Shopping for a Widebelt Sander: One Head or Two?
1. Would a single head, rubber contact drum/roller be better, worse, or the same vs a steel head (for our operation)? Achieving a flat, uniform surface is of prime importance.
2. We're looking at a machine with an inflatable platen. How does this compare to a felt platen? Would it do a better job at removing scratch marks left by the drum? Would it work on solid wood, or is the benefit only for veneered work?
3. If we stepped up to a two head machine, is it customary to run one head more than the other? We'd only run the first head to dimension and make flat and then run both on the final couple passes. Is this how this is customarily done?
4. How much does horsepower matter? We're typically gluing up 1/8" over-sized and then removing .01" per pass. This means approximately 10-12 passes to achieve final thickness. Our current machine is 18 hp and not capable of removing much more than that in one pass (with an 80g belt).
From Contributor J:
We run a two head 37" for sanding glued door panels/cabinet doors and face frames. The door panels are planed with a knife planer to 1/16 - 3/32 over finished thickness. We run 80 on the front and 120 on the rear with platen up until the parts are flush, flat, and smooth. This usually takes two passes each side. Then we switch to 150 front and 180 rear, lower the platen (felt) and make one pass each side. Then we finish sand with 150 in a RO. Both drums are rubber. We have never had a single head machine, and after using the two head, I don't think I would enjoy using a single head.
I have a light duty 43" SCM two head machine that would be the best of both worlds if it had more power (single 30 HP motor for both heads means smaller belt contact on the sheaves compared to a full 180 deg wrap for a single head setup, so it is easy to slip the belts on heavy loads). For cabinet doors though, it is just about perfect. We run one pass per side for both door panels and finished doors. For large uneven glue-ups it is not ideal, but the head configuration is.
It has a steel first drum for calibrating, rubber second drum, and a felt platen. The drums pivot with air cylinder actuators, so that it is fast and easy to activate and combination of drums and platen from the control panel. It is simple to run a rough pass with just the first head, then a second pass with two or three contact points. We standardized our grits and leave it set up for an accurate thickness reading off the second head. If we do go to a coarser grit on the first head, the readout is close enough because the belt is a bit thicker.
I won't argue that more hp wouldn't help in your situation, but there are many more factors. First, don't be afraid to use much coarser belts for the first few passes. They will create a much deeper scratch that will help the subsequent passes remove more material. I would at least buy a 40 grit belt for the first couple passes. See if that belt helps you to avoid bogging down your machine. You probably only have to conveyor speeds so just use number one.
Using just the drum on the planing passes is exactly right. Even if you get a two head I often advise to only use the first head since it is often very hard or steel. If you upgrade to a two head buy quality. Buy a drum, combi-head machine. I could write a book on why this configuration is better most of the time. None of the Taiwanese machines have air exclusion like a SCMI or Viet. With air exclusion a flip of a couple switches pulls both the drum and platen in the second head out of the way to protect them while you are abusing your first drum only. Air bladder plattens are nice but not great for the coarser work you are doing. They offer no real advantage and they do have issues with dry rot and leaking. The medium density rock hard felt that the better machine companies use is much better for your job.
If you set yourself up with a good two head with at least 25 hp on the first and 15 hp on the second you should be able to slow your feed down a lot and take a few .025-.030" passes with a 40 grit cloth. Cut the part to .040" over your final thickness. Switch that out for a paper 80 on the first head and paper 120 on the second head. Sand .020" per side and you are golden. Maybe 5 passes total. Hand sand with 120 grit and your sanding time should be very short. There is a trick to it though.
Planer sanders are great and a curse at the same time. Finding a well-designed one for cheap is very hard. Hold down rollers should be as close to the cutting heads as possible. There are cemco planer/sanders out there with 3ft between hold down rollers. Who thought that was a good idea? Timesaver does a good job on this. If you buy one get two sanding heads after the planer head. Run a minimum of 100 grit (better an 80) on the drum and 120 or 150 on the combi-head. They are not cheap but this configuration will give you one pass per side and light hand sanding.
From the original questioner:
To contributor W: Thanks for your reply. The two head machine is no longer available so Iím only left with a single head with an inflatable platen. This is okay as I was guessing a two head machine would still involve belt changes so we're left with changing belts either way. One question - this machine has a rubber contact drum. Aside from potentially screwing it up by feeding small pieces (not likely), would this be as good for our operation as a steel head? I'm guessing it would be a toss-up as it "should" (to my understanding of the differences) be less aggressive at stock removal but leave a better sanded surface. Is this thinking correct? Also, the inflatable platen should follow undulations (should they exist) more so than a felt platen?
The bladder platens offer no advantage at all for your application. They are usually set up with a bladder behind a piece of stainless steel with a piece of felt and graphite on it as the actual contact surface. They do not remove material well so they are only for light finishing. That is pretty much what a platen is for, but I am not the biggest fan of them unless they are needed for very precise control of the platen pressure. A good ole felt platen is just fine and it works with less chance for failure.
Timesaver made machines for years with 75 shore drums on the first head. I believe this to be a bit too soft for a multi head machine, but it is not terrible for a single head machine. 75 shore will cut plenty, but it is not going to produce the flatness of a 85-90 shore drum, or a steel drum. A softer drum will most definitely make a shallower scratch pattern. All of the features you seek in a single head machine are tradeoffs. A harder drum will cut flatter, but make scratches much harder for the platen to remove. A bladder platen will give you precise control over stock removal, but it will not remove enough material if you are trying to push your platen slightly beyond its design limits.
From contributor S:
I was wondering why you want a new sander. The sander you have is excellent for machining solid material. All the new wide belts have better features for sanding veneer. Ideally for solid you want a steel first roller for dimensioning, a second rubber roller can be used for a finer sandpaper so you can get a nicer finish with one pass. When I run solid with my SCM widebelt I turn off the rubber drum and the platen. How wide are your panels? You should be running them through a planer first. That would save you all the passes through the sander. If your pieces are wider then you should get a wide belt sander that has a planer in it.
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