KWIL | 17/05/2020 11:15:19 |
3681 forum posts 70 photos | I think I would start by stripping and cleaning/examination of all the jaw faces, guide slots etc. |
Pete Rimmer | 17/05/2020 11:31:40 |
1486 forum posts 105 photos | Posted by John Baron on 17/05/2020 10:45:57:
Hi Guys, Pete is the winner I think. [quote] The chuck body runs true but the jaws do not hold the part straight. If all of the above check out OK, this is the last option.[/quote] Comments below the pictures. By the way I somehow managed to take a video but the forum won't let me upload it. I used a 1" inch parallel to check how straight the jaw was. I used a piece of postcard and a small dowel to hold the parallel tight up against the jaw face. I set the dial gauge so that it was level with the edge of the parallel. In this picture you can see the arrangement. This is jaw No:4. I used the lower jaw to support the parallel. In this picture I've run the dial gauge down to the end of the parallel. It shows that its moved away by about 0.6 mm. Sorry about the colour, fluorescent lighting. This jaw No1 was the worst one. I stopped taking pictures at this point. All the jaws have some runout when measured in this way ! I'm not sure why but they all seem to run away from the tool holder. The worst ones were jaws 1 and 2. When I gripped the parallel by the flat sides you could see that the indentations in the cardboard were deeper on one side.
It also explains why I can set the work piece true at either end and its out at the other.
So it seems that I've got the source of the problem... badly or wrongly ground jaws. After all this time I doubt that there is any point in having a word with Pratt Burnard.
To answer some questions: Yes the chuck is the original slim body one that was supplied with the lathe when I bought it directly from Myford. They arranged with Denford to deliver and set the lathe up. At the time Denford made bridgeport mini mills that were badged and sold by Myford. Its had three homes since I bought it.
Thanks for all the comments and suggestions. Now to think about what I'm going to do about it.
Edited By John Baron on 17/05/2020 10:48:16 I'm sorry John but that test you show could be reading bed twist (or wear). All of the checks you make need to be rotational checks. Your issue is one of runout, so all your checks need to be for runout. Edited By Pete Rimmer on 17/05/2020 11:32:54 |
John Baron | 17/05/2020 11:57:38 |
![]() 520 forum posts 194 photos | Hi Pete, I'm getting a bit lost now ! The idea behind that last run of checks with the parallel were intended to check that the face of the jaws was square to the axis of the lathe. Which they don't seem to be. I cant get my head around being able to swap to the three jaw and turn a bar parallel over roughly five inches in length with less than a couple of tenths variation from end to end. The thing that really bothers me is why then when I check each jaw the parallel seems to move away from the gauge. To me that suggests that the jaws have a taper running away towards the back of the chuck. I've checked the guides in the chuck for each jaw and they are virtually the same reading from the outer to the inner. Maybe a division at most. I'm going to take the jaws out and see what I can find, I did notice that the two jaws with the greatest deviation were the most loose in the guides. Thanks for your help. Its very much appreciated.
Edited By John Baron on 17/05/2020 11:58:53 |
Neil Wyatt | 17/05/2020 12:53:13 |
![]() 19226 forum posts 749 photos 86 articles | If both sides of the parallel run away from the tool holder then it isn't the chuck at fault. If the parallel is parallel then opposite sides should go in opposite directions. Something is not right with your test. Neil |
Pete Rimmer | 17/05/2020 12:54:02 |
1486 forum posts 105 photos | John, Don't be dismayed it can take a bit of getting your head around. Firstly, you don't want to check that the jaws are parallel to the axis of the lathe, they need to be parallel with the axis of the spindle. The spindle might not (probably is not) parallel to the axis of the lathe. The bed might not be free of twist, it might (will) have wear or the headstock might not be exactly aligned with the axis of the lathe - easily possible with the the Myford 7 because of the flat ways. For all these reasons you cannot rely on any measurement which involves moving the saddle. Your problem is that the axis of the test bar is not parallel with the axis of the spindle when it's turning. I would suggest that you work through my points 1-5 above and eliminate each in turn. You could start by testing the face and OD of the chuck, because being a fairly good quality item so long as it has been handled well it should run very close to true. If the face runs out, you have a register issue on the chuck or on the spindle. If the OD runs out you might have a burr on either register or trapped swarf in the threads. In any case I would start with that check and if you find an anomaly, remove the chuck and check for dirt, swarf and bruised registers. If you check the chuck and find no runout on either the OD or the face, leave the jaws and check the bar on vee blocks. You are quite right in that you will never turn your test bar in the 3-jaw, that is a between-centres job.
