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dr pepper
05-06-2010, 04:53 PM
As I've allready mentioned I had a barn find, a model engineers bandsdaw, well also with the saw there was a power hacksaw, a mini model engineers version that uses standard hacksaw blades, well thats what I have fitted, the originals that came with it are 40 years old and rusted up, they are a little wider, from bottom to top not thickness.
The problem is the saw will not cut straight, even after taking it all apart and cleaning, lubing all the parts.
The saw starts the cut perfectly straight, but then starts to wander out to one side as the saw cuts through the job.
I checked the saw arm to the vice and with a square it looks quite accurate.
I tried both a new hacksaw blade and a couple of the ones that came with it in better nick and I still get the same issue.
Any ideas, I'm fairly new to little stuff like this.

Black_Moons
05-06-2010, 05:26 PM
Id think its the same issue that occures with a real hacksaw:
Is the blade tight enough? aka, stupidly tight. Look at the lever mechnism in a normal hacksaw, lots of tension.
Is the machine using too much downforce? Enough force will start to deflect the blade. often its adjustable, even in a spring loaded unit. (or maybe the springs have worn out or one in a set is broken)

Does the hacksaw arm 'ways' have much 'play' in it?

Mike Burdick
05-06-2010, 06:00 PM
dr pepper,

This may not solve the problem but I am curious... What's the tooth count of the blade? On a hand hacksaw the finer the tpi the more the tendency for it to wander off course. Have you tried something like 14 tpi?

dr pepper
05-06-2010, 11:47 PM
Hi mike, didnt know that, the new blade I put in is a fine one, its all I had at 24tpi, I have some 18tpi ones ordered, I'll try one tomoz.
Will also tighten the blade silly tight.
The slide mech of the saw is quite rigid, theres no noticeable slop in the motion, theres a weight that slides to vary the downforce on the blade, I've only used this in its lightest position.
There are some real course blades with it, like 10 tpi which look like they are for wood, might also try one of those, there is some warp in the blade so they probably will cut steel.
I spose a neat looking cut isnt all that easy then with these machines, unless you can get a tall blade.

J Tiers
05-07-2010, 12:39 AM
if too much weight is on the blade, it will wander. Slide the weight back. Some go on two ways, with one sliding back farther

using a narrower blade also allows wander, that is one reason the real blades are wide, as in an inch or so.

is it a Covel type? those should hold a good cut..... the one I refurbed for my FIL did for sure, although I never tried it with a narrow blade.

Also, look at the mounting for the blade. The narrower hand hacksaw blades may let the wavy area of the teeth set against a support.... in that case, the blade may be setting at a very slight angle, which will cut at an off-angle almost from the start.

And, the blade should be quite tight.... too loose tilts and wanders.

oldtiffie
05-07-2010, 12:47 AM
As I've allready mentioned I had a barn find, a model engineers bandsdaw, well also with the saw there was a power hacksaw, a mini model engineers version that uses standard hacksaw blades, well thats what I have fitted, the originals that came with it are 40 years old and rusted up, they are a little wider, from bottom to top not thickness.
The problem is the saw will not cut straight, even after taking it all apart and cleaning, lubing all the parts.
The saw starts the cut perfectly straight, but then starts to wander out to one side as the saw cuts through the job.
I checked the saw arm to the vice and with a square it looks quite accurate.
I tried both a new hacksaw blade and a couple of the ones that came with it in better nick and I still get the same issue.
Any ideas, I'm fairly new to little stuff like this.

Any chance of a few pics please as it will save a lot of guessing?

Richard Wilson
05-07-2010, 04:45 AM
The Kennedy mini hacksaw used 12" hand hacksaw blades broken into halves, and the recommended tooth count was 24TPI. I had another small hacksaw which used 10" blades, and, at the time, you could still get, with difficulty
10" machine hacksaw blades which were thicker and wider and did not wander as much as a hand hacksaw blade. It might be worth trying some of the older engineer's merchants to see if they still have some. you certainly won't get them at the likes of B&Q

Richard

DougA
05-07-2010, 06:20 AM
I believe Starrett still sells power hacksaw blades. See if they have one that will fit. They are much stiffer.

Richard Wilson
05-07-2010, 10:33 AM
The OP doesn't say what length of hand hacksaw blade his machine uses. Starrett in the US still list 12" power hacksaw blades, but not 10", although Starrett in the UK only list hand hacksaw blades. The Starrett on-line shop in the US apparently can only sell to customers within the US.

Richard

Hal
05-07-2010, 11:01 AM
DP

Does your power hacksaw have a cam that lifts the blade so that the blade doesn't drag on the back stroke.
If it doesn't try reversing the blade so it cuts on the pull stroke not the push stroke.
It's worth a try.

