Is screw too soft ?

I got some 1/2" OD 1/4" ID steel light duty reducer bushings from McMaster-Carr to use as adapter from a motor shaft to a lawn mower wheel for my new robotics
project. I drilled a small hole perpendicular into it for a "set screw" -- but am using some 3/4" long "regular" screws from Home Depot since I want it go through the lawn mower wheel shaft, too. When I tighten it down all it takes is a small amount of extra force to strip the screw. I am thinking the hole I drilled before I tapped it is too big, and/or the screw is too soft. ( I'll try another hole tomorrow )
What would I need to ask for in a screw to make sure it is hard enough to keep it's threads ?
The following URL shows a photo with inset of the bushing and screw for reference:
http://www.waycoolgear.com/ebay/wheel.jpg
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Can't you just drill, pin and loctite? or is this a use that requires the removal of the shaft? not sure what you want to do but just setscrewing that onto the shaft sounds like a fairly bad idea... (using the setscrew to transfer torque, that is.). In our application in robotics, it tends to dig a groove in the powered shaft until it is no longer transmitting torque, or snaps off..
Also, do you mean "strip the screw" as in the threads outside or the screw head or the screw threads? because I suspect its the outside threads that are stripping.. the stuff outside looks like soft plastic (which will strip..) and the inside metal could be overdrilled.. not sure what you're doing but standard zinc ones should hold (at least just by hand-tighetening)
Tatsu
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It's the threads of the screw that stripped off (worn down)
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I want to be able to remove the wheel now and then to get to the motor and mounting hardware. Thanks !
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    First mistake -- these are typically very soft mild steel. It is chosen to make the screws easier to make, not better.
    Go back to McMaster-Carr and get a box of the same size, but black finished Allen head capscrews, which will be much harder. (And probably a box of 100 will cost less than four or five from Home Depot. :-)

    What is the screw thread? It looks something like 10-32.
    What drill did you use to make the hole? IIRC, the tap drill for a 10-32 is a #21 "wire size" drill. No fractional size is close enough to do the job right.
    If you examine the screw after this happens, are the screw's threads stripped? If not, you can probably start with the size of the hole which you tapped. (You *did* use a tap to make the threads in the hole, did you not? You don't want to use any kind of self-tapping screw for this kind of thing, you really want to have a proper tap to cut the threads without the screw being involved.

    It is more a matter of *where* you get it. The ones which I described above should do quite well -- either from your McMaster Carr, or from MSC. I prefer MSC, simply because it is not like pulling teeth to get a catalog from them, and the catalog has pages of choices of screws. The black oxide finished ones tend to be rather painfully hard.

    But no photo of either the hole or the stripped thread, both of which would have been more useful in diagnosing the problem
    I apologize if I've gotten too basic in what I sad, but your wording of your question, and what you did not say suggested that you needed this level of detail.
BTW    If you don't yet have the tap, try to avoid getting taps from Home     Depot as well. Those tend to be high carbon steel, and you will     be better off with HSS (High Speed Steel) taps -- they are     tougher and a bit more difficult to break. Your McMaster Carr     should also have excellent quality taps, and for this style of     project (not a blind hole) I would suggest that you get a "gun     tap" (spiral pointed, and it chases the chips ahead of it, so     you don't have to back up the tap ever half turn or less to     avoid clogging it with chips and breaking it.
    Also -- if you don't have one, get a good tap wrench of     appropriate size from your vendor (*not* Home Depot) at the same     time.
    Good Luck,         DoN.
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I want to respond to your post in detail & with a different photo but since I need to get to bed right now I will do so tomorrow night. Thanks for all of the detail! It's what I need at this stage in my education !
I will answer this more completely sometime tomorrow night ( EST time. )
Thanks again - JCD
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:-)
Good idea. But to make sure I am not getting the same kind of metal, what could I look or ask for ?

8-32. That is based strictly on the tap I used. I don't have a screw thread gauge ( what do you call it ? ) right now, so I just place the screw against the tap until I get one that lines up with the threads perfectly, then I make a test run on something and see how the screw works. I do have a plastic "catch-all" gauge from Home Depot and the screw fits in the #8 hole.

The only markings I can see on the drill bit are 25 ST HS one under another just like I typed it above.
(You *did* use a tap to make the threads in the

Yep - I used a tap. I'm new but now that new! Pretty close, though! I think it is the size of the hole and also that the end of the screw is tapered too much (before I even use i) for the depth of the hole that has the threads I cut in it.

I'll give a thorough look over the next couple of days and post back here to see if I am on the right track.

I just posted a new photo in this thread, although I'm not sure how much it will help. I *had* some perfect ones and accidentally deleted them from the camera. It took me forever to get just the one I posted today so I went with it.