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Hopper | 17/05/2020 13:18:30 |
![]() 7881 forum posts 397 photos | Quick check. Whip the jaws out and check them with an engineers try square held up to the light to make sure the gripping surface is square to the base surface. And mike the distance fron the base to the slot that engages in chuck body to make sure slot runs parallel to base surface. I have heard of jaws that were ground out of square before. Vaguely remember someone setting them up on a surface grinder to correct Hard to see bed being out of line or twisted if your test cut along a bar held in three jaw with no tail centre turns parallel within a couple tenths of thou as you have said. PS if the lug that engages with the threaded chuck barrel interferes with the try square mount yhe jaw on parallels on a flat surface such as surface plate, sheet of glass or lathe bed itself and stand square next to it. Edited By Hopper on 17/05/2020 13:33:45 |
John Baron | 17/05/2020 14:40:12 |
![]() 520 forum posts 194 photos | Posted by Neil Wyatt on 17/05/2020 12:53:13:
If both sides of the parallel run away from the tool holder then it isn't the chuck at fault. If the parallel is parallel then opposite sides should go in opposite directions. Something is not right with your test. Neil Sorry Neil, but your post makes no sense ! A parallel is parallel. The in the set up the parallel is only pressed up against the face of the jaw nearest to the gauge. There is a dowel pin on the other side to ensure that the opposite face isn't affecting it only holding it square.
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John Baron | 17/05/2020 15:00:18 |
![]() 520 forum posts 194 photos | Hello Hopper, Guys, That is exactly what I have done ! The amount of muck and chippings that came out from behind the jaws surprised me. Thinking about it this is what I should have done in the first place. I've discovered that what I thought should be a matched set of jaws, isn't even though they are all stamped with the same number. Unlike the three jaw chuck where the three jaws have the same ID as the chuck. Anyway I believe I confirmed the results of the measurements from the dial gauge.
This is jaw number one set against a M&W engineers square. Jaw number 2. Jaw number 3. Jaw number 4.
Some more pictures. You can see that these are not even or parallel. If you look closely the third jaw is a fraction smaller and the ground edges are not symmetrical. A slightly different angle to get a better view. I think these are horrible ! Nothing like I expected. When I put them back in the chuck, I'm going to turn them round and check the other faces. My opinion is that these jaws are effectively worn out. They should all be the same hight and square. Thanks again.
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Pete Rimmer | 17/05/2020 15:14:41 |
1486 forum posts 105 photos | Give me the dimensions off your jaws I have three loose sets here. That is not to say that I am agreeing with your diagnosis, there could be any number of things going on there, but if I have a suitable set you are welcome to them. |
John Baron | 17/05/2020 15:35:47 |
![]() 520 forum posts 194 photos | Hello Pete, First I must thank you for your very kind offer and your wise words. I'll do a cad drawing of the jaws later. But before I take up up on your offer I am going to put them back in reverse direction in the chuck and go through the process of measuring them again. There are a lot of sharp edges on these as well that could do with fettling. I wouldn't have noticed the issue if I hadn't been in the process of making a grinding spindle and had machined one end of the body, turned it around and discovered that it was no longer true. Fortunately I've not attempted to machine the other end. I suspect that it will become a piece of expensive scrap. To all that have offered their help and comments, Thanks Guys.
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Oven Man | 17/05/2020 17:44:18 |
![]() 204 forum posts 37 photos | Should the slots in the jaws be the measuring datum rather than the base? Peter |
John Baron | 17/05/2020 20:12:01 |
![]() 520 forum posts 194 photos | Posted by Oven Man on 17/05/2020 17:44:18:
Should the slots in the jaws be the measuring datum rather than the base? Peter I wouldn't think it matters since the slot and the back are parallel, but I can check and measure it.
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Michael Gilligan | 17/05/2020 23:10:33 |
![]() 23121 forum posts 1360 photos | Posted by John Baron on 17/05/2020 15:00:18:
[…] Anyway I believe I confirmed the results of the measurements from the dial gauge. […] I think these are horrible ! Nothing like I expected. […]
. If they are as bad as they look in your photos : My suggestion of using Plastigauge to check the grip wouldn’t be much use !! ... I was expecting you to find perhaps five thou’ variation along the gripping length of each jaw. MichaelG. |
Hopper | 18/05/2020 01:44:15 |
![]() 7881 forum posts 397 photos | Posted by John Baron on 17/05/2020 20:12:01:
Posted by Oven Man on 17/05/2020 17:44:18:
Should the slots in the jaws be the measuring datum rather than the base? Peter I wouldn't think it matters since the slot and the back are parallel, but I can check and measure it.
