Startrite 301 bandsaw blade tension

[chipchaser]

I took my secondhand bandsaw apart to move it and decided to strip it down to check bearings etc. I had read that blade tension springs can be over compressed and decided to look at that too when the top wheel was off.

There is no spring inside the metal "box" that is moved up and down to adjust the top wheel position and hence the blade tension. The end of the adjusting screw simply pushes against the fixed part of the box.

I contacted Machine Spares Ltd., Brierley Hill (web address is Startrite.com) who kindly confirmed these saws did not have a tension spring!

Does anyone know how the manufacturers recommended the correct blade tension should be set?

Graham

 

[Pete Howlett]

I have a 301 and just jack it up to the max... the old pro way is to give the blade a tweek and it should stop yielding after a 1/4 turn. I'm not so sure about this blade tension thing on small saws but it is very important on large resaws etc.

 

[chipchaser]

Pete,

thanks for reply and advice. I cannot get any feel for the tension and generally just screw it up hard. I let it off when not in use. I ought to make the diy tension gauge described in "Workshop Machines" (Pub Taunton press ISBN 1-56158-765-6) from a bit of wood a couple of nails and some feeler gauges.

I have been resawing some part seasoned oak that I cut into quarter logs with my chainsaw. They just fit under the top guide, i.e. 6" thick maximum. The saw struggles a bit for power and has not always made a good vertical cut, both barreled and not following the fence. it should do better now I have reassembled it with the guides properly set. The original fence is very solid but difficult to set accurately.

I am actually not surprised it struggles as the motor is only about 550w and my chain saw (Husqvarna 350 petrol) works hard to rip along the grain whereas it cross cuts the same timber like the proverbial knife through butter.

I have a reasonable grip but the tension knob is small so I guess I am not overstressing it. The screw thread on the tensioner, which I estimate to be the weakest link, is showing no signs of abuse It appears to be M8 which should be good for 1500 lbs even if made in the lowest grade S235 steel. It is probably a higher grade steel.

I was very impressed by how simple and robust the Startrite is. I made some calculations and can't see any reason why the frame of the saw should fail to allow blade tensioning to at least the 15,000 psi often recommended. If I had time I might add an external tensioner to pull the knob and top wheel upwards. It wouldn't be difficult to do and it would be easily accessible for calibration. Another job for my to do list!

Graham

 

[lurker]

Pete,
I ought to make the diy tension gauge described in "Workshop Machines" (Pub Taunton press ISBN 1-56158-765-6) from a bit of wood a couple of nails and some feeler gauges.

Sounds interesting - do elaborate!

 

[chipchaser]

Lurker,

I expect that I would be in trouble if I posted a copy of the design in the book as Taunton Press, the author or both will have copyright. However, I think I can describe how it works. Apologies for how long this post has become.

I didn’t know that commercial blade tension meters are available until I read the article by John White in the book mentioned above. Here is an example http://www.toolcenter.com/62126.html

The principal of operation is to measure the distance between two known points when the blade is pulled into shape but with little tension and then between the same points again when tensioned.

Robert Hooke sussed out elasticity and the relationship between stress and strain in the 17th century. Google “Hookes Law” for details.

Apparently a 5” length of blade stretches 0.001” for each 6,000psi of tension that is applied.

The commercial instrument indicates tension directly on the dial.

The DIY model works the other way, you make a sliding gauge in 2 parts with positions to clamp the gauge to the blade 5” apart when closed. The gauge is clamped to the blade after taking up just enough slack to pull it straight between the measuring points. Clamp to the flat part of the blade behind the teeth.

If the gauge has a small known gap at the start the stress applied can be measured by re-measuring the gap after tightening the blade. Applying 15,000psi (the stress) would stretch the blade 0.0025” (the strain) and the gap would increase by this amount.
15,000 / 6,000= 2.5 times 0.001” which is 0.0025” or 2.5 thousandth of an inch
The change in the gap can be measured using feeler gauges.

In the article the body of the gauge was made of a piece of hardwood 5 3/8” long cut into two parts and joined to slide on a steel pin made from a nail. The second nail was used to form part of the gap into which the feeler gauge is inserted. Many imperial feeler gauge sets have a very limited range of blades of thickness in 0.0005” (half thou) steps. If a suitable screw were substituted for the second nail it could be used to adjust the initial gap to suit the feeler gauges available.

The saw should be warmed up before using the gauge to check the tension because blade and tyres will expand as they warm in use. The article suggests the DIY gauge read within 10 to 15% of the readings obtained using a commercial gauge.

