View Full Version : Worm gear question from a newb.

10-29-2004, 07:07 PM
Been playing "Home machinist" for about 6 months now (Grizzly 9x20 lathe and a Mini mill are my major players)

Would like to convert my mini bench top drill press (HarborFreight on sale for 40.00) into a hand tapping fixture. My thoughts are to remove the pulley from the top of the spindle and mount a gear in it's place on the shaft. I would then actuate it with a worm gear turned by a hand crank on the side. I would remove the spring tensioned Z axis, so that the chuck would rest on the work piece without assistance (make sense?).

To my question: What ratio of worm gear would you recommend for this type of use? 4:1? And by 4:1 I assume that one turn of the hand crank would turn the spindle 1/4 turn. Also, where would you recommend I find worm gears (I am not setup yet to hobb my own.) Thank you for any assistance!


[This message has been edited by jbyrd (edited 10-29-2004).]

10-29-2004, 07:30 PM
Smoked the motor, Eh? Skip the gears. Just adapt a crank on top. I tap with mine all the time. I just grab the chuck though. It give me a better feel. Haven't broken a tap yet.

10-29-2004, 07:45 PM
Oh yeah... The motor is toast! haha. And while I know I don't need to get really complicated into turning this into a tapping device, it is more a learning experience in the use and setup of worm gears and their ratios. I suppose I could use bevel gears too but I like worms gears and would like to put them to use in this instance. What ratio do you think might work for tapping into steel etc? 4:1 - or higher?


10-29-2004, 08:54 PM
I think the leverage that you would gain in a gear down drive chain would make it pretty easy to break taps, especially in the smaller ones


10-29-2004, 09:59 PM
Ditto what Walt says.
A 4:1 worm setup is going to be hard to find. That's very course. Most will be 20:1 and above. You might be better off going with bevel gears. That will give you the right-angle action you may be looking for.

As far as gearing to tap, I don't recommend it unless you're taping VERY large holes and need the leverage. You'll be turning a tap 1/4 turn or less and reversing. That's going to be a lot of cranking, even at 4:1.

Scavenge a scrap yard for old lawn tractors. I picked up a set of NOS beveled steering gears that would probably work. The large gear even has tangs on top that would allow a bar tool to be used too. (At least I think they were steering gears. The set came with two stand-offs and wired together.)

Forrest Addy
10-29-2004, 11:25 PM
If you smoked the motor while tapping I'd guess you were jogging the motor to control the process.

I suggest you replace the fried motor with an oversize three phase or DC motor so you could slow it way down with electronic controls, have instant reverse, and have control over accel and decel. It's a bit expensive but it solves a lot of problems and will ensure you never ever fry another drill press motor again.

Forget about the worm gear and stuff. I have a 2 HP motor and VFD on my 1946 Craftsman drill press. I can use it to tap up to 5/8 UNC in mild steel plate with a spiral point tap.

10-30-2004, 12:19 AM
While getting experience with worm gears would be good, I don't think this is the application for it. I find it easier to turn the chuck by hand, and I remove the drive belt before I do any tapping. This gives me two advantages, one, I can't accidentally turn the motor on and fry a tap and maybe the work, and two, I don't have to fight the friction of the belt drive when turning the spindle by hand. There's enough inertia in the spindle and chuck to spin the tap out more quickly for chip removal, and doing it by hand allows me the 'feel' of the operation. If I was converting a drill press mostly for tapping, I would alter the return spring so the spindle could pretty much 'float' wherever you raised or lowered it to. This way when you spin the tap in, you have better control over the axial force on the tap, and when spinning it out, the return spring won't be trying to rip the top thread or two out as the tap comes close to disengaging the threads. Also, when you let go of the feed handle, the workpiece won't want to lift off the table, which could easily break the tap if the work tilted when raising off the table.
You certainly could motorize the machine for tapping, but you'd have to have a means of getting a suitable speed and some kind of torque sensing so you wouldn't be breaking taps all the time.
I should qualify my whole post by saying that there's a wide range of application within the members of this bbs. Some are hobbiests working with very small stuff, and some work with projects which require fork lifts and hoists just to handle the raw materials. You don't need 2 horse if you're tapping 4-40 holes, and you don't want to hand turn the spindle with a 3/4 inch tap in it. My gut feeling is that if you're modifying a 40$ drill press for tapping, you won't be doing any holes larger than 1/2, probably less than 3/8, and not any production runs, either. Keep the whole thing simple, and use the worm gears for more suitable applications.

10-30-2004, 05:17 AM
Thank you all for the great information. The drill press motor was actually toasted while being used in a drilling process, not taping. So I am actually removing the motor and all from the machine. I should also clarify that I would be using the press for small jobs only as the previous post aluded to. 3/8 would generally be the largest. At this point I will concentrate on finding the appropriate bevel cut gears for this project. I will even get a few, in order to play with the ratio (1:1, 2:1, etc.). Thank you all for your insight and I appreciate being able to tap your knowledge banks (pun intended)

Take care.


Ian B
10-30-2004, 11:54 AM

For a quick way of turning the drill into a tapping machine, you might consider replacing the stepped pulley with something similar to the hand capstan that the drill already has for downfeed.

Nicer to use - when you retract the tap, the balls & levers should give enough mass to spin the tap out in one or two goes, rather than winding a handle backwards.