[widgitmaster]

I have been trying to measure a 6 foot long lead screw, except my vernier caliper is only 48 inch.

Does anyone have any tricks as to how I can do this within .005 or less!

I was thinking of taking a 7 foot long strip of MIC-6 1" thick, and milling a V down the middle, then press 1/4" drill bushings every 6" from a solid stop block on one end with a micrometer head mounted in the end center point.

This would prove to be rather expensive, as the MIC-6, 12 drill bushings, and the Micrometer head are all expensive! Not to mention the accumulation of error from hole to hole!

I have been making leadscrews in 6" increments, 6" to 36", but everyone is asking for 6 ft long screws! I really hate to sell something which is not made with some standard length or interchangability!

Now that I have a new lathe, I have made an extention tube on the back of the lathe out of PVC tubing, it works great with long bar stock!

Any ideas, the WidgitMaster needs help!

Eric

[Climatepro]

How about three split shaft clamps, one at each end (flush with the end) and one roughly in the middle of the screw? You can then measure from the outside edges of one side to the outside edge of the middle clamp. ditto for the other side, subtract the middle clamp thickness.

[Geof]

You can measure shorter than 48" with good accuracy. Get some lengths of 1" round bar, face them to lengths of say 36.000 +/- 0.001, 24.000+/-0.001 and 12.000 +/-0.001 and tap the ends something like 1/2-NF. Also make a flange to bolt on one end. Now you can screw them together and get 36, 48, 60 and 72 inches long reference bars accurate to within +/-0.003 or a bit better with the flange on one end. Butt the screw you are measuring up against the flange and then measure from the end of the reference bar to the end of the screw. This will give you the screw length to your desired tolerance.

[WhiteTiger]

You have a 4 ft vernier, make up a pair of clamp on trammel jaws to extend the reach an extra foot in each direction from the vernier jaws. Simple to make, easy to apply and remove.

You can find such extensions illustrated in almost any tooling catalog.


Tiger

[JFettig]

Question, why do you need such a long peice within that kind of tollerance?

because your putting bearings on it? only one end needs precision bearings, just put a floating bearing on the other end?

Jon

[NC Cams]

The domino effect only worse:

If you fasten one end of lead screw and let other end float, there is less deflection potential the closer the nut gets to the fixed end and vice versa as you move farther from fixed end.

As in a 1" rubber band only stretches X before it breaks, A 2" one of same material stretches 2X before it breaks. The longer it gets the more it stretches/can stretch.

One trick used for long lead screws was to "pre stretch" them, the closer to yield, the better. Reason, the applied load of the table would have to overcome stretch before it placed any appreciable stress and resultant deflection in the opposite direction on the screw.

This same principle is used a lot these days in torque to yield bolts.

If you prestretch the lead screw, your only error would/should be in thread pitch which you can't do squatt over anyway.

If you merely fasten one end, you have "stretch factor" plus lead error to compensate for or live with.

This does not include any issues of thermals which further exacerbate the problem.

The used of long 6" micrometer "standards" combined with precision ground shaft collars "hop skotched" down the shaft should get you pretty close.

We did that sort of thing when checking lead error on a ball screw for ball screw comp - used 4" gage std and moved it down the table as we went.

Or, get hold of laser and find out within a few light particles of what the true width/length is. At that point, call Spock as it is time for a Vulcan mind meld...

[dertsap]

WidgitMaster

what about going with you original idea but get a nice piece of prefinished aluminum angle extrusion

[WhiteTiger]

Here ya go. Second item down on the lefthand side of the page. Caliper extender


Tiger

[widgitmaster]

Wow! Great responses, so many good ideas!
Because the ends are turned to .3125 & .250 diameter, holding the long screw between 1/4" dowel pins with the "Extend-A-Cal" is too clumsy for this old man.

The idea of the clamp ring in the middle has plenty of potential!

