520 Miter slots

[teufelhund323]

Hello all. Spent the morning aligning my 520 main table, as per Nick's instructions found on the table ribbing post - using the dial indicator (and the dollar bill "shim" to prevent binding), and the results were great. Once I had the table aligned, I set up to align my rip fence. Here's the problem, I aligned the fence from the left hand miter slot using the dial indicator and got it aligned to .001" infeed to outfeed. Just for the heck of it, I set up to double check the alignment from the right hand miter slot and found a difference of .008" infeed to outfeed. I double checked, and then measured from both sides of the fence to insure that it wasn't a problem with either face of the fence. Same results. I am concerned that the miter slots are not perfectly parallel to each other. Any thoughts or ideas would be greatly appreciated here, as I am considering buying a miter sled and do not want to have alignment issues. I left the rip fence aligned to the left slot, and am having no problems at all ripping -very accurate results. Thanks in advance.

[charlese]

I missed something here, Tim. Did you do a check of both miter slots, when aligning the table? If you did and both are within .001" - that is terrific! Then if the rip fence doesn't come out the same on both sides - seems that your problem is with the alignment of the 520 rails.

If you find the miter slots to be off .001" left, to .008" right then the slots aren't perfect. If that is the case, my recommendation is to ask SS if this is within tolerances. If it is - then re-align the table and split the difference, so each miter slot is .0035".

Best wishes,

[dusty]

Are we talking about measuring with a micrometer in preparation for cutting with an axe?

Remember that accuracy to 1/64" is accuracy to .015625";

accuracy to 1/256" is accuracy to .00391"

A typical human hair is .003" in diameter.

The Shopsmith miter slot should be expected to move (side to side in the miter slot) from .004" to .006".

IMHO your miter slots and your alignment are pretty **** close.

Your fence aligns to within about 3 human hairs difference from infeed to outfeed.

[teufelhund323]

I know that the alignment is very good, even over the top for woodworking purposes. I am kind of obsessed when it comes to tolerances (obviously). With the rip fence aligned to the left slot, I am beyond happy with the accuracy of the the test rips I made. My concern here is that the miter slots themselves are not true to each other, with .001" at the left (infeed to outfeed) and .008" at the right. Will this difference cause a problem using a miter sled? I really want to increase my accuracy when cross cutting/mitering - especially with the repetitive cuts - which is why I am considering buying the sled. Thanks!

[dusty]

I seriously doubt that the miter slots are out of spec with relation to one another.

When you take your measurements, are you making certain that the miter bar is consistently tight against one side of the miter slot (that is to say, there is no side to side movement).

I have three main tables. Today I measured all six miter miter tracks against both sides of the one rip fence that I have for my 520. The maximum deviation was .004. The majority are .002/.003.

Furthermore, most miter sleds that I have seen/used have two miter bars. The installation/alignment procedures for the sleds tends to compensate for any variations in the main table. The miter accuracy is a function of the sled and not the main table. Of course, you must be properly aligned with respect to the blade.

[teufelhund323]

Yes, I did compensate for the give in the miter slots by making sure the miter bar that my micrometer is mounted to was consistantly tight to the same side of each slot every time I measured. Same results every time I measured, and believe me I checked, double checked and then checked again to make sure I wasn't mistaken. I know my measuring is on the money because my table alignment and rip fence alignment are dead on in relation to the sanding disc/saw blade. Table alignment was done following Nick's procedures to the letter. After I aligned the table and fence, I ripped some test peices of various lengths and widths with outstanding results. Since most sleds use two miter bars, there is my concern. If they (the miter slots) are not true to each other, will that affect the accuracy and operation of the sled? Thanks!

[scottss]

So if I am correct you are thinking that the miter bars might pintch?

[dusty]

If your worst concerns come true, what happens? You put a miter sled on the table and attempt to push it from infeed to outfeed and it binds up so you can't move it any farther.

What to do then? Loosen one of the miter bars so as to permit it to relocate itself, tighten it down and you're in business. One miter slot keeps your sled from moving right and the other prevents the sled from moving left.

Recheck the miter alignment and go build "fine" furniture.

If this was my concern, I'd build a simple sled with two miter bars and check it out. If it works like I think it will, then you are free to invest whatever you are inclined to spend on a miter sled with "little or no" risk.

[teufelhund323]

Dusty, thank you for the advice. I'll put it to good use.

[Nick]

Folks, some ancillary information from my factory snitches that might help to illuminate this discussion:

Because of the equipment and the method Shopsmith uses to the mill the miter gauge slots, it's absolutely impossible that they will not be parallel. They are both milled in a single pass at the same time on the same machine. The discrepancy in the alignment of Tim's fence with each of his miter gauge slots has nothing to do with the miter gauge slots themselves.

More likely, it is either the front fence mounting bar or the table bosses to which the front mounting bar attaches. The mounting bar, because it is an extrusion, is made from a very ductile form of aluminum. Although we purchase our extrusions from one of the top extruders in the US, they are not as precisely flat or straight as a milled surface. Quite possibly, there is a small anomaly in the mounting bar extrusion that causes the rip fence to swing .008" out of parallel when you tighten the fence clamps. If you move the fence an inch to the right or left, the rip fence may swing to a a slightly different position.

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It's also possible that one of the three bosses to which the fence mounting bar is attached may be slightly higher or lower than the others. Because the extrusion is ductile, it will easily bend to conform to the levels of the bosses when the mounting bolts are tightened.

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If your quest for precision is such that even though you are "beyond happy" with the cuts the machine is making, you just have to straighten out the rip fence, you can do this: Hold a precision straightedge against the front surface of the mounting bar and shine a light from below. If you see any light between the bar and the straightedge, this indicates the existence of a dreaded "low spot." Remove the bolt that holds the fence to the nearest boss, insert a shim between the fence mount and the boss, then replace the bolt.

However -- and this is an important "however" -- you will most probably have to purchase your shims through the aerospace industry. To straighten out your rip fence which, if I understand correctly, is .008" out of alignment over the 24" length of the fence, then a little trig tells me the thickness of the shim you need to insert is .001" or less. Good luck with that. And let us know if you get to that engineers' Nirvana where you're beyond beyond happy.

With all good wishes,