Inaccurate Rail Scale/Tape

[Ken Louis]

The rail scale on my Powermatic 64A fence starts drifting high at ~10" and eventually increases to 1/32" around 30". Powermatic is going to send me a new scale tape under warranty. The pic shows an 6" Incra ruler placed against the scale at the 18-24" position. Aligned the ruler at the 18-19" position. Note the error at 22-23".

Anybody have any tips on how I can install this new scale accurately on the first try? Since it appears to be vinyl tape I would imagine it would be very easy to inadvertently stretch it out of tolerance during installation.

I'd also given some thought about not using the Powermatic tape and possibly mounting an accurate, very thin, metal 36" ruler (similar to the Incra ruler) on top of the rail instead . . . if such a thing exists

Ideas? Thoughts? Thanks!
Powermatic Rail 18-23 Scale Error.jpg

[David B Thornton]

It looks like the 4" mark on your rule is off on the 22" mark of the tape. Looks to me that's what the problem is, not the tape itself?

[Rick Gibson]

I've got 4 tape measures in the shop and only 2 of the 4 are the same at the 3 foot mark. Found that as long as I stick to the same tape measure for the whole project things turn out ok.

[Justin Ludwig]

I'm with David. Your ruler is shy of both the 22 and 23 mark the same distance. The deviation from 4 to 22 and 5 to 23 is equal. Slide the ruler over about 1/128" (give or take) and your back on line.

For what it's worth, if my table saw tape were off the LITTLE, I wouldn't worry. I've replaced my tape twice (from me damaging it) with a Starrett.

[Mike Goetzke]

I had the same issues on my Uni until I added a Wixey digital scale.

[glenn bradley]

I picked up a metal self stick tape from starret that matches my wicket drop from 0 to 52".

[Kyle Iwamoto]

I compared my tape meaures with rail ruler at 52", and tossed out the tape rules that were off. I'm with Rick, as long as you use the same tape rules, and they match your rail ruler, it should be fine.
As a side issue, I'm thinking that PM does NOT have a different (more accurate) rule for the rail. You're getting another that has the exact same markings. And therefore the same error. Just my $0.02 of course.

[ian maybury]

There's precision tape measures available over here in Euroland: http://www.thetapestore.co.uk/tapes-...-tape-measures EC class1 is accurate they say to within 0.3mm in 2m.

Chances are there's an equivalent US standard. It's not coming up for me just now, but i've previously seen a US company doing precision tapes and self adhesive scales. This self adhesive Starrett item may (it doesn't seem to say?) be in this territory: http://www.amazon.com/Starrett-63168.../dp/B00175RTS8

My personal instinct would be to go for something like this if it fits - stretchy vinyl seems unlikely to be stable even if you get it on right….

[Jamie Buxton]

I'd also given some thought about not using the Powermatic tape and possibly mounting an accurate, very thin, metal 36" ruler (similar to the Incra ruler) on top of the rail instead . . . if such a thing exists..

Yes, such a thing exists. You can buy 36" versions of your 6" machinists rule, and even 48" ones. That's what I did to my table saw, mostly because I run my shop in decimal inches, and I could not find a stick-down tape like that. But you're right, the accuracy of an engraved steel rule will be better than a printed plastic tape. You can buy long machinists rules at places that cater to machinists, like Enco (www.use-enco.com) and MSC (www.mscdirect.com). A 36" one will cost you $70 or so.

[Ken Louis]

Really appreciate everybody's input on how to work around this problem. After an even closer took at the rail tape, it's worse than I thought. There would be 2 or 3 inches that were spot-on, then another 2-3 inches off, then back on again. Impossible to work around.

Jamie Buxton, thanks for the tip. Got lucky and found a metal 36" ruler at Woodcraft with 1/64"increments on sale (supposedly made by Starrett). About 95% of my measuring is 36" or less. Considered mounting the new ruler on the rail as I mentioned earlier, but decided it might be more useful as a standalone ruler.

In any event, this 36" ruler would certainly make installing a new rail tape much easier. Since it's probably going to be a few weeks before I get the new rail tape from Powermatic, decided to try and reposition the existing one using this new ruler. The old rail tape came off fairly easily. Powermatic used some kind of clear, gummy type of adhesive to hold the tape. Lucky for me it remained sticky enough to reattach the rail tape--at least temporarily.

Taped the 36" ruler to the rail, then started reattaching the rail tape. It was a bit more work than I anticipated. Had to manipulate and adjust every 1/2" of tape to get it to position exactly right. Since it's temporary I just covered it with some clear packing tape to make sure it doesn't come loose. Made a number of cuts along the entire length of the rail (0-33")---it's dead-on accurate now

If this Powermatic replacement tape doesn't work out, I may try the Starrett adhesive tape on Amazon that Ian Maybury recommended.

Thanks again for everyone's help!