Needs to be checked for sure. Or put a parallel in the slot and sit the square on the parralel. |
John Baron | 18/05/2020 08:51:05 |
![]() 520 forum posts 194 photos | Posted by Michael Gilligan on 17/05/2020 23:10:33:
Posted by John Baron on 17/05/2020 15:00:18:
[…] Anyway I believe I confirmed the results of the measurements from the dial gauge. […] I think these are horrible ! Nothing like I expected. […]
. If they are as bad as they look in your photos : My suggestion of using Plastigauge to check the grip wouldn’t be much use !! ... I was expecting you to find perhaps five thou’ variation along the gripping length of each jaw. MichaelG. Hi Michael, These look like what I would call "Friday night" work. Even if the chuck was second hand, I find it hard to believe that each jaw would be that bad. Using a spring clamp to hold the square I can pass a 20 thou feeler gauge through at the back on the worst one but only a 5 thou through the best one. Also I noticed when taking them out of the chuck the two worst ones were the loosest in the slots. Thanks for your suggestion. I do have some plastic strip that I was given that works like Plastigauge. But I've never used it.
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John Baron | 18/05/2020 08:57:43 |
![]() 520 forum posts 194 photos | Posted by Hopper on 18/05/2020 01:44:15:
Posted by John Baron on 17/05/2020 20:12:01:
Posted by Oven Man on 17/05/2020 17:44:18:
Should the slots in the jaws be the measuring datum rather than the base? Peter I wouldn't think it matters since the slot and the back are parallel, but I can check and measure it.
Needs to be checked for sure. Or put a parallel in the slot and sit the square on the parallel. Hi Hopper, Thanks for your suggestions. You and I seem to think alike
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Nigel McBurney 1 | 18/05/2020 10:53:38 |
![]() 1101 forum posts 3 photos | I had a problem with Burnerd 3 jaw,same as the four jaw,bought new when I bought the Myford dividing head,this allowed me to hold work in the head and use the lathe 3 jaw to hold the cutters,.twenty years later the 3 jaw on the lathe was a little worn and the dividing head was not used as I had a universal dividing head for the mill.It was then that I found that the new chuck had a runout of around 1.5 thou, the one that I had used on the lathe initially had virtually zero runout. Too late to have a go at Burnerd so I mounted the new one on a back plate and got it dead true,and is still a good chuck. Manufacturers do make errors,quality is/was always a uk problem.Had a similar problem with a very expensive bore micrometer,this had two sets of jaws to cover the measuring range,this was a company purchase so it went to the standards room for calibration,one set of jaws gave a perfect reading,the second set had a serious error, and accordingly rejected. I dont know what happened as I moved on to another project.So if you buy precision tooling for your workshop ,check it over if you have the equipment to do it. Now there is a way to correct these jaws in the four jaw burnerd,find someone with a surface grinder find out the run out error on each jaw set up the jaw in the vice on the grinder with a dial gauge set the jaw with same amount of run out and grind the jaw true,do the jaws one at a time, I once trued up a 10 inch four jaw in this manner so that it could grip round material.I do not believe in using tool post grinders on a lathe,grinding dust and lathes do not mix no matter how careful you are, I once had to use a toolpost grinder at work to grind a long cylinder,and then found how grinding debris gets everywhere, being at work I used it as I was told to ,though it was never used again all the time I was there. |
John Baron | 18/05/2020 11:16:17 |
![]() 520 forum posts 194 photos | Hi Nigel, Guys, Actually I've had a conversation with a friend of mine who is the works forman at a major engineering works discusing the issue with my chuck jaws. Normally he would have said "bring them in and I'll sort it" but because of the current problems few of the staff are in and he says that they are pulled out with work that is late and urgently wanted. His advice is, because it is an independent jaw chuck, to use a toolpost grinder and true each jaw individually whilst in the chuck and to use a "G" clamp across a pair to stop any movement, also to lock the spindle with a block making sure that it was horizontal. He also told me that it didn't matter if each jaw was very slightly different from each other. He suggested that 20 thou gap on the inside was probably due to dirt or something underneath when it was ground. Which if you think about it, it doesn't, because each jaw can be moved independently of the others.
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Pete Rimmer | 18/05/2020 17:57:16 |
1486 forum posts 105 photos | John, Give me the jaw width, slot width and tooth pitch of your jaws. I'll check what I have. Also, a better check than using the square is to put two jaws on your surface plate with the gripping faces against each other, that way you aren't measuring any error in the square. Finally, if those two jaws really are touching at the tips whatever you clamp in them (lightly) should rock about in the chuck if you wrench the other end about with your hand. You should easily feel (or hear) this. Does it happen? |
old mart | 18/05/2020 19:04:33 |
4655 forum posts 304 photos | The pictures of the chuck jaws and the square are totally meaningless. The rear of the jaws has probably been linished to remove burrs at the edge of the teeth. The squareness that matters is between the slots and the jaws and only then if they are in the chuck body and are stressed in the clamping direction. There was mention of soft jaws being bored to fit the test bar. Did that confirm an error in the lathe bed? |
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