Hal

dr pepper
05-07-2010, 11:50 AM
Yep well, er at first I had the blade in backwards, your right it is meant to cut on the back stroke, there is no cam that lifts the blade, however if you hold the saw frame while its running there is a definate lift effect on the non cut stroke, this is effected by the arrangement of the crank and the slide.
The machine takes what looks like 8" blades, I have drilled and cut 12" blades to fit, the blades that came with it look like they are made from heavy bandsaw stock.
I'll see if I can get a spare proper 1" tall power hacksaw blade and make it fit, in fact I'd make 2 maybe 3 from one blade.
I'll post some pics later, Ive been called out to work just now.

dr pepper
05-17-2010, 02:33 AM
OK back at work again, heres a couple of nasty pics of the saw in mention as requested.
The original question was that I'm having trouble getting the machine to cut straight, the blade wanders.

Oh yeah, thats a new stand I bashed from ply, I cleaned up the machines, they were scrap before I started, 30 years of pigeon poo and all that.

http://i807.photobucket.com/albums/yy352/widgidibbie/Picture412.jpg
http://i807.photobucket.com/albums/yy352/widgidibbie/Picture410.jpg

Richard Wilson
05-17-2010, 11:33 AM
OK, I can ID the hacksaw for you. Its a 'Duplex', and was a build it yourself job, described in Model Engineer in the UK around 1953. Until around 6 months ago, I had a set of drawings and a set of ME describing the build, but I sold them when I got the Kennedy. It was a perfectly good little hacksaw, and I think it was intended to use hand hacksaw blades. Check out the bearings where the frame runs on the long flat bar to make sure there is no slack, and check that the frame itself is not twisted.

Regards
richard

dr pepper
03-03-2013, 06:25 AM
Yes I'm reviving an old thread.

I have dug out this machine again after a house move.

I didnt know it went all the way back to '53, it does give it away a little when you look at the pictures on the tin that the spare blades are kept in.

I'm going to revisit this machine as now I have space to use it, the machine looks ok and doesnt show any major wear or out of straight in the frame, one thing I noticed is that the blade isnt sat perfectly straight as the teeth on the blade are making the blade itself sit squiffy, so I'll try grinding the teeth flat at the very ends where the blade contacts the holder.

The bandsaw also pictured maybe destined to the scrap bin, the blade guides look unsubstantial and theres no guides at all below the table, I think its probably only good for making jigsaws, I recently aquired another that more deserves the title bandsaw.

_Paul_
03-03-2013, 07:31 AM
I dont know if this will be any help but I have a 6" Qualters & Smith Donkey saw, unlike yours it takes propriety blades for this type of machine (14" 6TPI).
The one thing that is critical for a straight cut is blade tension it has to be correct in my case one and a quarter turns on the tensioner after it goes tight.

Paul

MrFluffy
03-03-2013, 07:52 AM
You can fit normal hand hacksaw blades in my little donkey saw, but when you do, it goes all over the place because of flex and the weight it puts on the blade. When I put real eclipse power hacksaw blades it cuts straight. The power hacksaw blades are the same length, but taller and thicker and this resists the twisting action.
I've been thinking about making a hacksaw frame take the power hacksaw blades to improve the straightness of my hand sawing too :)

dr pepper
03-04-2013, 05:15 PM
Might see if I can get a couple of finer power saw blades, like 15tpi, I have some in the 'shop here at work but I dont think the little machine would cope with rough ones.
I can drill holes in them with a reground masonry drill bit.
The saw doesnt cut well at all with handsaw blades, but its sort of ok with one of the blades that came with it.
I spose I need something around 1" tall.

velocette
03-04-2013, 11:20 PM
Hi Mr Pepper
What are cutting Pipe - RHS or solid square or solid round. Thin wall tubing is a nightmare to cut with a power hacksaw.

Check with a dial gage that the blade is running parallel with the vice. The second picture shows the blade on the same side as the vice and with an offset to the centre line of the connecting rod. You could try it on the other side of the frame.

Was there an improvement getting the blade in the true vertical after grinding off the contact area to the frame.

Check the motor rotation almost all small hacksaws cut on the "Pull Stroke" From Pic 2 rotation will be "Clockwise" and pulling the blade down into the cut.

Lots of tension and a sharp blade traveling with the blade parallel to the vice and traveling down at a right angle to the rise and fall piviot.

Weight on the blade needs to adjusted to suit. My own hacksaw I use between 1 kg and 5 kgs on top of the frame that weighs about 1.5 kgs @ 125 mm stroke.

