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Accuracy of BDMS

Was I unlucky?

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Oily Rag14/10/2021 19:47:05
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I have been asked to make a re-designed blades and shaft for a cremulator (do you really want to know what that is? ) as the current system regularly suffers from failed bearings on a 6 monthly basis. The current design has a 1.2kW motor driving a vertical shaft and supported by a flanged self aligning bearing outside of a drum, inside the drum the shaft is supported by a flanged oilite bush which quickly degrades with the abrasive nature of the contents. The blade bosses are attached via a 4mm keyway and grub screws.

I have therefore upgraded the blade shaft from 13mm to a 20mm shaft and changed the design to a square form for the blade locations. Blade bosses will be laser cut to give a square internal form, grub screws are deleted and the whole assembly is clamped by a M16 threaded nut on the shaft end. Under the bottom boss is a 8042RS sealed ball race in stainless within an alloy housing clamped to the drum base and a suitable 'O' ring sealing under this.

I ordered some 20mm BDMS and proceeded to cut to length and centre the ends prior to machining a keyway for the Fenner bottom drive adaptor (this is currently 20mm dia in the original design so making the whole shaft 20mm seemed a good idea and saved on the machining required ). When I came to cut the keyway I found the shaft was bent, making the square section an odd shape! as the flats of the square do not fully meet their adjacent flats. I then checked the bar and found it was bowed by 0.2mm over 140mm length (0.008" over 5 1/2" ). Picking up the rest of the bar it was twisted and bent - I took the piece back to the metal suppliers and asked for a decent bit of 20mm BDMS that wasn't like a dog's back leg, they had to dig out some old stock that was something like acceptable. The boss was muttering about the quality of the material they had been receiving lately and showed me a length of Aerospace specification alloy bar which had visible inclusions on the surface, and showed me a internal void exposed where they had cut the material for a RR order.

What is the generally accepted allowance for straightness in BDMS? The diameter was pretty low at 0.04mm undersize, the old stock was much better at 0.01mm under.

Two guesses at where the material originated!

Martin

Tony Pratt 114/10/2021 20:16:08
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No idea what the specs for BDMS are but it's not the best idea to use such things for your purpose 'straight from the mill' so to speak, you are likely to get size & straightness issues as you have found. Anecdotal evidence suggests engineering materials are not as good quality as they were back in the day, just like many other products.

Tony

JasonB14/10/2021 20:37:04
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The bend could just have well been bad storage or handling of the 6m bar as it could manufacture, sling a load of bars wrongly on stack them badly and they can sag and bend.

Even gauge plate (ground flat stock) has a flatness tollerence of 1mm/m 

Edited By JasonB on 14/10/2021 20:38:29

SillyOldDuffer14/10/2021 20:38:46
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Posted by Oily Rag on 14/10/2021 19:47:05:...

What is the generally accepted allowance for straightness in BDMS? ...

I don't think there is one. Bright Mild Steel is made by rolling, which is a rough process, not particularly accurate. Although Bright comes with a better finish than Black Mild Steel, both are cheap steels intended for structural work, maybe unsuitable for what you're doing. Buy a Ground Steel if accuracy is needed.

My experience of Bright Mild Steel for hobby purposes is it's 'good enough' rather than wonderful, but then it's not meant to be anything special.

Dave

Martin Connelly14/10/2021 20:50:15
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I have always assumed that BDMS comes with built in stress due to the cold drawing process. There is plenty of past advice saying that it needs stress relieving before use or that it needs even machining rather than one sided machining to avoid it moving after one stressed face is machined off.

Martin C

JasonB14/10/2021 20:51:47
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Straightness allowance chart for bright mild, yours was 1 in 700 so within allowable for low carbon

Edited By JasonB on 14/10/2021 20:57:10

Andrew Johnston14/10/2021 21:01:04
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Posted by SillyOldDuffer on 14/10/2021 20:38:46:
.....Bright Mild Steel is made by rolling......

