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Elninio
06-01-2013, 05:55 PM
Tangential tool holders make it easy to machine exterior ball grooves on cylinder, because round cutting bits have very smooth surface finishes. Solid carbide endmills with mirror finishes are cheap in the size of most balls in the small machine domain (<1/4" diameter).
http://i.imgur.com/EZZfkMC.jpg
http://i.imgur.com/ktwRslg.png

Full diagrams and static analysis is here: http://urobotics.urology.jhu.edu/pub/2006-stoianovici-uspto-07051610.pdf
Details like ball slack are also covered:
http://i.imgur.com/OmYTyGV.png

Where can I buy steel balls in Toronto? 100x0.2500" balls on ebay for <10$ shipped, but don't wan to wait a month ... Fastenal has some, in hamilton (2hour drive)
Can I have a study on cutting tip elasticity?
Is it a bad idea to weld HSS to a shank to make a tangential tool?

Steel balls are sold by grade, in which the grade number corresponds to millionths inch deviation from spherical form (grade 5 = 0.000005" spher. dev. ). Grade 25 seems common and acceptable.

Duffy
06-01-2013, 08:09 PM
Elninio, there has GOTTA be a few Fastenals in TO. Hell we have one in Ottawa! Try a bicycle shop; the selection of size may be limited, but they should have quantity. Also, one of the on-line bearing suppliers will have LOTS of sizes and be only a USPS package away.

Toolguy
06-01-2013, 08:12 PM
How does one make the recirculating passage through the middle of the barstock?

Elninio
06-01-2013, 09:56 PM
Elninio, there has GOTTA be a few Fastenals in TO. Hell we have one in Ottawa! Try a bicycle shop; the selection of size may be limited, but they should have quantity. Also, one of the on-line bearing suppliers will have LOTS of sizes and be only a USPS package away.
The fastenals are there, but they only carry more common stuff like nuts and bolts - you won't find cutting tools, oils, balls, or other such items there!

Elninio
06-01-2013, 09:59 PM
How does one make the recirculating passage through the middle of the barstock?

Drill a hole through it along the axis, then mount the block on a rotary table mounted vertically and roll into that channel with a ball endmill. Next, machine some retainers and use a ball end mill on them too to machine the other half of the channel.

Or, find some tubing of the right size and bend a channel, machining the ends appropriatly. Bending the tube will make its cross-section oval, but instrument (like trumpets) makers force balls through their instruments to restore the circular shape.

Jaakko Fagerlund
06-02-2013, 01:06 AM
Try VXB.com for the balls, they have almost anything.

LKeithR
06-02-2013, 02:11 AM
Try a bearing supply place. The stores out here carry a decent selection of bearing balls. Wajax, for example, has a couple of stores in Mississauga...

ScubaSteve
06-02-2013, 08:01 AM
Very interesting concept. I wonder if it would be possible to undercut the channel so that the balls are retained by the shaft itself (and thus eliminating the large retainer block). I'd guess you'd need a fair amount of ball "stickout" to engage the gear.

The question is, other than cool factor, would this method provide that much more smoothness? And, would it be trouble free? More moving parts=more room for error and mechanical failure.

MikeWI
06-02-2013, 11:47 AM
How does one make the recirculating passage through the middle of the barstock?
It's shown in the patent. The drive line is hollow and has a plug with the necessary grove cut into it inserted.

lazlo
06-02-2013, 12:58 PM
Tangential tool holders make it easy to machine exterior ball grooves on cylinder, because round cutting bits have very smooth surface finishes.

Solid carbide endmills with mirror finishes are cheap in the size of most balls in the small machine domain (<1/4" diameter).

I've re-read those two sentences several times, and I can't figure out what you're trying to do :)

A broad nose cutting tool has a smooth surface finish if the nose radius is broader than the feed rate:


Surface finish (uInch) = Feed rate (inches per revolution)^2 / (Nose radius * 24)

That surface finish is the same whether it's a conventional lathe tool, a tangential tool, or a milling cutter. You're effectively applying a record needle with a nose wider than the record grooves.

Machining a ball groove on a cylinder will require a round form tool. That may be the same tool as in the formula above, but you don't get the smooth surface finish benefits because you're not overlapping the feed. In fact, it will chatter like hell.

Where does the "solid carbide endmill with mirror finish" come into play?

The rest of the project, a recirculating ball worm, is quite ambitious! You might want to try to build a recirculating ball screw first. How are you going to hob the circular grooves on the worm wheel?

Jaakko Fagerlund
06-02-2013, 02:47 PM
In fact, it will chatter like hell.
No it won't, if you feed away from the spindle. Then it requires really trying to get it to chatter.

The problem lies in the fact that when used from right to left, the tool bends under cutting forces down and as the tool post and cross slide bends, it digs in more to the material and this is the recipe for chatter. But if you reverse feed, then the cutter tends to bend away from the workpiece, thus reducing the force, thus eliminating chatter possibility.

Figured this out early on when I started using a round R5 tool to make some really deep grooves around workpieces.

Other option is to reverse the tool and spindle rotation to feed from right to left.

