SPINDLE DRAWBAR FORCE GAGE - Frequently Asked Questions

Frequently Asked Questions

1) What is the difference between using a standard tool holder as the spindle adapter vs. using the specially designed Clamprite Spindle Adapters?

Answer: A standard end mill holder can be used as the interface for drawbar testing. Our Adapters cost less than most tool holders, and since there are no tool changer flanges, the 50 taper (one unit) adapter fits all CAT, BT, and ISO spindles. Finding the right holder can be time consuming because it is often back in use on a machine when it is needed.

But the best reason to use a Clamprite Spindle Adapter is because we have found tool holders to be out of tolerance to the gage line dimension of the spindle. In fact, some tool holders (from reputable manufacturers) were .0125 over the high tolerance limit. Some were nearly that amount below the low tolerance limits. This amount of variation can cause great differences in spindle drawbar force readings.

All Clamprite Spindle Adapters are inspected with spindle taper gages to verify gage-length accuracy. A spindle that has .001 wear on the surface will allow a tool to seat .0069 deeper into the spindle pocket -- well past the gage-line tolerance limits, and enough to cause reduced overall drawbar holding force!

2) You commonly refer to a "Retention Knob" as a "Pull Stud". Is there any difference?

Answer: No, they are the same. In the 60's and 70's "pull stud" was more common. But today, "retention knob" is used more. Both are correct, but "retention knob" seems more popular.

3) I have various machines with different pressure values up to 4600 Lbs. Can I use the 5000 PSI pressure gauge for all the smaller, lower pressure machines too?

Answer: Yes. However, reading the numbers on the 3000 PSI pressure gauge is easier than on higher pressure gages because there are fewer numbers on the dial. But a slight difference in reading accuracy is not that important because there is little concern about 10 or 20 Lbs -- or even 100 Lbs. The real concern is when a machine is 20% - 30% low on drawbar force. Technicians usually accept 10% variation even when rebuilding a drawbar.

4) What about the accuracy of the Clamprite Drawbar Force Gage versus the much higher priced products? It is mentioned how the others are “extremely accurate” in your sales brochure. Isn’t it better to have higher accuracy whenever available?

Answer: Yes ... and no! More accuracy is great when FREE, or slightly more expensive. We believe the electronic gages are overkill for the shop floor in terms of accuracy and cost. The extremely accurate and expensive electronic components (similar to those used in the electronic drawbar force gages) are what we use to certify the Clamprite Gage. Our accurate but rugged gage is better suited to a shop environment where delicate electronic gages are easily damaged. If a Clamprite Drawbar Force Gage is dropped, a pressure gage could cost $40. An electronic gage can cost over $1,000 to repair. So, a new Clamprite Drawbar Force Gage can be less than the repair cost of a more expensive electronic drawbar force gage.

5) My CNC Mill has a high speed HSK type spindle. Can I use the same CG to test this machine?

Answer: Yes, the CG used for testing the more common long taper spindles is the same for the machines using the HSK spindle system. However, a special adapter is required for HSK spindle testing. These special adapters, available for all HSK spindle sizes, are very accurate, durable and low in cost.

If using HSK equipped machinery, a Clamprite Gage is needed. HSK spindles must be tested frequently to assure that they are within 75% of spec at all times or the tool holders will not seat correctly. Besides higher drawbar force requirements, there are many more issues with the HSK system to address.

6) I can’t find the drawbar force for my machine anywhere in the manuals. Can you tell me what the drawbar force my machine should provide?

Answer: There are so many variations, even among very similar machines. Only the manufacturer can say for sure. Some general figures: 1800 to 2500 Lbs for most 40-taper spindles, 3800 to 4800 Lbs for most 50-taper spindles except heavy-duty versions which may be considerably higher. This is just a reference, NOT a guide for any specific machine or spindle. To obtain the specific pressure for any particular machine, get the model number, the serial number, and date it was built from the nameplate and consult the factory representative.

7) Your instruction manual of older Clamprite Gage kits (prior to Oct. 2005) shows a stepped transfer screw, and a straight one with 1/2-13 UNC-2A threads full length. Which is best to use?

Answer: Note: This does not apply to Clamprite Gage Kits manufactured after October 2005. The new style gage kits use a transfer screw coupling. For older kits the stepped down end of the D1009-1 and –2 Transfer Screws are used with 40-taper pull-studs and smaller. Use the D1009-3 Transfer Screw with 45 and 50 taper spindles. Additional transfer screws are available for nominal cost. The stepped down screw is provided with each set because of its versatility, unless otherwise requested when ordering. Do not substitute common threaded rod for these transfer screws. Clamprite screws are higher tensile strength, and threaded rod will likely break during use. Injury to operator or equipment can result.

8) I have a 50-taper spindle on a geared head machine designed for cutting steel. Can I use the 5000 PSI pressure gauge to test it?

Answer: Most 50-taper machines are below 5000 PSI, usually from 3800 to 4800 PSI. But some heavy duty 50-taper machines can go to 7800, or more. If you over pressure the gauge, it may become inaccurate. See precautions in the instruction manual. As a courtesy to our customers, Clamprite will replace a pressure gauge with another gauge at no labor cost, if shipping is prepaid both ways. The complete re-certifying procedure is done for a nominal fee of just $59.95 (includes certificate traceable to NIST).

9) One of our machines is very large and was designed for heavy duty use. Someone said it may provide over 10,000 Lbs of drawbar force. Can the CG be used to test its drawbar?

