02-25-2015, 10:32 PM
I've had a Harbor Freight 4x6 bandsaw for close to if not more than 25 years. (Jeez, I feel old just saying that!) Like most HF tools, it was a kit delivered in assembled form. It required a lot of tinkering to make it cut straight and not throw the blade. After tinkering and tuning, it worked acceptably well for many years.
Recently, however, it started throwing blades again, to the point of frustration: yesterday, I was shopping for a new 7x12 saw. (I was considering either the TurnPro from ENCO, or the Balleigh, which is a Taiwanese product. What I **really** want is an Ellis with mitering, but it's too big, and it's expensive.)
None of the usual tuning tricks helped, so I looked into the workings and found that a number of bits & pieces had worn quite badly. In part, because I never thought of it as requiring maintenance, and in part due to poor workmanship or poor design. I also decided to fix a couple of crappy production details.
Paying work was slow today, so I dove in. Here's what I found:
1) lower/driven wheel - drive surface had worn to a cone shape, this was the primary cause of throwing blades. The wheels for this type of bandsaw are cast iron, with a flange on the back edge, and no rubber tire. The diameter of the front edge of the wheel was .07" smaller!!
2) the drive shaft & drive wheel would move axially about .0625"
3) upper wheel bushing was thoroughly clapped out and wobbly on the shaft.
4) upper wheel was also cone-shaped, but less so - about .03".
5) the upper wheel retaining arrangement had become sloppy - there was probably close to .1" axial slop.
6) the tracking adjustment had become loose, and was rocking side to side considerably.
7) the blade guides have always needed a quick pass under the mill spindle to sit flat in the saw chassis and remove a slight twist in the blade due to the as-cast surface of the blade guides.
I tackled the easiest job first: the blade guides were removed, clamped in the mill vise, and cleaned up, successfully removing the twist in the blade. The nasty funk buildup on the guide rollers was removed in the parts washer, and re-set to the correct width.
The next job was to fix the lower wheel. A bit of cursing, prying, and beating removed the lower wheel without breaking anything or resorting to heat. I put a big chunk of hot roll in the Victor and whipped up a 15mm diameter mandrel to mount the lower wheel on. I threaded the end and captured the wheel on the mandrel with a 10x1.25 bolt and heavy washer. In addition to the cone-shaped wear, there was a small amount of runout on both the flange and diameter. Both were corrected with light cuts.
One of the real joys of Mainland Chinese Industrial Design is a complete disregard for servicing. One indicator of this is that the drive shaft is located only by the worm gear inside the gearbox. When you try to slide the lower wheel onto the shaft, it retreats into the gearbox, forcing you to either drain the gearbox or balance it on it's left side, in order to keep the gear oil in the open gearbox. To get the right amount of axial play, the drive wheel & shaft have to be pushed together from the gearbox side. Just as a belt and suspenders approach to eliminating the axial play, a washer of the correct thickness was modified to fit between the drive wheel and circlip on the drive shaft.
The bushing in the top wheel was pressed out, a new bronze bushing turned on the lathe, and pressed into place. It was then mounted on the same mandrel (returned to the lathe in a 4-jaw and dialed in to zero-zero). The top wheel was quite a mess: the flange had about .06" of runout, and the wheel had some significant radial runout - I didn't measure, but it was visible. Cuts were made to true up the flange and wheel, but also to reduce the thickness of the wheel by the amount taken off of the flange. The last item is important because the teeth of the blade have "set" - they're proud of the band - and if they run on a metal wheel, it will beat the set out of the blade, causing curved cuts.
Bronze washers were made up to bring the top wheel into alignment with the bottom wheel. Alignment was checked with a piece of square tubing clamped to the lower wheel.
Tracking was initially set for the upper wheel to be parallel to the lower wheel, and adjusted so the blade lightly touches the back flange. All the tracking fasteners were then tightened. I may add some blue loctite to prevent them from loosening in service.
Proof of the pudding is in the sawing, and it happily sliced through some 2" .032 wall stainless tubing - something which had given it fits previously.
