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Old 02-26-2016, 09:01 PM   #11
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N,

If you are going to get yourself a 40tpi tap & die set, then cut your internal thread first then gently come down in size for your adjusting screw, that way, you should get an almost perfect fit, and that shows itself by having very little backlash when adjusting.

John


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Old 02-27-2016, 07:14 AM   #12
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Default Turning cap screws

Should of used a different title for this thread, my bad

But while we're here. Can I just jump up one size then finish tap to correct size and still achieve low backlash? I planned on just buying the single size vs a whole set.

I like the 20 divisions. If I'm reading the plans correctly the dial is .500. All it give me is a . 249 radius.


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Old 02-27-2016, 09:05 AM   #13
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The thing is N is that you have no control when using a tap to cut an internal thread and finishing with a plug tap, what you cut with it is what you have to go with, this is where the die comes into it's own, it is adjustable to a certain degree so that you can gently bring it down to fit exactly the internal thread you cut with the tap, and so reducing backlash to an absolute minimum.

I gather you are not up to a stage where you can cut your own threads, if you were, you wouldn't have any problems at all. But forget that for now, you will pick that up the further you get into model engineering

When going with your larger sized thread of 20 TPI, to retain your accuracy of setting, which normally with a 40 TPI, you have 25 divisions on the head, so for every division moved, it will move the tip of the tool 0.001". If you use the 20 TPI with 25 divisions, each segment will move the tip 0.002", so halving your accuracy, and that was why I suggested going with 50 divisions, which will give you the same movement per segment as a 40 TPI thread, and as you have mentioned, the only down side is getting the head of the adjusting screw large enough to fit the 50 divisions. If you can squeeze them onto a 1/2" head, then you are cooking with gas.

It is such a shame you don't live in the UK, I could most probably done the job for you in the time it has taken this post to get to this stage.

I hope this helps

John
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Old 02-27-2016, 10:01 AM   #14
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Unfortunately, the gremlins or whoever got in and my posts disappeared. Of course, John is quite right but it is important to mention about avoiding or minimising backlash. You should aim to get perhaps an 80% engagement when drilling prior to tapping.

Again, it is difficult to achieve some of the demands from perhaps limited tooling and improving skills. I wrote to a newcomer about the use of a metallic filler to take up some of less accurate items. Little did I think that my 'new' little Myford would have had a broken lever arm to control reversing and forward driving my leadscrew. It was cracked in some alloy and the thread was 3/8th BSF( with 20TPI)
It is an awkward casting and now unobtainable but I went out and bought a tin . Clearly, I will have to make a new one. Whether steel or otherwise, I have yet to decide but that is what is 'working' at the moment.

So there is no harm in making 'adjustments' until things improve.

Regards

Norman


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