View Full Version : Advice sought
I am in the planning stage of a new project. This is something that requires two round ways, about 1 1/4" diameter steel. My local supplier has an excellent selection of materials for this. I have a choice of precision ground stock that they call "Atlas" steel. The machinist says that it is a cut above 1018. I haven't heard of it. Have you? I can get the same in chrome plate with microinch finish. I can get it in induction hardened 4140. I can get it in SS, don't know what alloy yet. I intend to use silicon bronze bearings to run on the rods with provision for bearing clearance adjustment. It is for a small CNC milling machine.
What would be the best choice for the round ways?
06-05-2004, 03:54 AM
How about some hydraulic shaft tubing. I had a piece of 1 1/2" that had been outside on the ground for years. There was no rust on the hard chrome plating. I measured it in several places and got exactly 1.500.
I've seen some nice lengths of TG&P chrome steel on Ebay...Maybe motorcycle fork tubes, most fork bushings are bronze. Thompson rod from Enco w/ a free shipping deal? I don't know if the free shipping applies to Canada though.
what more can you tell us about the application? you mention 'small' CNC machine (the engraver?) but you're looking at 1.25" ways.
can you take a stab at the loads it'll be seeing? and, if they're light, perhaps the (linear) speeds?
most situations fall into the "slow & heavy" and "fast & light" categories.
personally i'd stay away from chromed hydraulic stock unless you really are only engraving signatures on gold watches. it does look nice, but won't hold up well -- especially problematic if you switch to roller bearings in the future.
i'm sure you know they sell 'linear guides' (ie ways) in just about any bearing catalogue. going this route, you'll have alot of the rudimentry design work already done for you.
06-05-2004, 08:09 AM
1-1/4" Rails is what my old Excello mill has for linear ways on the nc table,is going to be a small mill with a 800lb table http://bbs.homeshopmachinist.net//biggrin.gif
Oh I get it,the rails will be unsupported right?Then it will be fine.You might try the hardened 4140,but most I've seen has a barber pole stripe in it that yu can feel running your hand over it.Another option if you can find it would be Nitro bar,the same stuff they use on shock absorbers,no chrome,but still rust resistant and cheaper than Thompson shafting.
Of coarse you could go with that Atlas shaft,never heard of it,but I'll bet its just some 1020 thats been ground to size,then again if drill rod is within the budget?
Yep Wierd, you got it. The rails will be unsupported over about 20 inches. I can't afford linear guides. I will work with what is available locally as that cuts the cost. No, the table won't be 800 lbs, should come in under 400 somewhere. http://bbs.homeshopmachinist.net//biggrin.gif This has nothing to do with the Unimat, this is a fresh design. I was playing with the Unimat in milling config the other night and was impressed but it isn't quite big enough to do what I want. I am thinking of a table travel of around 8" X 10" and a Z of around 6". Strictly CNC, I probably won't even put any handles on it. I am leaning to the Atlas rod as it has a nice finish but will hold oil. The machinist I talked to also said to avoid the chrome because it might tend to flake with time. BUT I like shiny stuff! Must be a common affliction with machinists.
Oh yeah, this hopefully will fall into the heavy and fast category.
[This message has been edited by Evan (edited 06-05-2004).]
Would dove tail slides work for you? I have a bunch of cast iron dovetail slides that came off a couple special purpose automatic machines. The slides were moved with air-oil metered slides so they don't have lead screws but they are standard slides with provision for a nut and screw. If I were using round rod slides I would make the rods as large as would fit to get as much stiffness as possible. Deflection is the biggest problem I have seen with that type of slide system with wear second since it is hard to compensate for that with rods.
06-05-2004, 12:17 PM
Just some rules of thumb to keep in the back of your mind as you design. Hard overlaps soft, think you have that covered. Hard has finer surface finish than soft, cuts down on file type of wear action. Cross-hatch lay on surface finish in the bearing bore to help spread the oil and to wash out fine dirt. Avoid annular lay on rod surface finish again to prevent file type of action.
06-05-2004, 01:19 PM
I don't know enough about alloys or surfaces to add to the above but my years of experience with the Unimat tells me one thing about this type of design: the rods will flex. I'm sure you do know that. You may be thinking of linear displacements of the table with this flexture but in my experience, the rotation that occurs when one rod is moved down and the other up is a worse problem. I would suggest that they be places as far apart as possible to minimize this rotation. You should also keep the axis of the quill centered between them.
Thanks for the offer, but I have this design fairly well formulated in my head. I want to use round ways for the low friction that is possible. Yep, deflection of round rods is the biggest drawback. I can go up to 1 1/2" for the X axis ways as the chunk of silicon bronze round stock that I have is 2" OD.
All excellent advice which is why I asked. Neil, so I should put a cross hatch groove on the ID of the bronze bearings? I suspected that would be right.
Paul, I intend to place the X ways as far apart as is possible for that exact reason. That is the biggest weakness of the Unimat design. The cool thing is that I can use really massive X ways without increasing the weight of the moving components much. I also have in mind a method to allow for adjustment of the bearing clearance in a very accurate manner, some what like gibs.
