Are you up for acme all-thread?

Not quite 'box store all-thread', but it can be done.

I was thinking about doing somthing very home-brew like take some 'clay' style epoxy and put a bead around a form (like a dowel), then run it over an oiled piece of all-thread placed orthogonal to the major axis of the dowel. Roll the dowel to get a very basic gear.

This is assuming that the form/dowel is an integer multiple of the pitch of the all-thread.

Does that seem reasonable? ...

I think an experienced do-it -yourselfer could do this.

One thing I would like to point out is that gears probably have a lower pressure angle than all thread. This would make gears made with all thread to have a larger component that pushes the gears apart. In low load and good lubrication applications it may still work. If they do not goof up the cost of your project too much acme threads (as already mentioned) may work better because their pressure angle is in the same range as gears.

Hello servant74:

Would that automatically give you the correct clearance?

Sound intersting, but do not know enough really. Will keep an eye on this post.

Take care and happy holiday................

Servant -- I couldn't resist thinking about this problem of yours and had a creative thought that expands on your epoxy idea. I can tell you understand the need for a starting diameter that will be the right "pitch" diameter to produce an integer number of teeth. That diameter will be slightly different from a conventional involute gear because of the shape of your teeth.

That said, my contribution here is to suggest that the material you use rather than epoxy be a wax used for investment casting.

Whether you can get it to a workable degree of softness that would still hold it's shape is something I can't answer myself. But others with this kind of experience may be able to do so. Another thing I'd like to point out here is that both clear styrene plastic as well as clear acrylic can be burned out of a plaster investment mold in much the same manner as wax is burned out.

I know of several small brass foundry operations that are used by the cottage industry that produces fine scale models and kits for scale model railroad hobbyists. There are likely others.

The castings may be a bit rough to work as decent gears. But I've toyed with but have not completely thought thru the way one might finish machine such a "gear" using a threading tap and a worm gear set with a few added gears to create a crude gear hobbing machine. A 20 tpi thread for the racks may be a good choice because of the variety of standard size tap diameters with that thread count. Some experimentation with tap diameters may be needed to get the best running and wearing combination because of the way your special gear wraps around the all thread especially if it has a small number of teeth.

Ed Weldon

Hello Ed Weldon:

I know you sent this detail to Servant74? Can't recall the name.

With regard to the wax: melt it in a paint kettle and add a little paraffin or thinners. Let it cool and if you have put enough thinners in it will stay soft and workable.

If you put too much thinners in, cut another chunk of candle and drop in and melt again. It is a little hit or miss, and I do not have any charts or figures of how much wax to thinners.

Hope this helps.

Take care, and have a great holiday...........

Hello Ed Weldon:

With ref' to softening of candle, to make it workable...Use Turpentine or Vaseline.

Sorry for the mistake.

Hello Ed Weldon:

If you can get the beeswax sheets, you can use a hairdryer to soften it to workable texture, OK?

http://www.google.co.uk/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGLJ_enGB294GB294&q=beeswax+sheet

Take care.........

PS. I was assuming you meant candle wax. Beeswax is easier to work with.

Ed & Babybear,

Creative solutions is WHY I post questions here. Brain storming is never a bad way to start solving problems.

I like the was idea. ... Hmmm, I guess I could even cast a wax 'rack' using the all-thread, if it was thin enough, it could be cut an wrapped around a form for the center of the 'gear'.

Sounds like a weekend project!

I was also thinking that once I get one gear that seems to work, cast it in RTV, a room temp vulcanizing rubber kind of stuff, then use that as a mold for doing epoxy versions of the gears. I have some carbon fiber left from another project, put it in with the epoxy and it becomes a pretty strong piece of solid stuff.

... Thanks for the ideas!

Hello servant74:

You sound like a real interesting man!

And it looks as if you have the methods all worked out?

With ref to the sheet beeswax, it can warmed and rolled like pastry.

You are molding direct, you may be able to use the Vaseline as a release agent?

Please let me/us know how you get on OK?

Take care and good luck, Happy holiday!

Would it have to mesh like a gear? It could have stiff bristles or a soft leather surface to press down into the threads, turn parallel to the all-thread, with an idler wheel behind for support maybe.

Hmmm,

The use I have in mind is to replace the function of a spur gear and rack as used on a CNC machine. The rack is relatively long (4 to 12 feet) and the cost of course all thread is less per foot than manufactured rack. The 'gear' I have in mind would replace the spur gear used against the rack. I am not going to spin the all-thread, but keep it stationary and rotate the gear for linear motion.

Since I am looking at putting some moderately significant amount of torque on these (think up to 300 in-oz or so, possibly more) I wonder about the ability of a compliant surface to pass the torque.

But I do like the idea, that takes my thinking in a way I have not considered.

servant74 -- What thread size of all-thread are you thinking of using in this 300 inch oz. application?

Ed Weldon

That is the size of stepper motor, 305 in-oz rating (actually its station holding torque), is what I have available.

This is to drive a gantry on a DIY CNC table. The desire is to have a table large enough to take a sheet of MDF (a working surface about 5'x9' to allow a little extra room, so the gantry needs to cover a space of about 6' x 10' to allow gantry and tool to fully cover the working area). This is for wood routing.

Weld a sprocket onto a nut the same size as the allthread and drive that with a motor and chain?

check out instructables.com and search for DIY x-y table. There are many examples of using 1/4"-20 or other common threaded rod for low force less accurate x-y table. If you want to go with your approach I would think that tapping the hub with a common thread and riding that on a threaded rod would work.

