Source for small precision shafting

Hi, I'm in need of some shafting that measures 3.08mm and is pretty true. I have some 3mm A-0 drill rod that is 3.18mm but can't turn it precise enough with my equipment to get the desired size.

I would really like something like 416 stainless.

Any ideas or help?

Thanks James

That's going to be a special and pricey. In 416, it'd probably have to be ground. You could try W.M. Berg

or Stock Drive Products They can quote specials and do small ground shafts.

Good luck.

Pete Keillor

Standard HSS drill blanks are available in 3.1mm x 70mm

A #31 drill or reamer blank is 3.05mm dia

If you need the diameter closer or longer lengths, check with a centerless grinding company. I've been treated well by Boston Centerless in the past...

Ned Simmons

One more. If 2" is long enough, pin gages are available in .01mm and .0025mm increments.

Ned Simmons

You do not have 3mm drill rod. You have 1/8" drill rod. If you buy some 3mm drill rod, it should be within .02mm or better.

Drill blanks are even better, but are not really machinable (can be done with ceramic, cermet, CBN) so you either have to grind them, or be happy with what you have. Also, they are a shatter hazard because of their great hardness.

Why do you need stainless? You need to decide, I think, because it will almost certainly be more expensive than tool steel (drill rod) or drill blanks.

Regards,

Robin

How much are you trying to shave off in one pass? How quickly does your cutter tip move in each turn? I woul dget some junk rod and experiment with higher speeds, smaller diameter reduction per one pass, and much slower move of the tip along the axis. Sorry if I am not using proper terminology.

i

Just need something hard and non-corroding. I'm using a shaft out of a computer part now. It's not as hard as the drill rod but doesn't corrode at all. The drill rod already has surface rust on it after a week of service. I'm assuming that since the drill rod isn't perfectly round that it's not ground. I need perfectly round because it will be spinning in bearings up to 50K RPMs also it's a press fit into a stamped steel part so has to be pretty close on the specs.

Thanks

James

Rob>

Sounds like you do need to find some stainless which is (or can be) ground. Centerless grinding does not result in a *perfectly* round shaft, although it's certainly more convienent than cylindrical grinding.

I don't have any concrete sources for you.. Perhaps somewhere like Mcmaster-Carr

MSC or Thomson Industries might have something

Regards,

Robin

Sorry, Robin. You have been misinformed. A properly set up centerless will yield what is, for all practical purposes, dead round parts. Only when they are run *on center* do they not. In fact, they are capable of very close work, with outstanding repeatability.

Harold

Try PIC Design.

Harold

What type of hardening should I get? Air, Oil or Water? Criteria are: Easy to machine, corrosion resistance and stiffness.

Thanks

James wrote:

We really need to talk about the corrosion resistance you mention. If it's not chemically induced, I'm having the least bit of trouble understanding why it's a problem. None of your choices will offer corrosion resistance, although 416 SS would, especially in the heat treated condition.

PIC has lots of sizes in 303 SS, which is a dream to machine, but, not heat treatable. It might be wise to mention exactly what you're doing (and perhaps even why) so we don't have to make assumptions. You may be chasing your tail unnecessarily. Do you have to heat treat? Are the parts in need of precision after heat treat? Have you ruled out making the parts with grinding in mind? All of these will determine the best choice of material. Air hardening is likely the overall best choice if you can live with little corrosion resistance.

