Unwanted Taper on Big End Bolts

[martyn nutlandParticipant @martynnutland79495]

Although this was recoverable by a bit of contrivance I would appreciate the experts' observations on what was wrong so as to improve my technique.

I am making automotive big end bolts from an 18 cm long, half inch bar of EN24T. They are 47mm long under the head (12.8 mm diameter) and I'm turning down the bar to 7.6mm with a view to cutting a 5/16 BSF thread down 15mm of the 47.

I have the bar in a collet so there is a respectable amount extending through the collet and into the lathe spindle. I have the opposite end supported by a revolving centre. I'm following the cut left to right using the compound with a straight indexable tool (90° to the tool post) with about 20mm protrusion from the four-way post. I have the compound set at zero, the cross-slide locked for each pass and the carriage/saddle locked.

I think this is a fairly rigid set-up.

However over the 47mm I'm getting a 0.8mm taper from thread end to head – smallest at the thread end. Can't understand that.

I'm running at 375 rpm and taking 0.1mm finishing cuts.

Many thanks in advance for any observations.

[martyn nutlandParticipant @martynnutland79495]

[LambtonParticipant @lambton]

Martyn,

My first thought is that perhaps your revolving centre is not properly aligned along the lathe axis.

Also big end bolts with cut threads are normally not considered suitable due to the stresses involved when the engine is running that can cause failure. Have you tried to get some HT rolled thread ones? Yes I realise BSF bolts of any type are hard to find. How many do you require as I might have a few?

Eric

[Anonymous]

I assume you're using the compound slide to make the cut, as the saddle is locked? Lock the compound slide and use the saddle/power feed instead.

Also spindle speed is pretty low for insert tooling. What is the nose radius on the insert?

Andrew

[Phil PParticipant @philp]

Martyn

I am guessing these are for the Austin 7 ?

I think I would be tempted to buy a set if it was me, they can be had from various suppliers complete with locking nuts.

Here's one I picked at random

http://www.a7c.co.uk/spares.php

Big end bolts need to be "right", there is too much to risk if you get a home made one failing in service.

I applaud you for having a go at making them, but please be careful.

Phil

Edited By Phil P on 06/01/2016 13:06:05

[JasonBModerator @jasonb]

As Andrew says I would use the carrage under power to do the 47mm cut. If you want to use the compound then don't go by the scale unless you know it is right, I always clock mine in with the DTI

I'd be going about 800-1000rpm

[Martin ConnellyParticipant @martinconnelly55370]

There are a lot of people that will take the view that if you are using a collet or chuck you should not also use a centre in the tailstock. Any slight error in the tailstock alignment will introduce a distortion in the bar and result in odd errors such as you are reporting.

As Andrew said you should be moving the saddle and with power feed if you have it.

Have you either bought or made an accurate test bar to check tailstock alignment? How did you align the tailstock or have you ever checked it in any way?

How accurate is the 1/2" diameter of the bar and is the collet an exact match to it. Is the collet and its seat perfectly clean and free of damage?

What rpm are you running at? Carbide inserts work best at high speed and the speed for a diameter like this with carbide inserts may need to be as high as 2000rpm for a good finish. They also need a high feed rate to get the best from them otherwise they can rub since they are relatively blunt compared to a well sharpened and honed piece of HSS.

If you have a plain bar to put a thread on you could turn it between centres. This gives the option to measure the thread and put it back in the lathe with the correct positioning to cut some more depth in the thread. Once again this requires that the tailstock is properly aligned but that can be checked as you start to reduce the bar, it should be parallel after each cut, A steady rest can be used if the bar is going to get a high slenderness ratio as its diameter reduces.

0.1mm cuts on a Ø7.6mm workpiece may be a problem for carbide inserts. The centre of the workpiece will try to move away from the tip and so cut less than at the constrained ends, have you any HSS to use for these finishing cuts? It will be probably be sharper than a carbide insert and the force pushing the workpiece away will probably be lower as a result.

Martin

[martyn nutlandParticipant @martynnutland79495]

Many, many thanks for all of that.

Extrapolating – I think it is deflection is the problem. The revolving centre I'm using is second hand, old and 'grumbly'.

The bar may not have been straight and I didn't check and should have.

