Getting things parallel

Finally got back in the workshop this aft after 2 or 3 months of working with my other modelling head on (this is what happens when a hobby becomes a job grrr)

Now I'm struggling to get a piece of metal totally parallel. Have cut a piece of 1" x 1/2" off a length and then it goes like this...

1. check for parallelism on the surface plate (a large piece of thick glass in my case)

2. OK its close but not spot on so place it in millling vice and skim one long side. This is now used as the reference for all the others.

3. Its quite narrow so only one parallel can be used and I turn it over and place it on the (homemade) parallel and mill to the correct width.

But when I check it on the surface plate theres a 0.2mm difference from one end to the other grrrrrr

In this case I've left it as its not massively important but its annoying as I like things to be right. Going on to mill top and bottom ends, I find that a square shows a little daylight between side and top, much more noticeable from one side than the other the implication being that my part 3 wasn't accurate. Could be the home made parallel but that showed square on the surface plate

Ideas anyone?

i think your milling vice is 'out'. not an uncommon problem, and even with a dodgy vice not difficult to sort out and deal with once the problem is know. often the error only occurs once the jaws are tightened.... everything appearing level till then, the raised sliding section lifting. if your parallels are parallel then it cant be them!

I found that although my Drummond is a very stiff small lathe I couldn't get good quality parallelism

I suppose the bigger and stiffer a machine is, lathe/mill, the better your results

My own solution was a small Adept 2A shaper

Even a small shaper is great for flatness and parallelism because you can skim off such tiny amounts

Otherwise hand finishing was all I could think of, and I aint going down that route

It was when I made a steel vice that I was alerted to the problem.

I needed the base flat and parallel with the bottom, and the jaws flat and parallel at 90 degrees at the same time

The tiny amount of vibration from milling was also fatal for a good finish, you can see the shaper finish

on the left and the milling marks on the right

The other big advantage I found was that I could skim off the teeniest weeniest amounts of metal when required, removing a ding or error only down to its minimum depth

I was quite impressed with the accuracy I could achieve, even as a noob, but shapers are specifically designed for this kind of job and make it relatively easy

Setting up a parallel job is also quite easy, because a shaper can scribe a very very light mark along the full length of a good edge on one side, then you can track across and do whatever needs doing on the other side or wherever

Edited By Ady1 on 17/06/2012 01:21:48

Thats nearly 8 thou difference, whatever is wrong should be fairly obvious.

To test your home made parallel simply turn it end for end use a DTI/re-cut and see where or if your error shifts.

If your mill has a moveable head have you trammed it? is the vice out? if it were me I would pop a DTI in the spindle and take a few measurements.

I hear what you are saying about wanting to get it right, though it's all too easy to waste time searching for that last tenth.

Hi Wolfie.

It sounds like the piece you are trying to machine is lifting when you tighten the vice .

Did you check to see if the parrallel is loose when you tighten the vice as the piece you wish to machine should be pulled down onto the parrallel and the parrallel should not be able to be moved easily .

I use a small plastic hammer (brass is also good) to lightly tap the piece down as i tighten the vice and then i check to see if i can move the parrallel and if i can i try again until the parrallel is tight.

You can set up a DTI over the piece and check for lift as you tighten the vice .

If your parrallel is good you can also use it tram the vice to make sure its fixed jaw is parrallel to either the x or y axis and the vice bed is parrallel to the top of the mill table .

You didn't mention what type of vice you are using as some are notorious for this problem and the one i got with my machine is now just a handy doorstop !

Ian

I tend to mill most things, especialy thin plate, held in aluminium soft jaws, where you machine the registers directly into the ali, so the plate must be laying completely flat and square in both the X & Y axis. To overcome moving jaw lift, clamp the jaws apart with a suitable piece of material, so the register cut will be exactly the width of the plate.

HiiWolfie,

Welcome back, I've missed your pointed questions and the debates they stimulate. As for milling parallel, forget the vice. I beleive that you have the Harold Hall milling books. In his books he shows a method of making parallels from mild steel by bolting a couple of mild steel support buttons to an angle plate clamped to the table. Then a skim cut is taken over the buttons and the material to be machined is supported on them and clamped to the angle plate. You can then skim your workpeice and it shold be parallel. Harold reckons that you should be able to get accuracy to within 0.002mm which is pretty good.

My Milling machine has a large enough table so that I can keep an angle plate and a vice permanently mounted and find I tend to use the angle plate most for certain operatiions where accuracy is important. I don't rely on any vice for precision - at least the ones I can afford.

Reference is "Milling: A Complete Course" - Harold Hall, page 39 on

Best regards

Terry

Edited By Terryd on 17/06/2012 09:09:58

Its a Soba milling vice I bought at Harrogate

@Terry, thats exactly how I made my parallels.

Edited By Wolfie on 17/06/2012 09:11:52

Hi Wolfie, there is one thing being missed here. Sometimes you get metal that has a 'Skin' on the outside and as you machine away one side it starts to 'Bow'.

Brass is known for this and the only answer is to anneal Brass.

You can heat your piece of steel to cherry red and let it cool down slowly.

This will take any stresses out of it and you may find it will machine flat and not warp.

Tapping down into the vice is also reccomended.

Clive

To anneal, it's handy to have a log burner. If its cold rolled steel, put it in the fire, and leave over night.

When milling, make sure you have ALL of any burr on the edge that is seated in the vice, inspect it with a magnifing glass, just to make sure. Ian S C

Wolfie, back to basics, clock the vice bottom face that the parallels are going to sit on and then clock the parallel top face. If both read zero-zero end to end then it follows your work will be parallel if indeed it is sitting correctly in the vice. As has been said .008" is a lot of error and something should soon be obvious, finally the head being in or out of "tram" won't effect the mills ability to cut parallel but will cause other problems, but one thing at a time!

Tony

There is no alternative, if you are milling anything from steel plate or bar and you really want/need to stay accurate after working it into a shape,, you simply must anneal it first before you attemt to do anything. The internal stresses caused by hot rolling and cooling are there throughout the lump, not just a skin and they need to be relieved.

Heat on a forge or similar hot working surface to bright red for 1 hour per i inch of thicknees, cover with firebrick, thermalite block or similar and allow to cool slowly until you can pick it up. Then cut to shape etc.

Bright mild steel is cold rolled and does retain internal stresses that are relieved locally by any type of machining causing distortion that is almost impossible to correct.

The trick is to use hot rolled "black" mild steel wherever possible as is almost totally free from internal stress. It is also a lot cheaper than bright mild steel.

I agree with Clive and others who suspect distortion; it's amazing how a bit of straight bar can go bananas when you skim one side.

Neil

An interesting method of producing parallel sides to thin section (according to size) is to machine the part close to size. Then generate a flat surface on a piece of spare material using a fly cutter,or similar.Using double sided adhesive tape, the part can be stuck to the slave plate. By using common sense and a bit of for thought, measuring points can be made accessible, and some cutting forces be met with the application of stops. Light cuts only!