Power Hacksaw

Before milling or turning, stock typically needs to be cut to length. My first tool for this task was a handheld power bandsaw lashed to a plywood frame.
This worked quite well, and is still functional. The reason that I have tried other means for cutoff is that it did not do well with large (say 3 inch diameter) round stock; the blade did not travel straight in the vertical plane. Part of the reason for this is that it is still a handheld tool and it is difficult to maintain constant pressure during a lengthy cut. Also, the wood frame is too compliant.

Next I purchased a bandsaw from Little Machine Shop.
This was better because the frame was stiffer. But after a year of occasional use, the rear guide assembly failed. Again, this is still a handheld tool and I was probably forcing it too much. LMS sent me a new guide assembly and it is again working.

Large, free-standing metal bandsaws have hands-free operation and should perform better than either of these tools. They also have limit switches to shut down the blade at the end of the cut. But I do not have room in my shop for this.

If you search for “power hacksaw” on Youtube, you can find videos of many homemade saws. Invariably, they involve a crank to achieve reciprocating motion. This kind of gadget has quirky appeal; I decided to try my hand.

Here is the result.
What is different in my design is the use of a large (NEMA 34) stepper motor. This avoids the cost of a gearmotor and the complexity of two stages of belt drive.

Two more photos:


The limit switch:

The motor control: a 48 volt supply, a step motor driver, and an Arduino Nano to generate pulses at a ramping rate.

This is an auxillary fixture to hold large round stock.

Here is an extra weight for the front end, but it was too much for the 640 in-oz rated step motor.

Finally, a short video of the operation. Click on the image and you will be taken to my Flickr site, where the video can be seen.

Design and Construction Notes:
1. The frame for the saw blade is from a Stanley hand saw (STHT20138). The additional bow under blade tension is appreciable and must be considered when drilling holes in the mounting brackets, i.e., have the blade under tension when you spot the hole locations.
2. The fit of the blade end holders in the frame is loose. I was able to improve the squareness of the saw cut by inserting shim stock into the gaps.
3. The base is made from 1.5 x 1.5 inch “T-slot” material. Guide rails are 5/8 diameter steel shaft.
4. I tried Oilite for the linear bushings, but could not align well enough to prevent binding. Final bushings are teflon filled acetal.
5. The motor is rated 640 in-oz (at 5.5 amps/phase) but I think this means holding torque. The relevant rating is 475 in-oz driving at 100 rpm and 48 volts. 48 volts is much more than needed to push 5.5 amps through the 0.43 ohm winding resistance, but is needed to obtain a sufficient rate-of-change of current (4 mH/phase).
6. The motor has 200 full steps/rev but its driver is set to “microstep = 2”, so 400 pulses are needed per revolution. I run at 1.0 rev/sec, so the pulse source is a maximum of 400/sec. On acceleration, I ramp from 40 to 400 pulse/sec. I think that a 400/sec constant source would be OK (maybe a 555 timer), since there is no problem recovering from a stall when the source remains steady at 400/sec.
7. In operation, 0.7 amp is drawn from the 48 volt source, or about 34 watts. Most of this is accounted for as ohmic loss: 2 phases * 0.43 ohm/phase * (5.6 amp)^2 = 27 watts. The supply for the Arduino is just a 5:1 resistive divider from 48 volts. Wiring for the limit switch should be shielded.
8. The crank length is 2.0 inch, so the push available to the blade is 475 in-oz / (16 oz/lbf * 2 in) = 14.8 lbf. The crank grips the motor shaft with a steel shaft collar. Link pivots are shoulder screws running in bronze bushings, oil lube.
9. The two guide shafts together weigh 3.5 lbm and are nearly centered over the cutting point. Including the saw frame, the total down force at the cut is about 5 lbf. The machine runs without stall at this cut pressure. But if I add the auxillary 1.8 lbm weight at the front end (increasing the down force at the cut to about 8.5 lbf), stall is a problem. I am using a 14 tooth/inch blade, but did not see much difference with 24/inch. NEMA 34 step motors of twice this rating are available and should allow use of the extra weight. I assume that this would increase the cut rate.
10. Cutting is slow but square. I can do other tasks while the saw runs.

Here is a parts list. I have not been careful about fastener quantities, and I did not even try on small items like standard flat washers, wires, switches, etc.
Hardware list

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