Chuck slips on taper

[ki7fq]

Hi - I have a Homier 03947 Mini-Mill that uses R8 holders. The chuck has a JT33 taper, I think. The chuck will sometimes slip or fall off completely. There appears to be some minor scoring on the JT33 taper, too. Any thoughts?

Thanks for your input.

Mike / ki7fq

 

[jumps4]

are you using the chuck for endmills?
the vibration from large endmill can jar the taper loose
or it may have been a poor fit to begin with some of the chinese tapers are off a bit or be too smooth. my new lathes tailstock and mt3 drill chuck would not hold i had to rough up the finish with steel wool to make it grip tight enough for big bits.
steve

 

[ki7fq]

are you using the chuck for endmills?
the vibration from large endmill can jar the taper loose
or it may have been a poor fit to begin with some of the chinese tapers are off a bit or be too smooth. my new lathes tailstock and mt3 drill chuck would not hold i had to rough up the finish with steel wool to make it grip tight enough for big bits.
steve

Steel wools an idea. Beats my thought of using blue Locktite... When I first got the machine, I tried a mill in the chuck. Bad idea, eh? I now have end mill holders - lots better. There was a lot of oil on the chuck to taper joint, too. How much is enough? I have wiped away all that I can without using solvent.

Thanks for the input!

 

[Tony Wells]

Get some lapping compound and use it with a twisting motion back and forth between the tapers. They should be very smooth, actually. Clean them thoroughly, then put the arbor in the freezer for a couple of hours and the chuck in the oven @250°f for an hour or so. Take them both out and jam them together and give them a sharp rap on the butt end of the arbor. Once the temperature equalizes, it will be very tight. The lapping is critical to mate the two tapers. Any scoring will not be a problem provided it is not "proud" from the taper surface, it's just a groove at that point and won't interfere with the fit.

 

[Alan Douglas]

Thanks, I have the same problem with a new (old) drill press I recently bought. The former owner installed a nice new chuck and all was well until it fell off while I was drilling a hole.

 

[WobblyHand]

Get some lapping compound and use it with a twisting motion back and forth between the tapers. They should be very smooth, actually. Clean them thoroughly, then put the arbor in the freezer for a couple of hours and the chuck in the oven @250°f for an hour or so. Take them both out and jam them together and give them a sharp rap on the butt end of the arbor. Once the temperature equalizes, it will be very tight. The lapping is critical to mate the two tapers. Any scoring will not be a problem provided it is not "proud" from the taper surface, it's just a groove at that point and won't interfere with the fit.

My apologies for the necro-post. Similar problem, so I hope it makes sense to continue the thread.

I have an issue with a poor fit up between an arbor and a drill chuck for my lathe. If one uses Prussian blue to check for coverage, what is the expectation for area fill of the taper? Are we talking >40-50%? A lot more? Obviously the more the contact area, the better the grip. I only seem to have contact on a narrow 1/8" ring around the base of the JT33 female taper and kind of a blotch on one side. Total contact area is under 10%. The chuck loosens on the arbor when I use it. The drill then either wanders or is no longer centered. Is there a way to fix this? Will lapping change the concentricity of the chuck?

 

[mikey]

Jacobs tapers are usually precision-ground and unless the male and female tapers are truly buggered up, they will usually fit if installed properly. Remember that this is an interference fit so when the two surfaces contact firmly they will stick BUT those surfaces need to be clean and have nothing sticking up above the surface.

Try this. Put a light coat of oil on each of the mating tapers and insert the male into the female gently. Then turn the male part at least one or more revolutions. Take the male out and look for a bright ring on both. If there is a burr or defect, it will create a bright reflective line. If the line is on the female side then the burr or defect is on the male side, and vice versa. If there is a burr, use a diamond stone/file and remove only that defect. Do not touch the rest of the taper. Then clean all oil out of both tapers with lacquer thinner and make sure they are really clean. From that point on, do not touch those tapers with your fingers.

Now, insert the male taper into the female taper with a gentle twisting motion until it seats. Be sure the jaws of the chuck are retracted, then place the nose of the chuck on a solid surface. Concrete or an anvil works well but put a piece of paper under the chuck to avoid marring the tool. Now take a hammer and give the end of the taper a firm hit straight down, in line with the arbor. This will seat the arbor in the chuck and it will not come off until you take it off. Do not hit the arbor hard with your hammer; it only takes a firm tap.

Do not use a freezer and oven, Loc Tite or a press to assemble a chuck and arbor. Do it right and it will run true. One day, if you have to remove that arbor, it will come off.

I'm attaching something to read.

 

[WobblyHand]

Thanks Mikey. To be honest, I don't think the chuck side (internal) taper is machined correctly. It's a Chinese chuck that came with my mini-lathe. That's why I was asking about % coverage. I used a thin layer of prussian blue, aka dykem in grease on the arbor to check for coverage of the chuck taper. As I said, there's only an 1/8" contact ring at the bottom and a thin erratic blob along one side when I do the twist thing. The blue makes it real easy to see if anything transferred. It's clear that there is 0 contact for > 90% of the internal taper in the chuck. It doesn't look like a burr, it looks more like the taper is not JT33. I know it sure doesn't match the arbor that well. And the chuck either wobbles or falls off under load. It might be ok for one operation, but a second operation breaks the contact quite easily. It makes most drilling total frustration.

I'll try a thorough cleaning, and a burr check, especially near the back. There seems to be a dark ring back there - can't tell if its just old grease or hidden damage. The arbor seems very clean. If you reflect light off of it, there doesn't seem to be any noticeable defects, it appears uniform.

 

[mikey]

Well then, it might be time to buy a decent chuck.

 

[WobblyHand]

Can't disagree with you about a decent chuck. And thanks for the attachment.

I do think a respectable supplier has the implicit responsibility that the parts supplied, chuck and arbor actually fit each other, especially if they were sold as a set. I just don't know how to provide the correct machinist terminology that says, "these don't match, and they should". To be fair, I'm working with them, but as of the moment, it's my responsibility to show that I mounted it right. I have to say, I've had to reinstall this chuck a dozen times already, because it's always loosening.

However, in the case I might be totally wrong, and my method doesn't work, I will do my due diligence and clean and fit the chuck to spec.

If they don't really fit, what is the established method to prove the degree of fit? I thought a metric might be contact area. If there is high contact area one could state the parts are matched to each other? If there was 100% contact (meaning uniform contact) that would establish best fit. (It would be a bear to separate...) 0% contact would mean there is no friction to hold anything together. Somewhere between 0 (useless) and 100 (impossible to remove) is the right answer. Is there a better or more accepted metric?