Victory was declared! All in all, a very satisfying day in the shop! Best of all, I didn't spend $1600 on a new saw!
Recently, however, it started throwing blades again, to the point of frustration: yesterday, I was shopping for a new 7x12 saw. (I was considering either the TurnPro from ENCO, or the Balleigh, which is a Taiwanese product. What I **really** want is an Ellis with mitering, but it's too big, and it's expensive.)
None of the usual tuning tricks helped, so I looked into the workings and found that a number of bits & pieces had worn quite badly. In part, because I never thought of it as requiring maintenance, and in part due to poor workmanship or poor design. I also decided to fix a couple of crappy production details.
Paying work was slow today, so I dove in. Here's what I found:
1) lower/driven wheel - drive surface had worn to a cone shape, this was the primary cause of throwing blades. The wheels for this type of bandsaw are cast iron, with a flange on the back edge, and no rubber tire. The diameter of the front edge of the wheel was .07" smaller!!
2) the drive shaft & drive wheel would move axially about .0625"
3) upper wheel bushing was thoroughly clapped out and wobbly on the shaft.
4) upper wheel was also cone-shaped, but less so - about .03".
5) the upper wheel retaining arrangement had become sloppy - there was probably close to .1" axial slop.
6) the tracking adjustment had become loose, and was rocking side to side considerably.
7) the blade guides have always needed a quick pass under the mill spindle to sit flat in the saw chassis and remove a slight twist in the blade due to the as-cast surface of the blade guides.
I tackled the easiest job first: the blade guides were removed, clamped in the mill vise, and cleaned up, successfully removing the twist in the blade. The nasty funk buildup on the guide rollers was removed in the parts washer, and re-set to the correct width.
The next job was to fix the lower wheel. A bit of cursing, prying, and beating removed the lower wheel without breaking anything or resorting to heat. I put a big chunk of hot roll in the Victor and whipped up a 15mm diameter mandrel to mount the lower wheel on. I threaded the end and captured the wheel on the mandrel with a 10x1.25 bolt and heavy washer. In addition to the cone-shaped wear, there was a small amount of runout on both the flange and diameter. Both were corrected with light cuts.
One of the real joys of Mainland Chinese Industrial Design is a complete disregard for servicing. One indicator of this is that the drive shaft is located only by the worm gear inside the gearbox. When you try to slide the lower wheel onto the shaft, it retreats into the gearbox, forcing you to either drain the gearbox or balance it on it's left side, in order to keep the gear oil in the open gearbox. To get the right amount of axial play, the drive wheel & shaft have to be pushed together from the gearbox side. Just as a belt and suspenders approach to eliminating the axial play, a washer of the correct thickness was modified to fit between the drive wheel and circlip on the drive shaft.
The bushing in the top wheel was pressed out, a new bronze bushing turned on the lathe, and pressed into place. It was then mounted on the same mandrel (returned to the lathe in a 4-jaw and dialed in to zero-zero). The top wheel was quite a mess: the flange had about .06" of runout, and the wheel had some significant radial runout - I didn't measure, but it was visible. Cuts were made to true up the flange and wheel, but also to reduce the thickness of the wheel by the amount taken off of the flange. The last item is important because the teeth of the blade have "set" - they're proud of the band - and if they run on a metal wheel, it will beat the set out of the blade, causing curved cuts.
Bronze washers were made up to bring the top wheel into alignment with the bottom wheel. Alignment was checked with a piece of square tubing clamped to the lower wheel.
Tracking was initially set for the upper wheel to be parallel to the lower wheel, and adjusted so the blade lightly touches the back flange. All the tracking fasteners were then tightened. I may add some blue loctite to prevent them from loosening in service.
Proof of the pudding is in the sawing, and it happily sliced through some 2" .032 wall stainless tubing - something which had given it fits previously.
Victory was declared! All in all, a very satisfying day in the shop! Best of all, I didn't spend $1600 on a new saw!