I made a sled similar to this.  I did use to guides to keep it straight just in case the humidity changes on you.  The unique thing that I did was to install a screw inside the back support piece.  I was cutting some shelves and noticed that they were not straight enough.  Yeah, they were off about 1/32" inch over 12" span.  Putting in the screw allows me to change the direction of the board just enough to make that kind of tweak. 

Just make sure that when you push through the target board that you move all the way back past the saw and watch your hand placement.  You don't want that spinning blade coming through the back board and cut your fingers or thumb.  When you make your first cut, you want the blade to come completely through the bed board.  Else that board will raise off the table and become a missile.  Please be careful with it.

Bompa, isn't two (2) of most everything better???? 2 pieces of cake or pie. 2 glasses of milk and cookies. 2 day's of rest and that's what is known as the week-end.
Then there is (as Craig would say it) 2 bottles of Beer is better then one. LOL

My point is this: My table saw measures 10" slot to slot with 3/4" slots. Here is what I experienced with two rails. When first built it slid quite smoothly. That was a dry, low humidity day. Come the rains and high humidity the 10" of material between the rails expanded enough to make the two rails spread apart and bind in the table saw slots.

I worked out some math. If you have 1/4% (Edit made.  I originally mistyped % as ".) of wood expansion, an individual 3/4" rail would swell by less than 0.002"; pretty much inconsequential. But the 10.75" between outside edges of the two rails would increase by 0.026"; almost 1/32" and plenty enough to bind if they were made with less than 1/32 of clearance to begin with. To me, anything over 1/64" (0.015+) of clearance for a guide rail would certainly be considered sloppy. (That is a 0.735" rail riding in a 0.750" slot)

I can't believe other people haven't experienced this problem. The solution, of course, was to remove one of the rails. Unfortunately, they were glued and removal was not easy but it wasn't impossible either.



IMHO, it is better to have one well fitting rail in one slot, rather than having to make two of them sloppy enough to accomodate expansion and contraction.

Actually Bob I believe you are onto a good idea/design. My sleds work OK but mainly because they are somewhat sloppy fitting. Well now that I think about it my crosscut sled slides in the right slot and the right edge of the table. I also know my left miter slot is not near perfect(probably neither one) My miter gauge(wich I seldom use does not work really well in the left slot.

Well, now, Bob and Bill, I had not considered that it was the two rails.  But, you bring up a great point.  Will work on my sled to see if I have that problem.  I did notice a time or two that the sawblade did not travel right down the center one time and I thought that I had lost my touch.  Thanks for that tip as I may need to make an adjustment to my sled.  If I do, I'll use the track that works best.  Will wax the wooden rail to make it glide easier.  Hmmmmm!  Why didn't I think of that?

Is there a reason that the rail must be made of wood?
2 rails would be best but like was said - the wood base in between can expand and contract.
As well as the width of the rails.
Maybe your melamine would inhibit this for the base, what about nylon or teflon rails for the slot guides?
Randy

Posted By randyinaloha on 08-08-2010 04:37 AM
Is there a reason that the rail must be made of wood?
2 rails would be best but like was said - the wood base in between can expand and contract.
As well as the width of the rails.
Maybe your melamine would inhibit this for the base, what about nylon or teflon rails for the slot guides?
Randy

Not really, my choice for rails would be UHMW, MDF would make a very stable, although heavy, base.

Point to be made here...
Measurements of 0.00X may seem very inconsequential to a novice woodworker reading this, but to some of the guys posting in here those 0.00x's do add up in the overall scheme of things.
Contrary to the belief of some, they are not nuts.
They are just a step or two up from the weekend, handyman type woodworker.
Just read what they say, think about it and admire them for their efforts in precision.
I know I do.