Drill Holders

[daniel banquo merrick]

The belts on the drill holders are very slimming and stylish but they keep the foam from sliding and also lock down the springs so you can't pull out the drill bit.

I've had a 1/4" bit bend before and it's super frustrating. there is no way to straighten one. I can't recall for sure if it was A-Taper or SDS but I think A-Taper.

I measured the force required to press fit the striking surface in my drill holders and got as much as 10,000 lbs. The striking pins can be knocked in with a hammer so I think we can generate a force that large when drilling. In a 1/4" bit with flutes that probably equates to something on the order of 300 ksi. This would mean you need a Rockwell C of something more than 60. Delicate balance between getting the steel strong enough and not too brittle.

What brand was it? I'd call them and send it back. I'd be curious to hear what they say. I've been contacting drill manufacturers about making a batch of short 3/8" SDS bits. The only conclusion I've come to is that all US manufacturers of percussion carbide bits are complete idiots. Like I say, I'd be curious to hear what they say about bent bits but keep your expectations low.

[daniel banquo merrick]

I ran a very simple impact FEM model to show how the impulse bounces back from the shoulder of the bit. In the animated GIF, compression is blue and tension is red. An impulse is applied to the left end and the right end is fixed. In the model, the cross section area of the small part is 1/2 the large part. The difference is larger in 1/4" SDS pits so the effect will be more pronounced.

http://engr.sjsu.edu/dmerrick/misc/hammer.gif

[John]

Nice model!

So it looks like a taper on the bit end of the holder would be beneficial too, correct?

I got like 20 hits out of the bent bit, so you bet your ass I am sending it back. I am disappointed though, I got it from McMaster Carr which I order from almost daily so I would have liked to order the bits from them again. Almost everything they have is really great quality with a few clunkers once in a very rare while. They only have one type in A taper and it sucked so that is a dead end for me.

I want to make my own bits for the A taper holder. The carbide tipped types are slightly oversized so splitshafts go in way too easily for my comfort. The ones I make are truly .250". I am thinking a John taper to A taper adapter will work fine.

[daniel banquo merrick]

So it looks like a taper on the bit end of the holder would be beneficial too, correct?

I think so. I really don't know what happens at the taper interface. If the taper fit is good, it may nearly act as continuous steel. If the fit is less than perfect, who knows what happens. The drift pin hole is also a problem but I haven't run any tests to see how much of problem.

The 3/8" A-Taper drilled faster than the 3/8" SDS but the drills were not the same length so I can't say A-Taper is better than SDS yet. The next test I plan to do is to compare 1/4" SDS and A-Taper with bits that are roughly the same length.

Bob wants to try a one off A-Taper holder without a drift pin hole and with a tapered snout. This is pretty easy to do because I wouldn't brother to heat treat the steel. Even if the taper fits well, there will be a gap at the end of the bit so it won't be perfect.

The perfect bit would have no change in diameter, no flutes and no holder.

[John]

Dan-
BTW, on the drill I have, the bit sunk too deep into the drift pin hole and i had to heat up the drill to get the bent bit out. The tapered hole needs to be shallower or perhaps the hex stock thicker.

All
We are having a shop night on Thursday at my shop in Concord if anyone is interested in coming out and making drills and bits and hangers and drinking beer. Contact me if interested.

[YETI]

I just purchased 2 ea of 3/8 1/4" a taper Carbide tipped from ICS.

Dan, I am going to try an mod 1 ea of these the way you did. 

Maybe get some better rock crunching going on.

The 1/4" has a DD of 1.75" or something, just like the rawl's.



"The perfect bit would have no change in diameter, no flutes and no holder"

Hard to beat the chisel tips we used in the rawl system, 2 out of 3 aint bad.

[daniel banquo merrick]

After having a sandwich for lunch, I went down to the garage and tried some 1/4" bits. Comparison is a 1/4" SDS (DeWalt, Germany)  with a 1/4" A-Taper (Relton, USA). First I sharpened both and I used the BD hammer for pounding.



They drilled at the same rate so I can't say one drills better than the other.Somewhere around 100 hits, the A-Taper bit started to bend. The bent bit actually still drilled as fast (The impulse followed the bend?) but after awhile it started to spall the top of the hole.

After the test, I hit the A-Taper a number of times about as hard as I could to see if it would bottom out. It didn't and it came out easy enough. I think the holder I sent to John was a prototype that had the taper cut with a boring bar and it was hard to get it just right. The new ones where I cut the taper with a reamer may be more accurate. I should send a replacement.

My SDS holder seems to work well. Perhaps because the striking surface is very close to the end of the bit which may keep things focused.

[YETI]

What version did I get?

I will just stick with 3/8ths if this will be an issue with the drill you sent me?  If not, I want to try the 1/4"

Thanks for the cool write-ups, and I will get back to you soon on this Dan.

[mungeclimber]

NO QUARTERS FOR YOU YETI!!!

Man up


LOL

Sincerely,
QuarterLength Munge

[daniel banquo merrick]

Yeti-

If I recall, you got an SDS holder not an A-Taper holder. A-Taper bits won't work in it if so. If you bought A-Tapers and need SDS, I'd be happy to trade you some. I'd like to try a different brand of 1/4" A-Taper bit.

[YETI]

Hey Dan-

You sent me the A taper, and it seems that the bits only go in 1/2 way, so maybe it is different depth than johns?

I just received my 1/4" and 3/8" in the mail, and on the way to SHOP NIGHT WOOOHOOOO!!!

Machines and brews, fingers will be lost..

[daniel banquo merrick]

I'd love to stop by this evening but Concord is soooo far away and I have a job in Half Moon Bay tomorrow early.

[John]

Too bad you can't make it. Hopefully we will end up with something we will want to share here later, other than missing fingers.

[mungeclimber]

planning on shop, but it's a long way. Don't drink all of them.

[susan]

Fun at shop night... and no finger loss!