I am generally happy with my Hammer N4400 bandsaw. It has plenty of power with it’s 4HP three-phase motor and good capacity at a reasonable price. Sorry, I don’t remember the price. I am very good at comparing prices when I am researching the market for a type of product, but as soon as I have purchased the product, I tend to forget the price almost instantly. It took some practicing to develop this skill, but I have found it most useful! Given the motor power and resaw capacity of the saw, I really wanted to test a carbide tipped sawmill blade. So I started to search the internet for such a blade and found many recommendations on the Lenox Woodmaster CT blade. It is available in 25 mm (1”) width and up. For some reason the manufacturer of the Hammer specifies the maximum blade width to 20 mm (3/4”). There is room for a wider blade in the saw and the gauge for blade tension has markings up to 35 mm (1 3/8”). So it was tempting to fit a wider blade and crank up the tension to the wider blade according to the gauge. I took a chance and ordered a 13 ft 1” x 0,035” 1,3 tpi Woodmaster CT blade from www.spectrumsupply.com
I fitted the blade, cranked up the tension to the 25 mm marking, tracked the blade, adjusted the blade guides and started to make a few wedges in a simple jig. This blade is extremely fast cutting. The cut surfaces are definitely smoother than what is achieved with an ordinary 19 mm (3/4”) 3 tpi blade. After the test cutting I released the tension and, as a always do, looked at the tension gauge. It surprised me that the tension gauge wouldn’t return to zero, but stayed on the 5 mm (1/4”) mark. It took me about 2 minutes to figure out what had happened. The blade tension spring had reached it’s yield point and become shorter. This is not supposed to happen on a properly tempered spring, but I couldn’t really complain to my Hammer dealer, since I had used the saw beyond the manufacturer’s recommendations. It would be easy to order a new spring as a spare part, but then I would be getting a new poor quality tension spring. Perhaps the manufacturer would argue that the spring is supposed tho yield so that the bandsaw frame doesn’t. It´s actually not a bad argument. There’s definitively a risk in modifying the bandsaw with a custom tension spring. So I measured and calculated and searched the internet for another evening and found a fairly local supplier http://www.fjadrar.se of a spring with the item number 105-712 from AS Raymond springs. http://www.asraymond.com/documents/ASRaymond_Die_Springs.pdf
To change the spring you need to disassemble the following parts, see the pictures below:
The crank wheel. Use a metric allen key (2)
The indicator needle (3)
Release the screws (4). These screws fix the carriage sideways. That means that they are used to adjust the upper wheel so that the blade gets parallell to the travel of the blade guides, i.e the rack and pinjon rod. Make sure by measurement or other means that you can get the screws back in the same position as before.
Remove the nuts (5) on the left side and pull out the axle to the right (6).
Verifying the spring constant of the original spring
It bothered me a little that the establishment of the original spring constant was more of an estimation than a proper calculation. So I came up with an idea to actually measure the spring constant of the original spring, using the new spring with a known spring constant. But first a little theory; the spring constant is a measure of the linear macro-elastic behaviour of a spring, i.e. how much does does the spring compress or elongate when it is placed under a certain load? This behaviour is linear, which in this case means that it takes the same extra load to compress the spring from 0 to -1mm, as it does to compress it from -10 to -11mm. Or, in a more formal way:
, where DeltaL is the movement of the spring, k is the spring constant and F is the force applied.
If I could apply the same load and measure the compression on both springs, it would be possible to calculate the unknown spring constant. This is the experiment setup I came up with:
I used a small piece of veneer to make sure that there was no contact between the MDF-pieces or the springs and the clamp body, as it would have made the measurement less accurate due to friction.
I started to compress the whole spring package a little to remove all slack in the system. Then I mounted the caliper and the indicator dial and zeroed both. I used the clamp to compress the package about 6 mm (1/4”) and noted the compression of each spring. I used the indicator dial simply because I don’t own two digital calipers. I noted the compression of the old spring, DeltaLold and the compression of the new spring DeltaLnew. Since the spring constant i known for the new spring, the load on the springs can be calculated. Now, If we know the load and the compression of the old spring, the spring constant of the old spring can be calculated. More formal after some rearrangement:
Since the spring constants were not equal, the scale on the band tension gauge had to be redesigned. Every mark on the scale had to be 3,43/3,00= 1,143 times longer. I measured the original scale
and simply used painter’s tape to make a new one. As a coincidence the new scale has a distance of exactly 10 mm between the marks.
Replace the spring and assemble everything in the opposite order from the disassembly. This is a good opportunity to lubricate everything with a high quality grease. Being a cyclist I used Finish Line Teflon grease I found in the bicycle maintenance drawer. Don’t use too much as it will attract sawdust. Use a non permanent thread lock product (I used Loctite No 248) on the screw (1)
and don’t tighten it too much as it will apply a too high bearing tension, shortening the bearing’s service life. I used a torque wrench at a setting of 4 Nm (40 lbf.in) (It’s not very much)
Test of the bandsaw frame
To make sure that I can use the 25 mm (1”) blade I decided to test the bandsaw frame for yield. First I fitted a 19mm (3/4”) blade on the saw and tracked it. Then I mounted an indicator dial on the cast iron table. The indicator dial was set to measure the deflection of the frame on the upper blade guide. The blade was slightly tensioned to the 10 mm mark and then the tension was released. Now the indicator dial was zeroed using the blade guide travel mechanism.
The idea is to tension and de-tension the blade repeatedly to higher tensions. First to the 5 mm (1/4”) and the back to zero and then to the 10 mm (3/8”) and back and so on. The deflection is recorded for each cycle, both for the tensioned and released blade. When (if) the frame starts to yield, de-tensioning it will not make the deflection go back to zero. The results are found in the table below.
|5 mm (1/4”)||10 mm (3/8”)||15 mm (5/8”)||20 mm (3/4”)||25 mm (1”)||30 mm (1 1/4”)|
|Deflection with tensioned blade, mm||0,37||0,70||0,97||1,22||1,46||1,70|
|Deflection after release of tension, mm||0,00||0,00||0,00||0,00||0,00||0,00|
I decided to stop the frame yield test when I had reached the 30 mm (1 1/4”) mark.
If the data points are connected, they make a nice line, except from the area close to the origin.
That can be explained by friction in the blade tension mechanism and blade tension gauge. Since the deflection reading returned to zero after every de-tension, it is concluded that the frame of the Hammer N4400 is strong enough to run a 25 mm blade.
Releasing the blade tension at the end of the day?
I have heard and read many times the recommendation to release the blade tension at the end of the day. The reason given is usually that the tension will slowly deform the machine frame or the blade or the bearings, when they are placed under load for a long time. The truth is that there is no such thing as material creep of metals at room temperature. Leaving the band tensioned will not shorten the service life of the bearings, not distort the frame permanently, not shorten blade life and not make the blade tension spring loose it’s spring constant. Unfurtunately, material creep is a fact in most polymeric materials. The tires on the bandsaw’s are made of a material that may creep in room temperature. As a materials scientist, I’m not too worried if I forget to release the blade tension for a few days. Compare the machine with a car. You don’t place your car on jacks at the end of the day, do you? Don’t worry -it will not affect the service life of the wheel bearings, distort the frame or distroy the suspension springs. If you don’t use the car for a few weeks, you will probably experience small vibrations due to the deformations in the tires. It usually goes away after a few kilometers. Don’t be too worried if you forget to release the tension of the blade!
©Text and graphic /Mattias Puide