Answer: The Clamprite Gage II is a heavy duty version of the standard Clamprite Drawbar Force Gage. The standard Clamprite Drawbar Force Gage will test up to 10,000 Lbs when fitted with the proper pressure gauge. The Clamprite Gage II will test up to 15,000 Lbs. Contact us for testing of forces over 15,000 Lbs. If you attempt to test the spindle and it is over 10,000 Lbs, the 10,000 PSI pressure gauge may be ruined and hydraulic fluid may squirt out causing injury. See precautions in the Instruction Manual. Your gage body can be re-fitted with a higher reading pressure gauge at no charge for labor if postage is paid both ways. There is a nominal charge for re-certification, recording and issuing a new certificate with traceability to NIST ($59.95).

10) Can a Clamprite Drawbar Force Gage fitted with a 10,000 PSI pressure gauge be used to test spindle with 2000 Lbs of force?

Answer: Yes, but reading a 3000 PSI gauge is slightly easier than reading a 10,000 PSI gauge because there are fewer numbers on the face of the pressure gauge but the 10,000 PSI gauge will show you the drawbar force that your machine is producing. Changing gauges should only be done by qualified hydraulic repair personnel. As a courtesy to our customers, Clamprite will replace a gauge with another one of different pressure at no charge, except for the gauge and re-certification. Labor is free. Use caution if you attempt replacement it yourself, as the fluid can squirt out and cause skin irritation, and improper bleeding of the gauge and body may cause false readings. Also, commercial gauges can vary greatly in accuracy. Clamprite uses only high accuracy stainless steel pressure gauges, and we also test and certify each assembly for accuracy on a certified electronic test bench.

11) We are a small shop with no customer requirements to verify our drawbar force. How long can we go without checking or re-certifying the Clamprite Drawbar Force Gage.

Answer: A lot of attention is being given to machine condition today. Some companies, especially aerospace, are even condition-coding machines and requiring accuracy tests such as “ball-bar testing” to be performed by vendors before parts can pass QC requirements. It is just a matter of time before they realize that a drawbar is just as critical to consistent machining as the axis drives and ball screw condition. But under your circumstances, you might go a few years without worrying about the gauge accuracy. If the Clamprite Drawbar Force Gage is not over-pressured, it should remain accurate for many years. However, Clamprite will check the accuracy of your gauge for free as long as you own it, if shipping is paid both ways. We do not record and issue a new Certificate of Accuracy when doing a free courtesy check. But it will verify that your gage is still in the original condition. There is a nominal fee of $59.95 for a complete re-certification.

12) In advertising and in your Instruction Sheet you spell the word gauge as simply “gage” at times. Why the difference?

Answer: Gage (spelled this way) is somewhat traditional in the metal working trades, except when specifically discussing a pressure gauge. The bourdon-tube type of pressure gauge has come to be known as simply “a gauge“, whereas many other gages (like radius gages, depth gages, etc.) are more like a method of measuring. Most tool making companies like the L.S. Starrett Co. for example, still use the older spelling of gage when describing their products. Most dictionaries list both spellings as acceptable.

13) My machine uses hydraulic pressure to hold the tools in the spindle. Will the Clamprite Drawbar Force Gage work for this application?

Answer: Yes. Whatever the means of creating tool retention force, that force can be measured with the CG.

However, there is some confusion about this. Nearly all machines use the spring stack method of tool retention . But the method of releasing the tool from the spring pressure comes from a hydraulic piston which is usually energized by an air cylinder. Compressed air is supplied to the air-over-hydraulic piston (usually located on top of the spindle) which acts like a small jack-type press for compressing the spring stack which in turn releases the clamping force acting on the tool holder’s pull-stud. Sounds a little complicated, but is actually fairly simple once you have seen all the parts laid out. Sometimes people are confused about how the hydraulics work. The air-over-hydraulic piston can be heard during a tool change, but it is usually used to release the tools, not to hold them.

14) Some of our operators are experienced mechanics, so why can’t we just repair our own drawbars and save the money.

Answer: Some shops do repair their own drawbars, but we recommend finding a qualified technician or factory trained personnel to adjust or repair drawbars. It usually takes about 5 hours, plus a few parts to restore drawbars to the original condition (about $800 - $1500). With all the wires, air lines, sheet metal, heavy parts, everything slippery and high off the floor in a place where it’s easy to drop things where they can cause more trouble later on. It’s one thing to be a good mechanic, but for this you also have to crawl on top of the greasy, slippery machine where everything is sharp and a fall could easily happen. Most repair facilities have one or two guys who specialize in drawbar repairs and have mastered the technique over the years. In the long run, they will probably save you money and maybe a major injury!.

Regular drawbar maintenance (about 2 to 5 year intervals depending upon the kind of use) is money well spent, and good insurance against disaster! But if you intend to repair your own machines, pay close attention to all the length measurements of the original drawbar assembly, and the method of spring stacking. Where and when to use grease or anti-seize is important. And where to use Loc-Tite (or similar products) is also important. Another thing, factory purchased springs are recommended over commercially available springs. Theoretically they are the same, but factory springs seem to last longer. Maybe they are pre-tested or have some specific quality procedures that others do not have. Some also have special coatings that promote longer life too. If you choose to repair your own, allow plenty of time on the first job! After you have finished one drawbar, the second and third time goes much quicker. But that first one can be a pain. One more thing; if the whole assembly is not adjusted properly on the bench (takes experience) you will have to start all over again, because there’s really no way to test the pressure or length settings before you replace the whole thing back into the spindle and hook everything back up to run.

15) You used to offer an adapter to test pressure on a Bridgeport mill. Why test pressure on a manual machine?

Answer: The Bridgeport Adapter was designed to give people a way to see and feel the kind of pressure that can be generated simply by turning a screw with a wrench. Most people are surprised when they use the CG on a Bridgeport type mill. It shows that most people apply about twice the tool holding force than most 40-taper CNC Machining Centers when they are new! It was a real eye opener and a lot of fun, and it might have saved some Bridgeports from damage, too.

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