My bench grinder/buffer spins @ 3600 rpm. I noticed, while reading through the Eastwood catalog, they recommend 1800 rpm for buffing plastic (I want to polish my taillights). How to slow down the buffer?

Harbor Freight Tools sells an in-line potentiometer (rheostat?) which they call a "Router Speed Control" (P/N 43584-3RGB) designed for "any universal AC/DC brush type motor, 15 amp and under...won't work with soft or slow-start motors). My buffer is 3/4 hp and 4.2 amps.

Should work, no?

 

Should work ok for polishing plastic. If you try to grind metal at a lower speed the grinder might 'bog' down though.

 

RPM without specifying the diameter of the wheel is meaningless. Why don't you just get a smaller buffing wheel? The speed at the edge of the wheel will be slower.

 

It's an 8" grinder/buffer. I assume the specified rating is with an 8" wheel installed.

So, you're telling me if I put 6" wheels on it, the effective rpm will be reduced? How do I calculate? Guess I should've stayed awake in Geometry class...

 

Yeah, I only want to slow it down when polishing plastic pieces (taillights, antique telephones) and "soft" metals (gold watch cases).

 

The rpm is the speed the wheel is going round at. A wheel will run it's entire circumference over your piece of work every revolution. Therefore a smaller wheel (with a smaller circumference) will run a smaller circumference over the work per revolution. If you want to work it all out the circumference is Pi * Diameter (22/7 * dia) & the entire "distance" of mop that runs over the work in a min is this value * rpms. Rather than worrying about all that, just use a smaller wheel, don't push the work up hard against it & do it in small "bursts" ie don't hold it on the wheel for ages. Practice on a gash piece of plastic. The faster grinder will work out better as, once you've got used to how much pressure to apply, you'll find that polishing will take a lot less time than with a slower bench grinder. Beware that polishing causes heat & with a higher rpm more heat is generated, so if you press the work against it hard for any length of time you could end up melting the plastic! Additionally, don't hold the work in one position, always move it around against the wheel otherwise you run the risk of polishing a dip in it. When doing things like aluminium this looks real dumb!

p.s. practice makes perfect!

 

Another thing that might be the cheapest alternative - since you are just doing tail lights, can't you polish them by hand? I've polished many smaller items like that...

Hey! Did they ever figure out Pi exactly???

 

Yeah, I could polish by hand, but I also have several other projects (antique telephones, etc.). I get carpal tunnel when I start doing stuff like this.

The last I heard, Pi was still 3.14159265358979323846264338327950288419 7169399375...

 

The speed at the circumference is 50% with a 4" wheel instead of a 8" wheel....

 

that would still be faster than they say

 

why ?

They recommend 1800 rpm with a 8" wheel (45237 inches per minute at the circumference)

With a 4" wheel at 3600 rpm you'll get again 45237 inches per minute at the circumference.


8" x 3.1415 x 1800rpm
or
4" x 3.1415 x 3600rpm

 

Back to the original thrust of the poster - go to Home depot and buy a dimmer switch and an outlet box and outlet and wire it up with an old lampcord. That's what I did.

 

Man, that's a pretty good Idea...

It needs to go in that Shop Tool Thread.....

 

When I embarked on my 69 resurrection, I screwed around with this subject a LOT! My final solution was to go to Harbor Freight and buy a REAL buffer. They aren't that costly and are made to do this job (i.e. long shafts so you can move bigger and odd shaped pieces around, the correct RPM with sufficient power to keep the wheels turning). If you use a speed reducer on a regular grinder, you will be constantly bogging down the motor to the point of aggravation! Believe me - I tried it all!

I bought the supplies from Eastwood. They have GREAT kits, some of which have everything you need to do a real nice job on all materials.

Save yourself grief and put the money you are going to spend on experimentation into a buffer - you'll want to polish everything in sight!

 

don't some electric motor's burn up like that ???? i tried that on a drill and smoked it

 

I know the machines you're talking about. Good ones are $$$. I do buy stuff from Harbor Freight, but not machinery. The "adjustable speed" buffer/grinders are $500 and up. I don't have room (or regular need) in my shop for two different machines. If I can slow down the 3600 machine to 1800 (or thereabouts), I've solved my problem.

 

i'm planing on making a pulley system on mine to change the speeds ,,3 different size's ,,need sure what yet but this is food for thought so i don't waste money

 

First off, your bench grinder is most likely an induction style motor. This means it will only operate at speeds that correspond to the frequency of the AC current that it is running on. i.e: a two pole induction motor runs at 3600 rpm on 60 hz AC, a four pole induction motor runs at 1800 rpm on 60 hz AC etc....

The rheostat you mention is for motors using brushes like a router, drill, etc. and it varies the voltage output not the frequency of the AC.

The fan speed control is close to what you need although most house fans use much less current than a buffer motor and have an amperage limit. The correct speed control you need is called a "Triac" which may be available at a power tools supply but I'd expect it to be rather expensive....probably more so than buying the correct speed buffer.

For the small number of plastic parts you're likely to polish, the smaller wheels are probably the best bet since the smaller diameter decreases the surface velocity and that's what really counts in buffing. Another alternative for you may be to just buy an arbor that allows you to chuck up the buff in a variable speed electric drill. I've done this and it works awesome. Use Meguire's plastic polish for clear plastics. "Autosol" from Eastwood also works very well on plastic as well as chrome, brass, etc.

Incidentally, I bought the Harbor Freight buffer/grinder and took the grinding wheel off and replaced it with another buff. This enables me use different compounds and/or buff materials; switching back and forth without having to stop and remove the buff each time. I'm really pleased with the HF buffer and think it was $69 very well spent. I figure I can replace it 7-8 times before I've caught up to the cost of a Baldor buffer. So far I'm still on the first HF buffer with no signs of it ever giving up. It really is a nice machine!

 

so what did you do ???

 

My Makita buffer has a very speed on it and I run it slow for buffing with a light misting of water. The variac should work. You are not pressing hard when buffing. When I go to the shop later today I will plug my buffer into one of my variacs and see if it works but I feel it should.
Go for it