Myford Super 7 Backgear eccentric screw

[Paul Bishop 4Participant @paulbishop4]

Hi all. Does anybody know what thread size the holding screw/pin for the eccentric cam is on a Mk1 Myford super 7 please? This is the one which is in the lower hole of the lever detent for the backgear lever.

An owner previous to me had managed to break off the end with the slot in to use the screwdriver, and I've had to drill it out.

 

Edited By Paul Bishop 4 on 31/12/2018 20:27:23

[Paul Bishop 4Participant @paulbishop4]

[DMRParticipant @dmr]

1/4" BSF. Only the outer 0.350" or so is threaded. The rest measures 0.193" dia or 4.9mm (just a BSF clearance) and is about 0.750" long with a bit of a lead-in taper at the inner end. Overall length is about 1.080". I have a used spare in my hand but new Myford will sell you one. Part A1987 Backgear eccentric screw. Same on Mk1 as the later ones up to metrication.

Good luck, Dennis

[Paul Bishop 4Participant @paulbishop4]

Thanks Dennis.

I have a new one, but when I've drilled it out it's killed the thread.

However when I try to put it in its tight.

Wondering if I get a 1/4 BSF tap whether it would clear it.

Other plan was to open the hole, and insert a brass rod, use red loctite to hold it, and tap that out.

[DMRParticipant @dmr]

Well now, what size drill did you use? BSF tapping is 5.3mm for a good fit. If you have drilled out much bigger than that you are going to have to form a new thread area at some bigger size and make a special screw to fit or do as you suggest yourself with a plug. If you have gone off-centre with your drilling (which is likely) then a plug would be better with a pre-drilled and tapped central hole. Since your new screw is tight there is a chance you can form a fresh 1/4" BSF thread in what is left, but it sounds as if you have drilled out too big to do that. Note that the hole/thread must be fairly square to the original hole for the eccentric shaft to lie in the right place. Whatever you do, you need a right size set of second and plug tap.

As a second thought your new screw from new Myford may have a 6mm metric thread.

 

 

Edited By DMR on 31/12/2018 22:51:47

[Paul Bishop 4Participant @paulbishop4]

Hi Dennis

Used a 5.5mm so may be able to get away with it.

I'l order a tap as ail of mine are metric (looks like a good set of imperials will come in handy).

I'll post the result

Paul

[Brian WoodParticipant @brianwood45127]

Hello Paul,

While you have the eccentric dismantled, if you have the ability to do so, run an end mill along the shaft that carries the combination reduction gear to act as a reservoir along the bearing surface on either side of the oil feed hole.

A previous owner on my lathe had pumped grease into it from the nipple, it had set stiff and semi rigid, thus severely restricting oiling and the gear had partly seized on the shaft as a result

Regards Brian

[Paul Bishop 4Participant @paulbishop4]

Hi Brian

Will do. good idea. That's just an example of what user forums are about.

Do you think it would be sacrilege to do it on a Chinese Mini Lathe?

[Brian WoodParticipant @brianwood45127]

Paul,

Absolutely, but I won't tell the ethics police if you are discrete about it !!

The slot doesn't need to be deep by the way, I cut mine to about 1/2 mm..

Brian

[Peter SansomParticipant @petersansom44767]

If the hole you drilled is too big to take a 1/4" BSF tap, you can use a 1/4"thread insert such as a Helicoil. The thread inserts an be used to convert a 1/4" BSF thread to M6. The tap drill fro an M6 insert it 6.3mm, from memory. I have looked at this option several times as it is almost impossible to obtain 1/4" BSF in Australia.

[Paul Bishop 4Participant @paulbishop4]

I need to wait until the suppliers are open before I make a final decision.

I could tap it out to 6mm thread, and then take some 6mm threaded rod and have a go at making one (on the Chinese)

Could harden it, might be worth having a go.

Wont tap the headstock until I see what the part I make comes out like.

[Brian WoodParticipant @brianwood45127]

Paul,

I wouldn't harden it, in fact I would even go to the trouble of putting a small centre hole in it before you cut the screwdriver slot across it, just in case it needs to be drilled out again in the future.

Neither does it need screwing down hard. Enough to touch the bottom of the groove in the main body of the eccentric and then back by half a turn is all that is necessary. The end of the screw needs to have the diameter turned down to be a clearance fit in the groove so that the eccenric can be used as designed..

Regards Brian

[Paul Bishop 4Participant @paulbishop4]

I'm going to copy a new one, and the centre hole is an excellent idea.

It's only to stop the cam from moving so as long as it's within a realistic tolerance then it should be fine.

At least I can rebuild my headstock then.

[Paul Bishop 4Participant @paulbishop4]

After checking, checking, and checking again, i might be able to use the new one that I got from Myford. It appears to catch on what threads are left. I'm going to run a 3rd tap through it, and on installation of the cam, if it works, I'll put the screw in with a smidgin (imperial smidgin of course) of blue loctite.

The likelihood of me having to disassemble this unit when I've finished refurbing is not very high.

I made a copy (honestly) in 6mm, and it looks like that it would work, after offering it up to the cam, but didn't want to tap out to 6mm. That's always plan B now.

[Brian WoodParticipant @brianwood45127]

Paul,

It would be nice if you can use the genuine screw, the anchoring duty is light in the extreme and it keeps that fitting as would be expected for anyone following on with it as another owner.

I dislike mixed thread form screws on a machine; some of them are nearly right in terms of engagement in one direction, but it is not of course reciprocated in the other. On my ML7R for example, the apron is held on with imperial cap screws, but the bed jib with the laminated shins behind it that controls lift is fastened with two very similar looking metric cap screws.

Mix then up in a rebuild and you find this out when one hand is holding the apron in place and the blasted bolt engages but goes no further.

Regards Brian

[Paul Bishop 4Participant @paulbishop4]

Agree Brian.

If I can use the genuine I will. Like you i don't like mixing formats, and even though it's a new screw, it maintains Myford provenance.

Blue loctite will take the "slack' up.

Quite pleased though as a beginner that I managed to make the metric equivalent. Gives me hope for the future.

Regards

Paul

[Howard LewisParticipant @howardlewis46836]

Agree with the sentiment of not using metric threads on an Imperial machine., only a source of confusion in the future for you or someone else.

Maybe I've misunderstood, but having used a 5.5mm drill, a) M6 would be a loose fit (Tapping drill should be 5mm) and 1/4 BSF will be 6,354mm OD anyway.

If a 1/4 BSF bottoming tap won't clean the threads sufficiently, my advice would be, if enough meat is present, to drill and tap to 3/8 (BSF or BSW). If you drill out in small increments, the drill should follow the original hole.

Make a bush with 3/8 external thread and 1/4 BSF internal and use an anaerobic sealant to lock it in place. As a fitting tool, put a nut on a 1/4 BSF setscrew and run the bush into place, flush with the surface, back off the nut and remove the setscrew. Once the anaerobic has cured, (allow 30 minutes minimum) reassemble (It may be worthwhile to grease the new eccentric screw, so that the thread is contaminated, and any stray anaerobic does not cure on to it) and you should be back in business.

Howard

[Paul Bishop 4Participant @paulbishop4]

Thanks Howard

Picked up a 2&3 tap today, so will see how it goes tonight.

I'm confident I can repair it now with so many methods available to me.

Paul

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