Building a Belt grinder – advice please.

[Pete.Participant @pete-2]

Hopper, I think it's because the only powered wheel is the motor, if something got jammed the belt would come off pretty easily, leaving only the motor wheel spinning,

The only accident I ever saw with a sander was in wood work class at 13 years old, 2 people were playing who could put their finger closest to the 12" disc while spinning, the winner went to bed that night with a finger 1/4" shorter than when he woke up.

[TerryBParticipant @terryb19868]

Posted by Peter F on 25/04/2019 23:44:51:

Paul, Terry said he made a design mistake on his, that was present on the first picture I posted, what is this design fault?

A bit difficult to explain with my poor Engrish skills.

The tracking wheel needs to have a slight crown to keep the belt centered. So basically you only need to angle it ever so slightly to make it track in the center.

The way they have designed that one, it moves at an angle to the belt, not perpendicular? If you can understand what I mean? That basically defeats the object of the crown. I'm guessing they are using a parallel wheel there and forcing the belt to move to the highest point, which in turn is putting a lot of stress on the edge of the belt. This is my observation.

[Paul LousickParticipant @paullousick59116]

Geremy Scmidt's web sit with drawings available for download: https://jerswoodshop.com/

For a BIG GRINDER check this Youtube video.https://www.youtube.com/watch?v=UXzoM-WXWRw

Paul

Edited By Paul Lousick on 26/04/2019 02:27:33

[Neil WyattModerator @neilwyatt]

I must admit, for what I paid for it my little disc and belt grinder is one of those tools I would replace straight away if it expired.

I've used if for everything from touching up HSS tools, putting a decorative finish on flat steel, re profiling wood and plastic items – I even used it to take 1/8" of a set of flight case corners I wanted to use at 90-degrees to their normal orientation!

The unsupported section of the belt flexes enough you can use it to carefully touch up convex shapes too.

I bought a supply of quality belts then discovered that one good belt outlasts a packet or two of cheap ones!

Neil

[SillyOldDufferModerator @sillyoldduffer]

Posted by Neil Wyatt on 26/04/2019 11:19:12:

I must admit, for what I paid for it my little disc and belt grinder is one of those tools I would replace straight away if it expired.

…

Me too. Small, basic, inexpensive and useful. I suppose, as always, it depends on what the tool is for. Eminently suitable for modelling, totally unsuitable if you're rewiring a factory and have 5000 lengths of steel conduit to square-off!

[jaCK HobsonParticipant @jackhobson50760]

The high power knife grinders can be reasonably dangerous but I guess losing whole fingers is less likely than a lathe. I've trapped a finger, ground the odd knuckle, and a common reported issue is a snapped belt slapping you in the face (never happend to me). Guarding is sensible but hugely impacts flexibility of the tool when using the slack portion of the belt. The Goset grinders like I linked to in my first post now have much better guarding with CE approval http://goset.com.pl/pl/szlifierki/ but they cost more. I really like them – well made, good design, and you get a lot for your money.

I would think you can at least half the power if you are using it for wood – the limit on belt grinding wood seems to be the heat generated which burns the wood. The big power is only required for rough shaping metal on coarse grit – if you are just finishing up a mitre on a 2×4 then you will not be needing anything like 1HP per inch!

[Robin GrahamParticipant @robingraham42208]

Posted by SillyOldDuffer on 25/04/2019 13:39:33:

Basically the power needed to drive a machine tool is related to the metal being cut and how fast you want to remove it. A small motor can do the same amount of work as a big one, it just takes a lot longer!

How much work is required to remove a cubic millimetre of common metals:

Aluminium Alloys – 0.3 to 1J
Brass and Bronze – 1.3 to 3.2J
Cast Iron = 1.1 to 6J
Steel = 1.9 to 9.2J

It takes about 2 Joules to remove a cubic millimetre of mild-steel and a Joule is a Watt-second.

Apologies to the OP if this is veering off topic, but these numbers interested me because I've been trying to make some similar calculations for a project unrelated to grinding.

There is a rule of thumb that 1 HP can remove 1 cubic inch of mild steel per minute assuming motor power is the limiting factor. That is what I've been working from.

1 HP is about 750W, so applied for a minute the work done is about 750 x 60 Watt-seconds or 45,000 Joules. Google tells me that a cubic inch is 16,387 cubic millimetres, so that gives around 2.75 Joules per cubic millimetre which is comfortingly close to Dave's figure. I'm surprised how close actually, because it seems to me (purely subjectively) that grinding creates more heat to remove a given amount of metal than turning or milling.

Where did those numbers come from Dave? Not disputing, just trying to educate myself.

Robin.

[PlasmaParticipant @plasma]

This is a quarter scale scratch built machine loosely based on the American machines popular with knife makers. Same principles of belt tracking and tension apply. Made with gauge plate fabrications it could easily be scaled up.

[PlasmaParticipant @plasma]

So a 2hp single phase motor 2400 rpm will be up to the task of running a contact wheel and belt no problem

Would a speed control through say an inverter drive be a benefit?

I have found a source of brand new old stock belts but pretty huge at 3.45 m long! Looks like I will need some decent idler pulleys set up.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Plasma on 28/04/2019 12:08:19:

This is a quarter scale scratch built machine loosely based on the American machines popular with knife makers. Same principles of belt tracking and tension apply. Made with gauge plate fabrications it could easily be scaled up.

.

Nice one

It looks potentially useful in its own right

… Just needs a finer abrasive on the belt.

MichaelG.

[Pete.Participant @pete-2]

Posted by Plasma on 28/04/2019 12:08:19:

This is a quarter scale scratch built machine loosely based on the American machines popular with knife makers. Same principles of belt tracking and tension apply. Made with gauge plate fabrications it could easily be scaled up.

That's quite a nice little machine, similar to what I want to build, but with the 2 wheels and and backing plate rather than a single contact wheel, did you crown the tracking wheel?

I'm still debating the idea of speed control.

[PlasmaParticipant @plasma]

Yes the drive and idler pulleys have a slight crown.

I just bought a 200mm dia rubber contact wheel off Ebay and there is a supplier doing idler, drive and other front end bits for these grinders if you cant produce them. Just have to supply motor and framework.

I'm hoping to make use of even the longest belts but even if I cant the price I paid means I still win if I use them as Emery tape.

Picked up a 2hp 2400 rpm motor so will try it without speed control.

But I do have a newton Tesla inverter drive motor package bought for a lathe project that might be very good for a belt machine.

Mick

[JohnFParticipant @johnf59703]

Peter — I have not read all the posts but came upon this by chance, maybe its of interest ? **LINK**

John

[Pete.Participant @pete-2]

Posted by JohnF on 01/05/2019 18:08:58:

Peter — I have not read all the posts but came upon this by chance, maybe its of interest ? **LINK**

John

Thanks John, yes that's very similar to what I have in mind, only using 4" wide belts not 2" and making it use belts 36" to 48" to keep it more compact.

[Pete.Participant @pete-2]

Plasma, report back with your views on the 2hp motor without speed control and let us know what you think, I have to say, I'm leaning towards just using a non speed controlled version on mine to start, see how it goes.

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