Use a nut on a threaded rod and turn the shaft using two toothed pulleys and a toothed belt as many home built CNC machines have......

driving 'thin' allthread very fast is problematic at significant lengths. The idea of using the allthread as a rack was to allow it to be supported along its length without having to support it by bearings on the end instead.

Yes, you are right, many do. I was trying to emulate what the MechMate (mechmate.com) plans show without the several hundred dollars in rack they use. Also ShopBot (shopbottools.com) uses rack and spur on their stepper motors too.

Then how about cable and pulleys, or a thin metal strap over pulley in the way mechanical typewriter carriages were handled?

That could work. The initial ShopBots were 'cable drive'.

In the interim I conceived of a belt over pulleys, sort of like a belt sander or a tank track that would interface with a much broader length of the allthread to give a greater strength enhancement feature for torque, stability and durability.

I am still mulling over the easier ways to interface the threads, such as imprinting the wider, flat side of a V-belt with red hot allthread and a vise. You get the picture, but the idea is still young.

cj and servant-- This isn't allthread but it's a "flip" of your tank tread idea:

The Berg catalog shows timing belts with a .080 pitch and lengths up to 32 inches. There are other manufacturers of these small pitch timing belts and the pulleys. Cut the belt, lay it out flat and glue it down to a flat metal strip. If more length is needed careful splicing should be able to get the match within a couple or three thousanths. There's your rack!! Then just pick whatever matching pulley fits your design. If necessary trim down the flanges on the pulleys. They can be used as constraints but maybe better without the friction drag depending on how much power is available in the stepper motors.

Ed Weldon

OMG!! Was this a disclosure? Well, I'm not going to do anything with it. Except I can't resist adding something that won't help servant's immediate problem but may be the basis for a product.....

Supposing a timing belt were to be molded with instead of a flat back side a "T" shape feature designed to fit in a "T" slot such as you might form in an aluminum extrusion? Or maybe grooves in the edges? Then the assembly does away with the adhesive.

What other types of molded configurations on the side opposite the teeth could be added to enhance the flat "rack" application? And would the rack necessarily have to be flat? How about a "roller coaster shape or something else? Even a circular gear or partial gear?

BTW, has anyone here ever seen an involute rack molded out of nylon, perhaps thin enough to be bent?

Ed Weldon

Yes. The rack had a T-handle molded on it, and it was used as a pull starter on one of my younger brothers toys years ago. It drove a gear to 'energize' a flywheel to power the toy. Was it nylon? I doubt it, as I think it was some toy version of RED if I remember correctly.

http://www.alibaba.com/product-gs/208457977/Nylon_Rack_metal_rack/showimage.html sells nylon and metal rack it looks like.

Hmmm, your commentes made me think about sdp-si.com -
https://sdp-si.com/eStore/Direct.asp?GroupID=214 - they also have a great small parts catalog will all kinds of gears, belts, chains, sprockets, etc.

Hello servant74,

This is to all who are interetsed.

I found I could not link the Alibaba site from your link, but found it in the end.

Here is the address of the plastic and metal 'garage door opener'.

http://baishengdoor.en.alibaba.com/product/200226470/208457977/Accessories/Nylon_Rack_metal_rack/showimg.html

=============================

This is just to allow you to see the item, OK?

Take care to all and happy holiday.............

Your idea is not bad, what will help it further is the fact that such belting is available in any length that you wish to order.....no need to splice.

We call them half-nuts.

You know the mechanism that drives the carriage on a lathe when cutting threads.

Split a the matching threaded longitudinally, construct mechanism to hold halves apart then together when engagement is needed.

well how would that work? If I am NOT TURNING the allthread, but using it as a rack?

I see how your suggestion would help if I was working on an anti-backlash nut.

If you wish to build a CNC machine of the size you are talking about, may I suggest that you use toothed belts on both sides of the gantry to move it back and forth, not what you are thinking of, too inaccurate and complex. Toothed belts are not cheap, unless you are lucky enough to find them on ebay, but easy to design and build with.

Have them linked at -both- ends with a fixed shaft with toothed pulleys. Have a large toothed pulley somewhere in the middle of one of the shafts, with a short toothed belt going to a much smaller toothed belt on your motor as "gearing".

Keep it simple by having exactly 4:1 or 5:1 or so, no odd ratios like 1.33:1 for instance, keep the numbers round. Have the end pulleys also an easy size say the same as the motor size (number of teeth).

Use motors and drivers (buy carefully or go to servo motors) with "microstepping" (easily possible up to 1/64 microstep or more), using 1.8° motors as large as you can get, or 0.9° for even more accuracy.

(Actually, with careful selection of the other parts in the drive train, you could buy cheaper 3.6° or 7.5° motors!!)

Using 0.9° motors, 1/64 microsteps, 8:1 pulleyMotor to shaft (but could be far more!) 1 revolution of the shaft = 400 x 8 x 64 = 204,800 steps!! That should be accurate enough!!!

Even without microstepping, you would have 400 x 8 = 3,200 steps.......

Have good bearings, keep the belts taut with some sort of adjusting mechanism (ball bearing on a bolt that can be moved in and out!), but do not over tension. Support the end pulleys on both sides of each pulley and each side of the motor driven pulley too - important!! Then the shafts do not need to be so thick that it makes pulleys too expensive!!

I prefer T5 belts on smaller CNC machines, but you may want to go to T10 or greater, oversize for a good long life.

Do not use normal gears in a home built machine, too difficult to get rid of any back lash....

If you need a diagram, just let me know, it won't be pretty, but it will give you the right idea.

Best wishes....by the way, I build my own CNC machines too, but far smaller.......