Wonky tapped threads

Hi guys, I'm hoping someone can come up with some ideas as to what is going on with a little issue I am having currently when tapping threads on my ML7.

So basically, any tap I use, drilled with correct tapping hole, when I run the tap through, then if I fit a bolt or something, its not running true. I have tried several tap wrenches and have a tailstock mounted tapping guide holding the end of the tap straight(not spring loaded version)

So far I have checked :-

Test bar fitted to spindle and clocked (0.001" )

Test bar run between centres and clocked(0.001" )

Test machining to check parallel cut to bed(spot on)

The fact it does not matter which drill, tap or wrench I use makes me think something lathe side isnt right, maybe a worn tailstock although I cant feel any play. The technicuqe I am using was learnt during my apprenctieship so I feel I know what I am doing, but this one has me very frustrated and puzzled. Any help would be greatly apprecieated. If everything is ok, all I can think is when I use first tap, I'm being a bit heavy handed and not going in straight and I'm not keeping up fast enough with teh tailstock pressure maybe, so have bought a springloaded guide to try(in the post)

All tapping is being done with no chuck spinning by the way.

Thoughts?

Edited By JasonB on 23/02/2018 08:15:43

Try tapping a hole with the tap held in the tailstock chuck, tailstock loose so you can apply pressure to the hand wheel to push it in as you turn the chuck.

That should eliminate any error caused by your guide method or uneven pressure on the tap wrench

So turn chuck by hand or with back gear you mean? I have toyed with doing this, will give it a try.

Forgot to add all taps are HSS and good quality i.e. Presto, Volkel etc

Hand will do and is what I usually do to start a tapped hole, external I do with the lathe on its slowest speed and using a tailstock die holder

Before tapping put a bit of rod or the drill shank into the hole and make sure your hole is true by running lathe slowly to see if rod/drill wobbles

Edited By JasonB on 23/02/2018 08:25:58

Well, you would think the tap would just follow the hole. So I reckon first up I would be checking if the hole is drilled straight by drilling a hole and sticking a neat-fitting plain rod into it and see if that runs out of true when you rotate the chuck. If it does, then you have maybe a tailstock alignment problem. Or maybe a headstock bearing loose problem. (Did you check for headstock bearing play?)

If the drilled hole is true, it must be the tap going in crooked, which must be down to tailstock alignment, you would think. But if your lathe is turning parallel between centres, that seems odd.

However, if your ML7 is a narrow guide model (pre 1971) they can wear quite a bit on the rear vertical surface of the front way, which also is the surface that guides the tailstock. Max wear seems to happen that few inches from the headstock end of the bad, right where you have the tailstock when using a tap wrench. When using your test bar between centres however, the tailstock is probably further to the right, in the unworn section of bed so gives a true reading.

You can measure wear on that surface with a 1 to 2" mike from the front of the front shear to the back of the front shear. Or just feel/measure shake between tailstock base and bed at the far end of the bed vs the headstock end. A small amount of wear hear allows a large amount of movement at the tip of a revolving centre or drill chuck and bit sticking out many inches in front of the base.

If you have wear in this area, it can be remedied by a wide guide conversion and then by using a vertical slide to hold a tool bit to the bed vertical surface while racking the saddle back and forth like a shaper. I have written both these processes up for Neil and they may be published in MEW at some point in the future.

Edited By Hopper on 23/02/2018 08:26:45

Hmm, strangely I hadnt thought of putting a rod in the hole right after drilling. Will check that too, good idea.

It's a 1973 so wider guide, but will measure it up regardless. No play in bearings, checked spindle with a clock and it showed no movement in any direction.

Wouldnt a tailstock alignment issue show up when runing the test bar between centres though. Unless it is wear to the inner face as you mention, but closer to working area. Hmmmm, curious!!

If it's already a wide guide model then its not so likely to have more wear on the those inner shears at the working end than at the other.

Have you checked tailstock height by comparing a centre in the tailstock with centre in the headstock spindle? I suppose it's possible to have a low tailstock but still read OK on your test bar in the horizontal plane.

I would first look at tailstock alignment too.

It could be out laterally, vertically, or possibly even at an angle to the headstock.

