Standard cut

Standard cut (here conical cut) for glass appliances

Open standard ground joint

Closed standard ground joint

Closed standard joint with joint clamp

The standard cut represents the most common connection between two glass devices in chemistry . A distinction is made between sleeves and cores on the devices, whereby one core always fits into an associated sleeve. The sleeve is located, for example, on the round bottom flask , the core on the corresponding additional equipment such as reflux condenser or dropping funnel . The core devices also include glass stoppers for closing openings that are currently not in use. The connection surface between the sleeve and the core is the joint , which is greased with a highly viscous joint fat during use or sealed with joint sleeves made of PTFE or Teflon tape . A joint clamp (joint clamp) prevents the connection from diverging. Different sized sections can be connected with transition pieces.

Sizes

The standard ground joints in conical shape according to DIN 12 242 are the sizes NS 5/13, 7/16, 10/19, 12/21, 14/23, 19/26, 24/29, 29/32, 34/35, 45 / 40, 60/46, 71/51 and 85/55; outside of the standard, but based on this, 40/38, 50/42, and 55/44 are also available. The first number indicates the upper diameter in millimeters, the second the length. The pitch of the standard joint is always 1:20, which corresponds to a taper of 1:10.

The taper with tolerance is specified by ASTM E676-02 with 1 ± 0.006 mm diameter: 10 mm length. There is also a measuring method for vacuum leak measurement and diameter tolerances for conical ground joints.

For long cuts the size NS 5/20 7/25 10/30 12/32 14/35 19/38 24/40 29/42 34/45 40/50 45/50 loud,,,,,,,,,, 50/50 and 55/50. They are mainly used for work in a vacuum because the sealing surfaces are larger.

Core grindings are also available with constriction, extension, with a drip ring or with a drip tip, and with drip funnels with wedge-shaped grindings to enable a finer dosage.
Double pieces (sleeve and core) for DIN 12 594 are available in the sizes core NS 14/23 and sleeve 14/23, core 19/26 and sleeve 19/26, core 29/32 and sleeve 14/23 as well as core 29/32 and sleeve 29/32.

Spherical ground joints (DIN 12244 Part 1) are available in the sizes below. S stands for (English) Spherical

size	Core dm.	Tolerance core	Bowl dia.	Tolerance shell	2-digit name
S 13	12,700 mm	- 11 μm	12.706 mm	+ 18 μm	13/2 & 13/5
S 19	19.050 mm	- 13 μm	19.057 mm	+ 21 μm	19/9
P 29	28.575 mm	- 13 μm	28.582 mm	+ 21 μm	29/15
P. 35	34.925 mm	- 16 μm	34.934 mm	+ 25 μm	(35/20)
P. 40	38,100 mm	- 16 μm	38.109 mm	+ 25 μm	40/25
P 41	41.275 mm	- 16 μm	41.284 mm	+ 25 μm
P 51	50,800 mm	- 19 μm	50.810 mm	+ 30 μm	51/30
P 64	63,500 mm	- 19 μm	63.510 mm	+ 30 μm	64/40

According to the supplier Lactan, the size in brackets does not comply with DIN

Transition pieces

DIN 12257 describes various transition pieces to change from one size to another. Only transition pieces for conical ground joints are standardized. When changing from a large core to a small sleeve, one speaks of a reducer, and conversely, of an expansion piece. In addition to the standardized pieces, a large number of other designs are available, including transition pieces from conical to spherical ground joint and vice versa, and transition pieces for spherical ground joints of different sizes.

Standardized transition pieces between small (k) and large (g):

Sleeve	on	core
14/23	k / g	19/26
14/23	k / g	29/32
19/26	k / g	29/32
29/32	k / g	45/40
19/26	g / k	14/23
29/32	g / k	14/23
29/32	g / k	19/26

Core and sleeve refer to the element on the transition piece.

Ground stopper

Ground-joint stoppers are used to close an open sleeve and exist as

Octagonal cover plugs (up to NS 24/29 solid, from NS 29/32 semi-hollow)
Hexagonal hollow stoppers with a pointed bottom
as before, but with a flat bottom
as before: amber glass
as above, but with a small handle diameter
Lid stopper made of polyethylene

Solid stoppers are made from a glass rod that is heated and molded into a glass blank by compression molding, which already contains the raw core and the handle of the stopper in one piece. In a further step, the actual standard section is produced on the core of the plug.

