FAQ
TL;DR: This DIY tapping arm uses a 400 kg magnetic lifter and two 120 N supports to carry a screwdriver-based head; one expert warning says linear bearings are "not suitable" when the arm also rotates on the shaft. It helps hobbyists build a low-cost tapping arm and avoid the main weak points: bearing play, poor support choice, and base limitations on non-magnetic work. [#21486567]
Why it matters: The thread shows how to build a practical tapping arm from cheap parts, then immediately stress-tests the design with real criticism about durability, balance, and where a magnetic base stops being useful.
| Option |
Holding method |
Best use |
Main advantage |
Main limit |
| Magnetic lifter base |
400 kg magnet on steel |
Fast setup on ferromagnetic parts |
Compact and low cost at 330 PLN |
Not practical on non-magnetic large sheet |
| Gravity + counterweight base |
Heavy steel blocks and lever balance |
Drilling and tapping large non-magnetic sheet |
Works on unlimited sheet size |
Bulkier and less portable |
Key insight: The arm concept works, but the decisive engineering choice is not the screwdriver. It is whether the guide and support system can handle both feed and rotational side loads without developing play.
Quick Facts
- The base was a 400 kg magnetic lifter bought for 330 PLN, which set the project’s low-cost foundation. [#21486240]
- The sliding assembly used an 80x80 profile, a welded 30 mm shaft, and linear bearings mounted on both sides. [#21486240]
- The arm itself used two 30x30 profiles supported by two 120 N actuators, showing that force balancing was built in from the start. [#21486240]
- The screwdriver holder was made from 15 mm sheet metal, and the first reported test reached M14 tapping. [#21486240]
- A gravity-based alternative used two heavy steel blocks and a counterweight lever, allowing drilling and tapping in non-magnetic sheet metal of unlimited dimensions. [#21487169]
How was the threading arm built using a 400 kg magnetic lifter, 80x80 profile, linear bearings, and a screwdriver handle?
It was built around a 400 kg magnetic lifter used as the base. A flat bar with a welded 30 mm shaft was bolted on top, then an 80x80 profile with linear bearings on both sides was fitted to slide on that shaft. The arm used two 30x30 profiles, supported by two 120 N actuators, and ended in a screwdriver holder cut from 15 mm sheet metal. The builder also made a tapping head for shafts and tested it up to M14.
[#21486240]
What is a koromyslo in the context of a drill or tapping machine, and how does it relate to a counterweight lever?
A koromyslo here means a balancing lever, not a drilling spindle part.
"Koromyslo" is a counterweight lever that transfers and balances load between machine elements, a key characteristic being that it reduces effective head weight during positioning and feed. In the thread, it described a double-sided, unequal-arm lever that shifted the weight of the linear bearing, crossbeam, and spindle onto the main column. That let the operator control drilling pressure more precisely.
[#21488478]
What is the white liquid shown at the end of the build video, and how is coolant used during tapping?
The white liquid was coolant. It is used during tapping to lubricate the cutting action and help the tap run through the hole more smoothly. In this thread, the builder identified the white liquid directly and did not describe a flood system or pump. The context shows it as a simple, practical aid used during the build’s tapping demonstration.
[#21502593]
Why are linear bearings considered a weak point in a tapping arm that also rotates on the shaft instead of only moving linearly?
They are a weak point because the arm imposes rotation and side loading, not only straight feed. One commenter said smaller linear bearings failed easily when forces acted differently from the maker’s intended direction, and they developed noticeable play. That matters here because the arm slides on a shaft but also twists during positioning. A bearing designed mainly for linear travel can loosen faster under that mixed load pattern.
[#21486567]
Which replacement works better than standard linear bearings in this kind of arm: 30x45 bushings in the housings or SK30/SK35 shaft supports with plain bushings?
Both replacements were suggested as better long-term options than standard linear bearings. One approach keeps the existing housings, removes the bearings, and fits 30x45 bushings, which also allows later clearance adjustment. The second approach replaces the bearing units with four SK30 or SK35 shaft supports plus 30x35 plain bushings. The thread does not rank one as universally superior, but both options target the same problem: play under non-ideal loading.
[#21487169]
How do you balance the forces in a tapping arm so the head can be positioned easily and thin drills can still be used safely?
You balance the masses with a counterweight lever so the column carries most of the head load. In the example shown, a double-sided lever transferred the weight of the linear bearing, crossbeam, and spindle onto the main column. The author said this balancing was necessary; without it, thin drills would be hard to use and tapping would also suffer. The practical result is easier head positioning with less uncontrolled downward force.
[#21488478]
What could cause an M14 tap to produce a thread that is not very deep even when a 12 mm tapping drill was used?
The thread itself reports that the M14 tap went in without trouble, yet the cut was still shallow. The likely cause identified in the discussion is not the 12 mm drill size, but the machine setup: the arm uses linear bearings in a job that also twists the guide shaft, which can introduce play and reduce stable axial feed. Any lost rigidity at the head can limit how positively the tap follows and deepens the thread.
[#21486567]
How do gas struts or 120 N actuators help support the arm, and how do you choose the right force for them?
