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  1. #1
    Join Date
    Jul 2008

    Default Drilling Holes with Rag

    Opening up a drilled hole with a bigger drill is often a real pain in the neck. The drill jumps around all over the place and the hole is usually oversize, jagged and /or triangular in shape. There are a multitude of chatter marks on the job and nothing seems to fix the problem. Well, not until now.
    The secret to opening up a hole from say 16mm to 25mm is easy. It can be done quite safely even with a 16mm drill stand provided it will run at slow revs. For a 25mm drill in steel, a speed of 80 feet/minute or less is needed. This equates to 300 rpm or as low as 100rpm. I would go for 100 rpm to start with.
    It is essential that the drill is ground correctly. The length of the cutting lips/flutes must be exactly equal. If they are not equal, the drill will cut oversize. The next important requirement is to grind the clearance under the lip to a minimal amount. Definitely no more than 1 degree clearance to start with. The clearance can be ajusted (larger) as you get used to drilling in this manner and your knowledge and experience grows, but keep the clearance to a small amount.
    To open the hole, align the 25mm drill in the 16mm pre-drilled hole as acurately as possible. Get some cotten cloth, a strip off your shirt or your mates shirt about 75mm wide and 150mm long will do the job. Fold this in half to form a square, 75 x75. This is now two layers. Fold again to a rectangle, (four layers). Fold again to a square, (8 layers) and place this on top of the 16mm hole to be opened up. You may need more layers (12 to 16) if the shirt material is too thin. A good dose of your favorite lube will help lots if placed on the underside of the cloth first.
    Lower the 25mm drill down and rest it on the rag. Turn on the drill and gently press down on the lever that extends the drill. The sharp edge of the 16mm hole will start to cut into the rag and the lip of the drill will start to "bite" into the steel. The drill cant chatter 'cos the rag is stopping it from digging in and the now compressed layers of rag under the lip will only allow the drill to feed in very slowly as the rag is cut gently away.
    The type of rag you use makes a huge difference to the success of the operation. Denium as in jeans, works extremely well. In fact it can work too well and the drill wont penetrate to the job. If this happens, remove some of the layers of material. Dont use wool or synthetic material, only use cotten either as in a shirt or as denium (sp).
    Using the thicker denium, you can also move the 16mm hole a milimeter or more to the side and the 25mm drill will drill at its new location. This is why it's important to have the drill aligned correctly at the start. Use a new piece of rag for each hole. You will be amazed at how good this works. It goes without saying that this is a job that must be clamped, bolted or welded to the drill table for safety. This technique also works extremely well in the lathe.


  2. #2
    Join Date
    Oct 2007


    Thanks, shelved in memory for a rainy day

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