Light duty "precision" impact screw driver?

[ChazzC]

I submitted my questions to Wera last night:

Subject

921 KRAFTFORM PLUS IMPACT SCREWDRIVER - DETAILS

Comment

Hi, I have several questions about the 921 Screwdriver: • The description states “The pound-thru blade ensures complete power transmission (zero loss).” How much of the hammer blow is transferred into rotary motion? I am concerned that when the screwdriver is “On” that there will be significant force applied down into the screw. • The impact is noted as producing “a loosening moment of up to 25 Nm.” What is the minimum torque that can be produced? I understand that lighter blows on the handle of the screwdriver will produce lower torque, but I assume that there is a lower limit due to friction within the mechanism. • What is the smallest screw that you would recommend that the 921 be used on? Thank you for your assistance, Charlie

Also asked Bosch if there was a chart of the torque vs. Clutch Setting on the PS21 (and if not, was there an estimated torque for the lowest setting).


Amazon (US) has the Wera 900 kit for US$88:

Just US$18 more than just the screwdriver, so US$1 per bit (and a free case).

I forgot to ask Wera about the warranty & broken bits (will do so in my reply to their response when I get it): does anyone know if they replace broken bits?

 

[ChazzC]

Response from Bosch:

“I want to inform you that an exact measurement of all the settings is not available. The minimum setting for each of these models is 20 inch lbs. The intermediate settings are distributed fairly evenly, but one cannot rely on a specific setting to guarantee it is accurately set.”

As the maximum torque is specified as 275 in-lbf, a 15:1 “turndown” seems to be reasonably in line with the “20+1” clutch settings.

20 in-lbf (2.26 N-m) is a little over twice the manual tightening noted by iFixIt for their 4mm electric screwdriver; since I have several duplicate sets of iFixIt bits, I may sacrifice some to the torque experiments with the PS21.

 

[PeterT]

You can get adjustable torque hex drivers, would that work? Looks like copycats are available. I remember the USA/euro ones being quite expensive & usually more limited torque range

 

[kstrauss]

Back to the original question of removing recalcitrant Philips screws:
I've had good results using an ancient Yankee screwdriver. It is somewhat less brutal than a hammer powered impact driver. I'm too lazy to take a photo but mine is just like the image below including the flaking red paint on the handle.

 

[Mcgyver]

You can get adjustable torque hex drivers, would that work? Looks like copycats are available. I remember the USA/euro ones being quite expensive & usually more limited torque range

I've got those Peter, the issue is screw removal when the torque required to remove is either more than you can muster or is head strippingly strong, The impact screw driver is the right tool imo, I think I just needed a light duty one which this thread provide

Ken I've not been inside one but suspect the action of a impact screw driver is similar to that. Some sort of helix that takes a downward force and converts a part of it into torque.

 

[Stuart Samuel]

One other consideration, depending on Philips drive size, is coated bits. I picked up a set of insulated Wera screwdrivers for work a few months ago, and the #2 has some sort of diamond coating. It's quite grippy, though I have to think it will either wear or become caked with debris and need cleaning. The #1 in that set does not, unfortunately, have the coating. I'll be they (and others) sell a driver bit that could be used in a handle, however.

The biggest issue I've had at work is removing slot screws. Trying to find driver bits that don't fail/twist before I torque the head off the screw has been a challenge (That was with 1/4-20 flat heads, in cast iron, from the north face of Union Station).

 

[ChazzC]

I've got those Peter, the issue is screw removal when the torque required to remove is either more than you can muster or is head strippingly strong, The impact screw driver is the right tool imo, I think I just needed a light duty one which this thread provide

Ken I've not been inside one but suspect the action of a impact screw driver is similar to that. Some sort of helix that takes a downward force and converts a part of it into torque.

This guy takes one of the more common types apart (starting st 4:30); not the best narration and captions are worse):

 

[thestelster]

If you watch this video from Wera it shows the operation of the internal mechanism. So it looks like the anvil? is moved through a 45° slot, so rotational forces are maybe equal to the downward forces?

Wera.TV

 

[PeterT]

I've had the pleasure of working out semi stripped cap screws. I have used my Wera mini hex ratchet with success. The nice thing is you can push down on the head to keep the bit engaged with your thumb on one hand & of course the handle length offers smooth torque as leverage on the other. The other thing is I have ground/modified a (now disposable) hex bit to better engage a mangled hex or slot or whatever. The trick is to make it as tight fitting as possible. In some cases I've Dremelled a slot across but that's not always possible like flush fastener. I've snapped the heads off some bolts up to #8 size so I know the ratchet mechanism is pretty solid.

