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DSEL74
4th Jul 2013, 06:49 PM
Hi Guys,I planning on making a shoulder plane/ rebate plane which I think is known as a "York Pattern" or "Yorkshire Pattern" Similar to the Norris 20.

I have a pattern for a 1½" wide one (280mm long approx) including two core boxes and I intend to make the same for a smaller version. It will require some machining up on a mill to square the outside and remove the 3deg draft. It will also take some filling to clean up the inside and shape the bed and cut the mouth. I have time and bench work doesn't require much space.

I had some errands to run today so I stopped in at a foundry I was passing (Strangely convenient, almost like I planned it). I got a quote off the guy to cast one for me in Phosphor Bronze, which was the same cost as the regular bronze they use but would take a little longer as he would have to get it in. I got a little look around the foundry and the thing that interested me most was the amount of fake aka reproduction antique petrol parafinallia that was being produced and the fact that they don't do any green sand work.

There is a massive skip out the back full of broken lumps of pink casting sand. All the moulds are made with a resin sand. So the mould goes off and is poured and thrown out. It is also expensive to have the used sand removed and disposed of. The sand was a 70 grit according to one of the workers and it is quite fine and gives great detail. The owner told me it is much cheaper to use green sand. He didn't say why they soley used the resin sand instead, maybe it produces better quality moulds and therefore more efficient.

His minimum price was $60 for a job and he claims it cost $40 just for the sand & disposal. I got him down to $50 but I still will have to save a few pennies before I could get it done. Although I'd be just as happy to cast it myself if I had a furnace.

DSEL74
5th Jul 2013, 08:44 AM
Pattern Allowances
The patterns are not made the exact size as the desired casting because such a pattern would produce undersize casting. When a pattern is prepared, certain allowances are given on the sizes specified in the drawing so that the finished and machined casting produced from the pattern will conform to the specified sizes. While designing pattern, the allowances commonly considered are discussed below.

Shrinkage Allowance
Generally metals shrink in size during solidification and cooling in the mould. So casting becomes smaller than the pattern and the mould cavity. Therefore, to compensate for this, mould and the pattern should be made larger than the casting by the amount of shrinkage. The amount of compensation for shrinkage is called the shrinkage allowance.
Generally shrinkage of casting varies not only with material but also with shape, thickness, casting temperature, mould temperature, and mould strength. Therefore, it is better to determine the amount of shrinkage according to the past record obtained from many experiences.

Typical shrinkage allowances for Bronzes 1.05-2.10
Contraction rule +12/1000 (tel:+12/1000) Aluminium alloys, bronze, cast steel (thickness 5-7 mm)

What is not mentioned is that the metal actually shrinks 3 times during the cooling process, Freezing, cooling





Machining Allowance
In case the casting designed to be machined, they are cast over-sized in those dimensions shown in the finished working drawings. Where machining is done, the machined part is made extra thick which is called machining allowance.
Machining allowance is given due to the following reasons:
1. Castings get oxidised inside mould and during heat treatment. Scale thus formed requires to be removed.
2. For removing surface roughness, slag, dirt and other imperfections from the casting.
3. For obtaining exact dimensions on the casting.
4. To achieve desired surface finish on the casting.
The dimension of the pattern to be increased because of the extra metal required (i.e. finish or machining allowance) depends upon the following factors:
1. Method of machining used (turning, grinding, boring, etc.). Grinding removes lesser metal than turning.
2. Characteristics of metal (ferrous or non-ferrous, hard and easily machinable or soft). Ferrous metals get oxidised, aluminium does not.
3. Method of casting used. Centrifugal casting requires more allowance on the inner side. Die castings need little machining, sand castings require more.
4. Size and shape of the casting. For long castings, warpage is more and greater allowance is required. Thicker sections solidify late and impurities tend to collect there. This necessitates more machining allowance.
5. Degree of finish required. A higher degree of finishing requires more machining allowance. The standard machining allowances for different metals and alloys are shown in Table 1.4.



Table 1.4 Standards of general machining allowance
Type of metal and alloys

Cast irons
(i) Large size castings (>1000 mm) 10mm
(ii) Medium size castings (<150 mm) 3.0mm

Cast steels
(i) Large size castings (>1000 mm) 12mm
(ii) Medium size castings (<150 mm) 4.3mm

Non-ferrous materials
(i) Large size castings (>1000 mm) 5.0mm
(ii) Medium size castings (<150 mm) 1.5mm







3. Draft Allowance or Taper Allowance
When a pattern is drawn from a mould, there is always a possibility of damaging the edges of the mould. Draft is taper made on the vertical faces of a pattern to make easier drawing of pattern out of the mould (Fig. 1.3). The draft is expressed in milimetres per metre on a side or in degrees.

The amount of draft needed depends upon (1) the shape of casting, (2) depth of casting, (3) moulding method, and (4) moulding material. Generally, the size of draft is 5 to 30 mm per metre, or average 20 mm per metre. But draft made sufficiently large, if permissible, will make moulding easier. For precision castings, a draft of about 3 to 6 mm per metre is required. Table 1.5 shows different taper allowances used for different moulding methods.

The amount of draft needed depends upon (1) the shape of casting, (2) depth of casting, (3) moulding method, and (4) moulding material. Generally, the size of draft is 5 to 30 mm per metre, or average 20 mm per metre. But draft made sufficiently large, if permissible, will make moulding easier. For precision castings, a draft of about 3 to 6 mm per metre is required. Table 1.5 shows different taper allowances used for different moulding methods.

Table 1.5 Approximate taper allowances used in different moulding methods.

Sand moulding - Wood
Height of pattern mm Shell Moulding Manual drawn
Up to 20 0° 45’ 3°
20 to 50 0° 30’ 1° 30’

100 to 200 0° 20’ 0° 45’





Rapping or Shaking Allowance
When the pattern is shaken for easy withdrawal, the mould cavity, hence the casting is slightly increased in size. In order to compensate for this increase, the pattern should be initially made slightly smaller.
For small and medium sized castings, this allowance can be ignored. But for large sized and precision castings, however, shaking allowance is to be considered. The amount of this allowance is given based on previous experience.




Distortion or Camber Allowance
Sometimes castings, because of their size, shape and type of metal, tend to warp or distort during the cooling period depending on the cooling speed. This is due to the uneven shrinkage of different parts of the casting. Expecting the amount of warpage, a pattern may be made with allowance of warpage. It is called camber.
For example, a U-shaped casting will be distorted during cooling with the legs diverging, instead of parallel. For compensating this warpage, the pattern is made with the legs converged but, as the casting cools, the legs straighten and remain parallel.

Warpage depends on the thickness and method of casting and it is actually determined by experience. Generally 2 to 3 mm is considered appropriate for 1 metre length. Table 1.6 shows typical distortion allowance used in different sized castings.