They support the arm by offsetting part of its weight, so the head feels lighter and easier to position. In this build, two 120 N actuators were installed on the two 30x30 arm members, showing that balance was designed into the mechanism rather than added later. The thread does not give a sizing formula, so the practical rule shown here is simple: choose enough support force to balance the head and arm without making the motion fight the operator.
[#21486240]
What are the pros and cons of using a magnetic lifter base versus a gravity-and-counterweight base for drilling and tapping machines?
A magnetic base is compact, quick to place, and cheap, but it only helps on magnetic work. The build used a 400 kg magnetic lifter costing 330 PLN, which makes it attractive for steel jobs. The gravity-and-counterweight alternative uses heavy steel blocks and a balancing lever, so it can drill and tap non-magnetic sheet metal of unlimited dimensions. That broader capability costs portability and simplicity.
[#21487169]
How can you adapt a cordless screwdriver into a tapping head for shafts or blind holes without damaging the tap?
Use the screwdriver as a low-force drive with a holder and adjustable clutch. 1. Mount the screwdriver in a rigid bracket, such as the 15 mm sheet-metal holder shown. 2. Fix that holder to the arm or crossbeam so the tool stays aligned. 3. Set the clutch low enough for the tap and use it especially for blind or deep holes. One user explicitly preferred a screwdriver here because the clutch limited overload during hobby work.
[#21489205]
What is a threading head, and how does a reversing tapping head with clutch or planetary gearbox work?
A threading head is a tapping attachment that helps feed, torque, and reversal during thread cutting. In the thread, it was described as using a planetary gearbox and a clutch-like mechanism, similar in behavior to an old bicycle rear hub. Under higher load, rpm drops while torque rises, and lifting the head changes the rotation direction. That gives controlled tapping without requiring a full production spindle system.
[#21489002]
Why does reversing the spindle make some tapping heads retract automatically, and what mechanism controls the feed?
Reversing retracts the tap because the head couples rotation direction to feed direction. One user described an older column tapping machine where clockwise motion brought the tap quickly to the hole, then the stroke adjusted itself to any required depth without automation. When rotation was reversed, the tap backed out automatically. In that account, the feed was self-adjusting through the head’s internal mechanism rather than a separate powered feed control.
[#21488823]
What safety issues should be considered when building a heavy DIY tapping arm with a magnetic base and long lever arm?
The main safety issues are overload, unintended movement, and wear that turns into play. This design starts with a 400 kg magnetic base and a long arm, so any shift at the base or guide multiplies at the head. Commenters specifically warned that linear bearings can fail and loosen when loaded outside their intended direction. A second limit is material choice: a magnetic base does not solve clamping on non-magnetic sheet.
[#21486567]
Where do linear bearing housings develop play in this kind of DIY machine, and how can clearance be adjusted or reduced over time?
Play develops in the linear bearing units when they are forced to handle twisting and side loads. In the thread, one fix was to remove the bearings from the housings and fit 30x45 bushings instead. The same commenter added that these housings can be cut along the axle line, which allows the clearance to be adjusted as wear appears. That makes the housing reusable instead of disposable.
[#21487169]
What should I use for tapping non-magnetic sheet metal of large size if a magnetic lifter base is not practical?
Use a gravity-based stand with counterweight balancing instead of a magnetic base. The example shown used two heavy steel blocks to press the structure against the ground and a balancing lever to control head pressure. That arrangement allowed drilling and tapping in non-magnetic sheet metal of unlimited dimensions. It is a better fit when the workpiece cannot hold a magnet or is too large for magnetic positioning to be practical.
[#21487169]
Generated by the language model.
Comments
Nice video of the build. I have a feeling that this arm would even manage this screwdriver together with the operator. [Read more]
I generally like the design, only these linear bearings, as the name suggests are not suitable for the kind of work you are doing. Rotation instead of feed along the shaft. I've used smaller ones and they... [Read more]
Beautiful construction. And you can see the super "balanced" forces needed to position the "head" in the right place. . With his wife and children on his lap. :D Robust construction. . And this... [Read more]
Elegant design. In case the unit has to work hard, you can possibly remove the bearings from the housings and insert 30x45 bushings there as pan. If you don't have time to do the work, you can buy some... [Read more]
I don't know what you're getting at with this corom... To quote my own statement from about a year ago. I know two meanings of the word and neither has anything to do with drills. 1. koromyslo - a carrier,... [Read more]
. I see a similarity here - the counterweight I used to fully control the pressure of the drill. You can't see it in the photo, but there is a double-sided lever (double-armed), the weight of the linear... [Read more]
Years ago I got a "column tapping machine" from a friend, unfortunately it later went to scrap, but in general I remember the construction, the head (something like an ordinary chuck for a column drill)... [Read more]
I think such devices are still produced as threading heads, as well as for fine drilling. Maybe I can find cross-section drawings of the construction. There's a satellite gearbox and something like a... [Read more]
For threading blind holes and deep holes I make my apologies with a screwdriver She 'buzzes' a bit but has an adjustable clutch so I attach her to the Koromysl crossbeam. I don't have to listen to music... [Read more]
Congratulations, you are in second place in DIY constructions: https://www.elektroda.pl/rtvforum/ranking.php And probably immediately in first place. Respect. If you allow me, I will send you a small... [Read more]
Forgive the layman's question - what is the measure given at the end of the film? [Read more]
. The white one? That's the coolant. [Read more]