interestedparty
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| posted on 24/2/11 at 03:11 PM |
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Help me design an off topic machine
Friend of mine is a cabinet maker, and has a job coming up which requires a machine upgrade if it's going to be economical to do, and he needs
to improve his crosscutting.
Basically the idea is to place the pieces against a fence that is at right angles to the travel of the sliding table which is munted to one side of
the saw blade. The fence has a movable stop, so that the pieces will be cot to an exact length. All this is traditional wood working stuff.
Now the problem is that wood is always fed against the rotation of the blade, which is the safe way to do it when hand feeding. If it is fed
the WRONG way (which is what the arrows indicate in the diagram) then that is what the Americans would call a climb cut, and the blade is inclined to
snatch the wood.
However, feeding the wrong way does stop the problem of the wood chipping out (blow out) on the underside (the bit where the blade is coming OUT of
the wood). The same effect can be seen on when cutting plywood with a jig saw, the top edge (where the blade is coming out) is always much rougher
than the bottom edge. Professional table saws have a small scoring blade in front of the main blade and in line with it. This blade turns the other
way and cuts a very fine slot which prevents the main blade causing blow out on the underside.
So, what we are trying to do is to come up with a way of power feeding the sliding table. If, for instance, the table was powered by a lead screw,
then the wood would advance forward at a speed controlled by the speed of the screw, and not only would that be perfectly safe (the screw would
prevent the blade snatching) but the finish on the cut would be better and blowout would be virtually eliminated.
But a lead screw would need a motor or drive mechanism that could be quickly and easily reversed, and which ideally would have the reverse motion
faster than the forward, cutting motion.
Better still would be a feed system that could be engaged, make the cut, and then be disengaged to allow the table to return. A lot of these cuts
would only be 100mm, and quite a lot might be 700 mm or so, so variability is needed too.
Anybody have any ideas that could be used?
[Edited on 24/2/11 by interestedparty]
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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tony-devon
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| posted on 24/2/11 at 03:23 PM |
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I always cut against the blade as is correct, and have found that by varying the height of the blade, you can alter the way that it breaks out the
bottom, think its to do with the angle and direction the tooth is travelling when it breaks thru
I cut wood and plastic, TCT saw blades make short work of it and leave a lovely finish
Im not very technical, but the teeth on my blades alternate in shape, Im assuming that they do this to help with chipping?
heavy is good, heavy is reliable, and if it breaks, hit them with it
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dinosaurjuice
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| posted on 24/2/11 at 03:25 PM |
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negative rake blade. use them all the time, no problems.
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interestedparty
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| posted on 24/2/11 at 03:28 PM |
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quote:
Originally posted by tony-devon
I always cut against the blade as is correct, and have found that by varying the height of the blade, you can alter the way that it breaks out the
bottom, think its to do with the angle and direction the tooth is travelling when it breaks thru
I cut wood and plastic, TCT saw blades make short work of it and leave a lovely finish
Im not very technical, but the teeth on my blades alternate in shape, Im assuming that they do this to help with chipping?
Thanks for the comments but I can assure you that although the blades are adjusted just right (ie just coming through the top side) and are sharp, and
TCT (is there anything else still on the market?) there is usually some blowout, it's just a question of how close you are looking.
There are specialist blades for all types of material, and he has experimented with many of them.
This is why all the expensive table saws (except ripping saws) have scoring blades.
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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nick205
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| posted on 24/2/11 at 04:10 PM |
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What type and format timber are you sawing?
I'm guessing a router table type set-up would be far too slow to be economical?
If the volume is high enough, could you push the cutting task (and finish quality requirement) back to the timber supplier? Might negate any tooling
investment and speed up the whole job to the point where higher material costs are overcome.
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hughpinder
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| posted on 24/2/11 at 04:22 PM |
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I'm nor sure why power feeding the table will stop the wood lifting when the cut occurs? The blade then exits at the top of the workpiece, thus
splintering then occurs on the top side of the wood rather than the bottom - if thats ok, just turn it over before you cut and problem solved.
Assuming you still want a power feed, could you put limit switches at either end of the tables travel, wired up as a 'flip flop', and
connected to a 12V supply and a suitable 12v drill -assuming its powerful enough.
Or a lever arangement, where pushing the lever forwards clamps the wood and pushes the table forwards, and reverses on the pull back?
The "old fashioned" way was to have a sacrifical piece of wood underneath the important bit - the first piece you cut trims this to the
exact length, and subsequent pieces can't splinter on the 'down' side as theres a bit of wood under it to stop the splinter
propogation, and the top stays neat as the blade is entering on that side. Obviously the offcut side will splinter, but thats usually waste.