I went into town this morning and picked up about seven square feet of 3/4" 6061 plate for slightly above scrap price. The biggest challenge will be accurately milling the edges of pieces that exceed the travel of my Strands "mill". I'll figure it out.
06-05-2004, 05:57 PM
Silicon bronze is a great tough and corrosion resistant material but not the best choice for a bearing in a way system, especally linear where a tapered hydrodynamic film is not possible. I'd suggest a leaded bearing bronze alloy like 660 running on a hardened (or at least heat treated) ground surface.
If yours is the typical CNC application you'll want close restraint in the way bearings. Bushings on round ways will get you there briefly but they're not well adapted for rapid repetitive positioning. The way bearing of choice is Turcite or a comparable material on hardened steel with distributed open cycle drip lubrication but hardened steel is a bit pricy.
On the other hand cold rolled steel rectangles and flats are fairly accurate, dimensionally, and not at all pricey. They afford a starting point from which to assemble a precise maachine axis structure given clever design and painstaking workmanship.
Consider a bolted structure (A "boltment?" A derivitive of "weldment?" You saw the term coined here folks.) comprised of cold rolled shapes carefully de-burred and stoned assembeled into rectangular ways. They should be fitted and doweled into alighnment and the way bearings could be of sheet brass simply attached with brass flat headed screws and bedded with LocTite. Scrape the bearings to fit and provide a brass keystock set-screw gib. Shim the retainers for clearance.
There you have a way system made of low cost materials, featuring dissimilar metals for bearing durability, suitable for hime shop cinstruction.
30 or more years ago I used this construction system to build a set of positioning tables for drilling furniture parts. They lasted for years in a couple days a week year 'round environment but I suspect that had more to do with Bijur automatic oiling system than the excellent or durability of my design. Wood is kind to machinery.
I suppose theyre junk now if not already melted down as scrap to make new products. That's the usual fate of cheap barbecues, tin cans and the finest machine tools.
[This message has been edited by Forrest Addy (edited 06-05-2004).]
06-05-2004, 07:31 PM
I would suggest tube instead of solid rod as the stiffness is what counts. This also means you don't need any high-strength steel, any alloy will due, as the elasticity of all steels are pretty much the same. Of course, you will need a slightly larger diameter than what you planned to acceive the same stiffness, but the weight will be much less. Check some standard engineering references as to the calculation of stiffness of beams of various cross sections.
The stiffness is inversely proportional to the moment of intertia (I). For a beam, I is proportional to the fourth power of the diameter. So, a tube 1.5" dia with .125" wall thickness should be just as stiff as a solid 1.25" rod. Assuming I did my math right :-).
I think that Mike W has a good idea with using the old hydraulic shafts. These have a fine finish to provide the hydraulic seal so they should work well as ways.
06-05-2004, 07:44 PM
Answer, no. Lube grooves are fine for dynamic bearings but your application is transitional. A linear/axial feed groove in a nonloaded zone with a wick to introduce oil will do. The x lay will spread the oil. SAE 660 leaded bronze is good stuff.
[This message has been edited by NAMPeters (edited 06-05-2004).]
06-05-2004, 09:30 PM
I used W-1 drill rod for my rails and linear bearings pressed into table supports. I had two 36"x30mm rails for a table (1x10x26) that would only move about 10". (That's all I needed) When I loaded the 120lb table with about 100lbs of cast iron engine block, it would deflect about .030. I thought WTF! http://bbs.homeshopmachinist.net//frown.gif
As it turned out, I had to modify my design and add two more rails and two non-loaded rails. (The rods and table sagged and the ends were being pulled inword.) I also switched to a piece of C10x30 channel as a table. This lightened the table and added linear support. The unloaded rails (stretchers) are mounted about 2" above and out from the bearing rails. The two new loaded rails were mounted about 1" below and inboard of the originals. This seems to tigheten up the frame considerably. The only time I see any movement (<.001) on my table gauge is during placment and block bolt-down. I suspect this is just vibrations.
Either you or I have had too much to drink. A boltment? Exactly what do you mean? My plan is to use the materials close at hand which includes silicon bronze for the bearings that slide on the ways. It will work harden as it is used. I am constrained by what is available and what I can afford. I plan to make the bearings slit so they can be adjusted with a fine thread bolt/screw to eliminate play in the structure. I intend to make a test piece first to see how it works.
A rod is marginally stiffer than a tube of the same diameter. I can buy rod locally but not tube. The X ways do not move and the more they weigh the better. Yes, the modulus of elasticity of iron is the same no matter what the alloy.
06-05-2004, 10:01 PM
Evan: Have you ever seen the Oneway woodworking lathes from Canada?
Not relevant to your 2 rod needs but just something fun to look at. The bed is a large cylinder with closely spaced welded supports for the flat way.
Also, Forrest brings up a good point that crossed my mind ... speed of traversing. And, after dealing with various way wear problems over the past year, I have new respect for rectangular, flat ways and the ease with which they can be made, re-ground, etc.