Elninio
06-02-2013, 05:50 PM
lalzo; You get the finish on the clearance face, this isn't cnc. This is also the point of the tangential tool; by grinding only the face, the tool has a factory ground clearance face, which leaves a nice finish.

lazlo
06-02-2013, 08:15 PM
lYou get the finish on the clearance face, this isn't cnc.

What does CNC have to do with anything? That's the formula for surface finish in every elementary machinist book.

If you're having a hard time visualizing it, here's a great picture from Sandvik showing the "record groove" concept. It applies to any cutting tool, whether lathe, mill or shaper, manual or CNC:

http://www.sandvik.coromant.com/SiteCollectionImages/Technical%20guide/Pablo/F%20Boring/090071.jpg


This is also the point of the tangential tool; by grinding only the face, the tool has a factory ground clearance face, which leaves a nice finish.

A tangential tool has no advantage of surface finish over a conventional tool. The advantage of a tangential tool is that it's much easier to sharpen: you have one facet to grind, since the toolholder angle provides the correct clearance angles.

darryl
06-02-2013, 08:25 PM
Before rack and opinion steering, the mechanism was a recirculating ball type. Ah, but then it still had sector gears. In your method, you would need a pretty custom ball control casing-

Paul Alciatore
06-02-2013, 09:47 PM
I think the point is that the highly accurate and nicely polished, round section (stem) of a milling cutter can be used in place of the more common square tool blank used in the common tangential holders. Only the end of that round stem needs to be sharpened. Thus, the accurate radius, which lacks any of the common scratches and other artifacts of shop sharpening techniques will be present in that radius. So, the radius of the groove that it cuts will also be highly accurate and hopefully have a better surface finish than a more traditional tool that has this radius created on the shop grinding equipment.

I can see another advantage in using a tangential style tool with a round cross section. Due to the deviation of the angle that the tangential tool is held from the vertical or true tangential position, the top face of the round tool will present a eclipse shape to the work instead of a true circle. If the diameter of the tool is a bit larger than the diameter of the balls that will be used, then the ball will ride on two points that are a short distance below the top of the "teeth". I believe this is a desirable shape for such balls to run in as if there were total contact in a true semicircular channel, then the friction would increase due to rubbing. A ball can not roll if multiple points are in contact with the surface it is to roll on. All but one such points will necessarily have slippage and thus, friction.

I see two problems in making this device in a small or home shop. First, the tangential tool may allow cutting the "worm" part of the mechanism, but the "worm" wheel part will need an entirely different set-up. Some kind of form cutting milling cutter would seem to be required.

Second, the patent description states that the grooves on the "worm" wheel part are wider at the edges than in the middle. I suspect that this is needed to allow the balls to enter at the edges. Cutting this feature will also be difficult. Perhaps the part could be rotated by a small + and - angle about the center point of EACH tooth after the form cutter has penetrated to full depth. The indexing mechanism (dividing head or RT) could sit on a rotating base to accomplish this.

lazlo
06-02-2013, 11:36 PM
I think the point is that the highly accurate and nicely polished, round section (stem) of a milling cutter can be used in place of the more common square tool blank used in the common tangential holders. Only the end of that round stem needs to be sharpened. Thus, the accurate radius, which lacks any of the common scratches and other artifacts of shop sharpening techniques will be present in that radius.

Ah, now I understand what he was trying to say! I really don't see that an endmill shank held in a tangential toolholder makes life any easier than simply grinding a round form tool. The slice across the top (and therefore the cutting edge) will still have the grind marks from "shop sharpening techniques", and as you say, the resulting shape will be elliptical (not round). Skimming through the patent, it suggests that the "double circular undercut" on the far right diagram is required for the worm, as opposed to the gothic arch used in a ballscrew (center picture).


I see two problems in making this device in a small or home shop. First, the tangential tool may allow cutting the "worm" part of the mechanism, but the "worm" wheel part will need an entirely different set-up. Some kind of form cutting milling cutter would seem to be required.

I mentioned that too. It's an extremely ambitious project. I've never seen anyone successfully construct a recirculating ballscrew, and this is several level of complexities beyond that. :)

Elninio
06-05-2013, 02:44 AM
Paul is correct, but also these carbide shanks are probably extremely low deviating from round. This project is easier than a ballscrew, since it doesn't require an internal too, which is more flexible, and an endmill shank's profile cannot be used - it requires a special tool. The tangential tool holder solves this geometry obstacle, but can only used on the exterior surface, which this projects has only exterior surfaces to machine (minus the channel recirculating the balls, but that's not of a critical dimension.

lazlo
06-06-2013, 12:13 PM
This project is easier than a ballscrew, since it doesn't require an internal tool,

Good point. How are you planning to hob the worm wheel gothic arches?

Elninio
06-08-2013, 12:30 AM
Mount the gear blank on a rotary table that I incline to the pitch of the screw, and using a cutter like a flycutter which holds the cutting bit like a tangential holder on a lathe.
http://i230.photobucket.com/albums/ee286/Arbalist/IMGP1237.jpg
edit: updated pic