18 Teeth Per Inch Hand Hacksaw blade with 3 to 5 mm tension from finger tight on the adjustment.

Please keep us all posted on progress.

Eric

dr pepper
03-05-2013, 05:01 PM
I'll be cutting all those I spose.
I cut a piece of 2mm wall box after flatting the ends of the blade, not as bad as it was but you can see its not true.
Yes the teeth are arranged as a pull cut.
I'll try the dial guage check on the blade tomoz when I return from work.
5mm pull on the blade, that sounds a heck of a lot, or are is it the frame giving a little.
I'd like to get the cut to 1mm over a piece of 50 by 25 so I can use the machine for fabricating.

velocette
03-05-2013, 09:50 PM
Hi Dr Pepper
Perhaps I should have explained a little better the frame on my "Heath Robinson" hacksaw is fabricated RHS frame the normal tension is Three full turns on a 1.25 mm pitch thread.

keep in mind that you need at least 1 1/2 teeth to be in the cut I. E. 1 Inch = 1000 / 80 thous or 2 mm = 12.7 * 1.5 = 18.75 teeth per inch for 2 mm

With a little more tuning you can achieve the accuracy you stated and much better.

I have posted my setup Thanks to "Dave" for a hijack on his posting on power hacksaws.

http://www.homemodelenginemachinist.com/f13/power-hack-saw-d-m-15704/

or direct to You Tube

http://www.youtube.com/watch?v=PkT5Gauwv9M

One of my old mentors sayings was "If your "Improvements " do not work you can put it back the way it was" it was working OK"

Eric

dr pepper
03-06-2013, 01:39 PM
I spent an hour or so today on the old machine.
I can cut 12mm round solid bar, with the usual runout problem.
However as soon as I try to cut anything bigger like 40mm box the saw jams up and either wrecks the blade or forces the belt to slip on the motor.
I tried 14 tpi by 3/4" blade stock, 18 and 24 tpi hacksaw blades, all showed up the same problem, even with enough tension to play a tune on the blade, I also tried removing the weight altogether.
Seems the machine has got worse.
When the saw frame starts the backstroke there is a lot of downpull, and I mean a lot you can just about hold it from doing so, this downpull causes the blade to dig right into the material and bind up.
I'm thinking now maybe the assembly isnt as per the original duplex design intended, the machine has a reduction gear from the input pulley I dont think this is original.
I'll continue trawling the net to see if I can get any kind of drawings for this machine, or maybe compare the figure 8 link geometry with the popular kennedy saw which looks similar (by figure 8 link I mean the bar that looks like a figure 8 with bearings either end that goes from the crank to the saw frame), it seems that the height of this link with respect to the crank and saw frame will have an effect on the amount of pull down on the blade when the cut stroke starts, maybe the original builder didnt make it to the drawings,

dr pepper
03-06-2013, 10:55 PM
I think I sussed it in the end.
Flipping motor was going the wrong way.
Thinking about it things were happening backwards, the blade was being pulled down into the work on the cut stroke and pushed on the non cut stroke fairly parallel.
Reversing the motor means that now the blade is lifted slightly on the non cut stroke and pulled back parallel on the cut stroke, exactly what you'd expect for a power hacksaw.
The cut still isnt perfectly straight, about 1/2 a mill off, but rather than be unpredictable and all over the cut is now the same every time, the wander is the same no matter what size is cut, meaning that now its probably just an aligment issue, I probably put something back wrong.
This saw must have never run proper, it was like this when I found it in a barn.

velocette
03-07-2013, 01:12 AM
Hi Dr Pepper
Good to see You are getting on top of the problem now. Nice one reversing the motor to get a better cut.

Just a Thought If the Piviot for the Guide frame is above the crank C/L then this MAY be the cause of the excessive down force on the blade on the pull stroke.

With my hacksaw the bottom of the swinging arms is below the C/L so my advice is not correct for all hacksaws.

I have done rebuild on a local built power hacksaw with a 300 mm blade with the piviot on the same axis as the crank and it was fitted with a switch to reverse it.

It was much better cutting on the "Pull Stroke" so that is how it now runs.

Eric

dr pepper
03-07-2013, 05:43 PM
Yes I had to semi dissect the motor and resolder the connections to the start and run windings, cleaned up the centripital switch and the bearings in the process.
With the motor going the right way if you slightly lift the frame while the saw is cutting you can feel the machine lift the blade on the forward stroke, and theres no discernable pull up or down on the cut stroke, so I think the pivot geometry is meant to do this, saves the need for a cam to lift the blade.
I now have the cut to less than 0.5mm from square, and the machine still runs ok with the weight in the forward most position.
I can now use the machine instead of fall over it while looking for things.