Please help

[DellParticipant @dell]

[DellParticipant @dell]

Well I have made a couple of lathe chuck keys for my Pultra four jaw chuck out of HT socket head cap bolts and happy with the finished product, my problem was or is that I started off using the crosslide but the finish was rubbish ( more like a super fine thread than a smooth finish) I tried using indexable carbide and HSS steel but no better, tried altering speed up and down, I ended up using the hand graver as I got a much better finish , I know I only have a small Pultra instrument lathe but I should be able to get a decent finish I must admit I very rarely use the crosslide because most of what I make for antique clocks is small and I much prefer using the graver, what am I doing wrong?
Dell

[Dave HalfordParticipant @davehalford22513]

You need the tool tip width to be greater than the 'thread' that you can currently producing.

Slow down your cross slide winding hand or use a broader tool tip. Bolt steel is extra stringy stuff and gives a torn finish unless tools are very sharp.

[JasonBModerator @jasonb]

You don't say what inserts you use. I'd suggest some **GT ones worked well for someone a month or so back I suggested it to wanting a good finish on Titanium using a Sherline lathe

[GeorgineerParticipant @georgineer]

Dell,

May I politely request that you use a meaningful thread title? That way, you are more likely to get the help you are asking for.

George

[Speedy Builder5Participant @speedybuilder5]

check tool centre height

Check saddle / top slide gibs are nipped up so that tool height doesn't change when cutting load comes on.

Check cutting speed for tool / work material.

Use correct lubricant if using HSS tooling.

[DellParticipant @dell]

Thanks for the replies

I have both indexable HSS and carbide I can’t remember what the designation is but they are both this pattern

https://littlemachineshop.com/products/product_view.php?ProductID=3574&category=

with glanze holders, I have checked and adjusted the gibs, used a lubricant , haven’t rechecked if bit on centre as it was previously but will check also will try slowing feed down although I thought I was doing it slowly.

George

point taken.

Dell

[Michael GilliganParticipant @michaelgilligan61133]

I don’t know whether such things are available as inserts [but I suspect Jason could advise] … If you could grind, or find, a ‘shear’ tool … that would better-approximate the cutting action of the graver.

MichaelG.

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Edit: __ You may like the toolholder design that I linked in this previous thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=132573&p=1 

Edited By Michael Gilligan on 14/01/2023 22:12:30

[david bennett 8Participant @davidbennett8]

If you are sharpening your hss tools to look like gravers ( ie with a sharp point ) that could be your problem. You need a radius on the cutting tip

dave8

[Michael GilliganParticipant @michaelgilligan61133]

Posted by david bennett 8 on 15/01/2023 01:37:10:

If you are sharpening your hss tools to look like gravers ( ie with a sharp point ) that could be your problem.[…]

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For clarity … this is the shape of insert that Dell told us he was using: **LINK**

https://littlemachineshop.com/products/product_view.php?ProductID=3574&category=

MichaelG.

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Edit: __ and here is someone using a graver properly, on blue steel:

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Note the cutting action.

Edited By Michael Gilligan on 15/01/2023 07:03:12

[JasonBModerator @jasonb]

Big inserts for a small lathe, I would suggest something like this which is the same overall size but has a smaller 0.2mm tip radius, sold in twos.**LINK**

[HopperParticipant @hopper]

HT socket head cap screws are often a cow to machine to a nice finish. No idea why. They just often are. Plus you are using a very small, not very rigid lathe on some very tough steel. Not a good combination. Try on some mild steel of known grade and see how it compares. If the finish on that is acceptable using the same tools and methods, the problem lies in the material.

[Martin ConnellyParticipant @martinconnelly55370]

Hopper, the steel used for a SHCS is chosen for its toughness and is then heat threated to upset and form the head followed, after it is cooled, by roll forming the thread. The steel is never really chosen to be easy to machine and it has a lot of its internal grain structure distorted by the forming and rolling processes. This produces a quality socket head cap screw that is fit for purpose as long as that purpose is not (as many of us know) to be raw material for something else.

