There has been some talk here and there about grit barrels being heavier than original blade barrels, thus putting more load on motors and belts. When my belt broke down after some heavy use with a grit barrel, I started wondering about the exact truth in this. So, today, as I was replacing the blade barrel with a grit barrel on my new Skil 100, I took the time to weigh both on a precision scale, just to know what I was talking about…
Here are both barrels, standing side by side. Same diameter but, just holding each in your hand, you seem to notice a difference in weight:
Ok, let’s weigh them. Blades, first:
And now, the grit barrel:
Let me add that the blades barrel is the original Skil barrel, the grit barrel comes from Shapers Australia. Others may not weigh the same. All weights are in Grammes.
1
gramme = 0.0352739907229
once
**The difference is almost 70 grammes. I’m no engineer, but I think it’s a lot. What’s your take on this?
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Pete, thanks for explaining that so even I could understand. I wish they would make a double spiral cutter like in some woodworking planers, ie: Porter Cable 126 door planer. Carbide of course and easily sharpened.
The weight is more, since the weight is probably evenly distributed, the moment of inertia will be higher.
The torque variation will be dampened by this moment of inertia, so there will be less vibrations (assumend they are both well balanced).
My guess would be that the heavier barrel would be better for your belt. Since it’s a net coupled universal motor the motor will always be running at maximum torque, so the load on the belt won’t be influenced by the barrel weight
I believe the balancing of the weight will have the biggest impact on the lifetime of the belt
I agree with you, hans, but only once it’s turning. At the start, there should be more force required to start moving a heavier barrel, don’t you think?
Chris: yes, probably more friction, too; the blades hit the foam only twice in a turn whereas the grit makes contact almost all the time (except for the two relief grooves).
I believe the friction is the same and depends on the speed you move the planer. If it stand still and is turing, the barrel won’t hit any foam.
The motor has no speed control, it ALWAYS runs at maximum torque. The heavier barrel will run slower and will take longer to reach it’s top speed.
See the picture below (the motor is the second one “series connected DC-motor” = “universal motor”) Don’t be confused the series connected DC-motor can run on AC
Made a mistake in post above, removed it (post is now correct).
Indeed when the rotation speed is lower the torque will be higher. So indeed the belt is loaded a bit heavier
Sorry if I confused a bit, but by writing it, I came to the conclusion that it is true that the belt is loaded heavier. But it’s because the motor’s characteristic. With an other motor it could have been that the belt was less loaded with a heavier barrel.
And indeed especially when spinning up, the belt is loaded for a longer period at a higher torque.
Just don’t move to fast with the planer and ther shouldn’t be any problems.
Hans is correct, the current load on the motor is greater during start up only. Once the barrel gains momentum, the current stabilizes. However, when the grit barrel comes in contact, there is a higher current draw since the friction is continuous vs. intermittent with blades. The loading of the drum is transmitted along the belt to the motor, so there is more stress on the belt and sprockets. The belt gives first since it takes a very long time to wear the teeth off the sprockets. This can be easily measured in comparison to a bladed drum using a clamp-type current meter; the kind that clamps over the power cord. The current will be proportionate to the mechanical loading. This is why sanders use a differently wound motor for higher torque at 2K rpm vs. a planer which is much lower torque at 16K rpm. A grit barrel is a sanding device, not a cutter.
There is another issue on the Skil relative to belt wear, in that the belt will either rub against the cover plate to the drum or try and run off the small sprocket at the motor. In the latter case, the washer under the sprocket tries to stop it, but doesn’t do a very good job. In the counter-clockwise rotation, the belt tries to move towards the planer. It was designed to do this so that the belt wouldn’t fly off the other way. Balsa, check for any rubbing at the big sprocket and any irregularity in the feel of the belt as you rotate it by hand at the small spocket. Email me if you find any and I’ll tell you how I fix this.
The planer with blades experiences much less drag as it shears the work, like Pete says, the grit drum is in a constant state of friction
in contact with the work.
I was burning up motors on my centrifugal blade exhaust fans, I took them to a motor repair/rewind guy and he immediately saw my problem, the centifugal fans need a baffle.
