![]() One of the most common ways to produce such threaded holes is to drill a hole of appropriate size with a drill bit and then tap it with a tap. In manufactured parts, holes with female screw threads are often needed they accept male screws to facilitate the building and fastening of a finished assembly. Manufacturing tolerances for reamers can be found by looking up DIN 1420.Below is a comprehensive drill and tap size chart for all drills and taps, imperial and metric, up to 36.5 millimetres (1.44 in) in diameter. Again remember that reamers are manufactured to a tolerance that will give you the longest possible life within the hole tolerance. Reamers are named after the hole they produce as in 12H7. Holes (Bore) have capital letters and shafts have small letters. How good your supplier understands the ISO system. Try not to order by giving a size like 9.98 or you could end up paying for a special depending on ![]() Metric reamers are bought by nominal size i.e. Now if you want to keep using m6 dowel pins (most common) and you need an absolute tight press fit every time than you must buy a reamer that is always below the ZERO LINE like a N7 or even P7.ġ2N7 = 12 -5/-23 and 12P7 = -11/-29 for a tighter press fit. H7 reamers are readily available everywhere. Hole size 12 +18 and pin size 12 +18 = stick/driving fit. Hole size 12 + 18 and pin size 12 +7 = sliding/push fit. Hole size 12 +0 and pin size 12 +18 = medium press fit. Hole size 12 +0 and pin size 12 +7 = light press fit. So here is what you can get by using this combination. IT grade 6 is a grinding tolerance and for 12mm it is 11/1000mm. How much above is designated by the number IT grade 6. The letter "m" puts it's tolerance field completely above the ZERO LINE. Again - "m" designates the position of the tolerance field relative to the ZERO LINE. Now the most used metric dowel pin has a tolerance of m6. This is the hole produced by a 12H7 reamer. Since the letter "H" designates FROM ZERO ON UP the correct hole size for 12H7 is 12 +0/+18. "IT Grade 7" specifies the size of the tolerance. ![]() "H" borders with the low limit right on the ZERO LINE and goes UP. The letter "H" gives the position of the tolerance field relative to the ZERO LINE. So the reamer needed to make this hole is called H7. The tolerance for IT 7 varies, depending on the diameter. Grinding will produce a IT grad 5 or 6 hole. Using a reamer will produce a "QUALITY (IT grade)7" hole. In this system the hole has a tolerance position "H" with "ZERO" as it's low That system is called: STANDARD HOLE (BORE). You use the system mostly use by the industry. I will try to explain again how to use the mostly used metric dowel pin tolerance m6. A 12H7 reamer is manufactured to 12 +8/+15. Reamers are manufactured to give you a hole that is "in tolerance" from the first time you use itĮxcample: A 12H7 tol. Reamer tolerances are not the same as hole tolerances. In the shop we use a P7 reamer for a tight m6 dowel pin. To keep using standard m6 dowel pins you need to select a reamer that gives you the same press fit -tight, medium and light mentioned in () above. The preferred press fit is H8/x8 for tight (-13 to -67 press), H7/s6 for medium (-10 to -39 press}and H7/r6 for light (-5 to -34 press). ![]() If you want to have all your reamers H7 than to get a tight press fit you will need a pin x8, medium press fit you will need a pin s6 or a light press fit r6. If you use a 12m6 pin (standard dowel pin size) the pin will be 12+7/+18 If you use a 12H7 reamer your (new) reamer will be 12 +8/+15 for a hole size 12 +0/+18. You only need one size reamer (H7) plus a Go-NOGO gage and a Micrometer to measure the shaft. Most of the industry uses the "Standard Hole (Bore)" system since it saves money on buying gauging equipment. Standard hole = the lower limit of the hole equals the nominal size. Standard shaft = the upper limit of the shaft equals the nominal size. You need to figure out what system you will use.
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