The machine is relatively new and I have been very careful with it so the compound and tailstock ought to be within acceptable limits.

It would be better to use the carriage, I agree. The handle for the compound is close to the tailstock and awkward to turn and thus to deliver an even cut. Am now using the carriage on automatic feed. Much better.

I have now lightly cleaned the bar and 'miced' it and am repeating that measurement periodically as I reduce the diameter and if there is any discrepancy taking a pass on the previous setting and that seems to have eliminated any taper and the finish is very good.

The bolts are for the Austin and I take the point about big end bolt breakage. In fact, the exercise is about that very point. I have the old bolts and I could put them back and they probably wouldn't ever break. After all, whoever dreamed of renewing big end bolts in 'the old days'. However, I'm thinking new bolts from new steel of the correct spec are a few percentage points less likely to 'let go' than an old one.

Thanks again for the guidance everyone.

Martyn

[duncan webster 1Participant @duncanwebster1]

No-one has pointed out that 5/16" is 7.94 mm, not 7.6, although I doubt losing the tops off the threads will matter, the shanks will be less good fit in the holes. Of course the originals could be 7.6mm, in which case I'm talking rubbish, not unusual!

[Russell EberhardtParticipant @russelleberhardt48058]

A7 big end bolts are not highly stressed unless you're buiding one of the racing engines so a machined thread is fine.

It's a good idea to neck the bolts like this:

Make a few extra for testing and tighten one up in stages, measuring the length before and after until you reach the torque at which there is a slight permanent stretch. Then you can safely torque the others to about 20% less when fitting. If you are using the split pin and castle nut variety don't overtighten to reach the next slot – undo and skim the nut!

Russell

[Anonymous]

Posted by martyn nutland on 06/01/2016 14:49:13:

However, I'm thinking new bolts from new steel of the correct spec are a few percentage points less likely to 'let go' than an old one.

That doesn't necessarily follow. It depends upon how the original threads were formed – rolled or machine cut?

Andrew

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by martyn nutland on 06/01/2016 14:49:13:

The machine is relatively new and I have been very careful with it so the compound and tailstock ought to be within acceptable limits.

However new the lathe, the zero mark for the compound slide (I'd call it the topslide) is only an approximation. As Jason has said, in order to turn parallel with the compound slide it needs to be set very carefully using a known parallel bar held between centres and a dial indicator, usually held on the toolpost, The compound slide is nudged around until there is no deviation on the dial indicator as it moves along the parallel bar.

You are clearly new to this game. May I suggest that you get one of the books on using the lathe. Many of them seem a bit old fashioned but they are all relevant. My particular favourite is "Using the Small Lathe" by Len Mason . Don't be put off by the "small" bit, it is relevant to any lathe found in the home workshop. However, do keep asking questions, we all have to learn and there are some good mentors on this site.

HTH

Rod

[Tony Pratt 1Participant @tonypratt1]

Posted by Martin Connelly on 06/01/2016 13:16:25:

There are a lot of people that will take the view that if you are using a collet or chuck you should not also use a centre in the tailstock. Any slight error in the tailstock alignment will introduce a distortion in the bar and result in odd errors such as you are reporting.

Sorry Martin anyone with turning experience would take the opposite view, I would either use a decent revolving centre throughout or switch to a dead centre for finishing, rough out with one tool then finish with a different 'finishing' tool, use carriage power feed & coolant if available.

You should be able to get rid of the taper by adjusting the tailstock 'set over'

Tony

[martyn nutlandParticipant @martynnutland79495]

Thanks for the further comments.

I think the problem was deflection, caused, as Eric said, by the revolving centre. I changed this for a brand new scroll type self-centring tailstock chuck, gripping on what will form the bolt head and on the third bolt of the batch had no discernible 'taper' and a mirror finish.

But a further lesson is in planning the job. On the first three I got two shanks end to end and that is clearly going to exacerbate any deflection. When I do the remainder, I will arrange the bar(s) so the bolts are machined head to head thus minimizing the length of thin section exposed to the tool.

Thanks again for the advice. A happy outcome for once in life!