Hollow stoppers are made from tubular glass, which is brought into the viscous state by heating and then manually blown into a stopper shape by a glass blower. Hollow plugs are characterized by a significantly lower weight, but are more complex and therefore more expensive to manufacture. As with the solid plug, the core receives its standard grinding outside in the grinding shop.

Conical ground joints - with a transverse bore - also occur on valves in or on glass apparatus, such as a burette . The hollow cone fused or cast onto glass tubes is made of glass, the rotatable inner cone can be made of glass (greased) or plastic. There are also three-way valves in this form. The glass core is typically held in the inner cone by a plastic screw or a rubber ring.

Small glass dropper bottles are used for the approximate dosage of liquid, for example for dental treatment. In the mouth of the bottle there are already blown two grooves up to a little over half the height of the ground cone. To open the drip function, the conically ground glass stopper with its cast-in half-length grooves at a higher point is turned so that the four grooves overlap and form an outflow duct and an air inlet duct (diametrically opposite).

Other cut shapes

In addition to the conical ground, spherical ground , cylindrical ground and plane ground are also common.

Ball joint connection, consisting of ball (left) and shell (right).

With spherical joints, the two devices have a hemisphere ( ball ) instead of a core and a shell instead of the sleeve. This form of connection allows a flexible connection. Ball joints are used in apparatus with larger volumes such. B. often used in pilot plants because the construction of such plants is easier and more flexible to implement. They are also commonly used on the receiving flask on rotary evaporators . Ball flange connections consist of ball and socket in the nominal sizes KF 15 and KF 25. Compared to core joints, ball joint / ball flange connections cannot "bake", ie they cannot tilt or stick to one another, or only with difficulty. The disadvantage is the higher price of the cuts, that a clamp always has to be used to ensure a connection and that they can only be kept tight against overpressure with greater effort.

Cylindrical grindings consist of a shaft and sleeve and are used in KPG stirrers .

Glass syringes are also ground cylindrically. In any case, the cylinder outside is made of glass. A mostly hollow glass stamp can be ground inside, but there are also steel pistons and plugs made of elastomer with an inserted piston rod. Syringes with volumes of 0.5-100 ml are used for medical injections of liquids. A conical Luer connection can be formed directly from conically ground glass at the outlet . A pairing of two hard materials requires grinding with a very small gap with a small tolerance, but results in a syringe with an extremely low pressing force hysteresis , i.e. very sensitive pressure feedback on fingers and thumbs.

Piston samplers (gas syringes) with a cylindrically ground sealing surface are used for clear, metered handling of small gas volumes.

Flat cuts can be found in desiccators , chromatography chambers for thin layer chromatography , glass reactors and vacuum bells , which are often sealed with a rubber plate on a solid metal plate.

seal

Conical ground joints must be lightly greased during use so that they do not stick together. Various types of fat are used for this purpose ( ground joint fat ). A sealing ring, a ground joint or a tape made of PTFE (Teflon) can also be used for this purpose. With today's cuts, this greasing is no longer used for sealing, but rather to be able to loosen them again after use, as they are manufactured so precisely that the adhesive forces can become so great that the cuts "bake". This happens even if there is no dirt that can gum up on the joints. Such "baked" cuts can usually be separated again by quickly heating the sleeve and possibly simultaneously cooling the core.

Joint clamps

Joint clips for conical joints made of plastic. The sizes are color-coded; NS29 red, NS24 green and NS14,5 yellow

Joint brackets are used to prevent conical joints from loosening in the event of slight pressure surges or slight tensile load. The simplest versions are made of plastic, which is less heat-resistant and rather less chemically stable than those made of metal.

Metal clips can be bent from a single piece of resilient steel wire. Clip-on clamps and clamps that can be adjusted with pliers are made from punched, bent and nickel-plated spring steel sheet. Fork clamps, like clothespins, have to be opened a little against spring pressure in order to lock the 2 C-shaped receptacles on both ground joints with radial clamping. If the clamp handle is omitted, the pair of forks compresses the joint partners axially thanks to a spring. This pressing force on the gripping edges of the glass parts can be increased significantly with a locking screw with a knurl.

Conical joints clamped in this way hold closed even with a certain internal overpressure.

Ball joints must generally be secured with joint clips. These are all provided with screw locks.