Whilst writing this it occurred to me that, if it is possible to fit a cheap digital caliper in the gap between saw table and top guides (or take the measurement on the other leg of the blade), it would be easy to modify the caliper do this job. Drill and tap a hole in each of the jaws of the caliper to allow fitting of a pinch screw. Put washers between the caliper jaw and the screw head to offset the tool enough to avoid the set of the blade Turn the caliper on and set it to 5” gap*. Clamp to the blade with the pinch screws. Press the zero button. Tighten the blade until the caliper indicates 0.0025 inches.

* Actual reading should be whatever gives 5” between the clamping points.

The distance used for measuring should, within reason, be as large as conveniently accommodated on the saw you have and the numbers adjusted accordingly. So if the measuring distance were 6” then 15,000 psi tension or stress would give 3 thou strain or elongation.

Hope that makes sense!

Graham

 

[Chris24]

I'm most impressed, but can't help thinking that's more engineering than woodwork........
stand back and wait for the flack!......

I have a 352, and wind it up until there is about 1/4" play, sorry, lateral movement, holding the blade in my fingers, and not pushing TOO hard.

I use this as my starting point, and tweek from there. I find that different blades, age of blade, wood being cut etc all play a part...sort of suck it and see?

Further to above posts, I often find that a cut will wander when trying use the fence. The probability of wander is directly proportional to the importance of the cut. Is that a Startrite trait? I wonder??, or just me?


Keep it going, there's lots more good stuff to come on this subject.

Chris

 

[misterfish]

II often find that a cut will wander when trying use the fence. The probability of wander is directly proportional to the importance of the cut. Is that a Startrite trait? I wonder??, or just me?

With my 352 I spend a bit of time making sure the rubbing blocks are set up OK and use a decent (Duredge) blade and wander is not a problem. As others will tell you the important thing is to use good quality sharp blades: I was amazed at the difference they make.

Misterfish

 

[Chris24]

I completely agree with that statement. Unfortunately, like many, I tend to be lazy, or think that I can get one more cut out of a blade. In the end, if you think that the blade needs changing, then it probably does. Otherwise quality will suffer.


How many people keep a selection of blades for different jobs, and actually change them back and forth?

chris

 

[johnnyb]

i completely agree with your opinion on the 2 smaller startrite saws, robust and workmanlike although the 301 seems underpowered it is not really meant for resawing. all (small)pressed steel bandsaws are devoid of tension springs as there is a natural give in the frame (some more than others!)cast iron machine have very little give and need springs. i would be interested to see the tension the 301 can exert, i would be surprised if it was 15000 psi i would be impressed at half that. they are a far superior machine than many far east imports and hark back to a time when hobby machines were properly made versions of the bigger models. although mine is badged record it has the startrite plate on it. this also was when startrite were made by startrite.... i cant see why a diy bandsaw gauges are not on the market as dti s are pretty cheap and the mechanism is nothing special. the tension would surely be restricted to the tension that the threads could withstand on the tensioner before stripping out. re tapping this to a coarser thread or an acme thread may work.

 

[chipchaser]

I finally found time to test my 301 blade tension. I used a digital caliper to measure blade stretch as in this thread on UKW: Shop made bandsaw tension meter.

https://www.ukworkshop.co.uk/forums/view ... sc&start=0

With a ½” blade fitted I wound up the tension knob as hard as I could. Max elongation was 0.002” which is about 12,000 psi. The blade dimensions between teeth are 0.415” x 0.026” so csa is 0.0108 square inches and tension in the blade about 130 lbs force. I believe that equates to a total of 260 lb force at the wheel spindles. I have not tried to calculate the stress or the strain in the bandsaw frame due to its complex shape (3 boxes welded together).

With an old 3/8” blade fitted I wound up the tension knob again. I managed 0.0025” which is about 15,000 psi. I then decided to try increasing the tension by wrapping the small tension knob in cloth to increase my grip and thus torque applied. The knob on my 301 is only 43mm diameter and smooth black plastic. I found that I could wind the knob fairly easily to give 0.0045” stretch on the blade but the effort was not increasing proportionally. I believe the blade was beyond its elastic limit and stretching in plastic deformation mode. If it was stretching elastically the stress would be 27,000 psi.

