Will UHMW Runners be precise enough for a crosscut sled?

[Thomas Colson]

Getting ready to make the ubiquitous cross cut sled, and I'm making it BIG! Saw is a delta 36-725 but I got a Sawstop RT to the right. I could either just stick with the two Delta miters or go big or get out and span the entire table and use the Sawstop slot. Either way, wondering if I'm going to be happy with UHMW bars as the runners in this set up. I am looking to be precise as possible, and depending on vendor and expectations of the reviewer, UHMW is hit or miss on actually fitting precisely in the miter slot.

[Bruce Page]

I made a large sled years ago with UHMW runners, the sled is very accurate. I don’t use it that often but I haven’t seen any noticeable wear. To keep the UHMW strips from deforming, I attached them with several truss head screws, snugly tightened.

[glenn bradley]

Short answer, yes. I have slides with Incra aluminum, QSWO wooden and UHMW runners. All perform well.

[Frederick Skelly]

Yes. Ive done it too.

[Tony Shea]

I guess it depends on how you plan on buying the material. Do you plan on purchasing the exact width you need and then hope that it matches the slot perfectly? I will say there are ways to deal with looser fitting runners. If you shim one side while the runner is in the slot then lower and mount the sled with the shim in place then the runners will always be tight against one side of the slot. That is only true if you plan on using both miter slots and the runners registered against the blade side of the slots.

Having said this I think a snug smooth fit is best which is the beauty of the using UHMW. You can get a relatively snug fit and still have a nice smooth action. I personally would buy the material oversized or in sheet form then rip out and fit to the slots. I would never dare buying the exact size material I wanted and hope there is no slop.

[Andy D Jones]

Snip...
I could either just stick with the two Delta miters or go big or get out and span the entire table and use the Sawstop slot.
Snip...


The distance between runners/slots matters a lot less than the engaged length of runner and slot.

In fact, if runners are farther apart, it can tend to work against you, since the drag on a farther-away runner will tend to want to angle and jam both runners. The main advantage of two runners over just one is that neither runner need precisely fit their slot, but between them they can be adjusted to push against opposite (e.g. both pushing on outer or both pushing on inner) sides thus preventing any side-side movement of the sled/workpiece.

Also, if you really want to use the router table's miter slot, make sure it is exactly parallel to your other miter slots. For most router tables/fences, parallelism between the miter slot and the edge of the router table is not critical (since the router bit does not care, and the fences can usually be adjusted while setting them to be parallel to the miter slot if necessary).

When the router table is mounted to the edge of the saw table you get a tolerance stack-up which may not work in your favor. It could probably be shimmed (between the router table and saw table) to get the miter slots parallel, but as I said above, using the RT slot for a sled runner is less preferable than using the two slots in the saw table.

-- Andy - Arlington TX - Home of the Texas RANGERS!

[Dan Friedrichs]

I wouldn't assume you can buy it with a precision fit, but it can be easily machined. If you just need to tweak the fit, a sharp block plane can peel off material very neatly.

[Bruce Wrenn]

I wouldn't assume you can buy it with a precision fit, but it can be easily machined. If you just need to tweak the fit, a sharp block plane can peel off material very neatly.


I built a jig to run it thru my old Delta lunchbox for an exact fit. But screws tend to cause it to bulge out.

[Larry Frank]

I have tried many different materials for runners and most work fine. However, I now use the Incra Miter Slider and love it. I use it on a cross cut sled and if the sled wears out, I move it to a new sled. I can adjust it for the perfect fit.

[Steve Demuth]

I built a jig to run it thru my old Delta lunchbox for an exact fit. But screws tend to cause it to bulge out.

UHMW Poly is the only thing I use for runners on jigs. It's perfect - dimensionally stable, tough, self-lubricating, and easy to machine. I see its plasticity and tendency to spread under a screw as another asset - I mount mine with traditional profile wood screws, with matching countersink. The conical counters spread the UHMW Poly effectively, so you can adjust the fit of the runner easily just by tensioning the screws.

Compared to:

Wood - Even quarter sawn runners change dimension enough in my dry as dust in winter, wringing wet in summer, non-air conditioned shop to be a pain and effectively useless for precision work after a couple of years.

Aluminum - Nice, but machining has to match your slots perfectly, and for precision work, they have a problem with the wide temperature swings I see in my shop (could be 50F in winter at times, and 95 in summer). Not as dramatic as the moisture content problem in wood, but still better in my experience to avoid.

In a climate-controlled shop, with stable humidity and temps, I don't think there'd be much difference in the materials, but the UHMW Poly would still win for me because of it's ease of machining and adjustability.

[Larry Frank]

Interesting comment about issue of thermal expansion. However both a cast iron table and slot will also be expanding, the issue is small. I calculate that the difference due to temperatures mentioned is less than half a thousandth.

[Thomas Colson]

Ok, UHMW it is! Any negative to getting what ever is cheap on AMZ?

[Steve Demuth]

Interesting comment about issue of thermal expansion. However both a cast iron table and slot will also be expanding, the issue is small. I calculate that the difference due to temperatures mentioned is less than half a thousandth.

Thanks for the calculation. Obviously if it's that small, whatever I was seeing wasn't just thermal. I like my tolerances close, but I'm obviously not running sub-thousandth tolerances on a jigs.

[Pete Staehling]

I have found that when making runners from HDPE that sneaking up on the fit using the thickness sander was a good way to go. I'd assume that would work with UHMW as well. I have one thickness sander set up with a fine grit, but if you have yours set up too coarse and don't want to change grits you could go to a slightly snug fit and then finish up by hand or on a stationary belt sander with a fine grit.