Standards and designations

For taper ground:

Commercial Standard CS-21 of the former National Bureau of Standards is obsolete; this standard allowed significantly larger dimensional tolerances. CS 21-30, -34, -36, -39, -58 appeared in the corresponding years 1930–1958, in the last edition spherical joints were standardized.
ASTM E 676 (USA) Full / Medium / Short Length Taper-Ground Joints
DIN 12 242
ISO 383

NS (standard cut)
Standard taper (symbol: T and S drawn one above the other, T standing a little higher)
SJ (Standard Joint) Designation at Büchi in the documentation of his Rotavapor

Standard sizes

Conical ground joints all have the same cone pitch of 1:20, so the diameter tapers 1:10 with the length along the axis. Standard sizes are specified by a pair of numbers. The first number indicates the largest diameter of the manderl. The second number after the slash is the length, also in millimeters. Different standard sizes sometimes fit together even if only the first number matches.

The following table was taken from the article Ground glass joint (en.wikipedia.org) and supplemented by the DIN-standardized values mentioned above (standard joint and (standard) long joint):

Full-length	Medium-length	Short-length	International-length	Standard cut	Long cut
ASTM E 676-02 (obsolete CS 21)			ISO 383 (ISO K-6 series)	DIN 12 242	DIN
5/12	5/8	5/13		5/13	5/20
7/25	7/15	7/10	7/16	7/16	7/25
10/30	10/18	10/7 & 10/10	10/19	10/19	10/30
12/30	12/18	12/10	12/21	12/21	12/32
14/35	14/20	14/10	14/23	14/23	14/35
19/38	19/22	19/10	19/26	19/26	19/38
			21/28
24/40	24/25	24/12	24/29	24/29	24/40
29/42	29/26	29/12	29/32	29/32	29/42
34/45	34/28	34/12	34/35	34/35	34/45
40/50	40/35	40/12	40/38		40/50
45/50		45/12	45/40	45/40	45/50
50/50		50/12	50/42		50/50
55/50		55/12			55/50
60/50		60/12	60/46	60/46
71/60		71/15	71/51	71/51
			85/55	85/55
			100/60
103/60

Individual evidence

↑ ASTM: Designation: E676-02: Standard Specification for Interchangeable Taper-Ground Joints: Pooblic home.aktor.qa, accessed March 28, 2020.

^ Gerhard Meyendorf: Laboratory equipment and chemicals. People and knowledge Volkseigener Verlag Berlin, 1965, pp. 50–52.

^ Walter Wittenberger: Chemical laboratory technology. 7th edition. Springer-Verlag, Vienna / New York 1973, ISBN 3-211-81116-8 , p. 40.

↑ Piston syringe (gas syringes ) ivohaas.at, Lehrmittelverlag, accessed June 5, 2017.

^ Walter Wittenberger: Chemical laboratory technology. 7th edition. Springer-Verlag, Vienna / New York 1973, ISBN 3-211-81116-8 , p. 39.

↑ D. Glindemann, U. Glindemann: Greaseless Taper Jointed Glassware and Containers hermetic tight with new PTFE Sealing Ring. (PDF; 280 kB). 2001.

↑ Conical joint clamps, Rettberg Clamps for Conical Joints idl-laborbedarf.de, accessed June 5, 2017.

↑ wilmad-labglass.com

↑ Withdrawn DOC Commercial Standards (CS) nist.gov, June 14, 2017, updated November 15, 2019, accessed March 28, 2020.

↑ Commercial Standard for interchangeable ground-glass joints, stopcocks and stoppers Technical News Bulletin of the National Bureau of Standards, No. 237, January 1937, p. 9. - About the contents of the issues from CS 21 to 1936.

↑ Ground Joints , accessed June 5, 2017, not available March 28, 2020.

↑ Rotavapor R-300, Technical data sheet Büchi, buchi.com, accessed June 5, 2017.

[1] ASTM: Designation: E676-02: Standard Specification for Interchangeable Taper-Ground Joints: Pooblic home.aktor.qa, accessed March 28, 2020.

[Meyendorf-2] Gerhard Meyendorf: Laboratory equipment and chemicals. People and knowledge Volkseigener Verlag Berlin, 1965, pp. 50–52.

[Wittenberger1-3] Walter Wittenberger: Chemical laboratory technology. 7th edition. Springer-Verlag, Vienna / New York 1973, ISBN 3-211-81116-8 , p. 40.