 

[thestelster]

This is the traditional method of removing or installing slotted screws into firearms. Brace and bit! The firearm body is held in the vise so that the screw you want to remove is horizontal. The head of the brace is in held just above your abdomen/bottom of your sternum. The screwdriver bit is in the screw slot. The right hand is holding the shell/chuck. All your weight is pushing on the brace, and therefore into the screw, and you hit the handle with the palm of your left hand.

 

[ChazzC]

If you watch this video from Wera it shows the operation of the internal mechanism. So it looks like the anvil? is moved through a 45° slot, so rotational forces are maybe equal to the downward forces?

Wera.TV

I’d missed that one: similar to the $20 units, but heavier duty construction and only provides CCW rotation (as noted somewhere on their website); I’ve never quite understood the rationale of using a hammer driven tool with no indication of torque to tighten a fastener (although the hammer blow would help prevent a Phillips bit from ramping out of a screw head).

 

[Susquatch]

I am concerned that when the screwdriver is “On” that there will be significant force applied down into the screw.

Why is this a concern Charlie? I believe it is a desirable feature of the driver to do this to stop the driver bit from climbing out of the screw when torque is applied and also to compress the screw longitudinally to release some of the thread fit friction. Screws can handle a lot more longitudinal force than torsional force. In fact that is how screws are properly torqued. The dry and oiled torques are the torques required to stretch the screw by design and thereby develop enough friction in the threads and head to prevent loosening of the threaded connection under design conditions. Too little torque and the connection can come loose - too much and the fastener can be stretched beyond yield and fail. Of course, there are certain one-time use connections where the fastener is deliberately yielded to a design level for application specific purposes.

Anyway, my biggest concern is that your chosen wording might cause Wera to be cautious in their response so I'd prefer to have left that out. I'd rather hear what Wera has to say on their own volition without leading their response. We will have to keep that in mind as we weigh what they have to say.

I quite agree with what @thestelster suggests. Even if friction biases the torque to thrust ratio away from a 50/50 spilt it won't be different enough to really matter. Even if it did affect it, I definitely prefer a higher thrust ratio than torque so in my opinion, it's all good.

 

[terry_g]

My dad bought this for me when I was 14. I worked on Japanese motorcycles a lot plus two summers as a mechanic at a Honda dealer so its been heavily used. I ground the hammer end as it was starting to mushroom. I took it apart and lubed it a couple times. Still going strong at 51 years old.

 

[ChazzC]

Why is this a concern Charlie? I believe it is a desirable feature of the driver to do this to stop the driver bit from climbing out of the screw when torque is applied and also to compress the screw longitudinally to release some of the thread fit friction. Screws can handle a lot more longitudinal force than torsional force. In fact that is how screws are properly torqued. The dry and oiled torques are the torques required to stretch the screw by design and thereby develop enough friction in the threads and head to prevent loosening of the threaded connection under design conditions. Too little torque and the connection can come loose - too much and the fastener can be stretched beyond yield and fail. Of course, there are certain one-time use connections where the fastener is deliberately yielded to a design level for application specific purposes.

My concern is in keeping with @Mcgyver ‘s original question dealing with the fragility (must be a word as it didn’t get highlighted) of the plastic housing on his router. I know from numerous applications in construction that compressive strength is significantly higher than shear strength (BTW, it is difficult to ascertain shear strength of some specialty fasteners from manufacturers as typically they deal with compression or tensile).

Seeing as how his Klein “Whack-A-Mole” (similar to my Craftsman) did the job this may be moot, but I always have liked to know things (does this mean that there is some terrier in my DNA, or am I channeling Dean Spanley?): I would drive my design teams close to the brink every time they told one of my ideas wouldn’t work - an advantage of being a degreed engineer, but not an Engineer.

Anyway, my biggest concern is that your chosen wording might cause Wera to be cautious in their response so I'd prefer to have left that out. I'd rather hear what Wera has to say on their own volition without leading their response. We will have to keep that in mind as we weigh what they have to say.

We’ll have to see what they say, but I guess I could have asked this in a more open-question manner.

 

[Susquatch]

My concern is in keeping with @Mcgyver ‘s original question dealing with the fragility (must be a word as it didn’t get highlighted) of the plastic housing on his router.

Ah, I see! I can understand his concern and yours, but that's yet another reason for using a device where you can control the level of the forces applied. You don't have to swing that hammer any harder than you want to. Light blows (might be better to call them taps) should suffice.

And no worries about your curiosity. Mine is likely much worse than yours. Good thing I'm not a cat.