Meddings dril tru Bench Drill Repair.

Hi, my name is Bob and I live in the West Wickham area.

I would like to ask for help for a part on a Meddings Dril Tru bench drill.

The machine has a serial number DBT 7742, apparently dated back to 1965 !

The part in question is the head clamp device that consists of two pieces of 1/1/4" round material, threaded on one half with 3/8" BSF, and these two halves are pulled together by a handle/rod that passes through them. Each half has an angle cut on it that will make contact with the pillar, once the head has been placed upon it. Turning the screwed rod locks the head.

I intend to make the parts but do not know the the angle to machine.

I have spoken to meddings, but they say that no drawing are available of this part.

Thanks.

Hello Bob,

The angle doesn't really matter, something in the order of 45 degrees will do perfectly well. The lock works by the camming action of the two taper faces being pulled together within a hole bored to contain them and reaction forces resisting the outward thrust from the tapers does the job of jamming and therefore holding the head locked to the pillar

In your design, include a short compression spring between them to help the two haves of the clamp release when the clamping bolt if freed off.

Regards Brian

Bob, I have measured my Meddings, it is 60 deg.

Alaine.

Thank you very much for your reply.

I had done a drawing, in 'real time' full measurements and worked out an angle of 64 deg, but your actual piece from the m/c is better and makes it correct.

Perhaps you could comment on the rod that pulls the two pieces together, which I understand is 3/8" BSF. thread.

I looks to me that this size of rod will come very close to the pillar and I am looking to reduce it to 5/16 BSF (Found some in a drawer ! )

Regards Bob.

Hi Brian Wood.

Thanks you for your response, good idea for the release spring.

Regards Bob.

Hi again Alain Foote.

Sorry about the E,, it was overenthusiastic Typing.

I have to say that I am delighted with the responses to my query, as this is the first time I have ever been on a Forum even though I have taken the 'Workshop: Magazine for many years.

Thanks.

Bob.

5/16 will work fine and allow a little room for a release spring.

From what i've seen normal British practice was to make the bolt pretty much as large as would fit in the space and not use a release spring. Common to need a quick donk to release. Especially if the last guy to lock it was channeling their inner gorilla!

The angle on the end determines the force distribution. Following Alains' photo steeper angles push more force outwards to the main casting and shallower ones put more force along the clamp thread. With steeper angles you really don't want to do things up any tighter than needed as the outward force rapidly becomes enough to jam everything up solid.

It is said that shallower angles aren't as effective.

But I've never seen any sensible engineering explanation.

I imagine its like everything else in engineering. So long as the basic concept is workable the effectiveness in practice is down to how well the details have been handled. Careful engineering oif a theoretically not so good concept beats slap dash implementation of a theoretically better way every time. Good enuf is good enuf.

Clive

This type of cotter clamp is far better than the basic grub screw but can have a weakness. Try a plastic or wood test piece and using engineers blue or felt tip check where on the angled face it contacts the pillar. The problem is if it is too close to the outside diameter the contact area is small so pressure high and it will dent the column when done up by your pet semi-trained gorilla. If so look at shallowing the angle. In the ideal situation it is flycut to match the column diameter as a whole, ie both halves, then split and shortened so it will hug the pillar.

Hello Bob I confirm that on my drill it is 3/8" BSF see pictures.

Edited By Alain Foote on 24/10/2022 16:16:19

Bob, do you want to make it or would you prefer to purchase one if I can find a spare? I have a lot of Meddings spare parts but my home workshop is somewhat chaotic at present due to recent purchases.

Dear All.

Thanks for all of your very helpful comments that have given me the information I needed to progress.

I like very much the comments about 'good Enuf' which is a mantra that I practice, often !

If I get lucky, I may be able to not have to make the parts.

Regards Bob.

The discussion revolve around chamfers, but of course the shallower the angle the more the effect of simply wedging the opposite wall of the head against the column.

I have seen clamps of this type that use concave contact=faces: they'd probably be best machined with the bore, having been installed first with shim washers between them. They could be made in a jig on a face-plate, or by using a boring-head.

I wonder if there is any advantage in this over flat facets?

(I do own a Progress 2G drill, but at some point I need make a new drop-bracket for a small horizontal mill with a single, plain round bar as over-arm.)