Wood-fired!

Restoring tyres on an old bandsaw

August 18th, 2007

Some years ago I bought a large old Italian bandsaw at auction. I had used large and powerful bandsaws and resaws at Art School but had always managed with the standard 14in Taiwanese bandsaw in my own workshop. These can perform well when properly tuned but will never have enough grunt to resaw wide boards or cut veneers of any great width.

So I snapped up this bargain when I saw it for $1300. It had a huge cast frame, nice big wheels, solid table and very substantial guides with a neat adjustment mechanism. After the trauma of getting it home and finding a location in my workshop I discovered one of the lower guides had been broken and re-welded so badly it would never work again.

The upper guide. The horizontal guides are very easy to adjust. The rear guide is not the original.

This could be replaced, possibly without loss of function, but the lower guide will never have the same sweet mechanism again.
I bought some new blades and tried to tune the bandsaw. I found I could not adjust the tracking successfully. The blade just wanted to wander off the wheels - a slightly scary prospect with a machine this size. I soon realised that the tyres on the wheels were very worn and effectively flat. This is bad.
Due to other obsessions I didn’t get around to dealing with this for some time. Eventually I needed the ability to resaw wide boards so I had to do something.
Research came first. It appears that most bandsaws rely on tyres that are convex in profile. That is, the middle of the tyre is thicker than the edges. It is somewhat counter-intuitive but the blade rides up to the highest point due to centrifugal force so if the highest point is in the centre of the wheel then you are in business!
So my task was to find and fit replacement tyres and then shape them to the appropriate profile.
Searching the internet for bandsaw tyres (or tires if you want to find the US sites) was not encouraging. Lots of product for 14″ bandsaws and not much else. I needed tyres for wheels that were 591mm or 23.3″ in diameter.
I eventually found Carter Products in the US via Cutting Edge Lasers in Perth. They claimed their 24″ tyres glued on with 3M industrial epoxy would do the job. So they were ordered and eventually arrived.

Removing the lower wheel with a gear puller. Note the state of the old tyre.

The first task was to remove the wheels, clean off all the old tyre material and true the wheels. The top wheel was easy but as always the last one was a bugger. The picture shows my cheap gear puller stretched to its absolute maximum trying to pull the lower wheel off the drive shaft. After the Eureka! moment it came off smoothly.
To true the wheels I found a shaft that the wheels fitted exactly and mounted it vertically on a board. The board was bolted to the table of my disk sander and the wheel dropped into position (these wheels weighed about 35Kg from memory so they were not so easy to manoeuvre.)

Wadkin disk sander set up to true the wheels. The white line in the middle of the face is a groove in the metal that is filled with the old glue at this stage.

My disk sander is a wonderful old Wadkin disk/bobbin sander with a sizable disk and a table that moves into and away from the disk on a smooth dovetail and can be finely adjusted by a handwheel. This meant I could bring the edge of the wheel up to the disk and rotate it by hand as I very gradually eased the wheel toward the disk. This worked very well and successfully removed the old tyre as well as truing the wheel. (This disk sander can remove steel as easily as it removes soft pine as long as you are gentle with it.)

Preparing to glue the tyre. Epoxy is seen in the background. The flange on the facing side of the wheel is the pulley for the drive belt.

Then to gluing on the tyres. I reused the same shaft held horizontally in my bench vise so that I could rotate the wheel freely. The tyre was easy enough to get onto the wheel. Following the instructions I found on the internet, I used a short length of plastic pipe placed between the wheel and the tyre to roll around the wheel and even out the tension. I used the same pipe to hold the tyre off the wheel as I applied the epoxy. I rolled the pipe around the wheel, gluing as I went. Apart from a few permanent drops of blue epoxy on the workshop floor this went smoothly and 24hrs later the tyres were well attached.

Once the epoxy is set the tyre can be trimmed.

