Electric motor for hacksaw machine 1

[piedog]

Hi all,

I work for a bikelock company and have been given a project to make a hacksaw machine. I am essentially copying the machine found in this video, skip to 1 minute and 17 seconds.

https://www.youtube.com/watch?v=Yh46F3we1-M

I have settled upon a 0.75kw three phase motor using a single to three phase inverter, allowing the speed to be adjusted between around 100 and 50rpm. What I cannot figure out is how much torque will be required, and therefore I do not know which motor to choose. From my own research it appears that 4 pole motor will be best, but does anybody know a way of calculating this to a reasonable accuracy so I can be sure, or is it is just obvious that one of these options is best?

Toque values
0.75kw 2 pole motor = 2.52Nm at 50Hz
0.75kw 4 pole motor = 5.08Nm at 50Hz
0.75kw 6 pole motor = 7.66Nm at 50 Hz

Thanks for reading and it will be great if anyone can help me.

Tom

 

[SuperSalad]

If this is to demonstrate resistance to the efforts of thieves, I suspect you'll need a significantly more powerful motor to be representative.

This is the best (read quickest) reference I could find to human torque capabilities: Nasa Study on Human Performance Capabilities

It mentions about 17 Nm of torque barehanded.

Andrew H.

http://www.mototribology.com

 

[3DDave]

The torque required to make the cut depends on the length of the lever arm that is used times the friction of cutting the metal. Since hacksaws have very small engagements the amount of friction force is relatively small. In that demo video the blade is not engaging the metal of the lock hasp so there is very little friction force. Multiply the friction force by the lever (which sets the stroke) and then by the rate in radians/sec to get the power.

I expect thieves will mostly lean towards cordless angle grinders and the typical manual tool is a bolt cutter.

 

[piedog]

Hi SuperSalad,

Thanks for the link, that is interesting.

Yes, we are trying to see how resistant our components are to hacksaw attacks and also just have a consistent method of testing. At the moment we are doing it manually and of course this is very inconsistent. Also there are companies who perform external validations that use this type of machine to test locks and give them ratings.

How much torque do you think the machine in the video is producing? The way I have got to this point was understanding that cyclists can produce around 800w whilst sprinting for a short burst, and that seemed like more than enough power for this application. Is this a case of making a free body diagram and working it out? From what I understand the greatest torque will be required on the backstroke when the arm needs to be pulled up and away from the component being cut.

Cheers for the response

 

[SuperSalad]

Again, if you are making it representative, I'd suggest not lifting it off for the backstroke like this one:

https://youtu.be/NH_9-Om0Txo

Thieves aren't going to be concerned with proper cutting technique and are just going to thrash it as fast as possible.

Andrew H.

http://www.mototribology.com

 

[piedog]

Hi 3DDave,

Thank you for that, I suppose the problem I have with performing your calculation is that I do not know how to calculate the frictional force. Assuming this is very low, however, I would assume that a 750w motor will be sufficient, providing that it has enough torque to complete the backstroke, when the arm needs to be pulled up and away from the component being cut.

 

[piedog]

SuperSalad,

My colleague, who has done lots of hacksaw testing, thinks that the most effective way use a hacksaw is putting your weight into the saw on the stroke, then taking it off one backstroke (so not lifting it up, but it is the same effect as doing so). This, combined with us wanting to imitate testing companies, is why we are doing it like this. Maybe what you are saying is right though and he is not doing it the most effective way...

 

[3DDave]

You can't calculate the force, you measure it by taking a hacksaw, putting whatever load you want, and then measuring the amount of pull to get it to move. The backstroke force is lower than the cutting stroke. There's no need to lift the blade except for longevity of the blade; not a concern for product testing.

Yes, most of a horsepower is larger than most bike thieves will produce.

 

[BrianE22]

If you matched the power of a Sawzall then you should be in the ballpark. I can't find published specs for the motors on these but the amps and voltage are listed. For 13 amps and 115 VAC and assuming a Universal type motor (maybe 70% efficient and .9 PF), you'd be looking at an output power of the motor of about 13 x 115 x .7 x .9 = 897 watts. You'd then have some losses in the mechanical drive system.

 

[LittleInch]

I think you want a scotch yoke mechanism.

The issue though is that the "test" is relatively meaningless without some agreed parameters between lock manufacturers. You might be able to judge between different materials but that's about it.

The two machines noted have virtually no weight or force being placed into the cut unlike what a human would do or would do with a hand held jig saw.

Therefore I think your torque is very low.

At 50 htz a 2 pole motor would normally do 3000 rpm, 4 pole 1500, 6 pole 750 (I think).

therefore your speed controller will need to operate at some very low Htz to get 50 to 100 rpm. Really not sure it's going to like that very much. I suspect the motor you reference has some gears inside the motor, but you might be good. but I would go for the 6 pole motor if I was you.

But why don't you just buy one of these and build a little jig to hold it onto the lock?

https://www.amazon.co.uk/Makita-DJR...04&hvtargid=pla-423631970638&psc=1&th=1&psc=1

Most of them do variable speed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moon161]

^I was gonna say just buy a sawzall.

 

[IRstuff]

Or a bandsaw

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[piedog]

BrianE22 - thanks for the info

LittleInch - yes you are right with the figures you gave, 4 pole is around 1500rpm. I forgot to mention that I will be using a 15:1 gearbox to get the speed to around 100rpm, and then use the inverter to vary it between 100 and 50rpm. What I didn't realise is that with the gearbox the torque will now be 15x more than the figure in the motor spec, so this amount should easily be enough. We did consider buying used machines or doing the sort of idea you have suggested, however for various reasons my boss wants to replicate the one I have shown in the video.

I had a discussion with someone who runs a business making machinery and he agreed that a 0.75kw 4 pole motor geared at 15:1 will be a good fit for the job. Maybe when it is finished I will post up a video of it on here. Thanks everyone for your input.

 

[Compositepro]

Test methods have to be identical to get comparable results. This includes the exact type of blade used (new blade for every test), cutting speed and force. I do not see why a variable speed would be desirable. It just provides another variable that could be set wrong.

 

[itsmoked]

Just buy one! A measly 160GBP

Keith Cress
kcress -

http://www.flaminsystems.com