Power Hack Saw help needed

**garage nut** · Oct 22, 2017, 11:59 PM

I did not change the fibre gear in the motor. did not know one has to change them. I will rather then develop a reduction system for a little 1Hp single phase I have sitting on the shelf.
I will prioritise the advice.
1) Stronger power supply,
2) Box or remake the legs of the frame. What will be the strongest....2 30 x 30 x 5 angles welded up to form a 30 x 30 box or a piece of solid 30 x 30?
3) I like the over centre tensioner. Would like to see some examples.
4) Design that lifting cam on the backwards stroke. I must say I have watched numerous videos and have never seen it on a DIY saw, but I am in for a challenge.
I think I did thank everybody for their input. That is why my heading was "HELP" and not just "Here is my Saw"

**Frank S** · Oct 23, 2017, 12:50 AM

Garage nut I am trying to map out the geometry of your saw as built to attempt to work out a design the cam system I mentioned
when dealing with a saw if this nature there are a lot of things going on at the same time and even slight changes in the geometry or placement of a return stroke lift could make the difference between a smooth cutting machine or one that slams the blade down on the cut stroke causing the teeth to rip off.especially since you have what I wold call a walking frame instead of a spine that the frame of the saw slides on. Without actually building one just like it to test any of my theoretical lift designs on is also going to be a challenge. I could dig out my old saw and refurbish it but since it has the slide motion a lift would be totally different

**garage nut** · Oct 23, 2017, 05:06 AM

Any good ideas how to make the sliding type, but with bearings or something to allow adjustment of the slide to the frame?

**olderdan** · Oct 23, 2017, 07:31 AM

I totally agree that a blade lift on the return stroke would be the ideal, and most industrial machines have this as well as a dashpot feed but these are heavily built affairs where such items are required. For the light amateur workshop machine I am not sure if this is really necessary, and a compromise can be achieved through geometry. As an example the machine I built http://www.homemadetools.net/forum/h...6648#post84095
does not have a start capacitor fitted and as such sometimes starts in reverse, if I run it like this it will hardly cut at all. Indicating to me that the geometry is working against it and not with it. As built the cutting stroke is a pull stroke with the motor arm rotating downwards and this works fine. In use the blade frame can be seen to lift slightly on the return as long as to much weight has not been added. I think what is happening is similar to using a hand hacksaw in that a reduced pressure is used on the back stroke rather than lifting the blade.
I freely admit that this was not factored in at the design stage and simply worked out that way (we can get lucky sometimes). I use standard by-flex blades which are cheap and plentiful and they seem to last a reasonable time and I use it a lot for quite large lumps of steel.
If I wanted something with a better performance I would go for a home built horizontal band saw for metal which gets rid of the problem.
I look forward to any ideas to enhance my machine that are not to complicated.

**tonyfoale** · Oct 23, 2017, 08:23 AM

Originally Posted by olderdan

... if I run it like this it will hardly cut at all. Indicating to me that the geometry is working against it and not with it. As built the cutting stroke is a pull stroke with the motor arm rotating downwards and this works fine. In use the blade frame can be seen to lift slightly on the return as long as to much weight has not been added. I think what is happening is similar to using a hand hacksaw in that a reduced pressure is used on the back stroke rather than lifting the blade.

Alan,
That's a practical example of what I was saying in my previous post. Nice to see that it works in practice even if it was accidental and probably not optimised.

**Frank S** · Oct 23, 2017, 08:45 AM

Olderdan; tonyfoale mentioned much the same thing in one of his posts. about the slight upward push the con. link can exert. This leads me to a thought of an inverterd pendulum type counter weight mounted at the crank arm pivot that would swing outward away and hit a stop on the end of the con. link as the crank arm reached the bottom of its stroke then as the angle of the con. link would change as it reached the apex of the stroke the weight of the pendulum would shift applying slight downward pressure to the con. link. This possibly could be automatically activated by gravity for slow speed rotation but for high speed rotation the over-center effect would require a cam to be mounted and a cam following bearing to be mounted on the weight to trip it back and forth.
for controlled down feed this would be accomplished via a simple hydraulic cylinder with an internal check valve on the piston and a metering valve with tubes leading from the top port to the bottom ports of the cylinder.

**olderdan** · Oct 23, 2017, 10:46 AM

Tony I meant my input to be a follow up on yours.
Now your posts have really got me thinking, it would be a simple matter for me to alter the mounting point of the con rod to the rear of the frame in increments, at the moment it is midway and if I am thinking right, raising it would maybe as you say optimize it to a degree, I will give it a try.

**tonyfoale** · Oct 23, 2017, 11:05 AM

Originally Posted by olderdan

Tony I meant my input to be a follow up on yours.
Now your posts have really got me thinking, it would be a simple matter for me to alter the mounting point of the con rod to the rear of the frame in increments, at the moment it is midway and if I am thinking right, raising it would maybe as you say optimize it to a degree, I will give it a try.

Alan,
This can all be worked out before cutting metal. If you email me a sketch of your saw with dimensions and weights I could soon give you some options. Oh yes and the crank RPM.
I had never thought about this until a couple of nights ago, largely because I don't need a power hacksaw. I use a wood chop saw or wood table saw for lumps of Aluminium as per a previous post. Most steel that I need cut is tubing and I use an abrasive chop saw for that. I would like a nice band saw but lack of space leads me away from that. I only have the little Burgess which I mentioned a while back.

**olderdan** · Oct 23, 2017, 12:05 PM

Thanks Tony thats an offer I cant refuse from the geometry guru, I will gather the info you need soon.
Regards
Olderdan

**garage nut** · Oct 24, 2017, 05:09 AM

Power Hack Saw help needed-motor-direction.jpg

Good day guys

1) Question still...If I want to make the legs stiffer, do I weld another piece if angle to the leg, forming a box or just use a piece of 30 x 30 solid. I have to go and buy either.

2) I can change the direction of my motor by removing a bridge and fitting a Cap and then fitting the bridge where I removed the bridge. What is the normal way of doing this with a switch? do I buy a second cap or is there a special switch I must ask for.

3) To reduce the speed of the motor I will used an idler shaft with pulleys, Will a piece of normal 20mm mild steel be ok for this shaft.

I want to use this power unit to drive my ring roller I am also in the process of making. Huge thing to roll 60mm 3mm walled pipe for a car chassis.