He hooked my motor up to an amp meter and it was drawing X amps, as soon as he slid a baffle infront of the exhaust port the amps dropped instantly, there is so much going on behind the scenes that as laymen, we just don’t see or understand
I was completely baffled too, but having the air flow slightly restricted, made the ampere draw drop dramatically and instantly, my thoughts are, unrestricted motor, no strain, better for it, but it is exactly the opposite and before surfboards I was studing to be an electrical engineer
What do you guys think about the effect of increased heat (from grit barrel/drum) on the bearings in Skils? I know it can fry even the best bearings in Hitachis, and cook the seats too. I overheated 2 or 3 Hitachis and ruined the bodies before I changed my routine (and alternated planers to allow for cool-down between boards). And those were with blades.
The guy who works on planers here in Melbourne told me 15 years ago that he was seeing lots more cooked with grit drums, from guys that didn't kill bearings using blades.
Nowadays I'm back to using one planer at a time. The vac system helps with cooling, and I don't ''rough out'' large batches continuously anymore. And I'm shaping EPS, which cuts a lot easier. And my ''blanks'' are stupid-close. Not much time on the planer.
The newer grit drums, with the spiral ''blades'' of grit, really intrigue me though. Could that help with heat issue? Anyone out there using them?
It alway draws the same amount of amps if Ra is constant (no matter what the speed is). Ra is only dependant on the temperature.
So if the cooling falls away, the amps drop because Ra rises.
So a freewheeling motor draws the same amount of amps as a loaded motor. But the rpm will be higher. And since power(watts) = rotational speed x torque, the torque will be lower. (power(watts) is proportional with current(amps) if voltage is constant)
In the case of the squirrel cage blower that Jim mentioned, these work by creating a pressure differential to move the air. Unlike a fan, a blower works better against a restriction to the flow since it has more time to build pressure when the air isn’t moving so fast through it. That’s why a baffle increased it’s efficiency which lowered the motor temperature and the current draw.
On the grit barrel, the real issue is the mechanical loading. The cutter drum or grit barrel shaft is always under axial load due to the tension of the belt; this loading tries to pull it at an angle from the normal axis of the shaft. Anyone who has used a grit drum knows that it tends to suck the planer into the foam, especially if the cut is deep. The loading of this is transmitted to the shaft bearings, the sprockets, belt, and finally to the motor and an RPM drop results. The energy balance is like Hans says: Power = RPM x Torque. Under load, the RPM drops and the torque increases so the power is still the same. However, that is steady state conditions. In real time, the current does increase momentarily to overcome loading then returns to normal. Most current meters don’t detect the spikes but average them over time, so that’s why a higher current is measured when loading occurs. All loading, mechanical or electrical (resistance), creates heat. This is what melts the bearing housings in Hitachi’s and prematurely wears the belts on a Skil. Electrically, bad switch contacts or brushes cause arcing and shorting due to the heating.
Shapers AU also sells a version of the grit barrel with a bare spiral space. They do caution that this results in an unbalanced spin, which may create more shaft loading that a full grit design. I think that the tungsten teeth are too aggressive. Probably only something on the order of 60-80 grit will work well. Maybe after I finish the other 200 things I have to do I can look at this.
Some shapers use two planers, one with blades and another with a grit barrel. The grit planer is only used for very shallow finishing. Other guys just use blades with shallow cuts, and sharpen very frequently. If you want to just use blades, I would recommend carbides and change them after 10-15 boards.
Pete, I was using my Hitachi with the continious grit drum today and I have always had difficulty of anything more than a few thousandths in depth and could not understand why it would suddenly plow down into the work when on such smalls cut, this explains it very well.
I have seen an Australian version with a Mfgd. cutter head that is Z shaped, with structured grit on the protruding flanges similar to the placement of blade orientation, this has looked a lot more functional to me.
The constant contact does not allow for removal of waste material like in the chip breaker zone of blade cutters, which the Z type grit appears to have
HAve had similar experience with diesel compressors. We use the compressors in concrete repair industry, the units have side doors that flip up that the guys would open the doors thinking it was cooling them off better but it ran them hotter. Doors closed with the fan cooling them keeps em right, baffling the flow.