Martin C

[old martParticipant @oldmart]

With a small lathe and difficult material, you should expect to have to hand finish the keys. You only make them once, and your normal work is very different and more suited to your machine.

[david bennett 8Participant @davidbennett8]

Michael, as I read it, he ended up using the inserts after an unsatisfactory finish.

dave.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by david bennett 8 on 16/01/2023 02:29:21:

Michael, as I read it, he ended up using the inserts after an unsatisfactory finish.

.

Then I think we must ask Dell to describe each of the attempts he made, in a little more more detail

… as I read it: he ended up using a hand graver [with which he is familiar] to get a decent finish.

MichaelG.

[Les Jones 1Participant @lesjones1]

I don't understand why a good finish is required on a chuck key. If you wanted a polished finish why not just use emery paper ?

Les.

[DellParticipant @dell]

Here is a picture of my setup but please bear in mind that I have set it up with the finished product, as I turned it down prior to milling the square, the first picture is the HSS bits I used and the carbide were the same pattern.
Dell

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the photos, Dell … they appear to support my original reading

MichaelG.

[JasonBModerator @jasonb]

That's a smaller size insert than your previous link, so you would want something like this for a CCGT option.

From the picture I would have the tool at right angles to the lathe axis, the more you angle it the more "pointed" the tool is.

[DellParticipant @dell]

JasonB

i had to have it at that angle otherwise it didn’t clear the live centre when turning down the end where the square is , also I moved the fixed steady near to where I was trying to cut .

thanks for replies, as I said it’s not something I will need to do very often, the next time I need to do something similar I will try the advice.

most of my work is with a graver IE making small screws, arbors, and other clock related parts.

many thanks Dell

[Howard LewisParticipant @howardlewis46836]

Going back to basics.

Your Pultra does not have a Leadscrew.

So it is not possible to arrange a power feed for sliding or facing. The feed rate is literally in your hands.

(Just like a training lathe such as the larger Lougborough )

At the risk of teaching granny to suck eggs.

The bad news is that if you are getting an obvious "screw thread", all other things being equal, (i e. the material is not some nasty exotic "Wont machineium" ) you are feeding too quickly. It is assumed that the tool is sharp, possibly with a small nose radius, and mounted exactly on centre height.

Bear in mind that moulded carbide tips are not absolutely sharp. A HSS tool can be ground and honed to be sharper.

Carbide tips are not ideal for shallow cuts at fine feeds, although they will do it, given the right conditions.. They were developed so that industry could remove lots of metal, as quickly a possible.

The objective is to feed at a lower rate per rev than the nose radius (Remember that a large nose radius tends to encourage chatter, so keep things as rigid as possible) .

Whatever the feed rate, you are going to generate a helix. The objective is is produce one of such fine pitch that the radiused cuts overlap without leaving any sharp peaks.

One of the first things that we were taught as Apprentices was how to produce a steady, slow, manual feed.

Spend time, practicing this, on "scrap" metal, gently feeding from one hand to the other You are trying to reproduce the steady consistent motion that a low power feed would give, with a shallow cut (Again less than 0.005" ).

For a fine finish, you are looking to feed at about 0.003 – 0,004" per rev. (As an example, to produce a 40 tpi Model Engineer thread the tool would need to be fed at 0.025 per rev ) And you are looking for something much finer than that.

So start learning with the lathe running at a low speed. Speed up as you become more proficient.

We are all learning!

Practice will make perfect. (A friend who recently bought his first, basic, lathe has learned this so well that now he is better at it than I am! )

Keep trying, and you will succeed.

Howard

[Nigel McBurney 1Participant @nigelmcburney1]

where I worked and was trained we had some plain lathes,including a small pultra,if anyone was caught by the boss turning something like a tough bolt,silver steel or indeed any steel type they would have got a right telling off for putting such a strain on an instrument making lathe,as I did when I started to make a silver steel pin as the Boxford was not avaailable,I got a right old bol….g I never did that again, misusing such a fine lathe should not be contemplated,although finishing clock spindle ends is acceptable using a graver as very little metal is removed,its more of a polish.

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