Enlarging Holes in Thick Plate

[awemawson]

I need to enlarge some 22mm mounting pin holes in my Twose 276 Hedge cutting Flail to take 1" pins

This is the Flail:

[awemawson]

And these are the mounting lugs with the holes in question:

[RussellT]

Turn some bushes for the holes on the tractor.

Russell

[porker]

Just lift it onto the mill and bore....

But seriously I think this is a job for a mag drill I would have thought. Don't know how accurate they are though so you may need to drill oversize and make some bushes.

[tekfab]

Do you have access to a mag drill ? If you're careful at the start then it'll do it no problem.  If you're not confident then you can tack weld a plate behind the hole to locate the spring centre.
Mike

[awemawson]

All a bit too big to put on the table of the Bridgeport 


The side plates with the holes are 1/2" mild steel plate. My plan was to cut a hole in another piece of 1/2" plate with a 1" Starrett hole saw. Clamp it over the 'to be enlarged' hole, centring using a suitable stepped plug, and using a mains hand drill cut the new holes.

I did some experiments with a couple of Starrett hole saws, and they were quite happy cutting 1/2" plate using their outer diameter as a guide.

Then I find I can no longer buy 1" cutters. Nearest metric equivalent is a 27mm, which actually when tested cuts a 27.5 mm hole. Now accuracy is not too critical - these are load bearing (but not revolving) pins - but 27.5 mm ie 2.3 mm is too much slack.

I am trying to keep to Starrett as they are good quality and seem up to the job and was amazed that 1" was no longer available

[awemawson]

OK chaps you got in with replies while I was typing !

Yes I have a mag drill, but the access precludes its use 

Yes I can turn bushes, I can make stepped pins and all sorts of other solutions, but I want to INCREASE the pin size not decrease it. There is a HUGE torque sideways when this is mounted, and I'm planning on using 1" EN24T shaft as pins

In a previous life this flail was just mounted on the 3 point linkage, and the differential torque on the lifting arms twisted and sheared the main cross shaft that the lifting cylinder acts on, and that was 1 1/2" steel of some sort. This new set up will mount directly on the tractor and not use the lifting arms at all.

[porker]

If they cut oversize would a 25mm cutter get you close to the 1" diameter?

[awemawson]

I had ruled out the 25mm, as oversize would be ok'ish but undersize would be a no-no. and 1" is 25.4 mm as we all know


.. . . .  until I spoke to Starrett Tech Support in Scotland 

It seems that the 1" Starrett constant pitch hole saw sold in the USA is the very same 25mm hole saw they sell in the UK 

What they do with the missing 0.4 mm is probably best left unanswered 

[John Stevenson]

Probably falls overboard on the way over the Atlantic. 

[BaronJ]

Can you not use a 1" Blacksmiths drill.  They have 1/2" shafts and come in imperial sizes.  I paid £4 each for mine.

[Manxmodder]

Hi Andrew, just going back to Russell T's suggestion of bushing the eyes on the tractor.

Yes it does reduce the pin diameter by 3mm but surely a slightly smaller pin made from a high grade steel could compensate for that.

A couple of other alternatives:Make up a piloted cutter with the pilot a snug fit in the holes and long enough to extend through both holes of the yoke. Drill one side out then with the aid of a bush with suitable external diameter to fit in the enlarged hole and internal corresponding to pilot size enlarge the second hole from the opposite side.
Also Bahco do some very good quality bi-metal hole saws,maybe you could find a suitable size in their range....OZ.

[Manxmodder]

Just another thought, what about using an expanding reamer,a little labourious but I think it would get you the result....OZ.

[awemawson]

Can you not use a 1" Blacksmiths drill.  They have 1/2" shafts and come in imperial sizes.  I paid £4 each for mine.

Thanks, but starting a 1" blacksmiths drill (which I have) in a 22 mm hole will inevitably snatch and do me some damage 

[awemawson]

Hi Andrew, just going back to Russell T's suggestion of bushing the eyes on the tractor.