The best suggestion is the 1" diameter bars that screw together, that's like a large set of "Space Blocks"

The floating bearing idea is good, except that all my customers are from eBay, which I have no idea of their applications or mechanical aptitude! To try and explain a floating bearing to them is just asking for endless email questions and complaints!

As for stretching a lead screw, well, I won't go there.

The extruded aluminum at 7' would be too easily flexed, especially with holes every 6", so that is not a practical idea.

My original idea combined with the space bars, will allow me to calibrate the distance between stop pins. I found a Micrometer head with 1" travel on eBay for $20, the drill bushings are around $5 each, the 7' long x 3" wide piece of MIC-6 1" thick is the expensive part, around $190.00 !

This is basically a large micrometer, at a cost of $270
That's how much I paid for the 48" Vernier Caliper!

My eBay sales should recover that in a month or two!

Thanks for all your suggestions, you guys are great!

Eric

[Geof]

You say aluminum extrusion as suggested by dertsap would flex especially with holes. True, but you can delete all except one end hole and also this measuring instrument is mostly likely going to be used laying on a bench with the workpiece brought to it.

The reason I say delete the holes because these are just for the bushing which hold the pins to give you range adjustment. But if you are going to make spacer blocks to assist with calibrating the pins why not make additional spacer blocks so you can combine them to give you length increments of 1". Then you stack spacer blocks against the end pin and then measure your screw between them and the micrometer head at the other. Less work to make, less expensive for material.

The one thing you do have to consider and this applies to your length of MIC-6 as well is thermal expansion. This is one time you should calibrate and use within a small temperature range.

[BobWarfield]

Widgit, what's your eBay ID?

BW

[One of Many]

Eric,

I have had some similar conditions to work under.

A couple of ideas I found uselful. Maybe not perfect, but useful.....

I created a standard from basic round bar. If you have a DRO on your lathe of sufficient length. You can rough in the length, check it againt the face of the chuck. Just zero the indicator against the chuck and the DRO. Then travel the DRO length away from the chuck and check the standard with that indicator on the carriage to find the excess length, then trim off the excess to obtain a good standard. This was for setting a fixture on the cheap using the same travel indicator used on the lathe to compare parts to the standard reference. That worked for round bar and lathe work.

A few weeks later, I needed another setup for mill work. Again taking a length of rectangle or angle iron. Cutting notches on every 24" center line(My longest caliper is a 24"er!, but I did use the mill DRO and slid the bar down, referencing the previous notch center etc.). Having verified and calculated the true distance to each notch, I could now mount that to the mill table extended off one end. Using the DRO, I could then finish the part with reasonable certainty off the reference way out in space.

Once the standards are made. Mounting one of those fairly inexpensive linear scales with the LCD readouts to a fixture, makes a very nice(read as CHEAP! ) direct reading measuring tool, once set with the reference of zero againts the standard.

DC

[widgitmaster]

A DRO for my lathe is on top of my wish list, but right now I have to rely on the dials!

My eBay handle is: eaaenterprises

[widgitmaster]

You say aluminum extrusion as suggested by dertsap would flex especially with holes. True, but you can delete all except one end hole and also this measuring instrument is mostly likely going to be used laying on a bench with the workpiece brought to it.

The reason I say delete the holes because these are just for the bushing which hold the pins to give you range adjustment. But if you are going to make spacer blocks to assist with calibrating the pins why not make additional spacer blocks so you can combine them to give you length increments of 1". Then you stack spacer blocks against the end pin and then measure your screw between them and the micrometer head at the other. Less work to make, less expensive for material.

The one thing you do have to consider and this applies to your length of MIC-6 as well is thermal expansion. This is one time you should calibrate and use within a small temperature range.

Thats a lot of good suggestions, thanks!
As for calibrating it, well I'm in Florida, and my garage goes from 96 to 80 when the AC is on, and when I leave, it heats right up!
There's not too much I can do to control that, but if I can get the parts within .005 then I'm happy!