Precision Straight Edge Advice

[Eric Porter]

I know that the subject of precision straight edges has come up several times on this forum, but even after looking over the old posts I still have a few questions.

1) The first being how precise is precise enough when it comes to a straight edge for setting up woodworking machinery? I would assume it needs to be with in .001" to .002".

2) As we all know these aren't cheap, and the longer it is the more expensive it is. So how long is most practical? I am really only thinking about a 36" or a 48".

3) It seems that the most popular brand is Starrett, are there other brands?

4) What are the tolerances on the Starrett straight edges?

5) Has anyone come up with an alternative/more economical solution to purchasing a precision straight edge?

I was thinking about purchasing a piece of 36"x1/4"x1 3/4" Starrett steel flat stock ( part # 73-416-112). The tolerances on this are +.000" / .015". My assumption is that this steel would be well within the tolerances since it is Starrett. What are your opinions on this?

Thanks,
Eric

[Brian Hale]

36" should be plenty.

+.000" / -.015" = 1/64" and that figure applies at any one point over the length of the of the stock. This is more than straight enough for any woodworking/machine setup you may be doing.

You should also look at Air Hardened Ground Flat Stock (www.mscdirect.com) which is Much cheaper.

1.75 wide x .250 thick x 36" long goes for $48 and the width tolerance is +.005"/-.000"
http://www.mscdirect.com/IWCatProductPage.process?Merchant_Id=1&Section_Id=788&pcount=15&Product_Id=75522&Keyword=Y

[Tim Sproul]

5) Has anyone come up with an alternative/more economical solution to purchasing a precision straight edge?


Yup.

Don't purchase one. I'm assuming you are working wood, not metals...

precision such as what you want to obtain is unnecessary. Metal machinery has been used long before such precision edges were widely available. I've yet to see anyone argue that today's furniture - either mass market or custom pieces - are any better than yesteryear's......

You are working wood. Even you mill it flat within a mil, as soon as you set the board down, it may now only be within 10 mils....heck, maybe even 100 mils....and I've even had some pieces go out by thousands of mils.... :eek:

[Jim Becker]

You are working wood. Even you mill it flat within a mil, as soon as you set the board down, it may now only be within 10 mils....heck, maybe even 100 mils....and I've even had some pieces go out by thousands of mils....
Very true with respect to milling wood. But for machine setup, the precision is still desirable, especially for making sure the jointer is coplaner in all directions. That said, most folks don't seem to make the investment for a precision straight edge...they are not inexpensive and unless you use them a lot, hard to justify.

[Mark Singer]

I have a Pacific Arc and it is very true and not that expensive...it was given to me so I don't have a source.

[Tim Sproul]

Jim,

That is one way. Another is to simply joint some wood and understand how a power jointer goes about doing its business. A bit of deduction can tell you which way to tweak the machine, if needed.

If a straight edge really is wanted....IMO the starett is way overboard for setting up woodworking machinery. Rob Lee...as in Pres or such of Lee Valley...has said they will be offering an aluminum straight edge again. Time is uncertain but supposedly sooner rather than later.

Here are some other links...
one source (http://www.traditionalwoodworker.com/catalog/product_info.php?cPath=35_168&products_id=942)


another (http://www.installertools.com/cgi-bin/INTstore.pl?user_action=detail&catalogno=05-113S)

I'd probably also see if Enco sells an "economy" straight edge...

[Chris Padilla]

Aluminum is better to set (for example) a jointer as you want to move the cutterhead back and forth and just scrap the straight edge...you don't want to do that with a Starrett steel straight edge as it'll trash the blade on the jointer.

Personally, I think it is invaluable to have a precision straight edge. I made a torsion box once and wanted it to be as flat as possible since it would be the basis of my workbench. I also fixed some high/low spots on my TS iron. A plain jane 36" straightedge from Starrett is only a 100-120 bucks if I recall...I don't think it's outrageous and I enjoy mine quite a bit.

However, all that said, I'll take my Starrett straight edge to the borg and see if I can find a nice, flat aluminum level in the 6-7' range. If Lee Valley is going to sell them, sign me up! :)

[Eric Porter]

Thank you to everyone who responded.

Tim, I agree with you that thousandths of an inch is not the proper increment to measure wood, as it will undoubtedly move over time. However, as Jim stated, I do want to set up my machines as accurately as possible.

Chris, do you know what the tolerances are on the Starrett straight edge? I can't seem to find them anywhere on the web, including Starretts' homepage.

Thanks,
Eric

[Chris Padilla]

Eric,

If I recall correctly, it is a half mil per foot: 0.0005 "/'.

[Steven Wilson]

Eric, a 48" precision straight edge is a very handy piece of equipment for setting up machines. Although I don't use it every day it periodically gets a workout. I've been using mine a lot the past week setting up a combo machine. Although wood will move on you and it will be out quite a few thou, you want your machines to be setup more accurately than the wood you're cutting and you want your measuring instruments to be more accurate than your machines. So, with your machines setup to less than .010" you want your measuring instruments to be in the .001"-.002" range. If your reference is only good to +- .010 then your machine setup will be off more than that. The other use of good reference instruments (precision straight edge, reference square, etc) is in checking the accuracy of your day to day measuring devices. I use the straight edge to check to flatness of my 4' level. As long as the level stays straight I can use it to check assemblies and as a quick check on machine setup. I also use the straight edge to check my winding sticks and to check smaller straight edges that I use more often (for example leveling guitar frets). If you don't have a reference how do you know if something is straight?

As for the steel flat stock, the tolerance you list is for width measurement at any point along the stock, it isn't a measure of the straightness of the stock (i.e. the same tolerance could apply to a piece of curved stock)

[Mike Cutler]

Eric. A precision straightedge/rule is more than worth the investment. I have a Starrett 6' machine rule, and it gets used alot. I use it to set up machines as well as do layouts with it. It is the standard to which I reference all other measuring devices. Is it overkill, yes. Do you absolutely need one, probably not. Would you use one if you had it, absolutely.
As far as "accuracy" is concerned, it is generally not as important as "resolution" and "repeatability" in most wood working projects. Most of the quality measuring devices available are very good, and accuracy is moot point unless the device can be traced back to a known standard ( this is what makes the Starrett products so expensive. They are traceable to NIST Standards and can be supplied with letters of conformance and traceability).
If you need a precision straight edge, without a rule, an alternative would be to have a local machine shop make you one. They can hold to tolerances that are more than acceptable for wood working applications. Have it made from a quality steel and it will last a few lifetimes.