Return spring tension for pillar drill

[aneevuser]

I'm making a pillar drill jig for my hand drill, and I want to add a return spring. However, I've not used a pillar drill for years, and am not at all sure what would be a sensible return tension to aim for.

The drill+carrying attachment weighs about 30 N, so I guess that I want an initial tension somewhat more than that; I'm not sure what is a reasonable tension at the end of travel though, bearing in mind that the user will have to maintain that while using the drill.

Anyone got any experience here?

[DaJMasta]

The user only has to maintain the return string force minus the weight of the head, so don't worry too much about it.  If you can keep the mechanism to lower it really low friction, even a tiny bit more than the weight would technically suffice.  While I don't have any numbers for you, I suspect the tolerance will be pretty generous to feel decent.

[aneevuser]

The user only has to maintain the return string force minus the weight of the head, so don't worry too much about it.  If you can keep the mechanism to lower it really low friction, even a tiny bit more than the weight would technically suffice.  While I don't have any numbers for you, I suspect the tolerance will be pretty generous to feel decent.

Having thought about this, I think you're probably right - I have a mass of about 3 kg at the end of the spring, and even if the nett upward force at end of travel (about 0.4 m max) is 3 N, then that gives an upward accln of 1 m/s^2 which would return the drill to the top in (2 x 0.4/1)^0.5 s = 0.89 s which seems more-or-less sensible.

I probably just need to buy a few springs of approximately the right length and k, and play with them a bit.

Thanks.

[KL27x]

It depends how much leverage or gear-reduction your handle is going to give. Most drill presses have a wheel or 3-4 handles that spin round and give a really high leverage, so the spring force can be pretty high and it won't be significant amount of your pressing force.

then that gives an upward accln of 1 m/s^2 which would return the drill to the top in (2 x 0.4/1)^0.5 s = 0.89 s which seems more-or-less sensible.

The spring is so strong in my drill press, I can't even bring myself to do that from the lowest position. But letting it go from half an inch results in a hard-ass acceleration ending with a solid smack!

In case it's helpful, one full revolution on the wheel would move the pillar ~2.2" on my press. The wheel has a radius of about 5.3" So I'm coming up with about 15:1 leverage or gear reduction. If you're just using a single lever that goes up down with a simple linkage, you'd be getting into the neighborhood of maybe 3-4:1?

[langwadt]

I'm making a pillar drill jig for my hand drill, and I want to add a return spring. However, I've not used a pillar drill for years, and am not at all sure what would be a sensible return tension to aim for.

The drill+carrying attachment weighs about 30 N, so I guess that I want an initial tension somewhat more than that; I'm not sure what is a reasonable tension at the end of travel though, bearing in mind that the user will have to maintain that while using the drill.

Anyone got any experience here?

counter weight? https://youtu.be/ZQblQWYQOq8

[aneevuser]

It depends how much leverage or gear-reduction your handle is going to give. Most drill presses have a wheel or 3-4 handles that spin round and give a really high leverage, so the spring force can be pretty high and it won't be significant amount of your pressing force.

The first version in fact will have no lever or gear - just me pushing on the drill handle - I don't want to add anything more complex until I've got a better feel for the forces involved. So whatever force is provided by the springs will be overcome by my fair hands alone.

The spring is so strong in my drill press, I can't even bring myself to do that from the lowest position. But letting it go from half an inch results in a hard-ass acceleration ending with a solid smack!

In case it's helpful, one full revolution on the wheel would move the pillar ~2.2" on my press. The wheel has a radius of about 5.3" So I'm coming up with about 15:1 leverage or gear reduction. If you're just using a single lever that goes up down with a simple linkage, you'd be getting into the neighborhood of maybe 3-4:1?

Thanks. This is very useful and exactly the kind of info that I was looking for. I'm pretty surprised that the springs are that strong though - seems way more than I recall from using one in the dim and distant past.

Yes, in the second revision, I'll probably using a lever mechanism with limited scope for high mechanical advantage - I don't want to go for a wheel as then I'll probably need some kind of rack and pinion system and the cost will go way up (can't find a cheap one in the UK at least) -  also I think this may be more difficult to retrofit. At the moment, I think I've got a very mechanically solid design for peanuts, and I and my wife are keen to keep it that way.

Also, now I think of it, the high gearing that you mention would seem to be a bit much for a hand drill, maybe? I'm wondering if my trusty Black and Decker would be a bit too feeble for the kinds of forces that the user could generate?

