PDA

View Full Version : DRO Magnetic Tape Encoders?



Stuart Br
08-05-2011, 05:15 AM
This months issue of the UK Model Engineers Workshop has a feature on magnetic tape encoders. At first glance they seem to overcome all the space issues of a glass scale, particularly on a lathe cross-slide. The aluminum housing for the tape is only 7mm (0.275in) deep x 25mm (1in) wide.
Anyone have any practical experience of this technology? I have always been put off a lathe DRO installation because of the bulkiness of the scales.

More info is at http://www.machine-dro.co.uk/digital-readout-systems/magnetic-tape-linear-encoders.html

Stuart

John Corden
08-05-2011, 06:28 AM
I have DROs with both glass and magnetic scales. The magnetic scales are much easier to fit, take up less space and are more resistant to swarf and coolant. The accuracy and resolution seem the same as glass. They are more expensive but worth the money I would say.
John

philbur
08-05-2011, 06:42 AM
The stated accuracy of: +/-0.02mm (+/-0.0008") on a cross-slide might be a problem as it will be double the stated erorr on diameter measurements. It all depends on their exact definition of accuracy. If it's a linear error over 300 mm then it shouldn't be a problem. If it's a random error within a 300mm envelop then that's not so good.

It may be no coincidence that the link to the technical data sheet does not work.

If you want compact then also look at Newall's microsyn scales. Here's an installation on my 10" x 20" lathe.

http://i186.photobucket.com/albums/x36/philbur/cross-slidescale.jpg

Phil:)

Peter.
08-05-2011, 06:46 AM
How tolerant are the tape encoders to variations in tension? Do they not stretch at all?

philbur
08-05-2011, 07:17 AM
I have read that they are less accurate if you install without the metal frame and also that they quote "typical" installed accuracy, which may be indicative of the elasticity issue.

Phil:)


How tolerant are the tape encoders to variations in tension? Do they not stretch at all?

Forrest Addy
08-05-2011, 08:04 AM
A small hijack on the topic of DRO's: 24 years ago, I wrote the spec for converting a large Niles planer into a planermill. Among the features was a shiney new Sony DRO system installed for all axes. That's four heads and the table - total of nine axes. The table was 36 ft long with about 35 feet of practical travel. The DRO scale for it was a magnetized tape stretched the full length with a stationary reader. The tape was tucked under the table safe from chips and other hazards.

It took several years to contract for the work and accomplish all that needed to be done. Come time to calibrate the scale in 1991 we set up the HP laser system and after running the table back and forth a few times we comped the scale for accurate reading over the full length. Then we looked at the accuracy segment by segment. Turned out there was no need for further calibration. The tape read accurate to 0.0002" anywhere on the scale according to the HP laser calibration system - and no hystersis; none; nada. We called a guy out of the metrology lab and showed him what we hand and whether we overlooked any thing. He shrugged and said words to the effect we probably had the most accurate positioning system of its size in the Pacific Northwest. Divide the least detectible error into the length of table travel and you had about 1 part in 780,000 resolution. That was in 1991.

I heard that a few years ago that same 60 year old Niles planer with the 40 year old Futuremill planer mill heads and state of the art (in 1991) DRO were used to check an array of holes bored to 0.0005" true location on a steel structure over an area of 9 x 31 feet (I think). It was machined on the new (in 1994) G&L floor mill. The discrepancies of location were so small it was a coin flip which to believe and yes temperature was considered when the final tally was assessed - that was the first question I asked.

philbur
08-05-2011, 09:16 AM
If my numbers are correct 0.0005" in 31 feet requires 0.2 dgree F. What kind of a procedure did they use for controlling and measuing temperatures on such a large machine. Assuming control and measurement would need to be at least an order of magnitude better that puts it at + or - 0.02 Degrees F?

Phil:)


I heard that a few years ago that same 60 year old Niles planer with the 40 year old Futuremill planer mill heads and state of the art (in 1991) DRO were used to check an array of holes bored to 0.0005" true location on a steel structure over an area of 9 x 31 feet (I think). It was machined on the new (in 1994) G&L floor mill. The discrepancies of location were so small it was a coin flip which to believe and yes temperature was considered when the final tally was assessed - that was the first question I asked.

hareng
08-06-2011, 07:46 PM
Have a look
http://i1140.photobucket.com/albums/n563/Jonhareng/Harrison%20M300/M300DROCABLING004.jpg
http://i1140.photobucket.com/albums/n563/Jonhareng/Jons%20machining/RamFix1protrusion26mm.jpg?t=1294450476
http://i1140.photobucket.com/albums/n563/Jonhareng/Jons%20machining/MagneticStripsilicon4.jpg?t=1294703548
http://i1140.photobucket.com/albums/n563/Jonhareng/Jons%20machining/DROCarriage.jpg?t=1294184953
http://i1140.photobucket.com/albums/n563/Jonhareng/Jons%20machining/DROCrossslideinstalled.jpg?t=1294185149
http://i1140.photobucket.com/albums/n563/Jonhareng/Harrison%20M300/M300DROCABLING005.jpg?t=1308043203
http://i1140.photobucket.com/albums/n563/Jonhareng/Harrison%20M300/M300DROCABLING006.jpg?t=1308043361
http://i1140.photobucket.com/albums/n563/Jonhareng/Harrison%20M300/M300DROCABLING003.jpg?t=1308042946
http://i1140.photobucket.com/albums/n563/Jonhareng/Harrison%20M300/M300DROCABLING002.jpg?t=1308042831