Clamping X-Y Stages together 1

[pal00]

We have built an X-Y mothion stage, that moves a fixture aroung. Travel is 40inches x 24 inches. Max velocity is 1.5meters/sec. The upper stage(24") weighs approx 60 pounds, the fixture about 10 pounds. Stages and saddle below are anodized aluminum, stages are secured with 4x 12.9 Grade M8 Socket Head Cap Screws, torqued to 40N.M.

When the tables are run into the hardstops, they slide on each other.

Can anyone suggest an interface material or joint surface preparation that would prevent slippage?

We currently are using some drywall sandpaper pads, but that is only a temp fix.

Thanks

Paul Labelle

 

[IRstuff]

To what accuracy? Why don't you just drill a pin through both stages?

Or, modify your program to not run into the stops. Frankly, running into stops doesn't sound like something that one should be doing.

TTFN

FAQ731-376

 

[MikeHalloran]

Remove the sandpaper and clean the joints thoroughly.
Assemble, align, torque normally.
Apply a few drops of Loctite 271 (green, low viscosity, wicking) to each joint interface. Give it a few hours to set up if you can.

Be careful to not get the Loctite into any crevices that you don't want >>permanently<< bonded.




Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Use locating dowels in place of, or in locations adjacent to, some of the fixing screws?


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[pal00]

Thanks for your comments. The reason that I asked the question is that I needed some help. I realize that running tables into hardstops is undesirable, but guess what... It can happen. When it does, I have to send a guy from Canada to Germany, next day, to do a full day table calibration. So until I can run a table into a hardstop, and not have it slide around, I will continue to test this way.

Table accuracy needs to be within 5 microns, banging into the hard stops produces some linear translation, but also some rotational.

Drilling and pinning after assembly is not an option, no room to get in.

Will this green loctite come apart if necessary?

Thanks

Paul

 

[MikeHalloran]

Loctite 271 is separable with heat, and with mechanical stress, like anything else. We don't know whether your components can withstand the necessary force or heat without damage. You can figure that out, using info here:

http://www.loctite.com/int_henkel/loctite_us/index.cfm

Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

If you have the room to screw and unscrew the stages, you should have plenty of room for a pin.

TTFN

FAQ731-376

 

[GregLocock]

In an ideal world you shouldn't use dowels (or keys) to react forces, they are not supposed to work in shear.

I suppose the idea of installing shock absorbers on the end stops is unnaceptable, and redesigning the bolted joints properly is also unacceptable?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[prohammy]

Just a quick suggestion....

In a similar situation before (not the hard stop scenario, but did need ensure that there was no tranlation and/or rotation between the X and Y stage) and what I was able to do was use drill and taps that were the side of each stages interface plate to mount restraining plates.

If the stage you have doesn't have these drill and taps, would it be possible to dis-assemble the stages and add them? If so you may be able to add 6mm braces on all four sides

Kevin

“Insanity in individuals is something rare, but in groups, parties, nations and epochs it is the rule” Nietzsche

 

[prohammy]

PS Greg is absolutely correct about NOT using dowel pins to prevent translation between two plate, they are NOT designed for shear

Kevin

“Insanity in individuals is something rare, but in groups, parties, nations and epochs it is the rule” Nietzsche

 

[pal00]

Thanks for all your helpful suggestions, dis-assembly is not an option, nor is there enough room for shocks. I need a Joint solution for this go round. I have 4 assemblies that need a solution.

We are considering the loctite solution, but are still looking for a non-messy mechanical solution.

Dis-assembly to drill and tap is not an option for the unit in the field, it is installed in a clean room.

We have slowed down the table speed on this unit, as a precaution, however, they table can still hit the hardstop and be sent out of calibration.

Paul

 

[GregLocock]

As an additional thought, can you whack whoever did the original specification around the head with a 2x4? Your design FMEA was obviously not taken very seriously.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[prohammy]

Nice Greg....another big star in the Locock night sky



Kevin

“Insanity in individuals is something rare, but in groups, parties, nations and epochs it is the rule” Nietzsche

 

[sreid]

Any chance you could install hydrolic snubbers to slow the slamming into the hard stops?

 

[Lcubed]

GregLocock and prohammy,
Please expand on your statements about dowels not working in shear. I don't understand.
Thanks.
Lcubed

 

[eromlignod]

Yeah, that's a new one on me too.

 

[GregLocock]

Dowel pins are designed for alignment during assembly, the shear forces in service are supposed to be taken by friction in the bolted joint.

Same with keys in shafts, where you should use a spline or a taper or a friction face to transfer the torque.

This is assuming efficient design of course, if you are not trying to exploit the maximum capacity of the material then you can use pins. But that is carpentry, not engineering.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Doweled joints are automotive practice, and ancient machine tool practice. Old machine tool practice is central keys. Current machine tool practice is ... I guess you'd call it external keys or corner keys. They violate your sense of symmetry, but they work great.

No dowel, no key, and a friction joint that you have to send a tech across an ocean to realign after a bump is ... unfortunate.



Mike Halloran
Pembroke Pines, FL, USA

 

[prohammy]

And again I am piggy-backing on Greg's post (been out of the office and didn't see Lcubed's post 'til this morning)

In the type of application that you (pal00) are designing the dowels are all about location, location, location (as any good house seller will tell you). There are any number of reasons to take an x-y stage assy apart, but only one way to get it back together to its calibrated position....dowel pins. However, if you are hitting your end stop with enough force to overcome the friction forces generated by 4 bolts, then you are in a bad place. If you do have dowels in this assy then they are either being deformed into a lovely S-shape and/or destroying their mounting holes rendering them useless.

Some further suggestions/things to look at.....

1. Are the bolts holding the stages together tightened to the correct torque settings?
2. Are you sure that the damages to the slide is from the impact and not from over-running the system on the end stops?
3. I would get your techies to check the drive and gearbox to make sure your crash hasn't damaged them as well.


Kevin

“Insanity in individuals is something rare, but in groups, parties, nations and epochs it is the rule” Nietzsche