Backplate

[Steve355Participant @steve355]

My main 3-jaw chuck is too big for my little lathe. It actually came with the lathe, but without a backplate, so a while back I made a cast iron backplate for it. It was one of the first projects I did with the lathe, and it worked out pretty well. But the fact remains that the chuck is too big.

So I’ve kept an eye out on eBay for a suitable, small, low profile chuck with the required 7/8” thread and recently one came up as 7/8” BSF, so I negotiated a cheap deal for it. I thought it likely that in fact it was BSW and the seller was wrong. However, it turns out that the seller was correct, so now I have a nice little chuck but with a 11 TPI rather than 9 TPI backplate.

So I have a few options:

1) run my 7/8” 9 TPI tap through it to “convert” it to BSW.

2) buy a piece of (bright steel?) rod slightly larger than the exactly 2.5” that it currently is and proceed in the usual fashion to make a backplate. Existing backplate can be used as a template for the holes.

3) CNC a new one

Thoughts ?

 

[duncan webster 1Participant @duncanwebster1]

Do it properly, make a new one. Whether by CNC or manual is irrelevant

[BazyleParticipant @bazyle]

The best thing would be to find out what lathe it is actually for so it can be passed on to someone who probably is desperately looking for one. Then start again.

We could do with a reference table on here (or Lathes.co.uk) of all nose threads.

[BazyleParticipant @bazyle]

we’ve been here before
https://www.model-engineer.co.uk/forums/topic/lathe-chuck-thread-size/#:~:text=jim1956,form%20as%20Whitworth)%20but%2011tpi.

 

[old martParticipant @oldmart]

Can you modify the backplate from the oversize chuck to fit the new one on?

[Howard LewisParticipant @howardlewis46836]

32/4 BSw x 9 sounds like an early Myford ML1, 2, 3, or 4.

Could you bore out the “new” backplate and bush, before boring and tapping to 3/4 BSW?

The tapping will be hard work, but not for too long.

One tapped, fit the backplate to the spindle,and skim the face and then the OD to be a snug fit in the chuck.

Hopefully, the holes in the backplate will line up with tappings in the chuck.

Let us know how you get on!

Howard

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Steve355, if you run your tap through it, you will probably have very little thread left in it to hold it on securely. Like Duncan says, make a new one, you can get a suitable unmachined one from Chronos

You could always flog the one you have maybe.

Regards Nick.

[DiogenesParticipant @diogenes]

If you have the tap, buy a slice of CI and make one.

[JasonBModerator @jasonb]

Whatever way you make it I would screwcut or thread mill the thread on the CNC, less chance of getting a wonky thread than doing it with a large coarse tap. At the very minimum do about 90% by machine then complete the final profile with the tap.

A slice of 65mm CI would be sufficient to make it from and 1/4 the price of a backplate casting.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, yes a slice would do it, I just mentioned the one from Chronos, as it has the normal shape, and therefore less machining and less waste of material.

I’ve used a slice of steel at times, but I’ve been lucky enough to get pieces of the right diameter from work. I agree with the way JasonB has said to thread it.

Regards Nick.

[DiogenesParticipant @diogenes]

Yes same here, apologies forgot to mention technique!

[cogdobblerParticipant @cogdobbler]

I heartily agree with Jason’s advice to screwcut the thread, and maybe use your tap to clean up the last  couple of thou. This is more likely to give you a thread that is straight, square and concentric. (Plus a lot less sweat involved !)

These days screwcutting boring bars and inserts are so  cheap and do such a good job it really is a doddle. And cast iron is nice material to screwcut.

[Steve355Participant @steve355]

I think what Jason is suggesting is to use the CNC and thread mlll it.

I am quite tempted to:

1) get a lump of slightly oversized cast iron or steel

2) Surface grind flat on both sides

3) Get the CNC to mill all 4 holes

4) use the CNC to thread the centre hole, test/finish with tap

5) finish off the backplate on the lathe – diameter, fit, face etc.

 

i don’t have a thread milling  bit and I’ve never done it before. But it’s good fun to try,

[JasonBModerator @jasonb]

I was suggesting that you don’t use the tap. Either screwcut on the lathe or threadmill on the CNC

No need to grind both sides as the chuck side gets finished when it is on the lathe. I’d also tend to mill the face that goes against the spindle register and do the thread in one setting ensuring that surface is true to the bore/thread. So mount on the CNC in the orientation you have it in your first image. Level the top, adaptive and finish cuts to form the spigot with a corner radius cutter to leave a fillet in the internal corner. Use the mill to bore the 5 holes and then threadmill, knock off edges with a chamfer mill. Mount on lathe and turn larger OD, face and any chuck locating spigot.

 

Edit. Looking at the cost of thread mills of that size (£70+) I’d screwcut it on the lathe. Though I suppose you could make a single point threadmill which would not be as costly as a carbide thread mill.

 

[Steve355Participant @steve355]

I’m pretty sure I screwcut the last edit backplate I did, before finishing it with the tap. The difficulty I had with that, as that it was strapped to the faceplate, which has quite a small centre hole, so I was constantly worried about crashing the screw cutter. So I did what I could and finished it off with the tap. In practice it is accurate enough for a 3 jaw.

Somewhere I still have the cutter I ground for making that screw. Tbh the problem might be that there could be a small amount of wear on the lathe lead screw, which could result in a slightly less than perfect thread. But I guess that’s where the tap comes in.

This backplate is much smaller and the blank can probably be held in my existing 3 jaw, which makes the whole thing much easier to do on the lathe. I remember fixing the last one to the faceplate was tricky.

The problem is the 3 locating holes. I’m pretty sure that last time, I simply used the chuck as a template, centre punched them and drilled them on the pillar drill one by one. To do them on the CNC could actually be tricky. It looks like their centres is 1 inch from the centre of the central mounting hole… and it looks like they are 120 degrees apart. But that may not be exactly the case! Using the chuck as a template seems like a good idea.

 

[cogdobblerParticipant @cogdobbler]

Put packing pieces between the job and the faceplate to create clearance for your boring bar to run into. Pieces of square HSS tool bit or key steel work well for this.

Machine the thread, the register recess and the back face all in one setting so they are all true to each other.

Wear on your lathe leadscrew will not be a factor unless you have a phenomenally clapped out lathe. It’s a short thread length so error will be negligible in practical terms. That’s why the halfnuts are brass or Mazak , so they wear but not the steel leadscrew. I have screwcut backplates and similar on lathes made before World War 2 with no problem. And your tap used to clean the last 2 or 3 thou out will correct any error as you say.

 

 

[Steve355Participant @steve355]

>>Put packing pieces between the job and the faceplate to create clearance for your boring bar to run into. Pieces of square HSS tool bit or key steel work well for this.

Obvious now you mention it! As I recall, I roughly drilled and tapped the blank so I could bolt it to the faceplate. Then I turned the spigot and turned the overall backplate to diameter, but I couldn’t turn it all the way due to the faceplate. Some packing would have been ideal.

[Author]

Posts