Harold

Harold, I'm trying to find a shaft material to use in small electric motors that I'm going to kit and sell. These motors are rebuildable and the reason for the corrosion resistance is that they will be used outside and exposed to the air, dirt and everything else you can find in a typical field or park. This shaft will ride in ball bearings and be fitted into a pressed steel end piece which is why the diameter is critical. The pressed steel end is pre-manufactured and I have no control over the size. A shaft I have that measures 3.08mm fits perfect. I could probably go a little larger if I were using a press to install the shaft into the pressed metal but I am trying to sell this in kit form so the end user will need to be able to press fit the shaft. The 3.08mm shaft is doable using a drill press to press it in. The motors are designed to spin upwards of 50K RPM I have some 303 SS in 2mm but it seemed very soft to me and I'm not sure it would stand up to the abuse. I am doing this out of my house. I don't have heat treating abilities and very limited machining abilities. Small lathe, drill press and bench grinder are about it. So far I've used up 3' of A-1 in the 3.18mm size making the kits but trying to get 3.18 down to 3.08 has been a challenge. I can not seem to get it to cut very well on the lathe and end up using emory cloth to get the final dimensions. Very time consuming.

I have ordered some #31 oversize drill rod (0.120 +0.0002) from McMaster

Should have explained all that in the first post.

Thanks

James

Harold and Susan Vordos wrote:

To what extent would they be rebuildable? Rewinding? Seems like a tall order for high speed motors. I'd think that balancing them would be a top priority.

Unless I'm missing something, the bearings would dictate the size needed, not the end, although they may be similar in size, if not identical. Bearings that are not a decent fit will often cease to function at high speeds, with the shaft slipping in the inner race. That's not a good thing. How does the shaft size you desire relate to the bearings? Good fit? Too loose?

A shaft I have

Yes, it does feel soft, but it has respectable tensile strength, especially if you buy it in condition B, which is a work hardened state. Still, the diameter you seek could be an ongoing problem for you unless you can find someone with a centerless grinder to size the material. Shafts are typically soft, not heat treated, so you'd be right in the pack where shafts are concerned. You'd have the option of using either 303 or 416, both of which would serve well for corrosion resistance under the circumstances you described. Your chances of hitting the desired size is much better if you have the parts cut to length and they start out reasonably straight (less than a couple though bowed). A centerless is very capable of straightening stock, but it must be straighter than the amount that must be removed, and it must be handled accordingly, which means several light passes through the centerless, each one getting closer and closer to a straight piece. Assuming your part is slightly bowed, the grinding starts to occur on one side of each end, and again @ 180°, in the center. Each pass brings the part closer to straight, and it's very visible. Horsing the pass forces the part, so it doesn't get straightened as well and will end up not round or straight, which is the typical condition of drill rod, to varying degrees. That's likely why Robin said what he did. Not true when properly applied.

I'm of the opinion that you may be over your head in that sizing the material won't happen easily without a centerless, whether it's yours or one you hire. You're likely already doing as much as you can expect otherwise.

Yes, it is, but that's likely your only option unless you can buy a material that suits your needs and doesn't have to be worked for diameter. Polishing for final size isn't cheating, that's how guys like us achieve close tolerance when needed, and we don't own a grinder of some kind. Turning to a given diameter with a tight tolerance, especially on a small lathe, is nearly impossible, especially if you want a good finish. Most material simply won't cooperate. It has little to do with you and your equipment.

That won't solve the corrosion problem, and may not solve the size problem, either. 3.08 is a full thou larger than that size, and is way too far undersized to make a decent bearing fit, assuming you really do want the

3.08 finished diameter. Again, unless I've missed something, that's not going to solve your problem.

Yeah, the more information you can provide, the better the chance you'll get an answer that suits your needs. The slightest thing can make otherwise very good suggestions not so great for a given situation. I'm of the opinion this is one of them.

You have some serious considerations with this project, one of which is balance. At the velocities you mention, I can't help but wonder if the end product won't vibrate considerably. If your shafts aren't very straight, they're likely too. Maybe not a problem for the application, hard for me to say.

Without knowing more, what I think I'd do is get a prototype worked out, and if you have need for a volume of shafts, buy material that's near size, say

1/8", cut them to length, chamfer the ends, then have them centerless ground. It's also possible that you can order the shafts to that particular spec and get them without any handling on your behalf. Centerless grinders are work horses that have the capacity to turn out thousands of finished parts per hour. Setup takes time, grinding (normally) does not. You might find that you could get a shaft sized for a few cents, especially if they don't have to be run through the grinder too many times. By starting with straight material that's near size, you'd eliminate the need to do so. It's possible that you could run them through the grinder no more than three times and have them sized perfectly, holding a tenth or less of tolerance. You could probably do it with one pass, but you'd risk getting parts out of round. You're at the mercy of the skill and caring of the operator.

