Sears Flex drive conversion
I'm new here, first post. But I've been involved for several weeks on a project to convert a Sears 10-inch Flex Drive table saw to a belt drive unit. This saw was free, given to me, by a friend. I'm retired, and not a wealthy person, so this seemed to represent a great gain in my newbie woodworking equipment. Until now I've only had a small 99-dollar Ryobi or Delta direct-drive portable. Retrieved from a falling-down (literally) old barn, where it had sat for years, with an enormous cast iron table, looking like a much greater precision saw than I have, I thought the gift would be worth 'renewing' it, fixing it up--building a bit of 'sweat equity' in it.
Because of the effort I've given this, and the stuff I've learned, and the dearth of information elsewhere, some seemingly conflicting information I've seen, I thought I'd offer what I've learned. Please note: ALL of what follows is based on my opinions, estimates, and evaluations. I'm something of a knowledgeable machine designer/engineer, (strong emphasis on the 'something') so the following is based largely on observation and personal estimate of what is or is not; and not on hard engineering calculations. I'm also reasonably competent in Autocad and have done many design drawings in the process, attempting to figure out the 'best way' to do the project.
First, to do this 'right', solidly, is for anyone a very daunting job. Any comparison with the other Sears belt driven 10 inch saws from the same era (about 1995-97), or discussion of parts interchangeability is next to useless, simply not relevant. They are totally different designs, and mechanisms, internally.
I've spoken with an individual who worked directly under the original designer of this Flex Cable drive. The company was Elliot who made the drive cable. According to him, the original design was excellent, worked very well. But, apparently, when the saw was put into production for the market, Sears, others, may have decided the original design was just too complicated and expensive ... so they changed a 'few' things. And thereby probably did great 'damage' to the original design. Hence the reputation in some circles that the saw was badly executed in production.
The flex cable, according to design limitation specifications (7/16th core diam) was 1) possibly borderline for the energy transfer required, and 2) was definitely flexed to the minimum limit of the cable; something less than 10 inches radius. Again, something that contributed to the many failures, and returned purchases Sears experienced.
As to the conversion possibilities: The internal arbor and blade carriage in these saws is an aluminum casting, NOT cast iron or steel, as in the belt driven saw. I've seen, taken many photos, studied the belt driven saw and the difference in the two saws is huge. There are massive cast or forged trunnions supporting the entire carriage--and the motor. This is a critical/crucial point. In the flex drive unit, there are two 1/2 inch steel rods with (I'm guessing) two #10 screws on either end (4 total) supporting the blade carriage and arbor--but NOT the motor. It is rigidly mounted to, and supported by the external cabinet enclosure of the saw. Any conversion will require that the added steel structure/support for the motor and the weight of the motor not be hung from those half-inch shafts and those four small screws. And the structure to support and pivot the motor should be very rigid/sturdy/heavy(!) to prevent torquing of the motor under startup and working loads. Note also that that lightweight cast aluminum carriage and the depth/bevel adjustment as well as the arbor housing itself were never meant to tolerate side loading/pull ... as from a belt driving from one side. I have no idea even now, if it will hold up under the side-load caused by a belt arrangement. This, by the way, is, I'm sure, one of the reasons for the tremendous difference in the steel/iron carriage frame structure in the belt-driven model, and the much lighter, simpler aluminum and screw design for the flex drive. The power transfer in the flex drive is ONLY axial ... in line with the arbor ... and not from the side as with a belt and pulley arrangement.
The result of this is that in any re-design/conversion of the saw to belt drive will require one of two concepts ... either a belt drive that can drive with severely misaligned sheaves, or the motor has to be mounted on its own frame, but somehow attached to the blade and arbor carriage to accurately follow the operator set bevel angle. Note, by the way, that the CCW rotation of the motor is reversed by the original flex drive cable to correctly rotate the saw blade. A belt drive will require now, that the motor be hung on the opposite side of the carriage/blade, to maintain correct blade rotation (CW) and this will also increase the moment/loading moment caused by the weight of the motor, which only further complicates the design of the motor mount, and increases the structural requirements (steel sizes/weight) for mounting the motor.