The tyre was trimmed to the width of the wheel as shown on the right.
I used the same jig on the disk sander to true the tyres before beginning the profiling. The table of the disk sander can be finely adjusted to tilt up or down so this should have been an easy solution to creating the profile.
However… After a flawless truing of the first wheel a tragedy of limited proportions happened on the second wheel. I was over-confident about the strength of the mounting of the axel to the board and hence the table. As I rotated the wheel I came to a section that was thicker than the rest and the disk grabbed it with more downward pressure. As soon as the wheel was pushed slightly out of square, the disk contacted more of the tyre. It was a runaway effect. The wheel was quickly wrenched out of its mounting and the tyre torn from the wheel. In less than a second the tyre was destroyed and I had a small (thankfully) flat spot on the wheel itself.
In retrospect it was careless to have not mounted a safety support near the edge of the wheel. This would have been simple to do and would have prevented the destruction. This is the nature of experience. Rather than moving straight on to the reward for all this work, I had to wait several weeks for the replacement tyre to arrive from the US - and of course do all the tyre removal, cleaning and re-truing again ready for the new tyre.

I could have simply added the safety support to my existing jig and with more care it may have done the job. But there was a fundamental weakness in the design. The disk sander applies a downward force cutting across the tyre at right angles to the plane of the wheel. This means that the wheel and jig are resisting the force in exactly the worst place because the 300mm radius of the wheel provides a great deal of leverage on the axel fixing.
I decided to try a different approach - one that, as it turned out, was both accurate and very safe.

The bobbin sander jig. The white stick in the foreground is the adjustable depth-stop. The jig slides in and away from the bobbin on splines. One of the splines is visible near the right end of the depth-stop. The dark part of the tyre is the surface that has been trued by rotating the wheel past the bobbin.

The Wadkin sander is a combined disk and oscillating bobbin sander. The bobbin table also tilts accurately. So if I could present the tyre to the bobbin while rotating the wheel by hand I would have the same result but with the force acting parallel to the plane of the wheel (tangential to the tyre). The drawback is that there is no built-in adjustment to gradually move the wheel towards the oscillating bobbin as there was on the disk table. So I had to build this myself. Two slabs of 1″ MDF are clamped to the bobbin table. A third slab slides back and forth between the two fixed pieces and carries the vertically mounted axel and hence the wheel. Two hardwood splines run in slots cut into the moving section and the fixed sections. The moving section slides tightly on these splines which hold it firmly down on the bobbin table. The moving section and the wheel can be slid slowly towards the bobbin. I used a depth stop to make sure the wheel could not advance more than I intended. After the wheel had been rotated a couple of times against the bobbin I would move the stop half a millimeter and then slide the wheel gradually up to the stop while rotating it. As shown in the picture, I did this until the tyre was trued for most of its width.

The bobbin table is tilted to sand a 5 degree bevel on each side of the tyre.

The final profile sanded onto the tyre.

Even though this configuration had little risk of the wheel tipping off its axis, after the previous disaster I added a safety support anyway - just inside the rim of the wheel.

It was then very easy to tilt the bobbin table at +5 degrees and then -5 degrees to sand the profile.

A makeshift straightedge is used to align the two wheels.

Wheels were then fitted back onto the bandsaw. To give the blade a good chance of running true, the wheels must be reasonably co-planar.
The lower wheel has coarse adjustment and was set to bring it into an acceptable range. A makeshift straightedge (it needed to be 2m long) was installed and aligned to the lower wheel by measuring the distance between the straightedge and the top and bottom of the wheel.
The top wheel was then finely adjusted so it ran in the same plane as the lower wheel.

A new blade was fitted, tensioned and tested. The blade ran true and cut beautifully. Finally I was rewarded with a powerful but accurate machine. I have cut veneers as thin as .3mm (translucent in Hakea) in narrow widths (1-2″). I have resawn hardwoods up to 300mm deep without much strain. I’ve not yet needed to go beyond this but I expect to be able to push it further.

The top and bottom panels of the Wedding Present (about 250mm wide) were cut from figured blackwood using this machine.

Entry Filed under: Wood