Martyn

[Martin ConnellyParticipant @martinconnelly55370]

Tony, I have used a centre with collets and chucks as well, I was referring to a discussion here http://www.practicalmachinist.com/vb/south-bend-lathes/turning-between-centers-vs-chuck-130393/

If the centre is not aligned to the spindle axis of rotation it can move the part off the centre of rotation and so cause problems, that was why I was asking if the tailstock alignment had been checked. The original question is under the heading of beginner questions so it seemed to me to be a legitimate query to the op.

Martin

[Tony Pratt 1Participant @tonypratt1]

Martin,

I am aware of the points made in the link you provided although the chuck/material concentricity is immaterial.

If the work is faced off in position and then carefully centre drilled in theory the centre drilled hole will be running concentric to the spindle rotation.

If then a perfectly aligned tailstock centre is then applied to the centre hole the part will not be bent off line, if the tailstock is not aligned correctly the part will be bent towards or away from the operator and a tapered part will be the result, this can be corrected by tailstock adjustment.

I still say tailstock support is the way forward or even between centres would be better but perhaps not for a beginner? If this work was attempted without support I am sure it would end badly.

I know the above is perhaps an over simplification of the situation but others who feel qualified are free to comment.

Tony

 

Edited By Tony Pratt 1 on 06/01/2016 19:41:59

[Martin ConnellyParticipant @martinconnelly55370]

So what is needed is a perfectly made centre drilled end in a perfectly straight bar mounted in a perfectly made chuck or collet perfectly mounted in the headstock with the end located by a perfect centre in a perfectly aligned tailstock and all this done by a person asking questions as a beginner. Perfect!

Martin

[Tony Pratt 1Participant @tonypratt1]

Posted by Martin Connelly on 06/01/2016 21:14:51:

So what is needed is a perfectly made centre drilled end in a perfectly straight bar mounted in a perfectly made chuck or collet perfectly mounted in the headstock with the end located by a perfect centre in a perfectly aligned tailstock and all this done by a person asking questions as a beginner. Perfect!

Martin

You said it not me, I hope my answer was of some help to the OP and he took it in the spirit that it was intended. I won't respond further to your infantile post.

Tony

[martyn nutlandParticipant @martynnutland79495]

Please don't let's get unpleasant about this.

I did take everything in the spirit it was intended including being told, rather unflatteringly, I was 'clearly new to the game' and should read a book! (I've been studying and referring to Sparey's The Amateur Slave (sic) for at least 10 years along with many others including Hall and Using the Small Lathe, and will continue to do so.)

The problem, as far as I can see, was the revolving centre. I was generously given it by a retired professional engineer who never claimed it was perfect. However, which I didn't realize, it was worn out and I will buy a new one. As I said earlier, when I supported with a brand new (because it's not something you use everyday) revolving scroll chuck in the tailstock my problems were largely solved.

The collet, the arbor and the clamping nut were virtually new and clean, as I pride myself on keeping my tools that way.

It was a mistake to work the compound and not the carriage, but I was addressing a space and logistics issue. When I did use the carriage, and was thus able to engage automatic feed, the finish was transformed. Any taper or lack of parallelism that is there now (and we are not looking for tenths of thous here – to loosely quote Brown & Sharpe in Instructions for Young Engineers: 'don't spend hours agonizing over a dimension that's not critical&#39 is down, I feel to the configuration. I.e don't work shank to shank, which means a pass of around 100mm with cut off allowances on a diameter of less than 8mm, but work head to head so the pass is half that and much less likely to deflect the work.

I'm happy. So let's not be saying people are infantile!

Martyn (with a 'y&#39

PS Liked the photo of the double-waisted bolts. Need one like that for the radius rod anchorage. Wonder how it's done?!

[steve de24Participant @stevede2433577]

Martyn, I side with the previous posters urging you to consider buying bolts with rolled threads. Machine cut threads could easily reduce the fatigue strength of the bolts by a factor of 2 or more (and that would be with aerospace quality machining/inspection etc). Making sure the bolts are correctly torqued up when fitted is the other important factor to get right with regard to the fatigue strength of the bolts. Steve

[martyn nutlandParticipant @martynnutland79495]

Okay.

Job abandoned.

Can we please leave it there now?

Martyn

[Ian S CParticipant @iansc]

They should have rolled threads, but they also should be "fitted" bolts, a close locating fit in the holes.

Ian S C

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