Well since I did this sort of thing for a living I think I'd have a pretty good idea of the design intent. I also think you know how to read prints so "we" would/could have a really good idea if not right on the money.
I was guessing you had selected the "left" side of the blade and I agree that is most likely the datum that should be used.
I have my second 520 apart (it has been for over a year now) so I'd like to try a new method of alignment I've come up with and I'd also be able to measure the motion that is in play. Not sure when that is going to happen, it didn't expect to have the machine down as long as it has been but such is how it is working out. Good thing I don't need it, well I guess if I did need it then I would get it going again wouldn't I?
Ed
dusty wrote:Ed, even if we had access to design data we would not necessarily know "right" or "wrong". We might know, at best, what some engineer along the way thought was right.
You are correct, however, in every thing you said in your response.
In that regard, I do not center the blade. I use the left hand face of the blade for the datum. The left hand side of the blade is located by the arbor and how the arbor is attached to the headstock. It is more or less in a fixed position. A major assumption being that the quill is fully retracted for ALL alignment procedures.
{Knight of the Shopsmith} [Hero's don't wear capes, they wear dog tags]
reible wrote:I see a few issues here. First and foremost is we do not have access to design data from shopsmith. So what is right or wrong or centered or not we don't know.
Seeing as how we don't know and they don't give us a hint as to needing to center the blade in the slot I'd say it doesn't matter. The holes on the table used to move it for adjustment will give it some range, my guess is that range is small and thus the possible locations of the blade are with in some small range. So long as all the parts meet specification the design is robust enough to work.
Now if we were to really want to center the blade we would have to ask what blade? There are thin kerf and then are standard kerf so if you want both of these to be centered then you will have to adjust for each one. Why? Well the normal set up is with the quill not extended and against the rubber bumper. A saw arbor mounts to that and the headstock side of that is a control surface or if you will datum. The width of the blade then extends from there. A wider kerf blade extends farther then a thin kerf and the centers will move depending on which blade is used.
If you happen to a zci user and I am, if you happen to use a regular width blade in a thin kerf zci it always trims to the out side, away from the head stock.
Given one could attempt to find the center of a blade and align to it but only if the prime directive is taken care of first. That being the table aligned to the blade. Then if there is some wiggle room to move the table in that axes it could be done. My guess is that it a very small window and with manufacturing tolerances it may not some thing that every set of hardware is able to to do. The are "conditions" called MMC's maximum/minimum material conditions and if you end up on one end or the other with the parts it might meet all other requirements but unless the centering of the blade is a requirement it might not meet that.
At this point other then as a point of discussion I don't see any need to make this a requirement for my alignments.
Ed
I believe that the engineer I worked with at Hamilton Sundstrand on aircraft generators called that a "stack up". All of the tolerances of all the parts in an assembly had to add up to result in the final tolerance of the end item tolerance (ie. the saw blade being centered).
John & Mary Burger
Eagle's Lair Woodshop
Hooper, UT
While on the subject of other then shopsmith alignment...... Parts distribution during manufacture are design so most but not necessary all the parts will be acceptable when checked for tolerance. This is commonly thought of as a bathtub. Most of the parts fall in the good or acceptable range. For the purposes of this post we will say that the distribution has 3% of the parts that are too large and 3% that are too small. Now if you make 100 parts there will be 3 too large and 3 too small. Those parts cost money to make and either become recycled or waste items. At this level of manufacture you don't normally sweat it.
Say now that you are making a million parts, well now the size of too large and too small parts adds up to a lot of parts, to the tune of 60,000 parts!
The auto industry started looking at that large number of parts and said well what if we could use more of them. One way to do that would be to take parts that normally would not pass inspection and combine them with other parts that also don't pass inspection and then match over sized with over sized and undersized with undersized (example only). Perhaps now we can use all but the most over/under sized parts. Perhaps we had now only 1000 parts that were waste....
Looks good on paper. Of course for some select parts this was a fine way to go and they saved big bucks. It did not work well for parts that had to be replaced. The parts bin was stocked with just the ordinary parts, no over or under sized ones. You then were stuck with things that might go together but were a poor fit and often failed soon and failed often.
Of course we learned from that, or did we?
We had to explain this sort of things to management when the bean counters had these bright ideas. Every year they would go off on one of these cost savings things and came up with some good ideas and some really bad ideas. Had to deal with that for about 10 years before I moved to forward looking work........
Ed
{Knight of the Shopsmith} [Hero's don't wear capes, they wear dog tags]
reible wrote:While on the subject of other then shopsmith alignment...... Parts distribution during manufacture are design so most but not necessary all the parts will be acceptable when checked for tolerance. This is commonly thought of as a bathtub. Most of the parts fall in the good or acceptable range. For the purposes of this post we will say that the distribution has 3% of the parts that are too large and 3% that are too small. Now if you make 100 parts there will be 3 too large and 3 too small. Those parts cost money to make and either become recycled or waste items. At this level of manufacture you don't normally sweat it.
Say now that you are making a million parts, well now the size of too large and too small parts adds up to a lot of parts, to the tune of 60,000 parts!
The auto industry started looking at that large number of parts and said well what if we could use more of them. One way to do that would be to take parts that normally would not pass inspection and combine them with other parts that also don't pass inspection and then match over sized with over sized and undersized with undersized (example only). Perhaps now we can use all but the most over/under sized parts. Perhaps we had now only 1000 parts that were waste....
Looks good on paper. Of course for some select parts this was a fine way to go and they saved big bucks. It did not work well for parts that had to be replaced. The parts bin was stocked with just the ordinary parts, no over or under sized ones. You then were stuck with things that might go together but were a poor fit and often failed soon and failed often.
Of course we learned from that, or did we?
We had to explain this sort of things to management when the bean counters had these bright ideas. Every year they would go off on one of these cost savings things and came up with some good ideas and some really bad ideas. Had to deal with that for about 10 years before I moved to forward looking work........
Ed
I absolutely agree that those things happen. However, all my experience is with the aircraft industry, in particular the military aircraft industry. Comparatively small numbers, cutting edge technology, higher tolerances, longer design life, longer MTBF, constant scrutiny on failure modes/rates, etc.. Certainly not the only reason but part of why military hardware costs so much. Let alone the world wide supply chain that has to be supported for spare parts in some very austere parts of the world.
John & Mary Burger
Eagle's Lair Woodshop
Hooper, UT
