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HMWPE runners expanding when screwed (for crosscut jig)

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Forum topic by JakeG posted 08-18-2017 01:06 PM 415 views 0 times favorited 12 replies Add to Favorites Watch
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JakeG

15 posts in 125 days


08-18-2017 01:06 PM

Topic tags/keywords: question jig

Hi,

I’m new to woodworking and to the forums.

I’m making a crosscut sled for my table saw and i wanted to do it “right” so I purchased some HMWPE runners online and trimmed them to fit snugly in my mitre slots.

Then I tried affixing them using a few of the methods online (screw from the top, screw from the bottom, double sided tape).

I predrilled and countersunk all holes.

When I put in some screws, the material expands and the width of the runner around the screw increases by as much as 3/100ths of an inch.

Then my slides don’t slide.

Today I will try using some hardwood that I have, but I was just wondering if anyone knows what I did wrong!

Thanks!


12 replies so far

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firefighterontheside

16748 posts in 1673 days


#1 posted 08-18-2017 01:19 PM

Do you have standard 3/4” slots? What size screws are you using? I’m thinking that screws with too big of a head will be close to the outer edges and when tightened will make the material bulge out. Also only countersink so that the head is just flush.

-- Bill M. "People change, walnut doesn't" by Gene.

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jbay

1800 posts in 716 days


#2 posted 08-18-2017 01:46 PM

Just file the bulges down.

-- If anyone would like to see my Portfolio, PM me and I would be glad to send you the link.

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JakeG

15 posts in 125 days


#3 posted 08-18-2017 01:58 PM

Thanks guys. My mitre slots are 0.739-0.741 or so. I trimmed the runners so that they would fit.

I used #10 head 3/4” wood screws. I bet that’s the issue. I countersunk more than I should have because at first I thought that the screw heads might be protruding and causing drag.

I tried filing the bulges down, but I did not have very much luck. Perhaps I didn’t give it enough time… I trimmed them narrower to compensate for the bulges but now of course they don’t fit as snugly in the slots, but when I tighten the screws they still cause a lot of drag! Best of both worlds.

I’m pretty sure that there is no way to fix these runners. I’m sure I will find something to use them for, but because I trimmed them they probably won’t be useful as sled runners anymore.

I was actually thinking that I could take advantage of their tendency to expand by using them as mitre slot expansion hold downs.

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TungOil

712 posts in 311 days


#4 posted 08-18-2017 02:58 PM

Polyethylene is very soft. any countersunk hole in PE will deform the material. You might have better luck using a screw with a flat bottom head such as a cap screw or a pan head, but it will still deform to some extent. It’s the nature of the material.

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

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Fred Hargis

4719 posts in 2310 days


#5 posted 08-18-2017 03:33 PM

Maybe go to a washer head type screw and counterbore the runner a little so the head is below the surface. That way you won’t have the taper trying to spread the UHMW…at least that’s how i did it.

-- Our village hasn't lost it's idiot, he was elected to congress.

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TheFridge

8163 posts in 1303 days


#6 posted 08-18-2017 03:40 PM



Polyethylene is very soft. any countersunk hole in PE will deform the material. You might have better luck using a screw with a flat bottom head such as a cap screw or a pan head, but it will still deform to some extent. It s the nature of the material.

- TungOil

Ditto

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

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Rick_M

10490 posts in 2197 days


#7 posted 08-18-2017 05:55 PM

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oldnovice

6393 posts in 3184 days


#8 posted 08-18-2017 06:07 PM

I have to agree with what TheFridge said, this plastic is soft!
No countersink and flathead screws, but counterbore and panhead screws.
Also watch how much you tighten the screws as the material will try to rotate with the scew and when you put in the next screw the material will bow along it’s length; place a straight edge along the length to watch for bowing.

That’s what I do!
Stupid stuff I learned while working with plastics very early in my career.
Don’t even ask me about “cold flow”!

-- "I never met a board I didn't like!"

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JakeG

15 posts in 125 days


#9 posted 08-18-2017 06:12 PM



Also watch how much you tighten the screws as the material will try to rotate with the scew and when you put in the next screw the material will bow along it s length; place a straight edge along the length to watch for bowing.

- oldnovice

That’s great advice, I already learned this the hard way. Actually, it seems like that’s how I’m learning everything about woodworking :).

I do not have a drill press, which makes all of this more difficult. I think the easiest approach (for next time) might have been William NG’s drill from the top approach.

Counterbore and pan heads. I actually think that the material is too thin and warped at this point for me to use it, but that is good info. Thanks!

View jbay's profile (online now)

jbay

1800 posts in 716 days


#10 posted 08-18-2017 06:25 PM

I just made a quick, single use, sled. I used oak.
Put them in the miter slots, put a bead of glue down each one,
set the sled on top and put some weight on it and let it dry. Slides pretty nice.

-- If anyone would like to see my Portfolio, PM me and I would be glad to send you the link.

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MrRon

4422 posts in 3060 days


#11 posted 08-18-2017 07:12 PM

My approach is to use a wood runner and use only one miter slot. When you use both slots, the chance of binding is much greater. Just for argument sake, measure the distance between slots in the table at both front and rear of the table. They should read exactly the same; also the width of each slot front and rear.

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JakeG

15 posts in 125 days


#12 posted 08-21-2017 02:42 AM

I think that a lot of the drag was coming from the extension wings. I sanded the bars a bit and put paste wax on the table, wings, and the sled and it really glides now. Sanding introduced a little play in the runners, so I just tightened the screws a little bit and it went away.

Thanks for the help. Just need to add some fences and I’m good to go!

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