leadscrew accuracy

[docsteve66]

Darryl: I think your qquestion about DRO accuracy wnet unanswered; The DRO has two pieces, one mounts on the piecethat moves. the other mounts on the peice that dont move. The DRO tracks how much they move in relation to each other. You could mount a DRO and use a hammer or ccrow bar to movethings. Might have trouble positioning to where you want it, but you WILL know where its at .

I think,for hobby purposes that most precision over say two /three inches is mainly for getting a warm fuzzy feeling. There is seldom need for great accuracy on big stuff. You may need a shaft 36" long and 1" dia. but in use it can probably be 36 plus minus a lot. and it needs to be 1" plus minus a little in two or three spots. The rest can be maybe smaller by a lot. Its niceto beable to make 6 holes equaly spaced, but in most cases there is no need for them to be exact- template and drill where the holes match. Gear is different matter, pulley can be way off except the anglle for belt needs to be fairly close. I tried to leave tolerance to max that would do the job for one off stuff.

For mass production thinngs change- parts must interchange- that needs precision especially when tolerance stack up.

The "Arkansas ruler" should be used when possible.
Steve

[darryl]

I get a warm fuzzy feeling just seeing all the responses people get to their inquiries on this forum. It's great. By the way, Lynnl, by the year 2038, I'll probably be dirt. If not, I probably won't know what glass is, or anything else, for that matter. If I'm still around, I'll drink to your health from one of those glasses, if they're still standing. I won't be.
Interesting stuff about making master leadscrews in the days of yore, Kevin. I'm always amazed at the ingenuity of our ancestors.

[Cass]

Daryl,

If you get the "Foundations of Mechanical Accuracy" book from Moore you can get answers to all your questions. The standard spec that Moore used to quote for their lead screws was 0.000,001"per inch. I bought machines that had straightness of travel of about 0.000,010" peak to valley over 18" of travel. Straightness of travel roll, pitch, yaw of course encompasses more than the lead screw. Moore had a very precise and very special tread grinding machine in a very special temperature controlled room for grinding the lead screws. The checked the quality of screws against a master leadscrew. They lapped the nut to the screw as a matched set and tested the assembly against a master step standard for accuracy with hand lapping correction to get the delivered accuracy of the assembly. This is takes care of the point to point "bump" or other error you asked about. Very expensive but you get what you pay for. Now days Moore has stopped making those Acme thread lead screws and uses precision ball screws and relies on laser interferometer position readouts or precision scale position feedback for positioning. Ball screws are cheaper and with proper preload have less back lash and they also are more efficient for faster slide travel which is a modern priority.

[Ragarsed Raglan]

An interesting thread this one. I like Forrests comments about old worn machinery making a better machinist out of folk who have to produce accurate components from them. During my apprenticeship the use of old worn out machinery was an essential part of the 'training' - and my time was well spent on a 1916 Herbert #16 Universal mill, and a 1941 Milwaukee Vertical.

The first machine was part of an order from the Czarist government of Russia, it had been rejected by their inspectors based at the factory - so was put to use in the same factory! The Milwaukee came as Lend Lease aid during the early part of the war and was removed from the freighter that delivered it to Liverpool docks from under water. The freighter had been sunk at the dockside in a bombing raid. The only way to dial in an accurate table movement, on either machine, was to use dial indictors on the axis of movement. I remember having to cut a slot to a Newall A limit (something like +0.0005"/-0.00025") in a 'fitters return', using dial and gauge blocks to get the table movement across the slot. I got a commendation off the foreman for this job on my indentures - apparently they didn't think it was possible to do this job and were ultimately prepared to strip the job down and replace the component that had the skipped operation on. For some reason the shop only gave 'fitters returns' to the senior apprentices - probably as they were an open ticket they could make more money on them by giving them to the 'cheapest' labour (i.e. the Apprentices!!).

As for leadscrews I remember seeing the leadscrews made by Matthew Bolton, for the Wall lathe he built at the Soho works around 1760. This machine is now housed in the Birmingham Science Museum in the West Midlands, UK, the leadscrew was made by marking out, and then hand chiselling. Three screws were produced and then each was corrected on the lathe against the two others to produce three accurate screws (in a similar fashion to producing a flat surface by hand scrapping three plates against one another). Parts of the marking out (by centre pops) is still visible on the screws at the unworn ends.

RR

[milmat1]

I know on my Smithy 1220xl, I have just gotten used to using indicators on evrything because the dials are just not accurate enough.I hope to install sony/red-lion digital readouts on it soon. (as soon as i can cabbage enough parts from work)..
When i read post like this and the ones about the temerature of the steel creating errors etc. I wonder how we manage to make anything fit.........matt