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David Brown S7 adjustable reamer

Anyone know what the retaining plate fixings are? 8BA or 2-64 UNF

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Oily Rag22/11/2020 11:58:49
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Good Morning all! Hope everyone is safe and keeping well, A quick question for Sunday morning:-

I have a David Brown S7 adjustable floating reamer, unfortunately it has had a hard life and has along the way lost its end retaining plate which is fastened to the body by 4 small socket head screws - 3 of which are broken off inside the body leaving just 1 tapped hole.

Removing the end plate off a smaller S5 reamer I have measured the screw o/d as 0.085". This puts it in line to be either 8BA or possibly #2-64 UNF. The thread gauges I have do not go down to such fine sizes for Imperial threads but it gets close to somewhere around 0.4mm - 0.6 mm pitch which may mean it is #2 - 56 UNC!

Anyone know what these fasteners are?

old mart23/11/2020 15:46:16
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I would hazard a guess that if David Brown is a British company, then the threads are more likely to be BA. If they are from the USA, then definately a Unified or National thread form.

Howard Lewis23/11/2020 16:04:16
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Tubal Cain's Model Engineers Handbook quotes 8 BA as being 0.0866" diameter Being basically a Metric thread, it is 59.1 tpi (0.42978 mm pitch )

Zeus Charts give 2 - 64 ANF as 0.086" diameter, and 2 - 56 ANC as 0.0860" diameter.

As British company, 8 BA would be my first port of call. So how does a 8 BA screw into the one free tapping?

Howard

Oily Rag23/11/2020 16:46:26
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Well 'Old Mart' and Howard, thanks for your inputs. I sort of guessed that it might be a BA thread given DB's history but I hoped someone may have the information for sure.

After much hunting around I did manage this morning to find a supplier with some 8BA socket head cap screws - managed to buy 10 for a 'tea kitty donation' and at 5/16th length they screwed nicely into the one free thread. Now the problem was getting the broken screws out! Fortunately they succumbed to a 1.5mm carbide drill centred on the old screws by way of the centering microscope in the mill (using the high speed head at 10,000 rpm). then using the tiny square tang on a needle file, gently tapped into the drill hole they came out like 'milk teeth'. Job done! Wizzed up a new end plate, bolted it on, after making a suitable counter bore with a 3/16th end mill modified by grinding down to a 0.095" pilot to suit a #41 hole.

All this to accurately size a 1.375" inner bushing that is a bearing carrier. That's this evenings job!

At least now someone in the future will have the information available if they do a search!

old mart23/11/2020 17:21:51
2238 forum posts
165 photos

Those solid carbide drills are marvelous. It is the extra stiffness that gives them the edge over HSS. I broke a 1/4" BSF taper tap in a hole that I had drilled undersize inadvertantly and a solid carbide drill for hard metal made removing the tap easy.

Oily Rag23/11/2020 17:42:48
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Yes - definitely worth investing in a few smaller sizes for the occasional broken fitting. The ones I bought were on special offer from J & L Industrial some time ago, they were long series with a 6mm shank but from the drill tip to the end of the fluting was only about 8mm. the waisted drill was then about 30mm long to the 6mm shank, which gives a good level of grip on the tool and an excellent view of what is happening at the working end. I just touched the drill tip on to the broken bolt end and gently eased the quill of the high speed head and watched for swarf. Once it had self centred it was quite happy to drill away with lots of 'pecking'. I think also that the high speed head is key to successfully using carbide in these small sizes.

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