At present I'm following and working on a concept to hang the motor from a 5/8th shaft bolted to the rear apron of the cast iron table. But the motor 1) must be allowed to 'float' to compensate for belt tension and 2) must be supported NOT by any part of that cast aluminum saw carriage. Not sure if I can make it work. But will keep you informed ... if anyone is still interested.
I thought I had a neat solution to most of the problems by simply returning to the fixed motor attachment (though with the motor set at least two feet away from the cabinet) by using round endless poly-reinforced belting. This belting can often run reasonably well, though with some transmission losses, with badly misaligned sheaves. Then, two days ago, a distributor informed me that NO one in the U.S. is still carrying round belts, and that it can only be purchased from some outfit in Japan. Sounded crazy ... but if true ... there goes that idea ... even if it might have worked.
Soo... enough. I have been wanting to communicate with others about this and appreciate the opportunity. And hope that I might save someone a bit of repetitive effort. Glad to share.
I have a number of hi-res photos of both saw internal mechanisms, illustrating the different mechanisms and can provide if anyone is interested. The saws are similar only in external appearance.
(Added thoughts: In response to a couple of (much) earlier posts in the thread:
One poster mentioned exchanging parts with the belt driven models. That is probably not easily done either. Reason: the underside of the cast iron tables are designed with specific boss locations for attaching trunnions, frames, enclosure/cabinet. These are cast into the bottomside of the tables, then drilled and tapped to receive the bolts to attach the various components/sub-assemblies. The castings for belt driven models are very different from those of the flex-drive unit. The boss pattern is very different. Hence, one could not take the 'guts' from a belt-driven model and transfer that to the flex-drive model, and simply bolt it in place. In fact, amazingly, I doubt any of the parts are interchangeable between the two saw models.
One caveat: The two saws I have compared have different horsepower motors. The flex drive, as someone pointed out, is a 1-1/16th (?) horsepower motor. The belt drive model I have studied has a 3 horsepower motor. That could account for the major structural differences in these two particular saws. It is possible there is a one horsepower belt driven model that is much closer in internal design than the one I have worked with.
One questioner asked about what a 'flex-drive' is? I've explained it to many by saying that it's identical in concept to the old, mechanical speedometer cables that were used on cars back in the fifties/sixties. A spiral-wound, flexible steel 'shaft', that spins/is turned inside an outer protective sheath. They're also most commonly used today, I think, on 'Weedeater' lawn trimming tools. They need to be lubricated regularly, and are or tend to be inherently weak in the real world for transmitting torque. They can tend to vibrate, unless designed and installed correctly, can be 'springy' in response as well. Note that they are generally not repairable, and unlike belts, tend to be very costly to replace.
Another point: Another poster mentioned the flex-drive itself. Believe me, I've checked and the Sears price at about 80 bucks, is not unreasonable--IF you can actually find one any longer. I've checked with the Sears Parts website, and they show almost no parts still available for this saw other than nuts and bolts. Also, instead of attempting to change it to a belt drive model, I've looked into going back to the flex drive system, using a much newer, more readily available third-party flex cable, up-sizing the flex drive cable to at least a half-inch cable, and buying one at least a couple of feet longer in order to greatly increase the bend radius in the cable. This would increase power and torque at the arbor, and probably increase the smoothness, lack of vibration at the blade. Would also absolutely install a soft-start on the motor to reduce the huge startup torque spike/shock-load from the motor. But, all of this would cost something in the range of three to four hundred dollars (284 for the new cable and another 80 for the soft-start) And that doesn't include another hundred or so, probably, for remaking, creating, a new arbor shaft to accept a 5/8th inch, keyed pulley or new, larger flex cable. The original arbor has an integral, puny-looking press/formed square hollow to accept that little drive cable end, and is too short to just cut if off and add a sheave.
Last edited by Greyeagle; 10-10-2009 at 11:56 PM.