Regards
Hugh
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jacko
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| posted on 24/2/11 at 05:07 PM |
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We had a Malone panel saw at work that had 2 blades the first small blade cut the under side the second blade cut the top and went right though the
wood
it sounds this is what you want to be looking at
Jacko
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jacko
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| posted on 24/2/11 at 05:48 PM |
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http://www.ehow.co.uk/how-does_4613345_scoring-saw-work.html
something like this
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interestedparty
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| posted on 24/2/11 at 05:49 PM |
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quote:
Originally posted by hughpinder
I'm nor sure why power feeding the table will stop the wood lifting when the cut occurs?
Power feeding simply allows the cut to tbe done in reverse, "back feeding" or "climb cutting" (Americans have good ways of
expressing things) without the work snatching.
The rough edge is on the underside, rather than the back edge. He's having to cut a lot of birch plywood, and it's and absolute bugger for
this, some of the time it's ok, then you will get a bit say six inches long where there is a load of 1 to 2mm long lines running with the grain
(ie across the saw cut) coming back from the cut line. Touch one of these and a chip comes out which leads to the piece being scrapped
The issue with the power feed is that sometimes only a short stroke is needed, and sometimes it's a full panel so quick variability is the key.
I too wondered about a 12 v drill driving a lead screw (or more likely a length of studding) but experiments have shown it to be a bit unwieldy, and
really a variable speed is necessary. It could be done of course, but we are hoping for something better
quote:
Originally posted by jacko
We had a Malone panel saw at work that had 2 blades the first small blade cut the under side the second blade cut the top and went right though the
wood
That'll be the scoring saw I referred to earlier. Good solution but expensive and takes up a lot of space. Plus keeping the things tracking
correctly can be fiddly, especially after sharpening when it all has to be set up again (he used to have one, and doesn't want to have to get
another one)
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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Steve Hignett
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| posted on 24/2/11 at 07:58 PM |
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quote:
Originally posted by interestedparty
The issue with the power feed is that sometimes only a short stroke is needed, and sometimes it's a full panel so quick variability is the key.
I too wondered about a 12 v drill driving a lead screw (or more likely a length of studding) but experiments have shown it to be a bit unwieldy, and
really a variable speed is necessary. It could be done of course, but we are hoping for something better
Surely that's simple to overcome?
On the table bed, there's a microswitch that wants to "pop up", covered temporari;y when the slide bed is back toward the person.
When the sliding bit is slid forward the cover is moved and the switch is held down by the wood on the slide (you would need a small notch cutting in
the slide), then when the wood passes the switch, it finally pops up into the grooved and the motor is thrown into reverse...
Would be much easier to draw, but I just CBA right now...
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plentywahalla
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| posted on 24/2/11 at 08:25 PM |
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I really don't like the idea of cutting 'with' the rotation of the blade. The problem is the upthrust generated by the cutting
action. This tends to lift the panel and as it lifts through an arc from wherever it is supported the blade jams in the cut slot and throws the
workpiece violently back and up in the air. I sometimes (but very rarely if at all possible) do reverse cutting, usually to cut a slot, and have had
some almost horrendous accidents.
I have a saw bench with a scribing blade but very rarely use it. I keep it retracted under the surface because operators forget that its there and
have run their hands over it when feeding panels through. We do however cut veneered ply very successfully without the scribing blade simply by
controlling blade height and a slow feed speed.
Alternatively how about some fixed Stanley type knife blades before the saw blade. The scoring only has to be through the face veneer which is usually
only 0.6mm thick.
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SteveWalker
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| posted on 24/2/11 at 09:07 PM |
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Would a featherboard above the item being cut help?
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dinosaurjuice
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| posted on 25/2/11 at 11:29 AM |
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ill say it again, use a negative rake blade. if still got problems use a sacrificial peice underneath. ive built 2 kitchens from birch ply and never
found the need to start faffing around with leadscrews and cordless drills...
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interestedparty
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| posted on 25/2/11 at 01:10 PM |
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quote:
Originally posted by dinosaurjuice
ill say it again, use a negative rake blade. if still got problems use a sacrificial peice underneath. ive built 2 kitchens from birch ply and never
found the need to start faffing around with leadscrews and cordless drills...
You can say it three times if you like
I'll pass your message on, but I'm pretty sure he has tried most types (he has quite a collection)
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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