You got it all perfect as far as I am concerned! Thanks!
I have a cheapo tap from Sears, and a few from a friend of mine that used to do this stuff in his younger days.
Thanks again for all of the help! Hope what I have written here helps you to help me !
Thanks! JCD
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    Well ... assuming that your screw was a 1" long one, and looking in the MSC catalog for the 8-32 size which you say you are using (and I would suggest 10-32 anyway for that), the prices for a box of 100 range from $5.64 for the "import" (which is still pretty good from MSC) up to $16.87 for the ones made by "Unbrako" (which are really tough steel).
    I see that they have the same screws in 25 screw packages ("Made in USA", but with no brand) for only $4.13
    The 10-32 screws in the same length range from $7.15 (for the import screws) to $17.17 for the Unbrako. (And with a similar 50-screw pack of unbranded "Made in USA" screws at $10.57.)
    Any of these would be much better than the ones which you picked up from the local Home Depot. *Some* local hardware stores will stock good screws, but it is hard to predict which will and which won't. I would not trust anything from Home Depot if you need strength.
    I don't have anything resembling a recent McMaster Carr catalog, so I can't get the prices for you from there.
    Anyway -- the description in the MSC catalog is "Alloy Allen head cap screws".

    There are thread pitch gauges (many folding blades with a sawtooth edge to match the thread pitch for checking), and there are plates into which the screws can be fitted for checking the size and pitch. The latter is sold under the name "Screw Checkr" (or perhaps "Screw Checker"), and I got mine from MSC -- they show up frequently in their sales flyers once you get on their mailing list. I have and use both.

    O.K. It is a #25, and the "HS" means "high speed steel". The "ST" might be a maker's mark.
    Anyway, looking up the recommended tap drill for an 8-32, the size should be a #29. (Except that for some reason, the table in Machinery's Handbook gives the same size for 8-32 and for 8-36, but another table gives differing sizes, both of which are close to the #29 drill bit.
    And the table does say that I correctly remembered a #21 for tapping a 10-32 thread. (Note that the larger the number, the smaller the drill bit in the wire (number) sizes.
    So -- you used too large a drill bit for drilling the hole for tapping. You now have little choice other than going up to the next screw size (10-32). And you'll need a set of Allen wrenches (also called "hex keys" for driving the screws which I suggested.
    Normally, I don't bother with the handbook, because the HUOT drill indexes in which my drill bits are stored have the sizes for common threads on the hinged metal base which holds the first row of drill bits.

    It has a tapered point? That almost sounds like a supposed "self-tapping" screw, which is designed for use in mild sheet metal, not in tapped holes.

    Try the MSC catalog on their web site. You have above the description to look for, and their URL is (IIRC)
        http://www.mscdirect.com/

    Glad to help.

    The ones from your friend may be better quality taps -- depending. I would not expect a good one from Sears, either.

    I think so. What I have not found is your newly-posted photo (though the URL might be in some other branch of this thread). I could not find it by working uphill from the previous URL, as indexing of the directories is turned off.
    Good Luck,         DoN.
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Man - thanks for the absolute WEALTH of information !
Here's the URL:
http://www.waycoolgear.com/ebay/screw_in_bushing.jpg
Anyway, I am copying your post to my notes and will be referring to it frequently, I am sure! Thanks! JCD
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On Wed, 1 Mar 2006 01:02:42 -0500, "pogo"

One problem you will have, doing what it appears you are, is that the threads on the screw will be damaged by the part of the hole that is drilled into the shaft. In fact, any screw of this type as shown will be damaged as soon as it gets tightened, and will become difficult/impossible to remove/loosen. A proper dog-point set screw would help a lot.
If you can drill into the shaft as you say, and use it the way the photo looks, why not just drill right through and then use a nut and bolt. Doing that allows a number of advantages, including a "larger" diameter bolt or at least stronger for the same size hole, and no need to tap through, and allows use of a bolt with a shank of unthreaded length long enough to go all the way through and no threads touch anywhere. For instance, if the diameter of the OD of the wheel hub shown is one-and-a-half inches, look for a bolt of about 2-1/2" long, that only has 1 inch of the end threaded.
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"Brian Lawson" wrote: (clip) why not just drill right through and then use a nut and bolt. (clip) ^^^^^^^^^^^^^^^^ Or drill right through and use a roll pin. Or drill and ream and use a taper pin.
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Thanks! I am considering that, too.
What does "dog point" mean ?
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Thanks to everyone kind enough to help me with this! Here is a (hopefully)better photo of the screw and also a somewhat blurry one showing the penetration of the screw into the bushing where it stops at the motor shaft flat. I hand tightened it on the motor shaft and slid it off to show this.
Here's the latest link:
http://www.waycoolgear.com/ebay/screw_in_bushing.jpg
When I screw it in more (with the motor shaft removed), it seems to bite in and hold just fine without stripping at all.
I'll try to get even better photos if you guys think it will help more. Just let me know exactly what you need to be of assistance.
Thanks !
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The very tops of the screw threads are knocked off cleanly. Either the screw is undersized near the tip, the screw is REALLY soft, or your hole was drilled oversized.
I'd try a REAL setscrew. These are black phospate covered, use an allen wrench to tighten, and have a suitable point to dig in. The point is sometimes called a "dog point", come in serveral flavors: you may find a sharp point, a cup, or a star shaped arrangment that looks like the character *
Any big box home store should have a display with small parts. Ask for a 'set screw'. Terrribly expensive if you need lots, easy to buy just one for $.39 Buy a longer one so it has some meat sticking out.
My guess is that the set screw will hold in the bushing even though the screw did not. But use the proper siced drill for your next experiment.
pogo wrote:

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It was the hole size! I drilled a smaller hole with a bit marked 27 and even using the stripped screw I couldn't budge it once I torqued it down to the motor shaft.
Thanks again ! JCD
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