Hacksaw tension

[srm_92000]

Hello all,

I just bought a new 12" hand hacksaw (Bahco 319) and wondered if anyone knew the proper method/spec for the tension on the blade.

My old one was so rough (25 yrs+ old) I'd just tighten it as much as I could but with this new thing I think you could break the blade if you did.

I was told in college to flick it with your finger and tighten till it pinged rather than a dull thud, but those blades must have been just full hard, unlike the new bi-metal types.

I don't really have a problem as it works great but just thought there must be some spec somewhere.

Steve.

[Pete]

This says 100kg


http://www.ebay.com.au/itm/BAHCO-319-HACHSAW-INC-BLADE-FREE-POSTAGE-MADE-SWEDEN-/260578217366

SINCE 1886, BAHCO HAS BEEN DEVELOPING AND MARKETING PROFESSIONAL TOOLS FOR USE IN A WIDE RANGE OF TRADES THE WORLD OVER.

A PROFESSIONAL AND STABLE HACK SAW FRAME. BLADE STORAGE INSIDE THE RECTANGULAR BACK SETION. CAN BE USED AS A COMPASS SAW. ALTERNITIVE 55° BLADE MOUNTING FOR FLUSH CUTTING. INCLUDES A 12" (300mm) SANDFLEX BI-METAL BLADE, 24 TPI.

- INCLUDES 12" (300mm) SANDFLEX BI-METAL BLADE, 24 TPI.
- SIZE: 300mm (blade)

HIGH BLADE TENSION + LIGHT WEIGHT AND COMFORTABLE +BLADE TENSIONING 100kg

[srm_92000]

Thanks, yes I had seen that but took it to be just sales blurb for the max it could take, should be some data for the blades somewhere but I can't find it so far.

Steve

[Lew_Merrick_PE]

Steve,

A hacksaw blade is nominally about .032 X .500 inches (= .016 in²).  Garden variety low-carbon merchant steel has a yield strength of 36,000 psi.  That means that it will not start yielding until a longitudinal (tensile) load of 576 lbs is placed on it.  Rc 50 tool steel has a yield strength in the 120,000 psi range.  That means that it will not start yielding until a longitudinal load of 1920 lbs is placed on it.  A power hacksaw blade is supposed to be tightened to (about) 70% of yield to allow for any "binding" in operation.  A hand hacksaw should be somewhat less than that as you are much more likely to "bind" a hand-held blade than a machine blade.  Right?

The only screw-tightened hand hacksaw I have left (having changed over to cam-lock type many years ago) has a 5/16-24 thread on it.  A very general torque value for a given load is given by: T = kPD (T = torque, k = friction factor, P = load, and D = major diameter of screw) or T = (.2)(576 lbs)(.3125 in) = 36 lb-in (= 3 lb-ft).  (Make no mistake, this is a really rough value calculation.  The k-factor incorporates friction, thread pitch, and surface finish into one value.  The variations from "calculated" can be quite high and this is really only an estimated value!)

I would be highly surprised if your blade did not have a much higher yield strength value than low-carbon merchant steel.  It is probably more like 60,000 psi for a cheap "soft back" type blade.  However, most people can only exert somewhere in the 10 lb-in range when "torquing" a wingnut by hand.  People with very strong hands might be able to hit 25 lb-in of hand torque on a wingnut.  (I actually tested this for SAE back in the mid-1970's).

Does this help?

[krv3000]

HI well I just ping it and alwas releve the tension when finisht

[srm_92000]

Thanks KRV'

I think that is probably what I will still end up doing,
I was just a bit surprised there was no guide with the new saw or blades.

Cheers.
Steve.

[srm_92000]

Hi Lew,

Thanks for the info, I'll sit and digest it another day    .

I was surprised when this arrived as it is just a screw tightening device, M6 thread with just over 1" leverage, seems very little friction, but you can only adjust to one full turn as the moulded head fits to the handle
Easily replaced with an M6 bolt though.
Blades I bought are the same brand as the saw (Bahco 3906-300-24) and don't get me wrong they cut superb.
I suppose from your info it's just not possible to over tighten then - frame will yield first at the 100kg as stated in the adverts - if your hands are up to tightening that much ?

Thanks again,
Steve.

[Lew_Merrick_PE]

Steve,

Being an incredible snob about such things, I only buy Lennox or Starrett blades for my handheld hacksaws.  I purchased eight of the SnapOn brand high tension cam-lock frames back about 1972 (when I made my living on the floor of the shop).  Four of the frames are "loaded" with 32 tpi to 13 tpi blades for steel.  Two of the frames are "loaded" with 16 tpi and 7 tpi aluminum blades.  One frame is "loaded" with a diamond round blade.  And the last is loaded with a 4 tpi blade that works a real treat on pot metal (I used to do a lot of die-casting development work).  I replaced the knurled tension nut in all of them with an Allen nut just so I could get higher tension -- but I set my blade tension by listening to the "twang" of the tightened blade...

[djc]

...A hacksaw blade is nominally about .032 X .500 inches (= .016 in²).

Those measurements seem to be the full blade size. Should you not calculate the cross-sectional area at the weakest point where the hole is? Roughly guessing here 2/3 that value?

[75Plus]

...A hacksaw blade is nominally about .032 X .500 inches (= .016 in²).

Those measurements seem to be the full blade size. Should you not calculate the cross-sectional area at the weakest point where the hole is? Roughly guessing here 2/3 that value?

Add to that the fact that only half the circumference of the pin is bearing on the blade concentrating all the load on a relativity small area the yield point is much lower.

Joe

[Lew_Merrick_PE]

...A hacksaw blade is nominally about .032 X .500 inches (= .016 in²).

Those measurements seem to be the full blade size. Should you not calculate the cross-sectional area at the weakest point where the hole is? Roughly guessing here 2/3 that value?

Add to that the fact that only half the circumference of the pin is bearing on the blade concentrating all the load on a relativity small area the yield point is much lower.

Joe & djc,

Actually, the mounting pin will be the weakest point.  I forget about such things as my saws have double-shear pins made out of fully hardened and lightly tempered 4340 pins.  A ø.136 pin @ 210,000 Sy in double-shear I have 5810 lbs of allowable tension before pin failure.  Most hacksaws have (A) pins in single shear, (B) pins with a not insignificant bending moment induced by the blade, and (C) a much less load-capable material.  (I was working a job shortly after I made the mass purchase of hacksaw frames where we were put on "work hold" for several days and there were several feet of ø1/4 4340 rod being thrown out...)

If the quality of the steel in the blade is at all decent, the bearing load in the blade induced by the pin would have to exceed (.032 X .136 X 68,000 =) 295 lbs before the area around the hole started yielding.  Good blades made from higher yield steel would carry more load.  Slowly yielding the hole area in bearing is not necessarily bad.  The area will work harden as it yields and could easily double the yield load before actually failing.  (Your mileage may vary...)

However, the bottom line is that a person using finger strength to tighten a wingnut is unlikely to overstress a hacksaw blade while tightening it...