[aneevuser]

counter weight? https://youtu.be/ZQblQWYQOq8

Hmm. Pretty interesting - that would give a constant return force of course, and be easyish to adjust - certainly something to think about. Might need a buffer to stop it bashing hard at the end though.

Thanks.

[Jay_Diddy_B]

Hi,

Google 'constant force springs'. These apply a constant force independent of the extension.

This may be useful in this application.

Jay_Diddy_B

[KL27x]

Leverage: It's not so much the peak force you can apply (which would be limited by some fraction of your body weight, depending on the position). I'm sure I'm strong enough to get the peak force to drill anything I need. With larger diameter bits or hole saws or drilling in steel, it's more that you can do it with 2 fingers rather than leaning on it with some real weight. Instead of the weight of my arm, held onto the wheel by two fingers, you're using shoulders and core to apply a significant amount of your body weight into the drill. And that hole might take a few minutes to make.

Even with a modern 12V cordless drill, you will probably run out of muscle before the drill does, in plenty of common use cases. You'll be able to stall it with your peak force. But the drill can keep up with most of what you can actually maintain in a practical manner. My opinion and things I find my drill press useful for!

[jpanhalt]

Having thought about this, I think you're probably right - I have a mass of about 3 kg at the end of the spring, and even if the nett upward force at end of travel (about 0.4 m max) is 3 N, then that gives an upward accln of 1 m/s^2 which would return the drill to the top in (2 x 0.4/1)^0.5 s = 0.89 s which seems more-or-less sensible.

I probably just need to buy a few springs of approximately the right length and k, and play with them a bit.

Thanks.

40 cm of travel?  That's a huge amount for a hobby drill press in my experience.  Precision drill presses for home (not industrial) will typically be much less.  Of course travel and the adjustable height on the column are different. 

Aside from that consideration, I agree with have very little net return force.  That will enhance your feel for spotting and starting holes.  In fact, exact balance would not be a problem for me.  I have a drag adjust on my drill press that lets me lower it, and it stays.  A drill press that came down on its own weight would be unacceptable.  You need to consider the weight of the heaviest drill/cutter the drill press will need to handle.

[aneevuser]

Hi,

Google 'constant force springs'. These apply a constant force independent of the extension.

This may be useful in this application.

Jay_Diddy_B

Thanks, however I don't think that a constant return force will be particularly desirable here - I think I'd be better off with initial acceleration then deceleration, contrived somehow.

[aneevuser]

...drilling in steel, it's more that you can do it with 2 fingers rather than leaning on it with some real weight.

Yes, good point that I shall bear in mind.

Even with a modern 12V cordless drill, you will probably run out of muscle before the drill does, in plenty of common use cases. You'll be able to stall it with your peak force. But the drill can keep up with most of what you can actually maintain in a practical manner. My opinion and things I find my drill press useful for!

You may well be right - I don't have a great feel for the forces involved, so I aim to be cautious at first - there will almost certainly be a Rev. 2 (not least as I've discovered that I badly need a pillar drill to make a pillar drill, so I intend to bootstrap) and I'll worry about the lever or wheel then.

[aneevuser]

40 cm of travel?  That's a huge amount for a hobby drill press in my experience.  Precision drill presses for home (not industrial) will typically be much less.  Of course travel and the adjustable height on the column are different.

 
Yes, that's the max spindle travel that I can currently build in. Will probably be less than that (but adjustable).

Aside from that consideration, I agree with have very little net return force.  That will enhance your feel for spotting and starting holes.  In fact, exact balance would not be a problem for me.

This is a very useful observation - with exact balance, I'd feel the force at the end of the bit directly through the lever/wheel/my hand on the drill. Thanks.

[Jay_Diddy_B]

Hi,
All the drill presses that I have ever own have a spring like these:



The quill mechanism is a rack and pinion gear. The spring acts on the pinion.

Jay_Diddy_B

[aneevuser]

I've finished this device, more or less, and I ended up using these springs:

https://uk.rs-online.com/web/p/compression-springs/0121286/

which give me a return tension of about 180N at the end of travel (about 11 cm). This is acting on a drill+drill carrier mechanism of about 7 kg - plus there some friction involved too, which I can't quantify too easily. This force seems to be acceptable both from the POV of returning the drill, and of the force that I have to exert - I have a lever mechanism with MA of about 4 to 5.