The latter could give a good parallel turning result, but the action of advancing the tailstock barrel would result in tapping at an angle.

Hi Phil,

That's an odd set of symptoms. Being me, ahem, I'm very aware of the possibility of operator error. I'd go so far as to admit mistakes cause most of the problems I run into! Could you describe the technique learnt during your apprenticeship? The devil may be in the detail.

Dave

Your lathe can be dead on, but if the drill you use is sightly off, unequal angles, or one edge longer than the other, it will pull off straight. Your tap will follow the hole.

Ian S C

Edited By Ian S C on 23/02/2018 10:43:00

From reading your suggestions I am starting to wonder if the tailstock is indeed at a slight angle.

I'm sure though if my memory serves me good, I put the test bar in the tail stock and with the clock on the saddle, wound it along the full length and it was good. Cant recall though if I tried various tailstock barrel positions.

Is that the best way to check its running true to the bed & headstock?

I will check the centre heights are equal tonight.

So the method I learnt was using a standard tap holder and a tail stock chuck with a guide in it to suit the tap being used. Offer up the tap to the hole, take up the slack with the tail stock, then as you wind the tap in and back off slightly, adjust tailstock accordigly to keep it true. It always worked no problems.

Phil

Check spindle and tailstock centres first, per Hopper's suggestion. Steel rule flat between centres as good a method as anything.

Put the test bar in the tailstock chuck, clamp the tailstock up tight, clock base on the lathe bed, clock the test bar (zero it at one end) whilst winding the tailstock barrel in and out.

Clock it both on top and the side of the test bar, and as close as possible to the position you were tapping in.

It is the relationship of the tap travel to the work you want to check, not the relationship of the saddle movement to the tailstock barrel.

Edited By David Standing 1 on 23/02/2018 11:14:01

Ok cool, will try that. Sounding promising from what you are saying and I never did that particular test when rebuilding it last year. Thanks!!

Its all to do with how the start of the thread is first cut, if out of alignment as in not square 90 degrees it will just follow eccentrically even if holes been bored.

If its 5mm or above i would just push tail stock in and let the job draw the tap in under power. Comes in to its own with larger taps and deep depths ie have an M16X1 extended often power in 5 to 6" deep.

Are you starting with a taper tap?

The OP isn't tapping under power.

Hi again, sorry for not getting back to you sooner, busy weekend.

I started from scratch with the test bar and checked everything.

So that's in head stock checking parallel with bed, between centres running parallel with bed, in tailstock running parallel(winding in and out and sliding entire tailstock). Also fitted clock to bed and did the same as was suggested. And I set clock to zero with test bar in headstock, then switched bar to tailstock gently to see if I could get the same reading. All parameters good within a thou. I also as suggested touched 2 centres with a small piece of aluminium between them. No noticeable sway in any direction and points touch as best as I can tell with my eyes.

One thing I did notice I have a few thou of play from side to side in the barrel, which could be an issue, but my threads are worse than double this effect. Not keen on buying either a new barrel or 2nd hand body till I know its deffo this.

I still need to try the bar in a hole test and also test out my new spring loaded tapping guide.

I am always starting with a N°1 tap and am using new presto drills. Forgot to mention I have tried 3 different Jacobs chucks and 2 different clocks.

Just to double check, if the tailstock was running at a taper, this is corrected by the gib strip adjusters isn't it? Both are set good and then clamped in place with the 2 allen bolts.

Thanks guys

There's two adjustments on the ML7 tailstock. The gib strip underneath is set to run in the ways with no slack and then the allen screws tightened. There are also two screws one on each side of the tailstock body that adjust the body sideways relative to the base. This provides the offset for taper turning and must be set to turn straight otherwise.

Sounds like it has all checked out pretty good and is more likely something to do with your tap wrench or the way its set up etc.

Let us know what you find.

Just a thought, are you sure the thread on the screw being tried in the hole is true?

Following on from AJS's comment. I think you need to define what you mean by "not running true". A screwed connection - particularly a stock bolt - is not a precision assembly. Which is why, if you need any reasonable accuracy it would be normal to use a spigot at the joint and not rely on the thread.