80/20 Specs - Lathe Construction

[Deviant]

Does anyone know how flat this stuff is, and over what length?

I don't have access to a large mill, so I was debating on using this along with some mic-6 aluminum plating to build a lathe using thk rails.

Anyone know if this is a good idea?

I saw that 5bears site used it for mill, but also that they took off a bit of the finish to flatten it out.

Any info would be great.

Thanks

[ViperTX]

Check the 80/20 site....I recall 0.001 per foot....I believe that's correct....I don't know if Swede encountered any built in stress being relieved when he machined his stuff.

[Deviant]

hmmm,

Do you know if that is straight enough for a lathe bed?

I'm thinking sure the the mic-6 is something like .005 or so.

I haven't built anything like this from scratch before, I'm concerned over finished tolerances.

[mwp]

This is a really good question.

I don't think that the straightness of 80/20 is quite .001/ft. Unfortunately, I can't find the number on their site.

However, here is the link to bosch rexroth's (the brand extrusion Swede used in his mill) catalog section with tolerances:

http://www13.boschrexroth-us.com/ca...ection%2017.pdf

According to that page, it looks like bosch extrusions could be out by up to 1.5mm over 2m (or ~.060 over ~6'). However, I don't think they are usually this bad.

Regarding the flatness of MIC 6, check out this document:

http://www.millproducts-alcoa.com/p...lloys/mic-6.pdf

From my reading, it appears that in the 3/4" inch and up thickness, the flatness tolerance is .005 measured over 8' in one direction and 4' in the other (tell me if you read differently).

So, based on those references, it appears that the extruded profiles are much less straight/flat than the tooling plate. My experience tells me the same thing.

If you try to straighten out the extrusion by machining, clamping it tends to pull it flat while being machined. Then it springs back as soon as it is released. Very frustrating.

I'm interested in hearing what anyone else has to say about this.

Good luck!

Mitchell

[Deviant]

The problem is, I don't have a machine large enough to mill in one pass. I'd have to do it in multiple little passes, with reclamping between.

I'm not sure I'd want to do it that way, might be easier to pick up a 4x4 peice of coldroll steel and have a professional shop facemill it for me. That or try my hand at scraping.

*shrugs*

[miljnor]

when I have straighten long peices in house, I have had excellent success, but when you clamp you have to do it 90 deg to the cutting surface. Several vices all trammed in the same planes works well. This way there is no inadvertant straitening done buy the clamps.

Or the best solutions is to send it to a grinder that specializes in dual surface prep. (not neccessarily cheap)

Of course its either time or money.

[mwp]

Originally Posted by miljnor Of course its either time or money.

Yes, this is usually true. But one measure of a good design is its ability to minimize both.

I've definitely been through the multiple setup milling thing. Yuk.

I've done some thinking about this situation, and come up with the following:

Since the tooling plate is much flatter than the extrusions, use it for the surfaces that need to be flat. But tooling plate in reasonable thicknesses will not be stiff enough for this duty by itself. So the moment of inertia needs to be increased somehow. This is done by attaching extrusion(s) to the tooling plate with epoxy (jb weld?) and moderate pressure (i.e. not enough to deform the tooling plate). Then, after the epoxy dries, bolts are inserted to reinforce the joint. This should keep the slightly bent/warped extrusions from pulling the tooling plate out of line and require no machining. This does mean that anywhere there is a rail mounted, it needs to have a tooling plate/extrusion under it (the rails can not be mounted directly to the extrusion). But you could use two or more extrusions attached to a plate vs one large one (cheaper?). The use of epoxy has the added advantage of increasing joint stiffness.

I'm interested in what anyone else has to say about this idea.

Mitchell

[Deviant]

hmmm

I'm not really excited about the jb weld option. If I'm looking at spending money on the extrusions, I'd prefer that they handle the job. Otherwise the money would be better spent on getting a steel tube milled.

I may change the dimensions as I research parts more, but....

My plan was to use two 2x4x30" extrustions parallel to each other with a 4" gap.

Attached to both sides would be a .5" or 1" x6x30" piece of plate aluminum.

This would allow me to have a 4x4x30" passage for a ballscrew.

On the top extrusion, I planned on attaching a .5 or 1" x4x24 plate of cold roll steel. I was going to cut a 45-dovetail the length on both sides to allow attachment of the head and tailstock. Between the head/carriage/tailstock would be a 6" wide bellow to protect the thk linear rails that would run down one of the aluminum plates for supporting carriage.

So, I'd guess really only the plate would need to be perfectly flat, and the steel plate also. I'm just not sure how much they plates will deform when bolted to the extrustions. Might be spending to much money to make it easier on myself.

Anyone know roughly how much it would cost to get a 4x4x30 steel tube milled flat and square on all sides? I'm not sure how long this would take to setup in a professional shop. ((I wouldn't think it would take long))

[miljnor]

don't know about the cost on milling but have you considered Blanchard grinding them insted? there setup is fairly neglible, so I would presume cost to be minimum. and the size is nothing for most blanchard grinding shops.

I would think 75.00-150.00 in that area would cover the blanchard grind. and if you are doning multiplu peices the blanchard grind won't increase significantly because all the peices can be done at the same time.

[ViperTX]

I'd total up the external charges before proceeding....

[mwp]

Hi, I looked up the actual 80/20 brand spec in my catalog at work--.012"/ft straightness. Not good enough for directly mounting thk rails (without first machining flat) if you ask me. Regarding the steel tube finishing, I recently was quoted $130 to machine rail mounting surfaces on two sides of a 40" X40" weldment made of 4x6 structural steel square tubing. Quote seemed a bit cheap given the amount of work involved, but I do quite a bit of other work with this shop....

Your machine sounds really interesting. What size rails do you plan to use?

mitchell

[Deviant]

I'm looking at using a thk shs25 rail.. Assuming I'm patient enough to pick up a pair on the cheap.

I haven't decided if I want to use linear rails on the carriage. May just use a standard gib/dovetail on it.

I'd guess that it would be accurate enough. I'll look more into it, after I get the base.

Originally Posted by mwp Hi, I looked up the actual 80/20 brand spec in my catalog at work--.012"/ft straightness. Not good enough for directly mounting thk rails (without first machining flat) if you ask me. Regarding the steel tube finishing, I recently was quoted $130 to machine rail mounting surfaces on two sides of a 40" X40" weldment made of 4x6 structural steel square tubing. Quote seemed a bit cheap given the amount of work involved, but I do quite a bit of other work with this shop.... Your machine sounds really interesting. What size rails do you plan to use? mitchell