Positional Tolerances For New Holes

There are three factors in deciding the accuracy of the holes.

The first is the machining of the hole using cutters, second the type of template used, and the third is the method of mounting the template on the machine.

    1. Machining the hole.

      We use bushes, which are a slide fit into the template, these are used to position our rig, and then to guide the various cutters when machining.

      There must be a clearance between the template hole, and the bush, otherwise the bush wont go in the hole.

      Likewise there must be a clearance between the bush and the cutter, otherwise the cutter wont go through the bush.

      Allowing for these clearances, we expect to produce a hole to within 0.05mm of the template bush hole.

    2. The type of template used.

      Let’s assume you want 4 holes, 1 in each quarter of the platen…..

      The simplest template, is a ¼ template that locates off a couple of bolt holes, and has a single bush hole in it, at the required position.

      It relies on the bolt holes for positioning.

      The strap can easily be moved around the platen to pick up the various positions, but requires positioning 4 times, so you’ll get 4 errors.

      The next type is a ½ template with 2 bush holes in it.

      It has the advantage that the spacing between holes is fixed by the template.

      It can be flipped over to give matching positions on the other half of the platen.

      This is still located off bolt holes, but the strap is longer, going across the platen.

      It needs to be positioned twice, so you’ll get 2 errors.

      The third type is a full template with all the holes required in a single plate.

      This type of template is the most accurate.

      It can be located off the die location bore, or located off widely spaced bolt holes.

      The position of every hole is fixed by the template.

      It is only positioned once, so you get 1 error.

      We normally make our templates with hole centres to within +/-0.02mm over 500mm.

    3. Mounting the template on the platen.

      When the platen is manufactured, the die location bore is machined, and all the bolt holes are drilled and tapped, ideally all in one operation.

      Even with the best machinery, there is always some tolerance allowed on the die location bore, and the position of the tapped holes, because nothing is perfect.

      On a small 50T machine platen, the position of the bolt holes may be quoted as +/-0.20mm all over the platen.

      If you imagine the platen from a 1500T machine, then you’re probably looking at a quoted accuracy of +/-0.8mm by the time you get out to the M24 holes at 840mm from the centre. However, the accuracy between adjacent holes is significantly better than this, maybe +/- 0.1mm

      There is always clearance between the bolt, and the tapped hole, otherwise the bolt wouldn’t screw in.

      The bolt is usually made undersize. (Eg. M24 bolts can range from 24.0mm down to 23.67mm)

      If we locate a ¼ or ½ template using bolts, then we always introduce a small error due to the bolt being undersize, however this error is often taken out by the error in the tapped hole positions.

      The template is now located on the platen, but the absolute position is related to the bolt holes being used, which may be +/-0.8mm from where they are supposed to be.

      If we locate a full template using widely spaced bolts, then again, the clearance in the bolt holes will often be taken out by the errors in the hole positions, and the template will sit smack on position.

      However, with a large machine, and large full template, the errors may take the bolt holes outside the holes in the template.

      If we locate a full template using the die location hole, we will be within a H8 tolerance for the position of the template on the platen.

      (eg 250H8 = 250.00mm to 250.08mm)

      However, it then needs to be decided, about what level reference to use.

      If a bolt hole is used then the template will be aligned with the platen, but maybe not the floor.

      If a machine level is used, then the template will be level with the floor, but maybe not with the platen.

      This accuracy assumes that the die location bore is perfect, not bruised by tools being hung on it !!!

      Everything depends on the platen accuracy, or the state of the die location bore.