Yes it does reduce the pin diameter by 3mm but surely a slightly smaller pin made from a high grade steel could compensate for that.

A couple of other alternatives:Make up a piloted cutter with the pilot a snug fit in the holes and long enough to extend through both holes of the yoke. Drill one side out then with the aid of a bush with suitable external diameter to fit in the enlarged hole and internal corresponding to pilot size enlarge the second hole from the opposite side.
Also Bahco do some very good quality bi-metal hole saws,maybe you could find a suitable size in their range....OZ.

Easiest solution is to turn a stepped pin and leave holes present size. However that is asking for a fatigue crack at the transition point.  I really want to beef up the pin so I will go with the 25mm but in reality 1" hole saw that I now have on order.

[awemawson]

Just another thought, what about using an expanding reamer,a little labourious but I think it would get you the result....OZ.

Yes I thought about that as well ! However access is very difficult, the machine is balanced precariously, and the thought of trying to turn an adjustable reamer, keep it in line, all lying on the floor under a heavy implement doesn't appeal

[Manxmodder]

It will be interesting to see how you solve this,please post up a couple of photos of the job in progress....OZ.

[tom osselton]

Can you make a boring bar that fits the existing holes? Then just set the cutter for the 1" dia with a 1" plug to slip on  for when doing the other side to keep the holes lined up, you should be able to use a drill to power it.

[BillTodd]

tom beat me to it

just a length of 22 mm bar with a hss or carbide single point cutter mounted in it. single point could be driven with a good battery drill if necessary.

clamp.on a couple of extra 22mm holes as  guides.

bill

[DaveH]

I would go with the hole saw and plenty of cutting fluid - remember it is a round hacksaw blade. 
In stead of of the 1/4" drill bit use a 22 mm diameter guide. The hole will probably come out a bit bigger than 25mm - well mine always do 
 
DaveH

[awemawson]

Yes Dave that's what I'm now going to do, now Starrett tell me their 25mm saws are actually 1"   

I'll make a guide by cutting a hole in a thick bit of plate and clamp it in place as I originally intended but was initially thwarted by the apparent lack of 1" saws 

[RotarySMP]

Comparing the hole producted by a 1" hole cutter in a hand drill, to line boring:

Line boring:
+ accurate size to whatever tolerance you want especially if you make guide bushings for the existing holes,
+ round holes
+ both holes concentric / aligned.
+  in the correct place (assuming the existing holes are reasonably round and in the correct place).
+ more controllable, adustable process,
+ safer for the operator. Only torque to be reacted.

- Have to make up the long boring shaft, at least one guide bushing, probably three, if you use a piece of 20mm or 3/4" stock for the boring bar.

Hole cutter (hand held drill, unpiloted - guided by 1/2 plate):
- no control of diameter. Starrett knows a thing or two about metrology, and considers a 25mm and inch hole made with these things as equivelent. That is a strong hint.
- no control of alignment between two sides.
- terrible surface finish in holes
- more danger to operator if it grabs
- have to make the 1/2' guide plate.

Regards,
Mark

[awemawson]

The job doesn't justify line boring, and also that would be very difficult to arrange 'in the field' although specialist kit is available to do it. If this was the sockets for rotating pins on, for instance, a back hoe then yes. However in this situation it's the hole saw - horses for courses.

Actually I was surprised how good the surface finish was on my test samples, but obviously not as good a boring

[bertie_bassett]

last time i had to do something similar, i  found a nice solid fallen tree branch and hammerd it in the origional hole as a plug. i then just used my normal hole cutter with pilot, centerd as best i could.
worked fine for me and got me out of trouble when all the shops were shut and i was miles from a workshop.

[awemawson]

Well the Starrett hole saw arrived today so I set to and enlarged the holes:

First clear the nettles, brambles and other detritus that I'd otherwise have to roll around in - I also hauled the huge hydraulic pump out of the way.