The clue is in the name - Bright Drawn Mild Steel.

Sections are formed by cold drawing the material through a die. That's why the outer skin gets stressed. Accuracy is better than hot rolled but not as good as silver steel or gauge plate. JB beat me to the straightness spec.

Andrew

Speedy Builder514/10/2021 21:04:00
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Perhaps Martin needs some "Dead straight" bar ?

Two hairs width over 5.5" - that's not so bad. Now if that was ground bar, I would be worried.

duncan webster14/10/2021 21:04:28
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I think the clue is in the name Bright Drawn Mild Steel

The diameter tolerance is here BDMS, it's good enough for many applications

Oily Rag14/10/2021 21:30:09
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Thanks for all the replies, and yes I guess it was in tolerance according to Jason's data. Although the bend was 0.2mm (0.008" ) in the centre of a piece 140mm long (i.e both ends touching the surface plate and an 0.008" feeler gauge inserted beneath in the centre )

Although the machine is not what you would describe as a 'precision' piece of engineering it has run for a number of years on the original design (which, to be honest, I never liked in the first place as the shaft was effectively overhung too far from the flanged self aligning bearing at the base of the drum - the oilite bearing was pitiful ). I wanted to support both extremities of the shaft but in this small machine it is difficult to achieve - a much larger machine with multiple contra rotating blades on 3 shafts has worked from day one without any trouble. These shafts are end supported. Guess next time I upgrade another I'll have to go for 22mm (7/8th) and turn it down for its whole length or get some centreless ground bar.

The 'cremulator' by the way is a machine for reducing the rather large 'clinker' left over post cremation into a neat pile of dust for storage in a jar on the mantlepiece! Horses are the most difficult needing the big ones, the small ones are for Pussies and Rover. Humans come in between.

Martin

Edited By Oily Rag on 14/10/2021 21:33:07

SillyOldDuffer15/10/2021 09:25:44
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Posted by Andrew Johnston on 14/10/2021 21:01:04:
Posted by SillyOldDuffer on 14/10/2021 20:38:46:
.....Bright Mild Steel is made by rolling......

The clue is in the name - Bright Drawn Mild Steel.

...

What on earth was I thinking? Oh dear...

Nurse!!!

blush

bricky15/10/2021 09:32:58
529 forum posts
68 photos

I have not seen this machine but I used to have to cut from a 100mm log of lignum viti a slice 1 1/2" thick and cut it square to fit in an angle iron support.The engineer who I did this for would drill the timber to take a verticle shaft in a large food mixer and as it is self lubricating they lasted a long time.I don't know if that might be of any help but it worked.I did ask him why not use ball bearings he replied that they always did it like this and it worked so why change it.Cutting the timber did,t do my band saw a lot of good it's hard.

Frank

Oily Rag15/10/2021 12:21:19
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Frank,

That is an interesting alternative to the oilite bush. Think I'll look into that as a possible alternative for a future update if this doesn't work out.

The problem seems to be the ash is rather like talcum powder and gets into everything. The sealed (2RS) bearings have been upgraded to 'agricultural' specification with stainless races and tracks and a double lip seal and are for use in slurry pump applications, so we will see how good they are in 6 months or so.

The replacement BDMS was much better and has about a 0.15mm bow over an unsupported 1 metre length, and without the twist. I machined it last night and it is not warping so looks 'good to go'. The size is better as well, as the bearings fell onto the original dogs leg BDMS, but is now a nice sliding fit.

I have some 1" and 1.5" dia by 10ft lengths of BD which literally 'fell off the back of a lorry' when a mate was following it down a country lane (he nearly went in the ditch trying to avoid the tumbling bars! ). He picked up 12 lengths and gave me half of them. Interesting that they have been pretty accurate despite their 'poor handling' history. They are not MS though as the colour code suggests they are EN16 or so.

Martin

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