Working out the required length of linear glass scales

[hotponyshoes]

Hi Guys,
I am trying to set up my lathe and mill with glass scales.

I started with the shortest axis (because it was the cheapest!)
This was the Y (crossfeed) on my lathe.

The actuall travel is 7 1/2" from end to end so I ordered the nearest size to it which was a 8" scale. (Acu-rite if that makes a difference)

Anyway, I got it out last night and the actual travel of the scale is 9 1/2".

From reading the book that came with it the scale is only guaranted to be precise over the 8" of the scale but hooked up to my dro the whole length of the thing looks to read out good enough for what I require.

What is the general rule for choosing scale lengths?

Should I fit this 8" scale to the Y axis of the mill (actual travel 9") and get a shorter (hence cheaper) scale for the lathe?

Is there a standard amount of 'extra' on these scales and how do I know?

I can find data for current scales but to save cash I am looking at used scales and they are all sold as they are marked.

What sort of length scale would I need for a long axis (37") - the nearest standard scale is 40" but looking at the one I have I may be able to use a 32"!

[hotponyshoes]

Also ;-)
When mounting these things (I know the bit about getting them square)

Looks like the easiest way is to mount the scale to the table and the head to the machine.
This would mean that the head is stationary and the whole scale moves with the table.

Is it ok to do it this way or should I mount the scale to the machine below the table and make a bracket that goes from the table to the head so only the head moves and the scale stays still?
Thanks!

[Pressfit]

If at all possible, always go with a scale longer than your travel. Because if you use a shorter scale you are essentually reducing the capacity of the machine.
You may never actually use the last 5 inches of that lathe travel for a job, BUT if you or someone else forgets and quickly wheels the carriage down to the end, you run the risk of busting your DRO scale off into the chipbed.
I know: I couldn't begin to count the times Iv'e had to repair them or just throw the scales in the trash.

It's pretty much the same with a mill.

Just thought I'd add:

-The best place to mount it for the crosslide on the lathe that I've found is towards the tailstock side. It's safer there from possibly dropping a chunk of stock on top of it or even throwing a chunk of metal out of the chuck onto it. The kits usually come with a hardstop so you can mount it below the reader so someone doesn't slam the tailstock into the glass bar or Reader. Be sure you try to think of everything that can happen to damage it.

-Drill and tap for your mounting bolts in a course threaded bolt style. Fine threads don't work too well in cast iron because of the porosity of it.

-On Mills I usually mount them on the back of the table as once again, they're usually safer ther from damage. Since they are mounted at the ends of the bar, there will be a gap between the bar and the table. Fill that and all gaps with Silicone Rubber caulking so chips don't start getting wedged down into the gap. If chips get in that gap they can actually start wedging the rail in a bow - which of course will affect your accuracy.

[hotponyshoes]

Hi,
Thanks for the tips!

Regarding the bit aout travel- I went for a scale longer than the actuall travel.

The cross slide on the lathe is 7.5" so I bought an 8" scale.

The actuall travel on the 8" scale is 9.5". !.

Thats how far the head moves from end to end although it is clearly marked as an 8" scale.

So, could I fit this to my mill Y axis (actuall travel on that is 9") and expect it to work correctly or will it only be accruate over the central 8" portion?
Cheers!

[Pressfit]

Far as I know the scales should be accurate over their entire travel of it's 'readable' length. I've never thought it would be to the contrary: within acceptable limits of good quality machining work, however.
How accurate mainly depends on the condition of your machine: meaning how worn the ways are and/or tight the mill is.
Some people I have installed them for have had the misconception it will turn their manual mill into a super-accurate machine capable of producing "Jigborer" type tolerances by relying solely on the readout.
If you have a question about the readout's ability to hold accuracy and repeatability. You can perform checks easily enough for yourself using a .0001" test indicator and a Gage Block stack.

Here's another thing I like to do with a lathe as my pic shows below. I mount the readout box to the carriage and not on the headstock. So the cables don't dangle into the chipbed and wait to get snagged by stringy chips and get ripped out. Plus, it puts the box where I am (in front of the carriage). It's especially convenient on a long bed lathe like this 20"x60" Victor seen here... 'cause I don't have 5 feet long arms.
The only thing I have yet to do is order some shorter cables from Mitutoyo so I can get rid of the bunched-up bundle of cables you see under the box.

[Chris hanson]

Acu-Rite Scales are accurate to 0.0005" per meter of travel and are laser calibrated and a report is sent with each scale. All scales are sold as a nominal length and have an over-travel allowance to ease installation and prevent over-travel accidents. the typical ENC150 and SENC150 scale have an 1.75 inch over-travel allowance. The scales continue to count accurately during the entire travel so an 8 inch scale works well with a 9 inch travel and still has a 3/4 inch over-travel buffer.

[hotponyshoes]

Cheers for the info,
I have checked some of these scales (for example mesuering a 8" nominal over the full 9.5" travel) and they are more than good enough for what we reguire, In fact, I don't have anything that is as repatable as these to check them against so I ended up with the sacles on the workbench and bolting the read head on the 8" to a 14" and plugging them both into the readout to see if they both moved the same amount!

Anyway,
Regarding the 'over-travel allowance', is this allowance published anywhere?
I have not been able to find it in the suppliers catalogue and can't find it on the acu-rite web site either.

I ask as I need a 37" (actuall bed movment) scale for my mill,
Standard thinking would tell me the next size up which is a 40" but it that turns out to be 45" (say) then I will end up with 4" of fragile scale sticking out each side of the mill waiting to be knocked into,
What I am trying to say is what is the shotest 'nominal' length scale that will give me 37" travel?
The mill bed has stops on each end so overtravel allowance is not really needed providing I set the scale up properly (I presume )

[Chris hanson]

A 36" measuring length will have an overall travel of 37.75 inches and an overall length of 42.375 inches. This would be the closest fit to 37 inches of required travel. By doing the math, I assume you have a mill with a 42 inch table. The recommended scale for a 42 inch table is a 32 inch scale. It has an overall length of 35.75 inches and a max travel of 31.125 inches. This scale will fit between the coolant return holes an the back of the mill table.

If you are attempting to push a 42 inch table to the bitter ends of its travel (37 inches) you will find that your accuracy at the ends of travel will drasticly fall off due to having all of the tables weight hanging off of one site of the saddle. It typically is not a good idea to expose the ways on the saddle while machining. It is better to machine what you can, then move your work over and finish. If your tolerances are super critical, you can mount a larger plate on top of the table then swing the head left to right. this prevents you from having to un-clamp your work.

Here is the link to the manuals on the acu-rite web site: http://www.acu-rite.com/index.cfm?Pa...7200A0CC271CB6

AR-5 Scales are the oldest ones you will find.
MiniScales are next
Mate Scales were the only scale marketed with an analog output.
ENC150 scales are next and have the same footprint as MiniScales
SENC150 are the current design and also share the same footprint.

Another class of scales is the MicroScale and now the SENC50. These scales are much smaller in profile for applications that do not have a lot of room for a scale to mount.

For distances longer than 120 inches the AR10 and now the ENC250 uses a metal tape held in a series of sections and pulled under tension.

Probably too much information but I say it's always better to have the information you need before you need it.