metal but you still will not get something like a blued mating of a gib with perhaps 25 spots per square inch which is what is required in such an application.

If I had to tackle the problem, I would file, scrape and blue the offending gib to a flat reference.
The process has been covered many times here and elsewhere and provided that you can sharpen and re-sharpen a scraper, it is fairly straightforward. Tedious, long and mucky but each scraping cycle will improve your gib. The cost- apart from effort- is minimal and an old file to convert and blue- Dykem or whatever are all that is needed.

Regards

Norman 

[Jonny]

Second what Norman said, its asking for trouble.

Linishing you will not get it square you will roll the eges no matter how tight the belt is effectively ending up with a large radius.

Normal way is to file or scrape, black and keep removing the high spots.
Fine 6" three square just the job they have a nice belly if a quality file.

[MetalMuncher]

Fegus, thank you for the information. However, the actual question posed here is how to slow the sander, not whether my project has merit. ;) It is an experiment, for my own reference. If they turn out as hoped, I'll use the new gibs. If not, I'll continue using the ones in the mill being used for the past 10 years.

When I am told I can't do something, I try harder. 

My good friend Ron, a pro toolmaker, when I initially consulted him while buying the Homier lathe and mill, laughed and told me both of these machines would never be useful for anything. I think every member here who uses mini machines knows better. These beasts often fly in the face of "conventional machining wisdom". I have read his textbook "Technology of Machine Tools" which I've had on semi-permanent loan for years. Although it bore some useful information, most of what it specifies are things that often don't apply to these mini machines.

Although I can work a file well, I would consider it the least likely to end up with a flat finish, especially on something only 1/2 wide, since only muscle control is all that keeps it anywhere near level.

Jonny, thank you for the information. Can you please convert your final sentence to English?

[BaronJ]

A shaded pole motor is for all intents a fixed rotational speed device.  Its speed is locked to the frequency of the AC supply !

Reducing the voltage will appear to slow it down but it will cease to turn as soon as load is applied.  If you can find a way of varying the frequency whilst still supplying enough power at the rated voltage.  Only then will you be able to change its speed and still drive the load that you want to apply.

There are inverters that are intended to drive three phase induction motors with a variable frequency so that the speed can be controlled.  So your problem is not insurmountable.

[MetalMuncher]

Thank you Baron.

As it turns out, using the sander at its default speed has not been a problem, as I had thought it might. So things have been working well. I have the Y axis strip finished, and am almost finished with the X axis first side.

One thing I learned along the way is that trying to apply pressure to make the material removal more aggressive does indeed tend to round the edges. But avoiding that, holding the strip with minimal pressure downward, just enough to keep the surface flat on the sander and letting it do the work, is working very well. I place the strip on the belt parallel to the long axis, with an end against the work stop. Start the sander while holding the strip lightly. Stop the sander before moving the strip. This prevents roll tendency. And a precision straight edge held across the surface, looking into a bright light, verifies the surface is remaining flat. The rest is just rinse and repeat and time spent. I use 600 grit to remove the manufacturing grinder marks from the strip surface. Then 1200 grit to remove the remaining marks, finishing with 2000 grit for a nice shine.

[MetalMuncher]

All finished, and the new gib strips are installed. Working very smoothly.

[Jonny]

Although I can work a file well, I would consider it the least likely to end up with a flat finish, especially on something only 1/2 wide, since only muscle control is all that keeps it anywhere near level.

I would love to see the 'flat edge', Drop a straight edge on it I'll be impressed.

In proper English theyre descriptive terms used when filing, every file will have a belly none are perfectly flat. You cant file flat with a flat file it will be doing the same thing as the linisher creating a radius!
You can file several square feet to microns should you wish its also a lot more accurate than scraping.
Filing you have to look at what your doing and react accordingly.

Three square is the proper name for a triangular file.
Just had a look on youtube, there is nothing on there for creating a flat surface accurately. Shame this deserves a thread of its own.

[DaveH]

I think there are many right ways to do the job right. 
 
DaveH

[Fergus OMore]

I think there are many right ways to do the job right. 
 
DaveH

No, not really! All the right ways follow a strict pattern of procedure.
At the end of the day or at the beginning, you have the famous three plates- or someone has that become the classic references to flatness.  Your follow Euclid and Pythagoras in the concept of lines which are parallel - and these concepts of the Triangle and the Square are empirical and axiomatic.

At while ago, I was sitting as a new boy on the block and there was the classic Squaw on the Hippotamus being equal the sons of the Squaws on the other two hides. I looked at my companion and said 'Bloody Hell. do I have to try and prove it?'

That, whether others may differ is what you have to know and  retain. The rest of the problem  is surprising easy

Quo erat Demonstrandum

Norman

[DaveH]

So Norman, at the middle of the day it all comes down to what is Right.

If someone makes something and it works and he/she is happy with how it works surely it was made right.

 
DaveH

PS it's Quod 

[MetalMuncher]

Jonny,

As I mentioned, I used a machinist square to monitor the progress. I did see a tendency to round the edges  IF I applied extra pressure trying to get done faster. Hitting it with much coarser grit and then working toward finer again fixed it. The gibs are installed, so you'll have to take my word for it.

DaveH, you are exactly right! This is a HOBBY. Not NASA. And even NASA screws up in their attempts to go "by the book". At the end of the day, if it works, who cares whether it makes some old timer happy because "that's the way it has always been done". Under those guidelines, we'd still be using copper chisels and stone tools.

I have no intention of this thread turning into some kind of trolling session. Nothing turns me off faster on a forum.

Thanks for the help. Over and out.