There was no sign of any distress in the frame of the saw. I hardly expected to see any as the csa of the steel box section column alone, ignoring the blade guard which adds another 25% to the section, is 0.94 sq inches which is 87 times as large as the saw blade. I was able to reduce the elongation reading by 0.0005” by applying my weight to the top of the saw (Need to loose some weight  ).

A steel frame may be more flexible than a cast iron one but I believe that is mainly due to the amount of material in the cast iron frame. Even a hollow casting probably has walls 3 times or more as thick as the light steel box section of the 301. I suspect that the absence of a tension spring is solely because the 301 was designed down to a price.

The tension bolt itself is I think M8 or the imperial equivalent 5/16”. Recommended maximum torque for a M8, 8.8 grade bolt is 23 Nm. From a bolt stress calculator on the web, the axial stress in the bolt tightened to 23Nm torque = 56952 psi which, if the core diameter is 6.3mm and the csa 31mm sq or 0.048 sq inches, applies a load of 2,728 lbs force. The lowest grade bolt is about 1/3 as “strong” as an 8.8 grade so even using that the load could be 900 lbs force which I think would be enough to tension a 1” blade.

All these calculations were made quickly and roughly using data from the web as I haven’t had to do this for myself for years. Please advise if you see any errors or omissions.

I am happy that the limit of blade tension on the 301 was set by the size of the tension knob which the manufacturer carefully chose to ensure the rest of the machine, especially the blade, was not readily over stressed. Screwfix saw blades that I have purchased carry a warning not to overtighten them but does not specify an actual maximum tension! My gut feeling is that I would like a bigger tensioning bolt even though the calcs show its not needed and an indicator of the blade tension.

As to whether a 301 is meant to be used for resawing my view is that if the wood fits under the guides it is ok. Having found that the angle to which my fence bolts is visibly bent I can now see one reason why the cut wanders away from the fence. I expect that explains why some cuts were parallel but others not and why a hand guided cut to a marked line is easier to achieve. I need a replacement (straight) angle and also to set that angle, and hence the fence, to the blade. Even then I don’t expect, or need, it to be very accurate.

Happy sawing

Graham

 

[johnnyb]

wow thats a lot of tension on alittle saw. my old electra beckum 315 had loads of spring in the frame and as such would not tension very well, another problem was the side to side stiffness of the upper section which was very apparent whilst sawing. although this is not a problem on the startrite at all. i also agree that the tension knob is quite small and difficult to grip but is obviously capable of tensioning a 1/2 inch blade nonetheless. the only thing about resawing on the 301 is the motor power. the electra had enough power but didnt have the frame/guides to saw at all straight.

 

[chipchaser]

I was pleased that it could tension the 1/2" blade to 12,000 psi but would have been happier if it could reach the 15,000 psi which seems to be generally recommended.

The 27,000 psi figure for the 3/8" blade is not valid because as I screwed it up tighter the effort I had to apply was not increasing proportionately or, it seemed, at all. It was a bit like that moment when you realise that the thread on the bolt you are tightening has stripped. I felt I could have kept on stretching the blade but decided caution was the better option. Maybe the blade was of poor material?

I have an idea for a tensioning device that will indicate what the tension is but need some time to get round to it :lol:

Haven't worked out how the fence support angle got bent, it must have taken some effort!

Graham

 

[wizer]

Maybe the blade was of poor material?

I don't know about 'poor' material, but certainly differing materials will produce differing results. I have heard a lot about over tensioning, but I wonder how much damage it really would do?

Let us know how you get on.

 

[chipchaser]

Hi Wizer,

I am unsure of what was happening as, according to the tech info, Yield Stress of ordinary steel, used for building structures i.e. beams etc, is 36,000psi and better steels have much higher Yield Stress commonly up to 80,000psi but, exceptionally more i.e 100,000psi for ASTM AS14 alloy steel.

If the caliper readings are correct, and I have no reason to doubt them, then at 0.0045" elongation and thus calculated stress of 27,000psi the blade should still be in the elastic zone and so I think I should have felt more resistance to progressive tightening. In Fine Woodworking article they accept that readings could be 10 to 15% out but worst case still only gives a max stress of just over 31,000 psi.

As you say differing materials could give differing results. What I don't know is what grade of steel produces the 6,000 psi per 2 thou relationship. Could it be for a steel better than common carbon steel used in standard band saw blades? If so the calculation could be wrong :roll: However, I don't think this is really true as from tables found on the net Young's modulus is not significantly different for differing grades of steel, typically plus or minus 5%.

I will stick with it but need to do some productive work before I can spend more time on it.

Cheers all

Graham