Not sure I've been much help, James, but know that turning something like you're trying to turn isn't easy, not even for guys with years of experience. That's why grinders were made. You can do it that way, but it will be labor intensive.

Be certain to check with PIC before making any long term decisions. I used to buy from them when I was building tools for guidance systems. They're a top notch outfit with excellent service, or they used to be, anyway. Haven't done business with them for years.

Harold

McMaster Carr also sells 1 inch long 1/8 inch diameter hardened 416 dowell pins. These pins would be cheap to have centerless ground to your desired size. The pins are about 25 cents each. I'm sure you could have them ground, in quantities, for much less.

James,

Perhaps a tool post grinder would work on your small lathe. You didn't mention the length of the shaft. You could grind the .02mm off with the grinder and then cut the piece off. Slide out the next section and repeat operation.

Regards, Bernd

Don't even think of going there. All you'll do is change the nature of your frustration. You'll solve nothing. Toolpost grinders are, at best, a PITA, slow, hard to manage, and very destructive to your lathe, especially if you run coolant, which is very important for a grinding operation. The chance of you holding the size you desire using a tool post grinder aren't good, especially if you want to do it time and again. You'd spend more time fighting the grinder than you would turning and polishing. I'm speaking from the position of having run precision grinders *and* toolpost grinders.

If they're long enough, the comments on the 416 dowel pins is the best possible scenario as I see it. That was a very good tip. You'd start out with pins that are already straight, heat treated, so reducing size would be a snap. Possibly only one pass through the centerless.

Harold

[ ... ]

Note that the shaft needs to fit the bearings *and* the press fit of the disc. A common ID of small bearings (at least in the US) is

1/8" (0.125"), which converts to 3.175mm.

I would suggest that your shafts *must* be sized to fit the bearings, and only the *end* needs to be reduced to the press-fit size for your steel plate. And at the speeds which you are talking about, I would suggest that a really serious press fit is needed, not something easy to accomplish just using a drill press as the press. After all, if this steel plate comes loose at 50k RPM, it could do a lot of damage.

What is the approximate size of this steel plate? This would help get a feel as to how likely it is to come loose. (Presuming that the shaft and the plate are what is rotating, and not the outer bearings and whatever they are mounted in instead.)

While a toolpost grinder is a pain, and hard on the lathe itself, perhaps it might be reasonable for turning down only the end of the shaft to accept the press fit. You have only a small lathe, and one may presume one of the inexpensive ones, at that, so perhaps a second lathe of the same sort, dedicated to the toolpost grinding. And for the grinder, you could probably take a flexible-shaft Dremel tool and set it up on the toolpost for the grinding which you need to accomplish.

Also -- how are you holding the shaft in your lathe? For those speeds, you will want your press fit ring to be truly concentric with the part which fits in the bearings, which excludes most 3-jaw chucks, and makes a 4-jaw chuck a real pain if you have a lot of these to true up and grind. Ideally, you would want a collet to hold the workpiece.

I presume that in your attempts to turn the whole shaft to your desired diameter, you have a lot of it extending from the chuck. Note that this allows flex of the workpiece to make your surface finish a lot worse. You want to have as little extending beyond the workholder (chuck or collet) as possible.

There are also other things which can be tuned up in the lathe to improve the surface finish, even while turning, not toolpost grinding. Perhaps some details as to *which* lathe would allow those familiar with that one to point out what is needed to make it have less flex in the critical areas. (Things like snuging up the gibs, and taking it apart to remove burrs from the dovetails so the fit of the gibs is better.

Good Luck, DoN.