Variac on a BT3 clone?

[wlee44]

I believe that in theory a Variac should be able to control the speed of a BT3 table saw. I have a 30 amp 0-140 volt variac and would like like to slow the speed of my Craftsman BT3 clone to do some aluminum cutting. The blades I have are for cutting non-ferrous metal and are rated at 5500 rpm, however when I use then at that speed the carbide tips break off even when cutting 1/8" thick 1" aluminum angle. I want to slow the rpm down to about 3500 to see if that works. Has anyone actually used a variac to slow the speed of a BT3 universal motor. I don't want to fry my motor based on what should work. Thanks for any help. Bill Lee

[Black wallnut]

Welcome to BT3Central wlee44. As a new member your first few posts require moderator approval prior to posting publicly. Rather than let you add to a year old discussion I thought it better to start a new one so that folks only deal with your question.

IMHO if you are having carbide teeth break you need a different blade. I've cut a fair share of aluminum with my BT3K with a fine cross cut wood blade without any broken teeth. That is at full speed, 10" x 60 tooth blade.

[LCHIEN]

I believe that in theory a Variac should be able to control the speed of a BT3 table saw. I have a 30 amp 0-140 volt variac and would like like to slow the speed of my Craftsman BT3 clone to do some aluminum cutting. The blades I have are for cutting non-ferrous metal and are rated at 5500 rpm, however when I use then at that speed the carbide tips break off even when cutting 1/8" thick 1" aluminum angle. I want to slow the rpm down to about 3500 to see if that works. Has anyone actually used a variac to slow the speed of a BT3 universal motor. I don't want to fry my motor based on what should work. Thanks for any help. Bill Lee

i've controlled universal motors with variacs before, it works as the engineering texts say it should (and I'm an electrical engineer). I have not controlled a BT3 with one, but it has a universal motor.

I wouldn't have a hesitation to use one on mine should i need to slow it down.

That said, it sounds weird you should be breaking carbide tips with aluminum. I have cut modest amounts of aluminum at full speed using both non-ferrous blades (Freud) and woodworking blades with no damage on the BT3000 and my compound miter saw.

Are you 100% sure that your angle is aluminum and not steel or some other metal?

Feed slowly and use dust collection.

[vaking]

Another option is getting smaller blade. If you step down from 10"blade to 7" blade it is roughly equivalent to slowing speed down 30%. 7" blades cost a lot less than 10" blades so if you need only a few cuts - this might be an economical solution.

[leehljp]

I use a variac on my BT3000 - for a different reason - to step the voltage up to 120 from 100. Works better! I cut aluminum on occasion at full speed. I have an original 40 TK blade on it and none of the teeth have come loose.

It does seem like there are two or three kinds of aluminum bars/extrusions, and one is much harder than the others. (I could have dreamed that.) Is it possible that you are using some harder aluminum? I would still check for a new blade, 7 inch at that.

[capncarl]

Since you destroyed your blade you are going to have to buy a new one anyway, go ahead and buy a blade for cutting metal. My Granger catalog lists them as Metal Devil Carbide-Tipped Circular Saw Blades-Steel Cutting. I purchased mine somewhere else but know that the can be found in mill supply stores, maybe good hardware stores but not big box home supplys. It looks just like a regular wood cutting blade and fits circular saws or table saws. I use 7 1/4" blade in a circular saw to cut 1/4" steel. It cuts like butter, throwing no sparks only metal chips. The cost was about $50. I also have a 10" blade for the table saw that is used when nails or other metal is suspected in the wood. It cuts wood remarkably well and I have considered leaving it in the machine for all but the finer cuts. Its cost was about $80. I see the prices are somewhat higher now. They do offer these blades marked for cutting aluminun, and are somewhat cheaper that the steel cutting versions.

capncarl

[pecker]

Like the others, I have cut aluminum at full speed, using a 72 tooth NFM cutting blade.
And my SMT fence has a nice cut in it from a 40 tooth general purpose blade.

Maybe it's your technique. When crosscutting metal I clamp the stock to the miter fence, so it can't move at all sideways. And it seems to help to raise the blade a bit higher than normal, and push the stock through quite slowly.

[wlee44]

Variac driving BT3

Thanks for all the info. I feel much more confident about running my saw through a Variac now that I know its been done without the motor going up in smoke.
I think it was my technique (or lack of) that cause the Al cutting blade to loose teeth. I remember that when the blade reached the vertex of the Al angle, that the piece jumped and I lost my grip. Next time I think I will run the vertex through first and go slow. BTW, what is a NFM cutting blade? This is the first time that I've posted to a forum. It's a positive experience to ask a question get helpful answers from knowledgeable people.
Thanks again, William Lee

[jlm]

NFM = "Non-Ferrous Metal", meaning a blade for cutting aluminum, brass, and other soft (i.e. not iron) metals.

I've hooked my BT3100 up to a cheapo Harbor Freight router speed control before, and it works just as expected to slow the motor. It was actually kinda freaky to see a table saw spinning very slowly!

[LCHIEN]

Thanks for all the info. I feel much more confident about running my saw through a Variac now that I know its been done without the motor going up in smoke.
I think it was my technique (or lack of) that cause the Al cutting blade to loose teeth. I remember that when the blade reached the vertex of the Al angle, that the piece jumped and I lost my grip. Next time I think I will run the vertex through first and go slow. BTW, what is a NFM cutting blade? This is the first time that I've posted to a forum. It's a positive experience to ask a question get helpful answers from knowledgeable people.
Thanks again, William Lee

I'm reading that you fed the angle as an inverted "V" to the blade.

Aluminum likes to be cut by shaving small pieces off and having the blade come down at a sharp shearing angle.

I suspect what happened is that the bladee (maybe set to a low height) was traveling forward and down when it encountered the back side of the angle and its angle more or less coincided with the angle of themetal at that point - making a very long traverse through the aluminum, that may have shocked the blade since all teh force was resting on that blade tring to cut through a lot of aluminum. You need to slow the feed rate drastically when passing the vertex as you call it because of the change in material being cut. Raising the blade as someone suggested will work better, too.